# Chemical hydrogen storage materials – boranes and silanes catalytic solvolysis and dehydrogenation: a mechanistic and regeneration perspective

Gizem Karacaoglan, Maxime Thibault, Julien Roger, Nadine Pirio, Katia Fajerwerg, Myrtil L Kahn, Umit B Demirci, Jean-Cyrille Hierso

<figure><img src="image_1.png" /></figure>

## To cite this version:

Gizem Karacaoglan, Maxime Thibault, Julien Roger, Nadine Pirio, Katia Fajerwerg, et al.. Chemical hydrogen storage materials – boranes and silanes catalytic solvolysis and dehydrogenation: a mechanistic and regeneration perspective. Coordination Chemistry Reviews, 2026, 548, pp.217094.
10.1016/j.ccr.2025.217094. hal-05343553

HAL Id: hal-05343553

https://hal.science/hal-05343553v1

Submitted on 3 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Chemical hydrogen storage materials – boranes and silanes catalytic solvolysis and dehydrogenation: a mechanistic and regeneration perspective ☆

Gizem Karacaoglan<sup>a,1</sup>, Maxime Thibault<sup>a,1</sup>, Julien Roger<sup>a</sup>, Nadine Pirio<sup>a</sup>, Katia Fajerwerg<sup>b</sup>, Myrtil L. Kahn<sup>b,*</sup>, Umit B. Demirci<sup>c,*</sup>, Jean-Cyrille Hierso<sup>a,*</sup>

## ARTICLE INFO

**Keywords:**

ammonia borane

Sodium borohydride

Hydrosilanes

Transition metals

Catalysis

Solvolysis

Dehydrogenation

Regeneration

## ABSTRACT

Hydrogen ($H_2$) has gained a lot of interest as an alternative energy vector, to reduce greenhouse gas emission issues caused by the fossil fuel industry. However, to make hydrogen a real energy carrier in a decarbonated economy, a secure and sustainable supply chain is needed. This approach requires notably safe storage and efficient strategies for recycling of raw materials. We discuss in this survey the state-of-the-art in the field of chemical hydrogen storage (CHS) materials, considering two possible vectors: ammonia borane and hydrosilanes. Regardless of the vector, to achieve real use, it is necessary to understand both the performance of the system and its life cycle, which relates to catalysts structure, and the activation of chemical bonds with efficient and complete catalytic cycles. We give herein an overview of hydrolysis and/or alcoholysis from metals, using coordination complexes, molecular supported catalysts or other materials, including nanocatalysts, with a focus on mechanistic information and understanding. Notably, the studies related to these two vectors can be considered somewhat complementary. Thus, the set of bibliographic report on ammonia borane is very documented in efficient catalytic systems, while its recycling remains at a very early stage. In comparison, hydrosilanes have been much less addressed specifically as a vector for hydrogen, while their reactivity at the molecular scale benefits from a relevant understanding from coordination chemistry studies. In addition, both hydrosilane polymerization and solvolysis reaction enables the release of $H_2$, and produces by-products of which added value is already established. This opening the way to economical strategies where recycling can be optional. Nevertheless, the reversibility of hydrosilanes chemistry in $H_2$ uptake remains attractive and is another option to develop.

## 1. Introduction

For the past two decades, hydrogen ($H_2$) has gained a lot of interest as an energy vector, to reduce greenhouse gas emission issues caused by the fossil fuel industry. Hydrogen is considered essential for the future energy mix, because of its natural abundance: 75 % by mass of known matter in the Universe, its high energy density by weight with 120 $MJ·kg^{-1}$ (vs 44 $MJ·kg^{-1}$ for gasoline), and its ability to release only water from its combustion [1]. However, this option also has some

drawbacks that need to be offset in order to make it viable. In fact, $H_2$ has the lowest energy density by volume, with $0.01 MJ·L^{-1}$ (vs 35 $MJ·L^{-1}$ for gasoline). It is a very diffuse gas, 3.8 times faster than natural gas in normal conditions, and it can inflame or explode in contact with air (flammability range of 4–75 vol% in air, and explosion limits of 13–65 vol%) [2]. Lastly, the hydrogen physical storage methods that are currently used, i.e. compression and liquefaction, in addition to the above-mentioned security issues, are expensive and not energy-efficient enough [3,4]. Therefore, chemical storage in liquids or solids is

<footnote><sup>a</sup> Université Bourgogne Europe, CNRS, Institut de Chimie Moléculaire de l'Université de Bourgogne (ICMUB) UMR 6302, 21000 Dijon, France</footnote>

<footnote><sup>b</sup> Laboratoire de Chimie de Coordination (LCC) UPR CNRS 8241, Université de Toulouse, INPT, 3100 Toulouse, France</footnote>

<footnote><sup>c</sup> Institut Européen des Membranes, IEM – UMR 5635, Univ Montpellier, CNRS, ENSCM, Montpellier, France</footnote>

<footnote>* This article is part of a Special issue entitled: 'CC in France' published in Coordination Chemistry Reviews.</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: myrtil.kahn@lcc-oulouse.fr (M.L. Kahn), umit.demirci@umontpellier.fr (U.B. Demirci), jean-cyrille.hierso@u-bourgogne.fr (J.-C. Hierso).</footnote>

<footnote n="1">¹ These authors contributed equally.</footnote>
envisaged as an alternative approach.

To guide general researches on hydrogen storage and H₂ delivery materials, the US department of energy (DOE) has set target gravimetric and volumetric capacities of hydrogen, which are needed for a large-scale use of H₂ in light-duty vehicles [5]. The gravimetric capacity refers to the weight percentage of H₂ effectively stored and produced by a liquid- or solid-state hydrogen carrier, while the volumetric capacity is the weight of H₂ per liter of the hydrogen carrier. The ultimate goal for the gravimetric hydrogen storage capacity, considered on the scale of the storage system as a whole, is 7.5 wt% of H₂ as lower limit; with respect to the volumetric hydrogen storage capacity [5], the ultimate target is 70 g·L⁻¹, and these capacities are expected to be applicable between -40 °C and 60 °C. Although intense research has been carried out over the past two decades, none of the materials studied have shown the capacity to store as much hydrogen and to release the stored hydrogen on demand; in other words, today, no storage technique meets all the DOE requirements [3].

Finding an industrially suitable hydrogen storage material remains a crucial matter for the future. Dense phases for storing H₂, either physically or chemically, have been thus envisioned, which include liquid and solid storage under earth ambient temperature and pressure. Accordingly, various liquid potentially-reversible H₂ storage materials at ambient temperature have been investigated since the 1980s [6], including formic acid or methanol, that remains out of the scope of the present review. Notably, concerning other recent liquid storage material candidates, liquid organic hydrogen carriers (LOHCs) are hydrogen-rich molecules, usually aromatic, that are able to reversibly release H₂ at moderate temperature and pressure conditions with the help of a catalyst. This can realistically be seen in fuel cell transportation applications, even though some technological enhancement is still needed; for instance, a central challenge, among others [7], is to develop low-cost and highly efficient catalysts to lower the temperatures of hydrogenation/dehydrogenation reactions, and therefore drastically decrease the overall energy consumption. As an illustrative example of the potential of LOHC, the properties of 2-aminoethanol NH₂(CH₂)₂OH is relevant. This amino alcohol dimerizes, generating 4 mol of H₂, between 105 °C and 135 °C in the presence of a ruthenium coordination complex as catalyst [6]. The process results in the formation of glycine anhydride that is valuably hydrogenated back into the initial material using the same catalyst (110 °C, under 10 to 70 bar of H₂). This 2-aminoethanol/glycine anhydride pair has theoretical H₂ storage capacities of 6.6 wt% and 66.6 g·L⁻¹, fairly satisfactory for H-content requirement, but with limitations concerning the expensive metal catalyst content and the high temperatures used for H₂ release, leading to potential safety issues.

Solid state storage materials are very attractive H₂ carriers as they display a wide panel of properties that could be improved to hopefully reach the DOE targets in the upcoming years [8]. Different solids are currently investigated as either physical or chemical potential H₂ carrier candidates. There are several types of sorbents for physical storing: this includes carbonaceous materials [9], zeolites [10], clays [11], metal-organic frameworks (MOFs) [12], and the large family of coordination polymers and highly porous organic materials [13–16]. These materials are used to adsorb H₂ because of their high surface area, and can then release it under certain temperature and pressure conditions [17,18]. These materials can reach gravimetric capacities in hydrogen ranging from 2 to 10 wt% H₂. However, they generally require cryogenic conditions (low temperature as that of liquid N₂, i. e. –196 °C) to adsorb and desorb H₂. Even in these conditions, some solid sorbents, like zeolites, display limited H₂ uptake with less than 2 wt% [10]. Concerning MOFs, for instance, a copper-carboxylate ligand-MOF, named NU-100, can uptake 9.95 wt% H₂ at –196 °C and 70 bar [19]. At higher temperatures, the MOFs storage capacity decreases, with performances generally lower than 1 wt% at 25 °C and 100 bar [20].

Other relevant H₂ storage materials are metallic and complex hydrides, which incorporate chemical bonds with elemental hydrogen H. They can store a fairly large quantity of H₂, and some of them are able to

reversibly release it under moderate temperature (0 to 300 °C) and pressure conditions (1 to 40 bar) [8,21]. This has been reported in the case of magnesium hydride MgH₂, which could approach the theoretical hydrogen capacity of 6.5 wt%, when MgH₂ microparticles of ca. 800 nm size were hydrolyzed in a 1 M MgCl₂ solution at room temperature (rt.) [22]. With multicomponent metal hydrides, the hydrogenation/dehydrogenation conditions largely depend on the material itself. For instance, lithium borohydride LiBH₄ releases 9.3 wt% H₂ when heated above 380 °C, but the hydrogenation of one related product LiBO₂ was only partially achieved with a 40 % yield hydrogenation by ball-milling process at rt. under close argon atmosphere in the presence of 2 equiv. of H₂O [23]. With the sodium counterpart, NaBH₄, hydrogen release is relevant in hydrolytic conditions, releasing four equivalents of H₂ at 20 °C, alongside sodium tetrahydroxyborate [NaB(OH)₄] as by-product [5,8].

Another relevant solid H₂ storage material strategy, is the use of so-called chemical hydrogen storage materials (CHS). Typical examples are coordination compounds, notably light-mass boron- and/or nitrogen-based species like ammonia borane (NH₃BH₃, abbreviated AB) that features specific bonding with their hydrogen atoms [24]. Boron tends to make H more hydridic, meaning it will bear a partial negative charge, while nitrogen makes H more protic, so with a partial positive charge. Such polarized B–H and N–H bonds tend to form a 3D network of hydrogen bonding that play a decisive role in the materials stability at ambient conditions, as well as in the release of H₂ in remarkable mild conditions, as discussed hereafter [25]. The release of H₂, however, is hardly reversible at affordable conditions, contrary to metallic hydrides and adsorbing porous materials; indeed, dehydrogenation of CHS materials is generally an exothermic process [24,26]. AB has been intensively investigated over the last two decades. Under ambient conditions, AB is a white solid that offers several advantages: it is stable when kept in pristine state under inert atmosphere, and it is fairly stable when kept at a temperature lower than 60 °C; otherwise, it is deemed a low toxicity, a low hazard and it is non-flammable, provided it is kept in solid state (or in water solution), and far from chemical or heating source. AB has a theoretical gravimetric hydrogen density of 19.6 wt%. It is known to dehydrogenate when heated above 80 °C or at lower temperatures in the presence of a catalyst [25]. By thermolysis, AB can release up to two equivalents of H₂ at about 150 °C, which means a theoretical H₂ storage capacity of 13 wt%. Similar dehydrogenation is achieved in the presence of a metal catalyst at around 100 °C or in an aprotic solvent at around 85 °C [26]. However, thermolysis of pristine AB results in a mixture of several boron and nitrogen-containing by-products that are difficult to identify and later treat for recycling and/or regeneration of AB [27]. It is worth mentioning the solvolysis of AB that is also attractive owing to the release of up to 3 equivalents of H₂ from its reaction with water or alcohols (as detailed hereafter). These reactions are achievable with very good efficiency at rt. [18], and though the gravimetric H₂ storage capacities are lower than the 19.6 or 13.0 wt%, they remain high with an ideal 7.1 wt% for the couple AB/3H₂O, for example.

From one hand, the performances of catalytic solvolysis of AB much rely on the choice of the transition metal and coordination complexes, or nanocatalyst, or supported solid materials chosen to promote the said reaction, an advanced field of research that is now fairly mature [27–31]. From the other hand, decisive challenges still remain for the large-scale use of AB as a H₂ carrier. Indeed, such CHS material needs to be efficiently regenerated from its solvolysis products [18,32–34]. However, only limited attention has been drawn to regenerate the solvolysis by-products of H₂ evolution, typically borates, as B–O containing species. This might be explained by the inherent thermodynamics to back regenerate B–H from B–O, the bond dissociation energy for B–O bond being of ca. 626 kJ·mol⁻¹ [35].

Alternative CHS materials, made of abundant and widely available resources are silicon–H (Si–H) bond-containing materials, notably organosilanes. These have shown an increased interest from industrial and academic approaches since the 2010’s, that might provide an
alternative to well-studied AB, for the specific use of H₂ delivery [36]. This is due to the easy and recognized dehydrogenation reactions of silanes: both the exothermic hydrolysis of Si—H from solvolysis, and the Si—Si polymerization of hydrosilanes, for which the reversibility in the reactivity with hydrogen are very attractive [37,38].

Notably, hydrolysis and alcoholysis of hydrosilanes are long-known metal-catalyzed processes, in which equimolecular H₂ evolves through various reactions involving hydrolysis of the Si—H bonds [39–43]. Si—H bond breaking is a thermodynamically favorable process, which, however, suffers from high activation barriers and for this reason, various catalytic systems based on transition metals, such as ruthenium and iridium complexes, have been developed [39–43]. Thus, the role of coordination chemistry and catalysis might be decisive towards a goal of H₂ delivery circular process based on the extension of silane industry.

Table 1 presents a selection of leading reviews related to CHS materials that are pertinent for broader [1,3–5,17], or more specific discussion on this topic [6–12,14–16,18,20,21,24–34,36]. Armaroli and Balzani’s contribution concisely highlights the advantages and drawbacks of using H₂ to replace fossil fuels [1]. Durbin and Malardier-Jugroot’s review compiles the different H₂ storage methods specifically for automotive applications [3], as mobility is indeed one of the most promising niches for decarbonated H₂ delivery. Preuster’s analysis of LOHCs [6] complete Ouyang’s paper focused on light-metal based H₂ storage materials [8] and McWhorter’s survey that focuses on CHS materials comprised within the DOE targets [17].

Demirci and Miele helped define more precisely the terms used when talking about H₂ storage targets [5]. Indeed, it is worth clarifying whether we are talking about the *materials* or *systems* capacity, and indicate if it is absolute (i. e. theoretical), or net (i. e. real, useable), or excess (i. e. effective) in case of a system-based capacity. Notably, AB has been intensively studied because of its high absolute H₂ storage capacity [18]. The class of amine borane coordination compounds is also subject of book chapters [24], dedicated to their reactivity and evoking some recycling strategies for B—O bond-containing materials, and polyborazylene (BNHₓ) waste products. As for the general reactivity of AB, it is concisely discussed in Li’s contribution [25], but also before in Stephens’ review [26]. Both introduce the use of catalysts for a faster and milder dehydrogenation of AB, which is discussed in several other pertinent reviews [27–30]. Notably, Wu et al. treated some mechanistic approaches relating to catalytic hydrolysis of AB, and reported on the various types of metal catalysts [31]. Finally, older surveys briefly discussed the crucial matter of the chemical regeneration of AB [32–34], which remains difficult to achieve in an economically viable way. This, may suggest an economic and industrial “no-go” for larger scale use of AB, at least at the present date, that needs to be overcome. However, the boom of H₂ economy has already started to change this *status quo* for the borane sodium borohydride NaBH₄: Velox Energy Materials Inc. announced an agreement with the Australian Renewable Energy Agency on 15 March 2024 a grant funding of A$ 5,000,000 aiming at developing NaBH₄ as safe non-flammable hydrogen carrier (Kotai Hydrogen Project [44]).

Organosilanes, as a far less studied CHS alternative have not been specifically reviewed as potential candidates in this area to the best of our knowledge [36]. The synthesis of polysilanes and their application to new technologies and renewable energies are pertinent in this regard. As for CHS, polysilanes (H₂Si)ₙ (with n from 20 to 40) have a gravimetric hydrogen density ranging from about 3.0 to 6.6 wt% H₂. However, under hydrolytic conditions at 300 °C, and when the weight of steam is not taken into account, while considering the hydrogen coming from this water, the amount of released H₂ would be higher than 20 wt%. From a practical viewpoint this theoretical approach remains debatable, especially if embarked systems need to convey water.

The present survey aims at providing a better understanding of the catalytic solvolysis of current relevant CHS materials focused on ammonia borane and organosilanes, and extends this approach to their current means of regeneration. Accordingly, this review treats also of the

<table><caption>**Table 1**
Selected reviews and topics pertinent for hydrogen storage general overview.</caption><thead><tr><td>Title</td><td>Authors</td><td>Year</td><td>Ref.</td></tr></thead><tbody><tr><td>The hydrogen issue</td><td>Armaroli et al.</td><td>2011</td><td>[1]</td></tr><tr><td>Review of hydrogen storage techniques for on board vehicle applications</td><td>Durbin et al.</td><td>2013</td><td>[3]</td></tr><tr><td>Recent advancements in hydrogen storage – Comparative review on methods, operating conditions and challenges</td><td>Bosu et al.</td><td>2024</td><td>[4]</td></tr><tr><td>Chemical hydrogen storage: ‘material’ gravimetric capacity versus ‘system’ gravimetric capacity</td><td>Demirci et al.</td><td>2011</td><td>[5]</td></tr><tr><td>Liquid organic hydrogen carriers (LOHCs): towards a hydrogen-free hydrogen economy</td><td>Preuster et al.</td><td>2017</td><td>[6]</td></tr><tr><td>Hydrogen storage by liquid organic hydrogen carriers: Catalyst, renewable carrier, and technology – A review</td><td>Chu et al.</td><td>2023</td><td>[7]</td></tr><tr><td>Hydrogen production via hydrolysis and alcoholysis of light metal-based materials: a review</td><td>Ouyang et al.</td><td>2021</td><td>[8]</td></tr><tr><td>Recent advances in carbon nitride-based nanomaterials for hydrogen production and storage</td><td>Pachaiappan et al.</td><td>2022</td><td>[9]</td></tr><tr><td>Hydrogen storage in low silica type X zeolites</td><td>Li et al.</td><td>2006</td><td>[10]</td></tr><tr><td>Hydrogen wettability of clays: Implications for underground hydrogen storage</td><td>Al-Yaseri et al.</td><td>2021</td><td>[11]</td></tr><tr><td>The application of MOFs for hydrogen storage</td><td>Zhang et al.</td><td>2023</td><td>[12a]</td></tr><tr><td>Nanopore-supported metal nanocatalysts for efficient hydrogen generation from liquid-phase chemical hydrogen storage materials</td><td>Sun et al.</td><td>2020</td><td>[14]</td></tr><tr><td>Highly porous organic polymers for hydrogen fuel storage</td><td>Cousins et al.</td><td>2019</td><td>[15]</td></tr><tr><td>Covalent organic frameworks-Based Nanomaterials for Hydrogen Storage</td><td>Kopac</td><td>2022</td><td>[16]</td></tr><tr><td>Moderate temperature dense phase hydrogen storage materials within the US department of energy (DOE) H₂ storage program: trends towards future development</td><td>McWhorter et al.</td><td>2012</td><td>[17]</td></tr><tr><td>Ammonia borane: an extensively studied, though not yet implemented, hydrogen carrier</td><td>Demirci</td><td>2020</td><td>[18]</td></tr><tr><td>Metal-organic frameworks (MOFs) as hydrogen storage materials at near-ambient temperature</td><td>Sutton et al.</td><td>2024</td><td>[20]</td></tr><tr><td>A review on metal hydride materials for hydrogen storage</td><td>Klopcic et al.</td><td>2023</td><td>[21]</td></tr><tr><td>Ammonia borane and related compounds as hydrogen source materials</td><td>Mertens et al.</td><td>2010</td><td>[24]</td></tr><tr><td>Ammonia borane, past as prolog</td><td>Li et al.</td><td>2014</td><td>[25]</td></tr><tr><td>Ammonia-borane: the hydrogen source par excellence?</td><td>Stephens et al.</td><td>2007</td><td>[26]</td></tr><tr><td>The hydrogen-storage challenge: nanoparticles for metal-catalyzed ammonia borane dehydrogenation</td><td>Hierso et al.</td><td>2021</td><td>[27]</td></tr><tr><td>Hydrogen production from ammonia borane hydrolysis catalyzed by non-noble metal-based materials: a review</td><td>Wang et al.</td><td>2021</td><td>[28]</td></tr><tr><td>Ammonia-borane and amine-borane dehydrogenation mediated by complex metal hydrides</td><td>Rossin et al.</td><td>2016</td><td>[29]</td></tr><tr><td>Amine-borane dehydrogenation chemistry: metal-free hydrogen transfer, new catalysts and mechanisms, and the synthesis of polyaminoboranes</td><td>Stubbs et al.</td><td>2013</td><td>[30]</td></tr><tr><td>Metal-catalyzed hydrolysis of ammonia borane: mechanism, catalysts, and challenges</td><td>Wu et al.</td><td>2020</td><td>[31]</td></tr><tr><td>Ammonia borane as a hydrogen carrier: dehydrogenation and regeneration</td><td>Smythe et al.</td><td>2010</td><td>[32]</td></tr><tr><td>Regeneration of ammonia borane from spent fuel materials</td><td>Summerscales et al.</td><td>2013</td><td>[33]</td></tr><tr><td>Advances with ammonia-borane: improved recycling and use as a precursor to atomically thin BN films</td><td>Whittell et al.</td><td>2011</td><td>[34]</td></tr><tr><td>Polysilanes – backbones for new silicon-based technologies</td><td>Auner et al.</td><td>2010</td><td>[36]</td></tr></tbody></table>
mechanisms of catalytic hydrolysis and alcoholysis of AB, and organo-silanes, in strong relationship with transition metals coordination chemistry from pertinent molecular complexes, heterogeneous catalysts, and nanocatalysts.

Since many informative reviews on AB solvolysis are already available, this survey will chiefly discuss the research articles published after 2013 for a cut-off of the second semester of 2024. As for silanes, we reviewed herein research articles for which hydrogen generation from the catalytic hydrolysis and alcoholysis are the main goals. Complementarily, other uses of these reactions, such as sol-gel and crosslinking, have been reviewed [37,38,45]. Accordingly, the focus herein is on the coordination complexes and metallic species (including nanocatalysts and supported materials) that are catalytically pertinent, and their features and performances. Clearly, a detailed examination of the modes of coordination and interaction of metal catalyst with amine boranes and silanes deepens our understanding of the mechanisms behind the dehydrogenation/(re)-hydrogenation processes with these promising clean hydrogen vectors. A review in this field is timely and needed.

## 2. Ammonia borane as hydrogen storage material

### 2.1. Ammonia borane synthesis and chemical properties.

Ammonia borane, according to its safety data sheet [46], is a non-hazardous and a non-toxic material, and it is crystalline and stable at ambient conditions. AB is currently commercially available for a price ranging in ca. 14 to 30 € per g (data from TCI Chemicals for 25 g, and Sigma Aldrich for 100 g, in January 2025). AB is a synthetic material, isolated for the first time in 1955 by Shore and Parry [24]. Salt metathesis is the most commonly used strategy for laboratory-scale AB preparation: it consists in the reaction of an ammonium and a borohydride salt, typically ammonium formate with sodium borohydride, in an anhydrous solvent, such as tetrahydrofurane (THF) or dioxane [24–26]. The second strategy that affords important AB yields is the direct reaction of ammonia gas with L·BH₃ adducts (L = BH₃, DMS, THF, among others) in an electron donor solvent such as dimethylsulfide (DMS) or ethers, resulting in NH₃ displacing L by a Lewis acid/base exchange. This

method is suited to larger-scale preparation (Fig. 1) [24].

AB includes three N—H bonds where H has a protic nature with a Mulliken charge of +0.21, three B—H bonds where H is hydridic with a Mulliken charge of -0.195 [47], and a dative polarized N—B bond, with a dipole moment of 5.6 D [48], in which the electron lone pair of nitrogen interacts with a low-energy empty orbital of boron [24–26]. The protic H can form hydrogen bonding interactions with electronegative atoms, but also interact with the hydridic H of another AB molecule in the solid, to form hydrogen bonds (ca. 2.02 Å) that are sensibly shorter than the corresponding Van der Waals distances (ca. 2.4 Å). This is related to the high stability of AB in the solid phase. Consistently, these bonding interactions also play a part in the dehydrogenation of AB. It was suggested that the protic H of one AB molecule tends to combine with the hydridic H of another one through thermal initiation because of the high chemical potential of the reaction. However, McGrady reported that homopolar B·H<sup>δ+</sup> · · · <sup>δ</sup>H·B interactions contribute to the thermal dehydrogenation of AB as much as the heteropolar ones, after AB melting point is reached, as depicted on Fig. 2 [25,49]. Furthermore, the electronic state of the H also determines the reactivity of AB, since the protic H would play the role of a “Lewis acid” in reactions with metals, while the hydridic H would act as a “Lewis base”, reactive with

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** From McGrady works [49], modelling of intramolecular interactions in AB dehydrogenation, with focus on the heteropolar interactions (in blue) as well as the homopolar ones (in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** General synthetic pathways of ammonia borane (AB); In blue (top) only a part of the Brown-Schlesinger process is described (further detailed in the section 2.4). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
nucleophilic substituents [25]. Concerning the N—B bond, similar processes would apply, in which NH₃ acts as Lewis base and BH₃ as Lewis acid, each having the corresponding reactivity towards external Lewis acid and base, respectively.

Regarding AB behavior in liquid solution, various solvents are able to dissolve AB (Table 2). The solubility of AB in ethanol is ca. of 6.1 wt% (i. e. 6.1 g per 100 g of solvent), in tetrahydrofuran 20.0 wt%, in methanol 23.0 wt%, in water 25.1 wt%, in diglyme 27.2 wt%, and in liquid ammonia 72.2 wt% [24,50]. The ability of AB to dissolve is an obvious requirement for using it efficiently in a solvolytic reaction approach that would release gaseous H₂. The use of water may be relevant for this reaction with regards to “green chemistry” issues, and for good solubility and predictable by-products.

The theoretical (maximum) gravimetric hydrogen densities of AB and other hydrogen storage materials (HSM) in solution are presented in Table 2. For solvents that enable the solvolytic dehydrogenation of AB, these gravimetric hydrogen densities (GHD) are compared with those of common solvolytic hydrogen carriers, including sodium borohydride, magnesium and its hydride, and two representative silanes. Notably, AB in water and methanol provides hydrogen densities of 3.94 wt% and 3.67 wt%, respectively. These values lie between the density of sodium borohydride in methanol and that of magnesium hydride reacting with water, supporting the potential of AB as hydrogen carrier in solvolytic conditions.

In water, AB solvolysis is especially favored by non-neutral conditions: for instance, in acidic conditions the N—B bond is broken by an electrophilic H⁺ attack, giving rise to a NH₄⁺ ion, and facilitating the formation of strong B—O bonds [24,26]. A chemically balanced equation of this general reaction that is admitted in first approximation is given in Eq. (1) [24,28,31,51]. Depending on the conditions, mainly, metal catalyst nature, pH, additives, concentration, etc. this global equation has to be refined [51,52]; Eq. (2) is complementary and is also to be refined depending on the conditions. From the authors' experience, no data strictly confirmed the formation of BO₂⁻. All the spectra collected in aqueous medium, and therefore in excess of H₂O, lean towards boric acid and its conjugate base, and polyborates.

$$NH_3BH_3 + 2 H_2O \rightarrow NH_4^+ + BO_2^- + 3 H_2 \quad (1)$$

$$H^+ + BO_2^- + H_2O \rightarrow B(OH)_3 \quad (2)$$

The solvolysis of AB in methanol is another attractive option to be developed [27]. This, especially for the lower operable temperatures than AB hydrolysis, technically down to -97 °C, and practically tested at 0 °C and slightly below [53]. In general, solvolysis reactions allow milder reaction conditions as well as better predictability of the borate by-products compared to the thermal decomposition of AB, in which different oligo- and polymeric species are formed (see below) [51,54].

<caption>**Table 2**</caption>

Solvent of AB and its solubility (given in brackets; in g(AB) per 100 g(solvent)), as well as its theoretical/maximum gravimetric hydrogen density in wt% H (taking into account the dehydrogenation path, either solvolytic or thermolytic). The AB's GHDs are compared to that of common hydrogen carriers (sodium borohydride, magnesium-based, and two silane examples) with potential in solvolytic dehydrogenation.

<table><thead><tr><th>HSM</th><th>Solvent</th><th>Solvent and reactant</th><th>Reactant</th><th>Dehydrogenation</th><th>GHD (wt% H)</th></tr></thead><tbody><tr><td>Ammonia borane</td><td></td><td>Ethanol (6.1)</td><td></td><td>Solvolytic</td><td>1.13</td></tr><tr><td>Ammonia borane <sup>a</sup></td><td>Tetrahydrofuran (20)</td><td></td><td></td><td>Thermolytic</td><td>3.27</td></tr><tr><td>Ammonia borane</td><td></td><td>Methanol (23)</td><td></td><td>Solvolytic</td><td>3.67</td></tr><tr><td>Ammonia borane</td><td></td><td>Water (25.1)</td><td></td><td>Solvolytic</td><td>3.94</td></tr><tr><td>Ammonia borane <sup>a</sup></td><td>Diglyme (27.2)</td><td></td><td></td><td>Thermolytic</td><td>4.20</td></tr><tr><td>Ammonia borane <sup>b</sup></td><td>Liquid ammonia (72.2)</td><td></td><td></td><td></td><td>8.23</td></tr><tr><td>Sodium borohydride</td><td></td><td>Methanol (16.4)</td><td></td><td>Solvolytic</td><td>2.26</td></tr><tr><td>Sodium borohydride</td><td></td><td>Water (55)</td><td></td><td>Solvolytic</td><td>5.68</td></tr><tr><td>Magnesium</td><td></td><td></td><td>Water</td><td>Solvolytic</td><td>3.35</td></tr><tr><td>Magnesium hydride</td><td></td><td></td><td>Water</td><td>Solvolytic</td><td>6.48</td></tr><tr><td>Triethylsilane</td><td></td><td></td><td>Water</td><td>Solvolytic</td><td>2.65</td></tr><tr><td>1,1,2-trisilylethane</td><td></td><td></td><td>Methanol</td><td>Solvolytic</td><td>4.55</td></tr></tbody></table>

To better control and accelerate the H₂ release rate from the solvolysis of amine boranes, numerous noble or non-noble metal-based catalysts have been tested [27,55]. The large majority of the studies for H₂ evolution from AB solvolysis focuses on the performances and cycling stability of the catalytic systems according a special attention to turnover frequency (TOF) of the catalyst (TOF is –in principle– the number of reaction products generated *per active site* per unit of time, for instance given in mol H₂ min⁻¹ mol⁻¹catal, also denoted min⁻¹, see Eq. (3)) [13,27]. Notably, the number of active sites may be difficult to estimate, or is often neglected, and the total number of atoms is considered in replacement.

$$TOF = \frac{n_{H_2}(t)}{t \times n_{\text{catalyst}}} \quad (3)$$

In general, a strict comparison of the catalytic systems, from one case to another, is delicate, since the reaction conditions are mostly not unified, showing significant differences in operating temperatures, reagents concentration, metal loading, mode of calculation of TOF performances (true active metal surface sites being identified or not), etc. Additionally, very often the detailed catalytic systems characterization is lacking, as well as the characterization post-catalysis or before/after catalysts recycling. Finally, TOF is only a formal indicator, still limited while the systems are not up-scaled as economically viable, and ecologically sustainable industrial processes. A road to this progress is from one hand a clearer understanding of the various mechanistic processes occurring depending on the conditions, and from the other hand the chemical regeneration of the systems. Accordingly, we focused our first section on the currently proposed mechanisms for hydrolysis and alcoholysis of AB in the presence of heterogeneous or nanoparticle (NP) metal catalysts.

## 2.2. Hydrogen release mechanisms from ammonia borane in the solid state

For a necessary contextualization, it is pertinent to refer to hydrogen thermal release mechanisms in the solid state that complement the solvolysis-oriented studies. Studies were dedicated to the mechanistic aspects of (non-catalyzed) AB dehydrogenation either in the solid state or in organic solvents like diglyme and THF [54]. From combined experimental and theoretical studies, by using NMR studies, Fourier Transform-Infrared (FT-IR) spectroscopy, and thermogravimetric analysis (TGA), the evolution of AB and products was monitored during the reaction at different temperatures (Fig. 3). The authors proposed, essentially, both in the solid state and in these solvents that acyclic dimeric intermediates are first formed from NH₂BH₂, as well as diammoniate of diborane (DADB). Then polymeric species (PAB) and tetracyclic intermediates such as cyclodiborane (CDB) may form, followed

<footnote><sup>a</sup> The GHD is calculated assuming the release of 3 equivalent H₂ per AB.</footnote>

<footnote><sup>b</sup> Dehydrogenation of AB in liquid ammonia is unrealistic as ammonia will evaporate with the increase of temperature; the given GHD is thus indicative.</footnote>
<figure><img src="image_5.png" /><figcaption>Fig. 3. AB dehydrogenation non-catalyzed processes studied in the solid state and organic solvent [54].</figcaption></figure>

by hexacyclic species such as cyclotriborane (CTB), evolving to borazine (BZ) and polyborazylene (PB). The cyclic intermediates are formed more easily in a solvent, which allows faster and lower temperature H₂ release compared to the solid-state situation. A notable complication of the general scheme is the propensity of reactive NH₂BH₂ to combine with the formed intermediates at each step.

## 2.3. Ammonia borane catalytic hydrolysis and alcoholysis mechanisms

In comparison to non-catalyzed dehydrogenation, the catalyzed AB solvolytic dehydrogenation has been investigated under more various conditions, by employing many different types of catalysts, from homogeneous to heterogeneous systems, and including innovative supported small clusters and nanocatalysts [27]. Thus, in the AB hydrolysis process, the intermediates and final products are, as expected, mostly different from those mentioned above [51]. Mechanistic discussions on acid-catalyzed ammonia borane hydrolysis have been early on introduced [56], while the works of Xu and Chandra [52,57–59] launched the study of H₂ generation system based on transition metal-catalyzed dissociation and hydrolysis of AB at rt., this by using the addition of a catalytic amount of a suitable noble metal such as Pt, Rh and Pd. These authors have shown that the type of ligand, or support, associated to a given metal, highly affects the catalytic activity due to factors like particle size and dispersion of the metal. Pt has been at that time disclosed as the metal that allows the highest catalytic activity [57]. Chandra and Xu reported the presence of different borates at the end of the hydrolysis reaction, depending on the pH. With the help of ¹¹B NMR studies, they assumed an equilibrium process between B(OH)₃, BO₂⁻ and other borate species, which would undergo rapid exchange between each other in the solution on the NMR time scale, that would account for the observed low field-shifted single ¹¹B resonance, that was more simply formalized by Eq. 1 and Eq. 2 given earlier.

Following these early pioneering works, mechanisms of metal-catalyzed hydrolysis of ammonia borane have been explored both from noble and non-noble metals [31]. Illustratively, a reaction

mechanism was proposed for ammonia borane hydrolysis on a Pt catalyst supported on conductive multiwall carbon nanotubes (MWCNT) [60]. A combination of thermodynamic analysis and FT-IR measurement supported that boron-containing by-products are mainly in the form of a [NH₄B(OH)₄/B(OH)₃] mixture rather than NH₄BO₂. Thus, a revised reaction was proposed involving altogether B—H, B—N, and O—H bond breaking. Isotopic experiments using D₂O instead of H₂O as reactant, or introducing D₂ into the reaction atmosphere, suggested that the O—H bond breaking is rate determining (kinetic isotopic effect KIE was determined to be 1.9). The results also show that this Pt/MWCNT catalyzed hydrolytic dehydrogenation of ammonia borane likely follows a surface Langmuir–Hinshelwood mechanism, where activated (NH₃BH₂)ads (adsorbed on the catalyst surface) may react with H₂O activated on catalyst surface for an overall activation energy of 41.5 kJ mol⁻¹. Accordingly, apparent activation energy in the corresponding hydrolytic reactions is reported mostly ranging between 12.0 and 80.0 kJ mol⁻¹ [27]. Notably, in this specific system the concentration in AB is described as having no influence on the rate measured in the initial period of the reaction, an unusual specificity compared to several other catalytic systems [52,57,58].

Concerning mechanistic approaches with other metal catalysts, for Fe, Co, Ni and Cu nanocatalysts, zeolitic imidazolate framework (ZIF-8) were synthesized from 2-methylimidazole and [Zn(NO₃)₂·6H₂O] in water, and used as a support. The resulting Ni supported nanocatalyst, formed from the deposition-precipitation method, was the most efficient in water using NaBH₄ for H₂ release [61]. This Ni NPs/ZIF-8 catalyst –with Ni NPs average size measured by transmission electron microscopy (TEM) ca. 3 nm displayed a TOF ca. 35 min⁻¹, which was notably improved to 85.7 min⁻¹ by the addition of NaOH. On the basis of isotopic experiments with NH₃BD₃ and a NH₃BH₃–D₂O system, and by calculation of the KIEs, O—H bond cleavage in H₂O was suggested to be the rate determining step (RDS). Thus, the mechanism proposed an indirect O—H bond cleavage promoted by AB in the presence of the nanocatalyst, leading to a B—N bond breaking and H₂ release.

On the other hand, by using a cobalt catalyst supported on nickel
foam for the hydrolysis of AB at 30 °C, the formation of B(OH)⁻₄ and NH₄⁺ ions was evidenced from the analyzed hydrolysates by using ¹¹B NMR, powder X-ray diffraction (XRD), FT-IR, TGA, and differential scanning calorimetry (DSC) [51]. These species are fully stable for 6 months at 30 °C when the catalyst is removed. Conversely, it turned into B(OH)₃ and NH₃ when the catalyst stayed in the solution due to pH increase. The presence of one or another of these by-products is thus dependent on the pH, and can be modulated, which may be of interest for further regeneration steps [51].

The specific coordination of AB with transition metal-based catalysts was tentatively modelized to clarify the occurring mechanisms [62]. The chosen catalysts gathered molecular complexes, including early (Ti) and late (Pd, Ir, Ni, Fe, Ru) transition metals of various oxidation degrees, having different 3d, 4d, and 5d frontier orbitals, and stabilized by carbene or phosphino-amine ligands (Fig. 4), namely: $[\text{Cp}_2\text{Ti}]^{2+}$ (I, Cp = cyclopentadienyl), $[\text{Pd}(\text{allyl})(2,4-hexadiene)]^{+}$ (II), $[\text{Pd-(allyl)}]^{+}$ (III), $[\text{Ir}(\text{P}_2\text{CH}_6)_2]^{+}$ (IV), $[\text{Pd}(\text{PH}_3)_2]$ (V), $[\text{Ni}(\text{NHC})_2]$ (VI), $[\text{H}_2\text{Ir}(\text{POCOP})]$ (VII), $[\text{Fe}(\text{CNP})_2]$ (VIII), $[\text{H}_2\text{Ru}(\text{CNP})_2]$ (IX), and $[\text{Ru}(\text{CH}_3\text{CN})_3\text{C}_13\text{N}_2\text{F}_3\text{BO}_2\text{H}_{11}]^{+}$ (X).

Density Functional Theory (DFT) analyses exploring Extended Transition State-Natural Orbitals for Chemical Valence (ETS-NOCV schemes), predicted various behavior depending on the metal-ligand coordination complexes involved. For cationic complexes I to IV, a general tendency was noted as a strong donation from the σ(B-H) orbital to the empty d orbital of the transition metal that causes B-H bond activation and results in an easy H₂ release from the BH₃ moiety (Fig. 5). The more the interaction of the occupied molecular orbitals of AB with the unoccupied molecular orbitals of the complex has a lower energy value, the more favorable is the interaction (given in kcal mol⁻¹ on Fig. 5). The neutral zerovalent coordination complexes, $[\text{Pd}(\text{PH}_3)_2]$, V, and $[\text{Ni}(\text{NHC})_2]$, VI, tend to activate the N-H bond by σ back-donation from the metal to the empty σ*(N-H) orbital, thus destabilizing the bond. When entropic factors are included into the calculations, a secondary interaction between AB and $[\text{Ni}(\text{NHC})_2]$ also becomes important: NH···π interactions with the NHC ligand, that end up leading to hydrogen abstraction from the NH₃ moiety to the ligand. This discriminates Lewis acidic and Lewis basic pathways for AB dehydrogenation.

For the iridium complex $[\text{H}_2\text{Ir}(\text{POCOP})]$ VII, both the BH₃ and NH₃ interact with the complex, and are dually activated. The $[\text{Fe}(\text{CNP})_2]$ and $[\text{H}_2\text{Ru}(\text{CNP})_2]$ catalysts, VIII and IX, have a behavior similar to $[\text{Ni}(\text{NHC})_2]$, as they also cause hydrogen abstraction from NH₃. The cationic $[\text{Ru}(\text{CH}_3\text{CN})_3\text{C}_13\text{N}_2\text{F}_3\text{BO}_2\text{H}_{11}]^{+}$ X, which contains three non-innocent acetonitrile ligands, is distinguished regarding its interaction with AB, since CH₃CN ligand may bind to both functions of AB, resulting in some hydrogen atom transfer from BH₃ to the CH₃CN ligands, and in

addition from NH₃ to the metal (or the boron atom), this suggesting an intricate concerted H₂ release. This school-case study helps highlighting various of the possible interactions of a given coordination complex (metal/ligand) with AB, that may ultimately lead to H₂ extraction. This approach also emphasized the electrophilic character of efficient catalysts, which activates the B-H bond for faster dehydrogenation. A limitation of this study is that the reaction of AB with the solvent, that obviously plays a major role in solvolysis, was not addressed. In addition, the role and influence of the ligand, as an important parameter and lever for the catalytic center's reactivity, should be also important to assess.

The catalytic hydrolysis of AB with MnO-doped Co nanofibers (with 78 wt% Co, 20 wt% MnO and 2 wt% C), synthesized by electrospinning polyvinylalcohol (PVA), cobalt acetate (CoAc), and manganese acetate, then drying and calcinating, achieved a modest efficiency but a complete H₂ extraction, with 3 equiv. H₂ released in ca. 35 min at rt. (TOF ca. 0.3 min⁻¹) [63]. Unfortunately, the morphology and intimate structure of the nanofibers was not fully characterized, though TEM and scanning electron microscopy was achieved. The authors describe the N-B dative bond in AB with N bearing a partial positive charge, while B bears a partial negative one, thus represented as two split species $H_3N^{2+}$ and $H_3B^{2-}$. The reaction shown in Eq. (4) is proposed at the surface of MnO with electron transfer to the electrophilic nitrogen moiety. Consequently, the acceptor group $H_3B^{2-}$ oxidize at the Co surface (Eq. (5)), these formal half-equations giving when combined the overall balance reported in Eq. (1).

$$H_3N^{2+} + 2e^- + H_2O \rightarrow NH_4OH \quad (4)$$

$$H_3B^{2-} + 2H_2O \rightarrow H^{+} + BO_2^{-} + 3H_2 + 2e^{-} \quad (5)$$

Bimetallic Co nanofibers doped with Pd-NPs (with 80 wt% Co, and 20 wt% Pd), synthesized by electrospinning of PVA, CoAc, and Pd NPs, then drying and calcinating, were used at rt. for AB hydrolysis [64]. The synthesized material includes 26 nm Pd NPs supported on long Co nanofibers, for which the detailed materials characterization was not provided. During AB hydrolysis, the use of this catalyst provides 3 equiv. of H₂ after 25 min in daylight (indoor), and approximately 12 min when catalyzed by sunlight photo-assistance (outdoor under direct sun radiation), reaching TOF values of respectively ca. 0.4 min⁻¹ and 0.8 min⁻¹ (N.B. photo-assisted catalysis is distinguished from photocatalytic reaction [64b]). This performance remains for seven consecutive cycles, in link with the recyclability of the catalyst. The authors analyzed the photodegradation of AB in water in the presence of the bimetallic catalyst, using UV-vis absorption spectroscopy. AB hydrolysis with Pd-doped Co nanofibers under sunlight was facilitated by the generation of electron-hole pairs in the metallic structure, as Co proved to be

<figure><img src="image_6.png" /><figcaption>Fig. 4. Transition metal-based catalysts coordinating AB [62].</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Modelling of the σ-π bond interactions of the catalysts I to IV with AB. Reprinted with permission from Organometallics 2013, 32, 4103–4113. Copyright 2013 American Chemical Society [62].</figcaption></figure>

photoactive. To prevent fast electron-hole recombination the photo-active specie is doped with the additional metal, herein Pd NPs dispersed into the Co nanofiber (Fig. 6). They act as an electron sink, accumulating negative charge, and leaving positive holes into the Co nanofibers. As for MnO-doped Co nanofibers, Pd provides electrons to the putative $H_3N^{2+}$ moiety (Eq. (4)). In parallel, $H_3B^{2-}$ is oxidized by the accumulation of holes at the Co surface (Eq. (5)). The overall reaction equation matches Eq. (1).

For AB hydrolysis on photo-assisted catalysts, manipulation of charge transfer offers an efficient approach to achieving efficient $H_2$ release. In this regard, adopting a Z-scheme or type-II charge transfer mechanism was attracting [65a]. MOF based-composite $Cu_2O$/$UiO-66-NH_2$ photo-assisted catalysts were obtained by mixing earth-abundant low-cost $Cu_2O$ and $UiO-66-NH_2$. The formation of heterojunction led to better charge separation efficiency and higher yields of $H_2$ than either pristine $UiO-66-NH_2$ or pure $Cu_2O$. Performances of 2.3 equivalents of $H_2$ produced with a TOF value of $0.55 \text{ min}^{-1}$ were reported, with 83 % of photo-assisted catalytic capacity conserved after six cycles of AB hydrolysis, while pure $Cu_2O$ nanoparticles maintained only 30 %.

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Photo-assisted catalyzed AB hydrolysis using Pd-doped Co nanofibers [64].</figcaption></figure>

Plasmon-enhanced AB methanolysis under light illumination has also been reported using Cu–Ag alloy nanoparticles with controlled compositions [65b]. It was deemed relevant to alloy Cu and Ag for enhanced photo-assisted catalytic activity, copper being one of the lowest cost transition metals and plasmon-active, while silver display a strong Surface Plasmon Resonance (SPR) effect. $Cu_{0.4}Ag_{0.6}$ NPs of 30 nm size were synthesized via a one-pot method using Ag(I) nitrate and Cu(II) chloride salts as precursors. The methanolysis of AB in the presence of the bimetallic catalyst was conducted at rt., under dark and light conditions. The catalytic activity for AB methanolysis was four times higher with visible-light irradiation, reaching from plasmonic contribution TOF $ca. 24 \text{ min}^{-1}$ against $7 \text{ min}^{-1}$ in the dark, at rt. In such processes, mechanistic approach is all the more delicate that it is difficult to directly monitor the specific interface interaction in the plasmonic field. Using a combination of experimental and DFT studies, the authors suggested that high-energy electrons were generated by the irradiation, which in turn help activating methanol from O–H bond weakening, which was assumed to be the RDS of AB methanolysis. Plasmon-induced interfacial charge transfer between single Cu–Ag NPs and reactant molecules was evidenced *in situ* by using a single-particle confocal microscope system.

As previously observed from the works on different platinum-based catalysts [57,58], the mechanism of AB solvolysis is dependent on the nature of the active catalyst as a whole. This, includes the influence of the support that may be not neutral in the hydrolytic processes. Accordingly, using different mixed composition of copper and cobalt oxides NPs ($Cu_xCo_{1-x}O$) supported on graphene oxide (GO) [66], Feng et al. evoked the occurrence of a “synergetic effect” between the particles and the support. Such samples all gave TOF values higher than 45 $\text{min}^{-1}$, significantly higher than the value of $ca. 9 \text{ min}^{-1}$ obtained for monometallic oxidized copper on GO. They reported the absence of
activity for both GO alone, and monometallic oxidized cobalt on GO (the nature of the monometallic oxides being uncertain). The TOF value increases up to $70 \min^{-1}$ for the composition $Cu_{0.8}Co_{0.2}O/GO$ with increasing Cu content in the bimetallic samples, suggesting that Cu metal is the main catalytic element. GO has surface $C=O$ and COOH functional groups that anchor NPs through metal–O–C bonding, as detected by synchrotron radiation-based X-ray absorption spectroscopy (XAS). According to the authors, the interaction between metal and GO may form a new “hybridized” electronic state, different from that in the metal or GO alone. This hybridized state would improve the overall catalytic process. From XAS detailed comparative analysis, the addition of Co in the bimetallic sample would play a decisive role to enhance the bonding strength between $Cu_xCo_{1-x}O$ NPs and GO. Moreover, in situ XAS experiments revealed an evolution of the electronic structure of the catalyst throughout AB hydrolysis. The GO support apparently strongly adsorbs water molecules, so when AB molecules approach and interact with the NPs, especially the Cu atom, the activated B–N bond is attacked by proximate firmly adsorbed water molecules, facilitating $H_2$ release.

Synergetic effects were also described from oxygen-doped porous carbon (OPC-300) supporting bimetallic NPs nickel-cobalt doped with phosphorus (Ni<sub>0.66</sub>Co<sub>0.19</sub>P<sub>0.15</sub>) [67]. These were produced by a chemical reduction of $NaH_2PO_2$, $NiCl_2·6H_2O$ and $CoCl_2·6H_2O$ with $NaBH_4$ in water, and in the presence of the support. The resulting system formed an active catalyst displaying TOF of ca. $68 \min^{-1}$ at 25 °C, which is not achieved by the components considered individually. 85 % of this TOF was maintained over 5 cycles of hydrolysis. The kinetic studies identified an activation energy of ca. 39 kJ mol<sup>-1</sup> for AB hydrolysis reaction. According to the tests performed adding different concentrations of NaOH, the $OH^-$ species plays an important role in the release of $H_2$: the catalyst would allow both the adsorption of AB and $H_2O$ on its surface (Fig. 7a), facilitating an activated hydroxyl group $OH_{ads}$ (OH adsorbed on the catalyst surface) to attack some proximate $BH_3$ and breaking the B–N bond through a proposed $S_{N2}$ mechanism (Fig. 7b). A hydride could then leave the boron atom and react with $H_2O$ on the surface of the catalyst to generate $H_2$ and another $OH_{ads}$. The B–N bond would quickly regenerate due to electrostatic interactions (Fig. 7c). These steps would then occur twice more, generating a total of three $H_2$ molecules (Fig. 7d), and leading to the borate by-products, which potentially poison the surface from there adsorption, in agreement with the TOF decrease and the recycling limitation observed to five cycles.

A similar mechanism has been assumed in several other studies involving NPs in porous framework supports [68,69]. These studies highlighted the O–H bond cleavage as the RDS, but also emphasized the role of surface adsorption in this mechanism, as it would facilitate the dissociation of a hydrogen in $H_2O$ (a and d steps).

While a wide variety of catalysts have been developed for testing the hydrolysis of AB, the studies focused on their performances, mostly from TOF evaluation. The in-depth mechanistic studies of these solvolysis processes remains comparatively scarce. Yet, for a large part of the systems described, several interactions with AB are recurrently evoked:

i) B–H bond and/or N–H bond activation by electronic transfer from charged or neutral coordinated species is operating; ii) the role of water dissociation in the furniture of H is very often detectable; iii) accordingly, the dependence on the pH of the solvolysis medium is pertinent. Notably, the adsorption of $OH^-$/$H^+$ species, either on the supports, or on the catalytic NPs themselves, play a determining role in the kinetic enhancement of the process. Additionally, the supported (nano)catalysts may also interact –as a whole– with both AB and water, through concerted interactions involving both the support and the dispersed metal. Thus, in-depth surface adsorption processes examined in situ may help explaining further the influence of the support, which can often be considered as a crucial part of the “catalytic system” itself. The presumed activating effects are thus identified as so-called “synergetic effects”, involving concertedly metal and supports.

Concerning further development in the field, an interesting lever of modification at the surface of heterogeneous and nanocatalysts is the bimetallic (or polymetallic) combination, as well as band gap electronic modification due to the formation of semi-conducting metal oxide or phosphide [70].

A possible limitation of some AB solvolysis catalytic processes is the concomitant release of ammonia $NH_3$ that needs to be carefully controlled for avoiding poisoning of catalysts [71]. Ammonia release from the reaction slurry during hydrolysis of the $BH_3$ moiety of AB is unavoidable, since aqueous $NH_3$ is in equilibrium both with its acidic form, $NH_4^+$, and with gaseous $NH_3$ [72a,b]. Consequently, the hydrogen produced is inevitably contaminated with gaseous ammonia. As this hydrogen stream is extracted, the equilibrium between dissolved and gaseous $NH_3$ shifts towards the gaseous phase. This effect is further exacerbated by rapid hydrolysis, as the exothermic nature of the reaction increases the slurry temperature. The “hydrogen fuel quality product specification” from the International Organization for Standardization (ISO), i. e. ISO 14687:2025 standard, specify a content of 0.1 ppm $NH_3$ threshold, specially related to protection of PEM (proton exchange membrane) fuel cell catalyst, and fuel cell tolerance to hydrogen fuel impurities [71a]. In addition to GC analysis using a suitable column with a lower detection limit allowing finding ppm of $NH_3$ from gas evolution [72c], several methods for ammonia contamination detection exist. Notably, Fourier Transform Infra-Red spectroscopy was employed to analyze the $NH_3$ released from AB hydrolysis [71b]. Bubbling the hydrogen stream in a cupric sulfate solution allows detecting $NH_3$ by colour change [72d,e]. The concentration of the generated $NH_4^+$ in the liquid phase can be determined by using a colorimetric indophenol blue method [71b]. A comparable approach is to bubble the contaminated hydrogen stream in water and to follow the pH [72f]. Another method is to collect the gases in a bag, and use Draeger-Tubes to detect and quantify $NH_3$ at the ppm level [72g]. At the surface of catalysts XPS analysis from post-catalysis analysis could be useful to detect remaining nitrogen or boron residue.

Accordingly, mechanistic approaches have also pointed out the role of catalysts surface poisoning by various borates [71c], that must be better controlled. Notably, deactivation processes observed along

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Oxygen-doped porous carbon (OPC-300) supporting bimetallic nickel cobalt phosphide (NiCoP) for AB catalytic hydrolysis [67].</figcaption></figure>
multiple recycling attempts can be somewhat delayed, over ten cycles, from smart recovering of materials, such as magnetic systems [72a]. In any case, the catalysts deactivation modes are a current limitation that deserves better understanding. Finally, the stability of the various borates formed, depending on the conditions (hydrolysis or alcoholysis, pH, water content, AB concentration, etc.), also encourages the examination of some efficient catalytic opportunities to be developed for the regeneration process of AB from these diverse hydroxy or alkoxy borate by-products, as discussed in the next section.

## 2.4. Pathways to boranes regeneration after solvolysis

In comparison to the works devoted to boranes solvolysis performances, the modes of regeneration of AB remain understudied [73]. As a pertinent related topic, the regeneration of sodium borohydride (NaBH₄) conversely provides more clues [5,74], though less than its solvolysis that is however out of the scope of the present survey. AB regeneration from its solvolysis by-products mainly concerns borates that contain very stable B—O bonds [31], and consequently strong reducing agents would be a privileged approach for a single-step regeneration process.

Sodium borohydride hydrolysis, despite being a potential system for H₂ release, is characterized by irreversibility, and the resulting related high-cost hampers, to date, large-scale economic applications. NaBH₄ is industrially produced through two main processes: the Brown-Schlesinger process and the Bayer process, as detailed in Fig. 8. [75] The Brown-Schlesinger process combines preformed NaH and B(OMe)₃ at temperatures around 250 °C, while the more energetically demanding Bayer process (700 °C) starting from borax, Na, H₂, and sand, is a one-step only procedure.

After AB hydrolysis, the main by-products were shown to be NaB

(OH)₄—also noted [NaBO₂·2H₂O]— and/or its more hydrated derivative [NaBO₂·4H₂O] [74–77]. These borates are challenging to regenerate, requiring multiple steps processes and strong reducing agents, such as NaH [74–77]. To reduce the cost related to NaH, a novel technology involving MgH₂ as reducing agent was developed for regeneration of NaBH₄ from NaBO₂ with up to 97 % yield reported [78]. This method requires high temperature and pressure conditions that possibly delayed industrial scaling up. In agreement, the most recent researches shifted their focus onto the development of processes i) using solventless ball-milling (BM) [79,80], ii) with reduced input of hydride reductant, and

<table><caption>**Table 3**
NaBH₄ regeneration conditions.</caption><thead><tr><th>Reagents</th><th>Specific Conditions</th><th>NaBH<sub>4</sub> yield / [ref.] – report year</th></tr></thead><tbody><tr><td>MgH<sub>2</sub> + NaBO<sub>2</sub></td><td>70 bar H<sub>2</sub>, 550 °C</td><td>97 %/[78] – 2003</td></tr><tr><td>MgH<sub>2</sub> + Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub> (anhydrous borax) + Na<sub>2</sub>CO<sub>3</sub></td><td>AC, BM</td><td>78 %/[83] – 2003</td></tr><tr><td>MgH<sub>2</sub> + anhydrous NaBO<sub>2</sub></td><td>AC, BM</td><td>76 %/[84] – 2009</td></tr><tr><td>Na + MgH<sub>2</sub> + B<sub>2</sub>O<sub>3</sub></td><td>rt., inert atmosphere, BM</td><td>25 %/[85] – 2011</td></tr><tr><td>NaH + MgB<sub>2</sub></td><td>120 bar H<sub>2</sub>, BM</td><td>14 %/[86] – 2013</td></tr><tr><td>Mg or Mg<sub>2</sub>Si or Mg<sub>17</sub>Al<sub>12</sub> + NaBO<sub>2</sub>·2H<sub>2</sub>O or 4H<sub>2</sub>O</td><td>rt., inert atmosphere</td><td>78 %/[79,80,87] – 2017, 2017, 2019</td></tr><tr><td>MgH<sub>2</sub> + NaBO<sub>2</sub>·2H<sub>2</sub>O or 4H<sub>2</sub>O</td><td>AC</td><td>90 %/[88] – 2017</td></tr><tr><td>Mg + Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>·xH<sub>2</sub>O (x = 5, 10) + Na<sub>2</sub>CO<sub>3</sub> (from aqueous solution of NaBO<sub>2</sub> to CO<sub>2</sub>)</td><td>AC, BM</td><td>80 %/[82] – 2020</td></tr></tbody></table>

AC = ambient conditions, i. e. under air at room temperature; BM = ball-milling (a ball mill is a type of grinder filled with grinding balls working on the principle of impact and attrition: for systems with multiple components, ball-milling has been shown to be effective in increasing solid-state chemical reactivity).

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Industrial processes for NaBH₄ synthesis from the raw [75].</figcaption></figure>
iii) with alternative co-reagents added [81]. Accordingly, Table 3 summarizes the main conditions reported for regenerating NaBH₄ from various type of borates. Several rt. reactions, either under air or inert atmosphere, and solventless ball-milling are described since the beginning of 2000s. The yields in these studies mostly account for crude products, without further purification, and they demonstrated the nature of the obtained products by FT-IR or powder XRD experiments. Only Zhu's group reported the purification of the products by dissolving in ethylenediamine, filtrating over polytetrafluoroethylene, then freeze-drying [82].

Notably, some processes using hydrated metaborate or borax, avoid the energy-intensive dehydration procedure to obtain anhydrous NaBO₂ or Na₂B₄O₇ without the requirement of high-pressure H₂ gas. This might suggest that protonic H⁺ present in the water that is bound to NaBO₂ may convert into the hydride of the NaBH₄ formed. A detailed historical perspective could be found in a recent review by Ouyang and collaborators [8], and NaBH₄ regeneration modes was also previously reviewed [89]. Since thermochemical regeneration of NaBH₄ from NaBO₂ is considered too costly to be due to metal hydride reagents, Paskevicius et al. explored an alternative regeneration route of NaBH₄ using NaB (OCH₃)₄ instead of the widely studied NaBO₂ [90]. In-situ synchrotron X-ray diffraction was employed to detect the formation of NaBH₄ either from NaB(OCH₃)₄ or from NaBO₂ using the metal hydride reducing agents NaH, MgH₂, Mg₂FeH₆, LiAlH₄, NaAlH₄, CaNi₅H₃, and LaNi₅Hₓ. NaBH₄ formation was detected using NaB(OCH₃)₄ with LiAlH₄ at 20 °C. Regeneration from NaBO₂ required higher temperatures. The critical role of hydridic hydrogen H⁻ in solid-state thermochemical reactions was underlined for NaBH₄ formation. Using alternative regeneration route from NaB(OCH₃)₄ may offer cost and energy savings.

These approaches for NaBH₄ provide important insights for retrieving other hydrogen supply challenging systems, notably AB. Ramachandran et al. reported a preparation of AB from commercial B (OMe)₃, which is assumed to be the main by-product of AB methanolysis reactions [51,91]. Under inert atmosphere, 4.8 mmol of B(OMe)₃ was reacted with 9.6 mmol of ammonium chloride using 5.7 mmol of LiAlH₄ in THF at 0 °C for 3 h (Fig. 9) [92]. The product was isolated by distillation of the solvent, then purified by resuspension in cold diethyl ether, filtration, and drying in vacuo.

This process yielded ca. 90 % of isolated NH₃BH₃ with a purity of +99 % [92]. Herein, AB methanolysis by-product proved easier to recycle than hydrolysis by-product [93], which required an additional step to convert B(OH)₃ into B(OMe)₃ before undergoing the aforementioned reaction. From an economic and industrial perspective, Li and Al precursors are used to form LiAlH₄ (LiH and AlCl₃), while LiCl and Al (OMe)₃ do not give easy access to the LiAlH₄ reactive hydride. Thus, initial steps to reduce LiCl into LiH, and converting Al(OMe)₃ into AlCl₃ would be needed, clearly making this solution economically more limited. Paskevicius et al. has analogous conclusions [90], concerning the viability of NaBH₄ regeneration from NaB(OCH₃)₄ and sodium alamate NaAlH₄ in diglyme, which produced NaBH₄ at an 80 % yield. Although feasible, this synthesis is not viable for economical exploitation as it employs NaAlH₄, which is more expensive and difficult to recycle than NaBH₄.

The regeneration of AB from borates without the use of LiAlH₄ can take place via the Brown-Schlesinger process (Fig. 8, Fig. 10). On the ground of the regeneration processes sustainability, several studies reported that undesired NH₃ gas could be released during the AB hydrolysis step [94–96], if not entangled somewhat as an ammonium salt or

<figure><img src="image_11.png" /><figcaption>Fig. 10. AB regeneration from the hydrolysis by-product borate B(OH)₃ [97].</figcaption></figure>

being efficiently solvated (the solubility of NH₃ in the medium being an issue). Besides gaseous contamination of the converted H₂ (unsustainable for operative fuel cells, for instance), this implies the need to add a supplementary source of nitrogen for the AB regeneration – such as NH₄Cl, or (NH₄)₂SO₄ –, this decreasing the economic relevance of the process.

Ramachandran et al. reported that some reaction conditions may decrease NH₃ release during AB hydrolysis [94]. When 6 wt% AB were hydrolyzed in the presence of 0.06 mol% RuCl₃ catalyst, slowly introduced over 45 min, no NH₃ evolution was detected by titration. While the reason remains unclear, such type of conditions might improve the viability of the AB regeneration process. Sanyal et al. also proposed to separate and recover NH₃ using membranes [96]. Alternatively, Hanauer et al. were able to recover NH₃ with a cold trap system (liquid nitrogen) [98]. Therefore, this matter needs to be addressed systematically for convincing approaches of AB regeneration.

As illustrated before in the comparison of hydrolysis, resulting in B (OH)₃ by-product, and methanolysis reactions, resulting in B(OMe)₃ formation, the nature of borate by-products is a determining element for AB regeneration. Focusing on hydrolysis, mainly B(OH)₃ and B(OH)₄ are produced from this reaction, the former being the acidic form of the latter. The pKa of this couple being 9.2, B(OH)₃ is predominant at lower pH, and can be interchanged by control of the pH of the solution. Powder XRD and ¹¹B NMR analyses evidenced the equilibrium existing between the B(OH)₄⁻ form and different polyborates, including B₃O₃(OH)₄⁻, B₄O₅(OH)₄²⁻, and B₅O₆(OH)₄ [96,98–101]. These species formed according to the equations summarized in Fig. 11.

Using ¹¹B NMR studies, Salentine et al. [99] reported a detailed study about the stability of each of these anions in an aqueous solution by dissolving different potassium borate salts under various conditions. First, they showed that the higher the boron concentration –meaning the more potassium salt is added–, the more polyborates formed. Indeed, when 0.05 M of potassium pentaborate is dissolved in water, over 90 %

<figure><img src="image_12.png" /><figcaption>Fig. 11. Polyborates formation equations [99].</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 9. AB regeneration from methanolysis by-product B(OMe)₃ [92].</figcaption></figure>
of it immediately dissociates into monoborate B(OH)₃/B(OH)₄, and a detectable amount of triborate B₃O₃(OH)₄. As the concentration increases to 0.10 M, a small amount of B₅ anion can also be detected in addition to the previous species. The concentration of formed borates is thus linked to the increase of concentration of the starting potassium pentaborate salt [99]. This effect of concentration was also confirmed in different solutions of NaBH₄ and AB mixtures by ¹¹B NMR after hydrolysis with a cobalt catalyst, where the formation of B(OH)₃/B(OH)₄, and B₃O₃(OH)₄⁻ was detected, and AB concentration was reported to favor higher polyborates formation [98]. Temperature-dependent ¹¹B NMR showed that temperatures higher than rt. cause polyborate dissociation, as they are more stable under 40 °C [99]. Finally, each species appears in a specific pH range: the triborate anion B₃O₃(OH)₄⁻ usually appears in the pH range of 7.0 to 9.0, the tetraborate B₄O₅(OH)₄²⁻ is present in solution from pH 8.0 to 10.0, and the pentaborate B₅O₆(OH)₄ is detected until pH 9.7. Thus, favoring an acidic pH would allow to recover B(OH)₃ for further treatment.

Recovering BH₃ directly from B(OH)₃ without the stoichiometric use of expensive (potentially polluting) strong reducing agents, such as common metal hydrides, would significantly improve the viability of AB regeneration, and thus of the overall use of AB as a CHS material. Although such specific reaction type has not yet been reported, one can draw inspiration from various studies that have focused on the catalytic reduction of alcohols or ketones to alkanes. Gürbüz et al. reported the conversion of 1-butanol to butane under 10–30 bar H₂ pressure and between 210 and 250 °C, in the presence of SiO₂ supported Ir, Pt, Ru or Cu clusters [102]. Cu clusters of 5 nm showed the highest selectivity towards C—O bond hydrogenolysis (TOF ca. 600 min⁻¹). Ir (7.1 nm), Pt (0.7 nm) and Ru (0.6 nm) favored C—C cleavage, although they allowed also significant turnover rates for C—O hydrogenolysis (around 60, 6, and 4 min⁻¹, respectively). This difference was explained by higher chemisorbed CO and C (activated CO* and C*) binding energies for Ir, Pt and Ru, as compared to Cu. As CO is chemisorbed at the surface of the catalysts, it inhibits the desired C—O cleavage reaction and rather favors C—C cleavage. Another envisaged explanation is that larger clusters tend to have more so-called terrace sites (edge and corner higher coordination sites), which are more selective towards hydrogenolysis. The authors suggest that other metals with low C* binding energies, such as Ag and Au, could also promote C—O hydrogenolysis. Transposed to B—O bond cleavage, the same metals (Cu, Ag, Au) might also have a low B* binding energy [35], and reveal pertinent for studies aiming at for B (OH)₃ to BH₃ conversion.

Zhang et al. reported the full reduction of acetophenone into ethylbenzene with 94 % yield under 10 bar H₂ at 120 °C in hexane (hexane boiling point is around 150 °C at 10 bar), using as catalyst Co NPs supported on ordered mesoporous carbon [103]. Milder catalytic ketone deoxygenation to alkane was studied by Argouarch, with the complete reduction of 4-acetyl biphenyl at ambient conditions [104]. This was achieved in dichloromethane using two equivalents of Et₃SiH as hydride source, and 1 mol% of the complex [Rh(µ-Cl)(CO)₂]₂. It was suggested that the ketone is first reduced to an alcohol, which is then hydrogenated to an alkane. Therefore, this reaction may inspire B(OH)₃ reduction, on the road to AB regeneration.

## 2.5. Prospective of ammonia borane as CHS materials

AB reactivity to deliver H₂ has been intensively investigated since the year 2000s, especially regarding catalysts for its solvolysis. Its high intrinsic hydrogen content, stability at ambient conditions, low toxicity, versatile reactivity with metals at very mild temperature, are collectively advantageous. However, it remains, under the current standards, difficult to industrially implement because of significant drawbacks. First, its H content does not represent its excess storage capacity in a system. In fact, this value is closer to 3.5 wt% in the current hydrolysis system, compared to the initially stated 19.6 wt%. As the product manufacturing chain has not been developed yet, the costs of AB remain

high. Additionally, the purity of the produced H₂ should not be neglected, while its contamination by ammonia would be detrimental to any operative implementation. Specific filters would be necessary to prevent such contamination, or even recover NH₃ for further treatment. Finally, the borate by-products formed during the hydrolysis or alkoholysis reactions, are still little considered in the studies, and no really efficient regeneration pathways have been proposed. Further research efforts concerning pure and applied chemistry should focus on this topic, for example by applying the use of the many studied transition metal catalysts to the regeneration processes under H₂ pressure to reduce boric acid back into BH₃. Obviously, in this perspective, decarbonated H₂ produced from water electrolysis using renewable “green” energy sources (solar conversion) is the ideal identified virtuous scenario.

# 3. Silanes for hydrogen storage and delivery

## 3.1. Introduction and context

The term “silane” refers very generally to silicon–hydrogen bond-containing compounds. Silicon is the second most abundant element on Earth, with 26.3 wt% occurrence [36]. Although silanes are not naturally occurring compounds, their syntheses are done at large industrial scale, starting before the 1980’s, and a wide range of them is commercially available. They are usually easily handled and stored. Concerning hydrolysis, the cleavage of Si–H bonds is thermodynamically favored, this reaction producing valuable chemicals as side products, such as silanols, polysilanes or polysiloxanes. Within the family of silanes, the simplest one is silane gas (SiH₄), which is among the highest hydrogen-containing species, with an absolute gravimetric hydrogen capacity of 12.5 wt%. Tetrasilylmethane (TSM), C(SiH₃)₄, is the lightest carbosilane and a volatile colorless liquid at rt. with a 8.8 wt% of hydrogen [105]. Silicon polymers such as polymethylhydrosiloxane [Si(HCH₃)O]ₙ (PMHS), or H-polysilanes [SiH₂]ₙ (HPS) are also high hydrogen content compounds. SiH₄ is commercially available for prices around 4000 € per kg, while TSM is more synthetically demanding [105]. PMHS price is ca. 600 € per kg, and HPS is not to date widely available from commercial sources. Because of the major importance of silicon electronic and silane polymerization industry, using silane sources as CHS material is an opportunity to envision since catalytic H₂ release from silanes’ solvolysis is a recognized process. Cheaper alternative to boranes may be also attractive.

SiH₄ and HPS industrial synthesis start from SiO₂ (Fig. 12). A mixture of sand and coal submitted to HCl gas produce SiCl₄, which is either reduced to SiH₄, or polymerized before being reduced to HPS [36,106]. SiH₄ is also obtained from magnesium silicide Mg₂Si and hydrochloric acid, among other methods [106].

For TSM, numerous preparation modes were described [105,107–109], typically starting from CCl₄ with tetrakis(4-aminophenyl)methane and reduction with LiAlH₄. PMHS is a current by-product of the silicone industry [110], which can be prepared by hydrolyzing methyldichlorosilane CH₃SiHCl₂.

Thus, organosilicons and especially hydrosilanes, as widely manufactured commercial products, may also be attractive as potential hydrogen carrier source. This may follow various chemical roads catalyzed by metals, including hydrolysis and alcoholysis of silanes (R₃Si–H, R₂SiH₂, etc.) [39–42,111–113], or their dehydrogenative oligomerization (like PhSiH₃ dehydrocoupling) [114–116].

## 3.2. Metal-catalyzed hydrolysis and alcoholysis of hydrosilanes

Hydrosilanes alcoholysis, yielding the corresponding alkoxysilanes and hydrogen gas, was already described in the 1950s’ [117], and the dependence on Si–H bond polarization of the rate of dehydrocondensation of hydrosilanes in the presence of nucleophilic or electrophilic catalysts was established. Silane alcoholysis reactions were further studied in the complete absence of catalysts, showing the
<figure><img src="image_14.png" /><figcaption>Fig. 12. General industrial procedure for the synthesis of SiH₄ and H-polysilanes [SiH₂]n HPS.</figcaption></figure>

limitation to perform these reactions at rt., although some hydrosilanes, monoorganosilanes, diorganosilanes and arylfluorosilanes could be converted [118]. Further studies were devoted to the alcoholysis of hydrosilanes in the presence of alkaline, acidic, or metallic catalysts [118]. Driven by the O-silylation reactions, which are very effective to selectively protect hydroxyl functional groups, the use of cobalt complex catalysts Co₂(CO)₈ [119,120], and [CoH(N₂)(PPh₃)₃] [121] was reported, showing similar reaction pathways which involve metal-silicon intermediates. The catalytic activity of some iron complexes has also been investigated, showing at that time lesser efficiency [122]. Some catalytic complexes based on heavier elements of groups 8 and 9 were also studied, and ruthenium with [RuCl₂(PPh₃)₃] and the rhodium Wilkinson's catalyst [RhCl(PPh₃)₃] showed effective activity [123,124]. [RhCl(PPh₃)₃] is a strong silylating agent for OH groups, reported as more active and selective than Co₂(CO)₈ towards the alcoholysis of diarylsilanes, and which allows catalyzing the O-silylation of phenols. However, for HSiEt₃ it is active only in refluxing benzene [123,124]. The group 10 chloroplatinic acid [PtH₂Cl₆·6H₂O] was among the most investigated complexes for dehydrocondensation, with an uncertainty on the homogeneous or heterogeneous nature of the genuine catalytic species [118,125,126]. Indeed, in many cases, the nature of the active catalytic species has not been unambiguously established [40], and it was reported in other circumstances that colloidal particles form from [PtH₂Cl₆·6H₂O] reduction (Speier's catalyst) [124] in the hydrosilation of olefins and ketones, or alcoholysis of silanes [123]. Curiously, the development of well-defined and controlled nanocatalysts, possibly very active and recoverable, did not emerge from these results to date. Concerning palladium, only some attempts have been reported since the early 1980s' with palladium complexes, which did not show decisive improvement in the field [118,127].

Conversely, in the group 9 metals, the first catalytic activity proved from iridium(I) complexes by Blackburn et al. held much promises [128]. Vaska's complex [IrCl(CO)(PPh₃)₂] allowed complete ethanolysis and methanolysis of SiHR₃ compounds (R = OEt, Et and Ph) at 23 °C within 25 to 50 min, using a catalyst ratio of 5 mol%. This complex also allows up to 95 % conversion in 20 s for the polysilane Me₃SiO[SiH(Me)O]nSiMe₃ (n ≈ 50). The mechanism proposed and supported by FT-IR, NMR, and kinetic studies, is schematized in Fig. 13 and involved two successive oxidative addition/reductive elimination (OA/RE) from Ir(I)/Ir(III) complexes, with the formation of a transient iridium hydride and halosilane, from which the deprotonation of alcohol is proposed. This hypothesis is supported by the identification of the complexes [IrHCl₂(CO)(PPh₃)₂] and fac-[IrH₃(CO)(PPh₃)₂] in the precipitate from FT-IR data, which indicates a chlorine redistribution and therefore a cleavage of the Ir-Cl at some stage of the reaction. Moreover, the reductive elimination of ClSiH₃ had already been shown to occur from [IrH₂(SiH₂Cl)(CO)L₂] [129].

Luo and Crabtree reported the high catalytic activity and selectivity of the cationic Ir(III) complex [IrH₂S₂(PPh₃)₂]SbF₆ (S = solvent, THF, MeOH) for silane alcoholysis [40]. Kinetic and mechanistic studies led to a different, redox neutral, mechanistic proposal, involving the activation of the silane through η²-bonding of the hydridic hydrogen to the metal

<figure><img src="image_15.png" /><figcaption>Fig. 13. Mechanism proposed of SiHR₃ alcoholysis catalyzed by Vaska's complex type catalysts [128].</figcaption></figure>

center, and thus without any OA/RE process (Fig. 14).

Although hydrogen generation was clearly not the aim at this time, its efficient monitoring during different HSiEt₃ alcoholysis allowed to obtain kinetic data. For most of the reactions, they observed a constant rate value of H₂ release ranging between 3.3 and 7.0 cm³.s⁻¹, which is more than two hundred times faster than rates obtained for the ethanolysis of Si(OEt)₃ catalyzed by Vaska's complex [128]. They accordingly determined TOF values for each reaction, and got impressive maximum values of TOF ca. 1830 min⁻¹ and 2170 min⁻¹ for the reaction

<figure><img src="image_16.png" /><figcaption>Fig. 14. Catalytic cycle of [IrH₂(ROH)₂L₂]⁺ with SiHR'₃. [40].</figcaption></figure>
of HSiEt₃ with 2-BuOH and i-PrOH, respectively. Kinetic isotopic effect has been examined by isotopic substitution of HSiEt₃ and MeOH. Methanolysis of HSiEt₃ revealed a small isotope effect value (k<sub>O-H</sub>/k<sub>O-D</sub> = 1.8), while the value obtained for O-silylation of MeOH did not indicate isotope effect (k<sub>Si-H</sub>/k<sub>Si-D</sub> = 1.1). These results suggested that the O-H bond cleavage was directly involved in the rate-determining step of the reaction mechanism. This was considered consistent with the harder achievement of the reaction using a slightly different catalyst ([IrH(OH₂)(bq)]SbF₆, bq = anionic 7,8-benzoquinolate), which can a *priori* produce a single empty coordination site, instead of two for [IrH₂S₂(PPh₃)₂]SbF₆ (holding labile S = ROH, or THF, Fig. 14). The authors deemed this to be an important structural feature because of the generation of two vacant *cis*-coordination sites, allowing to bind booth the alcohol and the silane simultaneously, to give the [IrH₂(ROH)(η²-SiH*R'₃)L₂]⁺ species. Thus, the following step involves an internal or intramolecular nucleophilic attack, where the bound CH₃OH attacks, from within the coordination sphere, the 2-HSiEt₃ coordinated to the same metal center. This attack is made possible by the availability of two lone pairs on the oxygen atom of CH₃OH.

The reactivity of [IrH₂S₂(PPh₃)₂]⁺ (S = H₂O), in water as solvent, was also demonstrated for performing the somewhat related dehydrogenation of alkanes, which produces H₂, albeit in moderate conversion below 50 % yield [130]. Interestingly, water, which was generally avoided in such high-valent metal-catalyzed reactions, turned out to react four to ten times faster than ethanol, in a reaction with HSiEt₃ and Wilkinson's Rh reagent, or [Pt(C₂H₄)(PPh₃)₂] as catalyst, to form the corresponding Si-O bond [131]. Accordingly, water has been increasingly used, instead of alcohols, to perform hydrosilanes hydrolysis rather than alcoholysis, with the aim to obtain silanols [39,132], or silyl ethers (siloxane formation) [133].

With the resurgence of an interest for H₂ as energy vector in the mid
2000’s, the catalytic hydrolysis of hydrosilanes for H₂ production was
further described by reacting silanes and water with unusual high-valent
metal complexes [134]. A cationic oxorhenium(V) complex, [Re(O)
(hoz)₂(OH₂)]⁺ (hoz = oxazoline) reached reaction completion in less
than an hour to selectively gave the expected silanol. The source of each
H-atom of was evidenced by performing the hydrolysis of Et₃SiD with
H₂O, and separately of Et₃SiH with D₂O. The produced gas, analyzed by
mass spectrometry, was found to be exclusively HD, thus confirming that
each of the reagents provide one out of the two hydrogen atoms of H₂.
This double sourcing of hydrogen from silane and water in the hydro-
lysis of hydrosilane using cationic rhenium was supported by DFT
calculations.

These results were complemented to some extent by the examination

of hydride iridium complex models [41]. The catalyst precursor [IrH
(CF₃SO₃)(NSiN)(coe)] (NSiN is fac-coordinated bis[pyridine-2-yloxy]
methylsilyl, and coe is coordinated cyclooctene) was selected for the
calculations (Fig. 15) because of its recognized high activity for H₂
generation by organosilanes hydrolysis (TOF 1610 min⁻¹ and 1790
min⁻¹, respectively, for the hydrolysis at 25 °C of [Me₂HSi]₂O and
SiH₂Et₂ in THF) [41].

The approach proposed a mechanism producing a doubly silylated L-Ir(η³-SiH₂R₃) complex as catalyst, that releases a silanol from water outer-sphere nucleophilic attack, while a formal trihydride is formed, and is further regenerated by H₂ release of an L-IrH(η²-H₂) complex [41]. The study, unsurprisingly, concluded that the rate, through the TOF performance, depends both on the solvent and the nature of the hydrosilane, where less hindered silanes are more active – in apparent agreement with outer-sphere limiting process. H₂ releasing reactions could be performed in solvent-free conditions (notably without THF), but appeared three orders of magnitude slower for [Me₂HSi]₂O hydrolysis [41]. While Ir(η³-SiH₂R₃) bonding is intriguing, concerning the coordination chemistry of silanes to metals, it is worth noting that the first example of a fully characterized cationic Ir-η¹-silane complex had been reported [135]. Complementary, the reactions of hydrosilanes with transition metal complexes, and the characterization of ensuing products, were reviewed in detail [136], discussing the characterized transition metal complexes with different topical M-H-Si bonding.

Still under solvent-free conditions, the ionic iridacycle [(2-phenyl-
epyridine-κN-κC)-IrCp*(NCMe)][BArF₂₄] (BArF₂₄ = tetrakis[3,5-bis
(trifluoromethyl)phenyl]borate, {[3,5-(CF₃)₂C₆H₃)₄B], Cp* = pentam-
ethylcyclopentadienyl) used as a catalyst precursor, achieved for the
alcoholysis of HSiEt₃ using benzylic alcohols PhCH₂OH and *p*-MeO-
PhCH₂OH, respectively, TOF *ca.* 2720 min⁻¹ and 3200 min⁻¹ (Fig. 16)
[113]. This iridacycle catalyst performed these reactions to reach
completion within 20 s, increasing the H₂ pressure from 0 to 5.3 bar in
the meantime.

Comparative catalytic runs of MeOD with HSiEt₃ and of MeOH with DSiEt₃ were performed to further examine KIE. The values obtained, kQ. H/kO-D = 2.7 ± 0.2 and kSi-H/kSi-D = 0.73 ± 0.01, although slightly different than Luo and Crabtree anterior report [40], led to similar conclusion. The RDS would be the RO-H bond cleavage during the formation of MeOSiEt₃. It most likely involves a reaction of [RO(H)(SiEt₃)]⁺ with a hydrido ligand bound to the metal center. The authors proposed an outer-sphere process with a nucleophilic attack of MeOH on the activated Ir coordinated silane (Fig. 16) consistent with previous proposals [40]. They succeeded to grow single crystals of the catalyst, and to correlate its structure, obtained by XRD, with the one optimized

<figure><img src="image_17.png" /><figcaption>**Fig. 15.** Proposed mechanism for the hydrolysis of HSiMe₃ catalyzed by [IrH(CF₃SO₃)(NSiN)(coe)], coe = cyclooctene [41].</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Fig. 16.** Mechanism of Ir-catalyzed $SiEt_3H$ alcoholysis, with a cationic Ir–Si···H intermediate [113].</figcaption></figure>

by DFT. This geometry, associated with the analysis of calculations, led to identify in the Ir–Si···H motif (close to Ir–$\eta^2HSiEt_3$ formulation) a donor-acceptor type of interaction, with a higher contribution of Si—Ir compared to Si···H, which corresponds to a hydride transfer from H–$SiEt_3$ to the Ir center, concerted with the bonding of the silylum cation to the new Ir–H unit formed, this as an alternative to an oxidative addition process.

An analogous pathway, based on a strict Ir–$\eta^1$-H-SiR$_3$ σ-complex, was also considered by Ventura-Espinosa et al. [137]. Their DFT calculation results indicated that the first step of the reaction is an electrophilic activation of the Si–H bond (Fig. 17). The complex [IrCp\*Cl$_2$(1-Me-3-pyrenylimidazole)], which turned out to be an efficient catalyst precursor for silane alcoholysis, was not supposed to react through such pathway, since the latter requires a vacant coordination site that is not present on the complex. However, $^1$H NMR analyses of [IrCp\*Cl$_2$(1-Me-3-pyrenylimidazole)], both in CDCl$_3$ and in CD$_3$OD, revealed an equilibrium between different metal complexes promoted by ligands exchange between MeOH molecules and chlorido ligands (Fig. 18). A coordination site occupied by a solvent molecule is prone to be vacant, thus this ligands exchange is decisive for effective silane methanolysis by going through the postulated Ir–$\eta^1$-H-SiR$_3$ intermediate. Such ligand exchange had previously been observed for Ru-arene complexes [138].

Notably, Ventura-Espinosa et al. also made a hybrid material [137], by immobilizing the molecular complex on a reduced graphene oxide (RGO) surface, based on π-stacking interactions between the RGO support and the pyrene moiety of the strongly Ir-coordinated imidazole

ligand. The supported catalysts indeed achieved silane alcoholysis (100 % conversion within 7 min), and could be convincingly reused ten times without activity loss, and matching the performances of the molecular catalyst precursor for the methanolysis of $HSiPhMe_2$ (100 % conversion within 3 min).

As mentioned before, Luo and Crabtree early on suggested that the catalytic activity of $[IrCl(coe)_2]_2$, reported earlier by Blackburn et al., could be mostly heterogeneous in nature [40,128]. However, none of these authors mentioned the “reuse” of recovered so-called “precipitate” post-catalysis. The first attempt of catalyst precursor recovering for such application was reported by Oro et al. that indeed succeeded to reuse two “molecular catalyst precursors” five times without significant activity loss, for the methanolysis of $HSiMePh_2$ [42]. Nevertheless, catalyst recovery is easier from a suitably designed heterogeneous catalyst. Accordingly, the solvent-free octanolysis of $HSiEt_3$ was achieved by using a dirhodium(II) perfluorocarboxylate catalyst immobilized on derivatized silica at 50 °C, allowing TOF ranging up to 30 min$^{-1}$ but with a single recycling reported [139]. The butanolysis of $HSiPhMe_2$ catalyzed by Au NPs supported either on chromium or indium tin oxide (ITO)-spattered quartz plates, and decorated with dodecanethiolate self-assembled monolayers was also described at 25 °C, resulting in TOFs around 0.020 min$^{-1}$ [140]. In this field, the previous examples suggest that the development of catalysts “recoverable-by-design” will probably progress in the near future, taking profit of now available operando characterization for better mechanistic approaches and understanding inspired by well-developed homogeneous systems.
<figure><img src="image_19.png" /><figcaption>Fig. 17. Mechanism of Ir-catalyzed methanolysis passing through Ir-η¹-H-Si⁺R₃ σ-complex [137].</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Fig. 18. Equilibrium of [IrCp*Cl₂(1-Me-3-pyrenylimidazole)] in a MeOH solution monitored by $^{1}$H NMR [137].</figcaption></figure>

To conclude on these mechanistic advances, the lastly reported work of Huertos and coworkers testifies to the possible involvement of hydrogen quantum tunneling mechanism (HQTM) in the strikingly fast hydrosilane methanolysis and hydrolysis reactions (Fig. 19) [111].

This effect discovered by studying the catalytic activity of a new Ir complex {Ir[SiMe(o-C₆H₄SMe₂)(H)(PPh₃)(THF)]-[BArF₂₄] in the

hydrolysis of HSiEt₃ in THF, was evidenced by the largest KIE ever reported for such a reaction, kSi-H/kSi-D = 346 ± 4, of several orders of magnitude bigger than any previously reported analogous system [40,113]. HQTM was confirmed by calculation of an unusually low Arrhenius factor ratio (A<sub>H</sub>/A<sub>D</sub> = 0.0004) and of an important difference in the activation energy of H and D, E<sub>aD</sub> – E<sub>aH</sub> = 34.07 kJ mol<sup>-1</sup>. DFT
<figure><img src="image_21.png" /><figcaption>Fig. 19. Mechanism of HSiEt₃ hydrolysis catalyzed by {Ir[SiMe(o-C₆H₄SMe)₂](H)(PPh₃)}⁺ in THF [111].</figcaption></figure>

calculations were also used to identify the RDS in which this effect occurs, and an electrophilic mechanism was proposed, different from earlier proposals (Fig. 19). It is noteworthy that KIE values were also examined for the hydrolysis and methanolysis of HSiPhMe₂ and HSiPh₃: k<sub>Si-H</sub>/k<sub>Si-D</sub> = 4.3 ± 0.5 and k<sub>Si-H</sub>/k<sub>Si-D</sub> = 16.8 ± 0.2, respectively. They are still high, thus indicating the break of Si—H bond in the RDS, but not enough to be convincingly attributed to HQTM [141–143]. The probability for HQTM to happen, thus, in addition to the catalytic effect, strongly depends on the hydrosilane nature, and should be further exemplified and studied in the relevant conceptual advance of combining tunneling and catalysis for reaction rate improvement.

### 3.3. Metal-catalyzed dehydrogenative oligomerization of hydrosilanes

Dehydrogenative oligomerization of silanes, also known as dehydrocoupling, produces H₂ when silane reagents contain one or several hydrogen atoms. This is not the case for the Wurtz-type coupling that was the current procedure for synthesizing linear polysilanes from dichlorosilanes with alkali metal dispersion. In this case, sodium radical reactivity is employed in an inert hydrocarbon solvent at temperatures higher than 100 °C [144,145]. Wurtz-type coupling are mostly poor yielding with low functional groups tolerance [144]. More attractive alternative were disclosed by Harrod, Tilley, and co-workers, who reported the catalytic activity of group 4 metallocene-based complexes for oligosilanes and derivatives production [112,115,146–149]. Some group 4 complexes that are able to rapidly form a hydride in the presence of hydrosilanes were found particularly efficient. However, the evaluation of dehydrocoupling catalysts activity is complicated because of the difference in reactivity of the functional groups of the entities that continuously build up the polysilane chains (i. e. monomers vs. oligomers of various sizes, linear vs. cyclized entities). The intrinsic very fast initial speed of group 4 complexes makes measurement difficult for most zirconocene catalysts. An inaccurate initial rate of H₂ releasing is potentially misleading as a measure of dehydropolymerization activity. Instead, a quantitative procedure based on gel permeation chromatography (GPC) is usually employed, which consists in analyzing the

polysilane molecular weight distribution (D = M<sub>w</sub>/M<sub>n</sub> ≥ 1, M<sub>w</sub> = average molecular weight in mass, M<sub>n</sub> = average molecular weight in number), as produced by the catalyst, under a standard set of reaction conditions. Polystyrene standards being the most often used (neat H₃SiPh monomer, room temperature, 1 bar of N₂ pressure, 1.0 mol% of catalyst). A polydispersity index (D) is equal to 1 only when all the molecules of the studied sample have the same molecular weight; higher is this value, broader is the molecular weight distribution, lower is the selectivity of the catalyst used for the reaction. The catalyst activity towards polymerization (vs. oligomerization and cyclization) can be estimated with both M<sub>w</sub> and M<sub>n</sub>; higher are these values, heavier are the polymers produced in average, and thus longer are the polymers in average. Branching can occur, but as discussed further, the nature of the reaction mechanism makes the reaction between the catalyst and –SiH₂R end groups of polysilanes easier, thus preferentially giving linear polysilanes.

The zirconocene complexes [CpCp\*ZrH₂]₂ and {CpCp\*Zr[Si(SiMe₃)₃]Me} emerged as very active catalysts, reacting at 23 °C, under 1 bar of nitrogen, with H₃SiPh to produce H₂ and H(SiHPh)₃H polysilanes with yields over 90 % and a reported polydispersity index D ≤ 2.45 (with D = M<sub>w</sub>/M<sub>n</sub> ≤ 13,880/5660) [148,149]. The reactivity of hetero-substituted complexes holding both Cp and Cp\* ligands was found superior to homo-substituted complexes holding Cp/Cp and Cp\*/Cp\* pairs [147], hypothetically because of the beneficial combination of lesser steric hindrance from one Cp and higher electron-donation from one Cp\*.

Tilley et al. disclosed sigma-bond metathesis mechanism for dehydropolymerization of silanes to polysilanes by d⁰ metal catalysts [112]. Accordingly, silyl complexes with d⁰ metal–Si bond showed high reactivity towards σ-bond metathesis (Fig. 20).

To allow a direct comparison between different catalyst precursors, D was used for the dehydrocoupling of H₃SiPh at rt., followed by the analysis of the products by GPC [112]. This method has provided evidence for the role of hydride complexes as the true catalysts. As a result, a direct reactive path mechanism was proposed involving a succession of two σ-bond metatheses: one for the dehydrosilylation of the catalytic
<figure><img src="image_22.png" /><figcaption>**Fig. 20.** Dehydrogenative polymerization of hydrosilanes by zirconocene and hafnocene derivatives [112].</figcaption></figure>

complex (A step, Fig. 20) and another one for the Si—Si bond formation (B step), each passing through a four-center transition state [112].

This stepwise growth polymerization mechanism, as a transition metal center-driven catalyzed condensation, generally lead to steric limitations of the molecular weights of the polysilanes produced, due to growth-center accessibility restriction. The conversion rate is also limited by the increase in viscosity of the reaction mixture along the polymerization. Accordingly, attempts to reduce this viscosity has been done by heating up the reaction mixture. This was fairly ineffective because of the increased formation of low molecular weight cyclic oligomers. Still, the molecular weights of linear polysilanes could be increased (D = 2.20, M<sub>w</sub>/M<sub>n</sub> = 11,660/5300) by adding solvent, and removing H<sub>2</sub> produced along the reaction by applying dynamic vacuum [149]. Longer silicon polymer chains (D = 1.90, M<sub>w</sub>/M<sub>n</sub> = 13,790/ 7270) have been obtained by using BuLi and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> in combination with Zr catalysts [150]. B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> reduced the overall activity of the catalysts but strongly decreased ring formation. Most likely, significant mechanistic differences exist between the cationic catalyst formed and the previous neutral ones, but no detailed mechanistic study was reported. These studies point out, however, that the H<sub>2</sub> production is (mostly) a neglected collateral phenomenon, and the studies related to rate, purity, recovering, or further use in tandem hydrogenation reaction would be an attractive reassessment.

The group 4 transition metal catalyst precursors [Cp<sub>2</sub>M(NMe<sub>2</sub>)<sub>2</sub>] and [Cp<sub>2</sub>ClC<sub>2</sub>/n-BuLi] (M = Ti, Zr or Hf) showed improved reaction times, notably concerning initial induction periods, with no initial induction periods except for [Cp<sub>2</sub>Hf(NMe<sub>2</sub>)<sub>2</sub>] (3 min) (D < 4366/2608 = 1.67)

[151]. Indeed, even though more efficient catalyst precursors had been reported otherwise, they often display an induction period time before the first observable H<sub>2</sub> release. The mechanism proposed by the authors for [Cp<sub>2</sub>M(NMe<sub>2</sub>)<sub>2</sub>] is displayed in Fig. 21.

In Path 1 proposal (Fig. 21, top), the precatalyst undergoes a metathesis at four centers with hydride transfer from the silane to the group 4 metal (M–H formed). The electronegative ligand Y is transferred to the Si atom in this concerted process. The reactivity of YM(Cp<sub>2</sub>)-H intermediate with another molecule of R<sub>3</sub>SiH is H<sub>2</sub> productive; alternatively reductive elimination of HY may generate [Cp<sub>2</sub>M]. Conversely, in Path 2 proposal (Fig. 21, bottom), the Si atom binds to the metal (M–Si formed) and HY is first eliminated. The reductive elimination of YSiR<sub>3</sub> from YM(Cp<sub>2</sub>)-SiR<sub>3</sub> is unproductive in H<sub>2</sub> and generate also [Cp<sub>2</sub>M]. Therefore, H<sub>2</sub> production allows to distinguish Path 1 and Path 2. The ligands Y = F, OPh and NMe<sub>2</sub> were originally selected due to their potential bonding strength to silicon favoring HSiR<sub>3</sub> evolution.

Late transition metal complexes have also been used to catalyze silane dehydrocoupling. Fontaine and Zargarian introduced the nickel complex [(1- MeInd)Ni(PPh<sub>3</sub>)Cl], (Ind = indenyl), which reacted with cationic initiators (AgBF<sub>4</sub>, AlCl<sub>3</sub> or methylaluminoxane MAO) or a metal hydride (LiAlH<sub>4</sub>), to produce intermediates that indeed catalyze dehydropolymerization of H<sub>3</sub>SiPh [152]. The intrinsic performances with reaction times mostly ranging between 24 h and several days were lesser than using the more active group 4 catalysts, however, yields above 90 %, and molecular weight ranging between 600 ≤ M<sub>w</sub> ≤ 7100 g·mol<sup>-1</sup>, and 500 ≤ M<sub>n</sub> ≤ 5900 g·mol<sup>-1</sup>, with polydispersity D ranging between 1.20 and 2.27. Among late transition metals, Ni catalysts have potential and are economically pertinent, but the cationic [(η<sup>3</sup>:η<sup>1</sup>-Ind(CH<sub>2</sub>)<sub>2</sub>NMe<sub>2</sub>)Ni(PPh<sub>3</sub>)][BPh<sub>4</sub>] was tested without success to produce any Si-based oligomers [153]. Conversely, Smith et al. succeeded to produce polysilanes with fairly low molecular weights, yet comparable to those obtained with group 4 metallocene catalysts, by using the dinuclear neutral nickel(I)-hydride complex [(dippe)Ni(μ-H)]<sub>2</sub> with D = M<sub>w</sub>/M<sub>n</sub> = 3200/1700 = 1.82 (dippe = bis(di-isopropyl)phosphino ethane, Fig. 22, top) [116].

Notably, the authors monitored the dehydropolimerization of H<sub>3</sub>SiPh through H<sub>2</sub> evolution, observing a decreasing rate and a conversion of 69 % after 90 min. They suggested that the reaction between organosilanes and late transition metals was likely to occur through oxidative addition, and give a metal-silyl complex. They proposed two mechanisms both starting from dinuclear H-double bridged Ni complex dissociation: the first one involving only PhSiH<sub>3</sub> was based on concerted four-center Ni–Si–H–H–σ-bond metathesis (Fig. 22-A), while the second one –that is directly H<sub>2</sub> productive– was based on the oxidative addition of H–SiH<sub>2</sub>Ph on a Ni(0) center (Fig. 22-B). The authors achieved [(dippe)Ni(μ-H)]<sub>2</sub> reaction with DSiEt<sub>3</sub>, knowing a priori that this precatalyst

<figure><img src="image_23.png" /><figcaption>**Fig. 21.** Pathways for hydrosilanes dehydrocoupling catalyzed by group 4 metallocenes [151].</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>Fig. 22. σ-bond metathesis (A) and oxidative addition (B) hypotheses by Smith et al. for Ni-catalyzed $H_3SiPh$ dehydrocoupling [116].</figcaption></figure>

does not catalyze itself dehydrocoupling of triethylsilane. Any initial detection of $H_2$ or HD formation would rather support the oxidative addition mechanism. Neither $H_2$ nor HD were formed, which was taken as a support for the σ-bond metathesis process, already operative in group 4 metal silane chemistry.

In rhodium chemistry, di-*n*-hexylsilane dehydrocoupling catalyzed by Wilkinson's catalyst [154] was monitored by NMR and ESI-MS analyses. The reaction of Wilkinson's catalyst [Rh($PPh_3)_3Cl$] with 1–2 equiv. of di-*n*-hexylsilane gives rise to a complex mixture of products, which has been analyzed by $ ^{31}P $ NMR and shown to include the hydrido complex [Rh($PPh_3)_3H$] (Fig. 23). Continuous sampling of the 1:1 reaction mixture by ESI-MS provided time-dependent speciation that tracks the formation of [Rh($PPh_3)_3H$] at the expense of the initial silane oxidative addition product Rh(III) silyl hydride complex [($PPh_3)_2Rh(Cl)]

($H$){Si(*n*-Hexyl)$_2H$}]. The subsequent 1st order disappearance of [Rh($PPh_3)_3H$] suggests it as the active species for dehydrocoupling of di-*n*-hexylsilane. No rhodium-(bis)silyl intermediate was observed, illustrating the absence of disilane reductive elimination. Thus, an alternative way to reductive elimination was proposed that postulates the formation and then the loss of a silylene ligand (dotted square on Fig. 23). This corresponds to an α-elimination reminding some description by Neale and Tilley in the dehydrocoupling of stannanes catalyzed by an hydrostannyl-hafnocene complex [155].

The formation of a silylene ligand at a metal center, followed by its loss, were also catalytic steps early on proposed by Gustavson et al. for the mechanism of transition metal catalyzed silanes redistribution [156]. This hypothesis was strengthened by the synthetic development of transition metal silylene complexes, which confirmed the possibility

<figure><img src="image_25.png" /><figcaption>Fig. 23. Proposed mechanism for silane dehydrocoupling catalyzed by Wilkinson's catalyst [154].</figcaption></figure>
for hydrosilanes to be converted into silylenes. This was described as a two-step process occurring when an empty coordination site is accessible at the metal center, including the oxidative addition of an Si—H bond and the α-hydrogen migration [157]. This mechanism, considered for late transition metal catalyzed silanes redistribution, can compete with silane dehydrocoupling. Accordingly, general conditions to promote silane dehydrocoupling over redistribution have been recently proposed by Mucha and Waterman [158]. By studying a range of bisphosphinite pincer-ligated iridium complexes (Fig. 24), they figured out a strong influence of steric hindrance for both substrate and catalyst on the reaction promoted.

The constrained complexes **1–4** turned out to be effective catalysts for primary and secondary silane modification. However, complex **1** is primarily a silane redistribution precatalyst, but dehydrocoupling catalysis is observed for sterically demanding silane substrates, or with active removal of H₂. The bulkier compounds (**2–4**) are silane dehydrocoupling catalysts that also undergo competitive redistribution with less hindered substrates (Table 4).

Products generated from the reactions utilizing **2–4** include low molecular weight oligosilanes (with for instance molecular weight M<sub>w</sub> = ranging between 190 and 720 for the dehydrocoupling of H<sub>3</sub>SiPh). Selectivity for redistribution vs. dehydrocoupling depends on the steric and electronic environment of the metal but can also be affected by reaction conditions. This observation supported a reaction pathway going through reductive elimination – known to be facilitated by the steric pressure applied by bulky ligands on the metal center – but no further mechanistic studies were performed.

Most recent dehydrocoupling of silane reactions producing H₂ involved active titanocene complexes. Edlova and Normand reported a good catalytic activity for amidometallocene cation towards H₂SiPh dehydropolymerization [159]. The new derivatives ([Cp₂Ti(NRR')][B(C₆F₅)₄]), based on the previously reported [Cp₂Ti(NPh₂)][B(C₆F₅)₄] [160], led to branched polysilanes, with a noticeable improvement regarding the length of the polysilanes obtained (D 1.40, M<sub>w</sub>/M<sub>n</sub> = 3018/2156) when compared to the parent complex (D 1.40, M<sub>w</sub>/M<sub>n</sub> = 1784/1274). The authors also tested PhSiH₃ dehydrocoupling from a zirconocene complex displaying an analogous structure: [Cp₂Zr(NPh₂)] [MeB(C₆F₅)₃]. The latter was found inactive, illustrating the possible difference in reactivity of Ti and Zr complexes towards silanes.

As illustrated above, the reactivity of a large variety of early and late transition metal complexes (including Ti, Rh, Ir, Ni, Zr, etc.) for H₂ release from dehydrogenative oligomerization of hydrosilanes makes these latter pertinent potential H₂ carrier for energy storing, distribution and conversion processes. The processing studies, mostly driven by the interest for the industrially relevant oligosilanes formed, gathered informative mechanistic studies that need now to be extended, especially for further developing *reversible* hydrogenation and depolymerization of the polysilanes. Breaking Si—Si bonds is thermodynamically accessible (reported bond dissociation energy, BDE = 226 kJ mol⁻¹) [161], and polysilanes are also known to eventually degrade by oxidation or irradiation with UV light [162].

Accordingly, Grützmacher’s group experimentally demonstrated the proof-of-concept of polysilane hydrogenation with nickel-hydride molecular catalysts [163], while other hydrogenative depolymerization of silicon-modified polyureas were also achieved recently [164].

Originally, Pribanic and Grützmacher et al. [163] assumed that since

<figure><img src="image_26.png" /><figcaption>**Fig. 24.** Structure of biphosphinite pincer-ligated iridium complexes [158].</figcaption></figure>

zerovalent [Ni(dme₂)] [dme = 1,2-bis(dimethylphosphino)ethane] produces linear polysilanes selectively [165], thus from the principle of microscopic reversibility [166], the opposite reaction of hydrogenation of Si—Si bonds is feasible. Based on experimentally determined bond energies, they estimated that the hydrogenation of Si—Si bonds [ΔH<sub>diss</sub> (Si—Si) = 73.1–88.0 kcal mol⁻¹] is a thermodynamically allowed process regarding ΔH<sub>diss</sub> (Si—H) > 90 kcal mol⁻¹ [167].

Additionally, an equilibrium constant of K<sub>c</sub> = 10⁴ for [Ph₂HSi–SiHPh₂ + H₂ ⇌ 2Ph₂SiH₂] was reported at rt. [168], which reinforced their expectation that the hydrogenation of Si—Si bonds is exergonic [169]. The hydrogenation of Si—Si bonds is very rare, and only disilanes with a single Si—Si bond were successfully converted [170–172] by using either a Pt complex or a binuclear Ni⁰ complex as catalysts, achieving TOFs of up to 20 min⁻¹.

In their works, Pribanic and Grützmacher et al. [163] synthesized a new class of nickel complexes of the general formula [Ni(X)(trop₂NR)] and [Ni(X)(trop₂NR)]⁻ (X = H, SiR₃; R = H or Me; trop₂NH = bis(5H-dibenzo[a,d]cycloheptene-5-yl)amine), and studied their catalytic activity for the hydrogenolysis of linear and cyclic oligo- and polysilanes. Contrary to divalent Ni(II) and monovalent Ni(I) hydrides, very few zerovalent Ni(0) hydride species are described to date. These could be anticipated as very reactive in reactions involving hydrogen. The pure isolated mononuclear Ni⁰ hydride complex Li[Ni(H(trop₂NH))] and the dinuclear [NiI₀(µ-H)(trop₂NR)₂] (R=H or Me, Fig. 25, left) used as catalysts at rt. under 1 bar of H₂ achieved the hydrogenolysis of the oligosilanes H(SiPh₂)nH (Fig. 25, right) providing secondary silanes, R₂SiH₂ as products in high purity. This process can be considered as the direct reverse reaction of the dehydrogenation of Ph₂SiH₂, also achieved with the same catalysts. Accordingly, if the Ni catalyst is not separated from the reaction mixture and H₂ release is permitted, Ph₂SiH₂ is slowly reconverted to the initial mixture of oligosilanes H(SiPh₂)nH (n = 2–4). Some reactions require small amounts of a nucleophilic additive, which is needed to break-up a Si—Si bond: [(R₂Si)n + Nu⁻ ⇌ (Nu)R₂Si—(SiR₂)n·₂—SiR₂] (with KNu = KOTf, or K[Me₂NBH₃]). This activation is not necessary when the substrate contains one Si–H bond that allows forming a η²-Si–H complex at the metal. The hydrogenation of Si—Si bonds in oligo- and polysilanes provides quantitative formation of Me₂SiH₂ from cyclic [(Me₂Si)₆] and linear monodisperse [(Me₂Si)n] (D = 1.22).

Control experiments for determining eventual heterogeneous Ni nanocatalysts activity (mercury test, high resolution transmission electron microscopy, addition of quenching strong phosphine ligand) overall suggested that very likely nickel complexes in homogeneous solution are the active species [163]. In addition to the remarkable efficiency of their zerovalent nickel hydride catalysts, the authors have performed DFT calculations and proposed a mechanism involving several intermediates and successive transition states, a simplified version with main barriers of this mechanism are provided in Fig. 26.

Overall, this reaction mechanism involves classical intermediates that undergo σ-bond metatheses with only low activation barriers below 8 kcal mol⁻¹ encountered along the path. According to the authors, the catalytic activity of such complexes would be enhanced by modifying the structure of the ligand in order to make the liberation of one coordination site easier.

Interestingly, Driess group also highlighted the potential of Ni hydride chemistry for other strong bond activation [173] with the silylene-nickel promoted cleavage of B—O Bonds, that somewhat relates also to the potential methods for regenerating boranes above-discussed (section 2.4). The authors reported the synthesis of a Ni⁰ complex bearing an acyclic amidochlorosilylene ligand that achieved the addition of H₂ at 20 °C and 1 bar to yield the corresponding hydrido(silyl) nickel¹¹ complex, which reacts with catechol borane (HBcat, cat = o-C₆H₄O₂) for generating an unprecedented hydroborylene Ni¹¹ complex through the activation of two B—O bonds.

Clearly, the development of the type of reversible system disclosed by Grützmacher group [163] as regeneration catalytic procedure is a
<caption>**Table 4**</caption>

Conversion of substrates (%) and ratio of competitive reactions depending on the catalysts (1)–(4).

<table><thead><tr><th rowspan="2">Substrates<br>(from less to more hindered)</th><th colspan="4">Total conversion (%)</th><th colspan="4">Redistribution (%)</th><th colspan="4">Dimerization (%)</th><th colspan="4">Oligomerization (%)</th></tr><tr><th colspan="4">Catalysts</th><th colspan="4">Catalysts</th><th colspan="4">Catalysts</th><th colspan="4">Catalysts</th></tr><tr><th></th><th>1</th><th>2</th><th>3</th><th>4</th><th>1</th><th>2</th><th>3</th><th>4</th><th>1</th><th>2</th><th>3</th><th>4</th><th>1</th><th>2</th><th>3</th><th>4</th></tr></thead><tbody><tr><td>PhSiH<sub>3</sub><br>(o-tol)SiH<sub>3</sub></td><td>100</td><td>100</td><td>100</td><td>100</td><td>66</td><td>3</td><td>11</td><td>15</td><td>34</td><td>0</td><td>0</td><td>6</td><td>0</td><td>97</td><td>89</td><td>79</td></tr><tr><td>MesSiH<sub>3</sub></td><td>100</td><td>100</td><td>100</td><td>100</td><td>100</td><td>7</td><td>4</td><td>8</td><td>0</td><td>66</td><td>79</td><td>26</td><td>0</td><td>27</td><td>17</td><td>66</td></tr><tr><td>PhMeSiH<sub>2</sub></td><td>97</td><td>19</td><td>26</td><td>58</td><td>8</td><td>1</td><td>1</td><td>2</td><td>37</td><td>18</td><td>24</td><td>56</td><td>52</td><td>0</td><td>1</td><td>0</td></tr><tr><td></td><td>100</td><td>20</td><td>68</td><td>56</td><td>100</td><td>13</td><td>48</td><td>36</td><td>0</td><td>7</td><td>20</td><td>20</td><td>0</td><td>0</td><td>0</td><td>0</td></tr></tbody></table>

<figure><img src="image_27.png" /><figcaption>**Fig. 25.** (A) Anionic Ni⁰ hydride complexes 1–2 and bridged dinuclear Ni⁰ complexes 3–4 (left). (B) Hydrogenation of oligo- and polysilanes catalyzed by nickel complexes (right). a Reaction conditions: silane (0.34–1.72 mmol of Si units) in THF, additive (0.01–0.1 equiv., with respect to Si units), [Ni] catalyst (0.2–1 mol%), reacted with H₂ (1–5 bar) at rt. b The substrate is a mixture of trisilane and tetrasilane in a 2:1 ratio. c The polydimethylsilane has a M<sub>w</sub> = 1100–1700. [163].</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Fig. 26.** DFT calculations [B3LYP with 6-31 g(d) for C and N atoms, 6 31 g(d,p) for H atoms, LANL2DZ for the nickel center] for the hydrogenolysis of a disilane HPh₂Si–SiPh₂H using [Ni(H)(trop₂NH)] type catalyst [163].</figcaption></figure>
decisive proof-of-concept that reinforces a bloom for a future increase of interest in hydrosilane/polysilane couple as CHS materials.

### 3.4. Regeneration and reversibility of hydrosilane solvolysis products

As mentioned in the previous sections, the products of H₂ evolution from silane solvolysis are themselves industrially valuable [174]. Nonetheless, some studies focusing on silanes as CHS materials also mentioned the regeneration of spent silanes, aiming to close the loop and enhance the environmental sustainability of this approach [175–177].

A series of linear or cyclic silanes were prepared and tested as potential H₂ storage materials in 2011 [177]. From a stoichiometric amount of MeOH added to a mixture of cyclic organosilane, $(CH_2SiH_2)_3$ (a) and $(CH_2SiH_2CHSiH_3)_2$ (b), and 5 mol% of NaOMe, quick H₂ delivery was monitored at rt. within 10 to 15 s. The hydrogen storage capacities of compounds a and b were estimated to be 3.70 and 4.04 wt %, respectively. To secure the complete methanolysis from organosilanes including MeOH evaporation at exothermic hydrogen release condition, two equivalents of methanol were used. The resulting methoxysilanes, $(CH_2Si(OMe)_2)_3$ (c) and $(CH_2Si(OMe)_2CHSi(OMe)_3)_2$ (d), were regenerated to the starting organosilanes in high yields by the action of lithium hydride. Linear organosilanes, $SiH_3CH_2SiH_2CH_2SiH_3$ (e), $SiH_3CH_2CH(SiH_3)_2$ (f), and $SiH_3CH_2CH(SiH_3)CH_2SiH_3$ (g) also showed fast hydrogen release kinetics at rt. with hydrogen storage capacities of 4.26, 4.55, and 4.27 wt% H₂, respectively; the corresponding methoxysilanes were successfully regenerated by $LiAlH_4$. Compound a was further tested as hydrogen source for fuel cell operation. Thus, a typical silane regeneration pathway from its alcoholysis products consists in reducing the siloxane product with $LiAlH_4$ in diethylether with a yield of 70 % [175,176]. Han et al. have investigated this regeneration process in greater details [177]. Different cyclic and linear organosilanes were used to produce H₂ with NaOMe as a catalytic compound. The regeneration of the methoxysilane products was then studied by treating them with $LiAlH_4$ during several hours at rt. in diethylether. According to the authors, the starting materials were regenerated with yields between 80 % and 91 %. However, the use of the strong reducing agent $LiAlH_4$ is once again a limitation of these procedures, as it was the case for AB. Other works also utilized over stoichiometric amounts of strong reducing agents for siloxane or silyl ether reduction to silane, under various situations, like for instance phase-transfer conditions

[42,178,179].

Alternative strategies using simpler H₂ and (de)hydrogenation active transition metal catalysts are more promising for providing more economically viable and “greener” solutions. The recently reported iron-catalyzed activation of Si—N and Si—O bonds to desilylate a large scope of silazanes and silylethers in the presence of pinacolborane (amine acceptor) has been reported [180]. An iron(II) β-diketiminate complex was used as a catalyst (5 mol%), with pinacolborane as a reducing agent (0.5 mmol), and 0.5 mmol of silazane or silyl ether. The reactions were conducted in benzene during 18 h, at rt. for Si—N bond activation and at 70 °C for Si—O activations. For silazanes, conversion from 50 to 99 % into amino boranes and hydrosilanes was achieved, while a slightly lower activity and scope was reported for silylethers reduction (Fig. 27).

Partial or almost complete depolymerization of model cyclic and linear polysilazanes (mass $M_n$ = 17.5 and 3.7 kDA, respectively) and polysilylethers (linear models with mass $M_n$ = 7.3 to 10.6 kDA) was also achieved with the same iron catalyst (5 mol%) at 80 °C for several hours to several days. Unfortunately, depolymerization of a commercially available poly(silazane) was unsuccessful; commercially available Durazane 180 underwent competitive cross-linking. Commercially available poly(dimethylsiloxane) also showed little evidence of depolymerization [180].

This study extended previous works that succeeded to reduce different alkoxysilanes into hydrosilanes employing a half-metallocene yttrium complex, again in the presence of pinacolborane [181]. As an example, 50 µmol of $Me_2PhSiOMe$ were reacted with 50 µmol pinacolborane in deuterated benzene, in the presence of 10 mol% of the yttrium catalyst [Cp₂Y(CH₂SiMe₃)(THF)], at 100 °C for 24 h, yielding 57 % of hydrosilane $Me_2PhSiH$. Similarly, $MePhSi(OMe)_2$ (50 µmol) with pinacolborane (100 µmol) yielded 59 % of $MePhSiH_2$.

Iridium was also successfully used for the hydrogenolysis of Si—O bond-containing species into hydrosilanes [182]. Silyl triflate $Me_3SiOTf$ was reacted at rt. with 4 bar of H₂ in deuterated toluene as a solvent in the presence of 1.1 equiv. of i-Pr₂EtN, 5 mol% PPh₃, and 5 mol% of an Ir-complex catalyst (Fig. 28).

The reaction reached 100 % conversion in 8 h, with a $Me_3SiH$ yield of 93 %. $Me_3SiOTf$ hydrogenolysis can also occur in the presence of a Ru-complex catalyst [183]. 0.1 mmol of $Me_3SiOTf$ with 4 bar of H₂, 1.1 equiv. of $NEt_3$ and 1 µmol of Ru-complex (Fig. 28) in 0.5 mL of $C_6D_6$ reached 99 % conversion to $Me_3SiH$ (85 % yield) after 18 h at rt.

The previous methods, exploiting the reactivity of transition metals

<figure><img src="image_29.png" /><figcaption>Fig. 27. Silazane and silyl ether catalytic reduction with pinacolborane [180].</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>**Fig. 28.** Silyl triflate conversion to hydrosilane with Ir and Ru complexes as catalysts, respectively [182,183].</figcaption></figure>

or lanthanides, are relevant for hydrosilanes regeneration in the context of their by-products valorization from post-catalytic H₂ delivery. However, addressing more extensively the conversion of Si–OH and Si–OR species to Si–H in the presence of metal catalysts, including under the form of nanoparticles, is needed to further increase the benefit of using silanes as CHS materials compared to AB. Notably, some of the numerous catalysts active in the dehydrogenation process of amine boranes might be useful candidates to test for such regeneration processes. The works of Grützmacher’s group nicely illustrated the success of this strategy since the monovalent nickel complex [Ni(TFA)( trop₂NH)] (TFA = trifluoroacetic acid) found as particularly active catalyst in the dehydrogenation of amino boranes under mild conditions [184], inspired their successful development of Ni derivatives for polysilanes Si–Si bond hydrogenation [163].

### 3.5. Prospective of hydrosilanes as CHS materials

Silanes possess objective advantages over amine boranes, silicon is one of the most abundant elements on Earth, silane compounds have high enough hydrogen gravimetric capacities to fulfill the current DOE requirements [185], and silicon industry is mature with an established supply chain.

The catalytic solvolysis of silanes can take place in very short period of time, thus rapidly releasing H₂, especially when it is catalyzed by transition metal complexes. The exceptional cationic iridacycle complex (Fig. 16) reported by Djukic group [113], and the NHC-ligated iridium complex (Fig. 17) reported by Ventura-Espinosa et al. [137], which allow completion of tertiary silane alcoholysis, respectively within 20 and 180 s, are among the most striking and promising results in this field of research. Further breaking through conceptual advances are also expected from additional HQTM involved in fast hydrosilane methanalysis and hydrolysis catalytic reactions (Fig. 19) [111].

The regeneration of hydrosilanes from silyl ethers (more pertinent from catalytic methods than using stoichiometric reductant such as LiAlH₄) is currently more advanced than AB regeneration, and as such is a relevant opportunity. Different transition metal catalysts, such as iron or yttrium [180,181], have been used for the reduction of Si–O into Si–H, paving the way for new catalysts to be used for that purpose as well, and increasing the overall viability of silanes as CHS materials.

Dehydrogenative oligo- and polymerization reactions present several interests: in addition to be H₂ productive at good rate, these reactions are efficiently achieved by the abundant and cheaper early metals Zr and

Ti. The control of the lengths and polydispersity of the oligo- and polymers formed is however a major goal to further exploit these materials in industrial processes that would combine H₂ delivery and functional polymers synthesis. In this context, mastering reaction selectivity against polymers cyclization or redistribution, can be very challenging. Complementary, the development of reversed polysilanes hydrogenolysis catalytic conditions, in which efficient depolymerization would be achieved, even on complex oligomeric mixtures, has a room for improvement, but is achievable, as exposed from striking Ni(0) hydride chemistry [163].

From the H₂ generation perspective, in any of these strategies, systematic kinetic data collection is crucial, this especially considering the last decade increase in machine learning assistance for designing catalysts [186]. Isotopic assays using deuterated reagent/solvent have been decisive in order to get insights about silanes hydrolysis and alcoholysis mechanisms, and would be to extend to silane dehydrocoupling.

The elementary steps involved in silane and hydride coordination chemistry towards metals have been explored both experimentally and from supporting DFT calculations. Starting from various coordination modes (from η¹- to η³-) of silanes, hydrogen, or Si–H bonds on the metal center, they notably include concerted σ-bond metathesis [112,116,148,149,151], oxidative addition of hydrosilanes [116,154], or formation and release of silylene moieties. Theoretical calculations also supported disruptive proposals, like HQTM [111], and will remain indispensable for progress, and especially for predicting ligands and complexes modification for better activity and selectivity.

Finally, an interesting feature of these transition metal catalyzed reactions that would be of interest is the recovery and reuse of catalyst precursors. Some attempts have already been done in silanes solvolysis with heterogeneous or immobilized catalysts [137,139,140] but also with homogeneous ones [42]. Recovering catalysts strategies include complexes tagged with pyrene and supported on RGO through π-stacking [137]. Thus, other well-known immobilization method of the organometallic surface chemistry [187] could be a potential starting point towards the development of recyclable catalysts. The development of nanocatalysts is also a large room of improvement with silanes, like it exists for amine boranes [27].

## 4. Key data and comparisons of CHS materials

### 4.1. Direct comparison of H₂ storage properties, regeneration feasibility, and economic potential

A direct comparison of the hydrogen storage properties, regeneration feasibility, and economic potential is relevant, and a comparative summary is provided in the Table 5. First, the gravimetric hydrogen storage capacity (GHSC) is a key data that refers to weight percentage of H atoms that could be recovered [5,188]. However, for practical applications it is more pertinent to distinguish various values, as in the following by decreasing hydrogen amount: i) the *absolute* GHSC that refers to the total weight percentage of all H atoms in the pure material, ii) the *excess* GHSC which designs the maximum weight percentage of H atoms that can be theoretically recovered from the dehydrogenation process (thermolysis or solvolysis, not all H being accessible depending on the mechanism), and iii) the *effective* GHSC that relates to the weight percentage of H that are effectively released. The comparative economic advantage of the CHS materials, of either amine borane or silane nature, are reported in terms of general advantages and drawbacks mostly from a large-scale implementation perspective. Finally, the essential physical properties in terms of thermodynamic change of state (melting point, boiling point) and molecular mass are also provided.

The hydrosilanes in Table 5 [105,110,189,190] were selected for the following reasons: i) SiH₄ is the hydrosilane with the highest mass percentage of hydrogen (discussed in section 3.1); ii) tetrasilyl methane TMS is mentioned in section 3.1 as the lightest carbosilane; iii) penta-silane is an H-polysilanes (HPS), a category discussed in section 3.1 as
<table><caption>**Table 5**
Comparison of hydrogen storage properties of AB and selected hydrosilanes.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">m.<br>p.<sup>a</sup><br>(°C)</th><th rowspan="2">b. p.<br>a<br>(°C)</th><th rowspan="2">Density<sup>a</sup><br>(g cm<sup>-3</sup>)</th><th rowspan="2">Molecular<br>mass<br>(g mol<sup>-1</sup>)</th><th rowspan="2">Excess<br>GHSC<sup>b</sup><br>(wt%)</th><th rowspan="2">Absolute<br>GHSC<sup>c</sup><br>(wt%)</th><th colspan="2">Economic potential<br>as CHS material</th></tr><tr><th>Advantages</th><th>Drawbacks</th></tr></thead><tbody><tr><td>NH<sub>3</sub>BH<sub>3</sub><br>Ammonia Borane [188]<br>(AB)</td><td>97</td><td>-<sup>e</sup></td><td>0.780</td><td>30.87</td><td>13.0</td><td>19.5</td><td><ul><li>High intrinsic hydrogen content (GHSCs)</li><li>Stable crystalline solid (purifiable) at ambient conditions</li><li>High volumetric and gravimetric density as solid</li><li>Reputed low toxicity (to further examine)</li><li>Very versatile and very mild reactivity</li><li>Easily/securely transportable and storable</li><li>Upon H<sub>2</sub> delivery, provide by-products that are easily recoverable (decarbonated – no gaseous CO, CO<sub>2</sub>)</li><li>Catalytic use could be developed with lower cost metals and derivatives (Ni, Cu, Ru, etc.)</li></ul></td><td><ul><li>Effective GHSC<sup>d</sup> is lower than excess GHSC in the current hydrolysis system (3.5 wt%)</li><li>High cost</li><li>Supply chain not available</li><li>Purity of H<sub>2</sub> delivered (NH<sub>3</sub> byproduct)</li><li>Regeneration methods very energy consuming and not developed from catalytic option</li></ul></td></tr><tr><td>SiH<sub>4</sub><br>Silane [110]</td><td>-185</td><td>-112</td><td>1.340</td><td>32.12</td><td>12.5</td><td>12.5</td><td><ul><li>GHSC values in accordance with DOE requirements (not for all)</li></ul></td><td><ul><li>Lower intrinsic hydrogen content</li></ul></td></tr><tr><td>C(SiH<sub>3</sub>)<sub>4</sub><br>Tetrasilylmethane (TSM) [105]</td><td>-102</td><td>26</td><td>0.648</td><td>88.23</td><td>8.8</td><td>8.8</td><td><ul><li>Lower cost</li><li>Supply chain existing from electronic industry</li><li>Industrial handling long time-mastered (silicon electronic and polymer industry)</li><li>Advanced catalytic regeneration methods: diverse and versatile</li><li>Regeneration by hydride known [177]</li><li>Potential valorization of by-products siloxanes and silanols</li><li>Effective GHSC<sup>d</sup> is higher than excess GHSC in the system H<sub>2</sub>O/NaOH (30 %) [110]</li><li>Purity of H<sub>2</sub> delivered (no gaseous byproduct like NH<sub>3</sub>)</li><li>Low-cost catalysts for dehydrocoupling exists (Ti, Zr, Ni)</li></ul></td><td><ul><li>Toxicity known from electronic industry (not for all)</li><li>Non-conversion of dehydrocoupling reactions</li><li>High polydispersity indexes for by-products dehydrocoupling oligomers</li><li>Recycling or re-using byproducts possible but to be developed (current hot topic of researches)</li><li>Catalytic use often employs expensive late transition metals (Ir)</li><li>Molecular catalysts should be extended to recoverable materials (to be developed)</li></ul></td></tr><tr><td>SiH<sub>3</sub>(SiH<sub>2</sub>)<sub>2</sub>SiH<sub>3</sub><br>Pentasilane [110]</td><td>-72</td><td>153</td><td>0.827</td><td>152.52</td><td>7.9</td><td>7.9</td><td></td><td></td></tr><tr><td>H<sub>3</sub>SiPh<br>Phenylsilane [110]</td><td>-68</td><td>120</td><td>0.878</td><td>108.22</td><td>2.8</td><td>7.4</td><td></td><td></td></tr><tr><td>Me<sub>3</sub>Si[SiH(CH<sub>3</sub>)O]<sub>1</sub>OSiMe<sub>3</sub><br>Polymethylhydrosiloxane (PMHS) [189]</td><td>-55</td><td>142</td><td>1.006</td><td>1500–2200<br>[189]</td><td>1.66</td><td>7.2<sup>f</sup></td><td></td><td></td></tr><tr><td>HSiEt<sub>3</sub><br>Triethylsilane [190]</td><td>-157</td><td>107</td><td>0.728</td><td>116.28</td><td>0.9</td><td>13.8</td><td></td><td></td></tr></tbody></table>

compounds with a high hydrogen content (N.B. we selected silane, tetrasilane and pentasilane Table 5 to illustrate the essential physical properties variations as a function of Si content); iv) phenylsilane is the most studied hydrosilane in dehydrogenating coupling (see section 3.3); v) PMHS is discussed in section 3.1 as high hydrogen-content compound; vi) triethylsilane is a very commonly studied hydrosilane for metal-catalyzed hydrosilane solvolysis (see section 3.2). Cyclic organosilanes are promising, but have been only marginally studied for the production of hydrogen by methanolysis [177].

## 4.2. Key regeneration routes: operating conditions, yields, and associated challenges

The solvolysis of AB is mainly achieved in water or methanol, using metal catalysts under ambient conditions, and releasing H<sub>2</sub> eventually contaminated with ammonia (Fig. 29, see detailed discussion in section 2.3). As discussed in more details in section 2.4, AB regeneration is to

date not achieved in an economically viable way. AB regeneration from its hydrolysis by-products mainly refers to borates that form very stable strong B—O bonds (Fig. 29, middle left), and consequently powerful reducing agents are needed for regeneration processes achieved generally in several steps. These approaches have been extended to more energy efficient solvent-free processes like ball-milling (Fig. 29, bottom left). AB alcoholysis by-products (from methanolysis notably) proved easier to recycle than hydrolysis by-product, which required an additional step to convert B(OH)<sub>3</sub> into B(OMe)<sub>3</sub> before undergoing the aforementioned reaction (Fig. 29, top right). LiAlH<sub>4</sub> as a strong hydride is suitable for regenerating B(OMe)<sub>3</sub>, generating LiCl and Al(OMe)<sub>3</sub>. However, lithium hydrides and aluminum precursors are employed stoichiometrically (or more) to form LiAlH<sub>4</sub> (from LiH and AlCl<sub>3</sub>), which objectively constitutes an economical limitation with regards to mining depletive lithium sources also engaged in current battery industry. Ideally, borate regeneration would benefit from the development of catalytic processes implemented from abundant metals and renewable

<footnote>a Under standard conditions of pressure and temperature (1 bar and 25 °C).</footnote>

<footnote>b Excess GHSC (GHSC = gravimetric hydrogen storage capacity) refers to weight percentage of H atoms that could theoretically be recovered [188].</footnote>

<footnote>c Absolute GHSC refers to the weight percentage of all H atoms of the pure material [188].</footnote>

<footnote>d Effective GHSC refers to the weight percentage of H atoms that are effectively released [188].</footnote>

<footnote>e AB does not have a gaseous phase, it starts to decompose upon melting [188].</footnote>

<footnote>f Value calculated from the molecular mass value of the literature [189].</footnote>
<figure><img src="image_31.png" /><figcaption>Fig. 29. $H_2$ production from AB: flow diagram summarizing operating conditions, key regeneration routes and limitations.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>Fig. 30. $H_2$ production from silanes: flow diagram summarizing operating conditions, yields, and key regeneration routes.</figcaption></figure>

energy using directly $H_2$. Ammonia industry is well-developed and its access is thus less problematic for current industrial processes (see also Fig. 1 for AB synthesis from $NH_3$ and $BH_3$).

The production of $H_2$ from silanes $R_xSiH_{4-x}$ ($x = 3$, or $x < 3$) is achieved following solvolysis, oxidative coupling or dehydrogenative polymerization, under homogeneous catalysis conditions mostly (Fig. 30). Thus, the shift to heterogeneous or recoverable catalysts is a necessary challenge to address, and an attractive opportunity. Silanes are easily available at large scale (established supply chain) due to microelectronic industry (thin films and semiconductors) developed after the 2nd World War, and the current growing silicon polymers industry. As shown in Fig. 30 (top) the regeneration pathways exist from silyl

ethers, due to Si—O bonds that are more easily activable than B—O. These modes are catalytic and can be achieved by a variety of transition metals (Ru, Ir, Fe,); Catalytic methods for silane polymers depolymerization (Fig. 30, middle right, Ni and $H_2$ 1–5 bar) are more pertinent than using stoichiometric reductants such as $LiAlH_4$, Na/K, or even high pressure (>40 bar) $H_2$ pressure hydrogenolysis above 300 °C [167].

### 4.3. Categorizing the reaction mechanisms: similarities and differences

Tables 6a and 6b compilate and categorize the different reaction mechanisms analyzed in the present review. The analysis of the similarities and differences for heterogeneous solvolysis of AB (Table 6a)
<table><caption>**Table 6a**

Reaction mechanism categorization and comparisons for solvolysis of AB and silanes.</caption><thead><tr><th>Reaction type</th><th>CHS material</th><th>Reaction detail (cat.)</th><th>Proposed mechanism</th><th>Experimental evidences</th><th>Non experimental support</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="12">Solvolysis</td><td rowspan="3">Pt/MWCNT-catalyzed hydrolysis</td><td rowspan="3">Ni NPs/ZF-8 catalyzed hydrolysis</td><td>Surface Langmuir-Hinshelwood mechanism with a reaction between activated AB<sub>ads</sub> and H<sub>2</sub>O<sub>ads</sub> molecules. O-H bond breaking being the rate determining step</td><td>• Thermodynamic analysis<br>• FT-IR measurements<br>• Kinetic isotopic effect (KIE) experiment</td><td>/</td><td>[60]</td></tr><tr><td>Indirect O-H bond cleavage promoted by AB in the presence of nanocatalyst, leading to a B-N bond breaking and H<sub>2</sub> release</td><td>KIE experiments</td><td>/</td><td>[61]</td></tr><tr><td>Generation of high energy electrons by irradiation, which activates MeOH by O-H bond weakening (RDS). Plasmon-induced charge transfer between single CuAg NPs and reactant molecules</td><td>In situ catalyst analysis with single-particle confocal microscope system</td><td>DFT calculations</td><td>[65]</td></tr><tr><td rowspan="3">AB<br>CuCoO NPs/Graphene oxide (GO)-catalyzed hydrolysis</td><td rowspan="3">NiCoP NPs-catalyzed hydrolysis</td><td>1. Adsorption of H<sub>2</sub>O molecule on GO surface</td><td>In situ catalyst analysis by synchrotron radiation-based X-ray adsorption spectroscopy</td><td>/</td><td>[66]</td></tr><tr><td>2. Nucleophilic attack of H<sub>2</sub>O<sub>ads</sub> on the B atom of the activated B-N bond of proximate AB molecule, leading to H<sub>2</sub> release</td><td>Kinetic studies</td><td>/</td><td>[67]</td></tr><tr><td>1. Adsorption of AB and H<sub>2</sub>O molecules on catalyst's surface<br>2. Nu attack of an activated OH<sub>ads</sub> group on a B atom of AB<sub>ads</sub>, leading to B-N breaking (SN<sub>2</sub>)<br>3. Hydride departure from BH<sub>3</sub>(OH)<sup>-</sup>, followed by its reaction with a H<sub>2</sub>O<sub>ads</sub> molecule to generate H<sub>2</sub> and another OH<sub>ads</sub></td><td>• Identification of byproducts through FT-IR analysis of raw mixture<br>• NMR monitoring<br>• Kinetic studies</td><td>/</td><td>[128]</td></tr><tr><td rowspan="3">Ethanolysis and methanolysis of HSiR<sub>3</sub> (R = OEt, Et, Ph) by Ir molecular catalyst</td><td rowspan="3">Alcoholysis of HSiEt<sub>3</sub> by Ir molecular catalyst</td><td>1. Two successive oxidative addition/ reductive elimination</td><td>KIE experiment</td><td>/</td><td>[40]</td></tr><tr><td>2. Formation of a transient iridium hydride incorporating a halosilane deprotonating an alcohol is proposed</td><td>• Comparative experiments with another catalyst having one less vacant site</td><td>/</td><td>[40]</td></tr><tr><td>1. Silane electrophilic activation through η<sup>2</sup>-bonding of the hydridic hydrogen to the metal center (M)<br>2. Internal Nu attack of ROH on Si atom<br>3. Formation of [MH<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>)L<sub>2</sub>]<sup>+</sup> complex and release of H<sub>2</sub></td><td>• KIE experiments<br>• XRD of catalyst [Ir-η<sup>2</sup>HSiEt<sub>3</sub>]</td><td>DFT calculation of catalyst structure (correlation with XRD)</td><td>[113]</td></tr><tr><td rowspan="5">Silanes</td><td rowspan="3">Hydrolysis of HSiR<sub>3</sub> by Re or Ir molecular catalyst</td><td>1. Silane electrophilic activation through η<sup>2</sup>-bonding of the hydridic hydrogen to the metal center</td><td>Isotopic exchange experiments, followed by mass spectrometry of gas produced</td><td>DFT calculations for the whole mechanism</td><td>[134]<br>(Ir), [41]<br>(Re)</td></tr><tr><td>2. Outer sphere Nu attack of H<sub>2</sub>O on Si atom</td><td></td><td></td><td></td></tr><tr><td>3. Formation of a [L-MH(η<sup>2</sup>-H<sub>2</sub>)]<sup>+</sup> intermediate and release of H<sub>2</sub></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Methanolysis of HSiMe<sub>3</sub> by Ir molecular catalyst</td><td>1. Silane electrophilic activation through η<sup>1</sup>-bonding of the hydridic hydrogen to the metal center</td><td><sup>1</sup>H NMR analysis monitoring (CDCl<sub>3</sub>/CD<sub>3</sub>OD)</td><td>DFT calculations for the whole mechanism</td><td>[137]</td></tr><tr><td>2. Outer-sphere Nu attack of MeOH on Si atom</td><td></td><td></td><td></td></tr><tr><td>3. Formation of a [M(η<sup>2</sup>-H<sub>2</sub>)L<sub>3</sub>]<sup>+</sup> intermediate and release of H<sub>2</sub></td><td>KIE experiments</td><td></td><td></td></tr><tr><td rowspan="3">Hydrolysis of HSiEt<sub>3</sub> by Ir molecular catalyst</td><td>1. Substitution of an H<sub>2</sub>O molecule from the coordination sphere of the metal center by the H-atom of the triethylsilane, where a M-H bond is formed</td><td></td><td>• Calculation of Arrhenius factor ratio<br>• Calculation of activation energy difference (H/D)</td><td>[111]</td></tr><tr><td>2. H-transfer from the silicon to the M as RDS</td><td></td><td></td><td></td></tr><tr><td>3. Quantum tunneling of hydridic hydrogen of silane towards the metal center</td><td></td><td>• DFT calculations for the whole mechanism</td><td></td></tr></tbody></table>
<table><caption>**Table 6b**
Silanes dehydrocoupling and oligosilanes hydrogenolysis mechanistic features.</caption><thead><tr><th>Reaction type</th><th>CHS material</th><th>Reaction detail (cat.)</th><th>Proposed mechanism</th><th>Supporting evidences</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="5">Dehydro-coupling</td><td rowspan="5">Silanes</td><td>Dehydrocoupling of H<sub>3</sub>SiR<sub>3</sub> by zirconocene and hafnocene derivatives</td><td>Two successive σ-bond metathesis steps:<br>1. Dehydrolysylation of catalyst producing H<sub>2</sub><br>2. Si-Si bond formation<br>Pathway 1:<br>1. Hydride transfer to the metal center through σ-bond metathesis<br>2. Reaction of hydride complex with another molecule of HSiR<sub>3</sub> to generate H<sub>2</sub><br>Pathway 2:<br>1. Silylation of the metal center through σ-bond metathesis<br>2. Reductive elimination of LSiR<sub>3</sub> without production of H<sub>2</sub><br>Redox mechanism:<br>1. Oxidative addition of H<sub>3</sub>SiPh on the metal center<br>2. Release of H<sub>2</sub> concerted with a Ni-Si double bond formation (silylene ligand)</td><td>Analysis of the oligomeric products by Gas Phase Chromatography (GPC)<br><ul><li>Near stoichiometric reactions of Cp<sub>2</sub>MY<sub>2</sub> with PhSiH<sub>3</sub>, PhMeSiH<sub>2</sub>, and Ph<sub>2</sub>MeSiH monitored by <sup>1</sup>H, <sup>19</sup>F and <sup>29</sup>Si NMR</li><li>Exchange products containing Si-Y were identified. Metal hydride species could be observed only with Cp<sub>2</sub>TiY<sub>2</sub></li></ul></td><td>[112]</td></tr><tr><td>Dehydrocoupling of HSiR<sub>3</sub> by group 4 metallocenes</td><td>3. "Propagation" step<br>4. Reductive elimination of cyclic (Si-Si bond formation) or linear (Si-H bond formation) oligo/polysilanes<br>Successive σ-bond metathesis:<br>1. Dehydrolysylation of catalyst producing H<sub>2</sub><br>2. Si-Si bond formation<br>Redox mechanism:<br>1. Oxidative hydrosilylation of H<sub>2</sub>SiR<sub>2</sub> on the metal center<br>2. Reductive elimination of ClSiHR<sub>2</sub><br>3. Second oxidative hydrosilylation of H<sub>2</sub>SiR<sub>2</sub> on the metal center<br>4. Reductive elimination of H<sub>2</sub><br>Redox mechanism through formation and loss of a silylene ligand:<br>1. Oxidative hydrosilylation of H<sub>2</sub>SiR<sub>2</sub> on the metal center<br>2. Reductive elimination of HCl<br>3. Oxidative internal hydrosilylation of the metal (M = Si double bond formation, silylene ligand)<br>4. Reductive elimination of the silylene ligand<br>5. Oxidative hydrosilylation of another H<sub>2</sub>SiR<sub>2</sub> molecule on the metal center<br>6. Reductive elimination of H<sub>2</sub></td><td><ul><li>Reaction performed with DSiEt<sub>3</sub></li><li>followed by the non-early-detection of HD (which support the σ-bond metathesis mechanism)</li></ul></td><td>[151]</td></tr><tr><td>Ni-catalyzed dehydrocoupling of H<sub>3</sub>SiPh</td><td></td><td></td><td>[116]</td></tr><tr><td>Rh-catalyzed dehydrocoupling of H<sub>2</sub>SiR<sub>2</sub></td><td></td><td></td><td><ul><li>Reaction with H<sub>2</sub>Si(n-hexyl)<sub>2</sub> monitored by <sup>1</sup>H NMR and ESI-MS analysis tracking and identifying intermediates</li></ul></td><td>[154]</td></tr><tr><td>Hydro-genolysis</td><td>Oligo-silanes</td><td>Ni-catalyzed hydrogenolysis of H (SiPh<sub>2</sub>)<sub>n</sub>H</td><td>1. η<sup>2</sup>-bonding of an Si-H bond of oligosilane on the metal center<br>2. σ-bond metathesis involving Si-Si bond breaking, Ni-Si and Si-H bonds formation<br>3. Loss of the η<sup>2</sup>-coordinated H<sub>2</sub>SiPh<sub>2</sub> molecule</td><td><ul><li>Silyl and silane complexes characterized by <sup>1</sup>H, <sup>29</sup>Si NMR, and 2D <sup>29</sup>Si-<sup>1</sup>H HMQC NMR; XRD</li><li>Control experiments to ensure the homogeneity of the catalysis: mercury test, HRTEM, addition of quenching ligand</li><li>DFT calculations for the whole mechanism and XRD crystals isolated</li></ul></td><td>[163]</td></tr></tbody></table>

indicate that AB dehydrogenation appears very versatile, with a large set of conditions established (multiplying metals, metals combination and oxidized derivatives, and as well notably various actively involved supporting materials). The role of the supports appeared very important, and a common point of all the mechanisms is the hydridic and protonic activation modes mentioned for H<sub>2</sub> production. Conversely, recent mechanistic studies of silanes solvolysis mostly concern iridium molecular catalysts used in a priori assumed homogeneous conditions. Tables 6a and 6b summarized key points of the mechanisms previously discussed in the corresponding sections, and it indicates also the mode of investigation of these mechanisms, sometimes combining experimental and computational approaches.

Pertinent mechanistic data related to the production of hydrogen from silane dehydrocoupling and organosilanes hydrogenolysis are gathered in Table 6b. As detailed in the section 3.3 and summarized in

section 3.5, the elementary steps of silane/hydrogen chemistry (metathesis, oxidative hydrosilylation at the metal center, reductive elimination, hydride formation, etc.) were studied in details in the context of homogeneous catalysis, notably by <sup>1</sup>H and <sup>29</sup>Si NMR. Both early (Ti, Zr, Hf) and late metals (Rh, Ni) are pertinent to achieve these elementary steps related to silane oligomers formation or depolymerization. The access to reversible reactions was thus evidenced.

## 5. General conclusion

The present survey discusses the state of research about ammonia borane catalytic solvolysis mechanisms and the possible ways of its regeneration. We also analyze the option of hydrosilanes employed as a viable vector for H<sub>2</sub> storage and convenient catalytically controlled release. The academic bibliography is reviewed with a cut-off in the
second semester of 2024.

The main AB advantages for H₂ storage are: i) a room temperature solvolysis giving potentially rise to three equivalents of H₂, ii) this solvolysis that is performed in innocuous water or methanol (or other alcohols, notably ethanol), MeOH being attractive for lower temperature processes down to -80 °C. Metal catalysts for AB solvolysis have been extensively studied in terms of turnover frequencies over the two past decades, however the mechanisms governing this reaction, according to the conditions employed, have been very often overlooked. Thereby, the activation of B–H bond and/or N–H bond through an electronic transfer from a charged or neutral coordinated species takes place. Additionally, water or alcohol dissociation gives off one H atom. Finally, and strongly correlated to the second point, the pH-dependence of the process is crucial, where nonneutral conditions often favors faster H₂ release, and notably basic conditions. That said, in order for this H₂ storage solution to be industrially viable, AB should be regenerated from its by-products. The recycling of AB is possible from reduction of the B (OR)₃ species and derivatives. Unfortunately, to date, and after a number of studies on the subject since 2010, regenerating AB from its by-products involves the stoichiometric use of expensive and rather dangerous highly reducing agents, mostly electropositive metal hydrides, such as LiAlH₄. Finding an effective catalytic way of activating the strong B–O bond is thus desirable [191]. A few examples from the bibliography support the examination of low-valent Ni hydride catalysts [173]. Indeed, low-valent nickel complexes [192] are intermediates in reactions where hydrogenolysis using pure H₂ is used to cleave strong chemical bonds [193], they have also evidenced some efficacy in the electrocatalytic production of H₂ [194], these species are thus a pertinent option to explore. In general, the approaches using directly the activation of H₂ as a source of hydrogen, based on effective hydrogenation catalysts notably the less expensive metals Ru [195], or Fe [196,197] to polarize and help producing H⁻ and H⁺, might help substituting H⁻ for OH⁻ in borate. In terms of innovative processes, mechanochemistry might be an attractive solvent-less mean for regeneration of borates under H₂ pressure [198], avoiding protic or polar solvent back-oxidation interference, provided that an efficient hydrogenation catalysts is used, and possibly Pt, Rh and Ir-based catalysts would be relevant to investigate in this field.

In the case of hydrosilanes, they have been little studied in the academic bibliography as CHS materials specifically. However, hydrosilanes alcoholysis, yielding the corresponding alkoxysilanes and hydrogen gas, was already described in the 1950's. The ethanolysis and methanolysis of SiHR₃ compounds has been studied in details for the last decades with a large variety of active metals (notably iridium molecular complexes providing very high TOFs), and the concomitant production of H₂ exploiting deuteration an KIE measurement allowed to gather a profitable understanding of the process, leading to recent evidences of unexpected hydrogen tunneling effects in the reaction [111]. While molecular complexes are thus well-known [136], further development of nanocatalysts and heterogeneous systems is predictable in the field specifically for immobilizing H₂-productive systems.

The hydrosilane high industrial value, notably for the preparation of silicon polymers, implied that an embryonic bibliography exists to envision Si–O activation and regeneration to Si–H bond involving the use of diverse late transition metals as well as lanthanides (Y [181], Fe [180], Ir [182], Pt [199], or even Au [200]).

In addition to their solvolysis, dehydrogenative polymerization of hydrosilanes (dehydrocoupling) benefits from the reactivity of a large variety of early and late transition metal complexes (including Ti, Zr, Rh, Ir, Ni, etc.). The processing studies, mostly driven by the interest for the industrially relevant oligosilanes formed, gathered informative mechanistic studies and evidenced variously efficient H₂-productive catalytic systems. A noticeable interest for CHS application is the reversibility of this reaction. Breaking Si–Si bonds is thermodynamically accessible, and the metal-promoted means for its activation are recognized [201]. Accordingly, we can exemplify the recently reported hydrogenolysis of

polysilanes catalyzed by very original zerovalent nickel hydride complexes, which gives access to low-coordinated d¹⁰Ni (hydride and silyl) complexes (formally 14 and 16 e⁻) that eagerly react with Si–H and H–H bonds. These complexes are remarkably effective in the hydrogenation of various Si–Si bonds in cyclic and linear oligo- and polysilanes. The almost quantitative formation of Me₂SiH₂ from [(Me₂Si)₆] and [(Me₂Si)₇] is especially inspiring. Further limitations to overcome should be clearly the extension to catalytic applications with complex mixtures of linear and cyclic oligo- and polysilanes.

The potential interest for industrial and academic approaches will probably be significantly increased in relation with lower-cost catalysts (earth-abundant, nontoxic) and/or mixed polymetallic species, that would be better processed: i) with high dispersion/stability maintaining high activity such as in ultrasmall nanocatalysts synthesis (MOFs nanoparticles, for instance), ii) using environmentally decarbonated energy processes including photo-assisted catalysis and mechanochemistry (as illustrated herein), and efficiently regenerated or recycled. Further developments with beneficial economic aspects are expected: i) silanes and siloxanes would be increasingly used to generate H₂; ii) with methods and devices for generating H₂ for fuel cells and for other applications such as fuel (or as supplementary fuel) for internal combustion engines. Some processes for producing and regenerating hydrogen carrier compounds, including siloxanes, are patented. Such technologies are developed as innovative solution to facilitate hydrogen transportation, by charging and releasing hydrogen in silane liquid carriers, thus tackling safety, regulations and supply chain issues. We anticipate the synthesis of processes where hydrosilane precursors are both used as materials (for instance, in chemical vapor deposition for thin films in silicon industry) and their evolving H₂ is recovered to achieve a virtuous H₂ delivery, fitting a circular economy principle. The interest of safer hydrogen carrier with high density storing such as AB and silanes would match in the coming decades the standard requirement, all the more that as a realism principle, these standards are susceptible to be revised down in the near future, depending on the cost of resources needed for water electrolysis (precious transition metals), nuclear plant energy (heavy radioactive waste), and the recognized continuous and hazardous depletion of fossil resources. The enormous advantage of easy recovering of by-products from AB and silane, for their treatment, meets the need of the circular economy, especially when is considered the difficulties for CO₂ recovering from fossil resources burning.

Overall, in this perspective of boranes and silanes CHS and their virtuous (re)cycling processes, the obstacles are certainly multiple, but the knowledge and expertise needed to overcome them are available.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## References

[1] N. Armaroli, V. Balzani, The hydrogen issue, ChemSusChem 4 (2011) 21–36, https://doi.org/10.1002/cssc.201000182.

[2] H. Dagdougui, R. Sacile, C. Bersani, A. Ouammi, Chapter 7 - hydrogen logistics: Safety and risks issues, in: H. Dagdougui, R. Sacile, C. Bersani, A. Ouammi (Eds.), Hydrog. Infrastruct. Energy Appl., Academic Press, 2018, pp. 127–148, https://doi.org/10.1016/B978-0-12-812036-1.00007-x.

[3] D.J. Durbin, C. Malardier-Jugroot, Review of hydrogen storage techniques for on board vehicle applications, Int. J. Hydrogen Energy 38 (2013) 14595–14617, https://doi.org/10.1016/j.ijhydene.2013.07.058.

[4] S. Bosu, N. Rajamohan, Recent advancements in hydrogen storage - comparative review on methods, operating conditions and challenges, Int. J. Hydrogen Energy 52 (2024) 352–370, https://doi.org/10.1016/j.ijhydene.2023.01.344.
[5] U.B. Demirci, P. Miele, Chemical hydrogen storage: 'material' gravimetric capacity versus 'system' gravimetric capacity, Energ. Environ. Sci. 4 (2011) 3334–3341, https://doi.org/10.1039/c1ee01612a.

[6] P. Preuster, C. Papp, P. Wasserscheid, Liquid organic hydrogen carriers (LOHCs): toward a hydrogen-free hydrogen economy, Acc. Chem. Res. 50 (2017) 74–85, https://doi.org/10.1021/acs.accounts.6b00474.

[7] C. Chu, K. Wu, B. Luo, Q. Cao, H. Zhang, Hydrogen storage by liquid organic hydrogen carriers: catalyst, renewable carrier, and technology – a review, Carbon Resour. Convers. 6 (2023) 334–351, https://doi.org/10.1016/j.crcron.2023.03.007.

[8] L. Ouyang, J. Jiang, K. Chen, M. Zhu, Z. Liu, Hydrogen production via hydrolysis and alcaloholysis of light metal-based materials: a review, Nano-Micro Lett. 13 (134) (2021) 1–30, https://doi.org/10.1007/s40820-021-00657-9.

[9] R. Pachaiappan, S. Rajendran, P.S. Kumar, D.-V.N. Vo, T.K.A. Hoang, L. Cornejo-Ponce, Recent advances in carbon nitride-based nanomaterials for hydrogen production and storage, Int. J. Hydrogen Energy 47 (2022) 37490–37516, https://doi.org/10.1016/j.ijhydene.2021.09.062.

[10] Y. Li, R.T. Yang, Hydrogen storage in low silica type X zeolites, J. Phys. Chem. B 110 (2006) 17175–17181, https://doi.org/10.1021/jp0634508.

[11] A. Al-Yaseri, D. Wolff-Boenisch, C.A. Fauziah, S. Iglauer, Hydrogen wettability of clays: implications for underground hydrogen storage, Int. J. Hydrogen Energy 46 (2021) 34356–34361, https://doi.org/10.1016/j.ijhydene.2021.07.226.

[12] (a) X. Zhang, P. Liu, Y. Zhang, The application of MOFs for hydrogen storage, Inorg. Chim. Acta 557 (2023) 121683, https://doi.org/10.1016/j.ica.2023.121683;

(b) Q. Yao, X. Zhang, Z.-H. Lu, Q. Xu, Metal-organic framework-based catalysts for hydrogen production from liquid-phase chemical hydrides, Coord. Chem. Rev. 493 (2023) 215302.

[13] H. Nasrallah, J.-C. Hierso, Porous materials based on 3-dimensional td-directing functionalized Adamantane scaffolds and applied as recyclable catalysts, Chem. Mater. 31 (2019) 619–642, https://doi.org/10.1021/acs.chemmater.8b04508.

[14] Q. Sun, N. Wang, Q. Xu, J. Yu, Nanopore-supported metal nanocatalysts for efficient hydrogen generation from liquid-phase chemical hydrogen storage materials, Adv. Mater. 32 (2020) 2001818, https://doi.org/10.1002/adma.202001818.

[15] K. Cousins, R. Zhang, Highly porous organic polymers for hydrogen fuel storage, Polymers 11 (2019) 690, 1–25, https://doi.org/10.3390/polym11040690.

[16] T. Kopac, Covalent Organic Frameworks-Based Nanomaterials for Hydrogen Storage, 2022, https://doi.org/10.1201/9781003206507-20.

[17] S. McWhorter, K. O'Malley, J. Adams, G. Ordaz, K. Randolph, N.T. Stetson, Moderate temperature dense phase hydrogen storage materials within the US Department of Energy (DOE) H2 storage program: trends toward future development, Crystals 2 (2012) 413–445, https://doi.org/10.3390/cryst20201413.

[18] U.B. Demirci, Ammonia borane: an extensively studied, though not yet implemented, hydrogen carrier, Energies 13 (2020) 3071, 1–45, https://doi.org/10.3390/en13123071.

[19] O. Farha, A. Yazaydin, I. Eryazici, C. Malliakas, B. Hauser, M. Kanatzidis, S. Nguyen, R. Snurr, J. Hupp, De novo synthesis of a metal-organic framework material featuring ultrahigh surface area and gas storage capacities, Nat. Chem. 2 (2010) 944–948, https://doi.org/10.1038/nchem.834.

[20] A.L. Sutton, J.I. Mardel, M.R. Hill, Metal-organic frameworks (MOFs) as hydrogen storage materials at near-ambient temperature, Chem. A Eur. J. 30 (2024) e20400717, https://doi.org/10.1002/chem.202400717.

[21] N. Klopčić, I. Grimmer, F. Winkler, M. Sartory, A. Trattner, A review on metal hydride materials for hydrogen storage, J. Energy Storage 72 (2023) 108456–1084473, https://doi.org/10.1016/j.est.2023.108456.

[22] J. Chen, H. Fu, Y. Xiong, J. Xu, J. Zheng, X. Li, MgCl₂ promoted hydrolysis of MgH₂ nanoparticles for highly efficient H₂ generation, Nano Energy 10 (2014) 337–343, https://doi.org/10.1016/j.nanoen.2014.10.002.

[23] K. Chen, L. Ouyang, H. Zhong, J. Liu, H. Wang, H. Shao, Z. Zhang, Converting H⁺ from coordinated water into H⁻ enables super facile synthesis of LiBH₄, Green Chem. 21 (2019) 4380–4387, https://doi.org/10.1039/C9GC01897B.

[24] F. Mertens, G. Wolf, F. Baitowal, Ammonia borane and related compounds as hydrogen source materials, in: M. Hirscher (Ed.), Handb. Hydrog. Storage, 1st ed, Wiley, 2010, pp. 215–247, https://doi.org/10.1002/9783527629800.ch8.

[25] H. Li, Q. Yang, X. Chen, S.G. Shore, Ammonia borane, past as prolog, J. Organomet. Chem. 751 (2014) 60–66, https://doi.org/10.1016/j.jorganchem.2013.08.044.

[26] F.H. Stephens, V. Pons, R. Tom Baker, Ammonia-borane: the hydrogen source par excellence? Dalton Trans. (2007) 2613–2626, https://doi.org/10.1039/B703053C.

[27] C.D. Mboyi, D. Poinsot, J. Roger, K. Fajerwerg, M.L. Kahn, J.-C. Hierso, The hydrogen-storage challenge: nanoparticles for metal-catalyzed ammonia borane dehydrogenation, Small 17 (2021) 2102759, 1–60, https://doi.org/10.1002/smll.202102759.

[28] C. Wang, J. Zhao, X. Du, S. Sun, X. Yu, X. Zhang, Z. Lu, L. Li, X. Yang, Hydrogen production from ammonia borane hydrolysis catalyzed by non-noble metal-based materials: a review, J. Mater. Sci. 56 (2021) 2856–2878, https://doi.org/10.1007/s10853-020-05493-7.

[29] A. Rossin, M. Peruzzini, Ammonia-borane and amine-borane dehydrogenation mediated by complex metal hydrides, Chem. Rev. 116 (2016) 8848–8872, https://doi.org/10.1021/acs.chemrev.6b00043.

[30] N.E. Stubbs, A.P.M. Robertson, E.M. Leitao, I. Manners, Amine-borane dehydrogenation chemistry: metal-free hydrogen transfer, new catalysts and

mechanisms, and the synthesis of polyaminoboranes, J. Organomet. Chem. 730 (2013) 84–89, https://doi.org/10.1016/j.jorganchem.2012.10.010.

[31] H. Wu, Y. Cheng, Y. Fan, X. Lu, L. Li, B. Liu, B. Li, S. Lu, Metal-catalyzed hydrolysis of ammonia borane: mechanism, catalysts, and challenges, Int. J. Hydrogen Energy 45 (2020) 30325–30340, https://doi.org/10.1016/j.ijhydene.2020.08.131.

[32] N.C. Smythe, J.C. Gordon, Ammonia borane as a hydrogen carrier: dehydrogenation and regeneration, Eur. J. Inorg. Chem. 2010 (2010) 509–521, https://doi.org/10.1002/ejic.200900932.

[33] O.T. Summerscales, J.C. Gordon, Regeneration of ammonia borane from spent fuel materials, Dalton Trans. 42 (2013) 10075–10084, https://doi.org/10.1039/c3dt50475a.

[34] G.R. Whittell, I. Manners, Advances with ammonia-borane: improved recycling and use as a precursor to atomically thin BN films, Angew. Chem. Int. Ed. 50 (2011) 10288–10289, https://doi.org/10.1002/anie.201103830.

[35] O. Mó, M. Yáñez, M. Eckert-Maksić, Z.B. Maksić, I. Alkorta, J. Elguero, Periodic trends in bond dissociation energies. A theoretical study, J. Phys. Chem. A 109 (2005) 4359–4365, https://doi.org/10.1021/jp0508570.

[36] a) S.K. Spear, D.T. Daly, R.P. Swatloski, M.D. Redemer, R.E. Paggi, Silane as source of hydrogen, 2007, PCT WO 2007/01972 A2 and (2010), US 2010/ 0255392 A1: the subject matter disclosed herein relates to silanes and silicides and uses thereof to generate H₂. Methods and devices for generating H₂ for fuel cells and for other applications such as fuel or a supplementary fuel for internal combustion engines, and reducing agents to improve catalyst efficiency are also disclosed;

(b) B. Burcher, V. Lome, R. Benoit, Process for producing and regenerating hydrogen carrier compounds, 2019, PCT WO 2019/211301 A1: a process for producing and regenerating siloxane as hydrogen carrier is described. Such technologies are developed as innovative solution to facilitate hydrogen transportation, by charging and releasing hydrogen in silane liquid carriers, thus tackling safety, regulations and supply chain issues;

c) S. Tat Tan, J.W. Kee, W.Y. Fan, Catalytic hydrogen generation from the hydrolysis of silanes by ruthenium complexes, Organometallics 30 (15) (2011) 4008–4013;

d) D.N. Auner, D.B. Pandey, in: H.A. Øye, H. Brekken, L. Nygaard (Eds.), Polysilanes – Backbones for New Silicon Based Technologies, in Silicon for the chemical and solar Industry X, Ålesund-Geirander, Norway, June 28 – July 02, 2010, pp. 141–156. ISBN 978-82-997357-6-6;

e) S. Gutiérrez-Tarríno, S. Rojas-Buzo, M.A. Ortuño, P. Óna-Burgos, Sustainable synthesis of silicon precursors coupled with hydrogen delivery based on circular economy via molecular cobalt-based catalysts, ACS Sustain. Chem. Eng. 10 (50) (2022) 16624–16633.

[37] J. Li, H. Bai, Z. Feng, Advances in the modification of silane-based sol-gel coating to improve the corrosion resistance of magnesium alloys, Molecules 28 (2023) 2563, https://doi.org/10.3390/molecules28062563.

[38] F.D. Osterholtz, E.R. Pohl, Kinetics of the hydrolysis and condensation of organofunctional alkoxysilanes: a review, J. Adhes. Sci. Technol. 6 (1992) 127–149, https://doi.org/10.1163/156856192X00106.

[39] M. Jeon, J. Han, J. Park, Catalytic synthesis of silanols from hydrosilanes and applications, ACS Catal. 2 (2012) 1539–1549, https://doi.org/10.1021/cs300296x.

[40] X.L. Luo, R.H. Crabtree, Homogeneous catalysis of silane alcoholysis via nucleophilic attack by the alcohol on an Ir(n²HSiR₃) intermediate catalyzed by [IrH₂S₂(PPH₃)₂]SbF₆ (S = solvent), J. Am. Chem. Soc. 111 (1989) 2527–2535, https://doi.org/10.1021/ja00189a026.

[41] K. Garcés, F.J. Fernández-Alvarez, V. Polo, R. Lalrempuia, J.J. Pérez-Torrente, L. A. Oro, Iridium-catalyzed hydrogen production from hydrosilanes and water, ChemCatChem 6 (2014) 1691–1697, https://doi.org/10.1002/cctc.201301107.

[42] M. Aliaga-Lavrijns, M. Iglesias, A. Cebollada, K. García, N. García, P.J. Sanz Miguel, F.J. Fernández-Alvarez, J.J. Pérez-Torrente, L.A. Oro, Hydrolysis and methanolysis of Silanes catalyzed by iridium(III) Bis-N-heterocyclic carbene complexes: influence of the wingtip groups, Organometallics 34 (2015) 2378–2385, https://doi.org/10.1021/om5011726.

[43] M. Lee, S. Ko, S. Chang, Highly selective and practical hydrolytic oxidation of organosilanes to silanols catalyzed by a ruthenium complex, J. Am. Chem. Soc. 122 (2000) 12011–12012, https://doi.org/10.1021/ja003079g.

[44] Kotai Hydrogen Project, Velox Energy Materials, Velox Energy Mater, 2025, https://veloxenergymaterials.com.au/kotai-hydrogen-project/. (Accessed 11 January 2015)

[45] A. Shimojima, K. Kuroda, Controlled synthesis of nanostructured silica-based materials from designed alkoxysilanes, J. Sol-Gel Sci. Technol. 46 (2008) 307–311, https://doi.org/10.1007/s10971-007-1662-3.

[46] Sigma-Aldrich, Material Safety Data Sheet: Borane-ammonia complex. https://www.sigmaaldrich.com/FR/en/sds/aldrich/682098?userType=anonymous, 2010. (Accessed 26 September 2023).

[47] W.E. Palke, Calculation of the internal rotation barrier and its derivatives in BH₃NH₃, J. Chem. Phys. 56 (1972) 5308–5312, https://doi.org/10.1063/1.1677036.

[48] J.D. Jill, P.V.R. Schleyer, J.A. Pople, Molecular orbital theory of the electron structure of organic compounds. XXIV. Geometries and energies of small boron compounds. Comparisons with carbocations, J. Am. Chem. Soc. 97 (1975) 3402–3409, https://doi.org/10.1021/ja00845a021.

[49] D.J. Wolstenholme, K.T. Traboulsee, Y. Hua, L.A. Calhoun, G.S. McGrady, Thermal desorption of hydrogen from ammonia borane: unexpected role of homopolar B-H···H···B interactions, Chem. Commun. 48 (2012) 2597–2599, https://doi.org/10.1039/c2cc16111g.
[50] D.J. Ingram, The Solvation and Regeneration of Ammonia Borane: a Hydrogen Storage Material, University College London, 2019, in dissertation for the degree of Dr. of Philosophy, consulted may 8 2025 at https://discovery.ucl.ac.uk/id/eprint/10079070/14/thesis.pdf, in: M. Hirscher (Ed.), Handbook of hydrogen storage: New Materials for Future Energy Storage, Wiley-VCH Verlag GmbH & Co. KGaA, 2010.

[51] G. Moussa, R. Moury, U.B. Demirci, P. Miele, Borates in hydrolysis of ammonia borane, Int. J. Hydrogen Energy 38 (2013) 7888-7895, https://doi.org/10.1016/j.ijhydene.2013.04.121.

[52] M. Chandra, Q. Xu, Dissociation and hydrolysis of ammonia-borne with solid acids and carbon dioxide: an efficient hydrogen generation system, J. Power Sources 159 (2006) 855-860, https://doi.org/10.1016/j.jipowsour.2005.12.033.

[53] H. Li, Z. Yao, X. Wang, Y. Zhu, Y. Chen, Review on hydrogen production from catalytic ammonia borane methanolysis: advances and perspectives, Energy Fuel 36 (2022) 11745-11759, https://doi.org/10.1021/acs.energyfuels.2c02314.

[54] A. Al-Kukhun, H.T. Hwang, A. Varma, Mechanistic studies of ammonia borane dehydrogenation, Int. J. Hydrogen Energy 38 (2013) 169-179, https://doi.org/10.1016/j.ijhydene.2012.09.161.

[55] F. Xu, J. Ren, J. Ma, Y. Wang, K. Zhang, Z. Cao, Q. Sun, S. Wu, G. Li, S. Bai, A review of hydrogen production kinetics from the hydrolysis of NaBH₄ solution catalyzed by co-based catalysts, Int. J. Hydrogen Energy 50 (2024) 827-844, https://doi.org/10.1016/j.ijhydene.2023.08.142.

[56] H.C. Kelly, V.B. Marriott, Reexamination of the mechanism of acid-catalyzed amine-borne hydrolysis. The hydrolysis of ammonia-borne, Inorg. Chem. 18 (1979) 2875-2878, https://doi.org/10.1021/ic50200a049.

[57] M. Chandra, Q. Xu, A high-performance hydrogen generation system: transition metal-catalyzed dissociation and hydrolysis of ammonia-borne, J. Power Sources 156 (2006) 190-194, https://doi.org/10.1016/j.jipowsour.2005.05.043.

[58] M. Chandra, Q. Xu, Room temperature hydrogen generation from aqueous ammonia-borne using noble metal nano-clusters as highly active catalysts, J. Power Sources 168 (2007) 135-142, https://doi.org/10.1016/j.jipowsour.2007.03.015.

[59] X.-B. Zhang, S. Han, J.-M. Yan, M. Chandra, H. Shioyama, K. Yasuda, N. Kuriyama, T. Kobayashi, Q. Xu, A new fuel cell using aqueous ammonia-borne as the fuel, J. Power Sources 168 (2007) 167-171, https://doi.org/10.1016/j.jipowsour.2007.03.009.

[60] W. Chen, D. Li, Z. Wang, G. Qian, Z. Sui, X. Duan, X. Zhou, I. Yeboah, D. Chen, Reaction mechanism and kinetics for hydrolytic dehydrogenation of ammonia borane on a Pt/CNT catalyst, Am. Inst. Chem. Eng. J. 63 (2017) 60-65, https://doi.org/10.1002/aic.15389.

[61] C. Wang, J. Tuninetti, Z. Wang, C. Zhang, L. Salmon, S. Moya, J. Ruiz, D. Astruc, Hydrolysis of ammonia-borne over Ni/ZIF-8 nanocatalyst: high efficiency, mechanism and controlled hydrogen release, J. Am. Chem. Soc. 139 (2017) 11610-11615, https://doi.org/10.1021/jacs.7b06859.

[62] M. Parafiniuk, M.P. Mitoraj, Origin of Binding of Ammonia-Borne to Transition-Metal-Based Catalysts: An Insight from the Charge and Energy Decomposition Method ETS-NOCV, Organometallics 32 (2013) 4103-4113, https://doi.org/10.1021/om400235e.

[63] N.A.M. Barakat, Effective Co-Mn-O nanofibers for ammonia borane hydrolysis, Mater. Lett. 106 (2013) 229-232, https://doi.org/10.1016/j.matlet.2013.05.043.

[64] (a) N.A.M. Barakat, Catalytic and photo hydrolysis of ammonia borane complex using Pd-doped co nanofibers, Appl. Catal. Gen. 451 (2013) 21-27, https://doi.org/10.1016/j.apcata.2012.10.034;

(b) Photocatalysis strictly refers to a process where photogenerated electrons and holes directly participate in the reaction after being excited by light, leading to the catalytic reaction. In the case of ammonia borane solvolysis the catalytic reaction generally proceeds also without light, but illumination may accelerate H₂ release. These reactions are photo-assisted catalytic reactions. For distinguishing photocatalysis and photo-assisted catalysis, see for instance: Y. Li, Y. Feng, H. Wang, J. Liao, Z. Guo, X. Chen, W. Zhou, M. He, H. Li, Visible light-assisted hydrogen generation from ammonia borane over Z-Scheme NiO-CuO heterostructures J. Colloid Interface Sci. 650 (2023) 1648-1658.

[65] (a) J.-P. Chang, C.-Y. Wang, Y.-J. Hsu, C.-Y. Wang, Cu₂O/UiO-66-NH₂ composite photocatalysts for efficient hydrogen production from ammonia borane hydrolysis, Appl. Catal. A, Gen. 650 (2023) 119005;

(b) F. Tong, X. Liang, Z. Wang, Y. Liu, P. Wang, H. Cheng, Y. Dai, Z. Zheng, B. Huang, Probing the mechanism of plasmon-enhanced ammonia borane methanolysis on a CuAg alloy at a single-particle level, ACS Catal. 11 (2021) 10814-10823, https://doi.org/10.1021/acsatalc.1c02857.

[66] K. Feng, J. Zhong, B. Zhao, H. Zhang, L. Xu, X. Sun, S. Lee, CuₓCo₁ₓ-O nanoparticles on graphene oxide as a synergistic catalyst for high-efficiency hydrolysis of ammonia-borne, Angew. Chem. Int. Ed. 55 (2016) 11950-11954, https://doi.org/10.1002/anie.201604021.

[67] X. Qu, R. Jiang, Q. Li, F. Zeng, X. Zheng, Z. Xu, C. Chen, J. Peng, The hydrolysis of ammonia borane catalyzed by NiCoP/OPC-300 nanocatalysts: high selectivity and efficiency and mechanism, Green Chem. 21 (2019) 850-860, https://doi.org/10.1039/C8GC03536A.

[68] Z. Li, T. He, L. Liu, W. Chen, M. Zhang, G. Wu, P. Chen, Covalent triazine framework supported non-noble metal nanoparticles with superior activity for catalytic hydrolysis of ammonia borane: from mechanistic study to catalyst design, Chem. Sci. 8 (2017) 781-788, https://doi.org/10.1039/C6SC02456D.

[69] X. Ren, H. Lv, S. Yang, Y. Wang, J. Li, R. Wei, D. Xu, B. Liu, Promoting effect of heterostructured NiO/Ni on Pt nanocatalysts toward catalytic hydrolysis of ammonia borane, J. Phys. Chem. Lett. 10 (2019) 7374-7382, https://doi.org/10.1021/acs.jplett.9b03080.

[70] Q. Yao, Y. Ding, Z.-H. Lu, Noble-metal-free nanocatalysts for hydrogen generation from boron- and nitrogen-based hydrides, Inorg. Chem. Front. 7 (2020) 3837-3874, https://doi.org/10.1039/DQI00766H.

[71] (a) Standard ISO 14687:2019, "Hydrogen fuel quality—Product specification", consulted at https://www.iso.org/obp/ui/fr/#iso:std:14687:ed:1:v1:en, in June 18, 2025;

[72] M.-J. Fang, Y.-C. Lin, J.-Y. Jan, T.-H. Lai, P.-Y. Hu, M.-Y. Kuo, Y.-H. Chiu, C.-W. Tsao, Y.-A. Chen, Y.-T. Wang, Y.-J. Hong, J.-Y. Wu, Y.C.S. Wu, Y.-G. Lin, T.-F. M. Chang, C.-Y. Chen, M. Sone, S.-M. Chang, C.-L. Chang, Y.-J. Hsu, Au@Cu₂O core/shell nanocrystals as sustainable catalysts for efficient hydrogen production from ammonia borane, Appl. Catal. B 324 (2023) 122198;

[73] C. M. Nagyházi, G. Turczel, P.T. Anastas, R. Tuba, Highly efficient ammonia borane hydrolytic dehydrogenation in neat water using phase-labeled CAAC-Ru catalysts, ACS Sustain. Chem. Eng. 8 (2020) 16097-16103, https://doi.org/10.1021/acssuschemeng.0c06696;

[74] D. Poinots, M. Bouzid, A. Burlot, C.D. Mboyi, P. Doulain, J. Paris, O. Heintz, B. Domenichini, V. Collière, M.L. Kahn, J.-C. Hierso, High recyclability magnetic iron oxide-supported ruthenium nanocatalyst for h₂ release from ammonia-borne Solvolysis, ChemNanoMat 8 (2022) e202200285, https://doi.org/10.1002/cnma.202200285.

[75] (a) P.K. Dasgupta, S. Dong, Solubility of ammonia in liquid water and generation of trace levels of standard gaseous ammonia, Atmos. Environ. 20 (1986) 565-570;

[76] (b) O. Johansson, M. Wedborg, The ammonia-ammonium equilibrium in seawater at temperatures between 5 and 25°C, J. Solution Chem. 9 (1980) 37-44, https://doi.org/10.1007/BF00650135;

[77] (c)See Ref. [97]. (d) H.T. Hwang, A. Al-Kukhun, A. Varma, Hydrogen for vehicle applications from thermothermolysis of ammonia-borne: hydrogen yield, thermal characteristics, and ammonia formation, Ind. Eng. Chem. Res. 49 (2010) 10994-11000;

[78] (e)See Ref. [94] and [95].

[79] (f)See Ref. [101]. (g) U.B. Demirci, Ammonia borane in chemical hydrogen storage - Ammonia release during hydrolysis of ammonia borane, an issue, in: proceeding at ANBRE13, 2013, p. 723. http://www.i-asem.org/publication_conf/abre13/T4C.5.MS518_534F.pdf.

[80] M. Liu, L. Zhou, X. Luo, C. Wan, L. Xu, Recent advances in noble metal catalysts for hydrogen production from ammonia borane, Catalysts 10 (2020) 1-35, https://doi.org/10.3390/catal10070788, 788.

[81] U.B. Demirci, The hydrogen cycle with the hydrolysis of sodium borohydride: a statistical approach for highlighting the scientific/technical issues to prioritize in the field, Int. J. Hydrogen Energy 40 (2015) 2673-2691, https://doi.org/10.1016/j.ijhydene.2014.12.067.

[82] C.-H. Liu, B.-H. Chen, The concept about the regeneration of spent borohydrides and used catalysts from green electricity, Materials 8 (2015) 3456-3466, https://doi.org/10.3390/m808063456.

[83] Q. Lai, D. Alligier, K.-F. Aguey-Zinsou, U.B. Demirci, Hydrogen generation from a sodium borohydride-nickel core@shell structure under hydrolytic conditions, Nanoscale Adv. 1 (2019) 2707-2717, https://doi.org/10.1039/C9NA00037B.

[84] H.X. Nunes, D.L. Silva, C.M. Rangel, A.M.F.R. Pinto, Rehydrogenation of sodium borates to close the NaBH₄-H₂ cycle: a review, Energies 14 (2021) 3567, https://doi.org/10.3390/en14123567.

[85] Y. Kojima, T. Haga, Recycling process of sodium metaborate to sodium borohydride, Int. J. Hydrogen Energy 28 (2003) 989-993, https://doi.org/10.1016/S0360-3199(02)00173-8.

[86] L. Ouyang, W. Chen, J. Liu, M. Felderhoff, H. Wang, M. Zhu, Enhancing the regeneration process of consumed NaBH₄ for hydrogen storage, Adv. Energy Mater. 7 (2017) 1700299, https://doi.org/10.1002/aemn.201700299.

[87] H. Zhong, L.Z. Ouyang, J.S. Ye, J.W. Liu, H. Wang, X.D. Yao, M. Zhu, An one-step approach towards hydrogen production and storage through regeneration of NaBH₄, Energy Storage Mater 7 (2017) 222-228, https://doi.org/10.1016/j.ensm.2017.03.001.

[88] T. Kemmitt, G.J. Gainsford, Regeneration of sodium borohydride from sodium metaborate, and isolation of intermediate compounds, Int. J. Hydrogen Energy 34 (2009) 5726-5731, https://doi.org/10.1016/j.ijhydene.2009.05.108.

[89] Y. Zhu, L. Ouyang, H. Zhong, J. Liu, H. Wang, H. Shao, Z. Huang, M. Zhu, Closing the loop for hydrogen storage: facile regeneration of NaBH₄ from its hydrolytic product, Angew. Chem. Int. Ed. 59 (2020) 8623-8629, https://doi.org/10.1002/anie.201915988.

[90] Z.P. Li, N. Morigazaki, B.H. Liu, S. Suda, Preparation of sodium borohydride by the reaction of MgH₂ with dehydrated borax through ball milling at room temperature, J. Alloys Compd. 349 (2003) 232-236, https://doi.org/10.1016/S0925-8388(02)00872-1.

[91] C.-L. Hsueh, C.-H. Liu, B.-H. Chen, C.-Y. Chen, Y.-C. Kuo, K.-J. Hwang, J.-R. Ku, Regeneration of spent-NaBH₄ back to NaBH₄ by using high-energy ball milling, Int. J. Hydrogen Energy 34 (2009) 1717-1725, https://doi.org/10.1016/j.ijhydene.2008.12.036.

[92] Ç. Çakanyildirim, M. Gürü, The production of NaBH₄ from its elements by mechano-chemical reaction and usage in hydrogen recycle, energy sources part recovery, Util. Environ. Eff. 33 (2011) 1912-1920, https://doi.org/10.1080/15567030930530175.

[93] S. Garroni, Mechanochemical synthesis of NaBH₄ starting from NaH-MgB₂ reactive hydride composite system, Int. J. Hydrogen Energy 38 (2013) 2363-2369, https://doi.org/10.1016/j.ijhydene.2012.11.136.

[94] C. Qin, L. Ouyang, H. Wang, J. Liu, H. Shao, M. Zhu, Regulation of high-efficient regeneration of sodium borohydride by magnesium-aluminum alloy, Int. J. Hydrogen Energy 44 (2019) 29108-29115, https://doi.org/10.1016/j.ijhydene.2019.05.010.
[88] W. Chen, L.Z. Ouyang, J.W. Liu, X.D. Yao, H. Wang, Z.W. Liu, M. Zhu, Hydrolysis and regeneration of sodium borohydride (NaBH₄) – a combination of hydrogen production and storage, J. Power Sources 359 (2017) 400–407, https://doi.org/10.1016/j.jpowsour.2017.05.075.

[89] L. Ouyang, H. Zhong, H.-W. Li, M. Zhu, A Recycling hydrogen supply system of NaBH₄ based on a facile regeneration process: a review, Inorganics 6 (2018) 10, 1–12, doi: https://doi.org/10.3390/inorganics6010010.

[90] A. Ibrahim, M. Paskevicius, A. Patel, A.M. D'Angelo, T.D. Humphries, C. E. Buckley, Thermochemical formation of sodium borohydride from sodium tetramethoxyborate, ACS Appl Energy Mater 7 (2024) 11206–11217, https://doi.org/10.1021/acsaeom.4c02514.

[91] K. Aydin, B.N. Kulabik, B. Coskuner Filiz, D. Alligier, U.B. Demirci, A. Kantürk Figen, Closing the hydrogen cycle with the couple sodium borohydride-methanol, via the formation of sodium tetramethoxyborate and sodium metaborate, Int. J. Energy Res. 44 (2020) 11405–11416, https://doi.org/10.1002/er.5761.

[92] P. Veeraraghavan Ramachandran, B.C. Raju, P.D. Gagare, One-pot synthesis of ammonia-borane and trialkylamine-boranes from trimethyl borate, Org. Lett. 14 (2012) 6119–6121, https://doi.org/10.1021/ol302421t.

[93] N. Kang, C. Wang, D. Astruc, Hydrogen evolution upon ammonia borane solvolysis: comparison between the hydrolysis and methanolysis reactions, Chemistry 5 (2023) 886–899, https://doi.org/10.3390/chemistry5020060.

[94] P.V. Ramachandran, P.D. Gagare, Preparation of ammonia borane in high yield and purity, methanolysis, and regeneration, Inorg. Chem. 46 (2007) 7810–7817, https://doi.org/10.1021/ic700772a.

[95] A. Brockman, Y. Zheng, J. Gore, A study of catalytic hydrolysis of concentrated ammonia borane solutions, Int. J. Hydrogen Energy 35 (2010) 7350–7356, https://doi.org/10.1016/j.ijhydene.2010.04.172.

[96] U. Sanyal, U.B. Demirci, B.R. Jagirdar, P. Miele, Hydrolysis of ammonia borane as a hydrogen source: fundamental issues and potential solutions towards implementation, ChemSusChem 4 (2011) 1731–1739, https://doi.org/10.1002/essc.201100318.

[97] C.-H. Liu, Y.-C. Wu, C.-C. Chou, B.-H. Chen, C.-L. Hsueh, J.-R. Ku, F. Tsau, Hydrogen generated from hydrolysis of ammonia borane using cobalt and ruthenium based catalysts, Int. J. Hydrogen Energy 37 (2012) 2950–2959, https://doi.org/10.1016/j.ijhydene.2011.05.022.

[98] J. Hannauer, U.B. Demirci, C. Geantet, J.M. Herrmann, P. Miele, Enhanced hydrogen release by catalyzed hydrolysis of sodium borohydride-ammonia borane mixtures: a solution-state 11B NMR study, Phys. Chem. Chem. Phys. 13 (2011) 3809–3818, https://doi.org/10.1039/c0cp02009g.

[99] C.G. Salentine, High-field boron-11 NMR of alkali borates. Aqueous polyborate equilibria, Inorg. Chem. 22 (1983) 3920–3924, https://doi.org/10.1021/ic00168a019.

[100] G.P. Rachiero, U.B. Demirci, P. Miele, Bimetallic RuCo and RuCu catalysts supported on γ-Al₂O₃. A comparative study of their activity in hydrolysis of ammonia-borane, Int. J. Hydrogen Energy 36 (2011) 7051–7065, https://doi.org/10.1016/j.ijhydene.2011.03.009.

[101] U.B. Demirci, P. Miele, Hydrolysis of solid ammonia borane, J. Power Sources 195 (2010) 4030–4035, https://doi.org/10.1016/j.jpowsour.2010.01.002.

[102] E.I. Gürbüz, D.D. Hibbitts, E. Iglesia, Kinetic and mechanistic assessment of alkanol/alkanal decarbonylation and deoxygenation pathways on metal catalysts, J. Am. Chem. Soc. 137 (2015) 11984–11995, https://doi.org/10.1021/jacs.5b05361.

[103] P. Zhang, N. Chen, D. Chen, S. Yang, X. Liu, L. Wang, P. Wu, N. Phillip, G. Yang, S. Dai, Ultra-stable and high-cobalt-loaded cobalt@ordered mesoporous carbon catalysts: all-in-one deoxygenation of ketone into alkylbenzene, ChemCatChem 10 (2018) 3299–3304, https://doi.org/10.1002/cctc.201800358.

[104] G. Argouarch, Mild and efficient rhodium-catalyzed deoxygenation of ketones to alkanes, New J. Chem. 43 (2019) 11041–11044, https://doi.org/10.1039/C9NJ02954K.

[105] H. Schmidbaur, J. Zech, Tetra(silyl)methane, (H₃Si₄)C, a volatile carbobilane for the chemical vapor deposition of amorphous silicon carbide thin films, in: Effic. Methods Prep. Silicon Compd., Elsevier, 2016, pp. 13–19, https://doi.org/10.1016/B978-0-12-803530-6.00004-4.

[106] W. Simmler, Silicon Compounds, Inorganic, in: Wiley-VCH (Ed.), Ullmanns Encycl. Ind. Chem., 1st ed., Wiley, 2000 https://doi.org/10.1002/14356007a.24_001.

[107] H. Schmidbaur, R. Hager, Prepn. of di-, tri- or tetra-silyl-methane - by reacting aryl-halo-silane with di-, tri- or tetra-halo-methane in presence of reducing metal, and reacting prod. with hydrogen halide, Application Patent, 1991, DE3941997C1.

[108] R. Hager, O. Steigelmann, G. Müller, H. Schmidbaur, H.E. Robertson, D.W. H. Rankin, Tetrasilylmethan, C(SiH₃)₄, das Si/C-inverse des tetramethylsilans, Si (CH₃)₄, Angew. Chem. 102 (1990) 204–206, https://doi.org/10.1002/ange.19901020217.

[109] H. Schmidbaur, J. Zech, An improved synthetic pathway to tetrasilylmethane and the synthesis of 2,2-disilylpropane, Eur. J. Solid State Inorg. Chem. 29 (1992) 5–21, https://doi.org/10.1002/chin.199313226.

[110] J.M. Brunel, Polysilanes: the grail for a highly-neglected hydrogen storage source, Int. J. Hydrogen Energy 42 (2017) 23004–23009, https://doi.org/10.1016/j.ijhydene.2017.07.206.

[111] N. Almenara, M.A. Garralda, X. Lopez, J.M. Matxain, Z. Freixa, M.A. Huertos, Hydrogen tunneling in catalytic hydrolysis and alcoholysis of silanes, Angew. Chem. Int. Ed. 61 (2022) e202204558, https://doi.org/10.1002/anie.202204558.

[112] H.G. Woo, J.F. Walzer, T.D. Tilley, σ-Bond metathesis mechanism for dehydropolymerization of silanes to polysilanes by d⁹ metal catalysts, J. Am. Chem. Soc. 114 (1992) 7047–7055, https://doi.org/10.1021/ja00044a015.

[113] M. Hamdaoui, M. Ney, V. Sarda, L. Karmazin, C. Bailly, N. Sieffert, S. Dohm, A. Hansen, S. Grimel, J.-P. Djukic, Evidence of a donor-acceptor (Ir-H)→SiR₃ interaction in a trapped Ir(III) silane catalytic intermediate, Organometallics 35 (2016) 2207–2223, https://doi.org/10.1021/acs.organomet.6b00248.

[114] T.D. Tilley, The coordination polymerization of silanes to polysilanes by a “σ-bond metathesis” mechanism. Implications for linear chain growth, Acc. Chem. Res. 26 (1993) 22–29, https://doi.org/10.1021/ar00025a004.

[115] V.K. Dioumaev, J.F. Harrod, Catalytic dehydrocoupling of phenylsilane with “cation-like” zirconocene derivatives: a new approach to longer silicon chains, Organometallics 13 (1994) 1548–1550, https://doi.org/10.1021/om00017a004.

[116] E.E. Smith, G. Du, P.E. Fanwick, M.M. Abu-Omar, Dehydrocoupling of organosilanes with a dinuclear nickel hydride catalyst and isolation of a nickel silyl complex, Organometallics 29 (2010) 6527–6533, https://doi.org/10.1021/om100887v.

[117] J.E. Baines, C. Eaborn, 291. Organosilicon compounds. Part XVII. The acidic solvolysis of organosilicon hydrides, J. Chem. Soc. Resumed (1956) 1436–1441, https://doi.org/10.1039/JR9560001436.

[118] E. Lukevics, M. Dzintara, The alcoholysis of hydrosilanes, J. Organomet. Chem. 295 (1985) 265–315, https://doi.org/10.1016/0022-328X(85)80314-4.

[119] A.J. Chalk, J.F. Harrod, Homogeneous catalysis. IV. Some reactions of silicon hydrides in the presence of cobalt carbonyls, J. Am. Chem. Soc. 89 (1967) 1640–1647, https://doi.org/10.1021/ic00983a020.

[120] A.J. Chalk, Group IV-cobalt complexes as catalysts for silylation and cyclic ether polymerization, J. Chem. Soc. Chem. Commun. (1970) 847–848, https://doi.org/10.1039/C29700000847.

[121] N.J. Archer, R.N. Haszeldine, R.V. Parish, Complexes containing cobalt–silicon metal–metal bonds, J. Chem. Soc. Chem. Commun. (1971) 524–525, https://doi.org/10.1039/C29710000524.

[122] R.N. Haszeldine, R.V. (Dick) Parish, B.F. Riley, Organosilicon chemistry. Part 25. Catalysis of the reaction between alcohols and silanes by hydrido-, dinitrogen-, and ethylene-iron complexes, J. Chem. Soc. Dalton Trans. (1980) 705–708, https://doi.org/10.1039/DT980000705.

[123] R.J.P. Corriu, J.J.E. Moreau, Selective catalytic route to bifunctional silanes. Catalysis by rhodium and ruthenium complexes of the alcoholysis of diarylsilanes and the hydrosilylation of carbonyl compounds, J. Chem. Soc. Chem. Commun. (1973) 38–39, https://doi.org/10.1039/C39730000038.

[124] I. Ojima, M. Nihonyanagi, Y. Nagai, Rhodium complex catalysed hydrosilylation of carbonyl compounds, J. Chem. Soc. Chem. Commun. (1972) 938a, https://doi.org/10.1039/C39720000938a.

[125] L.H. Sommer, J.E. Lyons, Stereochemistry of asymmetric silicon. XVI. Transition metal catalyzed substitution reactions of optically active organosilicon hydrides, J. Am. Chem. Soc. 91 (1969) 7061–7067, https://doi.org/10.1021/ja01053a028.

[126] L.N. Lewis, Nathan., Lewis, platinum-catalyzed hydrosilylation - colloid formation as the essential step, J. Am. Chem. Soc. 108 (1986) 7228–7231, https://doi.org/10.1021/ja00283a016.

[127] F.R. Hartley, M.B. Hodge, S.G. Murray, A.R. Bassindale, The alcoholysis of silanes: palladium(ii) complexes that promote silyl perchlorate formation, Jol. Catal. 26 (1984) 313–319, https://doi.org/10.1016/0304-5102(84)85103-2.

[128] S.N. Blackburn, R.N. Haszeldine, R.V. Parish, J.H. Setchfield, Organosilicon chemistry: XXX. Homogeneous catalysis by iridium(I) complexes of the reaction between silanes and alcohols or dideuterium, J. Organomet. Chem. 192 (1980) 329–338, https://doi.org/10.1016/S0022-328X(00)81223-1.

[129] E.A.V. Ebsworth, D.M. Leitch, Reactions of silanes and germanes with iridium complexes. Part I. Reactions with Vaska's compound, J. Chem. Soc. Dalton Trans. (1973) 1287–1292, https://doi.org/10.1039/DT9730001287.

[130] R.H. Crabtree, M.F. Mellea, J.M. Mihelcic, J.M. Quirk, Alkane dehydrogenation by iridium complexes, J. Am. Chem. Soc. 104 (1982) 107–113, https://doi.org/10.1021/ja00365a021.

[131] K.A. Brown-Wensley, Formation of Si-Si bonds from Si-H bonds in the presence of hyrosilylation catalysts, Organometallics 6 (1987) 1590–1591, https://doi.org/10.1021/om00150a040.

[132] E. Matarasso-Tchiroukhine, AreneCr(CO)₂η²-HSiPh₃) complexes as catalysts for the Si-H bond activation. Hydrolysis of the Si-H bond and dehydrogenative coupling between diphenylsilane and nucleophiles, J. Chem. Soc. Chem. Commun. (1990) 681–682, https://doi.org/10.1039/C39900000681.

[133] U. Schubert, C. Lorenz, Conversion of hydrosilanes to silanols and silyl esters catalysed by [Ph₃PCuH]₆, Inorg. Chem. 36 (1997) 1258–1259, https://doi.org/10.1021/ic961134+.

[134] E.A. Ison, R.A. Corbin, M.M. Abu-Omar, Hydrogen production from hydrolytic oxidation of organosilanes using a cationic oxorhenium catalyst, J. Am. Chem. Soc. 127 (2005) 11938–11939, https://doi.org/10.1021/ja053860u.

[135] J. Yang, P.S. White, C.K. Schauer, M. Brookhart, Structural and spectroscopic characterization of an unprecedented cationic transition-metal η¹-Silane complex, Angew. Chem. Int. Ed. 47 (2008) 4141–4143, https://doi.org/10.1002/anie.200705359.

[136] J.Y. Corey, Reactions of hydrosilanes with transition metal complexes and characterization of the products, Chem. Rev. 111 (2011) 863–1071, https://doi.org/10.1021/cr900359c.

[137] D. Ventura-Espinosa, S. Sabater, A. Carretero-Cerdán, M. Baya, J.A. Mata, High production of hydrogen on demand from Silanes catalyzed by iridium complexes as a versatile hydrogen storage system, ACS Catal. 8 (2018) 2558–2566, https://doi.org/10.1021/acs Catal. 8b04479.

[138] P. Csabai, F. Joó, Synthesis and catalytic properties of new water-soluble ruthenium(II)-N-heterocyclic carbene complexes, Organometallics 23 (2004) 5640–5643, https://doi.org/10.1021/om049511a.