# Electrocatalytic hydrogenation of unsaturated organic compounds with molecular complexes: Mechanistic views

Gabriel Durin, Nicolas Kaeffer, Walter Leitner

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Gabriel Durin, Nicolas Kaeffer, Walter Leitner. Electrocatalytic hydrogenation of unsaturated organic compounds with molecular complexes: Mechanistic views. Current Opinion in Electrochemistry, 2023, 41, pp.101371. 10.1016/j.coelec.2023.101371. hal-04919197

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Review Article

# Electrocatalytic hydrogenation of unsaturated organic compounds with molecular complexes: Mechanistic views

Gabriel Durin<fnref n="1" />, Nicolas Kaeffer<fnref n="1" /> and Walter Leitner<fnref n="1" />,<fnref n="2" />

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

## Abstract

Electrocatalytic hydrogenation of π-bonds in unsaturated organic compounds is a promising strategy for electrifying chemical synthesis using renewable energies. This approach not only offers an alternative to the use of molecular hydrogen but also has the potential to unlock original reaction pathways with higher efficiencies or featuring distinct selectivities or functional group tolerances. In this emerging field, such reactivity engineering can be ideally explored with molecular complexes based on a fundamental understanding of the underlying mechanisms. Recent developments on the hydrogenation of organic π-bonds *via* molecular electrocatalysis and related key mechanistic features are presented in this viewpoint. Perspectives are drawn to highlight the less explored areas of the catalytic space and point the way to improved catalytic systems.

## Addresses

Corresponding author: Kaeffer, Nicolas (nicolas.kaeffer@cec.mpg.de)

Current Opinion in Electrochemistry 2023, 41:101371

This review comes from a themed issue on **Organic and Molecular Electrochemistry (2023)**

Edited by **Cyrille Costentin**

For complete overview about the section, refer Organic and Molecular Electrochemistry (2023)

Available online 10 August 2023

https://doi.org/10.1016/j.coelec.2023.101371

2451-9103/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## Introduction

Catalytic hydrogenation is a fundamental and ubiquitous reaction in synthetic chemistry. Initiated by Sabatier over a century ago [1], this reaction has undergone a breadth of developments in both catalyst design and associated applications [2,3]. Catalytic hydrogenations of unsaturated π-bonds are canonically performed with molecular hydrogen in the presence of a

catalyst. However, hydrogen is still mainly produced by steam reforming of methane (grey hydrogen), which is not compatible with the de-fossilization objectives of today's chemical industry [4]. A virtuous alternative is to use hydrogen gas generated *via* water-splitting [5] using electrons provided by renewable electricity and protons obtained from water (green hydrogen).

An even more direct alternative is to bypass hydrogen gas by reacting electrons and protons at the unsaturated π-bonds [6–8]. This approach is the one lying at the core of electrocatalytic and photocatalytic hydrogenations, which have been the focus of recent research efforts, towards sustainable chemical production (Figure 1, a) [6,9–11]. Obviating the need for H₂ production, transport, storage and use, these methodologies may in principle offer safer (no handling of highly flammable gas) and more energy-efficient routes, provided the right catalytic system is available. A more salient feature is the ability borne in these methods to decouple the sources delivering electrons and protons, in contrast to hydrogenation with molecular H₂. This point brings the opportunity to unlock original reactivity pathways. However, these opportunities also come with important challenges, for instance the competing hydrogen evolution reaction (HER) that must be limited to maintain high energy efficiency. The oxidative counter-reaction is also an important factor to consider in these systems, particularly regarding the global efficiency and value of the synthetic process [12–15].

In the forefront, heterogeneous electrocatalysts have been employed to address electrohydrogenations of organic unsaturated bonds (C-E; E = C, O, N) [16–21]. While these examples are leading the field in terms of efficiency and applicability, the reactivity patterns often resemble the ones of thermal hydrogenation, for instance *via* (electro)generation of surface metal hydrides. More recently, molecular electrocatalysis has been dialed in for the electrohydrogenations of unsaturated π-bonds and in particular carbonyls, olefins and alkynes [22–30]. The approach is namely bringing original angles to the reactivity and operating mechanisms for these hydrogenation reactions. Considering the mechanistic standpoint, the broad majority of

<footnote n="1">¹Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany</footnote>

<footnote n="2">²Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany</footnote>
<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>

Electrocatalytic hydrogenation of π-bonds by molecular complexes and associated thermodynamic and mechanistic features [31].

hydrogenation reactions involve metal-hydride species M–H as key intermediates, whatever the sources of electrons and protons (molecular H₂, sacrificial reagents or hydrides, or an electrode and an acid). The reaction between the metal-hydride species and the unsaturated substrate (electron transfer through the hydride, *ETH* pathway 32) commonly proceeds *via* one of these elementary steps: migratory insertion (MI), hydride transfer (HT) or hydrogen atom transfer (HAT). Metal hydrides are also occasionally involved at a later stage of the hydrogenation mechanism through reductive

elimination (RE) (Figure 1, b). In the electrochemical case, the generation of such key hydrides may be problematic if these species react with protons, leading to the HER. Interestingly, recent examples of molecular electrocatalytic systems have shown that metal-hydride species can be dispensed, either by concerted proton-electron transfer (CPET) or by substrate binding to the reduced catalytic center prior to protonation (electron transfer through the substrate, *ETS* pathway 32; Figure 1, b). In this context, we articulate our discussion around mechanistic considerations found in the most
recent examples of electrochemical hydrogenation
catalyzed by molecular complexes and highlight
further perspectives.

Electro catalytic hydrogenation of C=O
bonds

Only a few examples of electrochemical catalytic hydro-
genation of carbonyl compounds by molecular complexes
have been reported [22–24,33–37] some of which have
already been reviewed [38–40]. This paucity contrasts
sharply with the breadth of molecular electrocatalytic
studies for the reduction of CO₂ to formic acid/formate
[41,32], which also consists of the formal hydrogenation
of a C=O bond. In addition, the standard reduction
potentials of carbonyl compounds are in general slightly
to substantially more positive than that of the CO₂/
HCO₂H couple (ca. [-0.01; +0.30] vs -0.09 V<sub>Fc</sub> in
MeCN, respectively) (Figure 1, c) [39]. Their proximity
to the H⁺/H₂ interconversion potential nevertheless
represents a challenge in terms of selectivity, since HER
is then also favored at applied overpotentials.

The first examples of electrohydrogenation of organic
carbonyls were reported by Moutet and coworkers and
later on by Meyer and coworkers, using rhodium or
ruthenium bipyridine-based complexes to reduce ke-
tones [33,34,37]. More recently, Waymouth *and coll.*
described the first electrochemical hydrogenation of
benzaldehyde using a Shvo-like molybdenum catalyst
(Figure 2, a) [22]. Unlike thermal hydrogenation, this
substrate represents a challenge in electrochemistry,
due to facile electrodimerization by pinacol coupling
[42,43]. In a strongly acidic medium ([DMFH][OTf]),
electrocatalysis proceeds at a very mild potential (–0.9
V<sub>Fc</sub>) albeit with moderate turnover (TON = 4.4) and a
low selectivity, since a mixture of benzyl alcohol and
dibenzyl ether is obtained in a summed 67% faradaic
efficiency (F.E.). In that case, HT to benzaldehyde
occurs from a ligand-protonated Mo(II)–H intermedi-
ate and is found to be a slow, rate-determining step. Of
remarkable note, the protonation of the hydride is also
likely slow despite the strongly acidic conditions.

In recent works, Siewert's group was the first to repur-
pose CO₂ reduction electrocatalysts for the electro-
chemical hydrogenation of aldehydes and ketones
(Figure 2, b) [23,24]. The seminal example describes a
catalytic system based on a manganese bimetallic com-
plex bearing a H-bonding/proton relay in the second
coordination sphere (phenolic group) [23]. A previous
study had shown that this proton relay facilitates hy-
dride formation [44] and this knowledge was harnessed
to electrogenerate active metal-hydride species that can
transfer to carbonyl compounds, with subsequent elec-
tron and proton transfers inducing catalytic turnover.
This protocol was applied over a wide range of aliphatic
ketones and aldehydes and is tolerant to alkene groups.

In the latest work on the topic, the same group disclosed
a monomeric bipyridine manganese catalyst with
pendant amino groups that performs carbonyl hydroge-
nation with improved performance (Figure 2, c) [24].
This system also inspires from the extensive precedents
on CO₂ electroreduction using catalysts of the manganese bipyridine-based family [45–49]. Here, the catalyst loading could be reduced down to 1 mol%. This performance translates into high TONs (up to 250) and excellent faradaic efficiencies for hydrogenation (>99%) of the model acetone substrate. An extended substrate scope shows good yields to alcohols (from 38 to >99%) and namely includes aromatic ketones. Importantly, relatively low overpotentials were required to achieve the conversions (ca. 200–300 mV) [50]. Using the relatively mild proton source trifluoroethanol (TFE; pKa = 35.4 in MeCN) [46,51] likely contributed to improve the functional-group tolerance and the protocol tolerates carboxylic acids, esters, non-conjugated alkenes or cyclopropane, but also halogeno- or cyano-aryl groups. From electrochemical, UV- and IR-spectroelectrochemical data, electrogenerated Mn hydrides are proposed as key intermediates for hydrogenation, paralleling the reactivity observed for the electroreduction of CO₂ to formate. In particular, a Mn¹ hydride [Mn¹(L)(CO)₃H] (L = derivatized bipyridine; see Figure 2, c), which readily forms with TFE upon 2-electron reduction of the native [Mn¹(L)(CO)₃Br] complex [46], is found as central intermediate. A first mechanism at the potential of this reduction was identified to imply the transfer of the Mn(I) hydride to the carbonyl. The absence of dimeric products (except for activated aryl ketones reducing at the electrode) or ring-opening of cyclopropane in α position to the carbonyl group suggests that this transfer is unlikely to occur *via* radical routes, such as HAT. Instead, the hypothesis of a mechanism coupling HT with protonation (H⁻/H⁺ mechanism) is favored. The absence of Mn-alcoholate intermediates evolving by reaction of a chemically generated analogue Mn–H with acetone suggests an outersphere HT. This H⁻/H⁺ mechanism thus points to a termolecular event between Mn(I)–H, carbonyl and TFE as rate-determining. Such a process would be consistent with the low catalytic rates observed, as namely testified by the absence of catalytic currents on the CV at 100 mV s⁻¹ and can explain that substantial TFE concentrations (2 M) are beneficial for turnover. A second mechanism was identified at more negative potential, which involves the reduction of the Mn(I) hydride before transfer and turns over at faster rates. Interestingly, electrolysis at this higher overpotential does not produce degraded hydrogenation F.E., showing the selectivity of the process, but also does not substantially increase yields and TONs. Thus, operation along the low overpotential mechanism provides a good compromise between activity and functional group tolerance. Of note, although this reactivity pattern is very close to thermal catalytic hydrogenation, such as
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>

Electrocatalytic hydrogenation of carbonyl compounds: systems, key intermediates and a proposed catalytic cycle.

the one with manganese pincer complexes [52,53] this complex shows no reactivity in the presence of H₂ and phenoxide as a base.

All the examples cited above rely on mechanisms based on the generation of metal-hydride species (via 2

e⁻ + H⁺). This strategy seems to be particularly well adapted for polarized π-bonds, in the reminiscence of the ionic H⁻/H⁺ hydrogenations. So far, the approach is effective for aldehydes and ketones, although improvement in terms of energy efficiency (overpotential) and availability of the proton source is still required. These
protocols could subsequently be applied to other func-
tionalities such as imines or even esters and amides,
which are more challenging. We note that catalytic sys-
tems operating *via* alternative mechanisms are
conceivable, especially as the electrochemical perspec-
tive enables monoelectronic transfer regimes, possibly
coupled or concerted with proton transfer. Along that
line, a recent example by Peters *and coll.* demonstrated a
CPET-mediated carbonyl reduction, although not
leading to full hydrogenation but to pinacol coupling
(Figure 2, d) [54].

Electrocatalytic hydrogenation of C–C
bonds

The electrochemical hydrogenation of C—C π-bonds
catalyzed by molecular complexes is also a relatively

nascent field, with the first studies being contemporary.
Compared to the carbonyl case, the hydrogenation of
C—C bonds is thermodynamically much more favorable
(ca. [+0.36; +0.82] V<sub>Fc</sub> in MeCN; *vide supra*, Figure 1, c)
than the reduction of protons, which theoretically sim-
plifies the selectivity of this reaction versus HER. In
2017, Shimakoshi, Hisaeda *and coll.* reported the first
electrohydrogenation of alkenes and alkynes using
catalysis by a vitamin B<sub>12</sub> model cobalt complex
(Figure 3, a) [55]. Based on spin trapping EPR experi-
ments, the mechanism is suggested to involve a
Co(III)—H species that performs HAT on the C—C
unsaturated substrate, which is also corroborated by
the substantial amounts of radical coupling by-products.
In this vein, in 2022, teams led by Baran, Sigman,
Reisman, Minteer, Blackmond and Abruña, and by Lin

<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>

Electrocatalytic hydrogenation of alkynes and alkenes with molecular complexes: systems and proposed catalytic cycles.
and Abruña exploited salen and bipyridine-based cobalt complexes for the electrochemical reduction of terminal alkenes and internal alkynes through the agency of Co–H as HAT mediators (Figure 3, a) [25,26]. These protocols prove highly versatile and afford a range of HAT-type reactivities from hydrogenation of C–C double or triple bonds, to olefin isomerization and hydrofunctionalization. As remarkable features, a broad tolerance of functional groups is obtained (esters, acetylates, ketones, ethers, various N-heterocycles for instance are not hydrogenated). Terminal alkenes are also selectively reduced with respect to internals [25]. However, over-reduction is observed in the case of alkynes [25] and faradaic efficiencies are relatively low. A fine-tuning of the catalytic system, changing from a bipyridine-based to a salen Co catalyst enables switching from a most common (Z)-selective alkyne hydrogenation to an (E)-selective version [25]. The rationale invoked is the relative destabilization of the Co–C bond in the Co-salen vinyl intermediate (BDE([Co<sup>III</sup>(salen)(vinyl)]) = 37.8 < 60.0 kcal mol<sup>-1</sup> ≤ BDE([Co<sup>N</sup>(bpy) (vinyl)]; bond dissociation energy (BDE)), which is thus best described as a pair of metal, vinyl radicals, whose C=C• moiety may be easier to isomerize towards the thermodynamically-favored (E) intermediate. A second HAT from the external proton source may also follow outer-metal sphere. In contrast, an inner-sphere mechanism shuttling *via* cobalt-vinyl protodemetallation is favored, according to computational data, in the case of the (Z)-selective procedure with Co bipyridine-based complexes. The Co-salen series could also be exploited to promote the deuteration of alkenes using cheap and readily available AcOD as deuterating agent [26]. Of note, the HER reactivity of the intermediate Co(III)–H is quenched in the presence of an olefin to which HAT proceeds at a faster rate than homo/heterolytic hydrogen evolution. A general observation is that the Co salen-based hydrides are more akin to react *via* (outer-sphere) HAT into a radical pair in equilibrium with the corresponding metal alkyl/vinyl species, whereas the Co bipyridine-based ones are prone to classical organometallic sequences of C–C π-bond MI into the metal vinyl/alkyl complex.

In the same year, Peters' group presented a system coupling a cobaltocene-derivatized CPET mediator with an homoleptic nickel-bisdiphosphine catalyst for the reduction of the C–C bond of methyl-phenylpropiolate, an activated alkyne [28]. Through the CPET-mediated hydride delivery to the Ni catalyst, the strategy bypasses the reduction of the Ni catalyst otherwise required to generate a competent Ni–H and accessed at a potential for which that hydride is further reduced into a very active HER intermediate. Mechanical studies indicate that the CPET between the mediator and the catalyst is rate-determining. Although not discussed in detail, the HT to the C–C π-bond is

suggested to occur outer-sphere from the Ni(II)–H intermediate (H<sup>-</sup>/H<sup>+</sup> pathway), HAT being likely ruled out on respective BDE arguments. The notably high (E)-olefin formation (Z/E ca. 1:1) also corroborates an outer-sphere pathway.

The same CPET mediator was previously used alone as a catalyst for the electrochemical C=C hydrogenation of fumarate esters (Figure 3, b) [27]. The catalysis develops at the proton-coupled reduction of the cobaltocenium CPET mediator and gives good yield to the hydrogenated product (86%). Extensive mechanistic studies indicate that a CPET between the protonated, reduced mediator and the substrate is at play and is rate-determining at low concentration of the latter. Kinetic data and DFT computations further intimate that the reduction of the thusly evolved succinyl radical likely yields the corresponding anion in strong interaction (*via* π-π stacking) with the oxidized mediator and from which protonation generates the saturated product. Whether this direct CPET-mediated hydrogenation applies to other classes of olefins and C–C π-bonds remains so far open. From this perspective, it is interesting to note that the CPET mediator alone is not effective for alkyne hydrogenation, as reported by the same group [28]. However, a striking difference between the direct use of a CPET mediator and previously discussed systems is that hydrogenative catalysis is here no longer bound to the generation of a catalytically competent metal-hydride species. This study thus opens developments on catalytic electroreduction systems that can demonstrate a reactivity truly distinct from, and complementary to, conventional catalytic hydrogenation protocols.

In that vein, our group has also recently reported the electrochemical hydrogenation of alkynes catalyzed by nickel bipyridine-based complexes (Figure 3, c) [29,30]. Aliphatic or acrylic alkenes are obtained in moderate to high yields (38–93%) and FEs (24–98%) and with high (Z)-stereoselectivity. Our mechanistic evidence supports a hydride-free route. The pathway involves binding of the substrate to an electroreduced Ni(0) complex, producing a nickelacyclopropene species, which we identified as the resting state. Successive proton and electron transfers at that organometallic fragment then induce catalytic turnover. According to kinetics and DFT calculations, an overall ECEC-type mechanism was found as the most favorable pathway and displays a protonation of the nickelacyclopropene into vinyl species as the rate-determining step. Such inner-sphere behaviors are often associated with improved selectivity and concertation. Interestingly, this feature allows to keep overhydrogenation below traces, even at full conversion of the alkyne. This intrinsic semihydrogenation selectivity is likely a consequence of the low affinity of the olefin with the Ni center and is at variance with systems relying on hydride intermediates,
<figure><img src="image_6.png" /><figcaption>Figure 4</figcaption></figure>

Perspectives on the electrocatalytic hydrogenation of π-bonds and beyond with molecular complexes.

which are not selective for semihydrogenation (at
full conversion).

Catalytic pathways are therefore quite varied in the electrochemical hydrogenation of C—C π-bonds, with expedient cobalt-mediated HAT strategies being prominent and more easily accessed for these bonds less polarized than the C=O ones. The exploration of reactivity also encompasses hydride-free routes based either on outer-sphere CPET delivery or genuine organometallic fragments. Bypassing metal-hydride species is also an inherent way to avoid HER secondary reaction. These approaches deserve further exploration, particularly in terms of applicability, given the selectivity advantage.

Conclusion

Molecular electrocatalysis for the hydrogenation of organic π-bonds is still an emerging field, as compared to the related CO₂ electroreduction. Yet, some systems already show promising features in terms of efficiency

and applicability. These systems have both been
inspired by previous electrocatalytic manifolds (HER,
CO₂ER) and pushed forward reactivities (e.g. HAT)
developed under chemical or photochemical conditions
(Figure 4). In some cases, distinct, uncommon
pathways can also provide complementary approaches
to thermal hydrogenations, with the potential to offer,
for example, different selectivities or tolerances
towards functional groups. This latter is particularly
true when the intrinsic reaction mechanism differs
from the catalytic pathway of thermal catalytic
hydrogenation, as with CPET-mediated or hydride-free
catalytic systems. In addition, these approaches may
offer an inherent selectivity over HER. Likely, some of
these protocols will find application in the hydro-
genative chemistry of other unsaturations, such as
imines in reductive amination, esters, amides or even
extend to σ-bond conversions (hydrodeoxygenations or
hydrogenolysis) [56–58]. Opening up to these sub-
strates must be accompanied by a detailed under-
standing of the mechanisms involved and may require
approaches other than those highlighted in this review. More broadly, this research also contributes to fertilization in the fields of electrochemical hydrofunctionalization or reductive couplings, which are accessible in a similar fashion with other electrophilic partners.

## 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.

## Acknowledgments

The authors gratefully acknowledge basic support from the Max Planck Society and the RWTH Aachen University.

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