# Grafting of Anionic Decahydro-Closo-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties

Manal Diab, Ana C. Mateo, Joumada El Cheikh, Zeinab El Hajj, Mohamed Haouas, Alireza Ranjbari, Vincent Guérineau, David Touboul, Nathalie Leclerc, Emmanuel Cadot, et al.

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Manal Diab, Ana C. Mateo, Joumada El Cheikh, Zeinab El Hajj, Mohamed Haouas, et al. Grafting of Anionic Decahydro-Closo-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties. Molecules, 2022, 27 (22), 10.3390/molecules27227663. [https://doi.org/10.3390/molecules27227663](https://doi.org/10.3390/molecules27227663)

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# Grafting of Anionic Decahydro-Closo-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties

Manal Diab 1,2, Ana Mateo 3 ID, Joumada El Cheikh 4, Zeinab El Hajj 1,2, Mohamed Haouas 1 ID,
Alireza Ranjbari 5 ID, Vincent Guérineau 6, David Touboul 6 ID, Nathalie Leclerc 1, Emmanuel Cadot 1,
Daoud Naoufal 2,* ID, Carles Bo 2,* ID and Sébastien Floquet 1,*

1 Institut Lavoisier de Versailles, CNRS, UVSQ, Université Paris-Saclay, 78035 Versailles, France

2 Laboratory of Organometallic and Coordination Chemistry, LCIO, Faculty of Sciences I, Lebanese University, Hadath 6573, Lebanon

3 Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, 43007 Tarragona, Spain

4 Equipe de Recherche et Innovation en Electrochimie pour l'énergie (ERIEE), Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO), UMR CNRS 8182, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France

5 Institut de Chimie Physique, CNRS, UMR 8000, Université Paris-Saclay, 91405 Orsay, France

6 Institut de Chimie des Substances Naturelles, CNRS UPR2301, Université Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette, France

* Correspondence: dnaoufal@ul.edu.lb (D.N.); cbo@iciq.cat (C.B.); sebastien.floquet@uvsq.fr (S.F.)

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**Citation:** Diab, M.; Mateo, A.; El Cheikh, J.; El Hajj, Z.; Haouas, M.; Ranjbari, A.; Guérineau, V.; Touboul, D.; Leclerc, N.; Cadot, E.; et al. Grafting of Anionic Decahydro-Closo-Decaborate Clusters on Keggin and Dawson-Type Polyoxometalates: Syntheses, Studies in Solution, DFT Calculations and Electrochemical Properties. *Molecules* **2022**, *27*, 7663. https://doi.org/10.3390/molecules27227663

Academic Editor: Xiaobing Cui

Received: 2 October 2022

Accepted: 2 November 2022

Published: 8 November 2022

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**Abstract:** Herein we report the synthesis of a new class of compounds associating Keggin and Dawson-type Polyoxometalates (POMs) with a derivative of the anionic decahydro-*closo*-decaborate cluster [B₁₀H₁₀]²⁻ through aminopropylsilyl ligand (APTES) acting as both a linker and a spacer between the two negatively charged species. Three new adducts were isolated and fully characterized by various NMR techniques and MALDI-TOF mass spectrometry, notably revealing the isolation of an unprecedented monofunctionalized SiW₁₀ derivative stabilized through intramolecular H-H dihydrogen contacts. DFT as well as electrochemical studies allowed studying the electronic effect of grafting the decaborate cluster on the POM moiety and its consequences on the hydrogen evolution reaction (HER) properties.

**Keywords:** polyoxometalate; hybrid; decaborate; DFT; NMR; hydrogen evolution reaction

## 1. Introduction

Polyoxometalates (POMs) and POM-based materials constitute a highly versatile class of compounds rich in more than several thousand inorganic compounds, which can be finely tuned at the molecular level. Because of their stunning compositions, diversified architectures and their rich electrochemical redox behaviors, they are known to display numerous properties or applications in many domains such as supramolecular chemistry [1–3], catalysis [4–6], electro-catalysis [7–9], and medicine, especially when POMs are functionalized with organic groups or complexes [10–13].

On their side, hydroborates represent a wide family of anionic clusters, for which many reports demonstrated their interest in different areas, especially in the biomedical domain [14–18]. This property thus makes the studies of borane derivatives of a great interest. In particular, the [B₁₀H₁₀]²⁻ cluster offers the possibility of various selective functionalizations [19,20] leading for example to *closo*-decaborate-triethoxysilane precursor, which can be coordinated to luminescent dye doped silica nanoparticles, hence facilitating the tracing of the *closo*-decaborate drug pathway in BNCT (Boron Neutron Capture Therapy) [21,22].
Driven by the synthetic challenge that constitutes the association of two anionic species with two complementary redox characters, reductive for hydroborates and oxidative for POMs, and by the biomedical applications which could be reached by associating these two families of compounds, this study aims to find the right strategy to design such POM-borate adducts and to study their chemical properties.

In a previous paper, we demonstrated that it is possible to covalently graft decorate clusters to an Anderson-type polyoxometalate functionalized with the well-known TRIS ligand (TRIS = tris(hydroxymethyl)aminomethane), namely [Mn<sup>III</sup>Mo<sub>6</sub>O<sub>18</sub>(TRIS)<sub>2</sub>]<sup>3-</sup> [23]. Nevertheless, the compound [Mn<sup>III</sup>Mo<sub>6</sub>O<sub>18</sub>(TRIS-B<sub>10</sub>)<sub>2</sub>]<sup>7-</sup> resulting from the coupling between both components revealed to be fragile, probably because of the rigidity of the linker and the close proximity of both anionic components. This weakness is confirmed by DFT calculations indicating an athermic or slightly exothermic process for the formation of the adducts with Anderson-TRIS hybrid POMs.

In the field of hybrid POMs, the organosilyl derivatives of vacant polyoxotungstates as [PW<sub>9</sub>O<sub>34</sub>]<sup>9-</sup>, [SiW<sub>10</sub>O<sub>36</sub>]<sup>8-</sup>, [PW<sub>11</sub>O<sub>39</sub>]<sup>7-</sup>, [SiW<sub>11</sub>O<sub>39</sub>]<sup>6-</sup>, or [P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>]<sup>10-</sup> offer large diversities of compounds exhibiting a wide panel of applications [24,25]. Among them, the divacant POM Keggin [SiW<sub>10</sub>O<sub>36</sub>]<sup>8-</sup> (noted hereafter SiW<sub>10</sub>) and the monovacant POM Dawson [P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>]<sup>10-</sup> (noted hereafter P<sub>2</sub>W<sub>17</sub>) derivatives are probably the most used because of their stability, their topology and the richness of their electrochemical properties in reduction. In particular, by reacting with aminopropyltri(ethoxy)silane (called APTES) they can provide two very useful platforms, noted respectively **SiW<sub>10</sub>-APTES** and **P<sub>2</sub>W<sub>17</sub>-APTES** (see Figure 1), for elaborating functional hybrid molecular architectures.

The aim of this study is to use these two different platforms to prepare new hybrid compounds associating an anionic decorate boron cluster (denoted hereafter B<sub>10</sub>) with Keggin and Dawson POM derivatives. The choice of polyoxotungstate moieties rather than Mo-based POMs is based on its stability towards reduction. The employment of a long and flexible linker as APTES is essential to tackle the challenge of combining efficiently a reduced anionic boron cluster with an anionic oxidized polyoxometalate. The use of APTES linker should limit the repulsion between the two components, while its flexibility allows more easily accommodating the two entities. Finally, as shown in Figure 1, due to monovacant and divacant characters of P<sub>2</sub>W<sub>17</sub> and SiW<sub>10</sub>, respectively, it is worth noting that the relative conformations of the chains are different. For SiW<sub>10</sub>-APTES, the two alkyl chains are oriented nearly in parallel, whereas the monovacancy of P<sub>2</sub>W<sub>17</sub> imposes divergent directions for the two alkyl chains. This topology is well adapted for designing triangular or square molecular species as evidenced by Izzet et al. [2,26], and in our case, we expect that these two kinds of conformation could lead to different types of adducts incorporating B<sub>10</sub> clusters. In this study, we thus report the synthesis, the full characterization in solution by various NMR techniques, the electronic, the electrochemical and the electrocatalytic properties of three new hybrid POMs. In the absence of XRD structures, DFT studies provide a fine structural description of these hybrids and rationalization of their properties.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Molecular structures (DFT-optimized geometry) of (A) **SiW<sub>10</sub>-APTES** and (B) **P<sub>2</sub>W<sub>17</sub>-APTES** platforms highlighting the two different topologies of the APTES linker, and of (C) [B<sub>10</sub>H<sub>9</sub>CO]<sup>-</sup> (X-ray diffraction structure from reference [27]). Legend: C in black, H in white, N in dark blue, Si in pink, O in red, B in blue, WO<sub>6</sub> octahedra in orange and PO<sub>4</sub> tetrahedra in green.</figcaption></figure>

**2. Results and Discussion**

**2.1. Syntheses**

The synthesis of hybrid POMs can be achieved through different strategies. In the present study, the best synthetic procedure to get the targeted hybrid POMs has been to react first the lacunary POMs "SiW<sub>10</sub>" and "P<sub>2</sub>W<sub>17</sub>" with two aminopropyltri(ethoxy)silane molecules (APTES) to give the two POM-APTES precursors (see Figure 2) of formulas (TBA)<sub>3</sub>H[(SiW<sub>10</sub>O<sub>36</sub>)(Si(CH<sub>2</sub>)<sub>3</sub>NH<sub>2</sub>)<sub>2</sub>O]·3H<sub>2</sub>O (denoted hereafter **SiW<sub>10</sub>-APTES**) and (TBA)<sub>5</sub>H[P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>(Si(CH<sub>2</sub>)<sub>3</sub>NH<sub>2</sub>)<sub>2</sub>O]·6H<sub>2</sub>O, denoted hereafter **P<sub>2</sub>W<sub>17</sub>-APTES**. The syntheses of these two precursors were adapted from Mayer at al. [28] by reaction of k<sub>8</sub>(γ-SiW<sub>10</sub>O<sub>36</sub>)·12H<sub>2</sub>O or K<sub>10</sub>α-P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>·20H<sub>2</sub>O with 3-aminopropyltriethoxy silane in presence of TBABr in H<sub>2</sub>O/CH<sub>3</sub>CN medium acidified by concentrated HCl (for more details see experimental section in Supplementary Materials). Note that for each, the proton usually written as counter-cation is in fact probably an ammonium arm R-NH<sub>3</sub><sup>+</sup>.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Evolution of the proportions of the products in the system **SiW<sub>10</sub>-APTES**/B<sub>10</sub>H<sub>9</sub>CO/DIPEA as a function of B<sub>10</sub>H<sub>9</sub>CO/SiW<sub>10</sub>-APTES ratio at fixed DIPEA/B<sub>10</sub>H<sub>9</sub>CO ratio of 2. The proportion of each species are determined by integration of the <sup>29</sup>Si NMR signals. Reproduced with permission from the doctoral thesis manuscript of Dr Manal Diab, University Paris Saclay/Lebanese University, May 2018.</figcaption></figure>
The synthetic strategy to get POM-borate adducts is then to combine the amines of these POM-APTES precursors with the reactive carbonyl of the decaborate cluster $[B_{10}H_9CO]^{-}$ (Figure 1C) to give an amide function connecting both components. Since the boron cluster can react with water for giving a carboxylic acid and since heating the synthetic mixture above 40–50 °C led to some degradation products or to some reduction in the Dawson derivative by the hydrodecaborate cluster, reactions have been conducted at room temperature and under nitrogen atmosphere. Furthermore, the coupling reaction needs the presence of a base both to help the deprotonation of the ammonium arm(s) of the POM-APTES precursors and to trap the proton produced by the coupling reaction. A moderate and a bulky organic base, diisopropylethylamine (DIPEA), was thus used to avoid the competition with APTES for the coupling reaction with $[B_{10}H_9CO]^{-}$.

To quickly circumscribe the optimal conditions for the synthesis of the POM-borate adducts, $^{29}$Si, $^{31}$P and $^{1}$H NMR titrations were conducted by varying the ratios of the three reactants $[B_{10}H_9CO]^{-}$/POM-APTES/DIPEA (all details are given in the Supplementary Materials).

For the $[B_{10}H_9CO]^{-}$/SiW$_{10}$-APTES/DIPEA system, the $^{29}$Si NMR studies in solution reveal that it is possible to modulate the coupling reaction between $[B_{10}H_9CO]^{-}$ and POM-APTES precursors by playing on the amounts of DIPEA and of $[B_{10}H_9CO]^{-}$. For this tri-reactants system, the successive formation of two POM-borate species identified as mono- and di-adduct compounds was demonstrated thanks to their molecular symmetries ($C_s$ versus $C_{2v}$). Besides, the crucial role of DIPEA in the reaction of $[B_{10}H_9CO]^{-}$ with POM-APTES precursors was clearly evidenced. No reaction occurs when no base is used. NMR titration studies allowed establishing that the optimal quantity of base was two equivalents for one equivalent of $[B_{10}H_9CO]^{-}$. The Figure 2 shows for instance the proportions of SiW$_{10}$-derivatives determined by the integration of the different peaks obtained by $^{29}$Si NMR in the system SiW$_{10}$-APTES/$[B_{10}H_9CO]^{-}$/DIPEA as a function of $[B_{10}H_9CO]^{-}$/SiW$_{10}$-APTES ratio at fixed DIPEA/$[B_{10}H_9CO]^{-}$ ratio of 2.

Starting from SiW$_{10}$-APTES, it evidences first the formation of a mono-adduct, which predominates for ratio $B_{10}$/SiW$_{10}$-APTES = 1, before being converted into a di-adduct. The NMR titrations studies allowed establishing that using proportions SiW$_{10}$-APTES/$B_{10}H_9CO$/DIPEA = 1/3/6 lead to the pure di-adduct denoted SiW$_{10}$-diB$_{10}$, while using 1/1/2 ratios lead to around 80% of mono-adduct mixed with some unreacted starting POM and the di-adduct. The separation of compounds has not been possible but considering the effect of the added DIPEA amounts, we succeeded to reduce the formation of the di-adduct and thus to get the mono-adduct compound denoted SiW$_{10}$-monoB$_{10}$ with a good purity by decreasing the quantity of DIPEA in the proportions SiW$_{10}$-APTES/$B_{10}H_9CO$/DIPEA = 1/1/1.5. The Figure 3 summarizes the experimental conditions used to isolate POM-borate adducts.

Similar NMR studies were also performed in solution with the Dawson derivative P$_{2}$W$_{17}$-APTES (see Supplementary Materials). In contrast to SiW$_{10}$ derivatives, the formation of mono- and di-adduct of the Dawson derivative are not so separated as for SiW$_{10}$. Therefore, we failed to isolate the mono-adduct as pure product. Nevertheless, we can obtain quantitatively the di-adduct compound in the reaction mixture when ratios P$_{2}$W$_{17}$-APTES/$B_{10}H_9CO$/DIPEA = 1/3/6 are used.

To summarize, the multistep coupling reactions have successfully been monitored by $^{29}$Si and $^{31}$P NMR, fully described in the Supplementary Materials, revealing that intermediate products can be followed and isolated. From these results, we established the experimental conditions allowing to selectively synthesize with good yields the mono adduct of SiW$_{10}$ POM and the di-adducts of both POMs as mixed TBA$^+$ and DIPEAH$^+$ salts, namely (TBA)$_3$(DIPEAH)$_3$[(SiW$_{10}O_{36}$)(B$_{10}H_9$CONHC$_3$H$_6Si$)(NH$_2C_3H_6Si)O]·3H$_2$O denoted SiW$_{10}$-monoB$_{10}$, (TBA)$_{6.5}$(DIPEAH)$_{1.5}$[(SiW$_{10}O_{36}$)(B$_{10}H_9$CONHC$_3$H$_6Si)_{2}O]·2H$_2$O denoted SiW$_{10}$-diB$_{10}$, and (TBA)$_6$(DIPEAH)$_4$[($P_2W_{17}O_{61}$)(B$_{10}H_9$CONHC$_3$H$_6Si)_{2}O]$·3H$_2$O, denoted P$_{2}$W$_{17}$-diB$_{10}$ (See Experimental Section in Supplementary Materials for more details). All adducts were isolated as powders and were characterized by FT-IR, TGA, elemental analysis, MALDI-TOF and NMR techniques. It should be noted that to our knowledge, SiW$_{10}$-
**monoB**₁₀ is the first example of a POM-APTES monoadduct isolated so far from the direct synthesis. All studies in the literature usually reported di-adducts with such types of hybrid POMs [29–31].

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Scheme of syntheses of POM-borates adducts. The optimal quantities of reactants were determined by NMR titration studies. The reactions are performed in dry acetonitrile, at room temperature under inert atmosphere. Molecular structures are optimized geometry obtained by DFT. Legend: C in black, H in white, N in dark blue, Si in pink, O in red, B in blue, WO₆ octahedra in orange and PO₄ tetrahedra in green.</figcaption></figure>

## 2.2. FT-IR Spectroscopy

FT-IR spectra are given in Figures S11 and S12 in Supplementary Materials. The FT-IR spectra of **SiW**₁₀-*monoB*₁₀, **SiW**₁₀-*diB*₁₀ and **P**₂**W**₁₇-*diB*₁₀ evidence that the integrity of the POM part is maintained compared to the POM-APTES precursors. Furthermore, the association of the [B₁₀H₉CO]⁻ cluster is demonstrated by the disappearance of the carbonyl CO band at 2098 cm⁻¹ in the B₁₀H₉CO⁻ cluster, while the broad band located at 2464–2470 cm⁻¹ typical for B-H vibration bands of the decaborate moiety within the three compounds **SiW**₁₀-*monoB*₁₀, **SiW**₁₀-*diB*₁₀ and **P**₂**W**₁₇-*diB*₁₀ is significantly shifted from that observed at 2517 cm⁻¹ for the [B₁₀H₉CO]⁻ precursor [22,23].

## 2.3. Characterizations by MALDI-TOF Mass Spectrometry

Mass spectrometry (MS) is a very efficient technique for the characterization of polyoxometalates in solution. In our case, we did not succeed in getting mass spectra with reasonable signal-to-noise ratio and exploitable data by the usual electrospray ESI-MS technique. On the contrary, Matrix-Assisted Laser Desorption/Ionization coupled to a Time-of-Flight mass spectrometer (MALDI-TOF) revealed to be an effective technique for hybrid POMs characterization, as shown for example by Mayer and coworkers on “SiW₁₀” and “P₂W₁₇” organosilyl derivatives [28,32]. MALDI-TOF technique is applied on samples which are diluted in a matrix solution (DCTB in our case, DCTB = Trans-2-[3-(4-tert-Butylphenyl)-2-propenylidene] malonitrile) and then co-crystallized on a conductive target. Thanks to a laser irradiation, it allows producing singly charged species (cationic or anionic) and presenting the great advantage to strongly limit the number of peaks in comparison with ESI-MS spectra, where multiply charged species are generated. In the present study, the experiments were performed in both negative and positive modes (see Figure S14 in the
Supplementary Materials for the example of SiW<sub>10</sub>-diB<sub>10</sub>). According to previous works in this field, the best results were obtained in the positive mode, although the anionic character of the POM [28,32]. Indeed, as seen in the Supplementary Materials for SiW<sub>10</sub>-diB<sub>10</sub>, the intensity reached in the negative mode appears lower, but the number of peaks is higher as there are more degradation species. Even thought our systems are polyanionic, they are more efficiently analyzed as monocationic species resulting from adducts between POMs and counter cations such as TBA<sup>+</sup> and H<sup>+</sup> in our case (H<sup>+</sup> coming notably from DIPEAH<sup>+</sup> cations or protonated amines). Furthermore, the monocationic character of the species is confirmed in all cases by the shift between peaks in the isotopic massifs.

The precursor SiW<sub>10</sub>-APTES and the compounds SiW<sub>10</sub>-monoB<sub>10</sub>, SiW<sub>10</sub>-diB<sub>10</sub> and P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub>, were thus analyzed by this technique in the positive mode. The results are gathered in Table S1 (see Supplementary Materials). The full spectra and a zoom on the target compounds with a spectrum simulated with IsoPro3 software are shown in Figure 4 for SiW<sub>10</sub> derivatives and in Figures S15 and S16 for P<sub>2</sub>W<sub>17</sub> ones.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Reflector positive ion MALDI-TOF spectra of (a) SiW<sub>10</sub>-APTES, (b) SiW<sub>10</sub>-monoB<sub>10</sub>, and (c) SiW<sub>10</sub>-diB<sub>10</sub>. Zooms of major peaks in the 3000–5000 *m/z* range are displayed with their respective simulated spectra.</figcaption></figure>

As shown in Figure 4 the spectrum of the precursor **SiW<sub>10</sub>-APTES** (Figure 4a) displays a major peak centered at *m/z* 4087.3 and a minor peak at *m/z* 4328.3. The first peak is assigned to the monocationic species {((TBA)<sub>3</sub>H<sub>2</sub>[(SiW<sub>10</sub>O<sub>36</sub>)O(SiC<sub>3</sub>H<sub>6</sub>NH<sub>2</sub>)<sub>2</sub>](CH<sub>3</sub>CN)<sub>2</sub>(H<sub>2</sub>O)<sub>8</sub>(DCTB)<sub>2</sub>}<sup>+</sup> (calculated *m/z* 4087.3), while the second peak is attributed to the species {((TBA)<sub>4</sub>H[(SiW<sub>10</sub>O<sub>36</sub>)O(SiC<sub>3</sub>H<sub>6</sub>NH<sub>2</sub>)<sub>2</sub>](CH<sub>3</sub>CN)<sub>2</sub>(H<sub>2</sub>O)<sub>8</sub>(DCTB)<sub>2</sub>}<sup>+</sup> (calculated *m/z* 4328.7). The two peaks correspond to the expected hybrid POM associated with some TBA<sup>+</sup> and H<sup>+</sup> cations, some solvates and two molecules of the DCTB matrix. Note that the presence of amines on the APTES part of the POM could probably favor the formation of intermolecular interactions with solvates and DCTB molecules. Such an adduct with DCTB is also observed with the precursor **P<sub>2</sub>W<sub>17</sub>-APTES** (Figure S15, Supplementary Materials) but not seen with the other POMs functionalized with B<sub>10</sub> clusters. The MALDI-TOF spectrum of **P<sub>2</sub>W<sub>17</sub>-APTES** indeed exhibits a major peak corresponding to the expected precursor associated with one molecule of the DCTB matrix at *m/z* 6058.7 (calculated *m/z* 6057.9 for (TBA)<sub>6</sub>H[(P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>)O(SiC<sub>3</sub>H<sub>6</sub>NH<sub>2</sub>)<sub>2</sub>](DCTB)<sup>+</sup>) and a minor peak at *m/z* 6300.0
(calculated *m/z* 6299.4 for (TBA)₇[(P₂W₁₇O₆₁)O(SiC₃H₆NH₂)₂](DCTB)]⁺). The attribution of the peaks is definitely confirmed thanks to the fitting of the isotopic distribution massifs. The latter are mainly due to the isotopic distribution of the 10 or 17 tungsten atoms of the POMs, which appears consistent with the experimental spectrum (see Figure 4a and Figure S15, respectively).

The spectrum of **SiW<sub>10</sub>-monoB<sub>10</sub>** depicted in Figure 4b shows only one experimental peak at *m/z* 3843.2 which is perfectly consistent with the calculated mass for the monocationic product {((TBA)₄H₃[(SiW<sub>10</sub>O₃₆)O(SiC₃H₆NH₂)(SiC₃H₆NHCOB<sub>10</sub>H₉)])(CH₃CN)(H₂O)₃}⁺ (calculated *m/z* 3843.1). It evidences the formation of the expected adduct **SiW<sub>10</sub>-monoB<sub>10</sub>** and thus indirectly the grafting of one (B<sub>10</sub>H<sub>9</sub>CO)⁻ cluster to SiW<sub>10</sub>-APTES. The simulated spectrum agrees well with the experimental data, which supports this assumption although the presence of one B<sub>10</sub> cluster does not modify significantly the isotopic massif.

The MALDI-TOF spectrum of **SiW<sub>10</sub>-diB<sub>10</sub>** shown in Figure 4c displays a major peak centered at *m/z* 4085.8, which fully agrees with the expected di-grafted compound {((TBA)₄H₅[(SiW<sub>10</sub>O₃₆)O(SiC₃H₆NHCOB<sub>10</sub>H₉)₂](CH₃CN)₂(H₂O)₆}⁺ (*m/z* calculated 4084.4) and a minor peak at *m/z* = 4330.2 consistent with the species {((TBA)₅H₄[(SiW<sub>10</sub>O₃₆)O(SiC₃H₆NHCOB<sub>10</sub>H₉)₂](CH₃CN)₃(H₂O)₄}⁺ (*m/z* calculated 4330.8). This result confirms the formation of the expected di-grafted compound.

Finally, the case of **P₂W₁₇-diB₁₀**, appears more complicated, certainly due to a higher charge of the hybrid POM (10-) and a larger surface, which both favor intermolecular interactions with solvent molecules and cations. For technical reasons, the MALDI-TOF spectrum shown in Figure S16 (see Supplementary Materials) was recorded in linear mode, which does not favor the high resolution in contrast with other compounds. The spectrum displays an intense and broad experimental peak centered at *m/z* 6048.1, while four smaller peaks are found, respectively, at *m/z* 6289.6, 6431.7, 6672.5 and 6813.8. All these peaks are consistent with di-grafted species of general formula {((TBA)ₓHᵧ[(P₂W₁₇O₆₁)O(SiC₃H₆NHCOB₁₀H₉)₂](CH₃CN)z(H₂O)t}⁺ (x + y = 11, z = 0–3 and t = 5–6). Regarding the main peak, the latter appears much broader than expected for only one species. Moreover, the resolution of the isotopic massif is lost. In fact, the experimental spectrum likely corresponds to a spectra superimposition of monocationic species of general formula {((TBA)₅H₆[(P₂W₁₇O₆₁)O(SiC₃H₆NHCOB₁₀H₉)₂](CH₃CN)ₓ(H₂O)ᵧ}⁺ with x ranging from 1 to 5 and y from 0 to 8 (*m/z* in the range 6035.70 to 6076.75). Some simulated spectra are given in Figure S16 in Supplementary Materials.

## 2.4. NMR Studies in Solution

In the absence of crystallographic data, the three obtained hybrid systems have been thoroughly characterized by multinuclear NMR spectroscopy in order to verify their structures in solution. ¹H, ¹¹B, ¹³C, ¹⁵N, ²⁹Si, ³¹P, and ¹⁸³W NMR spectra were recorded in CD₃CN at room temperature. The data are gathered in Table S2 (Supplementary Materials), while selected spectra are given in Figures 5 and 6 and in Figures S17–S32 (Supplementary Materials).

As shown in Figure 5a and in Figures S17–S20 (Supplementary Materials), ¹¹B{¹H} NMR spectrum of [B₁₀H₉CO]⁻ undergoes a significant change upon coupling with **SiW<sub>10</sub>-APTES** or **P₂W₁₇-APTES**. In particular, the signal at -44.4 ppm specific for the equatorial boron atom bearing the substituent CO in [B₁₀H₉CO]⁻ (B2 atom, see Figure 1c) is strongly shifted to ca. -25 ppm in the spectra of **SiW<sub>10</sub>-monoB<sub>10</sub>**, **SiW<sub>10</sub>-diB<sub>10</sub>** and **P₂W₁₇-APTES** in agreement with the grafting of the cluster on the POM.
<figure><img src="image_8.png" /><figcaption>**Figure 5.** (a) $^{11}$B{$^{1}$H} NMR spectra of $SiW_{10}$-monoB$_{10}$, $SiW_{10}$-diB$_{10}$ and TBA[B$_{10}$H$_{9}$CO] in CD$_{3}$CN. (b) $^{1}$H NMR spectra of $SiW_{10}$-monoB$_{10}$, $SiW_{10}$-diB$_{10}$ and $SiW_{10}$-APTES in CD$_{3}$CN. * indicates the signal of the protonated amine DIPEAH+ present as a counter-cation. Reproduced with permission from the doctoral thesis manuscript of Dr Manal Diab, University Paris Saclay/Lebanese University, May 2018.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 6. (a) <sup>183</sup>W NMR spectra of SiW<sub>10</sub>-monoB<sub>10</sub>, SiW<sub>10</sub>-diB<sub>10</sub> and SiW<sub>10</sub>-APTES in CD<sub>3</sub>CN. (b) <sup>183</sup>W NMR spectra of P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub> and P<sub>2</sub>W<sub>17</sub>-APTES in CD<sub>3</sub>CN.</figcaption></figure>

Concomitantly, the ¹H NMR spectrum of the mono adduct SiW<sub>10</sub>-monoB<sub>10</sub>, exhibits a splitting of the signals for the three methylene groups –CH<sub>2</sub>– of the APTES linker, denoted a, b, c (see Figure 5b), because of the lowering of the symmetry of SiW<sub>10</sub>-APTES from C<sub>2v</sub> to C<sub>s</sub>. In addition, a new peak at 6.12 ppm assigned to an amide function is observed.
For the remaining amine function, a broad signal is observed at 7.4 ppm (d), but together with two other broad signals at 5.70 and 6.33 ppm (d' and d''), attributed to the amine function in a frozen configuration in which the interaction with B₁₀ cluster generates two inequivalent protons as depicted in Figure 7a (DFT optimized structure). These assumptions are confirmed by ¹H-¹⁵N HMBC (Heteronuclear Multiple Bond Correlation) NMR spectrum (Figure S25) revealing two ¹⁵N signals at -251 ppm (amide) correlated to the proton signal at 6.12 ppm and at -272 ppm (free amine) correlated to the two protons peaks at 5.70 and 6.33 ppm.

<figure><img src="image_10.png" /><figcaption>**Figure 7.** Optimized molecular structures of the POM-borates derivatives. **SiW<sub>10</sub>-monoB<sub>10</sub>** (a) in «closed» form and (b) in «open» form; (c,d) two views of the most stable configuration of **SiW<sub>10</sub>-diB<sub>10</sub>**; (e,f) two views of the most stable configuration of **P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub>**. Dashed lines are given for shortest H-H contacts. Legend: C in black, H in white, B in blue, N in dark blue, Si in pink, WO<sub>6</sub> octahedra in orange and PO<sub>4</sub> tetrahedra in green.</figcaption></figure>
Grafting a second $[B_{10}H_9CO]^-)$ group on the **SiW$_{10}$-APTES** platform allows recovering the $C_{2v}$ symmetry and thus one set of peaks was observed for the linker and especially the protons “a”, in addition to an amide peak at 5.94 ppm (Figure S24, Supplementary Materials). The signals of the amine at 7.4, 5.7, and 6.3 ppm disappear in agreement with the reaction of $[B_{10}H_9CO]^-$ groups with this function. Similarly, the $^1H$ NMR spectrum of $P_2W_{17}-diB_{10}$ compared to that of $P_2W_{17}-APTES$ (Figure S27, Supplementary Materials) evidences the appearance of a sharp peak at 5.94 ppm assigned to an amide function, while the signal of the free amine at 7.03 ppm in the precursor $P_2W_{17}-APTES$ disappears.

To further confirm our assignments of the signal of the amide function, $^1H$-$^1H$ ROESY (Rotating frame Overhause Effect SpectroscopY) and $^{13}$C NMR experiments were performed on $SiW_{10}-diB_{10}$ and $P_2W_{17}-diB_{10}$ (Figures S28–S32, Supplementary Materials). Cross REO peaks involving the amide proton (5.94 ppm in both compounds) and some equatorial B-H protons of the $B_{10}$ cluster at 0.4 ppm and the protons “c” of the APTES chains can be seen in Figures S28 and S29 (Supplementary Materials). This demonstrates the spatial proximity between these protons that interact between each other through dipolar contacts. $^{13}$C NMR spectra of $SiW_{10}-monoB_{10}$, $SiW_{10}-diB_{10}$, and $P_2W_{17}-diB_{10}$ (Figures S30–S32) notably exhibits a signal at 203 ppm assigned to a carbon atom from an amide group, which is confirmed by 2D $^1H-$$^{13}$C HMBC NMR spectrum of $SiW_{10}-monoB_{10}$ evidencing a correlation between this $^{13}$C signal at 203 ppm and the $^1H$ amide signal at 6.12 ppm. Besides, in both cases of $SiW_{10}-monoB_{10}$ and $SiW_{10}-diB_{10}$ this $^{13}$C signal appears as a poorly resolved quadruplet with a coupling constant of 95 Hz consistent with a $^1J_{13C-11B}$ coupling.

Therefore, $^1H$, $^1H-$$^{15}$N HMBC, $^1H-$$^1H$ ROESY, $^{13}$C and $^1H-$$^{13}$C HMBC NMR experiments unambiguously confirm the formation of an amide group in our three adducts by reaction of the amines of POM-APTES precursors with the carbonyl group of the cluster $[B_{10}H_9CO]^-$. The modification of the $^{11}B${$^1H}$ NMR spectra of the boron cluster after its reaction with the POM-APTES precursors further confirms such results.

$^{29}$Si, $^{31}$P and $^{183}$W NMR probe the POM part in compounds $SiW_{10}-APTES$, $SiW_{10}-monoB_{10}$, $SiW_{10}-diB_{10}$, $P_2W_{17}-APTES$ and $P_2W_{17}-diB_{10}$ (see Figure 6 and Figures S21–S23 in Supplementary Materials). The unsymmetrical environment in the mono adduct $SiW_{10}-monoB_{10}$ is clearly confirmed through the appearance of two peaks for Si of the different linker arms at –61.9 and –63.3 ppm, while only one signal was observed at –62.3 ppm for the symmetrical di adduct $SiW_{10}-diB_{10}$ with only a small shift from the initial $SiW_{10}-APTES$ precursor (–62.5 ppm). In addition, for all the compounds, a single peak is observed for the Si atom in the central cavity of the $SiW_{10}$ POM moiety which is almost not affected by the grafting of the boron clusters and the resulting changes of symmetry of the adducts (Figure S21, Supplementary Materials). In case of $P_2W_{17}-APTES$ and $P_2W_{17}-diB_{10}$, both compounds exhibit only one signal assigned to the two equivalent Si atoms of the APTES linker (Figure S22, Supplementary Materials).

The $^{183}$W NMR spectra of precursors and adducts are given in Figure 6. For $SiW_{10}-monoB_{10}$ the $^{183}$W NMR spectrum displays five peaks of intensities 2:2:2:2:2 in agreement with the expected low $C_s$ symmetry, while three resonances are observed for the di adduct $SiW_{10}-diB_{10}$ and the initial precursor $SiW_{10}-APTES$ of intensities (4:2:4) consistent with their $C_{2v}$ symmetry (Figure 6a). Figure 6b shows the $^{183}$W NMR spectrum of $P_2W_{17}-diB_{10}$ which differs significantly from its precursor. Both compounds exhibit nine NMR lines of integration 2:2:2:1:2:2:2:2:2 in agreement with the expected $C_s$ symmetry, but their positions are slightly changed. This is due to the modification of the $P_2W_{17}$ moiety induced by the grafting of the two $[B_{10}H_9CO]^-$ clusters. Additionally, $^{31}$P NMR spectra of $P_2W_{17}-APTES$ and $P_2W_{17}-diB_{10}$ display two signals (Figure S23, Supplementary Materials), wherein one of them showed a common chemical shift, while the second exhibited a small shift from –13.4 ppm in $P_2W_{17}-APTES$ to –13.6 ppm in $P_2W_{17}-diB_{10}$.

In conclusion, these experiments focused on the POM part fully agree in terms of molecular symmetries with the formation of the expected mono- or di-adducts with $B_{10}$ clusters.
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Energetic profiles of the formation of mono- and di-adduct from the starting precursors in CD₃CN. (a) SiW₁₀-APTES, SiW₁₀-monoB₁₀ (open and closed isomers), and SiW₁₀-diB₁₀; (b) P₂W₁₇-APTES, P₂W₁₇-monoB₁₀, and P₂W₁₇-diB₁₀.</figcaption></figure>

The computed reaction free energies for the **SiW<sub>10</sub>-APTES** and P₂W₁₇-APTES derivatives are fully consistent with the experimental findings. For the keggin derivatives, the strong stabilization of the mono-adduct allows isolating both mono and di-adduct thanks to the formation of strong H-H contacts. In contrast, for the Dawson derivatives, the small difference of energies between mono- and bi-adducts (1.1 eV only) does not permit isolating the mono-adduct. Besides, an excess of [B₁₀H₉CO]⁻ (3 equivalents/POM instead of 2) is needed to get the pure di-adduct compound to avoid the formation of a mixture between mono and di-grafted adducts. A similar situation was previously obtained with
Anderson-type derivatives since the mono and di-adduct of B₁₀ with [MnMo₆(Tris)₂]³⁻ are only separated by 6 eV and it was not possible to get mono-adduct [22]. This result highlights the role of the topology of the POM-APTES compounds and their faculty to stabilize species thanks to intramolecular interactions.

Finally, DFT studies provided the frontier orbitals for each compound (see Figure 9 and Figure S33 in Supplementary Materials). The results obtained for Keggin and Dawson derivatives exhibit the same feature. For POM-APTES the HOMO is located on one (for **SiW**₁₀**-APTES**) or two (**P**₂**W**₁₇**-APTES**) amines of the APTES part, while the LUMO are localized on the W atoms of the POM part. By grafting the B₁₀ clusters, the LUMO levels are slightly affected. LUMO remains localized on W atoms and only minor changes in energy are observed.

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Frontier orbitals energies and energy gaps (eV) for the **SiW**₁₀**-APTES**, **SiW**₁₀**-monoB**₁₀ and **SiW**₁₀**-diB**₁₀ species. Color code: W green, O red, Si light brown, B dark brown, C grey, N blue, H white; HOMO: red/blue; LUMO: orange/cyan. MO surfaces plotted at a 0.03 isovalue.</figcaption></figure>

Conversely, the HOMO levels are drastically modified by the introduction of B₁₀ clusters. Electrons of the HOMO orbitals are now mainly localized on one grafted B₁₀ cluster. Interestingly, for **SiW**₁₀**-monoB**₁₀ the HOMO is delocalized between one B₁₀ cluster and the amine of the second arm, which strongly interacts with the B₁₀ through H-H contacts. The HOMO energy level increases in all cases within the range 0.78 to 0.92 eV. The HOMO-LUMO gaps, are thus significantly reduced upon the B₁₀ grafting. Indeed, for **SiW**₁₀**-APTES**, the gap decreases from 1.67 to 0.90 eV for the first substitution, and to 0.94 eV for the second. For **P**₂**W**₁₇**-APTES, the gap evolves from 1.56 to 0.77 eV for the first substitution, and to 0.76 eV for the second.

## 2.6. Electrochemical Properties

The electronic spectra of compounds **SiW**₁₀**-monoB**₁₀, **SiW**₁₀**-diB**₁₀ and **P**₂**W**₁₇**-diB**₁₀ recorded in CH₃CN containing 0.1 mol.L⁻¹ TBACIO₄ (TBAP, tetrabutylammonium perchlorate)
at room temperature and 2.10⁻⁴ mol.L⁻¹ concentration are depicted in Figures S35 and S36 (Supplementary Materials).

The electronic spectra of precursors **SiW<sub>10</sub>-APTES** and **P<sub>2</sub>W<sub>17</sub>-APTES** display absorption bands in ultraviolet region corresponding to transition between p-orbitals of the oxo ligands and d-type orbitals centered on tungsten [33,34], while the cluster [B<sub>10</sub>H<sub>9</sub>CO]<sup>-</sup> exhibits weak absorption band between 300 and 200 nm notably assigned to π-π* transitions [35]. Considering that the main contribution of the spectra comes from the LMCT band involving the W atoms and that the LUMO band centered on tungsten atoms are only slightly modified upon grafting of B<sub>10</sub> cluster, no drastic changes are expected in the POM-B<sub>10</sub> adducts. Indeed, the electronic spectra of **SiW<sub>10</sub>-monoB<sub>10</sub>** and **P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub>** match well with the sum of the spectra of **SiW<sub>10</sub>-APTES** or **P<sub>2</sub>W<sub>17</sub>-APTES** and one or two times that of [B<sub>10</sub>H<sub>9</sub>CO]<sup>-</sup>, respectively. The spectrum of **SiW<sub>10</sub>-diB<sub>10</sub>** slightly differs from the sum of the component's spectra probably due to a larger variation of LUMO energy level from **SiW<sub>10</sub>-APTES** to **SiW<sub>10</sub>-diB<sub>10</sub>** and additional constraints due to the vicinity of the two boron clusters.

Since no evolution of the spectra were observed within 24 h in such a medium, the compounds appear chemically stable in these experimental conditions. The cyclic voltammograms (CVs) were thus recorded for all the SiW<sub>10</sub> and P<sub>2</sub>W<sub>17</sub> derivatives and are given in Figure 10 and in Figures S37 and S38 (Supplementary Materials), while the anodic and cathodic potentials are gathered in Table S3 (Supplementary Materials).

<figure><img src="image_13.png" /><figcaption>**Figure 10.** Comparison of cyclic voltammograms (a) for the three compounds **SiW<sub>10</sub>-APTES**, **SiW<sub>10</sub>-monoB<sub>10</sub>** and **SiW<sub>10</sub>-diB<sub>10</sub>**, and (b) for **P<sub>2</sub>W<sub>17</sub>-APTES** and **P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub>** in the reduction part. The electrolyte was CH<sub>3</sub>CN + 0.1 M TBACIO<sub>4</sub>. Dashed lines are only guide for eyes.</figcaption></figure>

As depicted in Figure 10a and Figure S37, the CV of **SiW<sub>10</sub>-APTES** is poorly resolved and it is difficult to identify confidently all the reduction processes corresponding to the successive reduction in W<sup>V</sup> centers into W<sup>V</sup>, well-known to be monoelectronic in non-aqueous solvents [36]. These waves seem nevertheless reversible with processes better resolved in oxidation. Besides, an irreversible process attributed to the oxidation of amine function of the APTES linker is also observed in oxidation around +0.452 V vs. Fc<sup>+</sup>/Fc
(see Supplementary Materials). As for their parent precursor, CVs of **SiW**<sub>10</sub>-**monoB**<sub>10</sub> and **SiW**<sub>10</sub>-**diB**<sub>10</sub> display poorly resolved reversible electronic transfers, which appear shifted towards more negative potentials and one irreversible oxidation process around +0.452 V vs. Fc<sup>+</sup>/Fc assigned to the oxidation of the remaining amine function and/or of the B<sub>10</sub> cluster (see Figure S39, Supplementary Materials). This behavior agrees with the increase in the charge from 4- in **SiW**<sub>10</sub>-**APTES** to 6- in **SiW**<sub>10</sub>-**monoB**<sub>10</sub> and 8- in **SiW**<sub>10</sub>-**diB**<sub>10</sub> and the electron donating character of the boron cluster [37] but does not evidence a strong electronic effect of the boron cluster on the POMs electronic properties.

The CVs of **P**<sub>2</sub>**W**<sub>17</sub>-**APTES** and **P**<sub>2</sub>**W**<sub>17</sub>-**diB**<sub>10</sub> are given in Figure 10b and in Figure S38 (Supplementary Materials) and appears much more resolved than those of SiW<sub>10</sub> derivatives. The CV of **P**<sub>2</sub>**W**<sub>17</sub>-**APTES** displays four reversible electronic transfers with cathodic peak potentials assigned to successive mono- or bi-electronic reductions of W<sup>VI</sup> centers into W(+V) [38] and two irreversible oxidation processes at E<sub>pa</sub> = +0.452 and +0.759 V, assigned to the oxidation of the terminal amine groups of the APTES linkers. Conversely to the di-adduct compound **SiW**<sub>10</sub>-**diB**<sub>10</sub>, the CVs of the Dawson derivative **P**<sub>2</sub>**W**<sub>17</sub>-**diB**<sub>10</sub> give four reversible reduction processes significantly shifted towards the more positive potential compared to **P**<sub>2</sub>**W**<sub>17</sub>-**APTES** and one irreversible reduction process at E<sub>pc</sub> = -2.030 V vs. Fc<sup>+</sup>/Fc, which was not observed in the precursor. The opposite effect was expected. This effect probably results from a combination of a charge effect, the presence of protons (in DIPEAH+ cations) and of an electronic effect of boron cluster on P<sub>2</sub>W<sub>17</sub> moiety but at this stage it is difficult to have a clear explanation of the contribution of all these effects which can be antagonist.

Although reduction waves in the Dawson derivatives are not very well resolved, it can be observed that the di-substituted species (green line in Figure 10) is reduced at lower potentials than **P**<sub>2</sub>**W**<sub>17</sub>-**APTES**, in agreement with the fact that the LUMO and LUMO+1 raise in energy upon B<sub>10</sub> attachment. Also, the successive reduction waves seem just shifted left, which would conform with the almost constant difference in the LUMO and LUMO+1 energies along the series.

## 2.7. Electrocatalytic Properties for the Reduction in Protons into Hydrogen (HER)

Many POMs are known to catalyze protons reduction into hydrogen in aqueous or in non-aqueous conditions [7,39,40]. We verified by UV-Vis spectroscopy that [B<sub>10</sub>H<sub>9</sub>CO]<sup>-</sup> and its adducts with POMs are stable in CH<sub>3</sub>CN in the presence of excess acetic acid (20 equivalents). In these conditions, it was interesting to study the reactivity of these compounds in regard to the electro-catalytic reduction in protons into hydrogen. The experiments were performed in CH<sub>3</sub>CN + 0.1 M TBAP by using acetic acid as a source of protons, and as a weak acid in such a medium (pKa = 22.3) [41].

Figure 11 and Figure S40 (Supplementary Materials) show the evolution of CVs upon stepwise addition of acetic acid up to 20 equivalents of acid/POM for all P<sub>2</sub>W<sub>17</sub> and SiW<sub>10</sub> derivatives, respectively. For all the compounds, the addition of acetic acid, gives a new irreversible reduction wave, which grows gradually with the amount of acid, expressed as γ = [acid]/[POM]. As shown in Figure 11 and Figure S40, at a given potential of -2.2 V vs Fc<sup>+</sup>/Fc, a linear dependence of the catalytic current versus γ is obtained, a behavior featuring the electro-catalytic reduction of protons. However, the effect of the addition of acetic acid in the solution appears stronger for P<sub>2</sub>W<sub>17</sub> derivatives than for SiW<sub>10</sub> based compounds.
<figure><img src="image_14.png" /><figcaption>**Figure 11.** Cyclic voltammograms of (a) P₂W₁₇-APTES and (b) P₂W₁₇-diB₁₀ after addition of variable amounts of acetic acid. (c) Plots of the cathodic currents measured at −2.2 V vs Fc+/Fc as a function of the ratio [acid]/[POM] for P₂W₁₇-APTES and P₂W₁₇-diB₁₀. (d) Comparison of HER with and without catalyst P₂W₁₇-diB₁₀, and with platinum after addition of an excess of acetic acid corresponding to the quantity added for a ratio [acid]/[POM] = 20. In all cases, the electrolyte was CH₃CN + 0.1 M TBACIO₄. The reference electrode was a saturated calomel electrode (SCE). Reproduced with permission from the doctoral thesis manuscript of Dr Manal Diab, University Paris Saclay/Lebanese University, May 2018.</figcaption></figure>

To evidence the electrocatalytic process, linear voltammetry of P₂W₁₇-diB₁₀ in the presence of 20 equivalents of acetic acid was performed and compared to similar experiments performed without catalyst or on platinum electrode (Figure 11d). We notice that in the presence of the catalyst P₂W₁₇-diB₁₀, the current density is almost doubled and there is a 250-mV overvoltage decrease compared to the solution without catalyst. Indeed, the proton reduction with respect to platinum starts at −1.400 V vs. Fc+/Fc, while it starts at −1.750 V vs. Fc⁺/Fc with catalyst and at −2.000 V vs. Fc⁺/Fc without catalyst. Finally, the formation of hydrogen is unambiguously demonstrated by gas chromatography analysis during electrolysis performed at −2.200 V vs Fc+/Fc during 4.5 h (Figure S40, Supplementary Materials).

To compare the efficiency of all compounds, the catalytic efficiency (CAT) can be estimated using Equation (1):

$$CAT = \frac{100 * (J_{(POM+20 \text{ eq. CH3COOH})} - J_{(POM \text{ alone})})}{J_{(POM \text{ alone})}} \quad (1)$$

Table 1 summarizes the CAT values measured for our products at −2.2 V vs Fc+/Fc. Interestingly, the two precursors SiW₁₀-APTES and P₂W₁₇-APTES exhibit similar efficiency. Also, the efficiency of SiW₁₀-monoB₁₀ and SiW₁₀-diB₁₀ adducts are lower than that of SiW₁₀-APTES, while it is the opposite for P₂W₁₇-diB₁₀, which appears much more efficient than its parent precursor, in agreement with cyclic voltammetry experiments. Indeed, a less negative reduction potential of the POM part should facilitate the electro-catalytic reduction in protons.
<table><caption>**Table 1.** Electrocatalytic efficiency for the reduction of protons into hydrogen at E = -2.2 V vs. Fc+/Fc for 20 equivalents of CH<sub>3</sub>COOH added in CH<sub>3</sub>CN.</caption><thead><tr><th>Compound</th><th>Catalytic Efficiency (%)</th></tr></thead><tbody><tr><td>SiW<sub>10</sub>-APTES</td><td>827</td></tr><tr><td>SiW<sub>10</sub>-monoB<sub>10</sub></td><td>524</td></tr><tr><td>SiW<sub>10</sub>-diB<sub>10</sub></td><td>548</td></tr><tr><td>P<sub>2</sub>W<sub>17</sub>-APTES</td><td>854</td></tr><tr><td>P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub></td><td>1340</td></tr></tbody></table>

In terms of mechanism, three key steps have to be considered: the protonation, the reduction in the catalyst and the transfer of electron towards the protons to give dihydrogen. For protonation step, since catalysis is observed in all compounds, it must occur on the most basic sites, either on the oxo groups of the POM moiety, on the free amine groups in SiW<sub>10</sub>-APTES and P<sub>2</sub>W<sub>17</sub>-APTES or on boron clusters for SiW<sub>10</sub>-diB<sub>10</sub> and P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub>. DFT calculations evidence that the most nucleophilic sites are found on the oxo ligands of the POM parts which are consequently the preferential sites for protonation (see Figure 12 and Figure S34 in Supplementary Materials).

<figure><img src="image_15.png" /><figcaption>**Figure 12.** Two views of the molecular electrostatic potential in atomic units (a.u.) projected onto an electron density isosurface (0.03 e·au<sup>-3</sup>) for P<sub>2</sub>W<sub>17</sub>-APTES, P<sub>2</sub>W<sub>17</sub>-monoB<sub>10</sub> and P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub> species.</figcaption></figure>

For the reduction step, as seen in Figure S41c,d in Supplementary Materials, during electrolysis, the P<sub>2</sub>W<sub>17</sub> derivatives turned to blue as expected for the reduction in such species before returning back colorless when the current is stopped indicating that the reduced POM probably transfers electrons to protons to produce hydrogen. We understand well that if this reduction occurs at higher potential, it should favor the process. P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub> is thus logically the most efficient compound.

To sum up, even if the decaborate cluster is probably not directly involved in the HER process, it plays two indirect roles: (1) the covalent grafting on POMs increases the electronic density on the POM which should facilitate the protonation step, and (2) the covalent grafting can modifies the reduction potential of the POM moieties in POM-borate adducts, which favors the reduction step of the POM species when shifted towards more positive potentials as observed in P<sub>2</sub>W<sub>17</sub>-diB<sub>10</sub>.

### 3. Conclusions

In this work, we succeeded in combining covalently the anionic [B<sub>10</sub>H<sub>9</sub>CO]<sup>-</sup> cluster with anionic SiW<sub>10</sub> and P<sub>2</sub>W<sub>17</sub> derivatives using functionalized silyl derivatives (APTES) as a linker. The coupling between the two families of anionic parts appears much stronger
than that with Anderson-type POMs we previously reported [23] and detailed NMR study allowed establishing the optimized conditions for the synthesis of target compounds. Hence, the selective isolation of mono- and di-adduct compounds of boron cluster with **SiW<sub>10</sub>-APTES**, namely [(SiW<sub>10</sub>O<sub>36</sub>)(B<sub>10</sub>H<sub>9</sub>CONHC<sub>3</sub>H<sub>6</sub>Si)(NH<sub>2</sub>C<sub>3</sub>H<sub>6</sub>Si)O]<sup>6-</sup> and [(SiW<sub>10</sub>O<sub>36</sub>)(B<sub>10</sub>H<sub>9</sub>CONHC<sub>3</sub>H<sub>6</sub>Si)<sub>2</sub>O]<sup>8-</sup> was successfully achieved, while only the di-adduct [(P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>)(B<sub>10</sub>H<sub>9</sub>CONHC<sub>3</sub>H<sub>6</sub>Si)<sub>2</sub>O]<sup>10-</sup> was isolated with **P<sub>2</sub>W<sub>17</sub>-APTES**. To the best of our knowledge, it is the first time that a mono-adduct can be isolated directly from the synthesis by functionalization of the **SiW<sub>10</sub>-APTES** precursor. DFT studies supported by experimental NMR data evidenced that the formation of intramolecular H-H dihydrogen contact is the driving force for the preferred formation of the mono-adduct species and such a synthetic strategy could open the route toward the formation of hybrid POMs with two different functional groups.

All these compounds were fully characterized by multi-NMR techniques including ¹H, ¹¹B, ¹³C, ¹⁵N, ²⁹Si, ¹⁸³W and ³¹P as well as multi-dimensional correlations such as COSY, HMBC (¹H-¹³C and ¹H-¹⁵N) and ROESY NMR allowing focusing on each part of the adducts, i.e., POM, linker and boron cluster. These characterizations demonstrated unambiguously the formation of the targeted adducts and were also consistent with FT-IR and MALDI-TOF spectrometry data. DFT studies permitted to get optimized structures for all compounds consistent with the NMR data.

The electrochemical studies allowed studying the electronic effects of the grafting of the reducing boron cluster on some oxidized POMs with probable antagonist effect between charge effect and the variation of frontiers orbitals levels upon grafting of B₁₀ cluster. Finally, electro-catalytic reduction in protons into hydrogen was evidenced for these systems, the best efficiency being obtained with **P₂W₁₇-dibB₁₀**. The process appears mainly effective on the POM part while the boron cluster participates only indirectly to the process.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27227663/s1. Experimental section including general methods, the ²⁹Si, ³¹P and ¹H NMR titration studies (Figures S1–S10) and the syntheses of compounds used in this study; FT-IR spectra (Figures S11 and S12); MALDI-TOF data (Table S1) and spectra (Figures S13–S16); a summary of NMR data (Table S2), ¹¹B NMR studies (Figures S17–S20), ²⁹Si NMR spectra (Figures S21 and S22), ³¹P NMR spectra (Figure S23), ¹H NMR studies including COSY, ROESY and ¹H-¹⁵N HMBC experiments (Figures S24–S29); ¹³C NMR studies (Figures S30–S32); a DFT part including computational details, frontiers orbitals of P₂W₁₇ derivatives (Figure S33) and electron density maps on SiW₁₀ derivatives (Figure S34); electronic spectra (Figures S35 and S36), additional cyclovoltammograms (Figures S37–S39) and table of potentials (Table S3); Evidence of the production of hydrogen and picture of the reduced POM during the HER process (Figures S40 and S41).

**Author Contributions:** Synthesis and solution studies, M.D. and Z.E.H.; elemental analyses, N.L.; supervision of NMR studies, M.H.; MALDI-TOF experiments, V.G. and D.T.; DFT Calculation, A.M. and C.B.; electrochemical studies, J.E.C. and A.R.; supervision of the work and funding acquisition, E.C., D.N. and S.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a public grant overseen by the French National Research Agency as part of the «Investissements d'Avenir» program (Labex Charm3at, ANR-11-LABX-0039-grat), by the Paris Ile-de-France Region-DIM "Respore", and by a mobility program PHC CEDRE (project POMBORON n°42237UG). M.D. thanks AZM association and IUF for financial support and Z.E.H. thanks CampusFrance for Eiffel doctoral grant. We also thank CERCA Program of the Generalitat de Catalunya, the ICIQ Foundation, and the Spanish Ministerio de Ciencia e Innovación through project PID2020-112806RB-I00 and through the Severo Ochoa Excellence Accreditation 2020-2023 (CEX2019-000925-S, MCI/AEI).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** A data set collection of computational results is available in the ioChem-BD repository and can be accessed via https://dx.doi.org/10.19061/iochem-bd-1-217 (accessed on 1 November 2022).
**Acknowledgments:** We acknowledge the Centre National de la Recherche Scientifique (CNRS) and the Ministère de l'Éducation Nationale de l'Enseignement Supérieur, de la Recherche et de l'Innovation (MESRI) for their financial support. This study results from an international collaboration supported by IRN-CNRS 2019–2023.

**Conflicts of Interest:** The authors declare no conflict of interest.

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