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

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.

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 [1,2,3], catalysis [4,5,6], electro-catalysis [7,8,9], and medicine, especially when POMs are functionalized with organic groups or complexes [10,11,12,13].

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 [14,15,16,17,18]. 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 decaborate clusters to an Anderson-type polyoxometalate functionalized with the well-known TRIS ligand (TRIS = tris(hydroxymethyl)aminomethane), namely [Mn^IIIMo₆O₁₈(TRIS)₂]³⁻ [23]. Nevertheless, the compound [Mn^IIIMo₆O₁₈(TRIS-B₁₀)₂]⁷⁻ 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 [PW₉O₃₄]⁹⁻, [SiW₁₀O₃₆]⁸⁻, [PW₁₁O₃₉]⁷⁻, [SiW₁₁O₃₉]⁶⁻, or [P₂W₁₇O₆₁]¹⁰⁻ offer large diversities of compounds exhibiting a wide panel of applications [24,25]. Among them, the divacant POM Keggin [SiW₁₀O₃₆]⁸⁻ (noted hereafter SiW₁₀) and the monovacant POM Dawson [P₂W₁₇O₆₁]¹⁰⁻ (noted hereafter P₂W₁₇) 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 SiW₁₀-APTES and P₂W₁₇-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 decaborate boron cluster (denoted hereafter B₁₀) 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 P₂W₁₇ and SiW_10, respectively, it is worth noting that the relative conformations of the chains are different. For SiW₁₀-APTES, the two alkyl chains are oriented nearly in parallel, whereas the monovacancy of P₂W₁₇ 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 B₁₀ 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.

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 “SiW₁₀” and “P₂W₁₇” with two aminopropyltri(ethoxy)silane molecules (APTES) to give the two POM-APTES precursors (see Figure 2) of formulas (TBA)₃H[(SiW₁₀O₃₆)(Si(CH₂)₃NH₂)₂O]·3H₂O (denoted hereafter SiW₁₀-APTES) and (TBA)₅H[P₂W₁₇O₆₁(Si(CH₂)₃NH₂)₂O]·6H₂O, denoted hereafter P₂W₁₇-APTES. The syntheses of these two precursors were adapted from Mayer at al. [28] by reaction of k₈(γ-SiW₁₀O₃₆)·12H₂O or K₁₀α–P₂W₁₇O₆₁·20H₂O with 3-aminopropyltriethoxy silane in presence of TBABr in H₂O/CH₃CN 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-NH₃⁺.

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 [B₁₀H₉CO]⁻ (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 [B₁₀H₉CO]⁻.

To quickly circumscribe the optimal conditions for the synthesis of the POM-borate adducts, ²⁹Si, ³¹P and ¹H NMR titrations were conducted by varying the ratios of the three reactants [B₁₀H₉CO]⁻/POM-APTES/DIPEA (all details are given in the Supplementary Materials).

For the [B₁₀H₉CO]⁻/SiW₁₀-APTES/DIPEA system, the ²⁹Si NMR studies in solution reveal that it is possible to modulate the coupling reaction between [B₁₀H₉CO]⁻ and POM-APTES precursors by playing on the amounts of DIPEA and of [B₁₀H₉CO]⁻. 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 [B₁₀H₉CO]⁻ 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 [B₁₀H₉CO]⁻. The Figure 2 shows for instance the proportions of SiW₁₀-derivatives determined by the integration of the different peaks obtained by ²⁹Si NMR in the system SiW₁₀-APTES/[B₁₀H₉CO]⁻/DIPEA as a function of [B₁₀H₉CO]⁻/SiW₁₀-APTES ratio at fixed DIPEA/[B₁₀H₉CO]⁻ ratio of 2.

Starting from SiW₁₀-APTES, it evidences first the formation of a mono-adduct, which predominates for ration B₁₀/SiW₁₀-APTES = 1, before being converted into a di-adduct. The NMR titrations studies allowed establishing that using proportions SiW₁₀-APTES/B₁₀H₉CO/DIPEA = 1/3/6 lead to the pure di-adduct denoted SiW₁₀-diB₁₀, 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 SiW₁₀-monoB₁₀ with a good purity by decreasing the quantity of DIPEA in the proportions SiW₁₀-APTES/B₁₀H₉CO/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 P₂W₁₇-APTES (see Supplementary Materials). In contrast to SiW₁₀ derivatives, the formation of mono- and di-adduct of the Dawson derivative are not so separated as for SiW₁₀. 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 P₂W₁₇-APTES/B₁₀H₉CO/DIPEA = 1/3/6 are used.

To summarize, the multistep coupling reactions have successfully been monitored by ²⁹Si and ³¹P 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 SiW₁₀ POM and the di-adducts of both POMs as mixed TBA⁺ and DIPEAH⁺ salts, namely (TBA)₃(DIPEAH)₃[(SiW₁₀O₃₆)(B₁₀H₉CONHC₃H₆Si)(NH₂C₃H₆Si)O]·3H₂O denoted SiW₁₀-monoB₁₀, (TBA)_6.5(DIPEAH)_1.5[(SiW₁₀O₃₆)(B₁₀H₉CONHC₃H₆Si)₂O]·2H₂O denoted SiW₁₀-diB₁₀, and (TBA)₆(DIPEAH)₄[(P₂W₁₇O₆₁)(B₁₀H₉CONHC₃H₆Si)₂O]·3H₂O, denoted P₂W₁₇-diB₁₀ (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, SiW₁₀-monoB₁₀ 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 [29,30,31].

2.2. FT-IR Spectroscopy

FT-IR spectra are given in Figures S11 and S12 in Supplementary Materials. The FT-IR spectra of SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀ and P₂W₁₇-diB₁₀ 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 SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀ and P₂W₁₇-diB₁₀ 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-ter-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 SiW₁₀-diB₁₀). 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 SiW₁₀-diB₁₀, 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⁺ and H⁺ in our case (H⁺ coming notably from DIPEAH⁺ 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 SiW₁₀-APTES and the compounds SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀ and P₂W₁₇-diB₁₀, 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 SiW₁₀ derivatives and in Figures S15 and S16 for P₂W₁₇ ones.

As shown in Figure 4 the spectrum of the precursor SiW₁₀-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)₃H₂[(SiW₁₀O₃₆)O(SiC₃H₆NH₂)₂](CH₃CN)₂(H₂O)₈(DCTB)₂}⁺ (calculated m/z 4087.3), while the second peak is attributed to the species {(TBA)₄H[(SiW₁₀O₃₆)O(SiC₃H₆NH₂)₂](CH₃CN)₂(H₂O)₈(DCTB)₂}⁺ (calculated m/z 4328.7). The two peaks correspond to the expected hybrid POM associated with some TBA⁺ and H⁺ 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 P₂W₁₇-APTES (Figure S15, Supplementary Materials) but not seen with the other POMs functionalized with B₁₀ clusters. The MALDI-TOF spectrum of P₂W₁₇-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)₆H[(P₂W₁₇O₆₁)O(SiC₃H₆NH₂)₂](DCTB)}⁺) 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 SiW₁₀-monoB₁₀ 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[(SiW₁₀O₃₆)O(SiC₃H₆NH₂)(SiC₃H₆NHCOB₁₀H₉)](CH₃CN)(H₂O)₃}⁺ (calculated m/z 3843.1). It evidences the formation of the expected adduct SiW₁₀-monoB₁₀ and thus indirectly the grafting of one (B₁₀H₉CO)⁻ cluster to SiW₁₀-APTES. The simulated spectrum agrees well with the experimental data, which supports this assumption although the presence of one B₁₀ cluster does not modify significantly the isotopic massif.

The MALDI-TOF spectrum of SiW₁₀-diB₁₀ 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[(SiW₁₀O₃₆)O(SiC₃H₆NHCOB₁₀H₉)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[(SiW₁₀O₃₆)O(SiC₃H₆NHCOB₁₀H₉)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)_xH_y[(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)_y}⁺ 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 Figure 5 and Figure 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 SiW₁₀-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 SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀ and P₂W₁₇-APTES in agreement with the grafting of the cluster on the POM.

Concomitantly, the ¹H NMR spectrum of the mono adduct SiW₁₀-monoB₁₀, exhibits a splitting of the signals for the three methylene groups –CH₂- of the APTES linker, denoted a, b, c (see Figure 5b), because of the lowering of the symmetry of SiW₁₀-APTES from C_2v to C_s. 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.

Grafting a second [B₁₀H₉CO]⁻ group on the SiW₁₀-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 [B₁₀H₉CO]⁻ groups with this function. Similarly, the ¹H NMR spectrum of P₂W₁₇-diB₁₀ compared to that of P₂W₁₇-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 P₂W₁₇-APTES disappears.

To further confirm our assignments of the signal of the amide function, ¹H-¹H ROESY (Rotating frame Overhause Effect SpectroscopY) and ¹³C NMR experiments were performed on SiW₁₀-diB₁₀ and P₂W₁₇-diB₁₀ (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 B₁₀ 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. ¹³C NMR spectra of SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀, and P₂W₁₇-diB₁₀ (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-¹³C HMBC NMR spectrum of SiW₁₀-monoB₁₀ evidencing a correlation between this ¹³C signal at 203 ppm and the ¹H amide signal at 6.12 ppm. Besides, in both cases of SiW₁₀-monoB₁₀ and SiW₁₀-diB₁₀ this ¹³C signal appears as a poorly resolved quadruplet with a coupling constant of 95 Hz consistent with a ¹J_13C-11B coupling.

Therefore, ¹H, ¹H-¹⁵N HMBC, ¹H-¹H ROESY, ¹³C and ¹H-¹³C 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 [B₁₀H₉CO]⁻. The modification of the ¹¹B{¹H} NMR spectra of the boron cluster after its reaction with the POM-APTES precursors further confirms such results.

²⁹Si, ³¹P and ¹⁸³W NMR probe the POM part in compounds SiW₁₀-APTES, SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀, P₂W₁₇-APTES and P₂W₁₇-diB₁₀ (see Figure 6 and Figures S21–S23 in Supplementary Materials). The unsymmetrical environment in the mono adduct SiW₁₀-monoB₁₀ 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 SiW₁₀-diB₁₀ with only a small shift from the initial SiW₁₀-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 SiW₁₀ 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 P₂W₁₇-APTES and P₂W₁₇-diB₁₀, both compounds exhibit only one signal assigned to the two equivalent Si atoms of the APTES linker (Figure S22, Supplementary Materials).

The ¹⁸³W NMR spectra of precursors and adducts are given in Figure 6. For SiW₁₀-monoB₁₀ the ¹⁸³W 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 SiW₁₀-diB₁₀ and the initial precursor SiW₁₀-APTES of intensities (4:2:4) consistent with their C_2v symmetry (Figure 6a). Figure 6b shows the ¹⁸³W NMR spectrum of P₂W₁₇-diB₁₀ 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 P₂W₁₇ moiety induced by the grafting of the two [B₁₀H₉CO]⁻ clusters. Additionally, ³¹P NMR spectra of P₂W₁₇-APTES and P₂W₁₇-diB₁₀ 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 P₂W₁₇-APTES to −13.6 ppm in P₂W₁₇-diB₁₀.

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 B₁₀ clusters.

2.5. Computational Studies

The molecular geometries of SiW₁₀-APTES, SiW₁₀-monoB₁₀, and SiW₁₀-diB₁₀, as well as those of P₂W₁₇-APTES, P₂W₁₇-monoB₁₀ and P₂W₁₇-diB₁₀ were fully optimized at a DFT level including implicit solvent effects (see Figure 7 and Supplementary Materials for computational details). We considered the most relevant plausible conformers. Firstly, regarding SiW₁₀-APTES, it exhibits two main conformers as defined by the orientation of the two amine organic arms, which we called them open and closed forms. The small difference in their relative energy, less than 1 kcal·mol⁻¹ in favor of the closed form (represented in Figure 1a), forecasted that further substitution would easily overcome any initial geometric preference in the reactants. Indeed, upon B₁₀ incorporation a much more complex situation arises. For SiW₁₀-monoB₁₀ we characterized five conformers, two arising from the closed reactant and three species from the open reactant form. In the most stable conformer (Figure 7a), which arise from the closed form, the decaborate moiety interacts favorably with the amine hydrogens (d’ and d’’) of the unreacted arm through strong dihydrogen contacts. In the most stable open form (Figure 7b), although interaction between arms is almost neglected, the H amide atom develops other interactions. Overall, the most stable conformer given in Figure 7a is 11 kcal·mol⁻¹ below the second one (Figure 7b). All five conformers lie in a narrow 20 kcal·mol⁻¹ range. For the double substituted SiW₁₀-diB₁₀, since the additional repulsion arising from the negatively charged B₁₀ groups, we could only characterize two forms: an open (not shown) and a closed one (two views on Figure 7c,d). The energy difference between both species was computed to be just only 5 kcal·mol⁻¹. We highlight, as dashed lines in Figure 7, those hydrogen atoms of the organic arm and the B₁₀ moiety that lie close in three-dimensional space.

Due to the monovacant character of P₂W₁₇, the Si-O-Si angle of the APTES moiety grafted to the POM strongly differs from that observed for the divacant SiW₁₀ POM (see Figure 1). The topology of the two arms, and thus the connectivity of the two POM-APTES derivatives, strongly differs. Therefore, as seen in Figure 1, closed form is not possible for P₂W₁₇-APTES. Only one geometry could thus be considered. Then, for the mono-substituted P₂W₁₇-monoB_10, two conformers were characterized, one open and one folded, the folded one being more stable by only 2.2 kcal·mol⁻¹. For the di-substituted Dawson derivative, only one open shaped product could be characterized (see Figure 7e,f).

Hydrogen atoms of the decaborate moieties possess a hydride character. Consequently, they can establish hydrogen-hydrogen contacts with protic solvent or with functional groups like amines or amides. In the present structures, many H-H dihydrogen contacts between the amine organic arms from APTES moiety and hydrogen atoms from decaborate clusters were observed. For instance, for SiW₁₀-monoB₁₀ (Figure 7a), the hydrogen atoms d’ and d’’ from the «free» amino group are found 2.21 and 1.99 Å far from an H atom belonging to the B₁₀ cluster, which are quite short distances. This fact agrees with the couplings observed in the NMR experiments.

The thermodynamics of the formation of the mono- and di-adducts of the Keggin and the Dawson species is computed exergonic in all cases as seen in Figure 8. For SiW₁₀-APTES, both the formation of the mono- and bi-derivative were computed exergonic, 22.1 kcal·mol⁻¹ for the SiW₁₀-monoB₁₀, and 13.0 kcal·mol⁻¹ for the SiW₁₀-diB₁₀. The formation of SiW₁₀-monoB₁₀ is clearly favored and the strong dihydrogen contacts between amino group and the grafted B₁₀ cluster undoubtedly strongly stabilize such a species compared to the di-adduct SiW₁₀-diB₁₀. For P₂W₁₇-APTES, also the two substitutions are favorable, 6.8 kcal·mol⁻¹ for the first, and 7.9 kcal·mol⁻¹ for the second.

The computed reaction free energies for the SiW₁₀-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 SiW₁₀-APTES) or two (P₂W₁₇-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.

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 SiW₁₀-monoB₁₀ 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 SiW₁₀-APTES, the gap decreases from 1.67 to 0.90 eV for the first substitution, and to 0.94 eV for the second. For P₂W₁₇-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 SiW₁₀-monoB₁₀, SiW₁₀-diB₁₀ and P₂W₁₇-diB₁₀ recorded in CH₃CN containing 0.1 mol.L⁻¹ TBAClO₄ (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 SiW₁₀-APTES and P₂W₁₇-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 [B₁₀H₉CO]⁻ 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 B₁₀ cluster, no drastic changes are expected in the POM-B₁₀ adducts. Indeed, the electronic spectra of SiW₁₀-monoB₁₀ and P₂W₁₇-diB₁₀ match well with the sum of the spectra of SiW₁₀-APTES or P₂W₁₇-APTES and one or two times that of [B₁₀H₉CO]⁻, respectively. The spectrum of SiW₁₀-diB₁₀ slightly differs from the sum of the component’s spectra probably due to a larger variation of LUMO energy level from SiW₁₀-APTES to SiW₁₀-diB₁₀ 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 SiW₁₀ and P₂W₁₇ 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).

As depicted in Figure 10a and Figure S37, the CV of SiW₁₀-APTES is poorly resolved and it is difficult to identify confidently all the reduction processes corresponding to the successive reduction in W^VI centers into W^V, 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⁺/Fc (see Supplementary Materials). As for their parent precursor, CVs of SiW₁₀-monoB₁₀ and SiW₁₀-diB₁₀ display poorly resolved reversible electronic transfers, which appear shifted towards more negative potentials and one irreversible oxidation process around +0.452 V vs. Fc⁺/Fc assigned to the oxidation of the remaining amine function and/or of the B₁₀ cluster (see Figure S39, Supplementary Materials). This behavior agrees with the increase in the charge from 4- in SiW₁₀-APTES to 6- in SiW₁₀-monoB₁₀ and 8- in SiW₁₀-diB₁₀ 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 P₂W₁₇-APTES and P₂W₁₇-diB₁₀ are given in Figure 10b and in Figure S38 (Supplementary Materials) and appears much more resolved than those of SiW₁₀ derivatives. The CV of P₂W₁₇-APTES displays four reversible electronic transfers with cathodic peak potentials assigned to successive mono- or bi-electronic reductions of W^VI centers into W(+V) [38] and two irreversible oxidation processes at E_pa = +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 SiW₁₀-diB₁₀, the CVs of the Dawson derivative P₂W₁₇-diB₁₀ give four reversible reduction processes significantly shifted towards the more positive potential compared to P₂W₁₇-APTES and one irreversible reduction process at E_pc = −2.030 V vs. Fc⁺/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 P₂W₁₇ 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 P₂W₁₇-APTES, in agreement with the fact that the LUMO and LUMO+1 raise in energy upon B₁₀ 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 [B₁₀H₉CO]⁻ and its adducts with POMs are stable in CH₃CN 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 CH₃CN + 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 P₂W₁₇ and SiW₁₀ 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⁺/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 P₂W₁₇ derivatives than for SiW₁₀ based compounds.

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):

$$CAT=\frac{100\ast \left(J_{\left(POM+20 eq. CH3COOH\right)}-J_{\left(POM alone\right)}\right)}{J_{\left(POM alone\right)}}$$

(1)

Table 1. Electrocatalytic efficiency for the reduction of protons into hydrogen at E = −2.2 V vs. Fc+/Fc for 20 equivalents of CH₃COOH added in CH₃CN.

Compound	Catalytic Efficiency (%)
SiW10-APTES	827
SiW10-monoB10	524
SiW10-diB10	548
P2W17-APTES	854
P2W17-diB10	1340

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.

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 SiW₁₀-APTES and P₂W₁₇-APTES or on boron clusters for SiW₁₀-diB₁₀ and P₂W₁₇-diB₁₀. 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).

For the reduction step, as seen in Figure S41c,d in Supplementary Materials, during electrolysis, the P₂W₁₇ 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. P₂W₁₇-diB₁₀ 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 P₂W₁₇-diB₁₀.

3. Conclusions

In this work, we succeeded in combining covalently the anionic [B₁₀H₉CO]⁻ cluster with anionic SiW₁₀ and P₂W₁₇ 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 SiW₁₀-APTES, namely [(SiW₁₀O₃₆)(B₁₀H₉CONHC₃H₆Si)(NH₂C₃H₆Si)O]⁶⁻ and [(SiW₁₀O₃₆)(B₁₀H₉CONHC₃H₆Si)₂O]⁸⁻ was successfully achieved, while only the di-adduct [(P₂W₁₇O₆₁)(B₁₀H₉CONHC₃H₆Si)₂O]¹⁰⁻ was isolated with P₂W₁₇-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 SiW₁₀-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-diB₁₀. The process appears mainly effective on the POM part while the boron cluster participates only indirectly to the process.