**3. Conclusions**

In this work, we succeeded in combining covalently the anionic [B10H9CO]<sup>−</sup> 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<sup>−</sup> and [(SiW10O36)(B10H9CONHC3H6Si)2O]8<sup>−</sup> was successfully achieved, while only the di-adduct [(P2W17O61)(B10H9CONHC3H6Si)2O]10<sup>−</sup> 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.

**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'Education 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.
