*Article* **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, Joumada El Cheikh 4, Zeinab El Hajj 1,2, Mohamed Haouas 1, Alireza Ranjbari 5, Vincent Guérineau 6, David Touboul 6, Nathalie Leclerc 1, Emmanuel Cadot 1, Daoud Naoufal 2,\*, Carles Bo 2,\* and Sébastien Floquet 1,\***

	- <sup>3</sup> Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, 43007 Tarragona, Spain
	- <sup>4</sup> 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

**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] <sup>2</sup><sup>−</sup> 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
