Syntheses, characterizations and properties of [Mo2O2S2]-based oxothiomolybdenum wheels incorporating bisphosphonate ligands

Abstract

We report the syntheses and characterizations of the first polyoxothiometalate complexes isolated from the reaction of the oxothiocationic [Mo^V₂O₂S₂]²⁺ precursor and bisphosphonate ligands H₂O₃PCR(OH)PO₃H₂ (R = C₄H₅N₂, zoledronic acid; R = C₃H₆NH₂, alendronic acid). [(Mo₂O₂S₂(H₂O))₄(O₃PC(O)(C₄H₆N₂)PO₃)₄]⁸⁻ (Mo₈S₈(Zol)₄) and [(Mo₂O₂S₂(H₂O))₄(O₃PC(O)(C₃H₆NH₃)PO₃)₄]⁸⁻ (Mo₈S₈(Ale)₄) contain four Mo^V dimers connected via bisphosphonate ligands. These compounds offer a unique opportunity to compare the structures and properties of cyclic compounds obtained with [Mo₂O₂S₂]²⁺ and with [Mo₂O₄]²⁺. The oxothio compounds appear less stable in solution than the oxo analogue, confirming the higher lability and versatility of [Mo₂O₂S₂]-based compounds compared to [Mo₂O₄]-based POMs. Multinuclear and multidimensional solid-state NMR studies were carried out to complement X-ray diffraction analysis. Information on short-range interactions, dynamic behaviors, and local disorder within the crystalline materials are therefore reported. Furthermore, the electrocatalytic properties of Mo₈S₈(Ale)₄ and of the analogous [(Mo₂O₄(H₂O))₄(O₃PC(O)(C₃H₆NH₃)PO₃)₄]⁸⁻ (Mo₈O₈(Ale)₄) immobilized onto the surface of a glassy carbon electrode were studied, thus evidencing the ability of [Mo₂O₂S₂]-based cycles to promote the reduction of protons into hydrogen, whereas the oxo analogue appeared inactive.