Capping the Electronic Lone Pair of the As(III) Central Atom in the Keggin-Type Anion: From Experimental–Theoretical Interplay to Evidence

The stereochemistry of the polycondensation processes within the Keggin-type derivatives is mainly governed by the nature of the assembly group X, being either tetrahedral or trigonal. Commonly, the use of trigonal units {XO₃} gives rise to open molecular structures resulting from connections of {XM₉} subunits through metal-oxo cationic fragments. Nevertheless, we show here that condensation of the vanadate VO₃^- onto the B-type isomer α-[AsW₉O₃₃]^9- led to the saturated mixed-metal Keggin derivative α-[AsW₉V₃O₃₉]^6-. Experimental evidence was provided by multinuclear nuclear magnetic resonance (NMR) characterization, including ⁵¹V, ¹⁸³W, and ¹⁷O NMR, supported by XPS analysis revealing unambiguously the fingerprints of the As(III) atoms compared to those of As(V). Moreover, this unexpected structural feature is fully consistent with the versatile coordination of the V(V) atoms, which makes it possible for the μ₃-O bridging oxygen of the {V₃} cap to be absent in front of the electronic lone pair of the As(III) ion. In addition, DFT studies give consistency to the electronic structure of this new Keggin-type derivative. At last, structural, spectroscopic, and electrochemical properties of this new version of the Keggin structure were compared to other archetypal vanadium-containing anions and then discussed with regard to the geometry and vicinity of the vanadium atoms.