Bonding insights from structural and spectroscopic comparisons of {SnW5} and {TiW5} alkoxido- and aryloxido-substituted lindqvist polyoxometalates

Incorporation of {MX}ⁿ⁺ groups into polyoxometalates (POMs) provides the means not only to introduce reactivity and functionality but also to tune the electronic properties of the oxide framework by varying M, X and n. In order to elucidate the factors responsible for differences in reactivity between {TiW₅} and {SnW₅} Lindqvist-type hexametalates, a series of alkoxido- and aryloxido-tin substituted POMs (nBu₄N)₃[(RO)SnW₅O₁₈] (R=Me, Et, iPr and tBu) and (nBu₄N)₃[(ArO)SnW₅O₁₈] (Ar=C₆H₅, 4-MeC₆H₅, 4-tBuC₆H₅, 4-HOC₆H₄, 3-HOC₆H₄ and 2-CHOC₆H₄) has been structurally characterised and studied by multinuclear NMR (¹H, ¹³C, ¹⁷O, ¹¹⁹Sn and ¹⁸³W) and FTIR spectroscopy. Spectroscopic and structural parameters were compared with those of titanium-substituted homologues and, when coupled with theoretical studies, indicated that Sn−OR and Sn−OAr bonds are ionic with little π-contribution, whereas Ti−OR and Ti−OAr bonds are more covalent with π-bonding that is more prevalent for Ti−OR than Ti−OAr. This experimental and theoretical analysis of bonding in a homologous series of reactive POMs is the most extensive and detailed to date, and reveals factors which account for significant differences in reactivity between tin and titanium congeners.