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 (nBu₄ N)₃ [(RO)SnW₅ O₁₈ ] (R=Me, Et, iPr and tBu) and (nBu₄ N)₃ [(ArO)SnW₅ O₁₈ ] (Ar=C₆ H₅ , 4-MeC₆ H₅ , 4-tBuC₆ H₅ , 4-HOC₆ H₄ , 3-HOC₆ H₄ and 2-CHOC₆ H₄ ) has been structurally characterised and studied by multinuclear NMR (¹ H, ¹³ C, ¹⁷ O, ¹¹⁹ Sn and ¹⁸³ W) 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.