Nonaqueous Polyoxometalate Synthesis for Systematic Studies of Hydrolysis, Protonation, and Reduction

Errington, RJ

doi:10.1016/bs.adioch.2016.12.010

Nonaqueous Polyoxometalate Synthesis for Systematic Studies of Hydrolysis, Protonation, and Reduction

Lookup NU author(s): Dr John Errington ORCiD

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Abstract

Dynamic aspects of polyoxometalate chemistry are often overlooked because of difficulties in monitoring subtle structural and/or speciation changes in solution. This chapter features the non-aqueous approaches to polyoxometalate synthesis being developed in our laboratory as a platform for detailed and systematic reactivity studies. Methods based on metal alkoxide hydrolysis are described that provide access to a range of substituted {M₂W₄} and {MW₅} Lindqvist-type polyoxometalates containing reactive {MX}ⁿ⁺ heterometal sites, and examples are given that highlight the importance of ¹⁷O NMR as a tool for monitoring solution reactivity. Lindqvist-type {XTiW₅} and {XSnW₅} and related Keggin-type {XMPW₁₁} anions are compared and significant differences in their hydrolysis and condensation behaviours are discussed in relation to DFT studies. The solution reactivity observed upon halide abstraction from {ClTiW₅} or protonation of {(RO)TiW₅} species is rationalized in terms of a common di-functional reactive intermediate [TiW₅O₁₈]^2–, which has been characterized in its dimeric form. Following the identification of di-protonated [(µ-O)(TiW₅O₁₈H)₂]^4– a new type of polyoxometalate ligand behavior is proposed, based on the characterization of [(µ-O)(TiW₅O₁₈)₂(SnMe₂)]^4–, ¹⁷O NMR studies of reactions with electrophiles and DFT studies of [(µ-O)(TiW₅O₁₈)₂(AuCl)]^4–. The final section describes non-aqueous reductive aggregation reactions of (TBA)₃[PMo₁₂O₄₀] that have produced a family of electron-rich anions of the type a-[PMo₁₂O₄₀(ML_n)_z]^x–, and the reductive degradation of (TBA)₂[Mo₆O₁₉] to give the tetra-capped e-[Mo₁₃O₄₀H₄{Mg(dmso)₃}₄]²⁺.