Protonolysis and Condensation Reactions of Alkoxido-Substituted Lindqvist {MW5} and Keggin {MPW11} Polyoxometalates: Comparative Experimental and Modeling Studies

An understanding of proton transfer and migration at the surfaces of solid metal oxides and related molecular polyoxometalates (POMs) and metal alkoxides is crucial for the development of reactivity involving protonation or the absorption/binding of water. In this work, the hydrolysis of alkoxido Ti- and Sn-substituted Lindqvist [(MeO)MW₅O₁₈]^3– (M = Ti, 1; M = Sn, 2) and Keggin [(MeO)MPW₁₁O₃₉]^4– (M = Ti, 3; M = Sn, 4) type polyoxometalates (POMs) to hydroxido derivatives and subsequent condensation to μ-oxido species has been investigated in detail to provide insight into proton transfer reactions in these molecular metal oxide systems. Solution NMR studies revealed the dependence of reactions not only on the nature of the heteroatom (Ti or Sn) but also on the type of lacunary (W₅ or PW₁₁) POM and also on the solvent (MeCN or DMSO). Tin-substituted anions 2 and 4 were much more susceptible to protonolysis than the Ti analogues 1 and 3 while reactions of {MW₅} anions were generally faster than those of the {MPW₁₁} anions. Subsequent condensation of the resulting hydroxido derivatives [(HO)MW₅O₁₈]^3– (M = Ti, 5; M = Sn, 6) and [(HO)MPW₁₁O₃₉]^4– (M = Ti, 7; M = Sn, 8) was significantly more facile for 5 and 7 and, in all cases, condensation was inhibited in DMSO. Quantitative comparisons of equilibria and reaction rates were provided by analysis of NMR kinetic experiments, while DFT calculations on these and the analogous {NbW₅} reactions provided comparative energetics and reaction profiles that are consistent with experimental observations. These results add to the fundamental understanding of proton migration in metal alkoxide hydrolysis/condensation and related reactions at metal oxide surfaces.