Preparation of α1- and α2-isomers of mono-Ru-substituted Dawson-type phosphotungstates with an aqua ligand and comparison of their redox potentials, catalytic activities, and thermal stabilities with Keggin-type derivatives

Abstract

Both the α₁- and the α₂-isomers of mono-ruthenium (Ru)-substituted Dawson-type phosphotungstates with terminal aqua ligands, [α₁-P₂W₁₇O₆₁Ru^III(H₂O)]⁷⁻ (α₁-RuH₂O) and [α₂-P₂W₁₇O₆₁Ru^III(H₂O)]⁷⁻ (α₂-RuH₂O), were prepared in pure form by cleavage of the Ru–S bond of the corresponding DMSO derivatives, [α₁-P₂W₁₇O₆₁Ru(DMSO)]⁸⁻ (α₁-RuDMSO) and [α₂-P₂W₁₇O₆₁Ru(DMSO)]⁸⁻ (α₂-RuDMSO), respectively. Redox studies indicated that α₁-RuH₂O and α₂-RuH₂O show proton-coupled electron transfer (PCET), and the Ru^III(H₂O) species was reversibly reduced to Ru^II(H₂O) species and oxidized to Ru^IV(O) species and further to Ru^V(O) species in aqueous solution depending on the pH. Their redox potentials and thermal stabilities were compared with those of the corresponding α-Keggin-type derivatives ([α-XW₁₁O₃₉Ru(H₂O)]ⁿ⁻; X = Si⁴⁺ (n = 5), Ge⁴⁺ (n = 5), or P⁵⁺ (n = 4)). The basic electronic and redox features of Ru(L)-substituted Keggin- and Dawson-type heteropolytungstates (with L = H₂O or O²⁻) were analyzed by means of density functional calculations. Similar to the corresponding α-Keggin-type derivatives, both α₁-RuH₂O and α₂-RuH₂O show catalytic activity for water oxidation.