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†
Both the α1- and the α2-isomers of mono-ruthenium (Ru)-substituted Dawson-type phosphotungstates with terminal aqua ligands, [α1-P2W17O61RuIII(H2O)]7− (α1-RuH2O) and [α2-P2W17O61RuIII(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 RuIII(H2O) species was reversibly reduced to RuII(H2O) species and oxidized to RuIV(O) species and further to RuV(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.