Molten Re(VII)-Oxosulfato Complexes in the Re2O7-K2S2O7 System Studied by Raman Spectroscopy at 260-470°C

Dissolution reactions of transition metal oxides (e.g. V2O5, Nb2O5, MoO3, WO3) in molten pyrosulfate, and formation of the corresponding molten metal oxosulfato complexes [1-4] have expanded our knowledge on coordination chemistry of the pertinent transition metals, and provided a very useful family of coordination complexes as reference compounds for understanding the molecular structure of supported metal oxides, commonly used as catalysts [5]. Herein, we show that the oxide of a heptavalent transition metal, Re2O7 (m.p. 297°C), undergoes likewise a reaction-induced dissolution in molten potassium pyrosulfate. Large amounts of Re2O7 can be dissolved in molten K2S2O7 (i.e. a mixture with X(Re2O7) = 0.5 fuses readily at 260°C). The structural and vibrational properties of molten Re(VII)I oxosulfato complexes formed in binary Re2O7-K2S2O7 (as well as ternary Re2O7-K2S2O7-K2SO4) molten mixtures under O2 atmosphere and static equilibrium conditions are studied by Raman spectroscopy at temperatures of 260-470°C. The corresponding composition effects are explored in the X(Re2O7) = 0 - 1 range. A quantitative exploitation of the relative Raman band intensities, due to the species present at static equilibrium, allows to determine the stoichiometry of the reaction taking place in the binary system [Re2O7 + nS2O7(2-) = C(2n-)] pointing to n = 1. Temperature and composition dependent evolutions of molecular structures and configurations are discussed and consistent band assignments are proposed.