Di-oxo and tri-oxo Re(VII)-oxosulfato complexes in the Re2O7-K2S2O7 molten system. Molecular structure, vibrational properties and temperature-dependent interconversion

The reaction between Re2O7 and K2S2O7 in the molten phase is studied by Raman spectroscopy under static equilibrium oxidative conditions at temperatures of 260-470 degrees C and in the full composition range, X-Re2O7(0)= 0 - 1. Raman band intensity correlations are adequate for inferring a 1:1 stoichiometry for the complex formation reaction. Depending on temperature and on composition of the mixture, two isomeric Re-VII-oxosulfato complexes are formed: (a) ReO3SO4-, prevalent at low temperature (T < 300 degrees C), where penta-coordinated Re-VII takes on a tri-oxo termination configuration; and (b) (ReO2)(2)O-2 (SO4)(2)(2-), dominant at high temperature (T > 360 degrees C), with hexa-coordinated Re-VII in di-oxo termination configuration. The characteristic symmetric and antisymmetric stretching modes (v(s)/v(as)) of the Re( = O)(3)(+) and Re( = O)(2)(3+) cores are found at 975/938 cm(-1) and 989/944 cm(-1) for the tri-oxo and di-oxo sites, respectively. A fully reversible temperature-dependent 2ReO(3)SO(4)(-) (I) <--> (ReO2)(2)O-2(SO4)(2)(2)(l) interconversion equilibrium is evidenced. Consistent molecular structural models are proposed for the ReO3SO4- and (ReO2)(2)O-2(SO4)(2)(2-) complexes as well as for their associated/polymeric counterparts, (ReO3SO4)(n)(n-) and ((ReO2)(2)O-2(SO4)(2))(n)(2n-). Molten Re2O7 reacts with K2SO4 according to Re2O7 (l) + 2SO(4)(2-) -> ReO4- (l) + S2O72- (l). The Re-VII-oxosulfato complexes and their configurational, structural and vibrational properties are of interest as references for inferring corresponding properties for dispersed amorphous oxo-Re-VII species in supported rhenia catalysts.