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

Abstract The reaction between Re 2 O 7 and K 2 S 2 O 7 in the molten phase is studied by Raman spectroscopy under static equilibrium oxidative conditions at temperatures of 260–470 °C and in the full composition range, X R e 2 O 7 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) ReO 3 SO 4 − , prevalent at low temperature (T VII takes on a tri-oxo termination configuration; and (b) (ReO 2 ) 2 O 2 (SO 4 ) 2 2- , dominant at high temperature (T>360 °C), with hexa-coordinated Re VII in di-oxo termination configuration. The characteristic symmetric and antisymmetric stretching modes ( ν s / ν a s ) 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 − ( l ) ↔ (ReO 2 ) 2 O 2 (SO 4 ) 2 2- ( l ) interconversion equilibrium is evidenced. Consistent molecular structural models are proposed for the ReO 3 SO 4 − and (ReO 2 ) 2 O 2 (SO 4 ) 2 2- complexes as well as for their associated/polymeric counterparts, (ReO 3 SO 4 ) n n- and ((ReO 2 ) 2 O 2 (SO 4 ) 2 ) n 2n- . Molten Re 2 O 7 reacts with K 2 SO 4 according to Re 2 O 7 ( l ) + 2SO 4 2- → ReO 4 − ( l ) + S 2 O 7 2- ( 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.