Kinetic studies of triphenylphosphine substitution of sulfur-coordinated thiophenes in Cp'(CO)2Re(Th): a model for thiophene adsorption onto hydrodesulfurization catalysts

Abstract

The mechanism(s) of thiophene hydrodesulfurization on heterogeneous catalysts has been suggested to be S- or eta{sup 5}-bound forms for thiophene adsorption. Much evidence has been reported to support the eta{sup 5}-coordination and activation in organometallic complexes, but little has been previously reported to support S-coordination. Results of studies of the mechanisms and comparison of the rates of substitution of a series of methyl-substituted thiophenes by PPh{sub 3} are reported herein. It was found that the rate of PPh{sub 3} substitution of various thiophenes in the complex Cp'(CO){sub 2}Re(Th) (Cp'=eta{sup 5}-C{sub 5}H{sub 5} and Th= T, 2-MeT, 3-MeT, 2,5-Me{sub 2}T, Me{sub 4}T, and dibenzothiophene) were all independent of the PPh{sub 3} concentration and follow a first-order rate law. This suggests that the reaction proceeds by a dissociative mechanism, and that the rate determining step is dissociation of the thiophene from Re. The findings reported here suggest that S-coordination is not as unreasonable as previously thought. 2 tables, 23 ref.