EQUILIBRIUM AND KINETIC-STUDIES OF SULFUR-COORDINATED THIOPHENES (TH) IN CP(CO)2RU(ETA-1(S)-TH)+ AND CP(CO)(PPH3)RU(ETA-1(S)-TH)+ - MODELS FOR THIOPHENE ADSORPTION ON HYDRODESULFURIZATION CATALYSTS

A series of stable sulfur-bound thiophene complexes, CP(CO)2Ru(eta1(S)-Th)+, where p = eta5-C5H5 and Th = T, 2-MeT, 3-MeT, 2,5-Me2T, Me4T, BT and DBT, are synthesized from the reaction Of CP(CO)2RuCl with Ag+ and thiophenes. Equilibrium constants, K, for the displacement of thiophene (T) by methyl-substituted thiophenes and benzo[b]thiophene (BT), CP(CO)2Ru(eta1(S)-T)+ + Th half arrow right over half CP(CO)2Ru(eta1(S)-Th)+ + T, increase with an increasing number of methyl groups in the thiophene: T (1.00) < 2-MeT (3.30) < 3-MeT (4.76) < 2,5-Me2T (20.7) < BT (47.6) < Me4T (887). First-order rate constants (10(6)k1, s-1) for phosphine substitution of the thiophenes in Cp(CO)2Ru(eta1(S)-Th)+, Cp(CO)2Ru(eta1(S)-Th)+ + PR3 --> CP(CO)2Ru(PR3)+ + Th, by a dissociative mechanism decrease in the following order: 3-MeT (450) > 2-MeT (410) > BT (100) > 2,5-Me2T (23). Rate constants for thiophene dissociation in the analogous Cp-(CO)(PPh3)Ru(eta1(S)-Th)+ complexes decrease in a slightly different order, T (1400) > 2-MeT (220) > 3-MeT (170) > 2,5-Me2T (130) > BT (70) > DBT (17) > Me4T (5.8), due to steric repulsions between the bulky PPh3 and methyl groups in the 2- and 5-positions of the thiophene. In general, methyl groups on the thiophene (Th) increase K and decrease k1, which suggests that the electron-releasing methyl groups enhance thiophene binding to the metal. Thiophene binding on a Co-Mo/Al2O3 HDS catalyst also increases with the number of methyl groups in the thiophene. This trend is consistent with eta1(S) and/or eta5 coordination of thiophenes at metal sites on the catalyst surface.