Equilbrium and kinetic studies of sulfur-coordinated thiophenes (Th) in Cp(CO)[sub 2]Ru([eta][sup 1](S)-Th)[sup +] and Cp(CO)(PPh[sub 3])Ru([eta][sup 1](S)-Th)[sup +]. Models for thiophene adsorption on hydrodesulfurization catalysts

A series of stable sulfur-bound thiophene complexes, Cp(CO)[sub 2]Ru([eta][sup 1](S)-Th)[sup +], where 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, BT and DBT, are synthesized from the reaction of Cp(CO)[sub 2]RuCl with Ag[sup +] and thiophenes. Equilibrium constants, K[prime], for the displacement of thiophene (T) by methyl-substituted thiophenes and benzo[b]thiophene (BT), Cp(CO)[sub 2]Ru([eta][sup 1](S)-T)[sup +] + Th [r equilibrium] Cp(CO)[sub 2]Ru([eta][sup 1](S)-Th)[sup +] + 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-Me[sub 2]T (20.7) < BT (47.6) < Me[sub 4]T (887). First-order rate constants (10[sup 6]k[sub 1], s[sup [minus]1]) for phosphine substitution of the thiophenes in Cp(CO)[sub 2]Ru([eta][sup 1](S)-Th)[sup +], Cp(CO)[sub 2]Ru([eta][sup 1](S)-Th)[sup +] + PR[sub 3] [yields] Cp(CO)[sub 2]Ru(PR[sub 3])[sup +] + Th, by a dissociative mechanism decrease in the following order: 3-MeT (450) > 2-MeT (410) > BT (100) > 2,5-Me[sub 2]T (23). Rate constants for thiophene dissociation in the analogous Cp(CO)(PPh[sub 3])Ru([eta][sup 1](S)-Th)[sup +] complexes decrease in a slightly different order, T (1400) > 2-MeT (220) > 3-MeT (170) > 2,5-Me[sub 2]T (130) > BT (70) > DBT (17) > Me[sub 4]T (5.8), due to steric repulsions between the bulky PPh[sub 3] and methyl groups in the 2- and 5-positions of the thiophene. In general, methyl groups on the thiophene (Th) increase K[prime] and decrease k[sub 1], which suggests that the electron-releasing methyl groups enhance thiophene binding to the metal. Thiophene binding on a Co-Mo/Al[sub 2]O[sub 3]HDS catalyst also increases with the number of methyl groups in the thiophene. This trend is consistent with [eta][sup 1](S) and/or [eta][sup 5] coordination of thiophenes at metal sites on the catalyst surface. 38 refs., 1 fig., 6 tabs.