KINETICS AND MECHANISM OF THE HYDROGENOLYSIS OF A RUTHENIUM(II) ACYL COMPLEX

N,Nʹ-Dimethylacetamide (DMA) and toluene solutions of the acyl complex dicarbonylchloronor-bornenoylbis(triphenylphosphine)ruthenium(II), RuCl(COC7H9)(CO)2(PPh3)2 (1), react with 1 mol equiv of H2 to give RuHCl(CO)2(PPh3)2 (2) and the unsaturated aldehyde product 2-norbornene-5-carboxaldehyde, C7H9CHO (3). A subsequent, relatively slow hydrogenation of C7H9CHO by a further 1 mol of H2 to give the saturated aldehyde norbornane-2-carboxaldehyde, C7H11CHO (4), is catalyzed by 2. A detailed kinetic study on the hydrogenolysis of 1 in DMA at 65 °C reveals a first-order rate dependence on total Ru concentration, an inverse dependence on added PPh3 ligand concentration, and a first- to zero-order dependence on H2 pressure. A mechanism involving PPh3 dissociation as the first step followed by hydrogenolysis of the resulting intermediate accounts for these observations. Values of k1 and k-1/k2 in DMA have been evaluated at 65 °C (k1 = 4.5 X 10-5 s-1, k-1/k2 = 1.6) and the activation parameters for the k1 step determined (ΔH* = 69 ± 7 kJ mol-1, ΔS* = -126 ± 13 J K-1 mol-1); k1 and k-1 are the rate constants for the dissociation and reassociation of PPh3, and k2 is the rate constant for the hydrogenolysis step. The activation parameters for the corresponding k1 step in toluene solution are very different; ΔH* = 128 ± 13 kJ mol-1, ΔS* = +62 ± 6 J K-1 mol-1. Association of DMA in the transition state is proposed to account for the differences. Rough estimates of the rate of the hydrogenolysis step, and a Ru‒P solution bond dissociation energy (assuming the kinetic intermediate in toluene is five-coordinate), are presented, as well as a discussion on the nature of the hydrogenolysis step. © 1990, American Chemical Society. All rights reserved.