Kinetics of halide exchange in cyclopentadienylruthenium bis(triarylphosphine)halides

Abstract

The kinetics of halide exchange in cyclopentadienylruthenium bis(triarylphosphine) halide complexes CpRu(PAr3)2X (1a X = Cl, 1b X = Br, 1c X = I; X′= Cl, Br, Ar = C6H5, 2 Ar = p-CH3OC6H4, X = Cl, X′= Br and 3 Ar = p-FC6H4, X = Cl, X′ = Br) have been measured and the results interpreted with respect to two possible mechanisms: halide dissociation vs phosphine dissociation. Halide exchange between nBu4NX′ and 1–3 in fluorobenzene under pseudo-first order conditions is found to be first order in 1–3. Relative rates of halide exchange follow the order: 1a > 1b > 1c > 2 ≈ 3 with activation parameters ΔH† = 117 ± 6 kJ/mol and ΔS† = 44 ± 18 J/mol-K (1a, X′ = Br) and ΔH† = 120 ± 14 kJ/mol ΔS† = 50 ± 45 J/mol-K (1b, X′ = Cl). Plots of kobs vs [X′–] for 1 and 2 are consistent with a two-term rate law where kobs = (k1 + k2[X′–]) while reactions of 3 are independent of the halide concentration. The rate data are consistent with phosphine dissociation as the rate determining step followed by a slightly faster reaction with halide ions where the dissociated phosphine competes effectively with halide for the open coordination site in a CpRu(PAr3)X intermediate.