Mechanism of halide exchange in reactions of CpRu(PPh3)2Cl with haloalkanes

Abstract

Kinetic measurements of the reaction between CpRu(PPh₃)₂Cl (1a) and 1-bromobutane reveal a nearly first order dependence on the concentration of haloalkane and a negative entropy of activation, ΔS^† < 0. The rate of halide exchange (k_obs = 0.56 ± 0.03 × 10⁻⁶ s⁻¹ at 35 °C) is two orders of magnitude lower than the rate of phosphine dissociation (k_obs = 47.1 ± 1.1 × 10⁻⁶ s⁻¹ at 35 °C). The reaction rate decreases in the presence of excess PPh₃ and when 2-bromo-2-methylpropane is substituted for ⁿC₄H₉Br. The rate dependence on [ⁿC₄H₉Br] is consistent with a two term rate law for the reaction: rate = k_obs[1] = (k₁ + k₂[RBr])[1]. A mechanism where formation of CpRu(PPh₃)(RBr)Cl in a second order reaction (k₂ = 1.2 ± 0.1 × 10⁻⁸ M⁻¹ s⁻¹) is slightly lower rate than a subsequent first order reaction leading to the final product (k₁ = 4.6 ± 3.1 × 10⁻⁷ s⁻¹) is proposed. DFT calculations are consistent with either oxidative addition or σ-bond metathesis as lower energy pathways than single electron transfer and formation of a radical pair. The reaction between CpRu(PAr₃)₂Cl (PAr₃ = PPh₃, PPh₂(p-tolyl), P(p-tolyl)₃, P(p-FC₆H₄)₃ and P(p-CH₃OC₆H₄)₃, 1) and haloalkanes also provides an efficient route for the synthesis of the corresponding bromides and iodides CpRu(PAr₃)₂Br (2) and CpRu(PAr₃)₂I (3).