Mechanism of halide exchange in reactions of CpRu(PPh3)2Cl with haloalkanes†
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 (kobs = 0.56 ± 0.03 × 10−6 s−1 at 35 °C) is two orders of magnitude lower than the rate of phosphine dissociation (kobs = 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 = kobs = (k1 + k2[RBr]). 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).