Mechanism of halide exchange in reactions of CpRu(PPh 3 ) 2 Cl with haloalkanes

Kinetic measurements of the reaction between CpRu(PPh 3 ) 2 Cl (1a) and 1-bromobutane reveal a nearly first order dependence on the concentration of haloalkane and a negative entropy of activation, Δ S † < 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 PPh 3 and when 2-bromo-2-methylpropane is substituted for n C 4 H 9 Br. The rate dependence on [ n C 4 H 9 Br] is consistent with a two term rate law for the reaction: rate = k obs [1] = ( k 1 + k 2 [RBr])[1]. A mechanism where formation of CpRu(PPh 3 )(RBr)Cl in a second order reaction ( k 2 = 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 ( k 1 = 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(PAr 3 ) 2 Cl (PAr 3 = PPh 3 , PPh 2 ( p -tolyl), P( p -tolyl) 3 , P( p -FC 6 H 4 ) 3 and P( p -CH 3 OC 6 H 4 ) 3 , 1) and haloalkanes also provides an efficient route for the synthesis of the corresponding bromides and iodides CpRu(PAr 3 ) 2 Br (2) and CpRu(PAr 3 ) 2 I (3).