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

Kinetic measurements of the reaction between CpRu(PPh3)(2)Cl (1a) and 1-bromobutane reveal a neatly first order dependence on the concentration of haloalkane and a negative entropy of activation, Delta S-dagger < 0. The rate of halide exchange (k(obs) = 0.56 +/- 0.03 x 10(-6) s(-1) at 35 degrees C) is two orders of magnitude lower than the rate of phosphine dissociation (k(obs) = 47.1 +/- 1.1 x 10(-6) s(-1) at 35 degrees C). The reaction rate decreases in the presence of excess PPh3 and when 2-bromo-2-methylpropane is substituted for (C4H9Br)-C-n. The rate dependence on [(C4H9Br)-C-n] 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(PPh3)(RBr)Cl in a second order reaction (k(2) = 1.2 +/- 0.1 x 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 x 10(-7) s(-1)) is proposed. DFT calculations are consistent with either oxidative addition or sigma-bond metathesis as lower energy pathways than single electron transfer and formation of a radical pair. The reaction between CpRu(PAr3)(2)Cl (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)(2)Br (2) and CpRu(PAr3)(2)I (3).