Kinetics and mechanism of halide exchange in reactions of CpRu(PPh₃)₂Cl with alkyl halides: evidence for radical pairs

Halide exchange reactions between CpRu(PPh3)(2)Cl (1) and CBr4, CHBr3, CBr3CO2Et or CHBr2CO2Et yielding CpRu(PPh3)(2)Br (3) are facile and appear to proceed by a radical pair intermediate in a second order reaction. The relative reactivity of the bromocarbon compounds follows the order: CBr3CO2Et > CBr4> CHBr2CO2Et > CHBr3 >> C4H9Br. Activation parameters for the reaction of 1 with CHBr3 are measured as Delta H dagger = 65 +/- 5 kJ mol(-1) and Delta S dagger =-157 +/- 17 J mol(-1) K-1. Substitution of CH3C5H4Ru(PPh3)(2)Cl (4) for 1 in reactions with CBr4 increases the rate of halide exchange. The observation of ATRA reactivity of CBr4, CHBr3, CBr3CO2Et and CHBr2CO2Et with styrene when catalyzed by 1 and AIBN supports the formation of d5 RuIII intermediates. Density functional theory calculations on potential intermediates suggests that a pathway leading to CpRu(PPh3)(Cl)(Br)' is more likely than oxidative addition, which is preferred in reactions of 1 with bromoethane.