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

Halide exchange reactions between CpRu(PPh 3 ) 2 Cl ( 1 ) and CBr 4 , CHBr 3 , CBr 3 CO 2 Et or CHBr 2 CO 2 Et yielding CpRu(PPh 3 ) 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: CBr 3 CO 2 Et > CBr 4 > CHBr 2 CO 2 Et > CHBr 3 > C 4 H 9 Br. Activation parameters for the reaction of 1 with CHBr 3 are measured as Δ H † = 65 ± 5 kJ mol −1 and Δ S † = −157 ± 17 J mol −1 K −1 . Substitution of CH 3 C 5 H 4 Ru(PPh 3 ) 2 Cl ( 4 ) for 1 in reactions with CBr 4 increases the rate of halide exchange. The observation of ATRA reactivity of CBr 4 , CHBr 3 , CBr 3 CO 2 Et and CHBr 2 CO 2 Et with styrene when catalyzed by 1 and AIBN supports the formation of d 5 Ru III intermediates. Density functional theory calculations on potential intermediates suggests that a pathway leading to CpRu(PPh 3 )(Cl)(Br)&z.rad; is more likely than oxidative addition, which is preferred in reactions of 1 with bromoethane. Halide exchange reactions between CpRu(PPh 3 ) 2 Cl ( 1 ) and CBr 4 , CHBr 3 , CBr 3 CO 2 Et or CHBr 2 CO 2 Et yielding CpRu(PPh 3 ) 2 Br ( 3 ) are facile and appear to proceed by a radical pair intermediate in a second order reaction.