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

Abstract

Halide exchange reactions between CpRu(PPh₃)₂Cl (1) and CBr₄, CHBr₃, CBr₃CO₂Et or CHBr₂CO₂Et yielding CpRu(PPh₃)₂Br (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 ΔH^† = 65 ± 5 kJ mol⁻¹ and ΔS^† = −157 ± 17 J mol⁻¹ K⁻¹. Substitution of CH₃C₅H₄Ru(PPh₃)₂Cl (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 Ru^III 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.