Oxidative Addition of Ethyl Iodide to a Dimethylplatinum(II) Complex: Unusually Large Kinetic Isotope Effects and Their Transition-State Implications

The mechanism of oxidative addition of ethyl iodide to [PtMe2(2,2'-bipyridine)], 1, has been investigated by product analysis and by study of secondary deuterium kinetic isotope effects (KIEs), using the reagents C2H5I, C2D5I, CH3CD2I, and CD3CH2I. The reactions in acetone and benzene give [PtIMe2Et(bipy)], mostly as the product of trans oxidative addition, but with some of the isomeric product of cis oxidative addition and some [PtIMe3(bipy)], resulting from methyl group transfer. The reaction in benzene is light-sensitive, giving additional major products [PtI2Me2(bipy)] and [PtI2Me2-(OOEt)(bipy)], as well as several minor products indicative of a photochemically initiated free-radical reaction. No H/D exchange within the ethyl group was observed in any of the products. The dark reactions in acetone and benzene follow second-order kinetics, with large negative values of the entropy of activation, indicating the S(N)2 mechanism of oxidative addition of ethyl iodide to 1. However, for reaction with C2H5I vs C2D5I, values of the KIE k(H)/k(D) range from 1.32 to 1.72 in acetone and from 1.44 to 1.90 in benzene solution, and studies with CH3CD2I and CD3CH2I show that alpha- and beta-deuterium KIEs make about equal contributions to the overall KIE. These are the first reports of isotope effects on the rate of oxidative addition reactions of ethyl halides, and the high values of the secondary deuterium KIE were unexpected for the SN2 mechanism. Possible reasons for these observations are discussed.