Steric and Solvent Effects on the Secondary Kinetic α-Deuterium Isotope Effects in the Reaction of Methyl Iodide with Organoplatinum(II) Complexes: Application of a Second-Order Technique in Measuring the Rates of Rapid Processes

Abstract

The kinetic isotope effects (KIEs), kH/kD, have been determined for reaction of CH3I/CD3I with several organoplatinum(II) complexes [PtR2(NN)], in which the bidentate NN ligand is bpy = 2,2′-bipyridine, tBu2bpy = 4,4′-bis(tert-butyl)-2,2′-bipyridine, phen = 1,10-phenanthroline, or Me2phen = 2,9-dimethyl-1,10-phenanthroline, at different temperatures and in solvents having different polarities. The values obtained for the secondary α-deuterium KIEs are close to 1 and are dependent on the solvent; values of up to 7−10% larger are obtained for the reactions in the polar solvent acetone as compared to those obtained in the nonpolar solvent benzene. The data also indicate that the steric crowding around the square-planar coordination sphere of the platinum(II) complexes with the ligand Me2phen results in higher KIEs. The reactions involving dimethylplatinum(II) complexes, [PtMe2(NN)], were fast, and a 1:1 molar ratio (of complex and reagent) technique was successfully used to measure the rate constants accurately by conventional UV−visible spectroscopy. It is shown that there are significant advantages to measuring the reaction rates under second-order condition, as compared to the usual pseudo-first-order method.