Secondary kinetic deuterium isotope effect in oxidative addition reaction of cycloplatinated(II) complexes with MeI

Abstract

Oxidative addition reaction of CH3I or CD3I with the cycloplatinated(II) complexes [PtMe(CˆN)L], 1a–1e, in which CˆN = 2-phenylpyridinate (ppy) or benzo[h]quinolinate (bhq), and L = PPh3, PMePh2 or P(OPh)3, in CH2Cl2 solvent gave the organoplatinum(IV) products [PtMe2I(CˆN)L]. NMR spectroscopy (1H and 31P), and X-ray crystallography (supported by DFT calculations) clearly indicated that in each case the thermodynamic isomer products 2a–2e, with I ligand trans to C of CˆN rather than the related kinetic isomer in which I ligand is trans to Me, is obtained. Kinetics of the reactions were studied by using UV–vis spectroscopy and the reactions are suggested to occur by a common SN2 mechanism, on the basis of their clean second-order kinetics, failure of radical traps to inhibit the reactions, and the observation of large negative values of ΔS‡. Rates of the reactions are significantly dependent on nature of the phosphine ligand L with the trend being PMePh2 > PPh3 > P(OPh)3. The kinetic isotope effect (KIE) values, kH/kD, were obtained in the present study and are found to be close to the normal value of 1 in the corresponding reactions, showing that nature of the ancillary ligands containing phosphorous donor atoms and the cycloplatinated ligands, CˆN, are not effective on the values of kH/kD. For example, KIE values, in reactions involving the ppy complexes [PtMe(ppy)L], 1a–1c, at 20 °C are 1.06 (L = PPh3, 1a), 0.97 (L = P(OPh)3, 1c) and 0.92 (L = PMePh2, 1b). Note that nature of the CˆN ligands are found to be almost non-effective on both rates of the reactions and the related KIE values.