Reactivity comparison of five-and six-membered cyclometalated platinum(II) complexes in oxidative addition reactions

The compound [PtMe(bzpy)(DMSO)] (1; bzpy = 2-benzylpyridinate) was synthesized by reaction of cis-[PtMe2(DMSO)(2)] with 1 equiv. of bzpyH under reflux conditions in toluene through C-H activation of the carbon-hydrogen bond in 2-benzylpyridine. Then, the complex [PtMe(bzpy)(PPh3)], 2, was prepared by addition of PPh3 to complex 1. Complex 2 undergoes oxidative addition with methyl iodide to give [PtMe2I(bzpy)(PPh3)], 3. NMR spectroscopy (H-1 and P-31) and X-ray crystallography (supported by DFT calculations) clearly showed that the thermodynamic isomer product 3, with iodide trans to C of bzpy rather than the related kinetic isomer, 3, in which iodide is trans to methyl, is obtained. Mechanistic studies using UV-vis spectroscopy and DFT calculations indicate that the reaction occurs via a S(N)2 mechanism. The kinetic study of the oxidative addition of methyl iodide to the non-planar, six-membered cyclometalated complex with that of the five-membered cyclometalated [PtMe(ppy)(PPh3)], in which ppy -2-phenylpyridinate, shows that the ring size of the chelating unit has a significant impact on the rate of the reaction.