Kinetics and mechanism of oxidative addition of MeI to binuclear cycloplatinated complexes containing biphosphine bridges: Effects of ligands

Abstract

The binuclear complex [Pt2Me2(bhq)2(μ-dppf)], 1a, in which bhqH = benzo{h}quinoline and dppf = 1,1′-bis(diphenylphosphino)ferrocene, was synthesized by the reaction of [PtMe(SMe2)(bhq)] with 0.5 equiv of dppf at room temperature. The reaction of Pt(II)–Pt(II) complex 1a with excess MeI gave the Pt(IV)–Pt(IV) complex [Pt2I2Me4(bhq)2(μ-dppf)], 2a. The complexes were fully characterized using multinuclear (1H, 31P and 195Pt) NMR spectroscopy and elemental analysis. The kinetic and mechanism of the reaction of complex 1a with MeI was investigated in CHCl3 and based on the data, obtained from UV–vis and low temperature 31P NMR spectroscopies, a mechanism involving stepwise oxidative addition of MeI to the two Pt(II) centers is suggested. Reaction rates concerning the complex 1a, having bhq ligand, are almost 1.4 times slower than those involving the ppy complex [Pt2Me2(ppy)2(μ-dppf)], ppyH = 2-phenylpyridine, reported previously (Inorg. Chem. 2008, 47, 5441). This is attributed to the stronger donor ability of the ppy ligand as compared to that of the bhq ligand as is further confirmed using density functional theory (DFT) calculations through finding approximate structures for the described complexes. A comparative kinetic study of reaction of the dimeric platinum(II) complex [Pt2Me2(bhq)2(μ-dppm)], 1b, where dppm = bis(diphenylphosphino)methane, with MeI was also performed to investigate the effect of bridging biphosphine ligand on the kinetic and mechanism of the dimeric complexes with MeI. A double MeI oxidative addition was observed for which the classical SN2 mechanism for both steps, as well as the possible intermediates, is suggested.