Reaction of allyl bromide with cyclometallated platinum(II) complexes: Unusual kinetic behavior and a novel case of methyl and allyl C-C bond reductive elimination

Abstract

Reaction of the known cyclometallated platinum(II) complexes [PtMe(CˆN)(PPh2Me)], in which CˆN is either 2-phenylpyridinate (abbreviated as ppy-H), 1a, or benzo-h-quinolate (abbreviated as bhq-H), 1b, with excess allyl bromide, CH2=CH-CH2Br, in acetone gave the thermodynamic Pt(IV) product complex cis-[PtMe(CH2-CH=CH2)Br(ppy-H)(PPh2Me)], 2a, or cis-[PtMe(CH2-CH=CH2)Br(bhq-H)(PPh2Me)], 2b, respectively, in which the phosphine and the allyl ligands are situated trans to each other. Although these Pt(IV) complexes may possibly take up other isomeric forms such as one in that allyl ligand being situated trans to N atom or one that allyl ligand is located trans to Br atom, our full characterization of the complexes using 1H, 13C and 31P NMR spectroscopy, completely ruled out the latter possibilities. Our attempt to grow crystals of the Pt(IV) complex 2a was ended up to give crystals of the Pt(II) complex [PtBr(ppy-H)(PPh2Me)], 3a, resulting from reductive elimination of Me and allyl from 2a. Also the solution of the Pt(IV) bhq-H analogous complex 2b in acetone at 60 °C gradually gave the Pt(II) complex [PtBr(bhq-H)(PPh2Me)], 3b; these two new complexes 3a and 3b were also prepared directly and fully characterized and their NMR spectra were used to confirm the occurred reductive eliminations. These observations show that during the time a novel case of Me and allyl C-C bond reductive elimination, as determined by 1H, 13C and 31P NMR spectroscopy and X-ray crystallography, is happening. Kinetics of reaction of the Pt(II) complexes 1 with excess ally bromide were investigated by UV–vis spectroscopy and found that while the bhq-H complex 1b reacts by usual second order SN2 mechanism, first order in both the complex and the reagent, the analogous ppy-H complex 1a reacts almost completely by an unusual third order kinetics, first order on 1a and second order on allyl bromide reagent. Suggestions are made for these behaviors and DFT calculations were obtained to support the experimental observations.