Mechanistic Insight into the Protonolysis of the Pt−C Bond as a Model for C−H Bond Activation by Platinum(II) Complexes

Abstract

The kinetic and NMR features of the protonolysis reactions on platinum(II) alkyl complexes of the types cis-[PtMe2L2], [PtMe2(L-L)], cis-[PtMeClL2], and [PtMeCl(L-L)] (L = PEt3, P(Pri)3, PCy3, P(4-MePh)3, L-L = dppm, dppe, dppp, dppb) in methanol suggest a rate-determining proton attack at the Pt−C bond. In contrast, a multistep oxidative-addition−reductive-elimination mechanism characterizes the methane loss on protonation of the corresponding trans-[PtMeClL2] species. Tools that were particularly diagnostic in suggesting different reaction pathways for the two systems were (i) the different results of kinetic deuterium isotope experiments, (ii) the detection or absence of Pt(IV) hydrido alkyl intermediate species by low-temperature 1H NMR experiments, and (iii) the detection or absence of isotope scrambling and incorporation of deuterium into Pt−CH3, combined with the loss of a range of CHnDn-4 isotopomers. For all systems, the rates of protonolysis are retarded by ligand steric congestion, accelerated by ligand electron donation, and almost unaffected by the chain length along the series of chelate complexes. A straight line correlates the rates of protonolysis of cis-dialkyl and cis-monoalkyl complexes, the difference in reactivity between the two systems being almost 5 orders of magnitude (slope of the line = 6 × 104). Factors controlling the dichotomy of behavior between complexes of different geometry have been taken into consideration. Application of the principle of microscopic reversibility suggests the reason why platinum complexes with nitrogen donor ligands appear to be far more efficient than platinum phosphane complexes in activating the C−H bond.