C-H bond activation by cationic platinum(II) complexes: Ligand electronic and steric effects

A series of bis(aryl)diimine-ligated methyl complexes of Pt(II) with various substituted aryl groups has been prepared. The cationic complexes [(ArN=CR-CR=NAr)PtMe(L)](+)[BF4](-) (Ar = aryl; R = H, CH3; L = water, trifluoroethanol) react smoothly with benzene at approximately room temperature in trifluoroethanol solvent to yield methane and the corresponding phenyl Pt(II) cations, via Pt(IV)-methyl-phenylhydrido intermediates. The reaction products of methyl-substituted benzenes suggest an inherent reactivity preference for aromatic over benzylic C-H bond activation, which can however be overridden by steric effects. For the reaction of benzene with cationic Pt(II) complexes bearing 3,5-disubstituted aryl diimine ligands, the rate-determining step is C-H bond activation, whereas for the more sterically crowded analogues with 2,6-dimethyl-substituted aryl groups, benzene coordination becomes rate-determining. This switch is manifested in distinctly different isotope scrambling and kinetic deuterium isotope effect patterns. The more electron-rich the ligand is, as assayed by the CO stretching frequency of the corresponding carbonyl cationic complex, the faster the rate of C-H bond activation, Although at first sight this trend appears to be at odds with the common description of this class of reaction as electrophilic, the fact that the same trend is observed for the two different series of complexes, which have different rate-determining steps, suggests that this finding does not reflect the actual C-H bond activation process, but rather reflects only the relative ease of benzene displacing a ligand to initiate the reaction; that is, the change in rates is mostly due to a ground-state effect. The stability of the aquo complex ground state in equilibrium with the solvento complex increases as the diimine ligand is made more electron-withdrawing. Several lines of evidence, including the mechanism of degenerate acetonitrile exchange for the methyl-acetonitrile Pt(II) cations in alcohol solvents, suggest that associative substitution pathways operate to get the hydrocarbon substrate into, and out of, the coordination sphere; that is, the mechanism of benzene substitution proceeds by a solvent (TFE)-assisted associative pathway.