Mechanistic investigation of benzene C-H activation at a cationic platinum(II) center: Direct observation of a platinum(II) benzene adduct

The platinum(II) methyl cation [(N-N)Pt(CH3)(solv)]+BF4- (N-N = ArN=C(Me)C(Me)=NAr, Ar = 2,6-(CH3)(2)C6H3, solv = H2O (2a) or TFE = CF3CH2OH (2b)) is prepared by treatment of (N-N)Pt(CH3)(2) with 1 equiv of aqueous HBF4 in TFE. Reaction of a mixture of 2a and 2b with benzene in TFE/H2O solutions cleanly affords the platinum(II) phenyl cation [(N-N)Pt(C6H5)(solv)]+BF4- (3). Investigations of the kinetics and isotopic labeling experiments indicate that reaction of 2 with benzene proceeds via benzene coordination, reversible oxidative addition of benzene C-H bonds, reversible formation of a methane C,H-sigma complex, and final dissociation of methane. Under conditions where [(N-N)Pt(CH3)(H2O)]+BF4- (2a) is the major starting complex, rate-determining benzene coordination to 2b is implicated by the observed kinetic rate law (inverse first order in [H2O] and first order in [C6H6] to 3.8 M) and the small kinetic deuterium isotope effect for C6H6 VS C6D6 (k(H)/k(D) = 1.06 +/- 0.05 at 25 degreesC). When deuterated benzenes C6D6 and 1,3,5-C6H3D3 are used, almost full statistical scrambling of deuterium from one benzene into methane is achieved, indicating that the energetic barriers for dissociating benzene and methane are considerably higher than interconversions of intermediate hydrocarbon complexes and [(N-N)Pt(C6H5)(CH3)H](+). Protonation of (N-N)Pt(CH3)(C6H5) with HBF4 in TFE, which provides an independent route into the manifold of postulated intermediates, gives a mixture of 3 + CH4 (82%) and 2 + C6H6 (18%). Protonation of (N-N)Pt(CH3)(C6H5) with triflic acid in methylene chloride/diethyl ether mixtures at -69 degreesC allows direct low-temperature NMR observation of a fluxional pi benzene complex, [(N-N)Pt(CH3)(C,C-eta (2)-C6H6)](+).