The platinum(II) methyl cation [(N−N)Pt(CH3)(solv)]+BF4 - (N−N = ArNC(Me)C(Me)NAr, Ar = 2,6-(CH3)2C6H3, 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-σ 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 °C). 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 °C allows direct low-temperature NMR observation of a fluxional π benzene complex, [(N−N)Pt(CH3)(C,C-η2-C6H6)]+.

The C-H activation of toluene and p-xylene at cationic Pt(II) diimine complexes (N-N)Pt(CH(3))(H(2)O)(+)BF(4)(-) (N-N = Ar-N=CMe-CMe=N-Ar; 1(BF(4)(-)), N(f)-N(f), Ar = 3,5-(CF(3))(2)C(6)H(3)); 2(BF(4)(-)), N'-N', Ar = 2,6-(CH(3))(2)C(6)H(3)) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe)(+) > (N-N)Pt(p-tolyl)(NCMe)(+) > (N-N)Pt(o-tolyl)(NCMe)(+) for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N'-N')Pt(benzyl)(NCMe)(+) were concurrently formed in the C-H activation of toluene at 2. The C-H activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (N-N)Pt(xylyl)(NCMe)(+) and (N-N)Pt(p-methylbenzyl)(NCMe)(+) were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (N(f)-N(f))Pt(Tol)(2) species (3a-c; a, Tol = o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF(4) was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (N(f)-N(f))Pt(m-tolyl)(NCMe)(+) (4b) and (N(f)-N(f))Pt(p-tolyl)(NCMe)(+) (4c) from all three isomeric bis(tolyl) complexes 3a-c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl)(CH(4))(+) (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition-state structures for the solvent-induced associative elimination of methane from these intermediates.