Kinetics and Mechanism of Methane, Methanol, and Dimethyl Ether C−H Activation with Electrophilic Platinum Complexes

The relative rates of C−H activation of methane, methanol, and dimethyl ether by [(N-N)PtMe(TFE-d3)]+ ((N-N) = ArNC(Me)−C(Me)NAr; Ar = 3,5-di-tert-butylphenyl, TFE-d3 = CF3CD2OD) (2(TFE))
were determined. Methane activation kinetics were conducted by reacting 2(TFE)-13C with 300−1000 psi
of methane in single-crystal sapphire NMR tubes; clean second-order behavior was obtained (k = 1.6 ±
0.4 × 10-3 M-1 s-1 at 330 K; k = 2.7 ± 0.2 × 10-4 M-1 s-1 at 313 K). Addition of methanol to solutions of
2(TFE) rapidly establishes equilibrium between methanol (2(MeOD)) and trifluoroethanol (2(TFE)) adducts,
with methanol binding preferentially (Keq = 0.0042 ± 0.0006). C−H activation gives [(N-N)Pt(CH2OD)(MeOD)]+ (4), which is unstable and reacts with [(RO)B(C6F5)3]- to generate a pentafluorophenyl platinum
complex. Analysis of kinetics data for reaction of 2 with methanol yields k = 2.0 ± 0.2 × 10-3 M-1 s-1 at
330 K, with a small kinetic isotope effect (kH/kD = 1.4 ± 0.1). Reaction of dimethyl ether with 2(TFE) proceeds
similarly (Keq = 0.023 ± 0.002, 313 K; k = 5.5 ± 0.5 × 10-4 M-1 s-1, kH/kD = 1.5 ± 0.1); the product
obtained is a novel bis(alkylidene)-bridged platinum dimer, [(diimine)Pt(μ-CH2)(μ-(CH(OCH3))Pt(diimine)]2+
(5). Displacement of TFE by a C−H bond appears to be the rate-determining step for all three substrates;
comparison of the second-order rate constants (k(methane)/k(methanol) = 1/1.3, 330 K; k(methane)/k(dimethyl ether) =
1/2.0, 313 K) shows that this step is relatively unselective for the C−H bonds of methane, methanol, or
dimethyl ether. This low selectivity agrees with previous estimates for oxidations with aqueous tetrachloroplatinate(II)/hexachloroplatinate(IV), suggesting a similar rate-determining step for those reactions.