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

Abstract

The relative rates of C−H activation of methane, methanol, and dimethyl ether by [(N-N)PtMe(TFE-d_3)]+ ((N-N) = ArN═C(Me)−C(Me)═NAr; Ar = 3,5-di-tert-butylphenyl, TFE-d_3 = CF_3CD_2OD) (2(TFE)) were determined. Methane activation kinetics were conducted by reacting 2(TFE)-^(13)C 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 (K_(eq) = 0.0042 ± 0.0006). C−H activation gives [(N-N)Pt(CH_2OD)(MeOD)]^+ (4), which is unstable and reacts with [(RO)B(C_6F_5)_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 (k_H/k_D = 1.4 ± 0.1). Reaction of dimethyl ether with 2(TFE) proceeds similarly (K_(eq) = 0.023 ± 0.002, 313 K; k = 5.5 ± 0.5 × 10^(-4) M^(-1) s^(-1), k_H/k_D = 1.5 ± 0.1); the product obtained is a novel bis(alkylidene)-bridged platinum dimer, [(diimine)Pt(μ-CH_2)(μ-(CH(OCH_3))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.

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