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-d(3))](+) ((N-N) = ArN=C(Me)-C(Me)=NAr; Ar = 3,5-di-tert-butylphenyl, TFE-d(3) = CF(3)CD(2)OD) (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 x 10(-3) M(-1) s(-1) at 330 K; k = 2.7 +/- 0.2 x 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(2)OD)(MeOD)](+) (4), which is unstable and reacts with (RO)B(C(6)F(5))(3)](-) to generate a pentafluorophenyl platinum complex. Analysis of kinetics data for reaction of 2 with methanol yields k = 2.0 +/- 0.2 x 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 x 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(mu-CH(2))(mu-(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((dimethy)(l e)(ther)) = 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.