A Study of Ligand Substitution and its Importance in the C-H Activation of Methane and Methanol

Abstract

Pyridinium and indolium-derived aminocarbene complexes of platinum and palladium were prepared by oxidative addition of pyridinium and 2-chloro-indolium carbene precursors. These complexes were synthesized in order to study the degree to which aminocarbene ligands pi-bond with the transition-metals to which they are bound. X-Ray crystal structures show minimal multiple bonding in the indole examples and a measurable shortening of the pyridine-2-ylidene Pt–C distance (1.959(3) A) compared with typical Pt–C bonds (2.01(1) A). The kinetics of associative DMSO substitution trans to the pyridine-2-ylidene ligand indicate a stabilization of the trigonal bipyramidal transition structure that is due to pi-acidity of the carbene carbon. This pi-acidity is responsible for 4-orders of magnitude acceleration in the associative substitution rate compared with a structurally similar phenyl donor. The relative rates of methane, methanol and dimethylether C–H activation by [(N-N)PtMe(TFE-d3)]+ ((N-N) = ArN=C(Me)-C(Me)=NAr Ar = 3,5-di-tert-butylphenyl, TFE-d3 = CF3CD2OD) were studied by 1H and 13C NMR spectroscopy. Methane activation kinetics were conducted at 300-1000 psi of methane pressure in single crystal sapphire NMR tubes (k = 1.6 ± 0.4 x 10-3 M-1s-1, 330 K; k = 2.7 ± 0.2 x 10-4 M-1s-1, 313 K). Deuterium scrambling studies indicate that displacement of TFE-d3 from the platinum center by methane's C–H bond is slower than the subsequent C–H oxidative cleavage and hence the rate-determining step in methane C–H activation. The kinetics of methanol and dimethylether C–H activation were studied with 1H NMR spectroscopy and shown to be inhibited by a preequilibrium binding of the substrates oxygen lone-pair to the metal center. A small kinetic isotope effect (kH/kD = 1.4 ± 0.1) and the observed concentration dependence suggest that the reaction proceeds by rate determining displacement of the coordinated trifluoroethanol by the C-H bonds of methanol (k = 2.0 ± 0.2 x 10-3 M-1s-1, Keq = 0.0042 ± 0.0006, 330 K). A similar concentration dependence is observed in the activation of dimethylether (k = 5.5 ± 0.5 x 10-4 M-1s-1, Keq = 0.020 ± 0.002, 313 K). Comparison of these second order rate constants (k(Methane)/k(Methanol) = 1/1.3, 330 K; k(Methane)/k(Dimethylether) = 1/2, 313 K) shows that the selectivity of this ligand substitution step matches the selectivity previously reported by our group for oxidizing methyl and hydroxymethyl groups with aqueous tetrachloroplatinate (1/1.5). These data strongly suggest a similar rate-determining step under the Shilov conditions.