A combined experimental and density functional theory investigation of hydrocarbon activation at a cationic platinum(II) diimine aqua complex under mild conditions in a hydroxylic solvent

Controlled protonolysis of (N-f-N-f)Pt(CH3)(2) (1; N-f-N-f = ArN=CMe-CMe=NAr, Ar = 3,5-(CF3)(2)C6H3) with NBF4. Et2O in dichloromethane in the presence of small quantities of water gives the BF4- salt of the aqua complex (N-f-N-f)Pt(CH3)(H2O)(+) (6). When dissolved in trifluoroethanol (TFE), 6(BF4-) effects the activation of methane and benzene C-H bonds under very mild conditions. Thus, 6 reacted with benzene in TFE-d(3) at ambient temperature to quantitatively yield (N-f-N-f)Pt(C6H5)(H2O)(+) and methane after 2-3 h. The use of C6D6 led to multiple incorporation of deuterium into the methane produced and suggests the involvement of methane sigma -complex and benzene sigma- or pi -complex intermediates. When the solution of 6(BF4-) was exposed to (CH4)-C-13, an exchange reaction produced ca. 50% of (N-f-N-f)Pt((CH3)-C-13)(H2O)(+) and CH4 after ca. 48 h at 45 degreesC. The reaction was inhibited by added water, suggesting that water is reversibly lost from 6 before C-II activation takes place. The use of CD4 resulted in multiple deuterium incorporation into the methane produced, again implying a Pt-methane sigma -complex intermediate. Low-temperature protonation of 1 in dichloromethane-d(2) generated observable Pt(IV) hydride species (N-f-N-f)Pt(CH3)(2)(H)(L)(+). These decomposed via methane elimination, raising the possibility that the observed C-H activation proceeds by an oxidative addition pathway. The reaction between 6 and CH4 was investigated by DFT calculations using a model system with the HN=CH-CH=NH ligand. The C-H activation was investigated for oxidative addition and sigma -bond metathesis pathways starting from the four-coordinate methane complex (N-N)Pt(CH3)(CH4)(+). The oxidative addition pathway, thermodynamically uphill by 23 kJ/mol (ZPE-corrected data), was favored by 12 kJ/mol relative to the sigma -bond metathesis. When a H2O ligand was added to the five-coordinate oxidative addition product, the overall oxidative addition reaction was thermodynamically downhill by 33 kJ/mol (partially ZPE-corrected) starting from an H2O adduct of (N-N)Pt(CH3)(CH4)(+) with H2O electrostatically bonded at the diimine moiety. In this case, the oxidative addition pathway was favored by 20 kJ/mol. The calculations indicated that reductive elimination of methane from the six-coordinate (N-N)Pt(CH3)(2)(H)(H2O)(+) with the hydride and H2O ligands trans disposed occurred in concert with dissociation of the aqua ligand.