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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

2000; American Chemical Society; Volume: 122; Issue: 44 Linguagem: Inglês

10.1021/ja0019171

1943-2984

Hanne Heiberg, Lars Johansson, Odd Gropen, Olav B. Ryan, Ole Swang, Mats Tilset,

Asymmetric Hydrogenation and Catalysis

Controlled protonolysis of (Nf-Nf)Pt(CH3)2 (1; Nf-Nf = ArNCMeCMeNAr, Ar = 3,5-(CF3)2C6H3) with HBF4·Et2O in dichloromethane in the presence of small quantities of water gives the BF4- salt of the aqua complex (Nf-Nf)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-d3 at ambient temperature to quantitatively yield (Nf-Nf)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 σ-complex and benzene σ- or π-complex intermediates. When the solution of 6(BF4-) was exposed to 13CH4, an exchange reaction produced ca. 50% of (Nf-Nf)Pt(13CH3)(H2O)+ and CH4 after ca. 48 h at 45 °C. The reaction was inhibited by added water, suggesting that water is reversibly lost from 6 before C−H activation takes place. The use of CD4 resulted in multiple deuterium incorporation into the methane produced, again implying a Pt−methane σ-complex intermediate. Low-temperature protonation of 1 in dichloromethane-d2 generated observable Pt(IV) hydride species (Nf-Nf)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 HNCHCHNH ligand. The C−H activation was investigated for oxidative addition and σ-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 σ-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.