Mechanistic information on the reductive elimination from cationic Trimethylplatinum(IV) complexes to form carbon-carbon bonds

Cationic complexes of the type fac-[(L-2)(PtMe3)-Me-IV(pyr-X)][OTf] (pyr-X = 4-substituted pyridines; L-2 = diphosphine, viz., dppe = bis(diphenylphosphino) ethane and dppbz = o-bis(diphenylphosphino) benzene; OTf = trifluoromethanesulfonate) undergo C-C reductive elimination reactions to form [(L2PtMe)-Me-II(pyr-X)][OTf] and ethane. Detailed studies indicate that these reactions proceed by a two-step pathway, viz., initial reversible dissociation of the pyridine ligand from the cationic complex to generate a five-coordinate Pt-IV intermediate, followed by irreversible concerted C-C bond formation. The reaction is inhibited by pyridine. The highly positive values for Delta S-obs(double dagger) = +180 +/- 30 J K-1 mol(-1), Delta H-obs(double dagger), = 160 +/- 10 kJ mol(-1), and Delta V-obs(double dagger) = +16 +/- 1 cm(3) mol(-1) can be accounted for in terms of significant bond cleavage and/or partial reduction from Pt-IV to Pt-II in going from the ground to the transition state. These cationic complexes have provided the first opportunity to carry out detailed studies of C-C reductive elimination from cationic Pt-IV complexes in a variety of solvents. The absence of a significant solvent effect for this reaction provides strong evidence that the C-C reductive coupling occurs from an unsaturated five-coordinate Pt-IV intermediate rather than from a six-coordinate Pt-IV solvento species.