Computational Study of Reductive Elimination Reactions to Form C−H Bonds from Platinum(II) and Platinum(IV) Centers with Strongly Coordinating Trimethylphosphine Ligands

Abstract

B3LYP calculations have been performed on the effects of replacing PH3 with PMe3 ligands in the reductive elimination of methane from cis-hydridomethyl−bisphosphine platinum(II) and platinum(IV) model complexes. In both the Pt(II) and Pt(IV) complexes, the replacement of PH3 ligands by the more strongly basic PMe3 ligands is predicted to favor a direct mechanism for reductive elimination over one in which the initial step is phosphine ligand loss. However, the effect of the increased platinum−phosphine binding enthalpy on the ligand-predissociation mechanism was found to be partially canceled by an increase in the barrier height computed for the direct mechanism. It was possible to locate the transition structures for direct reductive elimination of methane from several isomers of Cl2(PMe3)2Pt(CH3)H. In contrast, the lower binding enthalpy of PH3, compared to PMe3, made it impossible to locate the transition structures for direct elimination from the corresponding isomers of Cl2(PH3)2Pt(CH3)H. Our computational results suggest that destabilizing cis interactions between the atoms of the axial and equatorial ligands that are coordinated to platinum are responsible for the lower phosphine binding enthalpy in six-coordinate Pt(IV), compared to four-coordinate Pt(II) complexes. The much greater propensity of Pt(IV) than of Pt(II) complexes to undergo reductive elimination via a ligand-predissociation mechanism can be attributed to these interactions. Our calculations suggest that Pt(IV) complexes with chelating trialkylphosphine ligands should be good candidates for undergoing C−H reductive eliminations by a direct mechanism, rather than by a ligand predissociation pathway.