Direct and Autocatalytic Reductive Elimination from Gold Complexes ([(Ph3 P)Au(Ar)(CF3 )(X)], X=F, Cl, Br, I): The Key Role of Halide Ligands.

Abstract

Kinetic and thermodynamic preferences for the reductive elimination of Caryl -CF3 , Caryl -X, Caryl -P, and CF3 -X bonds and competitive phosphine dissociation from a series of AuIII complexes [(Ph3 P)Au(Ar)(CF3 )(X)] (1 X ; Ar=4-Me-C6 H4 ; X=F, Cl, Br, I) are studied computationally. Kinetically, the most favorable pathways were found to consist of an initial phosphine dissociation from complex 1 X , which furnished the respective three-coordinate AuIII complexes [Au(Ar)(CF3 )(X)] (2 X ). The computed enthalpy barriers for various reductive elimination reactions from complex 2 X by a direct (or uncatalyzed) mechanism showed that Caryl -CF3 bond formation was the most favorable fate for any X group. When the direct elimination was compared with an autocatalytic mechanism that proceeded through the formation of a mixed-valent binuclear AuIII -AuI intermediate, the preference for the formation of a Caryl -CF3 bond is dependent on the nature of the bridging halide atom and follows the order F>Cl>Br>I. Concomitantly, the selectivity for the formation of Caryl -X bonds for various X atoms follows the opposite trend. The preference for the direct and autocatalytic processes is controlled entirely by the nature of the halide ligand. The predicted mechanisms and product selectivity trends for various halides show excellent agreement with recent experimental observation. The selectivity of various reductive elimination pathways was rationalized by using molecular orbital theory and distortion-interaction model analyses. Attractive interactions between the AuI complex and complex 2X were found to reduce the activation barrier for Caryl -X elimination and critically control the selectivity of the product formation.