Determining the Impact of Ligand and Alkene Substituents on Bonding in Gold(I)-Alkene Complexes Supported by N-Heterocyclic Carbenes: A Computational Study.

Abstract

The nature of the gold(I)-alkene bond in [(NHC)Au(alkene)](+) complexes (where NHC is the N-heterocyclic carbene 1,3-bis(2,6-dimethylphenyl)imidazole-2-ylidine and its derivatives) has been studied using density functional theory. By utilization of a series of electron-withdrawing and electron-donating substituents ranging from -NO2 to -NH2, an examination of substituent effects has been undertaken with 4-substituted NHC ligands, monosubstituted ethylene derivatives, and 4-substituted styrene derivatives. Natural population, natural bond orbital (NBO), molecular orbital, and bond energy decomposition analysis (EDA) methods have been used to quantify a number of important parameters, including the charge of the coordinated alkenes and the magnitude of alkene→[(NHC)Au](+) and [(NHC)Au](+)→alkene electron donation. EDA methods have also been used to quantify the strength of the [(NHC)Au](+)-(alkene) bond and the impact of both ligand and alkene substitution on different components of the interaction, including polarization, orbital, electrostatic, and Pauli repulsive contributions. Finally, molecular orbital analysis has been used to understand the activation of the alkenes in terms of orbital composition and stabilization within the [(NHC)Au(alkene)](+) complexes relative to the free alkenes. These results provide important insight into the fundamental nature of gold(I)-alkene bonding and the impact of both ligand and alkene substitution on the electronic structure of these complexes.