Acceleration of Stepwise Carbon-Polygold Bonding Cleavage in Hypercoordinated Carbon-Centered Gold(I) Clusters

Abstract

Hypercoordinated carbon-centered organometallic clusters extensively exist in polymetallic and metal surface catalysis. The investigation on their electronic structures and transformations is crucial for mechanistic studies. Herein, we try to clarify the reactivity difference between a hypercoordinated tetrametalated species [PA-C-Au4]+ and a classical trimetalated one [(PA-C-Au3) + H]+ based on experimental and theoretical studies. Under a protonolysis condition, the hypercoordinated carbon species [PA-C-Au4]+ experiences a sequential cleavage process of carbon-polymetal bonding through the trinuclear [(PA-C-Au3) + H]+ intermediate to end up with a methyl group. Theoretical studies indicate that the high symmetry of five-center bonding in [PA-C-Au4]+ promotes the delocalization of negative charges, finally resulting in less nucleophilicity of the central hypercarbon atom. In contrast, the low C3V triangular arrangement of gold atoms in the [(PA-C-Au3) + H]+ structure causes the decrease in orbital interaction between the Aun moiety and the central carbon atom and the increase in the negative charge on the central carbon atom. Along with the gradual cleavage of the carbon-polymetal bonding, the accumulation of negative charges and the reduction of steric hindrance around the central carbon atom jointly contribute to the synergistic multi-step protodeauration process. This work not only illustrates the reactivity difference of RC-Mn species in different degrees of metalation but also provides an essential structural basis for the design of synergistic polymetallic catalysts.