Au^I Complexes with Planar Tetracoordinate Carbon and Their Catalytic Activity for the Rearrangement of Allylic...

Since the experimental synthesis of C₂B₂H₂ in 2017, the general class of C₂B₂R₂ with variable substituent group R has been speculated and is expected. Here we computationally investigate two rhombus molecules C₂B₂Me₂ (a) and C₂B₂ tBu₂ (b) which are confirmed to have carbene characteristics arising from charge shift based on the ab initio valence bond (VB) computations. These novel carbene molecules are structurally similar to N-heterocyclic carbenes (IMe and ItBu). Using them as ligands, six complexes (C₂B₂R₂)₂Au^I, (C₂B₂R₂)AuCl and (C₂B₂R₂)Au^I (R = Me and tBu) are designed and found to be thermodynamically stable on the basis of density functional theory (DFT) calculations. NBO analyses show that these six complexes contain planar tetracoordinate carbons (ptCs). The first vertical energies of these complexes are consistently red-shifted compared to their ligands C₂B₂R₂. We further explore the reaction mechanism of the rearrangement of allylic acetates catalyzed by (C₂B₂R₂)Au^I (R = Me and tBu) and their analogues (NHC)Au^I (NHC = IMe and ItBu) for comparison. It is found that the rearrangement of allylic acetates catalyzed by (C₂B₂R₂)Au^I is feasible both thermodynamically and kinetically, and the concerned energy barriers in the process are lower than those catalyzed by (NHC)Au^I. Thus, C₂B₂R₂ are promising novel molecules that can be used to rationally design a variety of compounds with ptCs for potential applications in catalysis.