Boron-boron, carbon-carbon and nitrogen-nitrogen bonding in N-heterocyclic carbenes and their diazaboryl and triazole analogues: Wanzlick equilibrium revisitedElectronic supplementary information (ESI) available: Details of computational methods, geometric parameters, energies and Cartesian coordinates of all investigated systems. See DOI: 10.1039/c8nj00296g

A series of monomers and dimers of N-heterocyclic carbenes and their diazaboryl and triazole analogues were investigated using the M06-2X and M06 density functional methods as well as using a DLPNO-CCSD(T) approach. We show that asymmetric carbenes bearing a benzyl and a mesityl moiety are predicted to form stable dimers due to favorable off-centre parallel stacking interactions. We also show that the double bond of the imidazole core destabilizes carbene dimers, shifting the Wanzlick equilibrium towards monomers. We predict that the methyl-, ethyl- and mesityl-substituted diazaboroles are more stable as monomers, irrespective of the presence of the double-bond in their structure, as are triazoles. Additionally, we demonstrate that the HOMO-LUMO gap difference between the monomer and the dimer, which is relatively easy to obtain, is a good indicator of the relative stability of the dimer versus the monomer for carbenes. Diazaboroles and triazoles are predicted to be unstable as dimers, in contrast to carbenes with small alkyl substituents or flexible side-groups.