Cyclic (alkyl)(amino)carbenes in organic and organometallic methane C-H activation: a DFT and MCSCF study

Density functional theory (DFT) and multi-configurational self-consistent field (MCSCF) calculations are used to investigate the electronic and steric properties of cyclic (alkyl)(amino)carbenes (CAACs). Calculations show CAACs' diverse electronic characteristics in terms of its donor and acceptor capabilities. Reactions of CAACs in methane C-H bond activation via insertion and also as a supporting ligand (L) for L(Cp)M (M = Co, Rh, Ir) motifs via C-H oxidative addition are studied. The binding energies and buried volumes are calculated for selected CAACs as ancillary ligands to the L(Cp)Rh motif. Overall, CAACs show highly tunable electronic and steric properties by adjusting their substituents and backbones. Although CAAC may not be viable in activating inert small molecules with low polarity like methane, this class of ligand has great potential as ancillary ligands for transition metal complexes in catalysis. Calculation of C-H activation by L(Cp)Ir and L(Cp)Rh with CAACs as supporting ligands show CAACs can render the reaction more amenable to catalysis by destabilizing the oxidative addition products while keeping the reaction mildly exergonic, and the barriers reasonable.