Synthesis, structure, and thermochemistry of adduct formation between N-heterocyclic carbenes and isocyanates or mesitylnitrile oxide

Reaction of N-heterocyclic carbenes (NHCs) with isocyanates yields stable zwitterionic imidates/amidates in toluene solution. These compounds were fully characterized and the crystal structures of several species were determined by X-ray crystallography. The thermochemistry of binding of these and related species was studied by solution calorimetry. Comparison is made of the enthalpies of binding of NHC to isocyanates (RNCO) and isomeric nitrile oxides (RCNO) as well as CO₂. DFT calculations were performed to additionally assess the nature of bonding in these compounds.