Pyridine-, Pyridazine‑, Pyrimidine‑, and Pyrazine-Derived Carbenes as Ligands for Transition-Metal Complexes: Perspectives from DFT Calculations

A comprehensive theoretical analysis of the electronic structure, reactivity, and ligand properties of various types of carbenes derived from pyridine and diazines (pyridazine, pyrimidine, and pyrazine) was performed. These carbenes are divided into three classes: “normal” N-heterocyclic carbenes (NHCs) bearing one nitrogen atom at the ylidene carbon (1N-NHC), mesoionic carbenes (MIC), and remote-NHCs (r-NHC). These species have a singlet ground state. Carbenes under study bear high-lying lone electron pairs localized on the ylidene carbon atoms making them bases 2–18 pKaH units stronger than the reference imidazol-2-ylidenes. However, such carbenes are thermodynamically and kinetically less stable than conventional imidazol-2-ylidenes. Pyridinium and diazinium carbenes have exceptional ligand properties evaluated using a model (NHC)Rh(CO)2Cl complex: high metal–ligand binding energies and significantly higher electron donation and steric stabilization than those provided by its five-membered ring counterparts. Diazinium carbenes can act as ditopic ligands that bind Lewis acids to both ylidene carbon and nitrogen atoms. Protonation of diazinium carbenes transforms them from strong donors into strong π-acceptors comparable to phosphites. Theoretical calculations have shown that diazinium carbenes are promising ligands for the construction of complexes photoactive in the NIR region. A new type of chiral ligand C-PyBOX has been developed, promising for use in transition-metal-mediated enantioselective catalytic transformations.