Computational screening of PCP-type pincer ligands for Mo-catalyzed nitrogen fixation

Abstract

Abstract Computational screening of N-heterocyclic carbene-based PCP-type pincer ligands (PCP = 1,3-bis(di-tert-buthylphosphinomethyl)benzimidazole-2-ylidene) has been performed for the design of molybdenum-based molecular catalysts for nitrogen fixation. Previously, we theoretically and experimentally demonstrated that the introduction of electron-donating/withdrawing substituents to the original PCP ligand is a promising way to control the catalytic activity. Here, we investigate electronic and energetic properties of nitrogenous Mo intermediates bearing 40 substituted PCP ligands [MoI(NHx)(R-PCP)] (x = 1 to 3) that are involved in the rate-determining step in our proposed catalytic mechanism. Electron-withdrawing substituents enhance the π-accepting ability of R-PCP and effectively stabilize the lowest unoccupied molecular orbital (LUMO) of the corresponding Mo–nitride (Mo≡N) complexes, which is expected to be advantageous for the transformation of the nitride N atom via proton-coupled electron transfer. The introduction of strong electron-withdrawing substituents to the PCP ligand also increases the N–H bond energy of [MoI(NHx)(R-PCP)] evaluated using the bond dissociation free energy and the bond dissociation enthalpy. As a result of the computational screening, we newly propose an alternative strategy for designing PCP ligands with high π-accepting ability: the extension of the π-conjugated system of the PCP ligand by introducing fused benzene rings.