Computational Screening of PCP-Type Pincer Ligands for Mo-Catalyzed Nitrogen Fixation

Abstract

Computational screening of N-heterocyclic carbene-based PCP-type pincer ligands 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 [Nat. Synth. 2023, 2, 635]. Here, we investigate electronic and energetic properties of nitrogenous Mo intermediates bearing 38 substituted PCP ligands [MoI(NHx)(R-PCP)] (x = 1-3) 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 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 (PCET). The introduction of strong electron-withdrawing substituents to the PCP ligand also increases the N–H bond energy of [MoI(NHx)(R-PCP)] evaluated with the bond dissociation free energy (BDFE) and the bond dissociation enthalpy (BDE). 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.