Nitrogen Fixation Catalyzed by Dinitrogen-Bridged Dimolybdenum Complexes Bearing PCP- and PNP-Type Pincer Ligands: A Shortcut Pathway Deduced from Free Energy Profiles

Density-functional-theory (DFT) calculations are carried out for the proposal of a theoretically-plausible reaction pathway for nitrogen fixation catalyzed by dinitrogen-bridged dimolybdenum complexes bearing pincer-type PCP and PNP ligands. The free energy profiles of the entire reaction pathway calculated with a dispersion-corrected functional provide a catalytic mechanism energetically more efficient than the previously proposed one [Nat. Commun. 2014, 5, 3737]. In the newly-proposed mechanism the dinuclear Mo-N equivalent to N-Mo structure should be maintained during the catalytic cycle. Despite the coordination of the triflate group with a strong electron-withdrawing ability, dinitrogen coordinated to a Mo-I center has higher reactivity with a proton donor than that coordinated to an electron-rich Mo-0 center. The calculated results enable us to propose a shortcut pathway in which the regeneration of the dimolybdenum (MoMo0)-Mo-0 complex is not required. Intermetallic electron transfer between the two Mo centers induced by protonation effectively enhances the reactivity of coordinate N-2 with a proton donor. Our calculations reveal that the synergy of the Mo centers at the protonation step is essential for the catalytic performance of the dimolybdenum system.