Molybdenum pincer complex-based reduction of dinitrogen: mechanistic bifurcation at the stage of Mo(I)-N2 intermediates.

The synthesis and characterization of a new PPP ligand, bis(di-cyclopentylphosphinoethyl)phenylphosphine (PPP-Cyp), and its derived molybdenum complexes [MoBr3(PPP-Cyp)], [MoNCl2(PPP-Cyp)], [MoNBr2(PPP-Cyp)] and [Mo(η6-toluene)(PPP-Cyp)] are presented. The activities of these compounds towards the catalytic N2RR (nitrogen reduction reaction) and reductive N2 splitting are investigated. A combination of experimental results with DFT calculations elucidates the critical role of the alkyl substituents R at the terminal phosphine donors, which through steric and electronic interactions determine whether Mo(II)- and Mo(I)-N2 intermediates follow a bimolecular pathway, undergoing N-N cleavage, or a monomolecular pathway, undergoing further reduction to Chatt-type Mo(0) complexes. Importantly, these two pathways originate from a common intermediate, [MoX(N2)(PPP-R)], which thus is located at a bifurcation of molybdenum pincer-based N2RR mechanisms.