Diverse Behaviors of N on Mo Centers Bearing POCOP-Pincer Ligands and the Role of π-Electron Configuration in Regulating the Pathway of N Activation.

Activation of N through transition-metal complexes has emerged as a powerful strategy for N fixation under mild conditions. Dissociative route and associative route are considered as two major routes for N transformation on transition-metal complexes. Homolysis of N between two metal fragments is the crucial step of the dissociative route and has been proven to be an efficient approach to the terminal metal nitride, which is the key intermediate for both routes. Hence, the conditions for N cleavage have attracted much interest and discussion. Herein, we investigated the reactivity of N when coordinated on Mo centers bearing POCOP-pincer ligands and isolated and characterized many novel N-related intermediates such as [(POCOP)MoI](μ-N) (), (POCOP)Mo(N)(μ-N)MoI (), {[(POCOP)Mo(N)](μ-N)}[Na(crypt-222)] (), and [(POCOP)Mo(N)(μ-N)Mo(N)]Na (). The influences of the oxidation state of the metal centers, π electrons, reaction conditions, etc., on the N-reactivity were also studied both experimentally and theoretically. Accordingly, some fundamental understanding of the regulation of N activation pathways was proposed: an N-bridged Mo dimer without ligand to the bridging N is a preferred structure for N cleavage; having adequate electrons to be transferred into the σ-σ*-σ related orbital in the {MoNNMo} manifold is the key; and heating or electron excitation is advantageous to the dissociative route.