Mechanisms of Electrochemical N₂ Splitting by a Molybdenum Pincer Complex

Molybdenum complexes supported by tridentate pincer ligands are exceptional catalysts for dinitrogen fixation using chemical reductants, but little is known about their prospects for electrochemical reduction of dinitrogen. The viability of electrochemical N₂ binding and splitting by a molybdenum(III) pincer complex, (^pyPNP)MoBr₃ (^pyPNP = 2,6-bis(Bu₂PCH₂)-C₅H₃N)), is established in this work, providing a foundation for a detailed mechanistic study of electrode-driven formation of the nitride complex (^pyPNP)Mo(N)Br. Electrochemical kinetic analysis, optical and vibrational spectroelectrochemical monitoring, and computational studies point to two concurrent reaction pathways: In the reaction-diffusion layer near the electrode surface, the molybdenum(III) precursor is reduced by 2e^- and generates a bimetallic molybdenum(I) Mo₂(μ-N₂) species capable of N-N bond scission; and in the bulk solution away from the electrode surface, over-reduced molybdenum(0) species undergo chemical redox reactions via comproportionation to generate the same bimetallic molybdenum(I) species capable of N₂ cleavage. The comproportionation reactions reveal the surprising intermediacy of dimolybdenum(0) complex trans,trans-[(^pyPNP)Mo(N₂)₂](μ-N₂) in N₂ splitting pathways. The same "over-reduced" molybdenum(0) species was also found to cleave N₂ upon addition of lutidinium, an acid frequently used in catalytic reduction of dinitrogen.