The Haber Bosch process (HB) for the synthesis of ammonia from N2 was perhaps the most important technological invention of the 20th century. However, HB is responsible for ca. 2% of global fossil fuel consumption and the co-production of commensurate quantities of CO2. Toward the goal of replacing HB with more sustainable alternatives, we are investigating the development of molecular catalysts that can cleave N2 via a bimetallic pathway and then accept protons and electrons to give NH3. Complexes that can cleave N2 via bimetallic routes have been known since Cummins’ seminal report in 1995, but subsequent PCET of the resulting nitrides has not been well characterized. Herein we present synthesis, characterization and reactivity of a Mo complex, bearing Ozerov’s diarylamine-based pincer ligand (PNP), which is capable of splitting N2, presumably via an N2-bridged intermediate. Two-electron reduction of (PNP)MoBr3 under argon atmosphere results in the formation of the Mo-Mo quadruple bonded species [(PNP)MoBr]2. Reduction under 1 atm N2 in THF solvent affords a product which, based on single-crystal X-ray diffraction, spectroscopic evidence, and geometry optimization using DFT, is assigned as a Mo-imido complex. Thus, cleavage of N2 appears to have been accompanied or quickly followed by protonation and reduction, proceeding a step closer to the formation of NH3. Investigations are in progress to determine the source of the imido hydrogen atom and to elucidate the reaction mechanism. The imido complex can act as a catalyst, although turnover numbers are limited, for the reduction of N2 to ammonia using Cp*2Co and collidinium triflate as sources of electrons and protons.

Approaching nitrogen fixation using a diarylamine-based PNP-pincer molybdenum complexes

ACS Spring 2022

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