Dinitrogen reduction to ammonia with a pincer-Mo complex: new insights into the mechanism of nitride-to-ammonia conversion.

The thioether-diphosphine pincer-ligated molybdenum complex (PSP)MoCl (1-Cl, PSP = 4,5-bis(diisopropylphosphino)-2,7-di--butyl-9,9-dimethyl-9-thioxanthene) has been synthesized as a catalyst-precursor for N reduction catalysis with a focus on an integrated experimental/computational mechanistic investigation. The (PSP)Mo unit is isoelectronic with the (PNP)Mo (PNP = 2,6-bis(di--butylphosphinomethyl)pyridine) fragment found in the family of catalysts for the reduction of N to NH first reported by Nishibayashi and co-workers. Electrochemical studies reveal that 1-Cl is significantly more easily reduced than (PNP)MoCl (with a potential 0.4 eV less negative). The reaction of 1-Cl with two reducing equivalents, under N atmosphere and in the presence of iodide, affords the nitride complex (PSP)Mo(N)(I). This observation suggests that the N-bridged complex (PSP)Mo(I) is formed and undergoes rapid cleavage. DFT calculations predict the splitting barrier of this complex to be low, in accord with calculations of (PNP)Mo and a related (PPP)Mo complex reported by Merakeb Conversion of the nitride ligand to NH has been investigated in depth experimentally and computationally. Considering sequential addition of H atoms to the nitride through proton coupled electron-transfer or H-atom transfer, formation of the first N-H bond is thermodynamically relatively unfavorable. Experiment and theory, however, reveal that an N-H bond is readily formed by protonation of (PSP)Mo(N)(I) with lutidinium chloride, which is strongly promoted by coordination of Cl to Mo. Other anions, triflate, can also act in this capacity although less effectively. These protonations, coupled with anion coordination, yield Mo imide complexes, thereby circumventing the difficult formation of the first N-H bond corresponding to a low BDFE and formation of the respective Mo imide complexes. The remaining two N-H bonds required to produce ammonia are formed thermodynamically much more favorably than the first. Computations suggest that formation of the Mo imide is followed by a second protonation, then a rapid and favorable one-electron reduction, followed by a third protonation to afford coordinated ammonia. This comprehensive analysis of the elementary steps of ammonia synthesis provides guidance for future catalyst design.