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)MoCl₃ (1-Cl₃, PSP = 4,5-bis(diisopropylphosphino)-2,7-di-<i>tert</i>-butyl-9,9-dimethyl-9<i>H</i>-thioxanthene) has been synthesized as a catalyst-precursor for N₂ 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-<i>t</i>-butylphosphinomethyl)pyridine) fragment found in the family of catalysts for the reduction of N₂ to NH₃ first reported by Nishibayashi and co-workers. Electrochemical studies reveal that 1-Cl₃ is significantly more easily reduced than (PNP)MoCl₃ (with a potential <i>ca.</i> 0.4 eV less negative). The reaction of 1-Cl₃ with two reducing equivalents, under N₂ atmosphere and in the presence of iodide, affords the nitride complex (PSP)Mo(N)(I). This observation suggests that the N₂-bridged complex [(PSP)Mo(I)]₂(N₂) 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 <i>et al.</i> Conversion of the nitride ligand to NH₃ 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, <i>e.g.</i> triflate, can also act in this capacity although less effectively. These protonations, coupled with anion coordination, yield Mo^IV imide complexes, thereby circumventing the difficult formation of the first N-H bond corresponding to a low BDFE and formation of the respective Mo^III 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^IV 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.