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The thioether–diphosphine pincer-ligated molybdenum complex (PSP)MoCl3 (1-Cl3, PSP = 4,5-bis(diisopropylphosphino)-2,7-di-tert-butyl-9,9-dimethyl-9H-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-t-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 ca. 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 et al. 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, e.g. triflate, can also act in this capacity although less effectively. These protonations, coupled with anion coordination, yield MoIV imide complexes, thereby circumventing the difficult formation of the first N–H bond corresponding to a low BDFE and formation of the respective MoIII 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 MoIV 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. 

Proton-switchable access to seven-coordinate ONNO dicarboxamide and NNNN dicarboxamidate rhenium oxo complexes provides a platform for understanding thermodynamics and bonding in pentagonal bipyramidal complexes. 

Great progress has been made in the past decade in the use of pincer-ligated transition metal complexes for the reduction of dinitrogen. Such complexes, however, have required 'pre-activation' by a strong reducing agent like Na/Hg or KC8 to achieve reductive N2 splitting. In this study, non-innocent pincer molybdenum(III) trihalide complexes, (PhPN5P)MoCl3 and (tBuPPHP)MoBr3, bearing acidic E-H (E = N or P) protons on the ligand periphery, have been utilized to investigate deprotonative N2 splitting. These complexes can be activated in the presence of KOtBu, without the need for a strong reductant. Reaction with KOtBu presumably affords (PhPN5P*)MoCl2 and (tBuPPP)MoBr2 respectively, through the loss of HX across the E-M bond. N2 binding at the vacant coordination site on the metal is followed by splitting of N2 to afford nitrides (PhPN5P*)MoVI(N)Cl2 and (tBuPPP)MoV(N)Br. Previous studies have demonstrated the reduction of molybdenum nitrides to ammonia in the presence of chem. reductants and proton sources but little is known about the relative reactivity of various nitrides and the detailed sequence of events leading to ammonia formation and regeneration of the active species. We, therefore, have begun an investigation of such catalytic cycles for ammonia formation. Mechanistic studies of the new complex (iPrPSP)Mo and of Nishibayashi's (tBuPNpyP)Mo systems, including DFT and electrochemical studies, revealed characteristic roles of the halides in splitting dinitrogen. A new pathway leading to the formation of ammonia and regeneration of the catalyst was elucidated.