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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.
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Mechanistic Insights into Key Steps of Dinitrogen Reduction to Ammonia with Pincer ligated Mo Complexes: Case Study of Easily Reduced PSP Mo Complexes
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 in 2011 by Nishibayashi and co-workers. Under an atmosphere of N2 the reaction of 1-Cl3 with three reducing equivalents yields the dinuclear penta-dinitrogen Mo complex [(PSP)Mo(N2)2](-N2), 2. Electrochemical studies reveal that 1-Cl3 is significantly more easily reduced than (PNP)MoCl3 (with a potential ca. 0.4 eV less negative). The bridging-nitrogen complex 2 shows no indication of undergoing N2 cleavage to Mo nitride complexes. The reaction of 1-Cl3 with only two reducing equivalents, however, under N2 atmosphere and in the presence of iodide, affords the product of N2 cleavage, the nitride complex (PSP)Mo(N)(I). DFT calculations implicate another N2-bridged complex, [(PSP)Mo(I)]2(N2), as a viable intermediate in facile N2 cleavage to yield (PSP)Mo(N)(I). Conversion of the nitride ligand to NH3 has been studied. If considering sequential addition of H atoms to the nitride, formation of the first N-H bond is by far the thermodynamically least favorable of the three N-H bond formation steps. The first N-H bond was formed by reaction of (PSP)Mo(N)(I) with [LutH]Cl, where coordination of Cl– to Mo plays an essential role. Computations suggest that a second protonation, followed by a rapid and very favorable one-electron reduction, and then a third protonation, furnishes ammonia. In agreement with calculations, ammonia can be generated using either mild H-atom transfer reagents or mild reductants/acids. This comprehensive analysis of the elementary steps of ammonia synthesis and the role of the central pincer donor and halide association provides guidance for future catalyst designs.
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- PAR ID:
- 10531503
- Publisher / Repository:
- ChemRxiv
- Date Published:
- Subject(s) / Keyword(s):
- Nitrogen reduction Nitrides Ammonia Dinitrogen cleavage
- Format(s):
- Medium: X
- Institution:
- Rutgers University
- Sponsoring Org:
- National Science Foundation
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- https://doi.org/10.26434/chemrxiv-2024-2v9z2
