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1. Mechanism of Chemical and Electrochemical N ₂ Splitting by a Rhenium Pincer Complex

   https://doi.org/10.1021/jacs.8b03755  

   Lindley, Brian M.; van Alten, Richt S.; Finger, Markus; Schendzielorz, Florian; Würtele, Christian; Miller, Alexander J.; Siewert, Inke; Schneider, Sven (June 2018, Journal of the American Chemical Society)
2. Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System

   https://doi.org/10.1021/acs.inorgchem.0c03778  

   Weber, Jeremy E.; Hasanayn, Faraj; Fataftah, Majed; Mercado, Brandon Q.; Crabtree, Robert H.; Holland, Patrick L. (May 2021, Inorganic Chemistry)

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

   https://doi.org/10.1039/d5sc00454c  

   Mandal, Souvik; Zhou, Xiaoguang; Bruch, Quinton J; Allen, Rachel N; Giordano, Laurence W; Walker, Nicholas_J I; Emge, Thomas J; Hasanayn, Faraj; Miller, Alexander_J M; Malakar, Santanu; et al (January 2025, Chemical Science)

   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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4. Mechanisms of Electrochemical N ₂ Splitting by a Molybdenum Pincer Complex

   https://doi.org/10.1021/acs.inorgchem.1c03698  

   Bruch, Quinton J.; Malakar, Santanu; Goldman, Alan S.; Miller, Alexander J. (January 2022, Inorganic Chemistry)
5. A Cationic Pt(II) Dinitrogen Adduct: Synthesis and Characterization of a CCC-aNHC Pincer Pt Complex with Abnormal N -Heterocyclic Carbene Donor Arms

   https://doi.org/10.1021/acs.organomet.3c00411  

   Fosu, Evans; Le, Nghia; Abdulraheem, Taofiq; Donnadieu, Bruno; Patrick, Amanda L.; Webster, Charles Edwin; Hollis, T. Keith (February 2024, Organometallics)

   In contrast to the reported CCC-NHC pincer ligands that contain normal N-heterocyclic carbenes (NHC), herein we report an imidazole-based abnormal NHC (aNHC) pincer ligand, CCC-aNHC. The CCC-aNHC pincer Pt complex with two aNHC donors was synthesized via the in situ metalation and transmetalation methodology. The 1,3-phenylene(bis-2-phenyl-3-butyl imidazolium) diiodide salt was reacted with Zr(NMe2)4 to generate a CCC-aNHC pincer zirconium complex in situ. It was transmetalated to Pt using [Pt(COD)Cl2]. Electrospray ionization of the Pt pincer complex [(BuCa‑iCa‑iCBu)-PtI] in acetonitrile generated an intense peak at m/z = 696.2375, which was assigned to the dinitrogen adduct [M−I+N2]+ of the cationic CCC-aNHC pincer Pt(II) complex [(BuCa‑iCa‑iCBu)Pt− N2]+, representing a rare example of the platinum dinitrogen organometallic complex. The super electron-donating ability of the pincer ligands with abnormal NHC enabled the cationic CCC-aNHC pincer Pt(II) complex to selectively bind N2 over MeCN in a first-order analysis. A collision-induced dissociation (CID) study was conducted on the N2 and MeCN adducts, suggesting that more energy was required to dissociate N2 than MeCN. A computational study suggested that the N2 adduct was kinetically stable in the gas phase whereas the MeCN adduct was thermodynamically preferred. The computational results reconciled the mass spectral data experiment with an attempt to isolate the N2 adduct. DFT computation suggested that N2 dissociation is more challenging due to higher energy transition states, and there is a competitive pathway of N2 tumbling within the coordination sphere of the Pt. This tumbling path is not available from the MeCN ligand due to ligand structural differences. 

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