Proton-Coupled Electron Transfer. ChemRxiv. Preprint. The direct scission of the triple bond of dinitrogen (N2) by a metal complex is an alluring entry point into the transformation of N2 to ammonia (NH3) in molecular catalysis. Reported herein is a pincer-ligated rhenium system that reduces N2 to NH3 via a well-defined reaction sequence involving reductive formation of a bridging N2 complex, photolytic N2 splitting, and proton-coupled electron transfer (PCET) reduction of the metal-nitride bond. The new complex (PONOP)ReCl3 (PONOP = 2,6-bis(diisopropylphosphinito)pyridine) is reduced under N2 to afford the trans,trans-isomer of the bimetallic complex [(PONOP)ReCl2]2(μ-N2) as an isolable kinetic product that isomerizes sequentially upon heating into the trans,cis and cis,cis isomers. All isomers are inert to thermal N2 scission, and thetrans,trans-isomer is also inert to photolytic N2 cleavage. In striking contrast, illumination of the trans,cisand cis,cis-isomers with blue light affords the octahedral nitride complex cis-(PONOP)Re(N)Cl2 in 47% spectroscopic yield and 11% quantum efficiency. The photon energy drives an N2 splitting reaction that is thermodynamically unfavorable under standard conditions, producing a nitrido complex that reacts with SmI2/H2O to produce a rhenium tetrahydride complex and furnish ammonia in 74% yield. File list (2) download file view on ChemRxiv Bruch_PONOPReN2NH3_ChemRxiv_Manuscript.pdf (1.06 MiB) download file view on ChemRxiv Bruch_PONOPReN2NH3_ChemRxiv_SI.pdf (15.70 MiB)

Despite advances in the development of molecular catalysts capable of reducing dinitrogen to ammonia using proton donors and chemical reductants, few molecular electrocatalysts have been discovered. This Perspective considers the prospects of electrocatalyst development based on a mechanism featuring the cleavage of N2 into metal nitride complexes. By understanding the factors that control the reactivity of individual steps along the electrochemical N2 cleavage path, opportunities for new advances are identified. Ligand design principles for facile electrochemical N2 binding, formation of bridging N2 complexes, thermal or photochemical N2 cleavage, and conversion of a nitride ligand into ammonia are described, featuring recent advances and the authors’ collaborative work on rhenium complexes.

The Haber-Bosch process is a major contributor to fixed nitrogen that supports the world's nutritional needs and is one of the largest-scale industrial processes known. It has also served as a testing ground for chemists’ understanding of surface chemistry. Thus, it is significant that the most thoroughly developed catalysts for N2 reduction use potassium as an electronic promoter. In this review, we discuss the literature on alkali metal cations as promoters for N2 reduction, in the context of the growing knowledge about cooperative interactions between N2, transition metals, and alkali metals in coordination compounds. Because the structures and properties are easier to characterize in these compounds, they give useful information on alkali metal interactions with N2. Here, we review a variety of interactions, with emphasis on recent work on iron complexes by the authors. Finally, we draw conclusions about the nature of these interactions and areas for future research.

Iron complexes containing tetradentate monophenolate ligands have been found to be highly active for the electrocatalytic reduction of protons to hydrogen gas. Catalysis occurs at -1.17 V vs SCE in CH3CN with a turnover frequency of up to 1000 s(-1) and a 660 mV overpotential. Interestingly, the catalyst activity is enhanced in the presence of water, achieving turnover frequencies of 3000 s(-1) with an overpotential of 800 mV, making it one of the most active iron electrocatalysts currently reported. The catalyst is also capable of generating hydrogen from purely aqueous buffer solutions of pH 3-5 with Faradaic efficiencies of 98%.