Investigating the Interconversion of Dinitrogen and Ammonia: An Interrogation of Secondary Coordination Sphere Influences on Critical Intermediates
Warren, Timothy H
The near thermoneutral interconversion of dinitrogen (N2) and ammonia (NH3) sustains life and society on Earth. Altogether, the N2 / NH3 interconversion involves a nexus of high energy intermediates connected through challenging N-H bond forming and breaking reactions that form high energy intermediates such as diazene (N2H2). Therefore, it is paramount to consider the influences governing N2H2 electronic structure and reactivity. Molecular N2H2 is highly unstable above -165 °C and is prone to bimolecular disproportionation to N2 and hydrazine (N2H4). Remarkably then, N2H2 formation from the initial 2 e- / 2 H+ reduction of N2 is considered in the mechanism of NH3 synthesis via nitrogenase enzymes. Controversial assignments of N2H2 binding modes at nitrogenase cofactors, however, highlights that different N2H2 coordination motifs lead to different mechanistic outcomes. Regardless, it is structurally evident that dynamic microenvironment H-bonding via proximal N-donor residues may govern transient N2H2 formation at cofactor sites. The challenges of studying NxHy species at native nitrogenase require that such intermediates are interrogated at discrete model complexes. Therefore, we set out to examine the influence of H-bonding to N2H2 using a novel, modular [xHetTpCu]2(μ-N2H2) platform that hosts tunable, dual H-bonding interactions between a bridging trans-N2H2 ligand and two pendant N-heterocycles (xHet). Using stable [xHetTpCu]2(μ-OH)2 precursors, the influence of tunable redox innocent H-bonding was initially gauged. These findings accent characterization of less stable [xHetTpCu]2(μ-N2H2) complexes, which manifest redox non-innocent H-bonding via partial H-atom transfer between N2H2 and H- xHet pendants. These findings foreshadow that H-atom transfer via H-bonding xHet pendants may avail future low energy pathways to interconvert N2 and NH3. Seeking to deliver efficient electrocatalysts that oxidize NH3 at low overpotentials based off of Fc+, we developed a [b-dik]CpFe platform to examine NH3 oxidation. Computational studies of this system outline key mechanistic paramters pertinent to inner- sphere NH3 oxidation. Furthermore, computational modelling of related [b-dik]xHetCpFe(NH3) species indicate that H-bonding between NH3 and xHet pendants favorably lowers NH3 oxidation overpotentials. Altogether these strategies underscore the importance of H-bonding in efficient N2 / NH3 interconversion.
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