Functional LnFe-NxHy Models of Biological N2 Fixation

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Project Summary - Functional LnFe-NxHy Models of Biological N2 Fixation Nitrogenase (N2-ase) metalloenzymes mediate biological nitrogen fixation and as such are essential to life. Their study correspondingly attracts intense interest from the biology and chemistry communities. Nonetheless, the mechanism by which nitrogenase enzymes promote the biological reduction of nitrogen under ambient conditions remains enigmatic. The broad questions that motivate our NIH-supported research program are as follows: Can a single iron site mediate the critical bond-making and breaking steps relevant to catalytic N2 fixation in a synthetic model system and, by extension, in biology? If so, what are the key intermediates and pathways that are accessible? How can a secondary metal center and/or secondary sphere interactions impact such reactivity? This renewal application builds on extensive progress made in the last grant period, reported via 21 primary literature publications. We propose to continue to design and study Fe-NxHy model complexes to address the questions highlighted above. Our approach stresses functionally, rather than structurally, faithful models of the iron-molybdenum cofactor (FeMoco). Low molecular weight Fe-NxHy complexes will be developed to explore iron sites in low coordinate geometries that accommodate N2 and more reduced NxHy. Two limiting single-site mechanisms for biological nitrogen fixation have been emphasized: the first is an alternating mechanism, where successive H-atom transfers (via H+/e- steps) occur at the distal and proximal N- atoms of an Fe-N≡N subunit in an alternating fashion (e.g., Fe-N=NH → Fe-NH=NH → Fe-NH-NH2→ Fe-NH2- NH2 → Fe-NH2 + NH3); the second is a distal mechanism, where three H-atom transfers at the distal N-atom to liberate an NH3 equivalent (e.g., Fe-N2 + 3 e- + 3 H+ → Fe≡N + NH3) precede H-atom transfers to the proximal N- atom. We also explore a hybrid cross-over mechanism that interweaves both paths. Our synthetic model studies are used to test the viability of each of these pathways, and to understand how the local geometry and electronic structure of Fe-NxHy species controls their reactivity patterns, with the goal of developing increasingly efficient Fe-mediated N2-to-NH3 conversion catalysts. Regardless of the precise mechanism/s of nitrogen reduction, the assignment of enzymatic intermediates relies upon the availability of well-defined spectroscopic parameters for Fe-NxHy models. We will continue to collect such data, and to collaborate with researchers that specialize in spectroscopic studies, including within nitrogenase enzymes, to enable useful comparisons to be made. In sum, the functional Fe-NxHy model chemistry proposed herein will continue to play a critical role alongside current biochemical, spectroscopic, and theoretical model studies aimed at unraveling the chemical mechanism/s of biological nitrogen fixation.