Project Summary/Abstract Entry of nitrogen into the biosphere is crucial for the development and sustainability of life, as this element is utilized in the development of proteins, nucleic acids, and other cell constituents. Nature uses nitrogenase enzymes to mediate the transformation of the bioinactive atmospheric N2 into more reactive nitrogen sources such as NH3. Ubiquitous to all nitrogenase enzymes are multi-nuclear transition metal cofactors which act as the active site for N2 binding, reduction, and transformation into its reduced substrates. Despite the foundational role of the N2 fixation cycle to biology, the detailed mechanism of how nitrogenase enzyme metallocofactors facilitate N2 fixation is largely uncertain. Synthetic chemists are actively pursuing mechanistic elucidation of the N2 fixation by nitrogenase metallocofactors by using coordination chemistry to design molecular architectures which can bind and reduce N2. Significant progress has been made in this regard, especially in terms of the reduction of N2 with mononuclear transition metal compounds, but it is in question whether mononuclear systems provide accurate models for polynuclear metallocofactor sites. To this end, the use of polynuclear high-spin transition metal complexes as functional models for nitrogenase metallocofactors have been much less explored. The research program described herein involves the synthesis, characterization, and reactivity of new all monovalent [Fe3] molecular architectures which we propose could be functional models to explore the mechanism of the Fe- Mo cofactor (FeMoco) of nitrogenase. Initially, a new trianionic, hexadentate, ligand scaffold [NPL]3- will be synthesized which can support three monovalent first-row transition metal ions. The trimetallation of [NPL]3- with monovalent Fe(I) will be carried out to form the proposed monovalent tri-iron complex, (NPL)Fe3. Single-crystal X-ray diffraction, SQUID magnetometry, EPR spectroscopy, and cyclic voltammetry will be used to determine the structure, spin-state, and redox properties of (NPL)Fe3. The reactivity of the new [Fe3] cluster will be evaluated by reacting (NPL)Fe3 with simple chemical oxidants and nitrogenase substrates (i.e. N2 and CO). Compound (NPL)Fe3 will be subjected to reactions with oxidative N-group transfer reagents or N2 surrogates to establish the N-bound intermediates involved on the way to full N2 conversion. The oxidative group transfer reactivity of (NPL)Fe3 will also be explored using S-, O-, and C- group transfer reagents where the proposed sulfide complex, [(NPL)Fe3(µ3-S)]-, will be used to model the role of sulfide ligands in the N2 fixation of FeMoco. We will synthesize mono- and poly-hydride complexes, (NPL)Fe3H1-3, which could act as synthetic models for the E4 state of FeMoco, which is the intermediate proposed to bind N2. The reactivity of the hydride complexes will be explored with nitrogenase substrates such as N2, CO, and acetylene and the formation of t...