Nitrogenases generate bioavailable nitrogen by catalyzing the 6-electron reduction of N2 to NH3. This reaction is vital to life, and understanding its mechanism is therefore of great fundamental interest. In nitrogenase enzymes, N2 reduction occurs at metalloclusters, the most well-studied of which is the iron-molybdenum cofactor (FeMoco). In the resting state of the FeMoco, six central iron atoms are arranged around a biologically unprecedented central carbide (C4-) and are additionally coordinated by sulfides (S2?). Recent experimental evidence suggests that the carbide plays a structural role, and that the sulfides may dissociate in order to act as proton shuttles during turnover. However, these proposals are largely speculative since the understanding of the mechanism of nitrogenase is still at an early stage. In order to illustrate the fundamental chemistry of N2 reduction in biologically relevant iron coordination environments, we propose to generate simple model complexes and study their electronic structure and reactivity. Unlike known model complexes which generally contain abiological N and P donors, our complexes will contain exclusively biologically relevant C and S donors. To accomplish this, we will synthesize iron complexes of novel ligands that incorporate sterically bulky thiolates and a central N-heterocyclic carbene (NHC). We hypothesize that the strong Fe-NHC bond will act as an anchor (similar to the role proposed for the carbide in the FeMoco), increasing the overall stability of the complex and preventing ligand dissociation under the strongly acidic conditions required for catalytic NH3 formation. This will also allow the thiolates to mimic the proposed behavior of the sulfides in the FeMoco by dissociating from the iron center to act as proton shuttles and/or by stabilizing partially reduced and protonated N2 reduction intermediates through hydrogen bonding interactions. Starting from the Fe-N2 complex, we will perform stepwise proton and electron transfer reactions in order to determine the elementary mechanistic steps of NH3 formation in these compounds. We will also independently synthesize and spectroscopically characterize key intermediates in N2 reduction. These studies will illustrate the electronic structure and reactivity of N2 and N2-derived NxHy fragments in a sulfur-rich coordination environment and show whether the mechanism of N2 production proposed for the FeMoco is chemically reasonable.
Nitrogenase enzymes convert atmospheric N2 to bioavailable NH3 and are therefore a key part of the global nitrogen cycle, but their mechanism is not well-understood. We will synthesize model complexes that mimic the coordination sphere of the enzyme active site. Studying NH3 formation from N2 in these compounds will allow us to demonstrate the chemical feasibility of proposed steps in the mechanism of nitrogenases.
