Abstract

Our group has suggested previously that a hemi-labile role may be played by the interstitial light X-atom of the FeMo-cofactor, enabling a high degree of conformational and redox flexibility at a single iron N2 binding site. In this regard the central X-atom could act like a spring as the iron site coordinates various reduced nitrogenous ligands during turnover. This scenario would allow the iron center to modulate its local geometry by varying its degree of interaction with the light X-atom under crude local three-fold symmetry, possibly sampling trigonal bipyramidal, trigonal pyramidal, and/or pseudotetrahedral geometries as a function of the nature of the state of reduction of the nitrogenous ligand. Our group has been exploring a range of 4- and 5-coordinate iron complexes that exhibit local three-fold symmetry and feature a host of nitrogenous ligands (e.g., N2, N2H4 , NH3, NH2 , NR2 , N3-) in an axial substrate-binding site. These iron complexes will be discussed as simple inorganic models that can help us to consider and evaluate a single-iron site spring hypothesis for nitrogen reduction in biology.