ENDOR Characterization of (N)Fe(μ-H)Fe(N): A Spectroscopic Model for N Binding by the Di-μ-hydrido Nitrogenase Janus Intermediate.

The biomimetic diiron complex 4-(N), featuring two terminally bound Fe-N centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N. One is the Janus intermediate E(4H), which has accumulated 4[e-/H+], stored as two [Fe-H-Fe] bridging hydrides, and is activated to bind and reduce N through reductive elimination (RE) of the hydride ligands as H. The second is a possible RE intermediate. H and N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N) complex exhibits a fully delocalized, Robin-Day type-III mixed valency. The two bridging hydrides exhibit a fully rhombic dipolar tensor form, T ≈ [- t, + t, 0]. The rhombic form is reproduced by a simple point-dipole model for dipolar interactions between a bridging hydride and its "anchor" Fe ions, confirming validity of this model and demonstrating that observation of a rhombic form is a convenient diagnostic signature for the identification of such core structures in biological centers such as nitrogenase. Furthermore, interpretation of the H measurements with the anchor model maps the g tensor onto the molecular frame, an important function of these equations for application to nitrogenase. Analysis of the hyperfine and quadrupole coupling to the bound N of N provides a reference for nitrogen-bound nitrogenase intermediates and is of chemical significance, as it gives a quantitative estimate of the amount of charge transferred between Fe and coordinated N, a key element in N activation for reduction.