ENDOR Characterization of (N_2)Fe^(II)(μ-H)_2Fe^I(N_2)^−: A Spectroscopic Model for N_2 Binding by the Di-μ-hydrido Nitrogenase Janus Intermediate

The biomimetic diiron complex 4-(N_2)_2, featuring two terminally bound Fe–N_2 centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N_2. One is the Janus intermediate E4(4H), which has accumulated 4[e–/H+], stored as two [Fe–H–Fe] bridging hydrides, and is activated to bind and reduce N_2 through reductive elimination (RE) of the hydride ligands as H_2. The second is a possible RE intermediate. ^^1H and ^(14)N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N_2)_2 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 ^1H 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 ^(14)N of N_2 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_2 activation for reduction