ENDOR Characterization of (N₂)Fe^II(μ-H)₂Fe^I(N₂)^-: A Spectroscopic Model for N₂ Binding by the Di-μ-hydrido Nitrogenase Janus Intermediate

The biomimetic diiron complex 4-(N₂)₂, featuring two terminally bound Fe-N₂ centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N₂. 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 N₂ through reductive elimination (RE) of the hydride ligands as H₂. The second is a possible RE intermediate. ¹H and ¹⁴N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N₂)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 ¹⁴N of N₂ 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 N₂ activation for reduction.