Modeling the signatures of hydrides in metalloenzymes: ENDOR analysis of a di-iron Fe(μ-NH)(μ-H)Fe core

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L₃Fe(μ-NH)(μ-H) FeL₃ (L₃ = [PhB(CH₂PPh₂) ₃]^-) complex, 3, is a novel S = ¹/₂ type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. ^1,2H and ^14,15N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, -T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [-T, -T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum-iron cofactor of nitrogenase contains two [Fe-H ^--Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E₄). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E₄. This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.