| Abstract
| - Molybdenum-dependent nitrogenase binds and reduces N2 at the [Fe7, Mo, S9, X, homocitrate]iron-molybdenum cofactor (FeMo-co). Kinetic and spectroscopic studies of nitrogenase variants indicatethat a single Fe−S face is the most likely binding site. Recently, substantial progress has been made indetermining the structures of nitrogenase intermediates formed during alkyne and N2 reduction throughuse of ENDOR spectroscopy. However, constraints derived from ENDOR studies of biomimetic complexeswith known structure would powerfully contribute in turning experimentally derived ENDOR parametersinto structures for species bound to FeMo-co during N2 reduction. The first report of a paramagnetic Fe−Scompound that binds reduced forms of N2 involved Fe complexes stabilized by a bulky β-diketiminate ligand(Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522−4523). Treatment of a sulfidodiiron(II) complex with phenylhydrazine gave an isolable mixed-valence FeII−FeIII complex with a bridging phenylhydrazido (PhNNH2) ligand, and this species now has been characterizedby ENDOR spectroscopy. Using both 15N, 2H labeled and unlabeled forms of the hydrazido ligand, thehyperfine and quadrupole parameters of the -N−NH2 moiety have been derived by a procedure thatincorporates the (near-) mirror symmetry of the complex and involves a strategy which combines experimentwith semiempirical and DFT computations. The results support the use of DFT computations in identifyingnitrogenous species bound to FeMo-co of nitrogenase turnover intermediates and indicate that 14Nquadrupole parameters from nitrogenase intermediates will provide a strong indication of the nature of thebound nitrogenous species. Comparison of the large 14N hyperfine couplings measured here with that ofa hydrazine-derived species bound to FeMo-co of a trapped nitrogenase intermediate suggests that theion(s) are not high spin and/or that the spin coupling coefficients of the coordinating cofactor iron ion(s) inthe intermediate are exceptionally small.
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