Mo
lybdenum-dependent nitrogenase binds and reduces N
2 at the [Fe
7, Mo, S
9, X, homocitrate]iron-mo
lybdenum cofactor (FeMo-co). Kinetic and spectroscopic studies of nitrogenase variants indicatethat a sing
le Fe-S face is the most
like
ly binding site. Recent
ly, substantia
l progress has been made indetermining the structures of nitrogenase intermediates formed during a
lkyne and N
2 reduction throughuse of ENDOR spectroscopy. However, constraints derived from ENDOR studies of biomimetic comp
lexeswith known structure wou
ld powerfu
lly contribute in turning experimenta
lly derived ENDOR parametersinto structures for species bound to FeMo-co during N
2 reduction. The first report of a paramagnetic Fe-Scompound that binds reduced forms of N
2 invo
lved Fe comp
lexes stabi
lized by a bu
lky
![](/images/gifchars/beta2.gif)
le">-diketiminate
ligand(Ve
la, J.; Stoian, S.; F
laschenriem, C. J.; M&uum
l;nck, E.; Ho
lland, P. L.
J. Am. Chem. Soc. 2004,
126, 4522-4523). Treatment of a su
lfidodiiron(II) comp
lex with pheny
lhydrazine gave an iso
lab
le mixed-va
lence Fe
II-Fe
III comp
lex with a bridging pheny
lhydrazido (PhNNH
2)
ligand, and this species now has been characterizedby ENDOR spectroscopy. Using both
15N,
2H
labe
led and un
labe
led forms of the hydrazido
ligand, thehyperfine and quadrupo
le parameters of the -N-NH
2 moiety have been derived by a procedure thatincorporates the (near-) mirror symmetry of the comp
lex and invo
lves a strategy which combines experimentwith semiempirica
l and DFT computations. The resu
lts support the use of DFT computations in identifyingnitrogenous species bound to FeMo-co of nitrogenase turnover intermediates and indicate that
14Nquadrupo
le parameters from nitrogenase intermediates wi
ll provide a strong indication of the nature of thebound nitrogenous species. Comparison of the
large
14N hyperfine coup
lings 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 coup
ling coefficients of the coordinating cofactor iron ion(s) inthe intermediate are exceptiona
lly sma
ll.