CHEMISTRY: A Chain to Break Nitrogen

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Science  22 Jun 2007:
Vol. 316, Issue 5832, pp. 1671a
DOI: 10.1126/science.316.5832.1671a

Synthetic chemists continue to puzzle over how bacteria manage the feat of reducing triply bonded nitrogen to ammonia without the help of extreme temperatures or pressures. Among the clues teased out of nitrogenase enzyme studies is the possible involvement of paramagnetic iron hydride centers. However, low-valent iron model compounds tend to be diamagnetic. Sadique et al. have applied a sterically bulky β-diketiminate ligand (L) that, despite differing structurally from the sulfur ligands in the enzyme, does coordinatively stabilize highspin Fe(II) hydrides. Moreover, like nitrogenase, the resulting model complexes can fully cleave an N=N double bond. Through a series of careful experiments, the authors explored the mechanism whereby two of these paramagnetic LFe-H centers split azobenzene (PhNNPh) to yield LFeNHPh complexes. They showed first that reaction with excess azobenzene leads to an isolable intermediate, LFeN(Ph)NHPh (shown above), which on heating is transformed into LFeNHPh and half an equivalent of azobenzene. Kinetic studies on this latter step revealed it to be first order in iron, after an induction period, which ruled out a bimolecular pericyclic rearrangement to the products. Moreover, the intermediacy of LFe-H (stemming from β-hydride elimination) was inconsistent with trapping studies. The data were most consistent with a radical chain mechanism initiated by homolytic dissociation to LFe and PhNNHPh, a hypothesis supported by disappearance of the induction period on addition of the Fe(I) complex K[LFeCl] to the reaction mixture. The results argue for deeper consideration of single-electron chemistry in probing the enzyme mechanism. — JSY

J. Am. Chem. Soc. 129, 10.1021/ja069199r (2007).

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