A New Pair of Dice

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Science  07 Sep 2007:
Vol. 317, Issue 5843, pp. 1297
DOI: 10.1126/science.317.5843.1297b

The metal-sulfide clusters at the heart of the nitrogenase proteins pose questions for the biochemist—how are these assembled and inserted?—and for the inorganic chemist—what are the electronic properties that enable these enzymes to catalyze the reduction of dinitrogen under mild conditions? The P cluster (Fe8S7) and the FeMo cofactor (MoFe7S9) of the MoFe nitrogenase both contain a framework that can be described as two cubes joined at a vertex, which is either a hexacoordinate sulfur atom or an unidentified light atom (thought to be C, N, or O), with the central six iron atoms connected by two or three other sulfur ligands. Ohki et al. have synthesized a new [8Fe-7S] cluster where the incomplete cubanes are linked by a hexacoordinate sulfur and three anionic ligands. Bulky thiol substituents (R = dimesitylphenyl and triisopropylphenyl) and nonpolar solvents stabilized the dimeric [Fe2(μ-SR)2] core of the starting material, which after reaction with elemental sulfur for 4 days provided a Fe8S7 cluster in 28% yield. A cyclic voltammogram indicated that the cluster may be electrochemically active, and EPR measurements supported the assignment of the oxidation states as Fe(II)5Fe(III)3. The P cluster and the FeMo cofactor are but a few synthetic steps away, at least on paper, raising the possibility that this thermodynamically stable construct might also be accessed biochemically. — GJC

J. Am. Chem. Soc. 129, 10457 (2007).

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