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Characterization of the Self-Condensation Equilibrium of [Fe4S4(SH)4]2-:  Spectroscopic Identification of a Unique Sulfido-Bridged Acyclic Tricubane Cluster

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posted on 1997-09-24, 00:00 authored by H. R. Hoveyda, R. H. Holm
Our interest in higher nuclearity sulfido-bridged Fe−S clusters, because of their occurrence in several proteins including nitrogenase, prompted us to investigate the solution chemistry of the functionalized cluster [Fe4S4(SH)4]2- (1). (n-Pr4N)2[1] crystallizes in space group P21/n of the monoclinic system with a = 26.201(1) Å, b = 11.4999(5) Å, c = 28.090(1) Å, and β = 110.735(1)°. The X-ray structure reveals the conventional cubane-type geometry with an [Fe4S4]2+ core symmetry more closely approaching Td than the tetragonally distorted D2d symmetry reported for the (PPh4)2[1] (Müller, A.; Schladerbeck, N. H.; Bögge, H. J. Chem. Soc., Chem. Commun. 1987, 35). In solution, 1 exists in dynamic equilibrium with self-condensation products formed through elimination of H2S and formation of sulfido-bridged cluster oligomers, one of which (4) is prevalent. The self-condensation equilibrium is shifted toward cluster 1. When acetonitrile solutions of 1 were treated with thiols more acidic than H2S, it was possible to detect hydrosulfido terminal ligand substitution products of 1 as well as those of the major self-condensation product 4. Detailed analysis of the products in acetonitrile solutions of 1, as well as those generated in solutions of 1 treated with acidic thiol, by electrospray mass spectrometry, and both 19F and 1H NMR spectroscopy indicates the presence of a sulfido-bridged acyclic trimer of [Fe4S4]2+ clusters, i.e. {[Fe4S4(SH)3]2[Fe4S4(SH)2](μ-S)2}6- (4), a hitherto unprecedented Fe−S structural pattern, as the principal Fe−S cluster self-condensation product.

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