Super reduced Fe4S4 cluster of Balch's dithiolene series

A super reduced Fe4S4 cluster with a sulfur based radical, [NBu4](4)[(Fe3FeII)-Fe-III(mu(3)-S)(4)(mnt)(3)(6-)(mnt)(1-center dot)](4-center dot), (1) (mnt, maleonitrile dithiolate) which evolves H2S gas on treatment with acid under ambient conditions has been synthesized and structurally characterized. The Fe-S distances in 1 are in the range 2.246-2.383 angstrom, in stark contrast to that of the known n = -2 member of the series based on the [Fe-4(mu(3)-S)(4)(S2C2R2)(4)](n) unit (R = CF3, Ph) with Fe-S bond lengths of 2.149-2.186 angstrom. The EPR of 1 displays very weak signals at g, 4.03 and 2.38 along with a strong S-based radical EPR signal at g, 2.003 associated with five structured components tentatively assigned to hyperfine interaction arising out of the naturally abundant Fe-57 with < A > = 88 G. The EPR profile resembles the reduced Fe-S cluster of CO inhibited Clostridium pasteurianum W5 hydrogenase or the Fe4S4 centers of wild-type enzyme, IspH treated with HMBPP or IPP.