Sulfur K-edge X-ray absorption spectroscopy of a hydrogen-bonded elongated [Fe 4 S 4 ] 2+ cube is reported. The data show that this synthetic cube is less covalent than a normal compressed cube with no hydrogen bonding. DFT calculations reveal that the observed difference in electronic structure has significant contributions from both the cluster distortion and from hydrogen bonding. The elongated and compressed Fe 4 S 4 structures are found to have different spin topologies (i.e., orientation of the delocalized Fe 2 S 2 subclusters which are antiferromagnetically coupled to each other). It is suggested that the H-bonding interaction with the counterion does not contribute to the cluster elongation. A magneto-structural correlation is developed for the Fe 4 S 4 cube that is used to identify the redoxactive Fe 2 S 2 subclusters in active sites of HiPIP and ferredoxin proteins involving these clusters.

The Fe4S4 complex {(CH3)3NCH2CONH2}2[Fe4S4((tBuS)4] (1) was synthesized to replicate the ferredoxin active site with a subset of its N-H...S hydrogen bonds. The two cationic counterions mimic the polypeptide backbone of ferredoxin (Fd) as amide hydrogen-bond donors to sulfur atoms of the iron-sulfur cluster. X-ray crystallographic data show that the organic sulfur (Sgamma) of one tert-butylthiolate ligand and one inorganic sulfur of the cluster core serve as N-H...S hydrogen-bond acceptors. The cluster core of complex 1 is tetragonally elongated in contrast to that of Fd, which is tetragonally compressed. This is the first observation of an elongated [Fe4S4]2+ cluster core. Additionally, this is the first synthetic Fd model in which N-H...S hydrogen bonding to a cluster has been achieved.