Isolation and Characterization of Single and Sulfide-Bridged Double [4Fe–4S] Cubane Clusters with 4-Pyridinethiolato Ligands



Cuboidal iron–sulfur clusters, [4Fe–4S], are important electron-transfer (ET) sites in biology. In addition, more complex structures, usually consisting of modified or fused cubane clusters, are used as active sites in many important enzymes. For example, the Fe–Mo cofactor (FeMoco) of nitrogenase contains two fused cubanes. Here, we report the synthesis of three new para-pyridylthiolate ligated iron–sulfur cubane clusters, two single clusters (Bu4N)2[Fe4S4(SMePy)4] and (Bu4N)2[Fe4S4(SPy)4], and the sulfide-bridged double cubane (Bu4N)4[{Fe4S4(SPy)3}2S] with 4-pyridinethiolato exogenous ligands. The properties of these clusters were then explored by 1H NMR, IR, and UV/Vis spectroscopy, cyclic voltammetry (CV), and X-ray crystallography. Importantly, (Bu4N)4[{Fe4S4(SPy)3}2S] is the first example of a crystallographically characterized sulfide-bridged double cubane with all-thiolato exogenous ligands that is not supported by a large encapsulating ligand. This cluster shows a bridging Fe–S–Fe angle of 104°, its other structural parameters are in close agreement with those of the single-cluster analog (Bu4N)2[Fe4S4(SPy)4]. Finally, the one-electron-reduced forms of (Bu4N)2[Fe4S4(SPy)4] and (Bu4N)4[{Fe4S4(SPy)3}2S] were studied by low-temperature electron paramagnetic resonance (EPR) spectroscopy. Both clusters exhibit reversible one-electron reductions at –401 and –528 mV [vs. the normal hydrogen electrode (NHE)], respectively. The one-electron-reduced forms of both clusters show S = 1/2 ground states as evident from EPR spectroscopy at liquid-helium temperature. The temperature-dependent data for the double cubane further indicate that the extra electron is trapped in one of the clusters of the dimer and that a low-lying excited state is likely present in this complex, close in energy to the ground state.