A trans-1,2 End-On Disulfide-Bridged Iron–Tetracarbene Dimer and Its Electronic Structure

A disulfide-bridged diiron complex with [Fe–S–S–Fe] core, which represents an isomer of the common biological [2Fe–2S] ferredoxin-type clusters, was synthesized using strongly σ-donating macrocyclic tetracarbene capping ligands. Though the complex is quite labile in solution, single crystals were obtained, and the structure was elucidated by X-ray diffraction. The electron-rich iron–sulfur core is found to show rather unusual magnetic and electronic properties. Experimental data and density functional theory studies indicate extremely strong antiferromagnetic coupling (−J > 800 cm–1) between two low-spin iron(III) ions via the S22– bridge, and the intense near-IR absorption characteristic for the [Fe–S–S–Fe] core was assigned to a S → Fe ligand-to-metal charge transfer transition.