Synthesis and characterization of Fe₄S₄ clusters with a crown ether skeleton

Tetrathiol ligands attached to a 36-membered crown ether (7) have been prepared by reactions of 4,7,13,16,22,25,31,34-octaoxo-1,10,19,28-tetraazacyclohexatriacontane (5) and acylchlorides bearing a thiol function. By ligand exchange reaction of (Ph₄As)₂[Fe₄S₄(S^tBu)₄] (4c) and these tetrathiol ligands, Fe₄S₄ clusters with a crown ether type ring were synthesized as black powders (3). These clusters showed relatively good solubility and improved stability toward molecular oxygen as compared with the corresponding small thiolate Fe₄S₄ clusters (4). In addition, these clusters exhibited their redox potentials of a [Fe₄S₄]^1+/2+ couple in a polar solvent, whose potentials appeared in the region between those of the corresponding thiolate clusters with a methylene (1) or cyclophane (2) type ring and of the unclad clusters. This suggested that a hydrophobic environment is important in stabilizing the oxidized core.