Macrocyclic tetranuclear clusters with 28-, 32-, 36-, 40-, and 44-membered rings as high-potential iron-sulphur protein analogues

A new series of Fe₄S₄ active-site analogues for high-potential iron-sulphur proteins have been prepared from [Fe₄S₄(SBu^t)₄]^2- and macrocyclic tetra-aza tetrathiol ligands where the active-site cores are composed of an intramolecular hydrophobic domain formed by 28-, 32-, 36-, 40-, and 44-membered rings having methylene backbones. The compounds were obtained in good yields (70-90%) as black powders with m.p. >300°C. They dissolve in dimethylformamide and dimethyl sulphoxide, but are almost insoluble in most common organic solvents and water. A remarkable positive shift of the 1- to 2- redox couple has been observed for all the macrocyclic clusters examined. There is also a notable stabilization and ring-size effect upon the reaction of these clusters with molecular oxygen.