A new synthetic approach to the ferritin core uncovers the soluble iron(III) oxo-hydroxo aggregate [Fe11O6(OH)6(O2CPh)15].

Hydrolytic polymerization of iron(III) occurs in many reactions in vivo, for example, the formation of bacterial magnetite in magnetotactic organisms, biomineralization of iron and the synthesis of the metallic core of the iron-storage protein ferritin. The ferritin core contains aggregates of up to 4,500 oxygen-bridged, octahedrally coordinated, high-spin iron(III) centres and is attached to the protein shell through carboxylate groups of amino-acid side chains. The X-ray and electron-diffraction patterns of this core resemble those of the mineral ferrihydrite, a hydrated iron oxide formed in nature, inter alia, by iron-dependent bacteria. The preparation and structural characterization of such large poly-iron aggregates has been a challenge to inorganic chemists. We have recently shown that tri- and tetranuclear iron(III) oxo complexes of the type thought to be important in ferritin-core formation can be prepared by reacting mononuclear [FeCl4]- and binuclear [Fe2OCl6]2- components in aprotic solvents (ref. 9 and S.M.G., W. H. Armstrong and S.J.L., in preparation). Here we report the discovery of a remarkable new molecule, [Fe11O6(OH)6(O2CPh)15], obtained by hydrolysis of the [Fe2O]4+ unit in the presence of limited amounts of water and carboxylate salts. The synthesis and properties of this soluble iron(III) oxohydroxo aggregate should help to elucidate the mechanism of formation of poly-iron centres.