Nucleation process in the cavity of a 48-tungstophosphate wheel resulting in a 16-metal-centre iron oxide nanocluster

The 16-Fe-III-containing 48-tungsto-8-phosphate [P8W48O184Fe16(OH)(28)(H2O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li4K16[P8W48O184Fe16(OH)(28)(H2O)(4)]center dot 66H(2)O center dot 2KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9)angstrom, b = 13.9708(3) angstrom, c = 26.9140(7) angstrom, and Z=2) and on the corresponding mixed sodium-potassium salt Na9K11-[P8W48O184Fe16(OH)(28)(H2O)4]center dot 100H(2)O (NaK-1, monoclinic space group C2/c, a = 46.552(4) angstrom, b = 20.8239(18) angstrom, c = 27.826(2) angstrom, = 97.141(2)degrees and Z = 4). Polyanion 1 contains-in the form of a cyclic arrangement-the unprecedented {Fe-16(OH)(28)(H2O)}(20+) nanocluster, with 16 edge- and corner-sharing FeO6 octahedra, grafted on the inner surface of the crown-shaped [H7P8W48O184](33-) (P8W8) precursor. The synthesis of I was accomplished by reaction of different iron species containing Fell (in presence of O-2) or Fe-III ions with the P8W, anion in aqueous, acidic medium (pH approximate to 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH I through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe-III. centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mVs(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO., including nitrate. Magnetic and variable frequency EPR measurements on I suggest that the Fe-III ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.