title: Nucleation process in the cavity of a 48-tungstophosphate wheel resulting in a 16-metal-centre iron oxide nanocluster.
abstract: The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(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 Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 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 1 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 mV s(-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(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.
authors.affiliation: Jacobs University Bremen, School of Engineering and Science, P.O. Box 750 561, 28725 Bremen, Germany.
authors.forename: Sib Sankar
authors.initials: SS
authors.lastname: Mal
authors.forename: Michael H
authors.initials: MH
authors.lastname: Dickman
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kortz
authors.forename: Ana Maria
authors.initials: AM
authors.lastname: Todea
authors.forename: Alice
authors.initials: A
authors.lastname: Merca
authors.forename: Hartmut
authors.initials: H
authors.lastname: Bögge
authors.forename: Thorsten
authors.initials: T
authors.lastname: Glaser
authors.forename: Achim
authors.initials: A
authors.lastname: Müller
authors.forename: Saritha
authors.initials: S
authors.lastname: Nellutla
authors.forename: Narpinder
authors.initials: N
authors.lastname: Kaur
authors.forename: Johan
authors.initials: J
authors.lastname: van Tol
authors.forename: Naresh S
authors.initials: NS
authors.lastname: Dalal
authors.forename: Bineta
authors.initials: B
authors.lastname: Keita
authors.forename: Louis
authors.initials: L
authors.lastname: Nadjo
mesh_terms: D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry*; D008670:Metals / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D049329:Nanostructures / Q000737:chemistry*; D044367:Phase Transition; D010710:Phosphates / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D017973:Tungsten Compounds / Q000737:chemistry*
chemical_list: D005290:Ferric Compounds; D008670:Metals; D010710:Phosphates; D017973:Tungsten Compounds; C000499:ferric oxide