title: Chemical Stabilization and Electrochemical Destabilization of the Iron Keggin Ion in Water.
abstract: The iron Keggin ion is identified as a structural building block in both magnetite and ferrihydrite, two important iron oxide phases in nature and in technology. Discrete molecular forms of the iron Keggin ion that can be both manipulated in water and chemically converted to the related metal oxides are important for understanding growth mechanisms, in particular, nonclassical nucleation in which cluster building units are preserved in the aggregation and condensation processes. Here we describe two iron Keggin ion structures, formulated as [Bi6FeO4Fe12O12(OH)12(CF3COO)10(H2O)2]3+(Kegg-1) and [Bi6FeO4Fe12O12(OH)12(CF3COO)12]1+(Kegg-2). Experimental and simulated X-ray scattering studies show indefinite stability of these clusters in water from pH 1-3. The tridecameric iron Keggin-ion core is protected from hydrolysis by a synergistic effect of the capping Bi3+cations and the trifluoroacetate ligands that, respectively, bond to the iron and bridge to the bismuth. By introducing electrons to the aqueous solution of clusters, we achieve complete separation of bismuth from the cluster, and the iron Keggin ion rapidly converts to magnetite and/or ferrihydrite, depending on the mechanism of reduction. In this strategy, we take advantage of the easily accessible reduction potential and crystallization energy of bismuth. Reduction was executed in bulk by chemical means, by voltammetry, and by secondary effects of transmission electron microscopy imaging of solutions. Prior, we showed a less stable analogue of the iron Keggin cluster converted to ferrihydrite simply upon dissolution. The prior and currently studied clusters with a range of reactivity provide a chemical system to study molecular cluster to metal oxide conversion processes in detail.
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Omid
authors.initials: O
authors.lastname: Sadeghi
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Clément
authors.initials: C
authors.lastname: Falaise
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Pedro I
authors.initials: PI
authors.lastname: Molina
authors.affiliation: Department of Materials Science and Engineering, University of Washington , Box 352120, Seattle, Washington 98195-2129, United States.
authors.forename: Ryan
authors.initials: R
authors.lastname: Hufschmid
authors.affiliation: Bruker AXS Inc , Madison, Wisconsin 53711, United States.
authors.forename: Charles F
authors.initials: CF
authors.lastname: Campana
authors.affiliation: Bruker AXS Inc , Madison, Wisconsin 53711, United States.
authors.forename: Bruce C
authors.initials: BC
authors.lastname: Noll
authors.affiliation: Department of Materials Science and Engineering, University of Washington , Box 352120, Seattle, Washington 98195-2129, United States.|Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Nigel D
authors.initials: ND
authors.lastname: Browning
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: May
authors.initials: M
authors.lastname: Nyman