title: Spin-glass behavior of a hierarchically-organized, hybrid microporous material, based on an extended framework of octanuclear iron-oxo units.
abstract: Inspired by the stepwise addition of octanuclear iron units into mammalian ferritin, a "stop-and-go" synthesis strategy was used to prepare two microporous (Langmuir surface area, 490 m(2) g(-1); effective pore size, 4-5 Å) hierarchical materials {[Fe8(μ4-O)4(μ-pz)12Cl0.3(μ-O)1.85}n () and {[Fe8(μ4-O)4(μ-4-Me-pz)12Cl0.4(μ-O)1.8}n (), which are new members of the EO2 family of polymeric materials (E = C, Si and Ge). The secondary building units (SBUs) E = [Fe8(μ4-O)4(μ-4-R-pz)12] (Fe8) are nanoscale pseudo-spherical clusters, rather than single atoms, forming μ-oxo Fe-O-Fe linkages between Fe8-SBUs. The characteristic Fe-O-Fe asymmetric stretching mode in the infrared (IR) spectra of these compounds appearing at around 800 cm(-1) suggest the formation of approximately linear μ-oxo Fe-O-Fe linkages between Fe8-SBUs in and . We employ the concept of continuous random network (CRN) to describe for the first time the framework features of a Fe8-based amorphous materials, in which the average connecting numbers of each Fe8-cluster are ∼3.7 and ∼3.6 for and , respectively. (57)Fe-Mössbauer spectroscopic analysis provides insights to the intercluster connectivity of and on one hand and to their magnetic properties on the other, evident by a magnetic split sextet below 30 K. The combination of Mössbauer spectroscopy and magnetism measurements reveals a spin-glass behavior with Tg of ∼30 K. The hierarchical porous materials and straddle the gap between metal oxides and metal-organic frameworks (MOFs). This study may open an alternative way for the development of multifunctional materials based on high nuclearity metal clusters.
authors.affiliation: Department of Chemistry and Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, PR 00936-8377, USA.
authors.forename: Xin-Yi
authors.initials: XY
authors.lastname: Cao
authors.forename: Jeremiah W
authors.initials: JW
authors.lastname: Hubbard
authors.forename: Jennifer
authors.initials: J
authors.lastname: Guerrero-Medina
authors.forename: Arturo J
authors.initials: AJ
authors.lastname: Hernández-Maldonado
authors.forename: Logesh
authors.initials: L
authors.lastname: Mathivathanan
authors.forename: Carlos
authors.initials: C
authors.lastname: Rinaldi
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Sanakis
authors.forename: Raphael G
authors.initials: RG
authors.lastname: Raptis