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 m² g⁻¹; effective pore size, 4–5 Å) hierarchical materials {[Fe₈(μ₄-O)₄(μ-pz)₁₂Cl_0.3(μ-O)_1.85}_n (1) and {[Fe₈(μ₄-O)₄(μ-4-Me-pz)₁₂Cl_0.4(μ-O)_1.8}_n (2), 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 1 and 2. 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 1 and 2, respectively. ⁵⁷Fe-Mössbauer spectroscopic analysis provides insights to the intercluster connectivity of 1 and 2 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 T_g of ∼30 K. The hierarchical porous materials 1 and 2 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.