(CN₃H₆)[Fe^IIFe^III(SO_{4)3(H2O)3]: A Framework Iron Sulfate with a Mixed S=2 and S=5/2 Honeycomb Lattice}

A new guanidinium-templated hydrated iron sulfate, [CN3H6][(FeFeIII)-Fe-II(SO4)(3)(H2O)(3)] (1), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from (FeO6)-O-III and (FeO3)-O-II(H2O)(3) octahedra linked by sulfate bridges forming a [(FeFeII)-Fe-II(SO4)(3)(H2O)(3)](-) 3D framework with a layer-by-layer ordering of ferric and ferrous cations. The structural topology of the framework is related to the anhydrous rhombohedral mikasaite Fe-2(SO4)(3). The removal of part of the sulfate tetrahedra and the partial replacement of the Fe3+ cations in the [Fe-2(3+)(SO4)(3)](0) framework by Fe2+ provide a negative charge and allow the incorporation of the protonated organic species in the voids. The compound 1 has been characterized by single-crystal X-ray diffraction, TG and DSC analyses, UV-vis-NIR spectroscopy, magnetic susceptibility, M & ouml;ssbauer spectroscopy, IR and Raman spectroscopy, and density functional band-structure calculations. The magnetic behavior of 1 shows an interplay of Fe-II (S = 2) and Fe-III (S = 5/2) sublattices that exhibit different types of antiferromagnetic couplings, one Fe-III-Fe-III (J(1) similar to 6.1 K) and two Fe-II-Fe-III couplings (J(2) similar to 1 K, J(3) similar to 5.9 K) within corrugated honeycomb layers. These ferrimagnetic layers are coupled antiparallel to each other, resulting in an overall antiferromagnetic order below T-N = 31 K.