(CN 3 H 6 )[Fe II Fe III (SO 4 ) 3 (H 2 O) 3 ]: A Framework Iron Sulfate with a Mixed S = 2 and S = 5/2 Honeycomb Lattice.

A new guanidinium-templated hydrated iron sulfate, [CN 3 H 6 ][Fe II Fe III (SO 4 ) 3 (H 2 O) 3 ] ( 1 ), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from Fe III O 6 and Fe II O 3 (H 2 O) 3 octahedra linked by sulfate bridges forming a [Fe II Fe II (SO 4 ) 3 (H 2 O) 3 ] - 3 D 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 (SO 4 ) 3 . The removal of part of the sulfate tetrahedra and the partial replacement of the Fe 3+ cations in the [Fe 3+ 2 (SO 4 ) 3 ] 0 framework by Fe 2+ 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, Mössbauer 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 ∼ 6.1 K) and two Fe II -Fe III couplings ( J 2 ∼ 1 K, J 3 ∼ 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.