(CN₃H₆)[Fe^IIFe^III(SO₄)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][Fe^IIFe^III(SO₄)₃(H₂O)₃] (1), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from Fe^IIIO₆ and Fe^IIO₃(H₂O)₃ octahedra linked by sulfate bridges forming a [Fe^IIFe^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 Fe₂(SO₄)₃. The removal of part of the sulfate tetrahedra and the partial replacement of the Fe³⁺ cations in the [Fe³⁺₂(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, 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 (J₁ ∼ 6.1 K) and two Fe^II-Fe^III couplings (J₂ ∼ 1 K, J₃ ∼ 5.9 K) within corrugated honeycomb layers. These ferrimagnetic layers are coupled antiparallel to each other, resulting in an overall antiferromagnetic order below T = 31 K.