New iron tetrazolate frameworks: synthesis temperature effect, thermal behaviour, Mössbauer and magnetic studies

The exploration of the FeF₃/FeF₂-Hamtetraz-HF system in dimethylformamide by solvothermal synthesis evidences two isostructural 3D hybrid fluoroferrates. They are prepared from the same starting mixture at two different synthesis temperatures: 120 °C for [Hdma]·(Fe₄^IIFe^IIIF₈(H₂O)₂(amtetraz)₄) (1) and 140 °C for [Hdma]_1.5·(Fe_4.5^IIFe_0.5^IIIF₇(H₂O)(HCOO)(amtetraz)₄) (2). Both compounds are characterized by single crystal X-ray diffraction, X-ray thermodiffraction, TGA analysis, Mössbauer spectrometry and SQUID magnetometry. They crystallize in the monoclinic system and are built from two distinct chains connected by aminotetrazolate anions. The first chain _∞(Fe^IIFN₄) is common to 1 and 2 and can be found in numerous fluorides. In the second chain _∞(Fe₃X₁₂) (X = F, N, O), iron cations adopt both valence states Fe(II)/Fe(III). The hydrolysis of DMF implies the formation of a [Hdma]⁺ cation and a (HCOO)⁻ anion. The presence of Fe³⁺ in both phases is evidenced by ⁵⁷Fe Mössbauer spectrometry. The magnetic properties are studied and two transitions from a paramagnetic regime to a long range ordered state below 30 K and 5 K are identified.