Spectroscopic, structural and magnetic investigations of iron(II) complexes based on 1-isopropyl- and 1-isobutyl-substituted tetrazole ligands

Two partly new [Fe(intz)6](BF4)2 complexes with n = 3 and 4 (i3tz = 1-isopropyl-1H-tetrazole, i4tz = 1-isobutyl-1H-tetrazole) were synthesized and characterised by X-ray powder diffraction, magnetic susceptibility measurements, vibrational, electronic and 57Fe–Mößbauer spectroscopy as well as DSC. The [Fe(i3tz)6](BF4)2 complex was re-investigated and shows an incomplete spin transition at Tc ∼ 109 K, while the [Fe(i4tz)6](BF4)2 features a complete but rather gradual spin transition with T½ = 223 K. In the lack of suitable crystals of [Fe(intz)6](BF4)2 with n = 3 and 4, we synthesized crystals of [Ni(intz)6](BF4)2 with n = 3 and 4, determined their X-ray crystal structures, and proved them to be homeotypic with the respective Fe-complexes by X-ray powder diffraction. DSC measurements showed an endothermic peak for the [Fe(i3tz)6](BF4)2 around T = 260 K not corresponding to a spin transition and suggesting a structural phase transition at this temperature. A well-developed peak at Tp = 225 K matches the spin-transition temperature T½ = 223 K for [Fe(i4tz)6](BF4)2.