Synthesis, structure, spectroscopic and magnetic characterization of a novel spin-crossover iron(II) complex with 1-cyclopropyltetrazole ligands

For the first time a 1-cyclopropyl substituted tetrazole (C3tz) has been used as a potential ligand for iron(II) spin-transition complexes. The complexation of 1-cyclopropyltetrazol with iron(II) tetrafluoroborate yielded a fine powdered product of [Fe(C3tz)6](BF4)2 being poorly soluble in most common solvents. Single crystals of complex were grown in situ from a solution of ligand and iron(II) hexafluorophosphate, which yielded a hexagonal prismatic crystalline product of [Fe(C3tz)6](PF6)2. A comparison of XRPD data of the homologues [Fe(C3tz)6](BF4)2 and [Fe(C3tz)6](PF6)2 proves them to be homeotypic. The thermally induced spin-crossover phenomenon of [Fe(C3tz)6](BF4)2 complex shows very abrupt spin transitions, with a spin-crossover temperature T1/2 ≈ 180 K which is found to be ≈50 K above the T1/2 of all known iron(II) complexes with n-alkyltetrazoles as ligands. The T1/2 was determined by temperature-dependent 57Fe-Mössbauer, far FT-IR and UV–Vis–NIR spectroscopy as well as temperature dependent magnetic susceptibility measurements (SQUID).