Thermal and light-induced spin crossover in iron(II) complexes with monodentate tetrazole ligands carrying long alkyl chains in the 1-position

Abstract

The complex compounds [Fe(otz) 6](BF 4) 2 ( I), [Fe(dotz) 6](BF 4) 2 ( II) and [Fe(odetz) 6](BF 4) 2 ( III (otz=1- n-octyltetrazole, dotz=1- n-dodecyl-tetrazole, odetz=1- n-octadecyl-tetrazole) with monodentate tetrazole ligands carrying long alkyl chains in the 1-position exhibit thermal high-spin (HS, 5T 2)⇌low-spin (LS, 1A 1) transitions below 250 K. All of them show a gradual transition behaviour with transition temperatures T 1 2 (γ HS=0.5) of 148 K for I, 120 K for II and 152 K for III. Compound II shows a residual paramagnetism at low temperatures, while the transitions in I and III are complete. The LIESST effect (light-induced excited spin state trapping) can be performed with all complexes. The HS→LS relaxation after LIESST was followed by optical spectroscopy on compound I which was pressed to a translucent tablet. The relaxation of the metastable LIESST state deviates strongly from first-order kinetics due to inhomogeneities in the material.