THERMAL AND LIGHT-INDUCED SPIN-CROSSOVER IN IRON(II) COMPLEXES WITH MONODENTATE TETRAZOLE LIGANDS CARRYING LONG ALKYL CHAINS IN THE 1-POSITION

The complex compounds [Fe(otz)6](BF4)2 (I), [Fe(dotz)6](BF4)2 (II) and [Fe(odetz)6](BF4)2 (III) (otz = 1-n-octyl-tetrazole, dotz = 1-n-dodecyl-tetrazole, odetz = 1-n-octadecyl-tetrazole) with monodentate tetrazole ligands carrying long alkyl chains in the 1-position exhibit the thermal high-spin (HS, T-5(2)) half arrow right over half arrow left low-spin (LS, 1A1) transitions below 250 K. All of them show a gradual transition behaviour with transition temperatures T1/2(gamma(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.