Abstract:
Homoleptic complexes [Fe(4bt)3](ClO4)2 (1), [Fe(2bt)3](ClO4)2 (2), and [Fe(3tpH)3](ClO4)2 (3) were obtained
by a reaction between the Fe(II) precursor salt and the corresponding thiazole-based bidentate ligand (L =
4bt = 4,4’-bithiazole, 2bt = 2,2’-bithiazole, 3tpH = 3-(thiazol-2-yl)pyrazole). X-ray crystal structure determination revealed crystallization of solvent-free complex 1, a solvate 2·MeOH, and a co-crystal 3·2(3tpH).
The crystal packing of all these complexes is dominated by one-dimensional interactions between the
[Fe(L)3]
2+ cations. These interactions are stronger in 2·MeOH and 3·2(3tpH), leading to cooperative and
slightly hysteretic transitions between the high-spin and low-spin electronic configurations at ∼235 K and
159 K, respectively. In contrast, weaker intermolecular interactions in 1 result in a gradual spin crossover
above 300 K, with the maximum fraction of the HS state ∼25% achieved at 400 K. Complexes 2 and
3·2(3tpH) exhibit light-induced excited spin state trapping (LIESST) under irradiation with white light or a
532 nm laser at 5 K. After the photoexcitation, the trapped metastable HS state relaxes to the ground LS
state with the average relaxation temperature of 81 K and 68 K, respectively. Examination of the relaxation
dynamics by optical absorption spectroscopy on a single crystal of 3·2(3tpH) revealed the sigmoidal shape
of the relaxation curves at lower temperatures, attributed to cooperative effects, as well as a plateau at
∼10% of the HS fraction at intermediate temperatures, hinting at a more complex mechanism for the
relaxation of the LIESST phase in this material.
