posted on 2021-11-10, 20:04authored byNataliya G. Spitsyna, Maxim A. Blagov, Vladimir A. Lazarenko, Roman D. Svetogorov, Yan V. Zubavichus, Leokadiya V. Zorina, Olga Maximova, Sergey A. Yaroslavtsev, Vyacheslav S. Rusakov, Grigory V. Raganyan, Eduard B. Yagubskii, Alexander N. Vasiliev
A potassium salt of the N2S2O2-coordination Fe(III) anion K[Fe(5Cl-thsa)2] (1) (5Cl-thsa − 5-chlorosalicylaldehyde
thiosemicarbazone) is
synthesized and characterized structurally and magnetically over a
wide temperature range. Two polymorphs of salt 1 characterized
by the common 2D polymer nature and assigned to the same orthorhombic Pbcn space group have been identified. The molecular structure
of the minor polymorph of 1 was solved and refined at
100, 250, and 300 K is shown to correspond to the LS configuration.
The dominant polymorph of 1 features K+ cations
disordered over a few crystallographic sites, while the minor polymorph
includes fully ordered K+ cations. The major polymorph
exhibits a complete three-step cooperative spin-crossover transition
both in the heating and cooling modes: The first step occurs in a
temperature range from 2 to 50 K; the second abrupt hysteretic step
occurs from 200 to 250 K with T1/2 = 230
K and a 6 K hysteresis loop. The third gradual step occurs from 250
to 440 K. According to 57Fe Mössbauer, XRPD, and
EXAFS data, the spin-crossover transition for the dominant polymorph
is quite peculiar. Indeed, the increase in the HS concentration by
57% at the second step does not result in the expected significant
increase in the iron(III)–ligand bond lengths. In addition,
the final step of the spin conversion (ΔγHS = 26%) is associated with a structural phase transition with a symmetry
lowering from the orthorhombic (Pbcn) to the monoclinic
(P21/n) space group.
This nontrivial phenomenon was investigated in detail by applying
magnetization measurements, electron spin resonance, 57Fe Mössbauer spectroscopy, and DFT calculations. These results
provide a new platform for understanding the multistep spin-crossover
character in the Fe(III) thsa-complexes and related compounds.