Peculiar Spin-Crossover Behavior in the 2D Polymer K[FeIII(5Cl-thsa)2]статья
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Дата последнего поиска статьи во внешних источниках: 4 марта 2022 г.
Аннотация:A potassium salt of the N2S2O2-coordination Fe(III) anion K[Fe(5Clthsa)2] (1) (5Cl-thsa − 5-chlorosalicylaldehyde thiosemicarbazone) is synthesizedand characterized structurally and magnetically over a wide temperature range. Twopolymorphs of salt 1 characterized by the common 2D polymer nature and assigned tothe same orthorhombic Pbcn space group have been identified. The molecularstructure of the minor polymorph of 1 was solved and refined at 100, 250, and 300 Kis shown to correspond to the LS configuration. The dominant polymorph of 1features K+ cations disordered over a few crystallographic sites, while the minorpolymorph includes fully ordered K+ cations. The major polymorph exhibits acomplete three-step cooperative spin-crossover transition both in the heating andcooling modes: The first step occurs in a temperature range from 2 to 50 K; thesecond abrupt hysteretic step occurs from 200 to 250 K with T1/2 = 230 K and a 6 Khysteresis loop. The third gradual step occurs from 250 to 440 K. According to 57FeMössbauer, XRPD, and EXAFS data, the spin-crossover transition for the dominant polymorph is quite peculiar. Indeed, the increasein the HS concentration by 57% at the second step does not result in the expected significant increase in the iron(III)−ligand bondlengths. In addition, the final step of the spin conversion (ΔγHS = 26%) is associated with a structural phase transition with asymmetry lowering from the orthorhombic (Pbcn) to the monoclinic (P21/n) space group. This nontrivial phenomenon wasinvestigated in detail by applying magnetization measurements, electron spin resonance, 57Fe Mössbauer spectroscopy, and DFTcalculations. These results provide a new platform for understanding the multistep spin-crossover character in the Fe(III) thsacomplexesand related compounds.