Peculiar Spin-Crossover Behavior in the 2D Polymer K[Fe(5Cl-thsa)].

A potassium salt of the NSO-coordination Fe(III) anion K[Fe(5Cl-thsa)] () (5Cl-thsa - 5-chlorosalicylaldehyde thiosemicarbazone) is synthesized and characterized structurally and magnetically over a wide temperature range. Two polymorphs of salt characterized by the common 2D polymer nature and assigned to the same orthorhombic space group have been identified. The molecular structure of the minor polymorph of was solved and refined at 100, 250, and 300 K is shown to correspond to the LS configuration. The dominant polymorph of 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 = 230 K and a 6 K hysteresis loop. The third gradual step occurs from 250 to 440 K. According to Fe 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 (Δγ = 26%) is associated with a structural phase transition with a symmetry lowering from the orthorhombic () to the monoclinic (2/) space group. This nontrivial phenomenon was investigated in detail by applying magnetization measurements, electron spin resonance, Fe 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.