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

Abstract

A potassium salt of the N 2 S 2 O 2 -coordination Fe(III) anion K[Fe(5Clthsa) 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 T 1/2 = 230 K and a 6 K hysteresis loop. The third gradual step occurs from 250 to 440 K. According to 57 Fe Mossbauer, 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 HS = 26%) is associated with a structural phase transition with a symmetry lowering from the orthorhombic (Pbcn) to the monoclinic (P2 1 /n) space group. This nontrivial phenomenon was investigated in detail by applying magnetization measurements, electron spin resonance, 57 Fe Mossbauer spectroscopy, and DFT calculations. These results provide a new platform for understanding the multistep spin-crossover character in the Fe(III) thsacomplexes and related compounds.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes

“…The high Δ E Q value of the HS species is due mainly to the electronic contribution to the electric field gradient of the doubly occupied 3d orbital in the HS state. This is in contrast to the LS state, where the electronic contribution is almost zero due to the symmetric charge distribution caused by the doubly occupied t 2g orbitals, resulting in a low value for Δ E Q to be observed. ,,,− …”

First crystal structure of an Fe(iii) anionic complex based on a pyruvic acid thiosemicarbazone ligand with Li+: synthesis, features of magnetic behavior and theoretical analysis

“…Therefore, the study of the theoretical aspects of these compounds is an important step of SCO parameter control. Previously, similar theoretical studies were carried out for anionic iron(III) complexes: [Fe(5Br-thsa) 2 ] − , 14 [Fe(5Cl-thsa) 2 ] − , 17 [Fe(azp) 2 ] − and [Fe( phsal) 2 ] − . 53 In the present work, the DFT calculations were aimed at studying the [Fe(thpy) 2 ] − anionic complex in the HS (S = 5/2) and LS (S = 1/2) states (Fig.…”

“…Similar trends for the change in iron( iii )–ligand bond lengths to thsa complexes were noted earlier. 17 Furthermore, the molecular orbitals (MOs) with predominant contributions from the e g and t 2g -like d-orbitals of iron( iii ) were considered for the identification of the electronic structure of the [Fe(thpy) 2 ] − anion. To facilitate the identification of the MOs in the HS and LS states, the global Z axis was chosen along the N im –Fe–N im bond, while the X / Y axes were aligned with the S–Fe–O bonds.…”

“…It should be noted that among the transition metals with d 4d 7 electronic configuration, anionic complexes with the spincrossover effect are now known only for iron(III). 3,[14][15][16][17] Special attention is paid to cationic and anionic spin-variable complexes due to the possibility of hybrid system acquisition by replacing anions (for cationic complexes) and cations (for anionic complexes) with functionally active structural units. A great number of hybrid systems based on spin-variable iron(III) cationic complexes with magnetic ordering and electrical conductivity are described in the literature.…”

“…1). 21 It is worth noting that only ten structures of Fe( iii ) anionic thsa complexes with alkali metal cations and an ammonium cation are currently known, 17 but not a single crystal structure with Fe( iii ) anionic thpy complexes has been discovered yet. Obviously, this is due to the synthetic complexity of finding appropriate conditions for obtaining single-phase samples, as well as single crystals suitable for X-ray analysis, which has been repeatedly noted in a number of works.…”