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Hybrid ion-pair crystals involving hexadentate [Fe(III)(3-OMesal 2 -trien)] + spin-crossover (SCO) cationic complexes and anionic gold complexes [Au(dmit) 2 ] − (1) (dmit = 4,5-dithiolato-1,3dithiole-2-thione) and [Au(dddt) 2 ] − (2) (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were synthesized and studied by single-crystal X-ray diffraction, P-XRD, and SQUID magnetometry. Our study shows that both complexes have similar 1:1 stoichiometry but different symmetry and crystal packing. Complex 1 has a rigid structure in which the SCO cations are engaged in strong π-interplay with molecular surrounding and does not show SCO transition while 2 demonstrates a reversible transition at T sco = 118 K in a much "softer", hydrogen-bonded structure. A new structural indicator of spin state in [Fe(sal 2 -trien)] + complexes based on conformational analysis has been proposed. Aging and thermocycling ruined the SCO transition increasing the residual HS fraction from 14 to 41%. Magnetic response of 1 is explained by the AFM coupled dimers S = 5/2 with J 1 = −0.18 cm −1 . Residual high-spin fraction of 2, apart from a contribution of the weak dimers with J 12 = J 34 = −0.29 cm −1 , is characterized by a stronger interdimer coupling of J 23 = −1.69 cm −1 , which is discussed in terms of possible involvement of neutral radicals [Au(dddt) 2 ].have been undertaken to use the SCO compounds in magnetic memory or display devices, as well as multi-modal sensors [4][5][6]. Moreover, polyfunctional compounds combining the conductivity and the spin-crossover transition have been highlighted in connection with spin-dependent transport and single molecular switching [7][8][9]. Spin-lattice relaxation in the low-dimensional conducting networks of centrosymmetrical molecules is strongly suppressed. The hybrid structures implementing efficient exchange interaction between the SCO and conducting subsystems would facilitate the development of new molecular spintronic devices where the spin transport is controlled by spin-crossover complexes.Metal bis-1,2-dithiolene complexes, as organic donors and acceptors, possess a delocalized electron system as a planar central core M(C 2 S 2 ) 2 and present different formal oxidation states. This type of complex has been intensively studied as a component of molecular conductors [10][11][12]. In addition, metal dithiolene complexes have a rich variety of physical properties, such as Peierls instability of the low-dimensional systems and the quantum fluctuations [10,13]. Thus, both SCO and metal dithiolene complexes could undergo phase transition, and combining two components in one crystal structure might give rise to a novel molecular material with exotic phenomena.The interplay between spin-crossover and conductivity was already observed in some of such materials [14][15][16]. The majority of conducting SCO compounds are represented by the Fe(III) cation complexes with [M(dmit) 2 ] δ− anions [17-19]. As a first step in this way, Faulmann et al. have published a simple salt [Fe(sal 2 -trien)][Ni(dmit) 2 ] ...

The DFT calculations of the spin crossover complex [FeIII(Sal2-trien)]+ with different conformations of ethylene groups were performed. The results explain the presence of disordered structures/transition states of the ethylene groups observed in the [FeIII(Sal2-trien)]+-type structures found in the CCDC database.

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.