Iron(II) Complexes of 4-(Alkyldisulfanyl)-2,6-di(pyrazolyl)pyridine Derivatives. Correlation of Spin-Crossover Cooperativity with Molecular Structure Following Single-Crystal-to-Single-Crystal Desolvation

Abstract

The complex salts [Fe( L 1 ) 2 ]X 2 ( 1X 2 ; L 1 = 4-(isopropyldisulfanyl)-2,6-bis(pyrazolyl)pyridine; X – = BF 4 – , ClO 4 – ) form solvated crystals from common organic solvents. Crystals of 1X 2 ·Me 2 CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X 2 ·Me 2 CO cocrystallizes with other, less cooperative acetone solvates, which all transform into the same solvent-free materials 1X 2 ·sf upon exposure to air, or mild heating. Conversion of 1X 2 ·Me 2 CO to 1X 2 ·sf proceeds in a single-crystal to single-crystal fashion. 1X 2 ·sf are not isomorphous with the acetone solvates, and exhibit abrupt spin transitions at low temperature with hysteresis loops of 30–38 K (X – = BF 4 – ) and 10–20 K (X – = ClO 4 – ), depending on the measurement method. Interestingly, the desolvation has an opposite effect on the SCO temperature and hysteresis in the two salts. The hysteretic spin transitions in 1X 2 ·Me 2 CO and 1X 2 ·sf do not involve a crystallographic phase change but are accompanied by a significant rearrangement of the metal coordination sphere. Other solvates 1X 2 ·MeNO 2 , 1X 2 ·MeCN, and 1X 2 ·H 2 O are mostly isomorphous with each other and show more gradual spin-crossover equilibria near room temperature. All three of these lattice types have similar unit cell dimensions and contain cations associated into chains through pairwise, intermolecular S···π interactions. Polycrystalline [Fe( L 2 ) 2 ][BF 4 ] 2 ·MeNO 2 ( 2[BF 4 ] 2 ·MeNO 2 ; L 2 = 4-(methyldisulfanyl)-2,6-bis(pyrazolyl)pyridine) shows an abrupt spin transition just above room temperature, with an unsymmetrical and structured hysteresis loop, whose main features are reversible upon repeated thermal scanning.

Spin-Crossover in a New Iron(II)/Di(pyrazolyl)pyridine Complex with a Terpyridine Embrace Lattice. Thermally Induced Excited Spin State Trapping and Clarification of a Structure–Function Correlation

“…The large ΔΘ distortion in 1­[ClO 4 ] 2 is a property of its inner coordination sphere, which should not influence its thermal hysteresis unless it reflects a larger structure change in the molecule as a whole . Hysteretic spin-transitions in [Fe­(bpp R,Y ) 2 ]­X 2 derivatives can be associated with a rearrangement of the ligands about the metal ion during SCO, which manifests in their trans -N­{pyridyl}–Fe–N­{pyridyl} angle (ϕ; Tables and ). ,− However, the change in ϕ between the spin states of 1­[ClO 4 ] 2 , Δϕ = −0.55(9)°, is too small to be significant (Table S7). Rather, the SCO hysteresis shown by 1­[BF 4 ] 2 and 1­[ClO 4 ] 2 may reflect the steric influence of their fluoro substituents on their crystal packing, which leads to a significant conformational distortion of one L ligand (Figures S8–S9).…”

Iron(II) Mediated Supramolecular Architectures with Schiff Bases and Their Spin-Crossover Properties

“…Spiral architectures of iron(II) are an interesting area to investigate due to the fascinating structure and cooperative SCO behavior, as metal centers in those will lead up to three different spin states: LS-LS, LS-HS and HS-HS [ 54 , 55 , 56 ]. These spin states can be related to cooperativity.…”

“…Spiral iron(II) SCO architectures are interesting due to the fact that they have three-dimensional structures. As positive cooperativity occurs between metal centers, the SCO behaviors have been found in different forms compared as mononuclear analogues [ 32 , 54 , 55 , 56 ]. Positive cooperativity can lead to one-step spin switching of both metal centers (i.e., [HS-HS] to [LS-LS] or vice versa) in which a sharp, abrupt and/or hysteretic loop SCO can be attained ( Figure 2 b,c) [ 57 , 58 , 59 , 60 ].…”

An iron(II) complex shows bistable spin-state switching characteristic with T1/2 centred at room temperature

“…The cycling is also accompanied by a steady rise of χT value in the 5 K-to-275 K region, as recently observed for an iron(II)-BPP system by Halcrow and co-workers. 26 A fraction of the complex molecules in the mixed phase, obtained after the 11 th cycle, undergoes gradual SCO, as depicted in Figure 4. To further confirm the occurrence of temperature-induced spin-state switching in 1•CH3CN, differential scanning calorimetric (DSC) studies were performed, as shown in Figure 5a.…”