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

[Image: see text] 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.