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

The complex salts [Fe(L-1)(2)]X-2 (1X(2); L-1 = 4-(isopropyldisulfanyl)-2,6-bis(pyrazolyl)pyridine; X- = BF4-, ClO4-) form solvated crystals from common organic solvents. Crystals of 1X(2)center dot Me2CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X(2)center dot Me2CO cocrystallizes with other, less cooperative acetone solvates, which all transform into the same solvent-free materials 1X(2)center dot sf upon exposure to air, or mild heating. Conversion of 1X(2)center dot Me2CO to 1X(2)center dot sf proceeds in a single-crystal to single-crystal fashion. 1X(2)center dot sf are not isomorphous with the acetone solvates, and exhibit abrupt spin transitions at low temperature with hysteresis loops of 30-38 K (X- = BF4-) and 10-20 K (X- = ClO4-), 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)center dot Me2CO and 1X(2)center dot sf do not involve a crystallographic phase change but are accompanied by a significant rearrangement of the metal coordination sphere. Other solvates 1X(2)center dot MeNO2, 1X(2)center dot MeCN, and 1X(2)center dot H2O 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 center dot center dot center dot pi interactions. Polycrystalline [Fe(L-2)(2)][BF4](2)center dot MeNO2 (2[BF4](2)center dot MeNO2; 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.

Automated summary

This study investigates the solvated crystals of complex salts [Fe(L-1)(2)]X-2 (1X(2)) formed from common organic solvents and characterizes their crystal structure and spin-crossover properties.