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]X₂ (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 ·Me₂CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X ₂ ·Me₂CO 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 ·Me₂CO 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^- = 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 ·Me₂CO 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 ·MeNO₂, 1X ₂ ·MeCN, and 1X ₂ ·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···π interactions. Polycrystalline [Fe(L ²)₂][BF₄]₂·MeNO₂ (2[BF ₄ ] ₂ ·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.

SUBMITTER: Kulmaczewski R 

PROVIDER: S-EPMC9007408 | biostudies-literature | 2022 Mar

REPOSITORIES: biostudies-literature