The number and shape of lattice solvent molecules controls spin-crossover in an isomorphous series of crystalline solvate salts

Crystals of [FeL2][BF4](2)center dot nMeCN (L = N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide; n = 1 or 2) and [FeL2][ClO4](2)center dot MeCN are isomorphous. When n = 1 the compounds exhibit an abrupt, hysteretic spin-transition below 200 K, but when n = 2 the material remains high-spin on cooling. [FeL2]X-2 center dot EtCN (X- = BF4- or ClO4-) are isomorphous with the MeCN solvates and undergo their spin-transition at almost the same temperature. However this now occurs in two-steps via a re-entrant mixed-spin intermediate phase, which correlates with crystallographic ordering of the bent propionitrile molecule.

Automated summary

The study investigated the structure and spin-transition behavior of compounds containing the [FeL2] group at low temperatures. When the ligand solvent is MeCN, the material's spin transition exhibits a hysteretic phenomenon, while in EtCN solvent the spin transition occurs in a two-step process.