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

Abstract

Crystals of [FeL2][BF4]2·nMeCN (L=N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide;n= 1 or 2) and [FeL2][ClO4]2·MeCN are isomorphous. Whenn= 1 the compounds exhibit an abrupt, hysteretic spin-transition below 200 K, but whenn= 2 the material remains high-spin on cooling. [FeL2]X2·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-stepsviaa re-entrant mixed-spin intermediate phase, which correlates with crystallographic ordering of the bent propionitrile molecule.