An iron(ii) complex exhibiting five anhydrous phases, two of which interconvert by spin-crossover with wide hysteresis

Abstract

[FeL₂][BF₄]₂·2H₂O (L = 2,6-di{5-methylpyrazol-3-yl}pyridine) adopts a 1 : 1 high : low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1^AA) or by thermal dehydration (phase 1^BB). Upon cooling in vacuo, the latter undergoes a thermal spin-state transition centred near T_1/2 = 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1^BB, initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1^AA, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.