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

[FeL 2][BF 4] 2·2H 2O (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 A) or by thermal dehydration (phase 1 B). 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 B, 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 A, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry. © 2012 The Royal Society of Chemistry.