Solvent modified spin crossover in an iron(iii) complex: phase changes and an exceptionally wide hysteresis† †Electronic supplementary information (ESI) available: Crystallographic details, structural figures, VT-PXRD data and figures, Mössbauer spectroscopic data, DSC and TGA results. CCDC 1519938–1519948. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc05317c Click here for additional data file. Click here for additional data file

The impact of solvent on spin crossover is explored with T 1/2 varying by 200 K and hysteresis up to 80 K. Solvent effects in a series of Fe( iii ) spin crossover (SCO) complexes [Fe(qsal-I) 2 ]OTf·sol (sol = MeOH 1 , EtOH 2 , n -PrOH 3 , i-PrOH 4 , acetone 5 and MeCN 6 ) are explored. SCO is abrupt in 1 (following MeOH loss) and 2 , gradual for 3 ( T 1/2 = 199 K) and 4 ( T 1/2 = 251 K) and incomplete, even up to 350 K, for 5 and 6 . In [Fe(qsal-I) 2 ]OTf SCO occurs at T 1/2 ↓ = 225 K and T 1/2 ↑ = 234 K (Δ T = 9 K), while aged samples of 2 exhibit an exceptionally wide hysteresis of 80 K ( T 1/2 ↓ = 139 K and T 1/2 ↑ = 219 K). In contrast, fresh samples of 2 exhibit stepped SCO with hysteresis varying from 2 to 42 K. VT-PXRD (variable temperature powder X-ray diffraction) studies indicate a new phase, 2b , is formed upon cooling below 180 K along with a minor LS phase 2c . Phase 2c and the HS phase 2a undergo a spin transition at T 1/2 ↓ = 180 K and T 1/2 ↑ = 215 K with phase 2b exhibiting two-step SCO. Structural studies in both spin states, except 6 , show the cations are linked through extensive π–π interactions to form 1D chains. A combination of P4AE (parallel fourfold aryl embrace) and I···X (X = I, O, π) interactions create tightly packed 3D supramolecular networks. This study emphasizes that while solvent may result in only small structural changes SCO characteristics can be impacted dramatically.