Solvent modified spin crossover in an iron(iii) complex: phase changes and an exceptionally wide hysteresis

Solvent effects in a series of Fe(iii) spin crossover (SCO) complexes [Fe(qsal-I) ]OTf·sol (sol = MeOH , EtOH , -PrOH , i-PrOH , acetone and MeCN ) are explored. SCO is abrupt in (following MeOH loss) and , gradual for ( = 199 K) and ( = 251 K) and incomplete, even up to 350 K, for and . In [Fe(qsal-I) ]OTf SCO occurs at ↓ = 225 K and ↑ = 234 K (Δ = 9 K), while aged samples of exhibit an exceptionally wide hysteresis of 80 K ( ↓ = 139 K and ↑ = 219 K). In contrast, fresh samples of exhibit stepped SCO with hysteresis varying from 2 to 42 K. VT-PXRD (variable temperature powder X-ray diffraction) studies indicate a new phase, , is formed upon cooling below 180 K along with a minor LS phase . Phase and the HS phase undergo a spin transition at ↓ = 180 K and ↑ = 215 K with phase exhibiting two-step SCO. Structural studies in both spin states, except , 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.