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)(2)] OTf.sol (sol = MeOH 1, EtOH 2, n-PrOH 3, i-PrOH4, 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 down arrow = 225 K and T-1/2 up arrow= 234 K (Delta T = 9 K), while aged samples of 2 exhibit an exceptionally wide hysteresis of 80 K (T-1/2 down arrow = 139 K and T-1/2 up arrow= 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 down arrow = 180 K and T-1/2 up arrow= 215 K with phase 2b exhibiting two-step SCO. Structural studies in both spin states, except 6, show the cations are linked through extensive p-p interactions to form 1D chains. A combination of P4AE (parallel fourfold aryl embrace) and I/X (X = I, O, p) 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.