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

Abstract

Solvent effects in a series of Fe(iii) spin crossover (SCO) complexes [Fe(qsal-I)₂]OTf·sol (sol = MeOH <b>1</b>, EtOH <b>2</b>, <i>n</i>-PrOH <b>3</b>, i-PrOH <b>4</b>, acetone <b>5</b> and MeCN <b>6</b>) are explored. SCO is abrupt in <b>1</b> (following MeOH loss) and <b>2</b>, gradual for <b>3</b> (<i>T</i>_1/2 = 199 K) and <b>4</b> (<i>T</i>_1/2 = 251 K) and incomplete, even up to 350 K, for <b>5</b> and <b>6</b>. In [Fe(qsal-I)₂]OTf SCO occurs at <i>T</i>_1/2↓ = 225 K and <i>T</i>_1/2↑ = 234 K (Δ<i>T</i> = 9 K), while aged samples of <b>2</b> exhibit an exceptionally wide hysteresis of 80 K (<i>T</i>_1/2↓ = 139 K and <i>T</i>_1/2↑ = 219 K). In contrast, fresh samples of <b>2</b> exhibit stepped SCO with hysteresis varying from 2 to 42 K. VT-PXRD (variable temperature powder X-ray diffraction) studies indicate a new phase, <b>2b</b>, is formed upon cooling below 180 K along with a minor LS phase <b>2c</b>. Phase <b>2c</b> and the HS phase <b>2a</b> undergo a spin transition at <i>T</i>_1/2↓ = 180 K and <i>T</i>_1/2↑ = 215 K with phase <b>2b</b> exhibiting two-step SCO. Structural studies in both spin states, except <b>6</b>, 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.