An iron(ii) complex salt that crystallises in three crystal forms, one of which undergoes a sterically controlled incomplete spin-state transition on cooling

Abstract

The synthesis and crystal chemistry of [Fe(L³)₂][ClO₄]₂ (1; L³ = 2,6-di[3-hydroxymethylpyrazol-1-yl]pyridine) is described. This compound crystallises as a mixture of three different phases from MeCN/Et₂O: two solvent-free polymorphs, termed α-1 (space group P2/c) and β-1 (P2₁/c); and, a solvate of approximate formula 1·xCH₃CN (x ≈ 0.75). The coordination geometries of the complex dications in these three crystals differ significantly, reflecting differing degrees of an unusual angular Jahn–Teller distortion at their high-spin iron centres. The solvated compound 1·xCH₃CN undergoes a spin-state transition upon cooling, which is 42% complete at 100 K. This transition cannot probably proceed to more than 50% completeness, owing to a steric clash between neighbouring complex molecules when both are low-spin.