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

The synthesis and crystal chemistry of [Fe(L-3)(2)][ClO4](2) (1; L-3 = 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 alpha-1 (space group P2/c) and beta-1 (P2(1)/c); and, a solvate of approximate formula 1.xCH(3)CN ( x approximate to 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.xCH(3)CN 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.