Parallel Crystallization of a “Static” and a Spin‐Crossover Polymorph of an Iron(II) Complex from the Same Solution

Abstract

AbstractThe ligand 2,6‐bis(1H‐pyrazol‐1‐yl)‐4‐(thiocyanatomethyl)pyridine (L) has been prepared from the hydroxymethyl precursor by OH/Br exchange and subsequent thiocyanate substitution. Two polymorphs of the unsolvated iron(II) complex [Fe(L)2](BF4)2 crystallize at room temperature from the same solution (MeOH), and side by side. One (yellow) is high spin between 4 and 350 K, whereas the other (red‐brown) shows spin crossover, with a thermal spin‐transition centred at 272 K (width ca. 50 K). Both forms have been fully characterized by variable‐temperature structure determination and magnetic susceptibility measurements. At a given temperature, the solid‐state structures differ strongly with respect to the coordination geometry at iron(II), the orientation of the thiocyanatomethyl substituents, and cation‐anion contacts. The implications of these structural differences for the observed spin behaviour are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)