Coordination isomerism in spin crossover (SCO) materials

A new series of three spin crossover (SCO) Fe(II) complexes based on a cyanocarbanion and on the neutral quinolin-8-amine (aqin) ligands, [Fe(aqin)2(tcnsme)2] (1), [Fe(aqin)2(tcnset)2] (2), and [Fe(aqin)2(tcnspr)2] (3), has been studied. The three complexes display similar molecular structures consisting of discrete [Fe(aqin)2(tcnsR)2] complexes [R = Me (1), Et (2), and Pr (3)]. Infrared spectroscopy and magnetic studies, performed on the three complexes, revealed the presence of similar SCO behaviors which strongly differ by their transition temperatures [234 K (1) < 266 K (2) < 360 K (3)]. The increase of the transition temperatures when passing from 1 to 3 may be explained by electronic and packing effects. Thus, when passing from Me (1) to Et (2) and Pr (3), the electron donor effect increases, resulting in an increase of the ligand field and, accordingly, in an increase of the transition temperature. On the other side, examination of the different coordination modes of the cyanocarbanions in the three complexes clearly reveals the crucial role of these coordination modes on the crystal packing and, therefore, on the transition temperature. We can, therefore, attribute the important increase of the transition temperature from complexes 1 to 3 to both effects: the electron donor character of the alkyl groups and the packing effects of the versatile cycnocarbanion ligands.