Kinetic Trapping Effects in Amphiphilic Iron(II) Spin Crossover Compounds

In this work, the synthesis of four new iron complexes with a Schiff base-like amphiphilic equatorial ligand (L) and dimethylaminopyridine (dmap) or 4,4'-bipyridine (bipy) as axial ligands is reported. Three of the complexes ([FeL(dmap)₂] 1, [FeL(dmap)(MeOH)_0.5]·MeOH 2, and [FeL(bipy)] _n 3 have an iron(II) center, and two of those with an N₄O₂ coordination sphere (1 and 3) are spin crossover active. Both exhibit a thermal hysteresis (10 K with T_1/2 = 131 K for 3 and 23 K with T_1/2 = 161 K for 1) where the width depends on the velocity used for the measurement. Additionally, in both cases, the high spin state is trapped by rapid cooling, and a T_TIESST was determined to be 121 K (1) and 101 K (3). Single crystals suitable for X-ray structure analysis were obtained for the three different complexes with dmap as axial ligand (1S, 2, and [μ-O-{FeL}₂(dmap)] 4). The complex 1S has two dmap molecules in axial position, while the other two structures were obtained for a complex where a dmap ligand is exchanged by methanol and one where the iron(II) center is oxidized to iron(III) to form a dinuclear μ-O-complex. All three complexes were obtained under similar reaction conditions in the presence/absence of oxygen, and all three structures show the formation of lipid layer-like arrangements in the packing.