Fe(III) quinolylsalicylaldimine complexes: a rare mixed-spin-state complex and abrupt spin crossover

A new synthesis of (8-quinolyl)-5-methoxysalicylaldimine (Hqsal-5-OMe) is reported and its crystal structure is presented. Two Fe(III) complexes, [Fe(qsal-5-OMe)(2)]Cl⋅solvent (solvent = 2 MeOH⋅0.5 H(2)O (1) and MeCN⋅H(2)O (2)) have been prepared and their structural, electronic and magnetic properties studied. [Fe(qsal-5-OMe)(2)]Cl⋅2 MeOH⋅0.5 H(2)O (1) exhibits rare crystallographically independent high-spin and low-spin Fe(III) centres at 150 K, whereas [Fe(qsal-5-OMe)(2)]Cl⋅ MeCN⋅H(2)O (2) is low spin at 100 K. In both structures there are extensive π-π and C-H⋅⋅⋅π interactions. SQUID magnetometry of 2 reveals an unusual abrupt stepped-spin crossover with T(1/2) = 245 K and 275 K for steps 1 and 2, respectively, with a slight hysteresis of 5 K in the first step and a plateau of 15 K between the steps. In contrast, 1 is found to undergo an abrupt half-spin crossover also with a hysteresis of 10 K. The two compounds are the first Fe(III) complexes of a substituted qsal ligand to exhibit abrupt spin crossover. These conclusions are supported by (57) Fe Mössbauer spectroscopy. Both complexes exhibit reversible reduction to Fe(II) at -0.18 V and irreversible oxidation of the coordinated qsal-5-OMe ligand at +1.10 V.