Anionic Tuning of Spin Crossover in FeIII–Quinolylsalicylaldiminate Complexes



A series of FeIII complexes, [Fe(qsal-5-OMe)2]Y [Y = BF4 (1), PF6 (2), NCS (3), BPh4 (4)], have been prepared and structurally and magnetically characterized. The low-temperature structures of 1 and 3 as solvent adducts were determined by X-ray crystallography with LS FeIII centres found in both cases. π–π and C–H···π interactions between the cations lead to 2D sheets that are linked to one another through C–H···O and, in the case of 3, C–H···N/S interactions resulting in high cooperativity. Magnetic studies revealed abrupt and gradual spin crossover for 3 and 2, respectively. For 3, spin crossover with possible thermal hysteresis was observed although solvent loss (CH2Cl2) could occur. In contrast, 1 and 4 were found to be low and high spin only, respectively. The results of Mössbauer spectroscopic studies are consistent with the magnetic susceptibility data and indicate that there are two low-spin FeIII centres in 2. Finally, electrochemical studies showed reversible reduction to FeII at –0.22 V, whereas reversible oxidation of the qsal-5-OMe ligand was found to occur between 1.04–1.06 V.