Stepped spin crossover in Fe(III) halogen substituted quinolylsalicylaldimine complexes.

The reaction of Fe(NCS)3 prepared in situ in MeOH with Hqsal-X (Hqsal-X = 5-X-N-quinolylsalicylaldimine) in CH2Cl2 yields the FeIII complexes, [Fe(qsal-X)2]NCS·solvent (X = F 1; X = Cl, 2, Br, 3 solvent = MeOH; X = I, solvent = 0.25CH2Cl2·0.5MeOH 4) in moderate to good yields. IR spectroscopy confirms that NCS− acts as a counteranion only and that the qsal-X ligand is bound to the FeIII centre. SQUID magnetometric studies reveal stepped hysteretic spin crossover in 1 and 2, which is abrupt in both steps in latter compound. Mössbauer spectroscopic studies of 1 and 2 support these conclusions. The bromo derivative, 3, undergoes half spin crossover up to 340 K while 4 is low spin at all temperatures measured. The spin transition temperature, T1/2 is found to increase on moving from F to Br. UV-Vis and NMR spectroscopic studies indicate that 1–4 have spin states intermediate between HS and LS in solution. Structural studies show that 1, 2 and 3 crystallize in triclinic P while 4 is in monoclinic P21/c. Crystallographic studies of 1 at 100, 200 and 270 K show that spin crossover proceeds from a [LS–LS] state through a [LS–HS] intermediate to a [HS–HS] state (LS = low spin, S = 1/2, HS = high spin, S = 5/2). Similar results are found for 3 although this time a [LS–IS] state exists at 123 K while a [LS–HS] state is found at 295 K (IS = intermediate spin state where partial spin crossover has occurred). Both 2 and 4 are found to have LS FeIII centres although the latter contains two crystallographically independent FeIII centres in the asymmetric unit. The crystal packing in 1–4 consists of extensive π–π interactions through the planar qsal-X ligands and CH∙∙∙X (X = O, halogen) and/or X∙∙∙π (X = halogen) interactions which result in pseudo 3D supramolecular networks. This results in high cooperativity in 1 and 2 and is probably responsible for the hysteretic stepped spin crossover in these compounds.