Stepped spin crossover in Fe(iii) halogen substituted quinolylsalicylaldimine complexes

Abstract

The reaction of Fe(NCS)₃ prepared in situ in MeOH with Hqsal-X (Hqsal-X = 5-X-N-quinolylsalicylaldimine) in CH₂Cl₂ yields the Fe^III 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 Fe^III 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, T_1/2 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 Fe^III centres although the latter contains two crystallographically independent Fe^III 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.