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

摘要

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 = 0.25CH(2)Cl(2)center dot 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. Mossbauer 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 (1) over bar while 4 is in monoclinic P2(1)/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 pi-pi interactions through the planar qsal-X ligands and CH center dot center dot center dot X (X = O, halogen) and/or X center dot center dot center dot pi (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.