Iron(II) spin crossover complexes of tetradentate Schiff-bases: tuning T 1/2 by choice of formyl-heterocycle component.

Four new tetradentate Schiff-base ligands were prepared in situ from the 1 : 2 condensation of 1,3-diaminopropane and either 2-thiazolecarboxaldehyde (L2thiazole), 4-thiazolecarboxaldehyde (L4thiazole), 4-oxazolecarboxaldehyde (L4 oxazole ), or 5-bromopyridine-2-aldehyde (L5Br-pyridine), and complexed with [Fe(NCS) 2 (pyridine) 4 ] to give four monometallic Fe II complexes, [Fe(Lheterocycle)(NCS) 2 ]. Structural characterisation shows the expected octahedral Fe II centres in all cases, with Lheterocycle occupying the equatorial plane and the two thiocyanate ligands trans to each other, resulting in an N 6 coordination sphere. Solid state magnetic measurements showed that the two complexes with the thiazole-based ligands exhibit the beginning of a spin transition above 300 K, with T 1/2 = 350 K for [Fe(L4thiazole)(NCS) 2 ] and 400 K for [Fe(L2thiazole)(NCS) 2 ], whereas the 4-oxazole-based ligand gives [Fe(L4 oxazole )(NCS) 2 ] which remains high spin at all measured temperatures (50-400 K). Interestingly, [Fe(L5Br-pyridine)(NCS) 2 ] crystallised as two solvent-free polymorphs: magnetic measurements on samples with both polymorphs present showed a two step SCO with an abrupt transition at T 1/2 = 245 K assigned to the transition in polymorph A (as this was also seen in a sample of pure polymorph A), and a gradual transition at T 1/2 = 304 K assigned to polymorph B. These findings show that the order of increasing ligand field strength for these heterocycles is 4-oxazole ≪ 5Br-pyridine < 4-thiazole < 2-thiazole.