Abstract
Four new tetradentate Schiff-base ligands were prepared in situ from the 1 : 2 condensation of 1,3-diaminopropane and either 2-thiazolecarboxaldehyde (L-2thiazole), 4-thiazolecarboxaldehyde (L-4thiazole), 4-oxazolecarboxaldehyde (L-4oxazole), or 5-bromopyridine-2-aldehyde (L5Br-pyridine), and complexed with [Fe(NCS)(2)(pyridine)(4)] to give four monometallic Fe-II complexes, [Fe(L-heterocycle)(NCS)(2)]. Structural characterisation shows the expected octahedral Fe-II centres in all cases, with L-heterocycle 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(L-4thiazole)(NCS)(2)] and 400 K for [Fe(L-2thiazole)(NCS)(2)], whereas the 4-oxazole-based ligand gives [Fe(L-4oxazole)(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.