Iron(ii) spin crossover complexes of tetradentate Schiff-bases: tuning T1/2 by choice of formyl-heterocycle component

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 (L^5Br-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 N₆ 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(L^5Br-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.