The complexes [Fe(dm4bt)3][FeCl4]2 (1) and [Fe(dm4bt)3][FeBr4]2 (2) were prepared from the reaction of 2,2′-dimethyl-4,4′-bithiazole (dm4bt) with FeCl3·6H2O and FeBr3, respectively, in methanol. Both complexes were characterized by IR, UV-Vis and 1H NMR spectroscopy and their structures were studied by single-crystal diffraction. The methylated bithiazole led to high spin Fe(II) centers in the octahedral cation part of complexes 1 and 2 with Fe–N distance of 2.220 Å, in spite of the low spin octahedral Fe(II) complexes with unsubstituted bithiazole ligands. Crystal structure determination of 2 was performed at 90, 120 and 298 K. Temperature reduction to 90 K resulted in a decrease in the Fe–N bond length to 2.206 Å which is still in the range of high spin Fe(II). Complex 1 shows a reversible mechanochromic effect from the crystalline phase to powder form from red to yellow; it also displays reversible photochromism from yellow to green in solution under sunlight. The magnetic behaviour of the complexes was also studied at 2–300 K. The temperature dependence of χmT curves for the two forms of 1, crystal and powder, demands some changes in their magnetic behavior, causing different colors i.e. red and yellow. At low temperatures, χmT decreases where the decrease starts at around 65 K for compound 1, and at around 100 K for compound 2, due to different counterions. The two complexes exhibit antiferromagnetism at around 4 K.
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