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Four Fe(III) spin-crossover (SCO) compounds, [Fe(HL1)2](HCl4TPA) (1-Cl), [Fe(HL1)2 ](HBr4TPA) (1-Br), [Fe(HL2)2](HCl4TPA) (2-Cl), and [Fe(HL2)2](HBr4TPA) (2-Br) (HL1 = 4-chloro-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate; HL2 = 4-bromo-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate; HCl4TPA = 2,3,5,6-tetrachloro-4-carboxybenzoate; and HBr4TPA = 2,3,5,6-tetrabromo-4-carboxybenzoate), were synthesized to investigate the halogen substituent change effect in salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids in SCO complexes to the spin-transition temperature. Crystal structure analyses showed that these compounds were isostructural. In addition, a one-dimensional hydrogen-bonded column was formed by the dicarboxylic acid anion and weak hydrogen bonds between the Fe(III) complexes. From Mössbauer spectroscopy and magnetic property measurements, these compounds were confirmed to exhibit gradual SCO. The spin-transition temperature can be shifted by changing the halogen substituent in the salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids without changing the molecular arrangement in the crystal packing. [ABSTRACT FROM AUTHOR]

Two isostructural NiII compounds, bis{N-[1-(pyridin-2-yl-κN)ethylidene]pyridine-4-carbohydrazonato-κ²N′,O}nickel(II)-2,5-dichloroterephthalic acid (1/1), [Ni(C13H11N4O)2](C8H4Cl2O4), and bis{N-[1-(pyridin-2-yl-κN)ethylidene]pyridine-4-carbohydrazonato-κ2N0,O}nickel(II)-2,5-dibromoterephthalic acid (1/1), [Ni(C13H11N4O)2](C8H4Br2O4), were synthesized and their crystal structures determined. The pair of N,N′,O-tridentate N-[1-(pyridin-2-yl-κN)ethyl]pyridine-4-carbohydrazonate L ligands result in a cis-NiO2N4 octahedral coordination sphere for the metal ions. The asymmetric units consist of two half-molecules of the dicarboxylic acids, which are completed by crystallographic inversion symmetry. In the respective crystals, the 2,5-dichloroterephthalic acid (H2Cl2TPA, 1-Cl) molecules form zigzag hydrogen-bonded chains with the [Ni(L)2] molecules, with the hydrogen-bond distances in 1-Br slightly longer than those in 1-Cl. The packing is consolidated by aromatic π-π stacking between the dicarboxylic acid molecules and terminal pyridine rings in [Ni(L)2] and short halogen-halogen interactions are also observed. The qualitative prediction of the H-atom position from the C--N--C angles of the terminal pyridine rings in L and the C--O distances in the carboxyl groups show that 1-Cl and 1-Br are co-crystals rather than salts. [ABSTRACT FROM AUTHOR]