Crystal structures of two nickel compounds comprising neutral Ni^II hydrazone complexes and dicarboxylic acids

Two isostructural Ni^II compounds, bis{N-[1-(pyridin-2-yl-κN)ethylidene]pyridine-4-carbohydrazonato-κ² N′,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-κ² N′,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.