Crystal structures of two nickel compounds comprising neutral Ni^II hydrazone complexes and di-carb-oxy-lic acids

Two isostructural Ni^II compounds, bis-{N-[1-(pyridin-2-yl-κN)eth-ylidene]pyridine-4-carbohydrazonato-κ²N',O}nickel(II)-2,5-di-chloro-terephthalic acid (1/1), [Ni(C₁₃H₁₁N₄O)₂](C₈H₄Cl₂O₄), and bis-{N-[1-(pyridin-2-yl-κN)eth-ylidene]pyridine-4-carbohydrazonato-κ²N',O}nickel(II)-2,5-di-bromo-terephthalic 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)eth-yl]pyridine-4-carbohydrazonate L ligands result in a cis-NiO₂N₄ octa-hedral coordination sphere for the metal ions. The asymmetric units consist of two half-mol-ecules of the di-carb-oxy-lic acids, which are completed by crystallographic inversion symmetry. In the respective crystals, the 2,5-di-chloro-terephthalic acid (H₂Cl₂TPA, 1-Cl) mol-ecules form zigzag hydrogen-bonded chains with the [Ni(L)₂] mol-ecules, 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 di-carb-oxy-lic acid mol-ecules and terminal pyridine rings in [Ni(L)₂] and short halogen-halogen inter-actions 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.