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Two NiII compounds forming zigzag chain structures through O—H⋯N hydrogen bonds between di­carb­oxy­lic acids and NiII complexes were synthesized, and their structures were determined. They are co-crystals rather than salts., Two isostructural NiII 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.

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.

We synthesized new Fe(II) compounds consisting of [Fe(L)2] (HL: 2-Acetylpyridine isonicotinoylhydrazone) and dicarboxylic acids. Single crystal analysis shows that short hydrogen bonds are formed between [Fe(L)2] and dicarboxylic acids. The hydrogen bonding interactions between metal complexes and dicarboxylic acids are complemented by π-π interactions, forming two-dimensional network structures. Magnetic properties show that the Fe(II) compounds exhibit gradual spin crossover behavior around room temperature and a T1/2 value is dependent on the halogen substituent on the dicarboxylic acids.

Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Rahara, Kolkata-700 118, India E-mail : ictg_64@yahoo.co.in Department of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK Manuscript received 12 June 2009, revised 9 September 2009. accepted 14 September 2009 A monoclinic variety with C2/c space group of tile title complex [VvO(L)(OCH3)2 incorporating the doubly deprotonated benzoyl hydrazone of 2-hydroxy-S-methylacetophenone (H2L) was synthesized from the decomposition of [VIVO(L)(bipy)](where bipy representing the 2,2'-bipyridine) in methanol which has been reported very recently from our laboratory. In this paper we report another monoclinic variety of this complex with P2 1/n space group that showed some differences in bonding patterns in the solid state (but in solution they are almost identical) prepared by different synthetic method viz. from the equimolar reaction of [VIVO(acac)2], H2L and imidazole in CH3OH.