Diaquabis[3-(hydroxyimino)butanoato]nickel(II): a triclinic polymorph

The title centrosymmetric mononuclear complex, [Ni(C4H6NO3)2(H2O)2], is a polymorph of the previously reported complex [Dudarenko et al. (2010 ▶). Acta Cryst. E66, m277–m278]. The NiII atom, lying on an inversion center, is six-coordinated by two carboxylate O atoms and two oxime N atoms from two trans-disposed chelating 3-hydroxyiminobutanoate ligands and two axial water molecules in a distorted octahedral geometry. The hydroxy group forms an intramolecular hydrogen bond with the coordinated carboxylate O atom. The complex molecules are linked in stacks along [010] by a hydrogen bond between the water O atom and the carboxylate O atom of a neighboring molecule. The stacks are further linked by O—H⋯O hydrogen bonds into a layer parallel to (001).

Comment 2-Hydroxyiminocarboxylates of various metal ions are an interesting group of chelate complexes intensively studied during the past 15 years (Duda et al., 1997;Onindo et al., 1995). It was shown that 2-hydroxyiminopropanoic acid and other 2-hydroxyiminocarboxylic acids act as efficient chelators with respect to copper(II), nickel(II) and aluminium(III) (Gumienna-Kontecka et al., 2000;Onindo et al., 1995;Sliva et al., 1997a,b). The amide derivatives of 2-hydroxyiminopropanoic acid have been successfully used for the synthesis of metal complexes with efficient stabilization of trivalent oxidation state of Ni and Cu (Fritsky et al., 2006;Kanderal et al., 2005). Recently we reported the first crystal structure of a metal compound of the nearest homologue of 2-hydroxyiminopropanoic acid, 3-hydroxyiminobutanoic acid, a mononuclear complex with Ni (Dudarenko et al., 2010). In the course of our synthetic study we found that a slight change of experimental conditions, namely use of nickel(II) sulfate instead of nickel(II) nitrate and conduction of synthesis at room temperature resulted in crystallization of a polymorph modification of the title compound.
A distorted octahedral coordination geometry is found in the title complex with the Ni II atom lying on an inversion center (Fig. 1). Two O atoms and two N atoms from two chelating ligands define the equatorial plane, each ligand defining a six-membered ring with a nearly planar conformation, and the two trans-coordinated water molecules complete the octahedral coordination geometry. The Ni-O bond lengths [1.992 (2) (2) and 1.250 (2) Å] is typical for monodentately coordinated carboxylate groups (Wörl et al., 2005a,b). The C═N, C═O and N-O bond lengths are typical for 2-hydroxyiminopropanoic acid and its derivatives (Mokhir et al., 2002;Moroz et al., 2008;Onindo et al., 1995;Sliva et al., 1997a,b). In general, the geometrical parameters of the molecule are very close to those observed in the structure of the monoclinic modification of the title complex (Dudarenko et al., 2010).
The octahedral complex molecules are organized in the piles disposed along the b axis due to a hydrogen bond formed between the axial water molecule and noncoordinated carboxylate O atom of a neighboring molecule (Table 1). The Ni···Ni separation in the piles is equal to the unit cell parameter b. The piles are united in walls with the help of a hydrogen bond of different type (a bifurcate hydrogen bond formed between the water molecule and both coordinated and noncoordinated carboxylate O atoms belonging to a translational molecule). The walls disposed parallel to (0 0 1) are united in a three-dimensional structure only with the help of van der Waals contacts (Fig. 2).

Experimental
The title compound was synthesized by adding the solution of nickel(II) sulfate hexahydrate (0.1 mmol, 0.026 g) in water (5 ml) to a solution of 3-hydroxyiminobutanoic acid (0.2 mmol, 0.023 g) in water (5 ml). The resultant mixture was filtered and the dark pink filtrate was left to stand at room temperature. Slow evaporation of the solvent yielded lilac crystals of the title compound (yield 73%). 3-Hydroxyiminobutanoic acid was prepared according to the reported procedure (Khromov, 1950).

Refinement
O-bound H atoms were located from a difference Fourier map and refined isotropically. H atoms of methyl and methylene groups were positioned geometrically and were constrained to ride on their parent atoms, with C-H = 0.97 (CH 2 ) and 0.96 (CH 3 ) Å and with U iso (H) = 1.2(1.5 for methyl)U eq (C).