Diaquabis[3-(hydroxyimino)butanoato]nickel(II)

In the neutral, mononuclear title complex, [Ni(C4H6NO3)2(H2O)2], the Ni atom lies on a crystallographic inversion centre within a distorted octahedral N2O4 environment. Two trans-disposed anions of 3-hydroxyiminobutanoic acid occupy four equatorial sites, coordinated by the deprotonated carboxylate and protonated oxime groups and forming six-membered chelate rings, while the two axial positions are occupied by the water O atoms. The O atom of the oxime group forms an intramolecular hydrogen bond with the coordinated carboxylate O atom. The complex molecules are linked into chains along b by hydrogen bonds between the water O atom and the carboxylate O of a neighbouring molecule. The chains are linked by further hydrogen bonds into a layer structure.

In the neutral, mononuclear title complex, [Ni(C 4 H 6 NO 3 ) 2 -(H 2 O) 2 ], the Ni atom lies on a crystallographic inversion centre within a distorted octahedral N 2 O 4 environment. Two trans-disposed anions of 3-hydroxyiminobutanoic acid occupy four equatorial sites, coordinated by the deprotonated carboxylate and protonated oxime groups and forming sixmembered chelate rings, while the two axial positions are occupied by the water O atoms. The O atom of the oxime group forms an intramolecular hydrogen bond with the coordinated carboxylate O atom. The complex molecules are linked into chains along b by hydrogen bonds between the water O atom and the carboxylate O of a neighbouring molecule. The chains are linked by further hydrogen bonds into a layer structure.
Comment 2-hydroxyiminopropanoic acid and its amide derivatives have been intensively studied during the past 15 years as efficient chelate ligands forming stable complexes with various transition metal ions (Onindo et al., 1995;Duda et al., 1997;Moroz et al., 2008). The presence of an additional strong donor oxime function in the vicinity to the carboxylic group results in important increase of chelating efficiency as compare to structurally related amino acids. For example, 2-hydroxyiminopropanoic acid and other 2-hydroxyiminocarboxylic acids were shown to act as highly efficient chelators with respect to copper(II), nickel(II) and aluminium(III) (Onindo et al., 1995;Sliva et al., 1997a;Sliva et al., 1997b;Gumienna-Kontecka et al., 2000). Also, the amide derivatives of 2-hydroxyiminopropanoic acid possess strong σ-donor capacity and thus have been successfully used for preparation of metal complexes with efficient stabilization of Cu 3+ and Ni 3+ oxidation states (Fritsky et al., 1998;Fritsky et al., 2006). Surprisingly, that the complex formation properties of the nearest homologue of 2-hydroxyiminopropanoic acid -2-hydroxyiminobutanoic acid -have not been studied at all up to date, and no crystal structures of the corresponding coordination compounds have been reported. Herein we present the first crystal structure of a metal complex of 3-hydroxyiminobutanoic acid.
A distorted octahedral coordination geometry is found in (I) with the Ni atom lying on a center of inversion, Fig. 1.
Two four N atoms of two chelating oxime ligands define the equatorial plane, each defining a six-membered rings with a nearly planar conformation, and the two trans-coordinated water molecules complete the octahedral coordination geometry.
The Ni-O bond lengths in the equatorial plane, Table 1, are somewhat shorter than the Ni-N (1.999 (1) Å and 2.043 (1) Å, respectively). The O atoms of the protonated oxime group form intramolecular hydrogen bonds with the coordinated carboxylate O atoms forming five-membered rings and thus fusing two six-membered chelate rings in a pseudomacrocyclic structure. The difference in C-O bond lengths for the coordinated and non-coordinated oxygen atoms (1.271 (2) Å and 1.250 (2)) Å is typical for monodentately coordinated carboxylic groups (Wörl et al., 2005a,b). The C=N, C=O, N-O, bond lengths are typical for 2-hydroxyiminopropanoic acid and its derivatives (Onindo et al., 1995;Sliva et al. (1997a,b); Mokhir et al., 2002).
The octahedral complex molecules are organized in the chains disposed along b direction of the crystal due to H-bonds formed by the axial water molecules and non-coordinated carboxylate O atom O4 belonging to the translational molecule (Table 1). The chains are united in layers with the help of the H-bonds of different type (also formed by the water molecules and non-coordinated carboxylate O atom O4 belonging to another translational molecule). The layers disposed parallel to b direction of the crystal are united in three-dimensional structure only with the help of van der Waals contacts (Fig. 2).

Experimental
Compound (I) was synthesized by adding the solution of nickel(II) nitrate hexahydrate (0.1 mmol, 0.029 g) in water (5 ml) to a solution of 3-hydroxyiminobutanoic acid (0.2 mmol, 0.023 g) in water (5 ml) with consequent heating at 60°C boiling over 15 min. The resultant solution was filtered and the dark pink filtrate was left to stand at room temperature. Slow supplementary materials sup-2 evaporation of the solvent yielded lilac filtrate of (I) Yield 73%. 3-hydroxyiminobutanoic acid was prepared according to the reported procedure (Khromov, 1950).

Refinement
The O-H hydrogen atoms were located from the difference Fourier map, and their coordinates and isotropic thermal parameters refined freely. The hydrogen atoms of the methyl and methylene groups were positioned geometrically and were constrained to ride on their parent atoms, with C-H = 0.96 Å, and U iso = 1.5 U eq (parent atom) for the methyl groups, and with C-H = 0.97 Å, and U iso = 1.2 U eq (parent atom) for the methylene groups.

Diaquabis[3-(hydroxyimino)butanoato]nickel(II)
Crystal data [Ni(C 4 H 6  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.