Crystal structure of bis(μ-N-hydroxypicolinamidato)bis[bis(N-hydroxypicolinamide)sodium]

In the crystal, the coordination dimers are linked via N—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds and π–π stacking interactions into a two-dimensional framework parallel to (100).


Chemical context
Hydroxamic acids as a class of organic compounds originate from Lossen's invention (Lossen, 1869). The coordination ability of hydroxamic acids has led to their extensive use in coordination and supramolecular chemistry (Ś wią tek- Kozłowska et al., 2000;Dobosz et al., 1999). In particular, over the past two decades they have often been used as frameworks of metallacrowns (Golenya et al., 2012a;Safyanova et al., 2015;Stemmler et al., 1999;Jankolovits et al., 2013a,b) and as building blocks of coordination polymers (Gumienna-Kontecka et al., 2007;Golenya et al., 2014;Pavlishchuk et al., 2010Pavlishchuk et al., , 2011. They have also been studied intensively in biology and medicine due to their various biological activities, especially their metal-chelating ability and inhibition of a series of metalloenzymes (Codd, 2008;Griffith et al., 2005;Marmion et al., 2013). N-Hydroxypicolinamide (or picoline-2-hydroxamic acid, H 2 PicHA) has been used extensively for the synthesis of polynuclear complexes, especially various metallacrowns (Stemmler et al., 1999;Seda et al., 2007;Jankolovits et al., 2013a;Golenya et al., 2012a;Gumienna-Kontecka et al., 2013). A large number of polynuclear metal complexes based on this ligand has been investigated. The Cambridge Structural Database (Groom et al., 2016) contains data on the crystal structures of over 20 coordination compounds based on o-PicHA. The crystal and molecular structure of N-hydroxypicolinamide monohydrate was the subject of two recent independent investigations (Chaiyaveij et al., 2015;Safyanova et al., 2016).
In the course of the synthesis of hydroxamate metal complexes, especially metallacrowns, in some cases alkaline metal hydroxamates appear to be more preferable starting materials than the parent hydroxamic acids due to their better solubility in water. During our synthetic attempts using the sodium salt of N-hydroxypicolinamide, we noticed that the elementary analysis data differ noticeably from those expected for the monosodium salt or its hydrates, which might affect the reagent ratio in the synthesis of coordination compounds. In order to find out the reason for this deviation in the analytical data, we undertook a single crystal X-ray analysis of the sodium salt of N-hydroxypicolinamide. Herein we present the crystal and molecular structure of the title compound.
The deprotonated N-hydroxypicolinamide residue adopts a strongly flattened conformation with a dihedral angle of only 0.6 (2) between the hydroxamic group and the pyridine ring. At the same time, the corresponding dihedral angles in both neutral N-hydroxypicolinamide molecules are noticeably greater [17.5 (2) and 8.9 (2) ], indicating a deviation of the hydroxamic group from the plane of pyridine rings. The configuration about the hydroxamic C-N bond is Z and that about the C-C bond between the pyridine and hydroxamic groups is E for both the neutral and deprotonated hydroxamates. Intramolecular N-HÁ Á ÁN attractive contacts between the hydroxamate group and the nitrogen atom of pyridine ring [2.25 (2)-2.35 (3) Å ] are present in both the neutral and deprotonated N-hydroxypicolinamide molecules (Table 1).
The centrosymmetric molecular unit of the title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of undefined radius. Table 1 Hydrogen-bond geometry (Å , ).

D-HÁ
The bond lengths and angles within both the neutral and deprotonated hydroxamic groups are within normal ranges. The C-N and C-C bond lengths in the pyridine moiety are typical for 2-substituted pyridine derivatives (Moroz et al., 2012;Strotmeyer et al., 2003;Fritsky et al., 2004).

Synthesis and crystallization
The title compound was obtained by the reaction of N-hydroxypicolinamide (0.156 g, 1 mmol, dissolved in 5 ml of water) with sodium hydrogen carbonate (1 M aqueous solution, 1 ml). Colorless crystals suitable for X-ray diffraction were obtained from the resulting aqueous solution by slow evaporation at ambient temperature within 48 h (yield 78%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were found in the difference Fourier maps; H atoms of pyridine rings were constrained to ride on their parent atoms with C-H = 0.93 Å and U iso = 1.2U eq (C), and H atoms of the N-H and O-H groups were refined isotropically.

Bis(µ-N-hydroxypicolinamidato)bis[bis(N-hydroxypicolinamide)sodium]
Crystal data [Na 2 (C 6 H 5 N 2 O 2 ) 2 (C 6 H 6 N 2 O 2 ) 4 ] M r = 872.73 Triclinic, P1 a = 9.7997 (7)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.