Crystal structure of N-hydroxypicolinamide monohydrate

In C6H6N2O2·H2O, the N-hydroxypicolinamide molecule adopts a strongly flattened conformation. O—H⋯O interactions and π–π stacking interactions between the pyridine rings organize the crystal components into columns extending along the b axis while N—H⋯N hydrogen bonds link these columns into a two-dimensional framework parallel to (100).


Chemical context
Hydroxamic acids (HA) are weak organic acids with the general formula R-C( O)-NH-OH. HA can exist as keto and imino(enol) tautomers with two isomers, E and Z, for each form, and in the zwitterionic form (see Scheme below). They have found broad application in coordination chemistry due to their diversity and comparatively facile synthesis (Ś wią tek- Kozłowska et al., 2000;Dobosz et al., 1999). In addition, they exhibit biological activities related to their enzyme-inhibitory properties (Marmion et al., 2013). HAs are widely used in coordination and supramolecular chemistry as scaffolds in the preparation of metallacrowns (Seda et al., 2007;Jankolovits et al., 2013;Safyanova et al., 2015) and as building blocks of coordination polymers Golenya et al., 2014;Pavlishchuk et al., 2010Pavlishchuk et al., , 2011.
N-Hydroxypicolinamide, known also as picoline-2-hydroxamic acid (o-PicHA), has been used extensively for the synthesis of polynuclear complexes, especially in the synthesis of diverse metallacrowns (Stemmler et al., 1999;Seda et al., 2007;Jankolovits et al., 2013;Golenya et al., 2012;Gumienna-Kontecka et al., 2013). Presently, the Cambridge Structural Database (Groom & Allen, 2014) contains more than 20 entries of coordination compounds based on N-hydroxypicolinamide. ISSN 2056-9890 Our interest in N-hydroxypicolinamide stems also from the fact that in the course of synthesis of the title and related compounds from 2-picolinic acid esters (Hynes, 1970), the products are frequently contaminated with impurities that result from hydrolysis of the ester or hydroxamic groups to the carboxylic group. Structural information about the title compound will be helpful in controlling the purity of the synthesised ligand by powder diffraction.

Structural commentary
The molecular structure of the title compound is presented in Fig. 1. The crystal structure of the title compound consists of an N-hydroxypicolinamide molecule in the Z-keto tautomeric form in agreement with the C O and C-N bond lengths [1.234 (2) and 1.325 (2) Å , respectively] and a water molecule. The N-hydroxypicolinamide molecule adopts a strongly flattened conformation and only the O-H group H atom deviates significantly from the molecular best plane. The maximum deviation from this plane for non-hydrogen atom is 0.083 (1) Å for O1 and the hydroxyl group H2 atom is displaced from the mean plane by 0.80 (1) Å in the direction of the water molecule. The dihedral angle between the hydroxamic group and the pyridine ring is 5.6 (2) . The configuration about the hydroxamic group C-N bond is Z and that about the C-C bond between the pyridine and hydroxamic groups is E [torsion angles O2-N2-C6-O1 À0.4 (3) , N1-C1-C6-O1 175.6 (2) ].

Supramolecular features
The molecular components of the title compound are connected by O-HÁ Á ÁO and N-HÁ Á ÁN hydrogen bonds (Table 1) into a two-dimensional framework parallel to (100) (Fig. 2). The O-HÁ Á ÁO interactions andstacking interactions between the pyridine rings [centroid-centroid distance 3.427 (1) Å ] organize the crystal components into columns extending along the b axis while the N-HÁ Á ÁN hydrogen bonds link these columns into a two-dimensional framework parallel to (100) (Fig.2).

Database survey
A search of the Cambridge Structural Database (Version 5.36, last update February 2015; Groom & Allen, 2014) revealed two crystal structures of isomeric pyridine hydroxamic acids and the crystal structure of 2,6-pyridinedihydroxamic acid Makhmudova et al., 2001;Griffith et al., 2008).

Synthesis and crystallization
The title compound was obtained by the reaction of methyl 2picolinate and hydroxylamine in methanol solution according to a reported procedure (Hynes, 1970 The asymmetric unit of the title compound, with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius. The dashed line indicates a hydrogen bond.

Figure 2
A packing diagram of the title compound. Hydrogen bonds are indicated by dashed lines. H atoms not involved in hydrogen bonding have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y; Àz þ 1 suitable for X-ray diffraction were obtained from a methanol solution by slow evaporation at room temperature (yield 79%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal was modelled as a nonmerohedral twin with the volume ratio of two twin domains refined at 89:19. The C-H, N-H and O-H hydrogen atoms of the organic molecule were found from the difference Fourier maps but for further calculations they were positioned geometrically and constrained to ride on their parent atoms with C-H = 0.93 Å , N-H = 0.86 Å and O-H = 0.82 Å , and with U iso = 1.2U eq (C,N) or U iso = 1.5U eq (O). The H atoms of the water molecule were located in the difference Fourier maps, the O-H distances standardized to 0.85 Å and refined in riding-model approximation with U iso (H) = 1.5U eq (O).  (Agilent, 2013); cell refinement: CrysAlis PRO (Agilent, 2013); data reduction: CrysAlis PRO (Agilent, 2013); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refined as a 2-component twin. 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 > σ(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.