Crystal structure of 1-methylimidazole 3-oxide monohydrate

1-Methylimidazole 3-N-oxide (NMI-O) crystallizes as a monohydrate, in the monoclinic space group P21 with Z′ = 2 (molecules A and B). The imidazole rings display a planar geometry and are linked in the crystal structure into infinite zigzag strands of ⋯NMI-O(A)⋯OH2⋯NMI-O(B)⋯OH2⋯ units by O—H⋯O hydrogen bonds. These chains propagate along the b-axis direction of the unit cell.


Chemical context
Aryl-N-oxides are an important class of materials acting as highly efficient catalysts for the phosphorylation of alcohols  and also for the site-selective phosphoylation of polyols and peptides (Murray et al. 2014). One material in particular, 1-methylimidazole 3-N-oxide, (NMI-O), has been shown to be a highly efficient catalyst for both sulfonylation and silylation procedures . Until recently, NMI-O has been somewhat elusive in the literature. The synthesis of NMI-O and its use as a highly efficient catalyst for certain Morita-Baylis-Hillman reactions has been reported (Lin et al., 2005) although no conclusive information on the structural identity of the material synthesized was presented. A recent paper, directed at the synthesis of salts of 1-alkyl-imidazole 3-oxides for use as ionic liquids also reported the synthesis of NMI-O, however all attempts at crystallizing a sample of this material were unsuccessful although two crystalline adducts of NMI-O, a tris (2-thienyl)borane and a silver carbene hexafluoridophosphate, were structurally characterized (Laus et al., 2008). These authors also demonstrated by NMR and subsequent X-ray structural analysis of a related 1,2-dimethylimidazole semiperhydrate material that the likely product reported earlier (Lin et al., 2005) was the 1-methylimidazole semiperhydrate rather than NMI-O itself. We now present a simplified synthesis of MNI-O and the crystal structure of its hydrate.

Structural commentary
The asymmetric unit of the title compound is shown in Fig. 1. It contains two molecules of NMI-O and two fully occupied ISSN 2056-9890 and ordered water molecules, making the overall stoichiometry a monohydrate. A calculated least-squares plane through the five atoms of the imidazole ring (C1, N1, C2, C3, N2) for molecules A and B gave r.m.s. deviations from planarity of 0.0008 and 0.0002 Å , respectively, with the oxygen atoms of the N + -O À groups also residing close to the ring plane; O1A, À0.021 (4) Å ; O1B, À0.008 (4) Å . The methyl groups lie somewhat farther outside the plane of the ring with displacements of À0.073 (5) Å for C4A and À0.116 (1) Å for C4B. The dihedral angle formed between the least-squares planes of the A and B NMI-O molecules is 12.96 (16) . The present data were not of sufficient quality to determine the absolute structure.

Supramolecular features
In the crystal, the NMI-O and water molecules are linked by O-HÁ Á ÁO hydrogen bonds to form an infinite NMI-OÁ Á ÁOH 2 Á Á ÁNMI-OÁ Á ÁOH 2 Á Á Á chain propagating along the baxis direction of the unit cell. Each water molecule forms two hydrogen bonds, one to each of the N + -O À groups of NMI-O molecules A and B with the oxygen atoms of these groups acting as double acceptors from both water molecules (Table 1, Fig. 2). The NMI-OÁ Á ÁOH 2 Á Á ÁNMI-OÁ Á ÁOH 2 Á Á Á chains are cross-linked in the crystal structure by weaker C-HÁ Á ÁO interactions (Table 1) with HÁ Á ÁO contacts in the range 2.41-2.56 Å .

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.37 update February 2016; Groom et al., 2016) for the imidazole-3-oxide substructure yielded 16 hits, all of which were genuine examples of substituted imidazole-3-oxides.

Figure 2
View of the crystal packing down the a axis. The O-HÁ Á ÁO hydrogen bonds (see Table 1) are shown as dotted lines.

Figure 1
View of the asymmetric unit of the title compound with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.
The O-HÁ Á ÁO hydrogen bonds are shown as dashed lines.
hydroxylamine hydrochloride in the presence of sodium carbonate to afford the mono-oxime. This intermediate was immediately condensed with methylamine to give the corresponding imine, which cyclo-condenses upon exposure to aqueous formaldehyde to give NMI-O after acidic workup in $68% yield (Murray & Spivey, 2016). The previously reported synthesis also started from glyoxal but required eight steps (Laus et al., 2008). The material was concentrated in vacuo to afford a brown oil, which crystallized overnight as colourless laths in the freezer after exposure to air, forming a monohydrate species. The crystals as prepared were extremely hygroscopic, necessitating a rapid transfer to the cold stream of the diffractometer.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The four water H atoms were located in a Fourier difference map and freely refined. All the remaining H atoms were placed geometrically in idealized positions and allowed to ride on their parent atoms: C-H = 0.95-0.98Å with U iso (H) = 1.5U eq (C-methyl) and U iso (H) = 1.2U eq (C) for other H atoms. The data were not of a sufficient quality to reliably determine the absolute structure.

1-Methylimidazole 3-N-oxide monohydrate
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1A 0.5193 (