Crystal structure of (E)-2-hydroxy-1,2-diphenylethan-1-one oxime

The H atom of the oxime moiety is equally disordered over two positions, giving rise to two equivalent hydrogen bonds between adjacent molecules.

The title compound, C 14 H 13 NO 2 , is a commercially available material and can be used as a multidentate ligand. The molecule of the asymmetric unit has an R configuration, while the corresponding S-configured molecule of the racemic mixture is generated by a crystallographic centre of symmetry. Both hydroxy groups (the H atom of the oxime group is equally disordered over two positions) are involved in hydrogen bonding, leading to the formation of chains extending parallel to [001].

Chemical context
The title compound (E)-2-hydroxy-1,2-diphenyl-ethan-1-one oxime, C 14 H 13 NO 2 , is commercially available and can be used as a multidentate ligand for which many trivial names such as cuprone or alpha-benzoin, and abbreviations including AboH 2 , BzoxH 2 , are in use. Used for a long time for the determination of manganese or copper in steel (Feigl, 1923;Knowles, 1932;Kar, 1935), BzoxH 2 has attracted considerable attention nowadays in the coordination chemistry of transition metals for the preparation of molecular wheels and highnuclearity metal units with copper, manganese or nickel cations (Stamatatos et al., 2012;Vlahopoulou et al., 2009;Koumousi et al. 2010;Karotsis et al., 2009). In the course of a project to evaluate the reactivity of BzoxH 2 towards organotin(IV) compounds, we obtained high-quality single crystals of the title compound which we have used for structure determination by X-ray diffraction.

Structural commentary
BzoxH2 crystallizes in the centrosymmetric monoclinic space group C2/c with eight molecules in the unit cell and one molecule in the asymmetric unit. As the compound possesses an asymmetric carbon atom (C2), the molecule of the asymmetric unit has an R-configuration while the corresponding S-configured molecule of the racemic mixture is generated by a crystallographic centre of symmetry. Both molecules also ISSN 2056-9890 show the E configuration at the N C double bond of the oxime moiety (Fig. 1).
The length [1.278 (2) Å ] of the N C double bond (Table 1) is consistent with the value of 1.281 (13) Å found in other oxime moieties (Allen et al., 1987). In addition, this moiety is characterized by a bond angle of 115.5 (1) at the N atom and of 102.1 at the O atom. The central C-C bond of the molecule has a length of 1.525 (2), which is also in good accordance with a typical single bond between sp 3 (C2) and sp 2 (C1) hybridized C atoms. As a consequence of the different hybridization states, however, the bonds of these two carbon atoms to their phenyl groups are slightly different: 1.512 (2) Å for C2 and 1.484 (2) Å for C1, respectively. The hydroxy group attached to C2 shows a C-O bond length of 1.425 (2) Å , which also lies in the normal range (1.421-1.433 Å ) of a C 2 -CH-OH group (Allen et al., 1987).
The two phenyl groups exhibit a mean C-C bond length of 1.387 (

Supramolecular features
The molecule possesses two hydroxy groups which, in principle, can act as donors and acceptors for hydrogen bonding while the N atom of the oxime moiety can only act as an acceptor atom in the formation of hydrogen bonds. In fact, the crystal packing ( Fig. 2) with its clear separation of polar and non-polar moieties, results from two different types of hydrogen bonds (Table 2), giving rise to a one-dimensional tube-like arrangement of the molecules propagating along [001]. In the first type of hydrogen bond, only the hydroxy group attached to the carbon atom C2 is involved, acting both as hydrogen-donor and hydrogen-acceptor groups (Fig. 3). Since the oxygen atoms of the resulting hydrogen bonds are related to each other by a centre of symmetry [O2Á Á ÁO2 ii = research communications Acta Cryst. (2017). E73, 1062-1065 Reuter and Okio C 14 H 13 NO 2 1063 Table 1 Selected geometric parameters (Å , ).

Figure 3
Detail of the one-dimensional hydrogen-bonding system (

Figure 1
The asymmetric unit of the title compound, showing the atom-labelling scheme and displacement ellipsoids for the non-H atoms at the 50% probability level; split positions of the H atom attached to atom O2 are labelled H3 and H4.

Synthesis and crystallization
In a typical experiment, -benzoinoxime was refluxed with din-butyltin oxide, C 8 H 18 OSn, in ethanol for 2.5 h. Single crystals of the title compound suitable for X-ray diffraction were obtained from the ethanolic solution layered with n-hexane.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were clearly identified in difference Fourier syntheses. Those of the carbon skeleton were calculated assuming idealized geometries and allowed to ride on the carbon atoms with 1.00 Å for sp 3 -hybridized and 0.95 Å for aromatic H atoms, and with U iso (H) = 1.2U eq (C). The H atoms of the two hydroxy groups were modelled with a common O-H distance of 0.96 Å before they were fixed and allowed to ride on the corresponding oxygen atom with U iso (H) = 1.2U eq (O). Disorder of the hydroxy group attached to C2 was taken into account reducing the site occupancy of both H atoms to one-half. This suggestion was confirmed by difference-Fourier maps that clearly showed both positions.   (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg, 2006) and Mercury (Macrae et al., 2008).  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.