Crystal structures and Hirshfeld surfaces of four methoxybenzaldehyde oxime derivatives, 2-MeO-XC6H3C=NOH (X = H and 2-, 3- and 4-MeO): different conformations and hydrogen-bonding patterns

The crystal structures of four (E) -methoxybenzaldehyde oxime derivatives, namely (2-methoxybenzaldehyde oxime, 1, 2,3-dimethoxybenzaldehyde oxime, 2, 4-dimethoxybenzaldehyde oxime, 3, and 2,5-dimethoxybenzaldehyde oxime, 4, are discussed. The arrangements of the 2-methoxy group and the H atom of the oxime unit are s-cis in compounds 1–3, but in both independent molecules of compound 4, the arrangements are s-trans. The primary intermolecular O—H(oxime)⋯O(hydroxy) hydrogen bonds generate C(3) chains in 1 and 2. In contrast, in compound 3, the O—H(oxime)⋯O(hydroxy) hydrogen bonds generate symmetric (6) dimers. A more complex dimer is generated in 4 from the O—H(oxime)⋯O(hydroxy) and C—H(2-methoxy)⋯O(hydroxy) hydrogen bonds.

Benzaldehyde oximes, ArCH NOH, with their -CH N-OH functional group are ideally arranged for classical O-HÁ Á ÁO and/or O-HÁ Á ÁN hydrogen bonding. The last survey of the classical hydrogen-bonding patterns in benzaldehyde oximes reported in 2010 (Low et al., 2010) confirmed that the most frequently found arrangements, with the exception of salicylaldoxines, are R 2 2 (6) dimers and C(3) chains, Fig. 1. Aakerö y et al. (2013) reported the percentages of R 2 2 (6) dimers and C(3) chains found in non-salicylaldoxine to be ca 72 and 24%, respectively -similar percentages can be derived from a recent survey of the Cambridge Structural Database (CSD Version 5.39, August 2018 update; Groom et al., 2016). Hydrogen bonds are considered to be the strongest and most directional of intermolecular interactions in molecules (Etter, 1990) and thus play the major roles in determining the overall supramolecular structures. However, the involvement of weaker intermolecular interactions, such as C-HÁ Á ÁO hydrogen bonds,interactions and interactions involving the substituents, can have a significant influence on the supramolecular arrays generated. In a continuation of recent studies on aldoximes Gomes et al., 2018), we have determined the crystal structures of four methoxybenzaldehyde derivatives, namely 2-MeO-X-C 6 H 3 CH NOH where X = H in 1, X = 3-MeO in 2, X = 4-MeO in 3 and X = 5-MeO in 4. The aim of the study was to further investigate the occurrence of R 2 2 (6) dimers and C(3) chains in a series of related compounds.

Structural commentary
There are no unusual features in the molecular structures. Compound 1 crystallizes in the orthorhombic space group Pna2 1 with one molecule in the asymmetric unit (Fig. 2), compound 2 crystallizes in the orthorhombic space group P2 1 2 1 2 1 with one molecule in the asymmetric unit (Fig. 3), compound 3 crystallizes in the triclinic space group P1 with one molecule in the asymmetric unit (Fig. 4), and compound 4 crystallizes in the monoclinic space group, P2 1 /c with two independent molecules, Mol A and Mol B, in the asymmetric unit (Fig. 5). The geometry about the oxime moiety in all molecules is (E). In compounds 1-3, the 2-methoxy group and the hydrogen of the oxime moiety have an s-cis arrangement.
1554 Gomes et al. C 8 H 9 NO 2 , C 9 H 11 NO 3 , C 9 H 11 NO 3 and C 9 H 11 NO 3 Acta Cryst. Atom arrangements and numbering system for the two independent molecules, Mol A and Mol B, of compound 4. Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
Illustrations of the C(3) chains and R 2 2 (6) dimers formed by oximes Atom arrangements and numbering scheme for compound 1. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Atom arrangements and numbering system for compound 2. Displacement ellipsoids are drawn at the 50% probability level.

Figure 4
Atom arrangements and numbering system for compound 3. Displacement ellipsoids are drawn at the 50% probability level.
In contrast, in both molecules of compound 4, the 2-methoxy group and the hydrogen atom of the oxime moiety have an s-trans arrangement. The s-trans arrangement of the 2-alkoxy group and hydrogen atom of the oxime units in compound 4 is very much rarer than the s-cis arrangement found in compounds 1-3 and other non-salicylaldoximes. There is a conformational difference between the two independent molecules Mol A and Mol B of compound 4. This difference is in the orientation of the two methoxy groups, see Fig. 5: in Mol A the orientation is s-trans and in Mol B, it is s-cis. As expected for a 1,2,3-trisubstituted benzene derivative, compound 4 is the least planar of the four oxime derivatives, with the 2-methoxy substituent furthest out of the plane of the attached phenyl group, see Table 1.
spiral chains along the a-axis direction: together these hydrogen bonds form R 4 4 (22) rings. The tiltedstacks propagate in the c-axis direction. The involvement of the weaker C3-H3Á Á ÁO13 ii , C21-H21CÁ Á ÁO13 iii andinteractions, along with the stronger O13-H13 Á Á ÁN12 i hydrogen bonds, creates the three-dimensional structure for 1.
As in 1, molecules of 2 are primarily linked by strong O13-H13 Á Á ÁN12 i hydrogen bonds (Table 3), forming C(3) chains: as such chains are very similar to those in compound 1, see Fig. 6, an illustration has not been provided for the C(3) chain in compound 2. Other intermolecular interactions in 2 are the weaker C21-H21BÁ Á ÁO31 iii and C31-H31BÁ Á ÁO13 iv hydrogen bonds and a C31-H31CÁ Á ÁCg1 v interaction involving the C1-C6 ring. These three interactions combine to form the arrangement illustrated in Fig. 8. The C21-H21BÁ Á ÁO31 iii hydrogen bonds on their own generate C(6) chains, which propagate in the a-axis direction while the C31-H31BÁ Á ÁO13 iv hydrogen bonds generate spiral C(9) chains in the b-axis direction. Together these hydrogen bonds generate a network of R 4 4 (26) rings. The C31-H31CÁ Á ÁCg1 v interactions lead to chains along the a-axis direction. The involvement of the weaker C21-H21BÁ Á ÁO31 iii , C31-H31BÁ Á ÁO13 iv C and C-HÁ Á Á interactions, along with the stronger O13-H13 Á Á ÁN12 i hydrogen bonds, creates a threedimensional structure for 2. C4-H4Á Á ÁO12 ii hydrogen bonds also occur.
Cg1 is the centroid of the C1-C6 ring. rings acting as the rungs and the C41-H41CÁ Á ÁCg1 iii interactions as the supports.
In compound 4, each of the two independent molecules forms symmetric dimers, see Fig. 10. These are generated from combinations of O113-H113Á Á ÁN112 i and O113-H113Á Á ÁO121 i hydrogen bonds (Table 5) for Mol A and O213-H213Á Á ÁN212 ii and O213-H213Á Á ÁO221 ii hydrogen bonds for Mol B. In each case, the dimers contain three rings, two R 2 1 (6) and one R 2 2 (6). There are short NÁ Á ÁN distances across the R 2 2 (6) dimer rings, 2.8595 (12) Å for MolA and 2.8956 (12) Å for Mol B, each being less than the sum of the van der Waals radius (3.10 Å ) for two N atoms.
The links between the two different dimers of 4 are provided by a number of C-HÁ Á ÁO and C-HÁ Á Á interactions, listed in Table 5. Fig. 11 restricts the contacts to just the C-HÁ Á ÁO hydrogen bonds, namely C121-H12CÁ Á Á N212 iii , C111-H111Á Á ÁO251 and C151-H15AÁ Á ÁO113 iv . To facilitate the viewing of the connection in Fig. 11, the two different dimers are drawn in different colours.

Hirshfeld surface analysis
Hirshfeld surfaces (Spackman & Jayatilaka, 2009) and twodimensional fingerprint (FP) plots (Spackman & McKinnon, 2002), provide complementary information concerning the intermolecular interactions discussed above. The analyses were generated using Crystal Explorer3.1 (Wolff et al., 2012). The Hirshfeld surfaces mapped over d norm for 1-4 are illustrated in Fig. 12. The red areas on the surfaces correspond to close contacts. The fingerprint plots are shown in Fig. 13. In all of the FP plots, the pair of spikes pointing south-west relate to the N-H contacts, which in compounds 1 and 2 are involved in the C(3) chains, while in compounds 3 and 4, they are responsible for the creation of the dimers. In compound 3, the fins ending at d e , d i = 1.9,1.1 Å are due to C()Á Á ÁH/C()Á Á ÁH contacts. The FP plots for Mol A and Mol B of compound 4 are asymmetric because of the different interactions of each molecule. The double wings in the FP plot for Mol A in the second quadrant are complementary to those displayed in the fourth quadrant by MolB and relate to CÁ Á ÁH close contacts connecting the two molecules. The spike ending at d i , d e = 1.1 Å in Mol A is due to HÁ Á ÁH contacts.
The percentages of the various atom-atom contacts, derived from the fingerprint plots, for the four compounds are shown in Table 6. The fact that compound 1 has only one methoxy group while the isomers, 2-4, have two is reflected in the greater percentages of contacts involving the oxygen close contacts. The C(3)-chain-forming compounds 1 and 2 show higher percentages of HÁ Á ÁH and CÁ Á ÁC contacts, but a lower percentage of HÁ Á ÁC/CÁ Á ÁH contacts, than the dimer-forming compounds 3 and 4. Gomes et al. C 8 H 9 NO 2 , C 9 H 11 NO 3 , C 9 H 11 NO 3 and C 9 H 11 NO 3 1557 Table 5 Hydrogen-bond geometry (Å , ) for 4.

Figure 10
Compound 4. Symmetric dimers of (a) Mol A and (b) Mol B. Hydrogen bonds (see Table 5) are shown as dashed lines.

Figure 11
Compound 4. Symmetric dimers of Mol A (green) and Mol B (blue). Intermolecular interactions (see Table 5) are shown as dashed lines.

Database survey
A search of the Cambridge Structural Database survey (CSD Version 5.39, August 2018 update; Groom et al., 2016) revealed compounds similar to 2 and 3. The classical hydrogen bonds in 3,5-dimethoxybenzene oxime generate C(3) chains (VUZJAC; Dong et al., 2010). No benzene oxime derivative with only methoxy substituents has been reported in the database to form an R 2 2 (6) or related dimer. The structure has been reported of 3,4,5-trimethoxybenzene oxime (MEQDAO; Chang, 2006) in which classical hydrogen bonds, formed between the oxime unit and the 4-and 5-methoxy moieties, but not the 2-methoxy group, result in the formation of a tetramer. The water molecule in 3,4.5-trimethoxybenzene monohydrate (HESWUY; Priya et al., 2006) is strongly involved in the hydrogen-bonding arrangements.
There are 376 structures, (411 fragments) in the CSD database with oxime R 2 2 (6) dimers in which the NÁ Á ÁN distance across the ring is less than or equal to 3.10 Å , the sum of two N-atom van der Waals radii. The HÁ Á ÁO hydrogen-bond distance range was restricted to 1.739-2.285 Å to exclude improbable OÁ Á ÁH distances based on a statistical analysis in Mercury (Macrae et al., 2006). The NÁ Á ÁN distances range from 2.727 to 3.097 Å with a mean value of 2.987 Å . There are 27 structures within the range 2.838 to 2.909 Å in which our values of 2.8595 (12) Å for MolA and 2.8956 (12) Å for MolB of compound4 lie. Only single-crystal organic compounds were searched for with no limit on the R factor.

Synthesis and crystallization
The title compounds were prepared from hydroxyamine and the corresponding benzaldehyde in methanol in the presence 1558 Gomes et al. C 8 H 9 NO 2 , C 9 H 11 NO 3 , C 9 H 11 NO 3 and C 9 H 11 NO 3 Acta Cryst. (2018). E74, 1553-1560 research communications Table 6 Percentages of atom-atom contacts for compounds 1, 2, 3 and 4 (Mol A and Mol B).

Figure 12
Hirshfeld surfaces for compounds 1-4. In each case, the interactions related to the red areas are designated.

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.

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.

2,4-Dimethoxybenzaldehyde oxime (3)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.20 e Å −3 Δρ min = −0.19 e Å −3 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. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.32 e Å −3 Δρ min = −0.19 e Å −3 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.