Different classical hydrogen-bonding patterns in three salicylaldoxime derivatives, 2-HO-4-XC6H3C=NOH (X = Me, OH and MeO)

The crystal structures of three salicyaldoxime compounds are discussed together with Hirshfeld surface and fingerprint analyses.

Compounds 1 and 3 have been shown to have significant activity against Mycobacterium tuberculosis ATTC 27294. The full report will be published elsewhere (da Costa et al., 2018).

Structural commentary
There are no unusual features in the molecular structures. Compound 1 (Fig. 1) crystallizes in the monoclinic space group P2 1 /n with one molecule in the asymmetric unit. Compounds 2 and 3 crystallize in the monoclinic space group P2 1 /c with one molecule in the asymmetric unit (Figs. 2 and 3), all having an oxime unit with an (E) geometry. Bond angles and bond lengths in the phenyl and oxime fragments are all in the expected ranges.
In compound 1, the hydroxyl group is essentially coplanar with its attached phenyl group [displaced by 0.020 (1) Å ], while the interplanar angle between the C NO moiety of the oxime unit and the attached phenyl rings is 0.08 (9) . In compound 2, the hydroxyl groups lie essentially within the phenyl ring plane [O atoms deviate by À0.003 (1) and 0.006 (1) Å ], while the interplanar angle between the C NO moiety of the oxime unit and the attached phenyl rings is 1.08 (15) . In compound 3, the interplanar angle between the C NO moiety of the oxime unit and the attached phenyl rings is 6.65 (15) .

Hydrogen Bonding
In the crystal of 1, molecules are linked by O13-H13 Á Á ÁO2 hydrogen bonds into inversion-related R 4 4 (14) dimers (Table 1). As stated above, such dimers are the most frequently found arrangement for salicyldoxime derivatives. These R 2 2 (14), or R 4 4 (10) (via the intramolecular hydrogen bond) dimers are linked into two-molecule-wide chains, propagating in the a-axis direction by pairs of O13-  The molecular structure of compound 3, showing 80% displacement ellipsoids.
H13Á Á ÁO13 hydrogen bonds, thereby creating R 2 2 (4) rings, as shown in Fig. 4. The H13Á Á ÁO13 lengths in the O13-H13Á Á ÁO13 ii hydrogen bond are rather long [2.611 (16) Å ] with a small angle of 100.8 (12) . However, such data fits well with published findings for H 2 O 2 rings: a recent CSD (Groom et al., 2016) search revealed more than 500 entries for nonsolvated structures having centrosymmetric H 2 O 2 rings with H-O-H angles of 120 or less and HÁ Á ÁO distances up to the sum of the van der Waals contact radii, 2.72 Å , of oxygen and hydrogen atoms. The two-molecule-wide chains are further linked into a three-dimensional arrangement by C3-H3Á Á ÁCg iii and C11-H11Á Á Á Cg iv interactions (Table 1). Nointeractions can be identified. Compound 2 with two hydroxyl groups, as well as the oxime moiety, produces a much more complex classical hydrogenbonding arrangement than the one found for compound 1. The bonding arrangement in 2 can be readily considered to be composed of two elements: a C9 chain, generated from O13-H13(oxime)Á Á ÁO4(4-hydroxy) ii hydrogen bonds, propagating in the direction of the b axis, see Fig. 5, and secondly a zigzag C6 spiral chain formed from O4-H4Á Á ÁO2 i hydrogen bonds, see Fig. 6. The C6 and C9 chains combine to form a bimolecular sheet running parallel to the b axis which lies between 0-1 2 c and 1 2 -1 c. These sheets are further linked by moderately strongstacking interactions, involving all the phenyl rings in the sheet: the CgÁ Á ÁCg separation is 3.7242 (13) Å with a phenyl ring slippage of 1.586 Å . The lack of an R 2 2 (14) dimer in 2 is apparent and results from the preferential interaction of the oxime group with the 4-hydroxyl group rather than with the 2-hydroxy group.

Figure 5
Compound 2. Part of a C9 chain, propagating in the b-axis direction, formed by O13-H13Á Á ÁO4 hydrogen bonds.

Figure 6
Compound 2, part of a spiral C6 chain formed from O4-H4Á Á ÁO2 hydrogen bonds atom O41 involved instead of the hydroxy oxygen O4. Interestingly, the parameters of the two hydrogen bonds in the chains of compound 2 and 3 are very similar. The chains in compound 3 are linked into a two-dimensional array by C11-H11Á Á ÁCg (Table 3) andinteractions. The centroidcentroid separation in theinteraction is 3.7926 (12) Å with a phenyl ring slippage of 1.571 Å -again similar parameters are found in the interactions of compounds 2 and 3.
The lack of an R 2 2 (14) dimer results from the preferential interaction of the oxime group with the 4-methoxy group rather than with the 2-hydroxy group. The C141-H14BÁ Á ÁO2 ii and C3-H3Á Á ÁO2 iii hydrogen bonds link the molecules into centrosymmetric dimers across the centre of symmetry at ( 1 2 , 0, 1 2 ). The former hydrogen bond forms R 2 2 (14) rings, and the latter R 2 2 (8) rings. These link anti-parallel C9 chains, forming a corrugated ribbon which runs parallel to the a axis.

Hirshfeld Surface Analyses
The Hirshfeld surfaces (Spackman & Jayatilaka, 2009) and two-dimensional fingerprint (FP) plots (Spackman & McKinnon, 2002) provide complementary information concerning the intermolecular interactions discussed above. The analyses were generated using CrystalExplorer3.1 (Wolff et al., 2012). The Hirshfeld surfaces mapped over d norm for 1-3 are illustrated in Fig. 8. The intense red areas on the surfaces correspond to OÁ Á ÁH close contacts. The less intense red spot on the surface of 1 relates to a OÁ Á ÁO short contact. The fingerprint plots are shown in Fig. 9. The percentage contributions to the Hirshfeld surface of the various atomÁ Á Áatom contacts shown in Table 4   Compound 3, part of a C9 chain of molecules formed by O13-H13Á Á ÁO41 hydrogen bonds, propagating along the a-axis direction.

Figure 8
Views of the Hirshfeld surfaces mapped over d norm for 1-3. In each case, the red areas relate to classical hydrogen bonds.

Figure 9
The FP plots for 1, 2 and 3. The pair of southwest spikes are due to the OÁ Á ÁH /HÁ Á ÁO close contacts. The highest intensity of pixels in the FP plot for 2 at d e /d i = 1.8 Å includes CÁ Á ÁC contacts. Table 4 Percentages of atom-atom contacts for compounds 1-3.
There are some differences in the percentage of close contacts listed in Table 4 between the R 2 2 (14) dimer formed by compound 1 and the molecular chains formed by compounds 2 and 3. Thus compound 1 exhibits the highest percentage of HÁ Á ÁC/ CÁ Á ÁH close contacts, but no CÁ Á ÁC and NÁ Á ÁO/ OÁ Á ÁN close contacts, unlike compounds 2 and 3, and is the only one of the three compounds to have any close OÁ Á ÁO contacts, albeit a very small percentage. It has to be said that the different substituents, especially the number of hydroxyl units, and other interactions, such as C-HÁ Á Á andinteractions, will have significant effects on the hydrogen-bonding.

Database survey
A survey of the Cambridge Structural Database (CSD Version 5.39, May 2018 update; Groom et al., 2016) of the hydrogenbonding patterns of oximes confirmed the invariable occurrence for salicylaldoximes, R-CH N-OH (where R is a 2-hydroxyphenyl derivative) of the formation of intramolecular O-HÁ Á ÁNO(oxime) hydrogen bonds involving the ortho hydroxyl group. In addition, this hydroxyl group is also most frequently involved in intermolecular interactions producing inversion-related R 2 2 (14) dimers (Smith et al., 2003;Wood et al., 2006Wood et al., , 2008b. An earlier search (Low et al., 2010) indicated that the most frequently found hydrogen-bonding arrangements for oximes without a 2-hydroxyphenyl group are inversion-related R 2 2 (6) dimers and C3 chains.

Synthesis and crystallization
The title compounds were prepared from hydroxylamine and the corresponding benzaldehyde in methanol in the presence of potassium carbonate and were recrystallized from 1484 Gomes et al. C 8 H 9 NO 2 , C 7 H 7 NO 3 and C 8 H 9 NO 3 Acta Cryst. (2018). E74, 1480-1485 research communications

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 5. All hydroxyl H atoms were refined isotropically. Those attached to C atoms were refined as riding atoms with C-H = 0.95-0.98 Å and U iso (H) = 1.2-1.5U iso (C).  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.33 e Å −3 Δρ min = −0.20 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.