Different substituent effects on the supramolecular arrays in some (E)-halo- and nitro-benzaldehyde oximes: confirmation of attractive π(C=N)···π(phenyl) interactions

The crystal structures and Hirshfeld surface analyses are reported for four aldoximes, (E)-X–C6H4CH=N–OH [X = 3-Cl (1), 4-F (2), 2-O2N (3) and 4-O2N (4)]. The strong classical O–H · · · N hydrogen bonds involving the oxime group generate C(3) chains in compound 1, in contrast to the R2 2(6) dimers formed in compounds 2–4; such arrangements have been shown to be the most frequently found for oximes other than salicylaldoximes (2-hydroxybenzaldehyde oximes). In general, weaker intermolecular interactions involving the X substituents, as well as C–H · · · O and π · · · π interactions have significant effects on the supramolecular arrays generated in the aggegation. A further important interaction in compound 1, and to a lesser extent in compound 4, is a π(C=N) · · · π(phenyl) molecular stacking. A data base search has indicated that short Cg(C=N) · · · Cg(phenyl) distances, <3.3 Å (Cg = centre of gravity), have been found in various compounds, including other oximes. A theoretical study was carried out starting from the crystal structure data of compound 1, with optimisation at the BLYP-D3/def2-DZVP level, as well as at the higher PBE0/ma-def2-TZVP level. Breakdown of the interaction energy into separate contributions was achieved using SAPT (using the jun-cc-pvdz basis set). Overall, the calculations indicate that the π(C=N) ·· · π(phenyl) interaction is attractive, with a magnitude of 14–18 kJ mol−1.

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, namely 2-hy-droxy-4-methyl-benzaldehyde oxime, C8H9NO2, 1, 2,4-di-hydroxy-benzaldehyde oxime, C7H7NO3, 2, and 2-hy-droxy-4-meth-oxy-benzaldehyde oxime, C8H9NO3, 3, are discussed. In each compound, the hydroxyl groups are essentially coplanar with their attached phenyl group. The inter-planar angles between the C=N-O moieties of the oxime unit and their attached phenyl rings are 0.08 (9), 1.08 (15) and 6.65 (15)° in 1, 2 and 3, respectively. In all three mol-ecules, the 2-hy-droxy group forms an intra-molecular O-H⋯N(oxime) hydrogen bond. In compound (1), inter-molecular O-H(oxime)⋯O(hydrox-yl) hydrogen bonds generate R 2 2(14) dimers, related by inversion centres. In compound 2, inter-molecular O-H(oxime)⋯O(4-hy-droxy) hydrogen bonds generate C9 chains along the b-axis direction, while O-H(4-hydrox-yl)⋯O(2-hydrox-yl) inter-actions form zigzag C6 spiral chains along the c-axis direction, generated by a screw axis at 1, y, 1/4: the combination of the two chains provides a bimolecular sheet running parallel to the b axis, which lies between 0-1/2 c and 1/2-1 c. In compound 3, similar C9 chains, along the b-axis direction are generated by O-H(oxime)⋯O(4-meth-oxy) hydrogen bonds. Further weaker, C-H⋯π (in 1), π-π (in 2) and both C-H⋯π and π-π inter-actions (in 3) further cement the three-dimensional structures. Hirshfeld surface and fingerprint analyses are discussed.

A spontaneous single-crystal-to-single-crystal polymorphic transition involving major packing changes

4,6-O-Benzylidene-α-d-galactosyl azide crystallizes into two morphologically distinct polymorphs depending on the solvent. While the α form appeared as thick rods and crystallized in P21 space group (monoclinic) with a single molecule in the asymmetric unit, the β form appeared as thin fibers and crystallized in P1 space group (triclinic) with six molecules in the asymmetric unit. Both the polymorphs appeared to melt at the same temperature. Differential scanning calorimetry analysis revealed that polymorph α irreversibly undergoes endothermic transition to polymorph β much before its melting point, which accounts for their apparently same melting points. Variable temperature powder X-ray diffraction (PXRD) experiments provided additional proof for the polymorphic transition. Single-crystal XRD analyses revealed that α to β transition occurs in a single-crystal-to-single-crystal (SCSC) fashion not only under thermal activation but also spontaneously at room temperature. The SCSC nature of this transition is surprising in light of the large structural differences between these polymorphs. Polarized light microscopy experiments not only proved the SCSC nature of the transition but also suggested nucleation and growth mechanism for the transition.

3-Fluoro-salicylaldoxime at 6.5 GPa

3-Fluoro-salicylaldoxime, C(7)H(6)FNO(2), unlike many salicylaldoxime derivatives, forms a crystal structure containing hydrogen-bonded chains rather than centrosymmetric hydrogen-bonded ring motifs. Each chain inter-acts with two chains above and two chains below via π-π stacking contacts [shortest centroid-centroid distance = 3.295 (1) Å]. This structure at 6.5 GPa represents the final point in a single-crystal compression study.