Crystal structures of (E)-5-(4-methylphenyl)-1-(pyridin-2-yl)pent-2-en-4-yn-1-one and [3,4-bis(phenylethynyl)cyclobutane-1,2-diyl]bis(pyridin-2-ylmethanone)

Upon recrystallization from ethylene glycol in daylight, (E)-5-phenyl-1-(pyridin-2-yl)pent-2-en-4-yn-1-one underwent spontaneous [2 + 2] cycloaddition reaction while (E)-5-(4-methylphenyl)-1-(pyridin-2-yl)pent-2-en-4-yn-1-one remained photoinert.


Chemical context
Vinyl-substituted ketones are known to take part in photoinitiated reactions both in the solid state and in solution (Hopkin et al., 1991;Vatsadze et al., 2006). Both trans-cis isomerization and [2 + 2] cycloaddition reactions can be observed depending on the nature of the substituents on the alkyl chain (Vatsadze et al., 2006). Many of the compounds previously reported by us, including 1,5-diarylpentenynones (Golovanov et al., 2013;Vologzhanina et al., 2014;Voronova et al., 20161) and cyclic ketones with vinylacetylene fragments (Voronova et al., 2018) in crystals exhibit coplanar packing with a distance between the olefin fragments of less than 4.2 Å ; thus, they satisfy the Schmidt (1971) criteria for a solidstate [2 + 2] cycloaddition to occur. However, our numerous attepts to carry out [2 + 2] photocycloaddition in these compounds were unsuccessful. We aimed to synthesize pyridine-substituted representatives of this family in order to fix olefin fragments in photoreactive positions using hydrogen bonding or coordination bonding as described by Nagarathinam et al. (2008). Two novel pyridine-2-yl-containing ketones, 1 and 2 (Scheme and Fig. 1), were synthesized as described below, and recrystallized from ethanol. Singlecrystal XRD data for 2 could only be obtained using synchrotron radiation, while we failed to obtain a crystal structure of 1 using single-crystal or powder X-ray diffraction. Recrystallization of 1 and 2 from ethylene glycol afforded, respectively, a dimerization reaction product, 3, and the initial solid phase. ISSN 2056-9890

Structural commentary
The asymmetric unit of ketone 2 contains two independent molecules (Fig. 1). Their conformations are very similar to each other as shown in Fig. 2. Both molecules of 2 exhibit delocalization of charge density along the alkyl chain, as can be concluded from the bond lengths given in Table 1, the single bonds between a double and a triple bond being much shorter than the average value of 1.53-1.54 Å for a C-C bond. The corresponding values for the C O ketone fragments in 3 are similar to those in 2, while the absence of double bonds along the alkyl chain causes shortening of the allyl bonds and elongation of single bonds. The bond lengths in the cyclobutane ring of 3 are unequal: those corresponding to a previously 'double' bond are characteristic of a C-C bond (ca 1.55 Å ), while the single bonds between two 'monomers' are elongated to 1.575 (2) Å . Only the rctt isomer of a 1,2,3,4-tetrasubstituted cyclobutane was obtained of four theoretically possible (based on XRD data).
The conformations of the molecules of both 2 and 3 is probably affected by intramolecular C-HÁ Á ÁN contacts (Tables 2 and 3) involving the nitrogen atoms of the pyridine-2-yl rings and hydrogen atoms of ethenyl or cyclobutane moieties. The C-HÁ Á ÁN angle does not exceed 102 ; however, such a mutual disposition of the conjugated pyridine ring and a double bond was found not only in 2 and 3, but also in previously reported pyridine-2-yl-containing chalcones. The chalcones in the Cambridge Structural Database (CSD, Version 5.40, update of November 2019; Groom et al., 2016) [ABADUE (Fun et al., 2011b), AFOPOC (Chantrapromma et   The molecular structure of 2 and 3, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Selected geometry parameters (Å , ) for 2 and 3.

Supramolecular features
As the independent molecules of ketone 2 have similar conformations, their crystalline environment becomes of particular interest because it can rationalize why Z 6 ¼ 1.

sup-7
Acta Cryst. (2020). E76, 192-196 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.