DIMERIC MOLECULAR AGGREGATION MOTIF IN CRYSTAL OF 2,7-DIETHOXY-1-(4-NITROBENZOYL)NAPHTHALENE: CORRELATION OF SINGLE MOLECULAR STRUCTURE, MOLECULAR ACCUMULATION STRUCTURE AND NON-COVALENT-BONDING INTERACTIONS
Crystal structure of 2,7-diethoxy-1-(4-nitrobenzoyl)naphthalene, C21H19NO5, is reported and discussed on the characteristics of the spatial organization of single molecule and molecular aggregation as contrasted with a homologous compound. The molecular structures of these compounds differ only in the kind of alkoxy group of 2,7-positions of naphthalene rings, i.e., ethoxy groups for title compound and methoxy ones for homologue. In single molecular crystal structure, the 4-nitrobenzoyl groups of these molecules are attached non-coplanarly to the naphthalene ring. The congested situation makes bonds connecting naphthalene ring and carbonyl group fixed to turn stereogenetic, which allow the independent existence of atrope stereoisomers of (R)- and (S)-enantiomeric conformer molecules, contrary to their solute state where the fast interconversion disturbs the distinction of the enantiomeric molecules. The two pairs of the enantiomeric molecules are related by two-fold helical axis in the asymmetric unit of P21/n space group for title compound and P21/c for homologous compound, exhibiting the number of molecules (Z) is four for both compounds. In crystal of title compound, (R)- and (S)-enantiomers are connected to each other by pi…pi stacking interaction and two types of C–H…O=N non-classical hydrogen bonds, (sp2)C–H…O=N and (sp3)C–H…O=N non-classical hydrogen bonds along b-axis, forming centrosymmetric dimeric molecular aggregates. The dimeric units are stacked into columnar structure by (sp2)C–H…O=C non-classical hydrogen bonds between molecular unit of identical enantiomeric configuration along a-axis. The columns are also connected by (sp2)C–H…OEt non-classical hydrogen bonds between molecular unit of identical enantiomeric configuration along c-axis to give sheet-like aggregate composed of molecules of same enantiomeric configuration spreading on ac-plane. The sheets are piled up through (sp3)C–H…pi non-classical hydrogen bonds between opposite enantiomeric molecular units of next dimeric aggregates along b-axis. In crystal of the homologous compound, 2,7-dimethoxy-1-(4-nitrobenzoyl)naphthalene, centrosymmetric dimeric aggregate resulted from the association of (R)- and (S)-enantiomers via pi…pi stacking interactions are also observed. The centrosymmetric dimeric aggregates are unidirectionally lined by two kinds of non-classical hydrogen bonds between molecular unit of identical enantiomeric configuration, (sp3)C–H…O=C and (sp3)C–H…pi non-classical hydrogen bonds, giving columnar structure. The columns are accumulated giving a wavy sheet structure composed of stripes of respective enantiomeric configuration oriented alternatively and anti-parallelly through weak (sp2)C–H…pi non-classical hydrogen bonds. The difference of higher ordered structure between title compound and homologue is plausibly explained according to one CH2 margin in 2,7-dialkoxy groups, i.e., elongation of the least length in alkyl group of title compound compared to homologue brings about the sterically significant hindrance among dimeric aggregates that makes rather anisotropic intermolecular non-covalent bonding interactions resulting in accumulating sheet structure. For 1-monoaroylnaphthalene compounds, the most stabilized single molecular structure on condition that the sufficiently effective intermolecular interaction is absent is proposed the perpendicular alignment of naphthalene and benzene rings to prevent the steric repulsion of two aromatic rings. The smaller spatial volume of methyl group in homologue molecule is plausibly able to be merged without large alternation in the single molecular structure most stabilized. Contrarily, the additional methylene unit at 2,7-alkoxy group probably requires the rather large perturbation from the supposed structure most stabilized.
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