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 the naphthalene rings, i.e., ethoxy groups for title compound and methoxy groups for homologue. In single molecular crystal structures of these compounds, 4-nitrobenzoyl group is non-coplanarly attached to the naphthalene ring. The molecules exhibit axial chirality, with either R or S stereogenic axis. 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 one for homologue, showing the number of molecules (Z) is four for both compounds. In their molecular packing structures, (R)and (S)enantiomers are connected to each other by ... stacking interactions, forming centrosymmetric dimeric molecular aggregates. However, the aggregation structures of the dimers are apparently different between title compound and homologue. The dimeric units of title compound are stacked into columnar structure with (sp)C–H...O=C non-classical hydrogen bonds between identical enantiomers along aaxis. The columns are connected by (sp)C–H...OEt non-classical hydrogen bonds between identical enantiomers along c-axis to give sheet-like aggregation spreading on ac-plane. The sheets are piled up through (sp)C–H... non-classical hydrogen bonds between opposite enantiomeric molecules of next dimeric aggregates along b-axis giving layers. On the other hand, the dimeric molecular aggregates of homologue are connected into flattish column by (sp)C–H...O=C non-classical hydrogen bonds and (sp)C–H... nonclassical hydrogen bonds between identical enantiomers. The columns are linked into waving plate through weak (sp)C–H... nonclassical hydrogen bonds. In title compound, difference between ... stacking interactions and non-classical hydrogen bonds is smaller than homologue in contribution to the whole of molecular packing structure.


Introduction
2][3][4][5][6] Crystal engineering often use a concept of supramolecular synthon 7 for leading objective crystal structure.Supramolecular synthons are kinetically defined structural units that transfer the essential features of a crystal structure, and a critical assumption is that the synthon is a reasonable approximation to the whole crystal.The closer the structure of a small synthon is to the actual crystal, the more useful is this entire concept.Although design of strong interactions leading robust synthon is one of important strategies in crystal engineering, supramolecular synthon is not universal concept from the perspective of understanding the correlation between molecular structure and molecular packing structure.Because, organic molecules in crystal are linked by not only strong hydrogen bonds but also a number of weak hydrogen ones including van der Waals interactions, non-classical hydrogen bonds whose C-H groups form with electron rich atom/group, and … stacking interactions.3][14] One reason why is that a series of model compounds suitable for systematic investigations on weak interactions is difficult to obtain.[17][18] There are two common features in the crystals of 1,8diaroylated naphthalene compounds, 1) two aroyl groups are non-coplanarly located to the 2,7-dialkoxynaphthalene core and oriented in an opposite direction with a few exceptional compounds bearing unidirectional-alignment of aroyl groups and 2) the molecular packing of 1,8-diaroylated 2,7dialkoxynaphthalene compounds are mainly stabilized by weak hydrogen bonds, i.e., four kinds of non-covalent bonding interactions, (sp 2 )C-H   frequency.The features can be interpreted that the noncoplanarly accumulated aromatic rings structure disturbs formation of strong π…π stacking interactions.0][21] They have non-coplanarly accumulated aromatic rings structure.The authors envision that 1-aroylated naphthalene compounds show similar structural features to 1,8-diaroylated naphthalene compounds, whereas they might have flexible spatial organization rather than the 1,8-diaroylated homologue.Therefore, the authors planned to elucidate correlation between molecular structure and crystal structure of 1aroylated naphthalene compounds by tracing non-covalent bonding interactions.The information should be complementary knowledge to a concept of supramolecular synthon.Herein crystal structure of 2,7-diethoxy-1-(4nitrobenzoyl)naphthalene 22 is reported and discussed correlation among molecular structure, molecular packing structure and effective non-covalent bonding interactions through comparison with the 1-aroylated naphthalene homologue, 2,7-dimethoxy-1-(4-nitrobenzoyl)naphthalane, 19 and the related other compounds.

Materials and methods
Aluminium chloride was of commercial quality and was used as received from Wako Pure Chemical Industries, Ltd., Japan, purity greater than 98%.4-Nitrobenzoyl chloride was acquired from Tokyo Chemical Industry Co., Ltd., Japan, purity greater than 98% and purified by distillation under reduced pressure (135˚C/18 mmHg).Solvents were dried and purified using standard procedures. 23Synthetic methods and spectral data for starting material and homologue, 2,7-diethoxynaphthalene and 2,7-dimethoxy-1-(4-nitrobenzoyl)naphthalene, have been reported in literatures. 24,19 surements 1 H NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (300 MHz).Chemical shifts are expressed in ppm relative to internal standard of Me4Si (δ 0.00). 13C NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (75 MHz).Chemical shifts are expressed in ppm relative to internal standard of CDCl3 (δ 77.0).IR spectra were recorded on a JASCO FT/IR-4100 spectrometer (KBr tablet).High-resolution FAB mass spectra were recorded on a JEOL MStation (MS700) ion trap mass spectrometer in positive ion mode.

X-ray crystallography
For the crystal structure determination, the single-crystal of title compound was used for data collection on a four-circle Rigaku RAXIS RAPID diffractometer (equipped with a two-dimensional area IP detector).The graphite-monochromated Cu Kα radiation (λ = 1.54187Å) was used for data collection.The lattice parameters were determined by the least-squares methods on the basis of all reflections with F 2 >2σ (F 2 ).Crystal data, data collection and structure refinement details are summarized in Table 1.All H atoms could be located in difference Fourier maps, but were subsequently refined in optimized positions as riding atoms, with C-H = 0.95 (aromatic) and 0.98 (methyl) and with Uiso(H) = 1.2 Ueq(C).For data collection: PROCESS-AUTO 25 ; cell refinement: PROCESS-AUTO 25 ; data reduction: CrystalStructure 26 ; program(s) used to solve structure: SIR2004 27 ; program(s) used to refine structure: SHELXL97 28 ; molecular graphics: ORTEPIII 29 .The hydrogen bond geometries of title compound are listed in Table 2. Molecular structure of title compound with the atom-labelling scheme is displayed in Figure 1.

Results and discussion
Figure 1 exhibits single molecular structure of title compound. 22The aroyl group is non-coplanarly situated against the naphthalene ring.The interplanar angle between 4-nitrobenzene ring and 2,7-diethoxynaphthalene ring is 81.99(5)˚ [torsion angles = 112.12(15)˚(C2-C1-C15-O3) and 114.48( 14 Contrary to the achiral nature of title molecule in solution, the molecules in crystal exhibit atrope isomerism brought about by molecular stereogenic axis of carbon-carbon bond between the carbonyl moiety and the naphthalene ring.Therefore, a pair of (R)-and (S)-enantiomeric molecules exists in the crystal.The two pairs of the enantiomeric molecules are related by two-fold helical axis in the asymmetric unit of P21/n space group, exhibiting the number of molecules (Z) in a unit cell is four (Figure 2).
In molecular packing, (R)-and (S)-enantiomeric molecules are linked centrosymmetrically (Figure 3).Between the paired enantiomeric molecules, (4-    Several years ago, the authors reported crystal structure of 2,7-dimethoxy-1-(4-nitrobenzoyl)naphthalene, which is one of the homologues of title compound. 19The spatial organization of homologue is exhibited with title compound in

Table 3. Dihedral angles and torsion angles in title compound and homologue
The centrosymmetric dimeric molecular aggregates imbricate along ab-diagonal through two kinds of nonclassical hydrogen bonds between molecules of identical enantiomeric configuration, i.e., (      Table 4 shows the effective non-covalent bonding interactions observed in crystals of title compound and homologue with the order of interatomic length for each category of non-covalent bonding interactions.In Table 4, the data printed in blue characters denote the distances of non-covalent bonding interactions between molecules of same enantiomeric configuration and those in red characters indicate non-covalent bonding interactions between molecules of opposite enantiomeric configuration.C-H…O=C non-classical hydrogen bonds between same enantiomeric molecules and C-H… non-classical hydrogen bonds between opposite enantiomeric molecules are observed in both of title compound and homologue.However, the types of hybrid orbital of carbon atoms to which the hydrogen atoms involved in these non-classical hydrogen bonds attach are in reversed combination [(sp  Furthermore, the lengths of hydrogen bonds for title compound are shorter than those for homologue.In addition, there are more kinds of effective non-classical hydrogen bonds specific for title compound [(sp 3 )C-H…OEt nonclassical hydrogen bond = 2.56 Å, (sp 3 )C-H…O=N nonclassical hydrogen bond = 2.59 Å, and (sp 2 )C-H…O=N non-classical hydrogen bond = 2.68 Å] than those for homologue [(sp 3 )C-H… non-classical hydrogen bond = 2.91 Å].Red characters and blue ones express non-covalent bonding interactions between opposite enantiomers and those between identical enantiomers, respectively.
Dimeric molecular aggregation is a common structural motif in the crystals of title compound and homologue.However, the aggregation structures have substantial difference between these compounds in types of constituent non-covalent bonding interactions.
In crystal of homologous compound, there are practically no effective non-covalent bonding interactions other than … stacking interactions for aggregation of dimeric units.Contrarily, for title compound there are several kinds of effective noncovalent bonding interactions with medium to weak strength in addition to … stacking interactions for construction of aggregation of dimeric pairs.For homologue, predominant contribution of … stacking interactions seems substantially to determine the spatial alignment without assistance of other less effective interactions.For title compound, the relative significance of … stacking interactions among effective non-covalent bonding interactions is considered to be smaller than that in homologue.In this consequence, the relatively reduced priority of … stacking interactions over other interactions are probably compelled to allow perturbation of spatial alignment of molecular structure for many weaker noncovalent bonding interactions to perform cooperatively maximum net stabilization.As a result, many non-classical hydrogen bonds are formed with compensation of offset of smaller overlapping of aromatic rings in the dimeric molecular aggregation.

Conclusion
In the absence of an intermolecular interaction of superiority such as classical hydrogen bond, molecules are aligned as the most stable spatial organization achieved by cooperative accumulation of weak intermolecular interactions.Among the weak interactions, the strongest one can determine the molecular structural motif on condition that the difference in strength among interactions is rather distinct.On the basis of spatial structural data of crystals of title compound and homologue, the molecular structural motif and spatial organization of dimeric aggregates for two compounds are comparatively analysed.
In the crystal of homologous compound, overlapping of … stacked aromatic rings in dimeric aggregation unit is almost satisfactory to gain maximum stabilization.The noncovalent bonding interactions other than … stacking interactions observed are far weaker ones.It means that the other non-classical hydrogen bonding interactions only functions to pile the dimeric aggregates up without affecting the alignment of dimeric aggregate itself.Though the dimeric aggregate of molecules is observed in the crystal of title compound as well as homologue, several distinct differences are recognized.The aroyl group is connected more closely to perpendicular situation against naphthalene ring than homologue.It suggests the … stacking stabilization of title compound is smaller than homologue probably because the only one methylene-length elongation of methoxy groups to ethoxy ones at 2,7-positions of naphthalene ring enlarges the intramolecular steric repulsion resulting in prevention of suitable alignment of aroyl groups for more effective … stacking interactions.In this consequence, … stacking interactions and non-classical hydrogen bonds cooperatively realize the largest stabilization with optimization of number and level of nonclassical hydrogen bondings by perturbation of intramolecular spatial atomic positioning.Contrary to the classical hydrogen bonds governed crystal, the sequence of strength among non-classical hydrogen bonds drastically turns relative precedence, and the single molecular and accumulation structure in crystal demonstrate rather irregular change of apparent feature.
Figure1exhibits single molecular structure of title compound.22The aroyl group is non-coplanarly situated against the naphthalene ring.The interplanar angle between 4-nitrobenzene ring and 2,7-diethoxynaphthalene ring is 81.99(5)˚ [torsion angles = 112.12(15)˚(C2-C1-C15-O3) and 114.48(14)˚ (C9-C1-C15-C16)].The plane of the bridged carbonyl group [C-(C=O)-C] is tilted to both 4nitrobenzene ring and the naphthalene ring [dihedral angles; 26.80(7)˚ and 67.43(6)˚, respectively].On the other hand, the nitro group is coplanarly attached to the 4-nitrobenzene ring [torsion angles = 0.21(19)˚ (O4-N1-C19-C18) and -179.68(13)˚(O5-N1-C19-C18)].Two ethoxy groups at 2,7-positions of the naphthalene ring are oriented in different directions, i.e., the ethoxy group at 7-position of the naphthalene ring turns toward outer side of molecule, and that at 2-position comes closer to inner part of molecule.Contrary to the achiral nature of title molecule in solution, the molecules in crystal exhibit atrope isomerism brought about by molecular stereogenic axis of carbon-carbon bond between the carbonyl moiety and the naphthalene ring.Therefore, a pair of (R)-and (S)-enantiomeric molecules exists in the crystal.The two pairs of the enantiomeric molecules are related by two-fold helical axis in the asymmetric unit of P21/n space group, exhibiting the number of molecules (Z) in a unit cell is four (Figure2).

Figure 2 .
Figure 2. Molecular packing of title compound, with the symmetry elements [pink molecules and pale blue ones indicate (R)enantiomeric isomers and (S)-enantiomeric ones, respectively].

Figure 3 .
Figure 3. Spatial organization of centrosymmetric dimeric molecular aggregate of title compound.

Figure 5 .
In the similar manner to title compound, the homologous compound has single molecular structure of non-coplanarly accumulated aromatic rings.However, dihedral angles between 4-nitrophenyl ring and the naphthalene ring, between the bridging C-(C=O)-C plane and the naphthalene ring, and between the bridging C-(C=O)-C plane and the 4-nitrophenyl ring are smaller than those of title compound [Table 3; 61.97(5)˚ for homologous compound < 81.99(5)˚ for title compound, 56.68(6)˚ < 67.43(6)˚, and 12.54(7)˚ < 26.80(7)˚, respectively].The methoxy groups are oriented in the same manner with title compound, i.e., the methoxy group at 7-position of the naphthalene ring is oriented toward outside of molecule, whereas that at 2-position comes toward inner side of molecule (Figure 5, right).
2-methoxy)C-H…O=C non-classical hydrogen bonds [2.49Å] and (7-methoxy)C-H…(naphthalene) non-classical hydrogen bonds [2.91 Å] (Figure 7, top).The plates are aligned along b-axis forming waving textile through weak (naphthalene)C-H…(naphthalene) non-classical hydrogen bonds between opposite enantiomeric molecules [3.07 Å] (Figure 7, bottom).On the basis of above considerations concerning molecular aggregation structures, the authors next compare the spatial alignment of centrosymmetric dimeric molecular aggregates in order to clarify the molecular structural motifs for title compound and homologue.

Figure
Figure 8a displays the pictures of dimeric molecular aggregates of title compound (top) and homologue (bottom) with the direction that the planes containing the benzene rings of 4-nitrophenyl group are positioned perpendicularly to the paper surface.The interplanar distance of title compound is shorter than that of homologue, however, slippage of two benzene rings of title compound is larger than that of homologue [interplanar distances: 3.489 Å for title compound vs. 3.523 Å for homologue; centroid-

Figure 6 .
Figure 6.Spatial organization of centrosymmetric dimeric molecular aggregate of homologue.

Figure 8 .
Figure 8. Spatial organizations of dimeric molecular motifs: (a) the pictures of dimeric molecular aggregates of title compound (top) and homologue (bottom) with the direction that the planes containing the benzene rings of 4-nitrophenyl group are positioned perpendicularly to the paper surface; (b) the pictures of dimeric aggregates of two compounds with the direction that their benzene rings of 4-nitrophenyl groups are placed parallel to the paper surface.

Figure 9
Figure 9 exhibits the difference in the alignments of the dimeric molecular aggregates between title compound and homologue.Dimeric units of title compound are stacked into columnar structure with minor deviations, whereas those of homologue are arranged with large deviations.Large overlap between dimeric units in title compound indicates that the dimeric units need a number of noncovalent bonding interactions for accumulation.

Figure 9 .
Figure 9. Arrangements of dimeric molecular units: (a) the pictures of dimeric aggregates of title compound (top) and homologue (bottom) with their naphthalene rings parallel to the paper surface.(b) the pictures of dimeric aggregates of two compounds with their naphthalene rings perpendicular to the paper surface.

Table 1 .
Crystallographic data and structure refinement parameters of title compound.

Table 4 .
Non-covalent bonding interactions in title compound and homologue