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Crystal Structure Study on Non-Coplanarly Organized Accumulating Aromatic Rings Molecules: Spatial Organization of C,C,N-Triaryl Substituted Imines

The X-ray crystal structures of C,C,N-triaryl-substituted imine compounds, which have methoxy or hydroxy group adjacent to the imino moiety, are reported and discussed in comparison with those of the precursor ketone compounds, 1-(4-chlorobenzoyl)-2,7-dimethoxynaphthalene and 1-(4-chlorobenzoyl)-2-hydroxy-7-methoxynaphthalene. In crystals, three aromatic rings in a molecule of the methyl ether-retained imine compound are positioned almost perpendicularly to each other by giving non-coplanar spatial organization of the single molecular structure [dihedral angles: 85.32(18) ̊ for C-linked phenyl ring and naphthalene ring; 79.27(17) ̊ for N-linked phenyl ring and naphthalene ring; 84.78(17) ̊ for C-linked phenyl ring and N-linked phenyl ring]. Spatial organization of the analogous methyl ether-cleaved imine compound has essentially same topology [dihedral angles 80.39(6) ̊ for the C-linked phenyl ring and naphthalene ring; 82.35(6) ̊ for the N-linked phenyl ring and naphthalene ring; 87.09(7) ̊ for Cand N-linked phenyl rings]. These structural features of triarylimines apparently differ from those of the precursor ketones. Two aromatic rings in the methyl ether-cleaved ketone compound make smaller dihedral angle [58.10(6) ̊] by intramolecular hydrogen bond between ketonic carbonyl group and hydroxy group [2.5573(16) Å] than that of the methyl ether-retained ketone [72.06(7) ̊]. In molecular packing, the methyl ether-retained imine forms tubular molecular alignments composed of R—S dimeric molecular pairs, whereas the methyl ether-retained ketone affords consecutively stacks of one configurated molecules.


Introduction
Non-coplanarly accumulated aromatic-rings compounds, e.g., biphenyls and binaphthyls, have been demonstrated as unique building blocks in construction for many functional materials such as molecular catalysis and functional polymers [1][2][3][4][5][6][7][8][9][10][11][12]. Thus, minute spatial structural characterization of these compounds [13][14][15][16] has attracted attention of the chemists in the wide-range of organic molecular science and polymer materials fields. However, intra-and inter-molecular interactions that afford various functions to such molecular units still remain ambiguous. As one of the protocols to estimate such interactions, the authors have been investigating synthesis and X-ray crystal structure analysis of congested spatial organization of aromatic rings accumulating molecules.
Recently, the authors have reported specific and characteristic electrophilic aromatic aroylation of naphthalene derivatives, i.e., two aroyl groups are regioselectively and effectively introduced at the 1,8-positions of the naphthalene ring accompanying with simultaneously proceeding retroaroylation behavior [17,18]. The 1-aroylated naphthalenes, which correspond to the intermediates in the diaroylation, are also obtained by choice of acidic mediator.
X-ray crystal structure study has revealed that the aroyl groups in these peri-aroylated naphthalene molecules are non-coplanarly attached to the naphthalene rings by giving crowded molecular organization [19][20][21][22]. In a natural consequence, the authors have planned to introduce additional aromatic ring planes to the core part of the aroylnaphthalene molecules for realization of more crowded inner spatial situation in accumulated aro-matic-rings molecule. As one of the molecular transfor-mation approaches to obtain such spatial organization, the authors designed conversion of ketonic carbonyl group on 1-aroylnaphthalene to imino moiety by the re-action with aniline derivative. Imination of 1-aroylated 2,7-dimethoxynaphthalene with aromatic amines scarcely proceeded with conventional additives except for TiCl 4 and 1,4-diazabicyclo[2.2.2]octane (DABCO) mixture. In TiCl 4 -DABCO mediated imination, triaryl-substituted imine compounds were formed in moderate conversion with/without preceding methyl ether cleavage reaction of the starting compound (Scheme 1) [23]. The neighboring ketonic carbonyl group of peri-aroylated 2,7-dimethoxynaphthalene derivatives plausibly accelerates TiCl 4mediated scission of rather stable ether bonding.
In this article, the authors report and discuss the single molecular spatial organizations and the molecular packing characteristics of C,C,N-triarylated imine compounds by comparing with those of original ketone compounds: 1-aroyl-2,7-dimethoxynaphthalene and 1-aroyl-2-hydroxy-7-methoxynaphthalene.

Experimental
All reagents were of commercial quality and were used as received. Solvents were dried and purified using standard techniques.

1
H NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (300 MHz) and a JEOL ECX400 spectrometer (400 MHz). Chemical shifts are expressed in ppm relative to internal standard of Me 4 Si (δ 0.00). 13 C NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (75 MHz). Chemical shifts are expressed in ppm relative to internal standard of CDCl 3 (δ 77.0). IR spectra were recorded on a JASCO FT/IR-4100 spectrometer. Elemental analyses were performed on a Yanaco CHN CORDER MT-5 analyzer. High-resolution FAB mass spectra were recorded on a JEOL MStation (MS700) ion trap mass spectrometer in positive ion mode.

Synthetic Procedure
Starting material 1 and triarylimines were prepared as follows.

Electrophilic Aromatic Substitution Aroylation of 2,7-Dimethoxynaphthalene by AlCl 3
To a solution of 2,7-dimethoxynaphthalene (0.200 mmol, 68.2 mg) and 4-chlorobenzoyl chloride (0.220 mmol, 38.5 mg) in dichloromethane (0.5 mL), AlCl 3 (0.220 mmol, 29.3 mg) was added by portions at 0˚C under nitrogen atmosphere. After the reaction mixture was stirred at r. t. for 3 h, it was poured into iced water (20 mL) and the mixture was extracted with CHCl 3 (15 mL × 3). The combined extracts were washed with 2 M NaOH aq., sat. NaCl aq. and dried over anhydrous magnesium sulfate. The solvent was removed under reduced pressure to give powdery product. The crude product of 1-momoaroylnaphthalene 1 was purified by recrystallization (hexane, isolated yield 78%).   and monochlorobenzene (1 mL) were added by portions at 90˚C under nitrogen atmosphere. After the reaction mixture was stirred at 125˚C for 1.5 h, the resulting solution was filtrated to remove the precipitate. The solvent was removed under reduced pressure to give crude material. The crude product was purified by silicagel column chromatography (Chloroform; isolated yield: imine 3, 10%; imine 4, 10%, 2-hydroxy compound 5, 8%).

X-Ray Crystallography
For the crystal structure determination, the single-crystal of the title compounds were used for data collection on a four-circle Rigaku R-AXIS RAPID diffractometer (equipped with a two-dimensional area IP detector). The graphite-mono-chromated Cu Kα radiation (λ = 1.54187 Å) was used for data collection. The lattice parameters were determined by the least-squares methods on the basis of all reflections with F2 > 2σ(F2). The data collection and cell refinement were performed using PROC-ESS-AUTO software [24]. The data reduction was performed using CrystalStructure [25]. The structures were solved by direct methods using SIR2004 [26] and refined by a full-matrix least-squares procedure using the program SHELXL97 [27]. All H atoms were found in a difference map and were subsequently refined as riding atoms, with the aromatic C-H = 0.95 Å and methyl C-H = 0.98 Å, and with U iso (H) = 1.2 U eq (C). Molecular structures of the title compounds showing the atomic numbering schemes are shown in Figures 1 and 2. The crystallography details for the structure determination of the compounds are displayed in Tables 1 and 2.
The aroyl group of 1-aroylnaphthalene 1 is non-coplanarly attached to the naphthalene ring. The dihedral angle between the benzene ring and the naphthalene ring is 72.06 (7)˚. On the other hand, 1-aroylated 2-hydroxy-7-methoxynaphthalene (5) has an intramolecular O-H···O=C hydrogen bond between the carbonyl group and the hydroxy substituent on the naphthalene ring system [O2-H2···O1; 2.5573(16) Å]. The angle between the C=O bond plane and the naphthalene ring system is relatively small [20.96(8)˚]. Naturally, the angle between the benzene ring and the carbonyl group is rather large [35.65(9)˚] compared to that in the original ketone compound [1, 3.43(11)˚], which has 2-methoxy group instead of 2-hydroxy substituent. Consequence of this, two aromatic rings in the methyl ether-cleaved ketone 5 make smaller dihedral angle [58.10(6)˚] than the precursory ketone compound [1, 72.06(7)˚] in crystal.

Figure 2. Molecular structures of analogous imine 6 with the atom-labeling scheme and displacement ellipsoids drawn at the 50% probability level.
About methyl ether-retained imine 3, each of the aromatic rings is connected almost perpendicularly against both of other aromatic rings. The dihedral angles between two of the aromatic rings are close to 90˚ [85.32 (18)˚ for C-linked phenyl ring and naphthalene ring; 79.27 (17)˚ for N-linked phenyl ring and naphthalene ring; 84.78(17)˚ for C-linked phenyl ring and N-linked phenyl ring, in imine 3] compared to that of precursor 1-aroylnaphthalene 1. These angle values indicate that three aromatic rings in methyl ether-retained imine 3 are situated to avoid steric hindrance to each other. However, the spatial organization of triarylimine 3 based on perpendicular arrangement of aromatic rings is essentially same to that of precursor ketone 1. Figure 2 shows the single molecular structures of methyl ether-cleaved 1-aroylnaphthalene (5) originated triarylimine (6) [30] in crystal. Though preparation of satisfactorily qualified crystal for X-ray crystal analysis of N-(4-methoxyphenoxy)imine compound 4 was unsuccessful, the crystal structure of analogous imine compound (6) was determined. In the crystal of analogous methyl ether-cleaved imine 6, two molecules of imine 6 form a 2:1 set with a DABCO molecule. However, the spatial organization of the aromatic rings in methyl ether-cleaved imine 6 has essentially same topology to methyl ether-retained imine 3. The dihedral angles of the C-linked 4-chlorophenyl ring and the N-linked phenyl ring with the naphthalene ring are 80.39(6)˚ and 82.35 (6)˚, respectively. The dihedral angle between C-and N-linked benzene rings is 87.09(7)˚. The structural similarities between imines 3 and 6 strongly suggest that the single molecular spatial organization of three aromatic rings in perpendicular fashion is satisfactorily stable, regardless of whether triarylimine has a methoxy group at the 2-position of the naphthalene or a hydroxy one. Although both ketone 5 and imine 6 have 2-hydroxynaphthalene unit, imine 6 has molecular organization of perpendicular-based aromatic rings arrangement, which is clearly distinguishable against rather planar structure of ketone 5.
The molecular packing of the methyl ether-retained imine compound 3 is compared with those of methyl ether-retained ketone 1 and methyl ether-cleaved ketone 5. In molecular packing, 1-aroylnaphthalene 1 is mainly stabilized by van der Waals interactions. The molecules of methyl ether-retained ketone 1 are aligned consecutively in stacks along the a axis (Figure 3). Adjacent 4-chlorophenyl groups are exactly parallel, and the perpendicular distance between these planes is 3.660 (1) Å (Figure 4). Figure 5 shows the herringbone packing of    (Figure 7).

Open Access CSTA
A. OKAMOTO ET AL.
Open Access CSTA 145 methyl ether-retained imine 3 are apparently directed by various kinds of effective interactions compared to the precursor ketone 1. Especially, C-H··· and C-H···N interactions affording molecular pairs play a key role to govern the whole molecular packing. These interactions presumably maintain the spatial organization of an R-S pair of imine 3 molecules with minimized inner steric repulsions.
Methyl ether-retained ketone 1 has enough flexible molecular skeleton to perturb the spatial organization so that the suitable stabilized molecular stack is achieved leading the optimal molecular packing. On the other hand, the rigid conformation of methyl ether-retained imine 3 molecule should have little space for perturbation of configuration. As a result, predominant two interactions function within the same pair of imine 3 instead of sequential interactions resulting in formation of dimeric pairs. Although the semi-rigid conformation of the methyl ether-cleaved ketone 5 is similar to methyl etherretained imine 3, loose van der Waals interactions might restrict roughly the perturbation of configuration.

Conclusion
Conclusively, the single molecular organization of the two types of C,C,N-triarylimine compounds with 2-methoxy or 2-hydroxy group in crystal is displayed topologically same. The three aromatic rings are situated almost perpendicularly to each other, regardless whether triarylimine has a methoxy group at the 2-position of the naphthalene or a hydroxy one. On the other hand, the crystal structure of methyl ether-retained ketone clearly differs from the methyl ether-cleaved counterpart. There-  Symmetry code: (i) x + 1, y, z, (ii) 1 -x, 2 -y, 1 -z, (iii) -1/2 + x, 1.5 -y, -1 + z, (iv) -x, 2 -y, 1 -z, (v) -x, 2 -y, -z, (vi) -x + 1, -y, -z+1, (vii) x + 1/2, -y + 1/2, z + 1/2. fore, triarylimine compounds have proved enough stable by the aid of the adapting of steric hindrance releasing molecular organization, where three aromatic rings situate perpendicularly to each other. The molecular packings of 1-aroyl-2,7-dimethoxynaphthalene and methyl etherretained triarylimine clearly differ to each other, i.e., the piles composed of one configurated molecules for the ketone and the tubular molecular alignments composed of R-S dimeric molecules for the imine. The difference is interpreted on the basis of flexibility of the molecular skeletons governing the number and the strength of effective intermolecular interactions.

Acknowledgements
This work was partially supported by the Iron and Steel Institute of Japan (ISIJ) Research Promotion Grant, Tokyo, Japan.