CRYSTAL STRUCTURE OF 1 , 8-BIS ( 4-FLUOROBENZOYL ) NAPHTHALENE-2 , 7-DIYL DIBENZOATE : ROLE OF ( SP 2 ) C – H ... F HYDROGEN BONDING AS DISTINCTLY STRONG INTERACTION AMONG NON-CLASSICAL HYDROGEN BONDS CONTRIBUTING STABILITY OF THE CRYSTAL

In crystal of 1,8-bis(4-fluorobenzoyl)naphthalene-2,7-diyl dibenzoate, C38H22O6F2, the phenyl rings of benzoyloxy groups and the naphthalene ring demonstrate largely disproportionate interplanar angles [38.97(7)° and 52.62(6)°] different from those between 4fluorobenzoyl group and the naphthalene ring core [71.24(5)° and 78.85(6)°]. One of two benzoyloxy groups has three effective intramolecular interactions [(benzoyloxy)C–H(o-)···O(benzoyloxy), (naphthalene)C–H(6-)···O=C(benzoyloxy), and (benzoyloxy)C–H(o)···π(4-fluorobenzoyl) hydrogen bonds] and the other has no intramolecular interactions. In crystal, the molecules of identical enantiomeric isomer are unidirectionarly arranged along the b axis through (4-fluorobenzoyl)C–H(m-)···O=C(4-fluorobenzoyl) hydrogen bonding interactions forming columnar structure. Moreover, a column is connected with the mirror imaged column composed of the opposite enantiomeric isomers into centrosymmetric dimer aggregates by three types of complementary interactions, i.e., (benzoyloxy)C–H(m-)···F, (4-fluorobenzoyl)C–H(m-)···π(4-fluorobenzoyl), and (4-fluorobenzoyl)C–H(m-)···π(benzoyloxy) hydrogen bondings. The tubular structures thus formed are stacked parallel to the ac plane via (benzoyloxy)C–H(p-)...F, (benzoyloxy)C–H(m-)...(benzoyloxy), (naphthalene)C–H(6)...O=C(4-fluorobenzoyl), and (4-fluorobenzoyl)C–H(o-)...O=C(benzoyloxy) hydrogen bonds. In homologous compound, a fluoro groupfree derivative for title compound, the enantiomeric isomer and the opposite enantiomeric counterpart isomer are alternately arranged in a head-to-tail fashion through (benzoyl)C–H(p-)...O=C(benzoyl) hydrogen bonds along b-axis. The zigzagged columns are aligned along aaxis with inversion center to form a sheet structure. However, there are no effective non-covalent bonding interactions between the zigzagged columns. In other words, the molecular packing structure of the homologous compound is governed by solely one strong (benzoyl)C–H(p-)...O=C(benzoyl) hydrogen bonds, contrary to the organization of supramolecular architecture in title compound ascribed to cooperative unidirectional (4-fluorobenzoyl)C–H(m-)...O=C(4-fluorobenzoyl) hydrogen bonds and bidirectional (benzoyloxy)C–H(m-/p)...F hydrogen bonds.


Introduction
Supramolecular architecture [1][2][3] along with supramolecular chemistry [4][5][6][7][8][9] has become of interest in recent years from the viewpoint of green chemistry and novel phase of functional device material development.Various building blocks bearing unique functions might be tailored to supramolecular structure exhibiting desired chemical and physical properties without formation of covalent bonds.][16] These have been successfully employed for the preparation of numerous molecular assemblies.However, to grasp nature of weak hydrogen bonds including "non-classical" hydrogen bonds where C-H group acts as hydrogen donor, for example, has scarcely achieved probably because they are often hidden by strong hydrogen bonds.
The authors have reported single molecular structure and the structural features of the molecular packing for roughly eighty compounds having 1,8-diaroylated naphthalene skeleton or the homologous/analogous structure via the Cambridge Structure Database (CSD). 17,18 1][22] The molecular packing of 1,8-diaroylated 2,7dialkoxynaphthalene compounds are mainly stabilized by weak hydrogen bonds.Four kinds of noncovalent-bonding interactions, (sp 2 )C-H•••O=C hydrogen bond, (sp 3 )C-H•••O hydrogen bond, C-H•••π hydrogen-bonding interaction, and π•••π stacking interaction are observed in decreasing order of frequency.The features can be interpreted that the noncoplanarly accumulated aromatic rings structure disturbs formation of strong π…π stacking interactions.Therefore, the authors planned to elucidate systematically non-classical hydrogen bonds, weak hydrogen bonds between C-H group and electron rich atom/group, by the aid of structure analysis of 1,8-diaroylated naphthalene compounds.Herein, crystal structure of 1,8-bis(4-fluorobenzoyl)naphthalene-2,7-diyl dibenzoate 23 is demonstrated.
The compound has characteristic molecular structure of four aromatic ring containing groups located at one long edge of naphthalene ring in serial order (1,2,7,8-positions) with other positions of another long edge free of substituent.The crystal structure is discussed from the standpoint of clarification of the correlation among single molecular structure, non-covalent bonding interactions, and molecular packing structure through comparison with the fluoro group-free homologous compound, 1,8-dibenzoylnaphthalene-2,7-diyl dibenzoate. 24

Materials and methods
All reagents were of commercial quality and were used as received.Solvents were dried and purified using standard procedures. 25Synthetic methods and spectral data for the precursor, 2,7-diethoxy-1,8-bis(4-fluorobenzoyl)naphthalene, have been reported in literature. 26asurements 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 Me4Si (δ 0.00). 13C NMR spectra were recorded on a JEOL JNM-AL300 spectrometer (75 MHz) and a JEOL ECX400 spectrometer (100 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).Highresolution 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 fourcircle 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 F2>2σ (F2).
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 27 ; cell refinement: PROCESS-AUTO; 27 data reduction: CrystalStructure, 28 program(s) used to solve structure: SIR2004, 29 program(s) used to refine structure: SHELXL97, 30 molecular graphics: ORTEPIII. 31The hydrogen bond geometries of title compound are listed in Table 2.

Synthesis of 1,8-bis(4-fluorobenzoyl)-2,7-dihydroxynaphthalene
To a 100 mL flask, 1,8-bis(4-fluorobenzoyl)-2,7diethoxynaphthalene (5.0 mmol, 2.3 g) and toluene (40 mL) were placed and stirred at 90 °C.To the reaction mixture thus obtained, AlCl3 (30.0 mmol, 3.9 g) was added.After the reaction mixture was stirred at 90 °C for 1 h, it was poured into 2M aqueous HCl and the mixture was extracted with CHCl3.The combined extracts were washed with brine.The organic layers thus obtained were dried over anhydrous MgSO4.The solvent was removed under reduced pressure to give cake.The crude product was purified by recrystallization from AcOEt (62 % isolated yield).

Results and discussion
Single molecular structure of title compound is illustrated in    hydrogen bonds (2.52 Å)] are concerned with just the benzoyloxy group that makes three intramolecular hydrogen bonds described above and the remaining hydrogen bond is (naphthalene)C-H(6-)…O=C(4fluorobenzoyl) hydrogen bonds (2.46 Å) located between the 4-fluorobenzoyl group, which also acts a role of intramolecular hydrogen bond acceptors, and the naphthalene ring (Figure 4).
The authors' group has reported crystal structure of homologous compound, 1,8-dibenzoylnaphthalene-2,7-diyl dibenzoate. 24The molecular structure of the homologous compound corresponds to fluoro group-free derivative for the title compound.Single molecular structure of the homologue is shown in Figure 5. Four benzene rings are situated out of the plane of the naphthalene ring core as well as title compound.However, correlation between kinds of benzene rings and the symmetric nature is inverse to title compound, i.e., the two benzene rings of the benzoyloxy groups at 2,7-positions of the naphthalene ring are tilted symmetrically, whereas the two benzene rings of benzoyl groups at 1,8-positions of the naphthalene ring lean unsymmetrically.The interplanar angles between the benzene rings of benzoyl groups and the naphthalene ring system are 67.12(5)° and 85.15 (5)°, respectively.The two interplanar angles between the best planes of the benzoyloxy groups and the naphthalene ring are 71.47 (5)° and 76.41 (5)°, respectively.The interplanar angle between the two benzene rings of the benzoyl groups is 59.81 (6)°.Furthermore, the homologous molecule has no intramolecular hydrogen bondings differently from the title compound.
In the crystal packing, (benzoyl)C-H(p-)…O=C(benzoyl) hydrogen bonds (2.41 Å) and two types of (benzoyloxy)C-H(m-/o-)…(naphthalene) hydrogen bonds (2.65 Å and 2.96 Å) link the homologous molecules in faceto-back fashion forming a zigzagged column structure along b-axis (Figure 6).The zigzagged columns are aligned with inversion center into a sheet structure along a-axis.However, there are no effective non-covalent bonding interactions between the zigzagged columns.(benzoyl)C-H(p-)…(benzoyl) Hydrogen bonds are also observed along c-axis (2.94 Å) (Figure 7).In the crystal packings of title compound and the homologous compound, the molecules exhibit axial chirality, with either R,R-or S,S-stereogenic axis.   3 summarizes correlation between absolute configuration of molecule and non-covalent bonding hydrogen bonds in title compound and the homologous compound.As a common feature about non-covalent bonding interactions, both of title compound and the homologous one have (aroyl)C-H(por m-)…O=C(aroyl) hydrogen bonds.However, they are observed between R,Rand S,S-enantiomeric isomers in the molecules of the homologous compound (2.41 Å), whereas those interactions are found between identically enantiomeric isomers in title compound (2.22 Å).In the homologous compound, two kinds of (benzoyloxy)C-H(m-/o-)… hydrogen bonds are also formed effectively between opposite enantiomeric isomers (2.65 Å and 2.96 Å).
However, the role of (aroyl)C-H(p-)…O=C(aroyl) hydrogen bonds in the homologous compound for connection of identical enantiomers is severely hidden.Red characters = non-covalent bonding distances between same enantiomeric molecules; blue characters = non-covalent bonding distances between opposite enantiomeric molecules.Generally, C2 symmetrical molecules in crystal are considered to be best stabilized by formation of pairs of identical effective intermolecular interactions on condition that satisfactory room is present for conformation perturbation free from specific regulations.To realize such crystalline structure, the molecules in crystal should take either complimentary face-to-face interactions or unidirectional stacking by a face-to-back mode.Presumably, the mode of face-to-face or face-to-back is determined according to the stabilization energy afforded in respective aggregation modes.As the interactions that perform the largest stabilization energy are considered to be aligned with a higher symmetric feature, the nearly identical spatial structure should be constructed as face-to-face mode for dimer formation or face-to-back mode for unidirectional alignment.The secondly or less effective interactions should be arranged to obtain totally the largest stabilization energy.
In title compound, the strong (benzoyloxy)C-H(m-/p-)…F non-classical hydrogen-bonding interaction between opposite enantiomers is considered to predominate possible interactions between identical enantiomers.This brings about the restriction of the formation of comparatively effective non-covalent bonding interactions at the backside position of the molecules.For the compensation of rather poor effect of intermolecular interaction stabilization, the larger intramolecular stabilization is achieved along with reinforcement of the disproportionation of molecular spatial organization.Furthermore, the stabilization should be best achieved when strongest interactions functions complementally, i.e., these interactions require a highly symmetric spatial arrangement.
To consistent rather contracted molecular structural requisites, one pair of the aroyl groups showing stronger intermolecular interaction take the positions in a highly symmetrical manner and the other pairs of aroyl groups arrange themselves dissymmetrically to attain the largest intramolecular stabilization.
In this consequence, the characteristic disproportionation of spatial organization bearing highly symmetric alignment part for such molecule of symmetric molecular structure is generated.Under such characteristic structural regulation, the spatial organization of molecules should be performed to maximize the total stabilization by various weak interaction.
On the other hand, absence of superior intermolecular interaction in the crystal of the homologous compound presumably performs the largest stabilization when largest number of moderate and weak non-covalent bonding interactions among neighbouring molecules contributes effectively.
To realize such spatial circumstance, disproportion of single molecular spatial alignment should affect unfavourably.Then, the accumulation of molecules with rather isotropic spatial alignment is advantageous.
On the basis of above interpretation, presence or absence of (benzoyloxy)C-H(m-/p-)…F non-classical hydrogen bond is strongly suggested to determine both the molecular proportional properties and stabilization fashion between enantiomeric isomers or between identically configurated molecules of apparently C2 symmetrical molecules in crystal.

Figure 5 .
Figure 5. Single molecular structure of homologous compound.

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