The crystal structure of 1,8-bis(4-methoxybenzoyl)naphthalene-2,7-diyl dibenzoate, C₄₀H₂₈O₈

Source of material

The title compound was prepared by reaction of 1,8-bis(4-methoxybenzoyl)-2,7-dihydroxynaphthalene (0.2 mmol, 85.7 mg), which was obtained via P₂O₅–MsOH [7] -mediated diaroylation of 2,7-dihydroxynaphthalene and 4-methoxybenzoic acid, benzoyl chloride (0.44 mmol, 61.9 mg), and triethylamine (0.22 mmol, 0.03 ml) in dichloromethane (0.5 ml). After the reaction mixture was stirred at room temperature for 3 h, it was poured into water (30 ml) and the mixture was extracted with CHCl₃ (10 ml × 3). The combined extracts were washed with sat. NaHCO₃ aq. and brine. The organic layers thus obtained were dried over anhydrous MgSO₄. The solvent was removed under reduced pressure to give the crude product (94%). ¹H NMR δ (400 MHz, CDCl₃): 3.73 (6H, s), 6.72 (4H, broad), 7.25 (4H, t, J = 6.4 Hz), 7.46 (2H, t, J = 6.0 Hz), 7.56 (2H, d, J = 7.6 Hz), 7.63 (4H, d, J = 6.0 Hz), 7.69 (4H, broad), 8.10 (2H, d, J = 7.6 Hz) ppm ¹³C NMR δ (75 MHz, CDCl₃): 55.45, 113.52 (overlap), 122.11, 128.31, 128.48, 129.68, 129.99, 130.82, 131.38, 131.54, 133.61, 147.54, 163.64, 164.18, 193.36 ppm. IR (KBr): 3450 (C–H), 2838 (O–CH₃), 1735 (OC=O), 1653 (C=O), 1599 (Ar), 1507 (Ar) cm⁻¹. HRMS (m/z): [M+H]⁺ calcd. for C₄₀H₂₉O₈, 637.1863, found, 637.1910. m.p. = 184.6–185.0 °C.

Experimental details

All H atoms were found in a difference map and were subsequently refined as riding atoms, with C―H = 0.95 (aromatic) and 0.98 (methyl) Å, and with U _iso(H) = 1.2U _eq(C).

Comment

Weak interactions in solid organic molecular substances have acquired growing interests and the consequent revaluation over the last two decades [8], [9], [10]. Since the degree of freedom in arranging aromatic rings is small, crystals of compounds of non-coplanarly accumulated aromatic rings can be expected to distinguish contributions of rather weak non-covalent bonding interactions other than π⃛π stacking interactions or classical hydrogen bonds as structure determining factors [11], [12], [13].

In the molecular structure of the title compound, the benzene rings of 1,8-(4-methoxybenzoyl) groups and those of 2,7-benzoyloxy groups are twisted away from the naphthalene ring (Figure). Two 1,8-(4-methoxybenzoyl) groups are situated in anti-orientation. The dihedral angles between the benzene rings of 4-methoxybenzoyl groups and the naphthalene ring system are 67.41(7)° [C20–C25 ring, C7–C8–C19–O3 torsion angle = –116.81(15)°] and 73.77 (6)° [C12–C17 ring C2–C1–C11–O1 torsion angle = –119.46(15)°], respectively. The dihedral angle between the best planes of the two phenyl rings is 63.18(8)°. The two benzoyloxy groups at 2,7-positions of the naphthalene moiety are also situated in opposite directions. The dihedral angles between their benzene rings [C35–C40 ring and C28–C33 ring] and the naphthalene ring system are 72.13(9)° and 53.08(7)°, respectively. The two benzene rings make a dihedral angle of 77.55(10)°. The phenyl rings and carbonyloxy moieties are almost coplanar [O8–C34–C35–C36 torsion angle = −171.15(19)° and O6–C27–C28–C29 torsion angle = 167.41(16)°]. Several non-classical hydrogen bonds exhibiting almost similar interatomic distances are also confirmed: C–H(methoxy)⃛O=C(benzoyl) (2.57 Å, H18B⃛O1; 1/2−x, −1/2+y, 1/2+z), C–H(naphthalene)⃛O=C(benzoyl) (2.58 Å, H5⃛O3; 1/2−x, −1/2+y, 1/2+z), C–H(benzoyloxy)⃛OMe (2.60 Å, H39⃛O2; 1/2+x, 1/2−y, −1+z), and and C–H(benzoyloxy)⋯O(benzoyloxy) (2.60 Å, H31⃛O6; x, y, 1+z). The first three non-classical hydrogen bonds are parallel to the glide planes, and the last one is oriented along the c axis. The authors reported the crystal structure of the homologous compounds [2,7-dimethoxy-8-(4-methoxybenzoyl)-naphthalen-1-yl](4-methoxyphenyl)methanone chloroform monosolvate (I) [14] and 1,8-dibenzoylnaphthalene-2,7-diyl dibenzoate (II) [15]. The title compound and the homologue II shows unsymmetric spatial situation of 1,8-diaroyl groups on naphthalene ring [dihedral angles between phenyl rings and naphthalene: 67.41(7)° and 73.77(6)° for the title compound, 67.12(5)° and 85.15(5)° for homologue II, and 72.51(7)° and 73.33(7)° for homologue I]. Two benzoyloxy groups in homologue II are twisted to the naphthalene ring with almost same dihedral angles [71.47(5)° and 76.41(5)° vs. 72.13(9)° and 53.08(7)° for the title compound]. In these compounds, fewer non-classical hydrogen bonds than in the packing of the title compound are observed, e.g. C–H(naphthalene)⃛OMe for homologue I, C–H(benzoyl)⃛O=C(benzoyl) and C–H(benzoyloxy)⃛π(naphthalene) for homologue II. More effective factors stabilize their crystals such as co-crystallization with solvent molecules or π⃛π stacking interactions. In other words, continuous neighbouring substitution of 4-methoxybenzoyl groups and benzoyloxy group disturb the effective interactions other than non-classical hydrogen bonds in title compound. Concludingly, neighbourly aligned four benzene rings on naphthalene ring afford enough stabilization with the aid of maximum number of non-classical hydrogen bonds in the absence of π⃛π stacking. The magnitude of the stabilization satisfactorily compensates those expected by intermolecular interaction of crystal solvent.