Crystal structure of 1,2-bis(4-fluorophenyl)-1-hydroxy-2,3,8-trimethoxyacenaphthene: formation of a five-membered intramolecular O—H⋯O hydrogen-bonded ring

In the crystal of the title compound, the formation of an intramolecular O–H⋯O hydrogen bond between the hydroxy group and the methoxy group at the 1,2-positions of the acenaphthene ring core giving rise to a five-membered cyclic organization is observed. In the molecular packing, a pair of non-classical C—H⋯O hydrogen bonds forms centrosymmetric dimeric structures.


Chemical context
The chemistry of congested aromatic-ring-accumulation compounds has attracted continuous interest, especially in non-classical non-covalent bonding interactions other than classical hydrogen bonds. Steric factors of these compounds influenced by the presence of exocyclic bonds presumably bring about in-plane and/or out-of-plane deviations from the ordinary geometry of aromatic molecules. Consequently, the molecules undergo geometrical changes to release the strain in the molecular skeleton, which, in turn, modulates the -electron delocalization. These space-structural characteristics result in an alteration of the reactivity and properties of the near-by moiety of the molecule (Tannaci et al., 2007;Pascal, 2006;Downing et al., 1994;Biedermann et al., 2001). From the point of view of such structural properties, the authors have been investigating peri-substituted naphthalene and 1,2-disubstituted acenaphthene compounds, focusing on the molecular structure and packing of the above compounds and their analogues and homologues along with the reaction behaviour, including the formation reaction and the design of novel categories of highly performing and functional organic and polymer materials (Okamoto & Yonezawa, 2015).
The authors have found that peri-aroylnaphthalene compounds are selectively yielded via electrophilic aromatic diaroylation of a naphthalene derivative in the presence of a suitable acidic mediator (Okamoto & Yonezawa, 2009;Okamoto et al., 2011). In peri-aroylnaphthalene compounds, probably caused by steric hindrance, the aroyl groups tend to ISSN 2056-9890 be arranged nearly perpendicular relative to the core naphthalene plane. Bearing this in mind, the authors have continued their crystallographic study of homologous and analogous peri-aroylnaphthalene compounds for elucidation of the correlation between molecular structure, crystal packing and non-covalent bonding interactions. As one of the readily performable reactions of peri-aroylnaphthalene compounds, a Zn-mediated reductive coupling to 1,2-diaryl-1,2-acenaphthenediol has been discovered (Mido et al., 2017(Mido et al., , 2020. Herein, the crystal structure of 1,2-bis(4-fluorophenyl)-1-hydroxy-2,3,8-trimethoxyacenaphthene (I), a monoalkoxylated derivative of a pinacol-coupling product, is reported and its structural features are discussed through comparison with homologues.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. This compound consists of an acenaphthene ring system core with a hydroxy group and a 4-fluorophenyl group at the 1-position, a methoxy group and a 4-fluorophenyl group at the 2-position, and two methoxy groups at the 3-and 8-positions. In the title compound, the two 4-fluorophenyl groups at the 1,2-positions are located on the same side of the acenaphthene ring system plane (i.e. cis), and the 1-hydroxy and 2-methoxy groups are positioned on the other side. Moreover, the hydrogen atom of the hydroxy group at the 1-position is located in between the two oxygen atoms (O3 and O4) of the hydroxy and the methoxy groups at the 1,2-positions, with the methyl group oriented away. A puckering analysis (Cremer & Pople, 1975;Luger & Bü low, 1983) suggests that the fivemembered ring of the acenaphthene core has a half-chair conformation. The positions of the ring substituents can be described as bisectional for the hydroxy group at the 1-position, axial for the 4-fluorophenyl group at the 1-position, axial for the methoxy group at the 2-position, and bisectional for the 4-fluorophenyl group at the 2-position (Fig. 2). The two benzene rings of the 4-fluorophenyl groups are twisted out of the naphthalene plane (C1-C10) of the acenaphthene ring system. The C19-C24 benzene ring at the 1-position is more heavily tilted compared to the C12-C17 benzene ring at the 2-position, as indicated by the dihedral angles of the best planes through the benzene rings and naphthalene ring system, which are 87.02 (7) and 51.86 (8) , respectively. The dihedral angle between the two benzene rings is 43.47 (9) . Furthermore, the C12-C11-C18-C19 torsion angle [31.37 (15) ] indicates a large slippage between the two benzene rings. In addition, the five-membered ring C1,C9,C8,C18,C11 and the naphthalene ring system (C1-C10) are not coplanar, the dihedral angle between their best planes being 7.03 (7) .

Figure 1
The molecular structure of the title compound, with atom labelling and intramolecular O-HÁ Á ÁO and C-HÁ Á Á contacts (dashed lines). Displacement ellipsoids are drawn at the 50% probability level.
between hydroxy group O4-H4 and oxygen atom O3 of the methoxy group at the 1-and the 2-positions of the acenaphthene ring system (see Table 1). An intramolecular C-HÁ Á Á interaction between hydrogen atom H24 of one of the 4-fluorophenyl groups and the acenaphthene ring system is also observed (C24-H24Á Á ÁCg9 = 2.90 Å ; Cg9 is the centroid of the acenaphthene ring; see also Table 1). The possibility of intramolecular classical O-HÁ Á ÁO hydrogen bond formation as part of a five-membered cyclic arrangement in 1,2-acenaphthenediol was proposed several decades ago by infrared spectroscopy (Moriconi et al., 1959;Hayward & Csizmadia, 1963). Trotter and Mak have designed and synthesized cis-1,2-acenaphthenediol . However, in the crystal structure of the pinacol compound, no effective intramolecular interactions were observed. Instead, intermolecular O-HÁ Á ÁO interactions align the molecules sequentially to form a chain-like structure in the crystal packing and the formation of intermolecular O-HÁ Á ÁO hydrogen bonds took precedence over an intramolecular classical O-HÁ Á ÁO hydrogen bond. In contrast, in the title compound, the hydroxy and methoxy groups are presumably forced to form an intramolecular hydrogen bond, i.e., the spatial arrangement of the two benzene rings -probably restricted by the methoxy groups at the 3,8-positions -inhibits the approach of other molecules.

Supramolecular features
In the molecular packing, a pair of non-classical C-HÁ Á ÁO hydrogen bonds between hydrogen H20 of a 4-fluorophenyl group (2-positioned) and oxygen O4 of the hydroxy group at the 1-position of the acenaphthene unit connects two molecules of the title compound, forming a centrosymmetric dimer [C20-H20Á Á ÁO4 i , 2.49 Å ; symmetry code: (i) Àx, 1 À y, 1 À z] (Table 1
The asymmetric molecular structure, with only one of the hydroxy groups methoxylated, disrupts the spatial alignment observed when both hydroxy groups interact with adjacent molecules, forming chain structures . Instead of the stabilization energy obtained by forming a chain structure, the title molecules afford the centrosymmetric dimer as the packing motif. The intramolecular classical O-HÁ Á ÁOMe hydrogen bond is required to adjust the spatial arrangement for forming centrosymmetric dimers. The nonclassical hydrogen-bonding interactions connecting the dimeric aggregates contribute to further stabilize the molecular packing. The title compound has a cis-configuration. For 1,2acenaphthenediol and its derivatives, cis-and trans-configurations are found for 1,2-acenaphthenediol and its dinitrates (ACNAOL: Trotter et al., 1963;ZZZPKU and ZZZIWC: Mak et al., 1963;ANADON: Mak et al., 1964).