Crystal structure of 8-hydroxyquinoline: a new monoclinic polymorph

In an attempt to grow 8-hydroxyquinoline–acetaminophen co-crystals from equimolar amounts of conformers in a chloroform–ethanol solvent mixture at room temperature, the title compound, C9H7NO, was obtained. The molecule is planar, with the hydroxy H atom forming an intramolecular O—H⋯N hydrogen bond. In the crystal, molecules form centrosymmetric dimers via two O—H⋯N hydrogen bonds. Thus, the hydroxy H atoms are involved in bifurcated O—H⋯N hydrogen bonds, leading to the formation of a central planar four-membered N2H2 ring. The dimers are bound by intermolecular π–π stacking [the shortest C⋯C distance is 3.2997 (17) Å] and C—H⋯π interactions into a three-dimensional framework. The crystal grown represents a new monoclinic polymorph in the space group P21/n. The molecular structure of the present monoclinic polymorph is very similar to that of the orthorhombic polymorph (space group Fdd2) studied previously [Roychowdhury et al. (1978 ▶). Acta Cryst. B34, 1047–1048; Banerjee & Saha (1986 ▶). Acta Cryst. C42, 1408–1411]. The structures of the two polymorphs are distinguished by the different geometries of the hydrogen-bonded dimers, which in the crystal of the orthorhombic polymorph possess twofold axis symmetry, with the central N2H2 ring adopting a butterfly conformation.

Sciences in the framework of the program 'Theoretical and experimental study of chemical bonding and mechanisms of chemical reactions and processes' is gratefully acknowledged.
Supporting information for this paper is available from the IUCr electronic archives (Reference: RK2430).

S1. Comment
Cocrystallization represents a form of supramolecular synthesis where molecules are linked by non-valent intermolecular interactions without making or breaking covalent bonds (Aakeröy et al., 2009;Lemmerer et al., 2011). Cocrystals are distinctly different from solid solutions or mixed crystals, and can be considered as molecular complexes (Desiraju, 2003;Dunitz, 2003). The ability of organic compounds to form cocrystals is dependent on a range of variables, including the types of co-formers, co-former ratios, solvents, temperature, pressure, crystallization technique etc. A systematic exploration of a combination of relevant variables increases the chance of discovering cocrystals with favourable properties.
Unexpectedly, a new polymorph of 8-hydroxyquinoline, C 9 H 7 NO (I), was isolated, and its crystal structure was studied by X-ray diffraction analysis. However, no polymorphs of 8-hydroxyquinoline were found in Cambridge strustural database. The result presented here can be considered as a new example of so called "induced polymorphism" (Bernstein, 2002;Timofeeva et al., 2003).
The molecule of I is planar, with the hydroxyl-H atom forming the intramolecular O-H···N hydrogen bond ( Figure 1, Table 1). The crystal grown represents the new monoclinic polymorph in space group P2 1 /n. The molecular structure of the monoclinic polymorph of I is very close to that of the orthorhombic polymorph in space group Fdd2 studied previously (Roychowdhury et al., 1978;Banerjee & Saha, 1986). were dissolved in a 1:1 chloroform-ethanol solvent mixture (3 mL). The single crystals of I were obtained by slow evaporation of the solvents at room temperature.

S3. Refinement
The hydrogen atom of the hydroxy group was localized in the difference-Fourier maps and refined isotropically with fixed displacement parameters (U iso (H) = 1.5U eq (O)). The other hydrogen atoms were placed in calculated positions with C-H = 0.95 Å and refined within the riding model with fixed isotropic displacement parameters U iso (H) = 1.2U eq (C).   The H-bonded dimers in the orthorhombic polymorph of I, in which the molecules are related by the twofold axis. The hydrogen bonds are drawn by dashed lines.

Figure 4
A portion of crystal packing of the H-bonded dimers in the monoclinic polymorph of I. The hydrogen bonds are drawn by dashed lines.

8-Hydroxyquinoline
Crystal data Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.