Crystal structure of (RS)-(4-chlorophenyl)(pyridin-2-yl)methanol

A combination of O—H⋯N hydrogen bonds and C—Cl⋯π(pyridyl) interactions links the molecules of the title compounds into (100) sheets.


Chemical context
Simply substituted diphenylmethanols, RPh 2 COH, exhibit a very rich diversity of supramolecular arrangements, including isolated molecules, hydrogen-bonded dimers, trimers, tetramers and hexamers, as well as continuous hydrogen-bonded chains (Ferguson et al., 1992(Ferguson et al., , 1994(Ferguson et al., , 1995. The predominant mode of molecular association in these structures involves O-HÁ Á ÁO hydrogen bonds, although O-HÁ Á Á(arene) interactions are sometimes present. It is therefore of considerable interest to investigate the influence of an addition potential acceptor of hydrogen bonds as achieved, for example, by the replacement of one of the phenyl rings by an isosteric pyridyl substituent. Here we report the molecular and supramolecular structure of (RS)-4-chlorophenyl(pyridin-2yl)methanol (I) (Fig. 1), which shows some striking structural differences from the simpler, non-chlorinated analogue phenyl(pyridin-2-yl)methanol, whose structure has been reported recently (Kim & Kang, 2014;Tsang et al., 2015).

Structural commentary
The molecules of compound (I) contain a stereogenic centre at atom C1 (Fig. 1) and the reference molecule was selected as one having the R-configuration at atom C1. The centrosym- ISSN 2056-9890 metric space group confirms that compound (I) has crystallized as a racemic mixture.
Both of the rings are rotated out of the plane of the central C11-C1-C22 fragment, which makes dihedral angles of 70.69 (2) and 84.66 (9) with the phenyl and pyridyl rings, respectively. The dihedral angle between the rings is 74.34 (6) , and this value is very similar to the value of 71.42 (10) reported (Kim & Kang, 2014) for the corresponding angle in the non-chlorinated analogue, compound (II). The general conformational similarity between the molecules of compounds (I) and (II) is shown by the torsional angles O-C-C-C and O-C-C-N (Table 1), where the corresponding angles for the R-enantiomer of (II) [the reference molecule was actually selected (Kim & Kang, 2014) as one having the S-configuration] are 49.0 (4) and À150.6 (2) , respectively.
However, one point of difference between the conformations in compounds (I) and (II) centres on the locations of the hydroxyl H atoms. In compound (I), this atom is antiperiplanar to atom C11 (Table 1), but the corresponding torsional angle for the R-enantiomer of (II) is À67 (2) . This difference in hydroxyl group conformations is probably associated with the different patterns of hydrogen-bonded supramolecular aggregation in compounds (I) and (II), as discussed below.

Supramolecular interactions
The molecules of compound (I) are linked by O-HÁ Á ÁN hydrogen bonds (Table 2), forming zigzag C(5) chains running parallel to the [001] direction. The chain containing the reference molecule at (x, y, z) consists of molecules which are related by the c-glide plane at y = 1 4 , so that molecules of Rconfiguration and S-configuration alternate along the chain (Fig. 2). Two chains of this type, related to one another by inversion, pass through each unit cell.
The crystal structure of compound (I) contains neither C-HÁ Á Á hydrogen bonds norstacking interactions. There is, however, a single short C-ClÁ Á Á contact with geometric parameters ClÁ Á ÁCg i = 3.5280 (10) Å , CÁ Á ÁCg i = 5.1785 (19) Å and C-ClÁ Á ÁCg i = 157.79 (7) [symmetry code: (i) 1 À x, Ày, Àz] where Cg represents the centroid of the pyridine ring. This ClÁ Á ÁCg distance is slightly shorter than the average distance, 3.6 Å , deduced (Imai et al., 2008) from database analysis in a study which concluded that such interactions were attractive, with interaction energies of ca 2 kcal mol À1 , comparable to those typical of weak hydrogen bonds The molecular structure of the R-enantiomer of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. Table 1 Selected torsion angles ( ).

Figure 2
Part of the crystal structure of compound (I), showing the formation of a hydrogen-bonded C(5) chain containing alternating enantiomers and running parallel to [001]. For the sake of clarity, the H atoms bonded to the ring C atoms have been omitted. The atoms marked with an asterisk (*), a hash (#) or a dollar sign ($) are at the symmetry positions (x, 1 2 À y, 1 2 + z), (x, 1 2 À y, À 1 2 + z) and (x, y, 1 + z), respectively. (Desiraju & Steiner, 1999). In compound (I), this interaction links inversion-related pairs of molecules into cyclic centrosymmetric dimers (Fig. 3). The overall effect of the C-ClÁ Á Á interaction in (I) is to link the hydrogen-bonded chain containing molecules related by the c-glide plane at y = 1 4 directly to the two chains that contain molecules related by the glide planes at y = À 1 4 and y = 3 4 , respectively, and propagation by translation of this interaction links the hydrogen-bonded chains along [001] into a sheet lying parallel to (100) (Fig. 4), but there are no direction-specific interactions between adjacent sheets.

Structural comparisons with related compounds
It is of interest briefly to compare the supramolecular assembly in compound (I), mediated by O-HÁ Á ÁN hydrogen bonds and C-ClÁ Á Á interactions, with the assembly in some closely related compounds (II)-(VIII) (see Fig. 5), and particularly with compound (II), whose constitution differs from that of (I) only in lacking the chloro substituent.
The molecules of compound (II) are linked into C (5)  A centrosymmetric dimer in in the crystal of (I) in which the molecules are linked by C-ClÁ Á Á interactions, shown as hollow lines. For the sake of clarity, all of the H atoms have been omitted. The Cl atom marked with an asterisk (*) is at the symmetry position (1 À x, Ày, Àz).

Figure 4
A view of part of the crystal structure of (I), showing the formation of a sheet parallel to (001) built from hydrogen-bonded chains linked by C-ClÁ Á Á interactions. For the sake of clarity, the H atoms bonded to C atoms have all been omitted.  built from molecules related by 2 1 screw axes in space group Pna2 1 , whereas in (I) zigzag chains are built from molecules related by glide planes. Hence in compound (II) each chain is homochiral, with equal numbers of chains built only from molecules having the R-configuration or only from molecules having the S-configuration: in (I), by contrast, each chain contains an alternation of the two enantiomers (cf. Fig. 2). Similar homochiral C(5) chains are formed in each of the three isomeric carborane derivatives (III)-(V) (Tsang et al., 2015), regardless of whether they are crystallized as single enantiomers or as racemates. The structure of compound (VI), which is isomeric with (II) has been reported briefly (Shimada et al., 2003) but, unfortunately, no atomic coordinates have been deposited in the Cambridge Structural Database (Groom & Allen, 2014). The structure report on (VI) concerns enantiomerically pure forms, in space group P2 1 2 1 2 1 , so that the formation of homochiral helical chains of C(7) type, seems plausible.
Compound (VII), which differs from (I) and (II) in containing two unsubstituted phenyl rings but no pyridyl ring, crystallizes with Z 0 = 2 in space group P22 1 2 1 (Ferguson et al., 1995) and the molecules are linked by O-HÁ Á ÁO hydrogen bonds to form C 2 2 (4) chains, but with no direction-specific interactions between adjacent chains. Compound (VIII) is the pentafluorophenyl analogue of (VII) and the molecules are again linked by O-HÁ Á ÁO hydrogen bonds, but now forming cyclic R 6 6 (12) hexamers having exact 3 (S 6 ) symmetry (Ferguson et al., 1995).

Synthesis and crystallization
A sample of the title compound (I) was a gift from CAD Pharma, Bengaluru, India. Colourless blocks were grown by slow evaporation at room temperature of a solution in methanol, m.p. 478 K.