Three-centre hydrogen bonds in triphenylphosphine oxide–hydroquinone (1/1)

The title cocrystal, C18H15OP·C6H6O2, belongs to a series of molecular systems based on triphenylphosphine P-oxide. The O atom of the oxide group acts as an acceptor for hydrogen bonds from OH groups of two hydroquinone molecules which lie on inversion centres [O⋯O = 2.7451 (17) and 2.681 (2) Å]. The crystal structure is stabilized by weak C—H⋯O hydrogen bonds, forming a C 2 1(8) chain which runs parallel to the [100] direction.

The title cocrystal, C 18 H 15 OPÁC 6 H 6 O 2 , belongs to a series of molecular systems based on triphenylphosphine P-oxide. The O atom of the oxide group acts as an acceptor for hydrogen bonds from OH groups of two hydroquinone molecules which lie on inversion centres [OÁ Á ÁO = 2.7451 (17) and 2.681 (2) Å ]. The crystal structure is stabilized by weak C-HÁ Á ÁO hydrogen bonds, forming a C 2 1 (8) chain which runs parallel to the [100] direction.

Related literature
For related literature, see: Al-Farhan (1992); Etter (1990); Fuquen & Lechat (1992); Wallwork & Powell (1980 Table 1 Hydrogen-bond geometry (Å , ). (17) 175 (2) (Fuquen et al., 1992). In order to expand the crystallographic information of the TPPO complexes, to study the hydrogen bond character of the complex, and to analize its supramolecular arrangement, the structure determination of TPPO + hydroquinone (HQ), (I), system was undertaken. The free HQ molecule in the more stable form at room temperature (Wallwork & Powell, 1980) and the free TPPO molecule (Al-Farhan, 1992) can be taken as a reference systems to compare with the structural characteristics of (I). A displacement ellipsoid plot of the title hydrogen-bonded complex (I), showing the atomic numbering scheme is given in Fig. 1. The O1 atom of the P-oxide group of TPPO acts as an acceptor for hydrogen bonds from O-H groups of two hydroquinone molecules [O1···O2, 2.7451 (17), O1···O3, 2.681 (2) Å and O1···H25-O2, O1···H26-O3 angles of 175 (2) and 170 (2)° respectively, ( Table 2)]. These two HQ molecules are each disposed about a centre of symmetry. The title molecule shows a H25-O1-H26 bond angle close to the right angle, seeking an orientation with the minor repulsion between the rings of the molecule. The presence of the three centre hydrogen bond at O1 induces the lengthening of P-O bond length from 1.479 (2) Å in free TPPO molecule (Al-Farhan, 1992) to 1.5016 (13) Å in (I). Other bond lengths and angles of TPPO and HQ remain similar in the complex.
The title molecules of (I) are additionally linked by C-H···O hydrogen bonds. Indeed, atom C3 in the molecule at (x, y, z) acts as a hydrogen-bond donor to O3 iv atom in the molecule at (x, 1 + y, z), so generating a C 2 1 (8) chain (Etter, 1990) which is running parallel to [100] direction (Fig. 2, Supp.material). Other significant intermolecular hydrogen bonds are not observed in the crystalline structure.

Experimental
Crystals of the title compound (I), were obtained by slow evaporation of equimolecular quantities of HQ (1.826 g, 0.017 mol) and TPPO (4.725 g) in 150 ml of dry acetonitrile. After three days, colourless plates of a good quality suitable for X-ray analysis were obtained. Its melting point is 425 (1) K.

Refinement
All non-hydrogen atoms were identified by direct methods and the positions of all the hydrogen atoms were obtained from the use of difference Fourier maps. In the final refinement, all hydrogen atoms were constrained to geometrically sensible positions with a riding model (SHELX97), C-H= 0.95 Å, and U iso (H)= 1.5U eq (C), apart from H25 and H26, which were allowed to refine freely.