1-Hydroxy-1,1,3,3,3-pentaphenyldisiloxane, [Si2O(OH)(Ph)5], at 150 K

In the crystal structure of the title compound, C30H26O2Si2, one Si(Ph)3 residue is bound to another Si(OH)(Ph)2 residue via a nonlinear Si—O—Si bridge. The asymmetric unit is composed of four [Si2O(OH)(Ph)5] molecules. Each pair of adjacent molecules interacts via strong and highly directional O—H⋯O hydrogen bonds connecting neighbouring Si—OH units, and via inter-unit O—H⋯π contacts connecting the second hydroxyl groups with adjacent phenyl groups.

In the crystal structure of the title compound, C 30 H 26 O 2 Si 2 , one Si(Ph) 3 residue is bound to another Si(OH)(Ph) 2 residue via a nonlinear Si-O-Si bridge. The asymmetric unit is composed of four [Si 2 O(OH)(Ph) 5 ] molecules. Each pair of adjacent molecules interacts via strong and highly directional O-HÁ Á ÁO hydrogen bonds connecting neighbouring Si-OH units, and via inter-unit O-HÁ Á Á contacts connecting the second hydroxyl groups with adjacent phenyl groups.  Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) Àx; Ày þ 1; Àz.
We are grateful to Fundaçã o para a Ciê ncia e a Tecnologia (FCT, Portugal) for general financial support and also for specific funding towards the purchase of the single-crystal diffractometer.

Comment
In a separate crystallographic communication we reported the crystal structure of the title compound, (I), at 100 K in the triclinic P1 space group with two [Si 2 O(OH)(Ph) 5 ] molecular units comprising the asymmetric unit (see Coelho et al., 2008).
At 150 K, a decrease in overall long range symmetry, accompanied by an increase in size of the unit cell (by a factor of ca 2) and, consequently, in the number of crystallographically independent binuclear [Si 2 O(OH)(Ph) 5 ] molecular units (from two at 100 K to four at 150 K, see Fig. 1. It is important to stress that the reduction of overall symmetry seems to be essentially due to an increase of thermal motion of the coordinated phenyl groups. Indeed, even though the increase in temperature was only of about 50 K, since the intermolecular interactions between adjacent [Si 2 O(OH)(Ph) 5 ] complexes are of rather weak nature (van der Waals interactions alongside with a number of C-H···π contacts between phenyl groups belonging to adjacent complexes), equivalence between adjacent binuclear units is ultimately destroyed by a combined effect of rotation of the phenyl groups around the Si-C bond with in-plane thermal vibration of the carbon atoms ( Figure 1). In fact, the average value of U eq for the carbon atoms composing the phenyl groups increases from 0.047 Å 2 (at 100 K) to 0.052 Å 2 (at 150 K). }, the Si-C and Si-O bond lengths were found in the 1.842 (5)-1.875 (5) and 1.605 (3)-1.648 (3) Å ranges, respectively, in good agreement with those found in related materials and in our determination at 100 K. Each pair of Si centres is interconnected via a µ 2 -bridging oxo group, imposing Si···Si internuclear distances ranging from 3.113 (2) Å to 3.216 (2) Å. These distances are shorter than those registered for disiloxanes in which the two Si centres exhibit identical coordination environments (found in the 3.24-3.44 Å range; see Glidewell & Liles, 1978;Hönle et al., 1990;Suwińska et al., 1986). In fact, the presence of distinct coordinating moieties, and the type of intermolecular interactions in which they are involved with neighbouring species, leads to a deformation of the binuclear units through the µ 2 -bridge, ultimately imposing shorter Si···Si interatomic distances. The Si-O-Si bond angles for (I) were found in the 145.1 (2)-169.5 (2)°r ange ( Fig. 1 & Table 1) and, as described for the determination of (I) at 100 K, are distributed over two markedly distinct ranges. On the one hand, the high range values are in good agreement with the angle reported by Wojnowski et al. (2004) Fig. 1 that the thermal motion of the phenyl groups involved in these interactions is significantly more limited than those of the remaining phenyl groups.
As for the structure at 100 K, at 150 K supramolecular entities formed by the combined effected of the O-H···O hydrogen bonds and O-H···π contacts arrange themselves in an ordely fashion in the ac plane of the unit cell forming layers, which close pack along the [010] direction of the unit cell to give the crystal structure of (I), Fig. 2. We also note the presence of a number of C-H···π contacts between phenyl groups (not shown) which help to mediate the crystal packing of individual [Si 2 O(H)(Ph) 5 ] molecular units.

Experimental
Crystals of the title compound were isolated from the same batch as those used for the determination at 100 K of the title compound (see Coelho et al., 2008).

Refinement
A small number of single-crystals of (I) could be indexed at 100 K with the unit-cell parameters summarized in the Experimental Table given in the previous paper (Coelho et al., 2008). Those results led us to infer that the increase of thermal motion, in particular that associated with the coordinated phenyl groups, could, to some extent, reduce overall symmetry.
Preliminary measurements for several different crystals at 150 K confirmed the increase of the size of the triclinic unit cell by approximately a factor of 2.