(2,4,6-Trimethylphenyl)boronic acid–triphenylphosphine oxide (1/1)

In the crystal structure of the title compound, C9H13BO2·C18H15OP, there are O—H⋯O hydrogen bonds between the O atom of triphenylphosphine oxide and one hydroxy group of the boronic acid. Boronic acid molecules form inversion-related hydrogen-bonded dimers in an R 2 2(8) motif. The structure is consolidated by intermolecular C—H⋯O bonds and C—H⋯π interactions.

In the crystal structure of the title compound, C 9 H 13 BO 2 Á-C 18 H 15 OP, there are O-HÁ Á ÁO hydrogen bonds between the O atom of triphenylphosphine oxide and one hydroxy group of the boronic acid. Boronic acid molecules form inversionrelated hydrogen-bonded dimers in an R 2 2 (8) motif. The structure is consolidated by intermolecular C-HÁ Á ÁO bonds and C-HÁ Á Á interactions.

Comment
Boronic acids are widely used as starting materials in Suzuki-Miyaura cross-coupling reactions (Suzuki, 2011), as sensors or binders for carbohydrates (Yang et al., 2011) or building blocks for covalent organic frameworks (Furukawa & Yaghi, 2009).
Although there are a large number of reported crystal structures for aromatic boronic acids, the structure of mesitylboronic acid has not been reported yet. We present here the crystal structure of the mesityl boronic acid -triphenylphosphine oxide (1:1) adduct.
In the title compound there is an O-H···O bond between the hydrogen atom of a hydroxy group and the oxygen atom of the triphenylphosphine oxide ( Fig. 1). The triphenylphosphine oxide molecules are connected into chains by weak C-H···O hydrogen bonds along the c axis direction (Table 1 and Fig. 2). The P═O bond length (1.479 (2) Å) is in the range of the values found for triphenylphosphine oxide (1.4863 (12) Å) (Sivaramkrishna et al., 2007), or triphenylphosphine oxide hemidydrate (1.4871 (15) Å) (Ng, 2009).
The molecules of the mesitylboronic acid assemble into centrosymmetric dimers through a pair of O-H···O bonds between the hydroxy groups ( Fig. 2). In contrast to the structure of phenylboronic acid where the centrosymmetric dimers are interconnected to one-dimensional chains (Rettig & Trotter, 1977;Cyrański et al. 2008), in the structure of the title compound, the triphenylphosphine oxide molecules block further assembly of the dimers.
The mesitylboronic dimers and the triphenylphosphine oxide chains are interconnected, additionally to the O-H···O bonds, through C-H···π interactions (Table 1 and Fig. 2). In the crystal there are alternate layers of mesitylboronic acid and triphenylphosphine oxide along the a-axis (Fig. 3).

Experimental
The title compound was serendipitously obtained in a Suzuki-Miyaura cross-coupling reaction between mesitylboronic acid and tert-butyl N-(tert-butoxycarbonyl)-N-(2,4,6-tribromophenyl)carbamate using tetrakis(triphenlyphosphine)palladium as catalyst. Prior to column chromatography of the crude product mixture a solid precipitated from the mobile phase (diethyl ether: petroleum ether = 1:8). Colourless crystals were obtained by recrystallization of the precipitate from hot toluene.

Refinement
Hydrogen atoms were placed in calculated positions with isotropic thermal parameters set at 1.2 times the carbon atoms directly attached for aromatic atoms and 1.5 for hydrogen atoms of the methyl groups and of the hydroxy groups. Methyl hydrogen atoms were allowed to rotate but not to tip. The hydrogen atoms of the hydroxy group were allowed to rotate about the O-B bond and their positions were calculated from the electron density. The C-H bond lengths were set at 0.93 Å for the aromatic groups, 0.96 Å for the methyl groups. The O-H bond lengths were set at 0.82 Å. Fig. 1. Crystal structure of the title compound with ellipsoids of non-hydrogen atoms drawn at the 25% probability level.  (2,4,6-Trimethylphenyl)boronic acid-triphenylphosphine oxide (1/1)