Mesityl(2,4,6-trimethoxyphenyl)borinic acid

In the title molecule, C18H23BO4, the dihedral angle between the least-squares planes of the aromatic rings is 84.88 (3)°. The B atom deviates by 0.202 (1) Å from the least-squares plane of the mesityl ring. All of the methoxy groups are approximately coplanar with the 2,4,6-trimethoxyphenyl ring, whereas the BOH group is twisted with respect to it by 19.5°. The borinic OH group is engaged in an intramolecular O—H⋯O hydrogen bond with one of ortho-methoxy groups. The molecular structure is stabilized by weak C—H⋯O contacts. In the crystal, molecules are linked by weak C—H⋯O and C—H⋯π interactions, generating a three-dimensional network.

In the title molecule, C 18 H 23 BO 4 , the dihedral angle between the least-squares planes of the aromatic rings is 84.88 (3) . The B atom deviates by 0.202 (1) Å from the least-squares plane of the mesityl ring. All of the methoxy groups are approximately coplanar with the 2,4,6-trimethoxyphenyl ring, whereas the BOH group is twisted with respect to it by 19.5 . The borinic OH group is engaged in an intramolecular O-HÁ Á ÁO hydrogen bond with one of ortho-methoxy groups. The molecular structure is stabilized by weak C-HÁ Á ÁO contacts. In the crystal, molecules are linked by weak C-HÁ Á ÁO and C-HÁ Á Á interactions, generating a three-dimensional network.
The X-ray measurements were undertaken in the Crystallographic Unit of the Physical Chemistry Laboratory at the Chemistry Department of University of Warsaw. This work was supported by the Aldrich Chemical Co. through donation of chemicals and equipment, and by Warsaw University of Technology.

Comment
The ability of arylboronic acids to form supramolecular assemblies due to intermolecular hydrogen bonding is well known.
The interest in our group has focused on ortho-alkoxyarylboronic acids (Dąbrowski et al., 2008;Luliński, 2008), which are related to the title compound, (I). It represents the first structural characterization of borinic acid possessing two different aryl rings. The molecular structure of (I) reflects the significant steric hindrance around the boron atom. The angle C3-B1-C15 is 126.90 (9)°, whereas the dihedral angle between the least squares planes of aryl groups is 84.88 (3)°. The boron atom is deviated from the least squares plane of the mesityl ring by 0.202 (1) Å. The borinic OH group is engaged in an intramolecular O-H···O hydrogen-bond with one of ortho-OMe groups. All OMe groups are approximately coplanar with the 2,4,6-trimethoxyphenyl ring, whereas the BOH group is significantly twisted with respect to it. Molecules are linked by C-H···O contacts, of which there are several types (Table 1). Finally, C-H···π interactions occur between ortho-OMe groups and the mesityl ring: the distances of H10A and H14A from the ring centroid are 2.79 Å and 2.88 Å, respectively (Tab. 1). As a result, a three-dimensional network is formed.

Experimental
A solution of 2-bromomesitylene (4.0 g) in THF (30 ml) was treated with n-BuLi (2.0 M solution in hexanes, 10 ml) at 198 K. The mixture was stirred for 15 min followed by the addition of (2,4,6-trimethoxyphenyl)diethoxyborane (5.30 g). The mixture was quenched with HCl (2.0 M solution in diethyl ether), 10 ml). The resulting suspension was filtered. Evaporation yielded an oil which was dissolved in hexane (30 ml). The solution was washed with water (5 ml). The solvent was removed and the residue was triturated with hexane (10 ml). The product was filtered and washed with hexane (10 ml). Crystals suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of a solution of (I) (0.2 g) in toluene (5 ml).

Refinement
All hydrogen atoms were located geometrically with C-H = 0.95 and 0.98 Å for aryl and methyl type H-atoms, respectively, and O-H = 0.84 Å, and were included in the refinement in the riding model approximation with U iso (H) set to