(2-Methoxy-1,3-phenylene)diboronic acid

The molecular structure of the title compound, 2-CH3O—C6H3-1,3-[B(OH)2]2 or C7H10B2O5, features two intramolecular O—H⋯O hydrogen bonds of different strengths. One of the boronic acid groups is almost coplanar with the aromatic ring, whereas the second is significantly twisted. Molecules are linked by intermolecular O—H⋯O hydrogen bonds, generating infinite chains cross-linked to form a two-dimensional sheet structure aligned parallel to the (01) plane.

The molecular structure of the title compound, 2-CH 3 O-C 6 H 3 -1,3-[B(OH) 2 ] 2 or C 7 H 10 B 2 O 5 , features two intramolecular O-HÁ Á ÁO hydrogen bonds of different strengths. One of the boronic acid groups is almost coplanar with the aromatic ring, whereas the second is significantly twisted. Molecules are linked by intermolecular O-HÁ Á ÁO hydrogen bonds, generating infinite chains cross-linked to form a twodimensional sheet structure aligned parallel to the (011) plane.
The X-ray measurements were undertaken in the Crystallographic Unit of the Physical Chemistry Laboratory at the Chemistry Department of the University of Warsaw. This work was supported by the Warsaw University of Technology and by the Polish Ministry of Science and Higher Education (Grant No. N N205 055633). Support by Aldrich Chemical Co., Milwaukee, Wisconsin, USA, through continuing donations of chemicals and equipment is gratefully acknowledged. (2-Methoxy-1,3-phenylene)diboronic acid M. Dabrowski, S. Lulinski and J. Serwatowski

Comment
The ability of arylboronic acids to form supramolecular structures via hydrogen-bonding interactions of B(OH) 2 groups is well documented (Rettig & Trotter (1977). The presence of two or more boronic groups in a molecule provides an increased potential for the extended supramolecular organization (Fournier et al., 2003;Maly et al., 2006;Pilkington et al., 1995;Rodríguez-Cuamatzi, Vargas-Díaz & Höpfl (2004). The promising properties of di-and polyboronic acids in crystal engineering prompted us to determine the structure of the title compound.
The molecular structure is shown in Fig. 1. One of two boronic groups is almost coplanar with the benzene ring whereas the second one is significantly twisted ( Table 1). The methoxy group is twisted almost perpendicularly with respect to the aromatic ring. Both boronic groups have an exo-endo conformation. The endo-oriented OH groups of both boronic moieties are engaged into intramolecular O-H···O bonds with the methoxy O atom. As a result, a nearly planar six-membered ring is formed by the boronic group coplanar with the benzene ring. This motif has already been observed in structures of related ortho-alkoxyarylboronic acids (Yang et al., 2005;Dabrowski et al., 2006;Serwatowski et al., 2006) and seems to be typical.
The interaction of the second (twisted) boronic group with the methoxy O atom is much weaker [H7···O4 at 2.317 (14) Å]. The molecules are linked via almost linear O-H···O bridges in a "head-to-head, tail-to-tail" fashion, i.e., equivalent groups interact with each other forming two alternate centrosymmetric dimeric motifs. As a result, an infinite, zig-zag chain is formed (Fig. 2). A similar situation is observed in 1,4-phenylenediboronic acid Rodríguez-Cuamatzi, Vargas-Díaz & Höpfl, 2004) and its tetrahydrate (Rodríguez-Cuamatzi, Vargas-Díaz, Maris, Wuest & Höpfl (2004). However, in the former structure both boronic groups are conformationally equivalent whereas in the latter they are almost coplanar with the aromatic ring. The one-dimensional supramolecular architecture extends through cross-linking O-H···O bonds between twisted boronic groups. As a result a two-dimensional network is formed, aligned parallel to the (01-1) plane. Unlike the structure of related diboronic acids (Maly et al., 2006;Rodríguez-Cuamatzi, Vargas-Díaz & Höpf, 2004), only one boronic group is active as a linker for chains.
In conclusion, the intermolecular hydrogen-bonding interactions of boronic groups are operative to form the chain structure whereas their contribution to further secondary supramolecular organization is strongly affected by competitive intramolecular hydrogen bonds.

Experimental
A solution of 2,6-dibromoanisole (5.32 g, 20 mmoL, prepared using the published procedure: Dorman, 1966) in Et 2 O (20 ml) was added under argon to a solution of nBuLi (10 mol, 4.5 ml, 45 mmol) in THF (60 ml) at 203 K. The mixture was stirred for 30 min at 233 K and then cooled again to 203 K followed by rapid addition of trimethyl borate (5.2 g, 50 mmol).
The mixture was stirred for 30 min at 273 K and then it was quenched with HCl (2 M solution in ether, 22 ml, 44 mmol). The resultant mixture was concentrated and the residue fractionally distilled in vacuo to give 2,6-bis(dimethoxyboryl)anisole as on oil (2.50 g, 50%), b.p. 377-381 K (0.5 Torr). It was hydrolyzed with water (0.9 g, 50 mmol) in acetone (20 ml Crystals suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of a solution of the acid (0.2 g) in ethyl acetate/acetone/water (20 ml, 10:10:1).

Refinement
All hydrogen atoms were located in difference syntheses and refined freely.
Figures Fig. 1  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq B1 0.2297 (