(2-Butoxyphenyl)boronic acid

The title compound, 2-(CH3CH2CH2CH2O)C6H4B(OH)2, exists as a centrosymmetric hydrogen-bonded dimer. Dimers are linked via C—H⋯π and π–π [with closest C⋯C contact of 3.540 (3) Å] interactions to produce a two-dimensional array.

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). This work was supported by the Aldrich Chemical Company through the donation of chemicals.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: TK2240).

Comment
The boronic acid group is known to support supramolecular organization due to intermolecular hydrogen bonding. ortho-Substituents in the aryl ring may significantly influence the structural properties of arylboronic acids. There are a few structures of ortho-alkoxyarylboronic acids available in the literature, i.e. those reported by Yang et al. (2005),  and Dabrowski et al. (2006). We were interested in studying the effect of a longer alkoxy chain on the structural characteristics of the related compound, (I).
The molecular structure of (I) shows the entire molecule to be essentially planar, Fig. 1 & Table 1. The mean planes through the boronic and butoxy groups are approximately co-planar with the aromatic ring. The boronic group has an exo-endo conformation. The endo-oriented OH group forms an intramolecular O-H···O bond with the butoxy-O atom, Table 2. A s a result, a nearly planar six-membered ring is formed. This motif has been observed in the structures of related ortho-alkoxyarylboronic acids. Monomeric molecules form hydrogen-bonded centrosymmetric dimers typical of boronic acids (Rettig & Trotter, 1977). The crystal packing in (I) features a parallel arrangement of hydrogen-bonded dimers (Fig.   2). It is stabilized in terms of CH-π interactions between the H7a atom of the butoxy group and the aromatic ring of the adjacent molecule: the distance of H7A from the ring centre is 2.777 (11) As a result, a centrosymmetric dimeric motif can be distinguished. In addition, weak π-π interactions between a pair of aromatic rings lead to their face-to-face center-to-edge stacking with the shortest contact between two C atoms C1-C1 iii = 3.540 (3) Å [symmetry code (iii): −x, 1 − y, 1 − z]. The other two π-π interactions are C1···C2 iii = 3.594 (5) Å and C1···C6 iii = 3.819 (3) Å. Thus, alternate CH-π and π-π interactions result in formation of a two-dimensional array. In conclusion, the hydrogen-bonded dimeric structure of (I) is typical of boronic acids whereas the unique secondary supramolecular assembly is achieved due to weaker CH-π and π-π interactions.

Experimental
Crystals suitable for the X-ray diffraction analysis were grown by slow evaporation of a solution of the acid (0.2 g) in acetone/water (20 ml, 1:1).

Refinement
All H atoms were located in difference syntheses and refined freely.  Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level.   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.