Crystal structure of 4-(methoxycarbonyl)phenylboronic acid

In the crystal of the title compound, molecules are linked via O—H⋯O and C—H⋯O hydrogen bonds, forming undulating sheets parallel to (10). The sheets are linked via C—H⋯π and offset face-to-face π-interactions, between inversion-related molecules [inter-centroid distance = 3.7843 (16) Å], forming a three-dimensional structure.


Structural commentary
The title molecule, Fig. 1, is almost completely planar with the methoxycarbonyl group inclined to the benzene ring by 7.70 (6) . The angle around atom B1, O1-B1-O2 is 118.16 (9) , very close to the ideal value of 120 . The bond lengths and angles are similar to those reported for 4-carboxyphenylboronic acid derivatives (SeethaLekshmi & Pedireddi, 2007) in which the carboxylic group is rotated from the plane of the benzene ring by ca 13. 83-26.44

Supramolecular features
In the crystal of the title compound, there are hydrogen bonds between the carbonyl atom O4 and the hydroxy group O2-H2A of the boronic acid and atom O2 of the boronic acid with a DÁ Á ÁA distance of 2.753 (1) Å ( Fig. 2 and Table 1). The hydroxy group O1--H1A of the boronic acid is in an inversionrelated hydrogen-bonded network with the oxygen O2 of the boronic acid at a distance of 2.762 (1) Å ( Fig. 2 and Table 1).
The presence of the methoxy group on the carbonyl removes hydrogen-bond donation of the carboxylic acid seen in related structures. Atom C8 creates a shield around atom O3, removing its ability to participate in hydrogen bonding due to steric effects. It is noteworthy that the methoxy protecting group is small compared to other protecting groups and therefore exhibits no steric effects on the hydrogenbonding capabilities to atom O4 in this structure, as seen in other examples. As exemplified by the work of Lemmerer (2012), most literature examples exhibit an almost exclusive head-to-tail hydrogen-bonding network between the boronic and carboxylic acids whereas the title compound exhibits exclusively head-to-head hydrogen-bonding interactions with the boronic acid subunit. This is due to the steric effects and removal of hydrogen-donating abilities in the methoxycarbonyl subunit (Fig. 2). The group can still act as a hydrogen acceptor, as shown in the packing diagram (Fig. 2). Atom O4 can accept H atoms from the hydroxy group O2-H2A to create an offset face-to-face overlap in the packing unit. Yang et al. (2005) published a structure of the boronic ester derivative of the title compound. This structure showed similar effects, however no hydrogen bonds were visible in the reported structure.
Hydrogen bonding and -stacking within the unit cell forms a strong set of dimeric pairs. This can be easily observed in Fig. 3. These dimeric pairs line up to form a zigzag stacking pattern with a consistent spacing throughout the unit cell. This is aided by the hydrogen bond between O4 and O2-H2A The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).
Cg is the centroid of ring C1-C6.

Figure 3
View of the -aggregated structure as viewed approximately along the a axis.
consists of four molecular units which form -aggregated pairs in a head-to-tail fashion and are stabilized through offset faceto-face -interactions [inter-centroid distance = 3.7843 (16) Å ; inter-planar distance = 3.3427 (4) Å , offset = 1.744 Å ]; see Fig. 4. The hydroxy group O1-H1A of the boronic acid is in an inversion-related hydrogen-bonded network with the oxygen O2 of the boronic acid at a distance of 2.762 (1) Å , forming a head-to-head hydrogen-bonded network with adjacent molecules (Fig. 5). There are also C-HÁ Á Á interactions (Table 1) present between the undulating sheets parallel to (102). The sum of all of these intermolecular interactions leads to the formation of a three-dimensional structure.  (Varughese et al., 2011). Likewise, the relevant values in the structure of a cyclopentanaminium 4-(dihydroxyboryl)benzoate trihydrate were ca 29.67 and 126.53 , respectively (Lemmerer, 2012). Finally, methyl 4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)benzoate showed similar features to the title compound with a methoxycarbonyl deviation from the ring plane of ca 4.97 (Yang et al., 2005). A view approximately along the b axis, showing the offset face-to-faceinteractions involving inversion-related molecules.

Figure 5
Crystal packing diagram of the title compound viewed along the c axis, showing the offset face-to-face -interactions involving inversion-related molecules (dashed lines; see Table 1 for details).

Synthesis and crystallization
The compound was purchased from Alfa Aesar and was purified with silica gel column chromatography using CH 2 Cl 2 :MeOH (19:1). The compound was then crystallized from a solution of 1% MeOH in CH 2 Cl 2 layered with hexane to give a single crystal suitable for X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The donor OH H atoms were located in a difference Fourier map and freely refined. The Cbound H atoms were placed in their expected calculated positions and refined as riding: C-H = 0.95-0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U iso (C) for other H atoms.