Solid-State Structures of 4-Carboxyphenylboronic Acid and Its

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CRYSTAL GROWTH & DESIGN

Solid-State Structures of 4-Carboxyphenylboronic Acid and Its Hydrates

2007 VOL. 7, NO. 5 944-949

Nanappan SeethaLekshmi and Venkateswara Rao Pedireddi* Solid State & Supramolecular Structural Chemistry Unit, DiVision of Organic Chemistry, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India ReceiVed NoVember 29, 2006; ReVised Manuscript ReceiVed March 1, 2007

ABSTRACT: Solid-state structures of 4-carboxyphenylboronic acid (1a) and its mono- and quarter-hydrates (1b and 1c, respectively) obtained by crystallization from different solvents, acetone, methanol, and 2-proponal, respectively, have been reported. In the anhydrous and monohydrate structures (1a and 1b), the H-atoms on the -B(OH)2 functional group exist in syn-syn conformation, but syn-anti conformation was found in the quarter-hydrate structure. While 1a and 1b crystallize through interactions between -B(OH)2 and -COOH groups (heteromeric), homomeric interactions of the functional groups (COOH‚‚‚COOH and B(OH)2‚‚‚ B(OH)2) were observed in 1c. In addition, although similar interactions prevail in 1a and 1b, the latter crystallizes in a noncentrosymmetric space group Ccc2, registering itself as a unique structure with unusual catenation features with the aid of water molecules. However, 1a and 1c show many common features in the molecular arrangement in the crystal lattices with the formation of sheet structures. Introduction Boronic acids, with a general structural representation R-B(OH)2 (R ) alkyl or aryl), have attracted a wide range of researchers because of the challenges involved in their synthesis1 as well as for their applications as facile reagents to perform various organic transformations,2 high utility in the pharmaceutics3 polymer industry,4 etc. Taking into account the bioactivity of boronic acids and their interactions with various biologically important compounds, such as sugars,5 we recently initiated molecular recognition studies involving boronic acids by synthesizing a wide variety of supramolecular assemblies. Thus, we have reported some molecular complexes of boronic acids, with aza donor compounds such as 4,4′-bipyridine, 1,2bis(4-pyridyl)ethane, 1,2-bis(4-pyridyl)ethene, etc. formed exclusively due to the O-H‚‚‚N hydrogen bonds, exemplifying the ability of the -B(OH)2 moiety to form those bonds like -COOH and -OH groups.6 From our experiments and from the crystal structure determination of various boronic acids done by others,6,7 it is clearly understood that the -B(OH)2 moiety adopts mainly three different types of conformations, as shown in Scheme 1, which yield different hydrogen-bonding networks. Among these arrangements, while syn-anti conformation is observed in a majority of structures, the syn-syn and anti-anti conformations are relatively uncommon.8 It is, indeed, an interesting feature as the contemporary frontier research areas such as polymorphism,9 the synthesis of functional solids through noncovalent interactions, etc.10,11 are mostly directed by the flexibility of formation of different types of molecular frameworks by a given compound. This is well demonstrated in the case of 4-hydroxybenzoic acid. 9m Thus, we have carried out solid-state structural studies of 4-carboxyphenylboronic acid to obtain novel assemblies, and the results are described in this article. One of the challenges in the development of functional solids is the synthesis of noncentrosymmetric structures, which is often difficult not only due to the nontrivial reasons such as the complexity of the molecules, commercial viability etc., but also * To whom correspondence should be addressed. Fax: 91 20 25902624. Tel: 91 20 25902097. E-mail: [email protected].

Scheme 1

often because of the generation of inversion symmetry in bulk structure. Even the attempts through noncovalent synthesis by inducing molecular recognition between different functional moieties such as -COOH and -CONH212 were not successful, as crystal lattice(s) of the resultant assemblies adopted centrosymmetric packing in a three-dimensional (3D) arrangement. Since the -B(OH)2 moiety has a more flexible conformation than amides and moreover one of its conformations (syn-anti) mimics -CONH2, we have chosen 4-carboxyphenylboronic acid to evaluate the COOH‚‚‚B(OH)2 interactions, as depicted in Scheme 2. Results and Discussion Crystallization of 4-carboxyphenylboronic acid from acetone gave single crystals, 1a, that are suitable for structure determination by X-ray diffraction methods. The crystal lattice adopts a centrosymmetric monoclinic space group (C2/c), with one molecule in the asymmetric unit (see Figure 1a). The structural and crystallographic parameters are listed in Table 1. The packing analysis in the crystal structure reveals the formation of one-dimensional (1D) molecular chains through a noncentrosymmetric hydrogen-bonding pattern as shown in Scheme 2i, due to -COOH‚‚‚B(OH)2 interaction. Further, the recognition pattern between -COOH and -B(OH)2 shows that both the functional moieties, indeed, have adopted a rare arrangement with the former being completely disordered and the latter in an anti-anti conformation. The hydrogen bond distance (H‚‚‚O) observed in the cyclic network is 1.75 Å, and the other characteristics are given in Table 2.15 The molecular chains thus formed interact in two-dimensional (2D) arrange-

10.1021/cg060860c CCC: $37.00 © 2007 American Chemical Society Published on Web 04/13/2007

4-Carboxyphenylboronic Acid and Its Hydrates

Crystal Growth & Design, Vol. 7, No. 5, 2007 945 Scheme 2

Table 1. Crystallographic Parameters for the Crystal Structures of 1a-1c 1a CCDC247776

1b CCDC 247777

1c CCDC 247778

formula

C7H7BO4

formula wt crystal habit crystal color crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) T (K) (λ) Mo KR µ (mm-1) 2θ range (deg) limiting indices

165.94 block colorless monoclinic C2/c 11.378(5) 9.825(5) 7.245(3) 90 120.08(7) 90 700.8(6) 4 1.573 133(2) 0.71073 0.126 46.60 -12 e h g 7 -10 e k g 10 -7 e l g 8 344 1444 505 388 61 1.084 0.0758 0.2161 0.628, -0.380

(C7H7BO4): (H2O) 183.95 block colorless orthorhombic Ccc2 7.566(5) 11.929(7) 9.769(6) 90 90 90 881.7(9) 4 1.386 298(2) 0.71073 0.116 54.34 -9 e h g 9 -14 e k g 13 -10 e l g 11 384 2925 495 435 69 1.433 0.0713 0.1587 0.230, -0.245

(C7H7BO4): 0.25(H2O) 170.44 needle colorless orthorhombic Fddd 13.140(3) 18.138(5) 25.328(7) 90 90 90 6037(3) 32 1.500 133(2) 0.71073 0.122 56.56 -17 e h g 17 -23 e k g 23 -33 e l g 33 2832 12537 1797 1181 137 1.058 0.0523 0.1335 0.387, -0.333

F (000) reflns measured unique reflns reflns used no. of parameters GOF on F2 R1 [I > 2σ(I)] wR2 final diff Fourier map (e Å-3) max, min

Figure 1. ORTEP drawings, showing the conformations of -B(OH)2 in the crystal structures of (a) 1a, (b) 1b, and (c) 1c.

ment constituting a planar sheet structure, as shown in Figure 2a, which are stacked in 3D crystal lattice (see Figure 2b). The interaction between the chains is established through C-H‚‚‚O hydrogen bonds (H‚‚‚O, 2.51 and 2.68 Å). Since it is well-known that the nature of intermolecular interactions between the functional groups are often influenced by the solvent of crystallization,9,13 we continued our experiments by dissolving 4-carboxyphenylbornic acid in different organic solvents, with a dual purpose that either a noncentrosymmetric structure would be realized or it may obtain polymorphs, which is much more exciting, especially if the target compound(s) have pharmaceutical importance. We were successful in obtaining good quality single crystals from methanol (1b) and 2-proponal (1c), and X-ray diffraction studies reveal that both 1b and 1c are two different hydrate forms of 4-carboxyphenylboronic acid. Further, while 1b crystallizes in a noncentrosymmetric space group, Ccc2 (see Table 1) 1c adopts a centrosymmetric space group, Fddd (see Table 1). We present the asymmetric units of both forms as ORTEP drawings in Figure 1, panels b and c, respectively. Since 1b and 1c have different water compositions, they can be represented only as pseudopolymorphs or solvates rather than as polymorphs.

Table 2. Characteristics of Hydrogen Bonds in the Crystal Structures of 1a, 1b, and 1ca hydrogen bond

1a

1b

1c

O-H‚‚‚O

1.75 2.70 161 2.08 2.91 141

1.66 2.61 163 1.88 2.82 160

C-H‚‚‚O

2.51 3.48 149 2.68 3.61 143 2.85 3.57 124

2.93 3.75 132 2.97 3.84 137

1.64 2.62 178 1.67 2.65 173 1.72 2.69 173 1.84 2.72 148 2.65 3.68 158 2.82 3.69 137 2.83 3.71 139 2.90 3.80 140

a In each column, the three numbers correspond to distances H‚‚‚O, O(C)‚‚‚O, and angle O(C)-H‚‚‚O.

In the crystal structure of 1b, the adjacent molecules are held together by noncentrosymmetric hydrogen bond couplings like in 1a, forming similar molecular arrangements both in one and two dimensions as that of 1a. The 2D arrangement of molecules is shown in Figure 3a. In the 3D arrangement, water molecules are inserted between the layers of boronic acid, like in the inorganic pillared structures (See Figure 3b). These water molecules interact with the boronic acid molecules present in the adjacent layers, as shown in Figure 4a, with each water molecule holding four acid molecules, utilizing its dual nature of acceptor and donor properties, by forming O-H‚‚‚O hydrogen bonds with an O‚‚‚O distance of 2.82 Å. Furthermore, such an association leads to the formation of a catenated network as shown in Figure 5.

946 Crystal Growth & Design, Vol. 7, No. 5, 2007

SeethaLekshmi and Pedireddi

Figure 2. (a) Molecular arrangement of a typical layer in the crystal structure of 1a. (b) Stacking of layers in the 3D crystal lattice. (c) Schematic drawing for the generally observed layer structures in the literature represented by 1a.

In contrast, the crystal structure of 1c is quite distinct from 1a and 1b in many ways, although global packing features remain the same about the formation of molecular chains, sheets and stacking. A prominent deviation is that in each molecular chain, the -COOH and -B(OH)2 moieties have formed centrosymmetric hydrogen bond couplings independently (homomeric) as shown in Scheme 2ii, through O-H‚‚‚O hydrogen bonds. In addition, unlike in 1a and 1b, both functional groups adopted well-known conformations: -COOH as a fully ordered and -B(OH)2 in a syn-anti conformation. The typical molecular chain observed in 1c is shown in Figure 6a. Thus, two types of cyclic couplings are observed in each molecular chain, COOH‚‚‚COOH and B(OH)2‚‚‚B(OH)2, with H‚‚‚O distances of 1.64 and 1.72 Å (Table 2), respectively. These molecular chains are held together by C-H‚‚‚O hydrogen bonds (H‚‚‚O, 2.65, 2.82, 2.83 Å) constituting planar sheets (Figure 6a), which are stacked in three dimensions, inserting water molecules in between the layers (Figure 6b). The water molecules occupy a crystallographic special position

and also show disorder resulting in a 0.25 composition. Water molecules interact with the acid molecules lying in the juxtaposed layers as in 1b but only as a donor due to the disorder nature (Figure 4b). Thus, a centrosymmetric relation was established between the layers, unlike in 1b. However, the juxtaposed layers are not stacked in either a parallel or antiparallel manner, as generally observed in many centrosymmetric structures (see Figure 7). A schematic representation of arrangement of the layers is shown in Figures 7b and 2c for the patterns observed in 1c and 1a (a typical stacked layer structure), respectively. Thus, in the crystal structure of 1c, the pattern adopted is ABAB‚‚‚, while similar structures, in general, form the AAAA‚‚‚ arrangement (see Figure 2c). In further experiments of crystallization, we were able to obtain single crystals from a few solvents only, for instance, tetrahydrofuran, 1-proponal, etc. with unit cell dimensions well matched with that of 1c. It is interesting to note that crystallization of boronic acid from water also (both at room temperature and under hydrothermal conditions), surprisingly, adopts

4-Carboxyphenylboronic Acid and Its Hydrates

Crystal Growth & Design, Vol. 7, No. 5, 2007 947

Figure 3. (a) 2D arrangement of boronic acid molecules in the crystal lattice of 1b. (b) 3D arrangement of the molecular sheets with water molecules inserted between the sheets.

Figure 5. (a) Catenation of the adjacent layers through water molecules in 1b leading to the creation of a noncentrosymmetric arrangement, due the tilting of layers. (b) A schematic representation of catenation by reducing each molecule to a point.

Conclusions

Figure 4. Interaction of water molecules with four molecules of 4-carboxyphenylboronic acid in (a) 1b and (b) 1c. Notice the disorder of the water molecule in 1c.

1c, as confirmed by the single-crystal X-ray diffraction methods, rather than 1b. Thus, the influence of methanol solvent is distinctly visible for the formation of the noncentrosymmetric form of 4-carboxyphenylboronic acid.

In conclusion, we have illustrated the structural variations in 4-carboxyphenylboronic acid by varying the solvent of crystallization. In particular, formation of noncentrosymmetric crystals from methanol in the form of a hydrate structure is unique. Also, the preparation of two different hydrate structures would pave the way for the exploration of further salient features of this compound in the directions of polymorphism and solvated structures. We have been working with various types of other boronic acids to explore these salient and intriguing features, and also many competitive experiments in conjunction with other functional moieties such as -COOH and -CONH2 are in progress.

948 Crystal Growth & Design, Vol. 7, No. 5, 2007

SeethaLekshmi and Pedireddi

Figure 6. Packing of molecules in the crystal lattice of 1c. (a) 2D arrangement of the molecular chains yielding a planar sheet structure. (b) Stacking of the planar sheets inserting water molecules in between the layers. within 3 days. The crystals, thus obtained, were separated from the mother liquor by filtration followed by washing with the respective solvents and subsequently dried under vacuum and used for the analysis by single-crystal X-ray diffraction methods. Crystal Structure Determination. The single crystals obtained in the above processes were analyzed using a Leica microscope equipped with a CCD camera, and good quality crystals were chosen for structure determination by X-ray diffraction methods using a Polaroid detector. The crystals were mounted on a goniometer by gluing them to a glass fiber using adhesive (cyanoacrylate), and the crystal data were collected on a CCD diffractometer with an APEX detector. The intensity data were processed using SAINT software of the Bruker suite of programs. The structures were determined and refined using the SHELXTL package,14 and no anomalies were observed at any stage of structure solutions. The final crystallographic details and data collection strategies are given in Table 1. All the calculations of intermolecular interactions, shown in Table 2, were done using PLATON.15

Acknowledgment. We thank Dr. S. Sivaram and Dr. K. N. Ganesh for their constant support and encouragement. We thank LTMT, BRNS, and DST for the financial assistance. Supporting Information Available: Crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. Figure 7. Representation of the arrangement of layers in the crystal lattice of 1c. (a) As observed. (b) Schematic drawing.

Experimental Procedures 4-Carboxyphenylboronic acid was procured from Aldrich and used as such without any further purification. The crystals 1a-1c were prepared by dissolving the boronic acid in acetone, methanol, and 2-proponal, respectively, and by allowing the resultant solutions to evaporate at ambient conditions. In all cases, single crystals were found

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CG060860C