Symbiosis in Solid State Interconversion and ... - ACS Publications

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Symbiosis in Solid State Interconversion and Synthon Modularity in Hydroxybenzoic Acid−Hexamine Adducts Ramanpreet Kaur, Bannur V. Lalithalakshmi, and Tayur N. Guru Row* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India S Supporting Information *

ABSTRACT: Systematic cocrystallization of hydroxybenzoic acids with hexamine using liquid-assisted grinding shows facile solid state interconversion among different stoichiometric variants. The reversible interconversion caused by varying both the acid and base components in tandem is shown to be a consequence of hydrogen-bonded synthon modularity present in all representative crystal structures. Among a total of 11 complexes, three are salts and eight are cocrystals. The insulated synthons appear as conserved tetrameric motifs in the structures, and the mechanism of interconversion is closely monitored by the synthon modularity. The interconversion is consistent with the theoretically computed stabilization energies of all the tetramers found in this series of cocrystals based on atoms in molecule calculations.

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INTRODUCTION Rational design and synthesis of crystalline materials with desired physiochemical properties have been attempted over several decades. In recent years, the propensity to utilize cocrystals as possible design elements for the generation of functional materials has been evaluated.1 Cocrystals have been explored for applications in the pharmaceutical industry because they offer better solubility and bioavailability profiles than the APIs.2 Therefore, the goal is to construct and screen the cocrystals with ease and in an environment friendly fashion for further applications. In this direction, mechanochemical methods that involve kneading and grinding using limited amounts of solvents [liquid-assisted grinding (LAG)] have become the preferred techniques for cocrystal growth.3 It is observed that the resulting product might possess the potential to form polymorphic variants depending on the choice of solvent,4 the propensity of intermolecular interactions among coformers. Studies aimed at solid state interconversion between cocrystals of variable stoichiometries using LAG/NG (neat grinding), keeping one of the components either acid or base as a constant while varying the other, have provided clues for determining the mechanistic pathways in the literature.5 With the components identified as A (acid), B (base), and H2O (water), the effects of the variations on stoichiometric ratios have been investigated with different viewpoints. For case 15a in Scheme 1a, A (dicarboxylic acids) and B (nicotinamide) show reversible interconversion between A1B1 and A1B2 using LAG, with A1B1 being kinetically less stable than A1B2. In case 25b in Scheme 1b, the pamoate/DABCO/H2O system shows a reversible conversion of A1B1·2H2O to A1B2·3H2O resulting in generation of a new polymorphic form that cannot be obtained from solvent evaporation. In case 35c in Scheme 1c, reversible interconversion of A1B1 to A1B2·H2O, the relative © 2014 American Chemical Society

Scheme 1. Stoichiometric Reversible Conversions in Cases 1−3a

a

LAG refers to liquid-assisted grinding in cases 1 and 2, while NG refers to neat grinding in case 3.

stabilities of the individual compounds have been evaluated from quantum mechanical calculations. Even though such mechanochemical experiments have resulted in products that cannot be obtained from solutionReceived: March 3, 2014 Revised: April 1, 2014 Published: April 2, 2014 2614

dx.doi.org/10.1021/cg500308m | Cryst. Growth Des. 2014, 14, 2614−2620

Crystal Growth & Design

Article

based crystallization, the rationale for the facile formation of required stoichiometric variants and more so the mechanism of interconversion among them is hardly understood. In addition, it has always been suggested that the design of cocrystals can be planned in such a way that the functional groups provide a priori information about the necessary synthons6 for building the supramolecular assembly. The concept of “template switching” was proposed in this context,7 and in recent studies, it has been pointed out that finding conserved supramolecular synthons with the same functional groups in a variety of crystal structures led to “synthon modularity”.8 Thus, the cocrystal can be built on the basis of the inherent information in synthon modularity; at the molecular level, interactions can be based on moieties (hydrogen- or halogen-bonded) for differently oriented repeatability to generate new polymorphic forms in a given system. In this article, we have crystallized a series of stoichiometric variant adducts of mono-, di-, and trisubstituted hydroxybenzoic acids (HBAs, component A) with hexamine (Hx, component B) in an effort to link the mechanistic implications of the LAG/NG approach to repeating structural motifs (synthon modularity) occurring in crystal structures. HBAs are bioactive molecules and have been studied in the context of a crystal engineering and pharmaceutical approach.9 HBAs offer the most ubiquitous functional groups, namely, carboxylic acids and phenols, while hexamine provides competing nitrogen atoms on its framework to generate strong O−H···N hydrogen bonds. In particular, trihydroxybenzoic acid (gallic acid) shows various polymorphic forms,10 thus offering a variety of intermolecular interactions for monitoring the stoichiometric variations among cocrystals. Thus, HBAs and hexamine form a model series that can be used to improve our understanding of the formation of variable stoichiometric adducts given the competition between acid and phenol under different conditions. Indeed, in these attempts, the variation of both components A (acid) and B (base) is allowed, unlike in earlier mechanochemical studies.5 Theoretical calculations based on the quantum theory of atoms in molecules (QTAIM)11 provide supporting evidence in terms of interaction energies for evaluating the propensity of formation of stoichiometric variants and their interconversion potential.

Figure 1. Chemical structures of mono-, di-, and trihydroxybenzoic acid (component A) and hexamine (component B), where 3-HBA is 3-hydroxybenzoic acid, 4-HBA 4-hydroxybenzoic acid, 3,4-DHBA 3,4dihydroxybenzoic acid, 3,5-DHBA 3,5-dihydroxybenzoic acid, 3,4,5THBA 3,4,5-trihydroxybenzoic acid, and Hx hexamine.

The structure of 1b (2:1) is monoclinic, in space group C2/c. Proton transfer is activated from the acid moiety to the base, resulting in a salt formation via the N+−H···O− hydrogen bonding motif. A tetrameric unit (hereafter T2) is formed with an O−H(hydroxyl)···O(carboxylate) hydrogen bond, with further stability offered through bridging T2 with carboxylic acid homodimers as shown in Figure 2b. Though crystals of 1a and 1b were obtained from solvent evaporation, all stoichiometric variants could not be obtained by this approach. Mechanochemistry was sought as an alternate approach, including neat grinding as well as solvent grinding. Both neat grinding and LAG of 1:1 acid:base molar ratios resulted in the formation of 1a. However, the other stoichiometric conversions were achieved only by LAG. The addition of 1 mol of 3-HBA along with methanol to 1a forms 1b. 1b reverts to 1a after the addition of 1 mol of Hx with solvent grinding (Figure 3). However, this experiment could not be reproduced with repeated solvent grinding experiments with methanol or with other solvents like tetrahydrofuran (THF), ethanol, and acetone. The resulting powder when recrystallized by solvent evaporation produced the structure of 1c (Figure 2c), which is a hydrated form of 1b. Figure 3 also shows that 1c also produces 1a after the addition of 1 mol of Hx followed by LAG (methanol), i.e., reversible interconversion. Structure 1c (2:1:1) (Figure 2c) displays a positional disorder at the meta position in the phenolic moiety, leading to the masking of tetrameric unit T2 of 1b. Instead, there is a pseudotetramer involving water with the disordered units of phenolic groups. Interestingly, hydrated form 1c cannot be obtained by solvent evaporation, a result that supports the use of mechanochemistry as a means for obtaining unexpected products. 4-HBA-Hx (2a12). The structure of 2a12 forms an infinite one-dimensional chain of alternating acid and base molecules via O−H···N hydrogen bonding (Figure 4). Attempts to obtain stoichiometric variants via liquid-assisted grinding using solvents with different polarities were unsuccessful.

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RESULTS AND DISCUSSION Figure 1 and Table 1 provide a list of compounds used to establish the interconversion among stoichiometric variants of hydroxy-benzoic acids with hexamine. Three stoichiometric variants of 3-hydroxybenzoic acid (3HBA-Hx, hereafter labeled 1a−c), one variant of 4-hydroxybenzoic acid (4-HBA-Hx, hereafter 2a, reported previously12), three variants of 3,4-dihydroxybenzoic acids (3,4 DHBA-Hx, hereafter 3a−c), one variant of 3,5-dihydroxybenzoic acid (3,5DHBA-Hx, hereafter 4a), and two variants of 3,4,5-trihydroxybenzoic acids (3,4,5-THBA-Hx, hereafter 5a and 5b), all with hexamine, have been analyzed. Structural Details and Mechanochemistry. Variants of 3-HBA-Hx (1a−c). The anhydrous cocrystal 1a (1:1) belongs to an orthorhombic system, in space group Pccn. The primary repeat motif in the crystal structure is a tetramer (hereafter T1) composed of two acid and two hexamine molecules hydrogen bonded at both acid and phenolic groups. O−H···N hydrogen bonds (Figure 2a) stabilize the tetrameric units with assistance from weak C−H···O and C−H···N interactions. 2615



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Table 1. Successful Crystallization Experiments (denoted with √, studied here) and Unsuccessful Ones (×)a component A:component B 1 2 3 4 5

3-HBA:Hx 4-HBA:Hx 3,4-DHBA:Hx 3,5-DHBA:Hx 3,4,5-THBA:Hx

1:1 A:B √ √ √ √ √

cocrystal cocrystal cocrystal salt cocrystal

2:1 A:B

2:1:1 A:B:H2O

1:1.5 A:B

1:1:1 A:B:H2O

√ salt × √ cocrystal × ×

√ salt × × × ×

× × √ cocrystal × ×

× × × × √ cocrystal

a

Abbreviations: 3-HBA, 3-hydroxybenzoic acid; 4-HBA, 4-hydroxybenzoic acid; 3,4-DHBA, 3,4-dihydroxybenzoic acid; 3,5-DHBA, 3,5dihydroxybenzoic acid; 3,4,5-THBA, 3,4,5-trihydroxybenzoic acid; Hx, hexamine.

Figure 2. (a) Tetramer (T1) in 1a formed through O−H···N hydrogen bonds of both acid and phenolic functional groups. (b) Formation of acid homodimers between symmetrically dependent acid molecules and interaction of carboxylate moieties with hexamine via N+···H−O− hydrogen bonding in 1b. Tetramer T2 in this structure is highlighted. (c) Masking of synthon T2 (i.e., pseudotetramer) occurs via bridging water molecules in 1c (oval).

Figure 3. Comparison of experimental and simulated powder patterns of the 3-HBA-Hx cocrystal showing reversible solid state interconversions. 1a represents the 1:1 3-HBA-Hx cocrystal; 1a simu is its corresponding simulated pattern produced on the basis of the coordinates from the single crystal. 1b is the 2:1 adduct, while 1b simu is its simulated counterpart. 1c is the hydrated form (2:1:1), and 1c simu is its simulated counterpart.

Stoichiometric Variants of 3,4-DHBA-Hx (3a−c). The 1:1 cocrystal of 3a (space group P21/c, monoclinic system) has two tetrameric assemblies formed between acid and base moieties with N−H···O hydrogen bonds linking the phenolic and acid functional groups. Apart from tetramer T1 found in 1a, tetramer T3 is formed with the participation of the meta- and para-hydroxyl groups of the acid moiety with hexamine (Figure 5, 3a). The 1:1.5 cocrystal (3b) crystallizes in space group C2/c and consists of one molecule of acid and one and one-half molecules of hexamine in an asymmetric unit. The crystal packing displays exclusively tetramer T1 as the main structuredirecting synthon. In addition, O−H···N hydrogen bonds link the T1 units through the symmetrically independent hexamine molecule (Figure 5, 3b). The 3c complex (2:1 cocrystal) is in space group P21/c with tetrameric unit T1 as the basic synthon in the crystal packing. Formation of phenolic homodimers between acid moieties via O−H···O hydrogen bonds generates the supramolecular assembly in the lattice (Figure 5, 3c). LAG experiments using a 3,4-dihydroxybenzoic acidhexamine system provided 3a−c depending on the stoichio-

metric ratios used. Addition of 0.5 mol of the base to 3a followed by grinding with a few drops of MeOH resulted in 3b, which reverts to 3a upon addition of 0.5 mol of acid with LAG (Figure 6). In a separate experiment, 1 mol of the acid component was added to 3a assisted by methanol-based LAG, resulting in the formation of 3c (Figure 6). Addition of 1 mol of the base component proves the reversibility associated with this process by regenerating 3a. It is important to note that reversible stoichiometric variations of both components are observed in the solid state interconversion of the cocrystal for the first time. Variants of 3,5-DHBA-Hx (4a). The 1:1 complex (4a) belongs to a triclinic system, in space group P1̅ with two molecules of acid and two molecules of hexamine in the asymmetric unit. There is proton transfer indicating salt formation in the structure. O−H···O hydrogen bonds formed 2616



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Figure 4. One-dimensional chains formed in 2a through O−H···N hydrogen bonds.

Figure 5. Tetramer T1 is common in structures 3a−c. 3b has an additional tetramer T3.

only between acid molecules; carboxylate−phenolic group interactions expand the structure into a four-component supramolecular assembly (Figure 7). The carboxylate groups of both acid molecules are linked to base molecules via N+− H···O− hydrogen bonding. Once again, attempts to generate other stoichiometric variants via both solution crystallization and LAG/NG were unsuccessful. Variants of 3,4,5-THBA-Hx (5a and 5b). The anhydrous cocrystal (5a) belongs to monoclinic system P21/n with two molecules of gallic acid and two molecules of hexamine in the asymmetric unit. Tetramer T1 dominates the crystal packing involving symmetry-related moieties, color-coded as red, green, blue, and yellow pairs (Figure 8, 5a). The hydrated form 5b (1:1:1) crystallizes in space group P21/c with one molecule each of gallic acid, hexamine, and water in an asymmetric unit. Tetrameric motif T3 along with the pseudotetramer (shown in the blue circle in Figure 8, 5b) involving water packs the molecules in the unit cell. Attempts to generate other variable stoichiometric cocrystals via LAG and/or via solution crystallization were unsuccessful. However, LAG using methanol, THF, or acetone of the 1:1 mixture of the two components results in the formation of 5b. This product is transformed to the anhydrous form (5a) upon

Figure 6. Comparison of experimental and simulated powder patterns of the 3,4-DHBA-Hx cocrystal. 3a represents the 1:1 3,4-DHBA-Hx cocrystal, and 3a simu is its corresponding simulated pattern produced on the basis of the coordinates from the single crystal. 3b is the 1:1.5 cocrystal, while 1b simu is its simulated counterpart. 3c is the 2:1 cocrystal, and 1c simu is its simulated counterpart.

being heated to 85 °C for 5 h (Figure F2 of the Supporting Information) and reverts to 5b after exposure to the atmosphere in ∼8 h (Figure 9). Mechanochemical Interconversion. The reversible interconversion in 3-HBA cocrystals 1a−c involves the stoichiometric variation of A1B1 (1a) to A2B1 (1b) and also to A2B1·H2O (1c) (Scheme 2). The conversion pathways are governed by the interchange of tetramers T1 and T2; however, in 1c, the formation of T2 is masked because of the intervention of water molecules resulting in the formation of a pseudotetramer (Figure 2c). It is noteworthy that both T2 (1b) and the pseudotetramer (1c) convert to T1 (1a) upon 2617



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Figure 7. Tetrameric four-component supramolecular assembly formed in 4a.

Figure 9. Comparison of experimental and simulated powder patterns of 3, 4, 5-THBA/Hx cocrystal: 5a and 5b represent the experimental patterns while 5asimu and 5bsimu represent the simulated patterns.

Scheme 2. Conclusion of the Solid State Interconversion via LAG in the 3-HBA-Hx System

Figure 8. Tetrameric motifs in 5a and the water-incorporated tetrameric assembly 5b crystal structure.

Scheme 3. Conclusion of the Solid State Interconversion via LAG in the 3,4-DHBA-Hx System LAG with 1 mol of Hx. It is of interest to note that upon addition of 1 mol of base (Hx) to 1a LAG does not produce any new product (Scheme 2). The interconversion involving 3a−c, the first example of mechanochemical interconversion in which both components are alternatively changed, occurs with the retention of tetramer T1 during reversible interconversion (Scheme 3). An additional tetramer, T3, appears in the packing of 3a in the crystal lattice. It appears that tetramer T1 is the key synthon, and hence, evaluating the relative strengths of all the tetramers is worthwhile. The QTAIM11 method is employed to establish the nature of interactions in the tetramers and to quantify the interaction energies in terms of the EML approach.13 These calculations (details in the Supporting Information) are performed on tetramers isolated from the neutron-normalized crystalline geometries in the second-order Møller−Plesset perturbation method (MP2)14 with the 6-31G(d,p) basis set using Gaussian 09.15 The pseudotetramer in structure 1c is not included in the calculation because the molecules in this motif are disordered. The interaction energies are −69.6, −65.0, and −50.0 kcal/ mol for T2, T1, and T3, respectively. The bond path diagrams (Figure 10) indicate additional bond critical points (BCPs) corresponding to weak intermolecular C−H···O interactions in the case of T2 that provide additional stability to this motif.

The fact that interconversion occurs with the retention of tetramer T1 indicates that the thermodynamically favored arrangement in this cocrystal system is the crystal form dominated by T1. This factor supports the conversion of 1b to 1a upon LAG after addition of 1 mol of Hx. In addition, the pseudotetramer in 1c also reverts upon LAG after addition of 1 mol of Hx to 1a. It needs to be noted that the conversion of 1a to 1c is more frequent than the conversion to 1b, which clearly supports the observation that the structure of 1b is a kinetically stable form.5a In the case of structures 3a−c, where T1 is conserved in the packing, 3a is the thermodynamically stable system because of the occurrence of an additional tetramer T3 in the packing, adding to the stability. The structures of 4a, 5a, and 5b were not obtained from mechanochemistry and hence are not discussed in this context. However, all structures except 4a can be rationalized in terms of synthon modularity.8 2618



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Figure 11. (a and b) Basic hydrogen-bonded motifs in the crystal structures of 1a and 2a, i.e., tetramer T1 and zigzag chains, respectively, and (c) an amalgam of motifs described above in the 3a cocrystal.

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CONCLUSION In summary, this work supports the importance of mechanochemistry in accessing the cocrystal forms with ease, avoiding concomitance in general, with good quantitative yields. This study shows that this model series (hydroxybenzoic acids with hexamine) can be used to control and screen the different stoichiometric variants more efficiently. The stoichiometric interconversion can be achieved by varying both acid and base components in tandem unlike earlier mechanochemical studies. The interconversions between various forms and the mechanistic pathways upon LAG have been investigated. In particular, the retention of tetramer T1 in these conversions provides a guideline in determining these mechanistic pathways. The mechanism of interconversion between different stoichiometric variants quantified on the basis of the interaction energies associated with tertramer motifs has been rationalized in terms of the observed synthon modularity. Thus, this detailed study of hydroxybenzoic acid-hexamine adducts successfully describes the development of the desired solid form by mechanochemical methods.

Figure 10. Bond critical points between all the strong hydrogen bond interactions and weakly bonded motifs shown in tetramers T1 (a), T2 (b), and T3 (c) (red for oxygen, gray for carbon, orange for hydrogen, and blue for nitrogen).

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Synthon Modularity. It is noteworthy that the packing characteristics of all these structures that are governed by the use of tetramer T1 as a basic building unit led to the evaluation of synthon modularity. The O−H···N hydrogen bond plays a key role in generating the structures either by providing chains like in the crystal structure of 2a12 or by forming tetramers (T1−T3 and pseudo) as in structures 1a−c, 3a−c, and 5a. The structure of 3a (Figure 11) is formed by a combination of the two features, the chain and tetramer T1, with the assistance of the third tetramer, T3. The modularity in this structure is effectively observed with the O−H···N chain motif of structure 2a in combination with tetramer T1 of the structure of 1a. The fact that these structures can also be obtained by solvent evaporation augments the presence of synthon modularity in these structures and provides a possible basis for the smooth mechanochemical conversions observed via LAG. The presence of tetramers T1 in 5a and T3 in 5b provides evidence of synthon modularity. However, mechanochemical conversions in 5a and 5b are not observed by NG/LAG because of the presence of water molecules and its enhanced affinity for gallic acid.16

ASSOCIATED CONTENT

S Supporting Information *

Experimental section, crystallographic tables, interaction tables, and thermal analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India. E-mail: [email protected]. ernet.in. Fax: +91-080-23601310. Telephone: +91-08022932796. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.K. thanks IISc for a fellowship, Mr. Pavan for help with AIM calculations, and Dr. Suryanarayan for useful discussions. T.N.G.R. thanks DST for the award of a J. C. Bose fellowship. We thank the DST, India, for the funding under DST-FIST 2619



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(Level II) for the X-ray diffraction facility at SSCU (IISc, Bangalore, India).

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