Structural Insights into Proton Conduction in Gallic Acid–Isoniazid

Communication pubs.acs.org/crystal

Structural Insights into Proton Conduction in Gallic Acid−Isoniazid Cocrystals Ramanpreet Kaur, S. S. R. R. Perumal, Aninda J. Bhattacharyya, S. Yashonath, and T. N. Guru Row* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India S Supporting Information *

ABSTRACT: Hydrated cocrystal of gallic acid−isoniazid displays a single crystal-to-single crystal transformation upon dehydration, resulting in a difference of three orders of magnitude in proton conduction. The conduction pathway is shown to follow the Grotthus mechanism, supported by theoretical (DFT) calculations.

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roton-conducting materials like Nafion and Nafion-based polymers are extensively used in fuel cell technology.1 The search for less expensive materials involving not too complex synthesis has resulted in materials based on polymeric, inorganic, and inorganic−organic hybrid compounds.2 Metal organic frameworks (MOFs) and channel cocrystals with encapsulated solvents (hydrates) are of special interest as their proton conductivity3 values are in range of 10−3 to 10−7 Ω−1 cm−1. Organic cocrystals/salts have attracted growing interest in pharmaceutical industry and in molecular electronics4 since past few years. The ease of synthesis makes cocrystal based materials desirable and the possibility of tuning structural architectures using the principles of crystal engineering is advantageous.5 Nevertheless, they have not been extensively explored as potential candidates for fuel cell technology. Several recent articles6,7 focus on obtaining higher proton conductivity and discuss the structural consequences due to hydration. Interestingly, an exceptionally large value of 5.5 Ω−1 cm−1 was reported for the crystalline nanoassembly of hydrated trimesic acid/melamine6,8 system. However, it was shown to be superfluous as it exceeds the predicted limit of proton conductivity.9,10 Recent studies on a cocrystal of isoniazid: 4-aminosalicylic acid cocrystal11 from variable temperature X-ray diffraction studies show that the protons become labile with increasing temperature. A study based on both hydrated and dehydrated samples of gallic acid: isoniazid system would enable us to address the proton conductivity and to authenticate the role of water in terms of the structural aspects. In addition, we have also examined the possibility of Grotthus mechanism operating through water molecules in this system, supported by preliminary first-principle Density Functional Theory (DFT) calculations. The two forms of gallic acid/isoniazid system were formed by slow evaporation of 1:1 molar mixture of the two components in methanol/water and ethanol/water solution. The crystals were obtained concomitantly (Figure 1) as yellow blocks and colorless needles, former being the hydrated form (GINZH, hereafter) and the latter as anhydrous form (hereafter, GINZA) of the system as identified by X-ray diffraction studies. Several attempts to resolve the concomitant © 2014 American Chemical Society

Figure 1. Concomitant growth of GINZH and GINZA showing morphological differences between two forms (image is at 50× resolution).

Figure 2. (a) Structure of GINZH showing intermediate proton position between gallic acid and isoniazid and the hydrogen bonding pattern with water. (b) Arrangement of water molecules along crystallographic b-axis (screw axis) in GINZH. (c) 2D-layered arrangement in GINZA showing the hydrogen bonding pattern in the crystal structure and pseudo 21 axis along crystallographic bdirection (dotted line).

Received: December 17, 2013 Revised: January 13, 2014 Published: January 14, 2014 423

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Figure 3. SCSC transformation: bright yellow GINZH crystal (left) at 25 °C transforms to white opaque crystal of GINZA (right) at 150 °C.

growth (involving variations in solvent polarity, molar ratios and variable temperature crystallization) failed and hence the two forms were separated manually on the basis of morphology. GINZH crystallizes in a monoclinic system, space group P21 with two sets of gallic acid: isoniazid: water in an asymmetric unit, thereby resulting in the ratio 2:2:2. It is interesting to note that both independent sets represent a partial proton transfer from acid to base moieties at room temperature while at 100 K a complete proton transfer is indicated resulting in the formation of a salt (Supporting Information; F3, F4). At room temperature, the hydrogen bonding parameters are N−H = 1.272 Å, O−H = 1.293 Å, and N···O = 2.564 Å) for one set while N−H = 1.292 Å, O−H = 1.265 Å, and N···O = 2.551 Å for the other, indicating partial proton transfer. The structure is stabilized via O−H···N and O−H···O hydrogen bonds connecting para-hydroxyl group of gallic acid to the isoniazid moiety and to the water molecule respectively (Figure 2a). A π−π interaction between the acid and isoniazid molecules provides additional stability to the molecular assembly (Supporting Information; F5). The water molecules are arranged along the 21 screw axes (Figure 2b) (the longest

Figure 5. Proposed pathway showing step by step proton migration in GINZH.

axis in the unit cell) while bridging the two components of the cocrystal. Thermal analysis and hot stage microscopy (Supporting Information; F9, F10) suggest that GINZH converts to GINZA on heating to around 140 °C. GINZH converts to GINZA (anhydrous form) on heating to 150 °C as a result of an irreversible single crystal-to-single crystal (SCSC) transformation which was monitored in situ on the diffractrometer. The color of the crystal is observed to change from transparent yellow to opaque (Figure 3). It is of interest to note that there is a change of 5.5% in volume upon dehydration with the cell dimensions reducing to half along the b-axis while doubling the one of the axes (Supporting Information; T1) in the space group P1̅. Since the water molecules are arranged along the 21 screw axis in GINZH (Figure 1b), the removal of water results in the component

Figure 4. (a) Nyquist plot for proton conductivity at 307 K in 98% humidity for GINZH. (b) Proton conductivity plots of GINZH at variable temperature. (c) Arrhenius plot of activation energy for GINZH showing activation energy of 0.2 eV. (d) The Nyquist plot for proton conductivity at 308 K in 98% humidity for GINZA. 424

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DFT results indicate that this initial movement is almost barrier less (step 1, Figure 6). Once the proton hops to the gallic acid moiety forming a −COOH, the next step involves transfer of the proton to the meta-hydroxyl group of the adjacent gallic acid moiety in the crystal packing (step 2, Figure 5), which is also nearly barrier less (Figure 6).Since the meta-hydroxyl group cannot have two protons, it becomes facile to transfer one of them to the water molecule resulting in the formation of a hydronium ion (step 3, Figure 5). This transfer involves 17.4 kcal/mol (Figure 6), which is well within the estimated limits required in proton conduction.15 Further, the proton transport proceeds toward the carbonyl group of isoniazid in the intermolecular region with an energy requirement of 3.95 kcal/mol (step 4, Figures 5 and 6). The proton transfer proceeds through a tautomeric transformation in an intramolecular fashion followed by the transfer from the hydrazide moiety (step 5, Figure 5,6) with an additional expense of 14.11 kcal/mol (well within the limits for a N−H···O intramolecular transfer2b,16) to migrate to the carboxylic end of the gallic acid. Once step 1 is initiated, transport of proton proceeds through the lattice by repetition of steps 2−5. It is to be noted that in the DFT calculations the electronic structure energy for the hopping mechanism is principally a function of the coordinate of the proton. The energy profiles were computed along the shortest path connecting the two moieties. In the crystal environment the influence of other degrees of freedom including angular distortions in the hydrogen bonded motifs are stronger and they might assist the proton hop thereby enhancing the conductivity values. It must be mentioned that the DFT calculations are at a preliminary level to obtain guidelines and specific correlation effects including dispersion have not been included. In conclusion, the analysis of the two forms in gallic acid/ isoniazid system allows for the rationalization of the observed higher proton conductivity in the hydrated form as compared to anhydrous form. The conduction pathway explained on the basis of Grotthus type mechanism clearly indicates the role of water molecules in facilitating the long-range proton hopping in molecular cocrystal.

Figure 6. Calculated proton pathway barriers in the proposed mechanism: steps 1 and 2, no barrier; steps 3−5,: 17.4, 3.95, and 14.11 kcal/mol, respectively (see details in Supporting Information).

molecules competing to form hydrogen bonded networks within themselves leading to the loss of the 21 symmetry. Indeed there is a pseudo-21 axis in the observed packing (Figure 2c) in GINZA with two gallic acid and two isoniazid molecules in the asymmetric unit. N−H···O hetero dimers formed between the carboxylic group of gallic acid and pyridine nitrogen of the isoniazid moiety dominate the packing. The meta-hydroxyl groups of gallic acid interact with hydrazide moiety and another acid molecule via O−H···N and O−H···O interactions resulting in a two-dimensional layered packing (Supporting Information; F6, F7, F8). It may be that the hydrogen bonding potential dominates the packing leading to the change in the space group from P21 to P1̅. The phase purity of both the GINZH and GINZA was confirmed by powder X-ray diffraction experiments followed by profile fitting refinements before performing measurements (Supporting Information; F11, F12). Ideally, the impedance measurements to evaluate the anisotropy in conductive behavior of the system should have been carried out on a single crystal; however large sized crystals could not be obtained for the measurements. Hence, impedance spectroscopy on pressed powders of GINZH and GINZA in the form of pellets were performed. These Complex impedance measurements were done in the frequency window of 1 Hz−1 MHz between 298 and 318 K, which show significant differences in proton conductivity values (Figure 4). The value of the conductivity for GINZH is 1.1 × 10−4 Ω−1 cm−1 at 307 K, while the corresponding value for GINZA is 4.2 × 10−7 Ω−1 cm−1 respectively (98% relative humidity). It is observed that the conductivity of the GINZH starts decreasing after 307 K, similar to the behavior exhibited by MOFs.12 The computed activation energy of GINZH is 0.2 eV (Figure 4c) suggests that the behavior follows the Grotthus mechanism.13 We propose a pathway for the proton migration based on the structural features while providing a theoretical basis for this pathway from DFT calculations performed at B3LYP functional with 6-311G++ (d,p) basis set using the Gaussian suite of programs.14 The hydrogen bonding network in the structure of GINZH which incorporates the water of crystallization provides the route for the transport of the proton. Indeed, the movement of the proton associated with the salt to cocrystal regime with temperature, initiates the conduction mechanism (Figure 5).

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic table, interaction table, thermal and HSM data, PXRD patterns and details of crystal structure, figure of variable temperature distances in crystal structure GINZH and Fourier difference maps for proton location and theoretical calculations.This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-080-23601310. Tel: +91-080-22932796. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.P.K. thanks IISc for a fellowship and Mr. Somnath Pal (SSCU, IISc) for useful discussions on conductivity experiments. T.N.G. thanks DST for the award of a J. C. Bose fellowship. We thank the DST, India, for the funding under 425

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Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (15) Scheiner, S. Acc. Chem. Res. 1985, 18, 174−180. (16) Scheiner, S. J. Phys. Chem. A. 2000, 104, 5898−5909.

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

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