Salts of purine alkaloids caffeine and theobromine with 2,6-di­hy­droxy­benzoic acid as coformer: structural, theoretical, thermal and spectroscopic studies

The study of various forms of pharmaceutical substances with specific physico­chemical properties suitable for putting them on the market is one of the elements of research in the pharmaceutical industry. A large proportion of active pharmaceutical ingredients (APIs) occur in the salt form. The use of an acidic coformer with a given structure and a suitable pK_a value towards purine alkaloids containing a basic imidazole N atom can lead to salt formation. In this work, 2,6-di­hydroxy­benzoic acid (26DHBA) was used for cocrystallization of theobromine (TBR) and caffeine (CAF). Two novel salts, namely, theobrominium 2,6-di­hydroxy­benzoate, C₇H₉N₄O₂⁺·C₇H₅O₄⁻ (I), and caffeinium 2,6-di­hydroxy­benzoate, C₈H₁₁N₄O₂⁺·C₇H₅O₄⁻ (II), were synthesized. Both salts were obtained independently by slow evaporation from solution, by neat grinding and also by microwave-assisted slurry cocrystallization. Powder X-ray diffraction measurements proved the formation of the new substances. Single-crystal X-ray diffraction studies confirmed proton transfer between the given alkaloid and 26DHBA, and the formation of N—H⋯O hydrogen bonds in both I and II. Unlike the caffeine cations in II, the theobromine cations in I are paired by noncovalent N—H⋯O=C inter­actions and a cyclic array is observed. As expected, the two hy­droxy groups in the 26DHBA anion in both salts are involved in two intra­molecular O—H⋯O hydrogen bonds. C—H⋯O and π–π inter­actions further stabilize the crystal structures of both com­pounds. Steady-state UV–Vis spectroscopy showed changes in the water solubility of xanthines after ionizable com­plex formation. The obtained salts I and II were also characterized by theoretical calculations, Fourier-transform IR spectroscopy (FT–IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and elemental analysis.

1. Introduction

Research into different crystalline forms of active pharmaceutical ingredients (APIs) is one of the inter­ests of chemists and engineers in the pharmaceutical industry. Each form of a given drug, such as polymorphs, solvates, hydrates, salts, cocrystals, amorphous forms, etc., have different physico­chemical properties due to the mol­ecular arrangement in the solid state (i.e. stability, bioavailability, tabletability, permeability, mechanical properties and dissolution rate) (Carstens et al., 2020). The preferred form of a substance is the crystalline form with the most stable arrangement of molecules (Ghadi et al., 2014). In turn, amorphous substances have limited pharmaceutical use despite their proven better solubility com­pared to crystalline forms, due to their poor stability and the possibility of transformation into other phases (Hancock & Parks, 2000; Yu, 2001; Banerjee & Brettmann, 2020).

The search for the appropriate form of a drug is often difficult and time-consuming because, on the one hand, its pharmacological action must be maintained or improved and, on the other hand, its physico­chemical properties, which have a direct impact on its activity, should be improved (Kumar & Nanda, 2018; Yadav et al., 2009). Many methods are known to improve the physico­chemical properties of an API, such as particle size reduction (Fang et al., 2020), synthesis of solid dispersion (Nair et al., 2020; Sareen et al., 2012), nanoparticles (Kumar et al., 2020), self-emulsifying drug delivery (SEEDS) (Pehlivanov, 2020), nanosuspension (Stefan et al., 2020) or advanced lipid technologies (ALT) (Lopez-Toledano et al., 2019). Another method is to introduce guest mol­ecules into the crystal structure of an API, which is still a very popular method (Dai et al., 2018). This gives an opportunity to create various types of two- or multi-com­ponent systems. One of these systems involves mol­ecular com­plex formation, in which the mol­ecules of both an API and a guest (coformer) are in the neutral form. When both of these substances and the com­plex which they form are in the solid form, under normal conditions, we can obtain a cocrystal (Byrn et al., 2017). There is also the possibility of proton transfer between an API and coformer, leading to salt formation when certain conditions are met, such as a suitable ΔpK_a value or the structure of the com­pound (type and position of functional groups). The formation of com­plexes of this type (multi-com­ponent crystal) in com­parison to pure substances (single-com­ponent crystal) most often leads to changes in physico­chemical pro­perties due to rearrangement of the mol­ecules in the crystal lattice (Schultheiss & Newman, 2009).

The use of acidic coformers with a given pK_a value for purine alkaloids containing an imidazole basic N atom may lead to the formation of an ionizable com­plex. In this article, 2,6-di­hydroxy­benzoic acid (26DHBA), the strongest of the di­hydroxy­benzoic acids, was used for cocrystallization with theobromine (TBR) and caffeine (CAF) (Fig. 1). The crystal structures of the title salts were determined by single-crystal X-ray diffraction. The powders were also characterized by powder X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, UV–Vis and Fourier-transform IR spectroscopy, and elemental analysis. Theoretical studies were also performed for the title salts.

Figure 1 The structures of 2,6-di­hydroxy­benzoic acid and the alkaloids used for cocrystallization.

2. Experimental

2.1. Materials

Theobromine (TBR) and caffeine (CAF) were purchased from Swiss Herbal and Coffeine Shop, respectively. 2,6-Di­hydroxy­benzoic acid (26DHBA) was obtained from TriMen Chemicals. All substances were used without prior purification. Millipore distilled water (18 MΩ) was used in all UV–Vis analyses.

2.2. Synthesis and crystallization

2.2.1. Cocrystallization from solution

Theobromine (13.3 mg, 0.074 mmol) and 2,6-di­hydroxy­benzoic acid (11.5 mg, 0.075 mmol) were dissolved in an iso­propanol–water solution by heating, and single crystals of I were obtained by slow evaporation. Caffeine (27.5 mg, 0.14 mmol) and 2,6-di­hydroxy­benzoic acid (21.7 mg, 0.14 mmol) formed II by slow evaporation from an aceto­nitrile solution.

2.2.2. Cocrystallization by grinding

Neat grinding experiments were performed using an oscillatory ball mill Retsch MM300. TBR (8.8 mg, 48.8 µmol) with 26DHBA (7.5 mg, 48.7 µmol) and CAF (10.8 mg, 55.6 µmol) with 26DHBA (8.4 mg, 54.5 µmol) were ground with a 6.35 mm stainless steel ball. Each experiment lasted 30 min at a frequency of 25 Hz.

2.2.3. Microwave-assisted slurry cocrystallization

The Discover LabMate reactor was used as a microwave irradiation source for cocrystallization experiments. The alkaloid and the acid in stoichiometric ratios were placed in glass vials together with a magnetic stirrer bar, and then a specific volume of the appropriate solvent was added. Water, methanol, aceto­nitrile and ethyl acetate were used as solvents. The amounts of the substrates and solvents used in the microwave-assisted cocrystallizations are given in Tables S1 and S2 (see supporting information). The experiments with the slurries containing theobromine and 2,6-di­hydroxy­benzoic acid were carried out at 60 °C for 5 min with continuous stirring at 300 rpm. In turn, the experiments with samples con­taining caffeine were carried out at 80 °C for 3 min with a stirring speed at 300 rpm. The microwave potency needed to reach the desired temperature and maintain it during the reaction for a certain time was adjusted automatically by the apparatus. The solids obtained were dried under ambient conditions and characterized by powder X-ray diffraction (PXRD).

2.3. Refinement

Crystal data, data collection and structure refinement details for (TBR-H)⁺·(26DHBA)⁻ (I) and (CAF-H)⁺·(26DHBA)⁻ (II) are summarized in Table 1. H atoms were located in a difference Fourier map and refined with isotropic displacement parameters. In the final refinement model, chosen distances were restrained in both structures.

Table 1
Experimental details

For both structures: Z = 4. Experiments were carried out with Cu Kα radiation using a Rigaku OD SuperNova Single source diffractometer with an Atlas detector. Absorption was corrected by multi-scan methods (CrysAlis PRO; Rigaku OD, 2019). All H-atom parameters were refined.

	I	II
Crystal data
Chemical formula	C7H9N4O2^+·C7H5O4^−	C8H11N4O2^+·C7H5O4^−
Mr	334.29	348.32
Crystal system, space group	Monoclinic, P21/c	Monoclinic, P21/n
Temperature (K)	132	131
a, b, c (Å)	6.9579 (3), 16.5845 (6), 12.4718 (5)	14.8560 (3), 6.95591 (11), 15.8927 (3)
β (°)	95.691 (4)	115.413 (3)
V (Å^3)	1432.06 (10)	1483.39 (6)
μ (mm^−1)	1.06	1.05
Crystal size (mm)	0.34 × 0.24 × 0.08	0.3 × 0.17 × 0.12
Data collection
Tmin, Tmax	0.649, 1.000	0.773, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	12055, 2970, 2782	11899, 3076, 2815
Rint	0.023	0.025
(sin θ/λ)max (Å^−1)	0.631	0.630
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.055, 0.147, 1.11	0.034, 0.096, 1.05
No. of reflections	2970	3076
No. of parameters	273	290
No. of restraints	2	1
Δρmax, Δρmin (e Å^−3)	0.51, −0.24	0.28, −0.18
Computer programs: CrysAlis PRO (Rigaku OD, 2019), SHELXT2018 (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

2.4. Powder X-ray diffraction (PXRD)

Low-temperature powder experiments using an Oxford Diffraction SuperNova diffractometer with a Cu Kα radiation source for samples from a solvent, neat grinding and microwave-assisted slurry cocrystallizations were performed. CrysAlis PRO was used for data collection (Rigaku OD, 2019). Experimental conditions: scanning inter­vals = 0–50° (2Θ), time per step = 0.5 s and step size = 0.01° θ. Theoretical powder patterns were determined using Mercury (Macrae et al., 2020). Parameters used for simulation: step size = 0.01° θ and width at half height (FWHM) = 0.2. The Kdif software (http://kdiff3.sourceforge.net/) was used for analysis of the powder patterns.

2.5. Vibrational spectroscopy (FT–IR)

FT–IR spectra were recorded in KBr pellets in the range 4000-400 cm⁻¹ using a Bruker IFS 66v/S instrument, with a resolution of 2 cm⁻¹. Each spectrum was accumulated by the acquisition of 64 scans.

2.6. Theoretical calculations

Density functional theory (DFT) calculations were per­formed using the GAUSSIAN16 program package (Frisch et al., 2016). The APF-D hybrid DFT method including dispersion (Austin et al., 2012) and the 6-311++G(d,p) basis set (Wiberg, 1986) were employed to obtain the optimized geometry and vibrational wavenumbers. The APF-D method was chosen as one of the best for determining the mol­ecular geometry of organic mol­ecules, hydrogen-bond inter­actions and IR spectra, and it had the best com­promise between accuracy and com­putational cost (Foresman & Frisch, 2015). The X-ray geometries of I and II were used as starting points for the calculations. All calculated IR frequencies were real, which confirmed that the optimized structures corresponded to a local energy minimum. The potential energy distribution (PED) of the vibrational modes was established using the VEDA 4 program (Jamróz, 2004, 2013). Only contributions to PED greater than 10% were considered.

2.7. Thermal analysis

The thermal stability of the described salts was investigated with a TG 209 F3 Tarsus thermogravimetric analyzer (NETZSCH–Geratebau GmbH, Norderstedt, Germany) in the temperature range from 30 to 600 °C. About 10 mg of sample was placed in a platinum crucible and analyzed with a 10 °C min⁻¹ heating rate under a nitro­gen atmosphere (purge of 10 ml  min⁻¹ of N₂ protection gas and 20 ml min⁻¹ of N₂ sample gas). Melting temperatures (T_m) were measured by DSC (Mettler–Toledo DSC1 instrument, Greifensee, Switzerland) under a nitro­gen atmosphere with a heating rate of 10 °C min⁻¹ in the temperature range from 30 to 220 °C and were determined from the transition peaks.

2.8. Steady-state absorption spectroscopy

The concentrations of (TBR-H)⁺·(26DHBA)⁻ (I) and (CAF-H)⁺·(26DHBA)⁻ (II) in aqueous samples were determined via spectrophotometric assays using an Agilent 8453 UV–Vis spectrophotometer with a scanning range between 200 and 1000 nm, and 1 nm increments. The investigation of the solubility of the salts was performed using UV–Vis spectrophotometric assays. A series of standard solutions of I and II in distilled water at concentrations of 0.0025, 0.005, 0.01, 0.015 and 0.02 mg ml⁻¹ were prepared and used for the preparation of the calibration curves. The recorded UV–Vis spectra exhibited maxima for I and II at 274 nm. The prepared calibration curves were subsequently used for the calculation of the solubility of the salts by measuring saturated solution absorbance. Saturated samples of I and II were prepared by mixing 15 mg of the alkaloid derivative with 3 ml of distilled water at room temperature for 1 h, using an ultrasound bath. Saturation of the solutions was indicated by the presence of undissolved material. The spectrophotometric measurements were made in triplicate.

3. Results and discussion

3.1. Crystal structure design – Cambridge Structural Database (CSD) analysis

In the initial stage of designing the crystal structure of the two title substances, it was significant to know the forms in which they occur (neutral or ionized), the formation of possible supra­molecular synthons and which of them are preferred. It was difficult to unequivocally predict the nature of the product based only on the determined ΔpK_a values for the discussed systems (Table 2), as they are in the range −1 to 4 (Cruz-Cabeza, 2012). However, much information can be gained by an analysis of solids containing substances that have been deposited in the CSD (Groom et al., 2016) so far, which will be used for the cocrystal synthesis.

Table 2
Calculated ΔpK_a values for the title salts

The 2,6-di­hydroxy­benzoic acid pK_a value of 1.29 was taken into account for the ΔpK_a calculation (Papadopoulos & Avranas, 1991).

Alkaloid	pKa(protonated base)	ΔpKa (Cruz-Cabeza, 2012) = pKa(protonated base) − pKa(acid)
TBR (theobromine)	0.12 (Pereira et al., 2016)	−1.17
CAF (caffeine)	0.6 (Pereira et al., 2016)	−0.69

2,6-Di­hydroxy­benzoic acid is the strongest of the di­hydroxy­benzoic acids, due to the formation of two inter­molecular hydrogen bonds in its structure, with the participation of the hy­droxy groups and the carboxyl group (pK_a = 1.29) (Papadopoulos & Avranas, 1991). In the crystal structure of the pure acid, we observed either 2,6-di­hydroxy­benzoic acid mol­ecules in which the carboxyl groups adopt the syn (motif C, monoclinic polymorph; Gdaniec et al., 1994) or anti [motif D, ortho­rhom­bic polymorph (MacGillivray & Zaworotko, 1994) and monohydrate (Gdaniec et al., 1994)] conformations (Fig. 2). In cocrystals of this acid, the carboxyl group more often adopts the syn (9 out of 10 entries) than the anti conformation (Table S4 in the supporting information) (Bruno et al., 2002). This acid, however, is more likely to form salts than cocrystals. Of 50 deposited acid–base systems, 44 structures display proton transfer from 2,6-di­hydroxy­benzoic acid.

Figure 2 The observed motifs for the intra­molecular hydrogen bonds in the 2,6-di­hydroxy­benzoate anion (motifs A and B) and 2,6-di­hydroxy­benzoic acid (motif C is the syn-COOH conformation and motif D is the anti-COOH conformation) (D'Ascenzo & Auffinger, 2015).

In the two deposited structures where 2,6-di­hydroxy­benzoic acid is in the anionic form, only one intra­molecular hydrogen bond with the participation of one hy­droxy group was formed, while the proton from the second hy­droxy group does not form any strong contact (motif B). However, the formation of two intra­molecular noncovalent inter­actions with the participation of both hy­droxy groups is much more preferable, as can be seen in 43 of 44 structures containing the 2,6-di­hydroxy­benzoate anion and in all structures where this acid is in the neutral form (Tables S3 and S4, respectively, in the supporting information). This observation is consistent with one of the Etter rules of the preferential formation of intra­molecular hydrogen bonds over inter­molecular inter­actions in six-membered rings (Etter, 1990). The hy­droxy O atoms can also be proton acceptors if additional groups, such as nitro­gen (e.g. –NH₃⁺, primary or secondary amine group) or oxygen donors (e.g. –COOH or –OH group, or H₂O mol­ecule), appear in their vicinity. In 22 entries containing the 2,6-di­hydroxy­benzoate anion, at least one hy­droxy group acts as a proton acceptor and is involved in the formation of an inter­molecular hydrogen bond (details are presented in Table S3 of the supporting information).

Caffeine has only good proton acceptors (carbonyl groups and an imidazole N atom) but does not have good donors. Theobromine, unlike caffeine, has one good donor at the pyrimidine ring. It can participate in the formation of strong amide–amide dimers, which are observed in many theobromine–carb­oxy­lic acid systems (Figs. 4a and 4b) (Groom et al., 2016). The imidazole N atom of purine alkaloids most often inter­acts noncovalently with the carboxyl group of an acid. In several structures containing theobromine and a carb­oxy­lic acid derivative, formation of the amide–carb­oxy­lic acid synthon was observed (Figs. 3a and 3b). However, proton migration from the carboxyl group to the imidazole N atom is expected (Fig. 4c), so we do not take into account the possibility of the formation of an amide–carb­oxy­lic acid heterosynthon in the theobromine–2,6-di­hydroxy­benzoic acid system.

Figure 3 The supra­molecular carb­oxy­lic acid–amide heterosynthons observed in theobromine–carb­oxy­lic acid systems. Synthons A [observed in structures with CSD refcodes UKOLAK (Gołdyn et al., 2020) and CSATBR (Shefter et al., 1971)] and B [HIJYEF (Karki et al., 2007), MUPPET (Clarke et al., 2010) and ZOYBOG (Jacobs & Amombo Noa, 2015)] represent synthons with the participation of the exo- and endo-carbonyl O atom, respectively.

Figure 4 The predicted supra­molecular synthons with theobromine (A, B and C) and caffeine (C). Synthons A and B represent amide–amide homosynthons with the participation of the exo- and endo-carbonyl O atom, respectively.

Based on the above considerations, it can be concluded with a high degree of probability that caffeine with 2,6-di­hydroxy­benzoic acid will form an ionized binary system maintained by N—H⋯O⁻ hydrogen bonding. In the crystal lattice of the theobromine–acid com­plex, it is envisaged that an imidazole–carb­oxy­lic acid motif (Fig. 4c) and the amide–amide homosynthon between two xanthine units will be observed. There are two possibilities for the formation of this dimer, i.e. with the exo or the endo carbonyl group of the pyrimidine ring of the alkaloid (Figs. 4a and 4b, respectively). The CSD analysis indicates the preference for the formation of the amide–amide synthon with the participation of the exo-carbonyl group (Groom et al., 2016).

3.2. Synthetic approach

Cocrystallizations with theobromine and caffeine using 2,6-di­hydroxy­benzoic acid as coformer were performed. Theobromine with this acid can form a salt hydrate or a salt depending on the conditions of cocrystallization from solution or by milling. It has been shown that the monohydrate form (TBR-H)⁺·(26DHBA)⁻·H₂O was obtained by slow evaporation from an aceto­nitrile–water solution and by water-assisted grinding (Gołdyn et al., 2019). In turn, cocrystallization by slow evaporation from an iso­propanol–water solution leads to the anhydrous form (I). Caffeine with 2,6-di­hydroxy­benzoic acid forms only the salt (II). The described salt was obtained by cocrystallization in aceto­nitrile solution. A com­parison of the powder patterns shows the possibility of the formation of both discussed com­pounds via grinding under solvent-free conditions (Figs. 5 and 6).

Figure 5 Comparison of the powder diffractograms for (TBR-H)^+·(26DHBA)^−. The theoretical powder pattern (black line) and the powder patterns of the materials from grinding (green line) and solution cocrystallization (red line) are presented.

Figure 6 Comparison of the powder diffractograms for (CAF-H)^+·(26DHBA)^−. The theoretical powder pattern (black line) and the powder patterns of the materials from grinding (green line) and solution cocrystallization (red line) are presented.

The use of microwave irradiation seems to be a promising method for obtaining cocrystals or salts. Both of the discussed com­pounds were obtained by microwave-assisted slurry cocrystallization. Solvents such as water, methanol, aceto­nitrile and ethyl acetate were used for these experiments. The powder diffraction patterns for the solids obtained by the synthesis using the above method were com­pared (Figs. 7 and 8). The microwave cocrystallization of caffeine and 2,6-di­hydroxy­benzoic acid in each case resulted in the formation of II. The use of water and aceto­nitrile to cocrystallize theobromine with this acid resulted in the desired product I. In turn, when methanol and ethyl acetate were used, peaks not only from salt I, but also from theobromine, were observed, which indicates incom­plete conversion of the substrates. In these cases, the reaction should probably be carried out longer or at a higher temperature.

Figure 7 Compilation of the powder diffractograms for (TBR-H)^+·(26DHBA)^−. The theoretical powder pattern and the powder patterns of the materials from microwave-assisted cocrystallization are presented. The powder diffraction patterns of the substrates are also included.

Figure 8 Compilation of the powder diffractograms for (CAF-H)^+·(26DHBA)^−. The theoretical powder pattern and the powder patterns of the materials from microwave-assisted cocrystallization are presented. The powder diffraction patterns of the substrates are also included.

3.3. Structural analysis

Single-crystal X-ray diffraction measurements allowed the determination of the ionic nature of the title substances. The location of the proton in a difference Fourier map is the simplest determinant of the nature of the analyzed com­plex. However, we should also pay attention to the geometry of the relevant functional groups, since we are dealing with a pair of acid–base com­pounds. There is a significant difference in the C—O bond lengths in the carboxyl moiety, as opposed to the carboxyl­ate anion, in which these values are similar.

In theobromine salt I, the C=O [C7—O1 = 1.241 (3) Å] and C—O [C7—O2 = 1.297 (3) Å] bond lengths suggest the presence of a carboxyl­ate group. A similar situation occurs in caffeine salt II, where the difference in the C—O bond lengths in the carboxyl group of 2,6-di­hydroxy­benzoic acid is 0.041 Å [C7—O1 = 1.2530 (16) Å and C7—O2 = 1.2940 (15) Å]. Inter­estingly, such significant differences result from the different number of strong hydrogen bonds formed by the carboxylate O atoms and the nature of the donors (Chumakov et al., 2006). In I and II, each of the carboxylate O atoms in the 2,6-dihydroxybenzoate anion forms strong intramolecular O—H⋯O hydrogen bonds, while one of them additionally forms a strong hydrogen bond with the imidazole N atom. The formation of the ionic com­plex is the result of proton migration from the acid group to the imidazole N atom, which is a basic group. There is then a slight but distinct change in the geometry of the imidazole ring. Changes in the values of the valence angles in this ring can confirm the presence of the cationic form of the alkaloid mol­ecule in the crystal lattice (Fig. 9). The list of values of the α, β and γ valence angles in the discussed com­plexes, together with those already obtained for theobromine and caffeine com­plexes with hy­droxy­benzoic acids, are presented in Tables S6 and S7, respectively, in the supporting information.

Figure 9 Changes in the values of the valence angles in the alkaloid imidazole ring as a result of proton transfer from the acid mol­ecule to the imidazole N atom. It has been shown that the values of the α and γ valence angles are greater in the protonated forms (α1 < α2 and γ1 < γ2), while the value of the β valence angle decreases (β1> β2) (Gołdyn et al., 2021).

3.3.1. Theobromine with 2,6-di­hydroxy­benzoic acid

Theo­bromine and 2,6-di­hydroxy­benzoic acid cocrystallize as a salt, I, in a 1:1 stoichiometric ratio in the monoclinic space group P2₁/c (Fig. 10a). In the crystal structure, finite four-com­ponent systems formed by two alkaloid and two acid mol­ecules are observed (Fig. 10b). The hy­droxy groups in the 2,6-di­hydroxy­benzoate anions are involved in intra­molecular O—H⋯O⁻ hydrogen bonds and two S(6) systems are formed (Table 3). As predicted, the acid–alkaloid motif is maintained through charge-assisted N—H⋯O⁻ hydrogen bonds (Fig. 4c). Each pair of theobromine cations forms a centrosymmetric R₂²(8) homodimer via N—H⋯O inter­actions with the participation of the endo-O atom (Fig. 4b). The above supra­molecular motifs are consistent with earlier assumptions, however, a less expected amide–amide synthon is observed (Figs. 4b and 10b). Weak C—H⋯O hydrogen bonds and π(TBR)⋯π(26DHBA) inter­actions (Table S8 in the supporting information) are responsible for stabilization of the three-dimensional structure (Fig. 10c).

Table 3
Experimental and com­puted hydrogen-bond parameters (Å, °) in the TBR·26DHBA (I and Ia) and CAF·26DHBA (II and IIa) systems

Alkaloid·26DHBA	D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
Experimental I	O3—H3⋯O2	0.975 (10)	1.632 (16)	2.558 (2)	157 (3)
(TBR-H)^+·(26DHBA)^−	O4—H4A⋯O1	1.00 (4)	1.62 (4)	2.550 (2)	153 (3)
	N1—H1⋯O6^i	0.85 (3)	2.04 (3)	2.881 (2)	169 (3)
	N4—H4⋯O2	0.94 (4)	1.62 (4)	2.557 (2)	169 (4)
Calculated Ia	O3—H3⋯O2	0.972	1.727	2.579	144
TBR·26DHBA	O4—H4A⋯O1	0.984	1.670	2.559	148
	O2—H4⋯N4	1.026	1.587	2.612	178
Experimental II	O3—H3⋯O2	0.94 (2)	1.72 (2)	2.5737 (13)	149 (2)
(CAF-H)^+·(26DHBA)^−	O4—H4A⋯O1	0.91 (2)	1.69 (2)	2.5440 (13)	156.1 (19)
	N4—H4⋯O2	0.989 (15)	1.555 (15)	2.5441 (14)	179 (2)
Calculated IIa	O3—H3⋯O2	0.972	1.725	2.579	145
CAF·26DHBA	O4—H4A⋯O1	0.984	1.668	2.558	148
	O2—H4⋯N4	1.028	1.579	2.607	178
Symmetry code: (i) −x + 1, −y + 1, −z.

Figure 10 (a) The asymmetric unit of (TBR-H)^+·(26DHBA)^−, with displacement ellipsoids drawn at the 50% probability level. (b) The 2D structure consisting of four-com­ponent systems of (TBR-H)^+·(26DHBA)^−, inter­connected by C—H⋯O hydrogen bonds. (c) The three-dimensional structure of I stabilized by π(TBR)⋯π(26DHBA) inter­actions.

3.3.2. Caffeine with 2,6-di­hydroxy­benzoic acid

Caffeinium 2,6-di­hydroxy­benzoate salt II crystallizes in the monoclinic space group P2₁/n with a caffeine cation and a (26DHBA)⁻ anion in the asymmetric unit (Fig. 11a). These ionic species are hydrogen bonded via N—H⋯O⁻ inter­actions (Fig. 4c and Table 3). As expected, in the crystal structure of II, the alkaloid and acid form discrete two-com­ponent building blocks stacked in a `head-to-tail' manner, sustained by π(CAF)⋯π(26DHBA) forces (Fig. 11b and Table S8). In the anion, two intra­molecular O—H⋯O⁻ hydrogen bonds involving hy­droxy groups are formed. The O3—C2—C3—C4 and O4—C6—C5—C4 torsion angles [−178.9 (1) and 179.8 (1)°, respectively] show that the phenol groups are practically in the same plane as the arene ring of the 2,6-di­hydroxy­benzoate anion. C—H⋯O inter­actions occur between neighbouring two-com­ponent building blocks and these additionally stabilize the (CAF-H)⁺·(26DHBA)⁻ crystal structure (Fig. 11c).

Figure 11 (a) The asymmetric unit of (CAF-H)^+·(26DHBA)^−, with displacement ellipsoids drawn at the 50% probability level. (b) The two-com­ponent units inter­connected by π(CAF)⋯π(26DHBA) forces. (c) The two-dimensional structure sustained via C—H⋯O inter­actions.

3.4. Optimized structures and IR spectra

The optimized structures of the isolated mol­ecules of TBR·26DHBA (Ia) and CAF·26DHBA (IIa) are presented in Fig. S2 (see supporting information). The geometrical parameters (bond lengths, bond angles and selected torsion angles) are com­pared with the XRD results in Table S9.

The mean absolute differences (MAD) between the experimental and calculated bond lengths are 0.011 and 0.013 Å for Ia and IIa, respectively. The MAD of the bond angles for the theobromine com­plex is 0.94° and for the caffeine com­plex is 0.99°. In the crystal, the proton is transferred from the acid to the TBR or CAF mol­ecule, which is in contrast to the isolated mol­ecule. Additional calculations for CAF·26DHBA with aceto­nitrile or water (Fig. S2c) as solvents revealed also the transfer of the proton to the imidazole N atom. We conclude that the localization of the proton depends on inter­molecular inter­actions.

Electron-density distribution can be described in detail with the quantum theory of atoms in mol­ecules (QTAIM) (Bader, 1994; Lu & Chen, 2012). The (3,−1) critical points and the bonding paths between two atoms indicate the existence of hydrogen bonds. Bonding paths and critical points in the structures of Ia and IIa are shown in Fig. 12.

Figure 12 Critical points (black dots) and paths indicating inter­actions through hydrogen bonds for (a) TBR·26DHBA (Ia) and (b) CAF·26DHBA (IIa).

In order to inter­pret the measured IR spectra (Fig. 13), model spectra were calculated for the optimized structures. Details of these calculations are included in the supporting information (Figs. S3 and S4, and Tables S10–S13). For the salts, in both cases, there are hydrogen bonds of medium strength, and in the experimental IR spectra, broad absorption with three ABC bands was observed. All bonds are noncentrosymmetric, especially N—H⋯O, hence the N—H stretching vibration bands are also visible. In salt I, in the range 3600–2000 cm⁻¹, the absorption is more intense com­pared to the spectrum of II, which is related to the presence of a weak N—H⋯O (2.881 Å) hydrogen bond in the TBR salt. The most intense band predicted theoretically is connected with the O—H⋯N mode.

Figure 13 IR spectra in the range 1800–1200 cm^−1 for (a) CAF·26DHBA and (b) TBR·26DHBA.

The carbonyl group is characterized by a strong absorption band due to C=O stretching vibrations. In the IR spectrum of caffeine, two bands are observed in the carbonyl region at 1701 and 1660 cm⁻¹ (Srivastava & Singh, 2013). After the formation of salt II, these bands are moved towards higher wavenumbers at 1718 and 1673 cm⁻¹, similar to the caffeine and theophylline com­plexes (Gunasekaran et al., 2005). Stretching modes νCC and νCN in the purine ring are observed at 1565 and 1530 cm⁻¹ for II, and at 1560, 1529 and 1516 cm⁻¹ for I. The imidazole ring νCN vibrations are observed at 1322 cm⁻¹, whereas for TBR·26DHBA they are at 1335 cm⁻¹.

IR spectroscopy provides valuable information on salt formation. For 2,6-di­hydroxy­benozic acid, the νCOOH mode is observed at 1685 cm⁻¹ (Solomon et al., 2017). The formation of the salt causes the disappearance of this band and two bands are observed for the carboxyl­ate anion. In the experimental spectra of I and II, carboxyl­ate bands are observed at 1579/1390 and 1587/1390 cm⁻¹. Similar bands were observed for salts of 2,6-di­hydroxy­benzoic acid with nitro­gen heterocycles (Solomon et al., 2017) and Dapsone (Li et al., 2020).

3.5. Solubility tests

The modification of active pharmaceutical com­pounds should lead to an improvement in their physico­chemical properties. Much attention has been paid to solubility, an increase of which can affect other properties, such as permeability, bioavailability, dissolution rate and others (Roy & Ghosh, 2020). Solubility measurements were carried out for I and II using UV–Vis spectroscopy. Theobromine is characterized by relatively low solubility in water, i.e. 0.33 mg ml⁻¹ (0.00183 mmol ml⁻¹). Cocrystallization of this alkaloid with 2,6-di­hydroxy­benzoic acid results in the formation of salt I with a solubility of 1.44 g l⁻¹ (0.00431 mmol ml⁻¹) (Table 4). Thus, this com­pound is more than twice as soluble in water com­pared to the pure alkaloid. Earlier studies have shown that the monohydrate equivalent (TBR-H)⁺·(26DHBA)⁻·H₂O has a 52-fold greater solubility than theobromine (Gołdyn et al., 2019). The anhydrous form of this com­plex is less soluble than the hydrate analog, which shows that the presence of water in the crystal lattice can significantly improve water solubility.

Table 4
Comparison of the water solubility of the alkaloid derivatives with 2,6-di­hydroxy­benzoic acid described in the literature

Alkaloid–26DHBA com­plex	Alkaloid solubility in water (mmol ml^−1)	Absorption solubility (mmol ml^−1)^a
(TBR-H)^+·(26DHBA)^−	0.00183 (Sanphui & Nangia, 2014)	0.00431 (×2.36) (this work)
(TBR-H)^+·(26DHBA)^−·H2O		0.09452 (×51.65) (Gołdyn et al., 2019)
(TPH-H)^+·(26DHBA)^−·H2O	0.0457 (Sarma & Saikia, 2014)	0.0526 (×1.15) (Sarma & Saikia, 2014)
(CAF-H)^+·(26DHBA)^−	0.108 (Lilley et al., 1992)	0.009 (×0.083) (this work)
Note: (a) the change in water solubility in relation to the alkaloid solubility is shown in brackets.

Caffeine salt II shows a greater than 12 times lower water solubility (3.133 g l⁻¹, 0.009 mmol ml⁻¹) com­pared to pure caf­feine (20.973 g l⁻¹, 0.108 mmol ml⁻¹). It is worth mentioning the salt hydrate of theophylline with this acid, which, according to UV–Vis studies by Sarma & Saikia (2014), is about 1.2 times more soluble in water than the pure alkaloid (Table 4).

3.6. TGA and DSC analysis

Thermogravimetric analysis (TGA) was used to investigate the thermal stabilities of (TBR-H)⁺·(26DHBA)⁻ (I) and (CAF-H)⁺·(26DHBA)⁻ (II). Theobromine salt I displays two decom­position steps. The first, occurring at 194 °C, can be attributed to the loss of 2,6-di­hydroxy­benzoic acid (weight loss calculated 45.8%, determined 43.7%). The next step at 292 °C is the release of the alkaloid mol­ecules (Fig. 14a). The melting point of this solid, determined by DSC analysis, is 202 °C (Fig. S5 in the supporting information). For com­parison, the hydrate form (TBR-H)⁺·(26DHBA)⁻·H₂O melts at 193 °C (Gołdyn et al., 2019). The decom­position of caffeine salt II is divided into two stages. The first step, related to decom­position of the acid, and the second, related to the decom­position of caffeine, are observed at 181.3 and 238.2 °C, respectively (Fig. 14b). However, it is not possible to determine the loss of mass occurring during the first stage, due to its continuity, and this, in turn, does not allow the method of decom­position of the caffeine salt to be determined. On the DSC curve, a sharp endothermic peak, indicating a melting point of II, is observed at T_m = 183 °C (Fig. S6). 2,6-Di­hydroxy­benzoic acid melts at about 173 °C (Liao et al., 2010) and the melting points of the salts are between those of this acid and the given alkaloid [the melting points of caffeine and theobromine are 235.6 (Klímová & Leitner, 2012) and 348 °C (Martin et al., 1981), respectively].

Figure 14 TG (solid red line) and DTG (dotted black line) curves for (a) (TBR-H)^+·(26DHBA)^− and (b) (CAF-H)^+·(26DHBA)^− over the temperature range 30–400 °C.

4. Conclusions

Cocrystallization of selected purine alkaloids with 2,6-di­hydroxy­benzoic acid as a coformer leads to the ionic com­plexes I and II in a 1:1 ratio. The reactions were carried out in solution, by milling and by microwave-assisted slurry cocrystallization, and the products obtained were confirmed by PXRD analysis. Structural studies showed practically com­plete success in obtaining the predicted basic supra­molecular synthons responsible for the noncovalent association of mol­ecules in the described purine derivatives. Both hy­droxy groups in the 2,6-di­hydroxy­benzoate anion form intra­molecular O—H⋯O⁻ hydrogen bonds. Proton migration from the carboxyl group to the imidazole N atom, confirmed by the presence of the carboxyl­ate group and the imidazole-ring geometry, leads to N—H⋯O⁻ hydrogen-bond formation (alkaloid–acid heterosynthon). However, the amide–amide dimer found in I was not formed with the exo-carbonyl group but with the endo-carbonyl group, which is less frequently observed in theobromine–carb­oxy­lic acid systems.

Following the assumptions based on the ΔpK_a values and an analysis of the CSD, ionic com­plexes were obtained. Theoretical calculations carried out for the isolated system reflected the structure obtained by the single-crystal X-ray diffraction method, apart from the proton transfer from the 2,6-di­hydroxy­benzoic acid to the alkaloid mol­ecule. The difference in the nature of the com­pounds obtained by the above methods may result from the role of inter­molecular inter­actions in the solid, which are not taken into account in theoretical calculations. Only additional calculations in which proton transfer was simulated in the presence of a solvent as an environment (aceto­nitrile or water) gave results consistent with our results and the literature data. IR spectroscopy further confirmed the formation of the salts.

Theobromine with 2,6-di­hydroxy­benzoic acid forms a salt monohydrate (1:1:1 ratio) or anhydrous salt I (1:1 ratio) depending on the cocrystallization conditions. Both are more soluble in water than the pure alkaloid by 2.4 and 52 times, respectively. Caffeine forms anhydrous ionic com­plex II with this acid, which is 9 times less soluble in water than caffeine. Theophylline forms with 2,6-di­hydroxy­benzoic acid a slightly more water-soluble salt hydrate (1:1:1 ratio).

In addition to the above differences, we also observed different thermal properties. Both salts described in this article have melting points higher than the melting point of the coformer but lower than the melting point of the alkaloid. The decom­position of both com­plexes takes place in two stages, with the release of acid mol­ecules in the first stage. In contrast to the theobromine salt, the release of acid coincides with the degradation of the alkaloid in the caffeine salt, so it was impossible to determine a definite way of this salt decomposition.

Supporting information

Computing details

For both structures, data collection: CrysAlis PRO (Rigaku OD, 2019); cell refinement: CrysAlis PRO (Rigaku OD, 2019); data reduction: CrysAlis PRO (Rigaku OD, 2019); program(s) used to solve structure: SHELXT2018 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

1,3,7-Trimethyl-2,6-dioxo-2,3,6,9-tetrahydro-1H-purin-7-ium 2,6-dihydroxybenzoate (caf-26dhba-1-1b)

Crystal data

C8H11N4O2^+·C7H5O4^−	Dx = 1.560 Mg m^−3
Mr = 348.32	Melting point: 183 K
Monoclinic, P21/n	Cu Kα radiation, λ = 1.54184 Å
a = 14.8560 (3) Å	Cell parameters from 6420 reflections
b = 6.95591 (11) Å	θ = 3.1–75.9°
c = 15.8927 (3) Å	µ = 1.05 mm^−1
β = 115.413 (3)°	T = 131 K
V = 1483.39 (6) Å^3	Block, clear light colourless
Z = 4	0.3 × 0.17 × 0.12 mm
F(000) = 728

Data collection

Rigaku OD SuperNova Single source diffractometer with an Atlas detector	3076 independent reflections
Radiation source: micro-focus sealed X-ray tube, SuperNova (Cu) X-ray Source	2815 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.025
Detector resolution: 10.5357 pixels mm^-1	θmax = 76.4°, θmin = 3.4°
ω scans	h = −18→18
Absorption correction: multi-scan (CrysAlis PRO; Rigaku OD, 2019)	k = −8→8
Tmin = 0.773, Tmax = 1.000	l = −19→19
11899 measured reflections

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.034	All H-atom parameters refined
wR(F^2) = 0.096	w = 1/[σ^2(Fo^2) + (0.0539P)^2 + 0.4452P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max < 0.001
3076 reflections	Δρmax = 0.28 e Å^−3
290 parameters	Δρmin = −0.18 e Å^−3
1 restraint

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O6	0.87775 (7)	0.94531 (14)	0.71150 (6)	0.0295 (2)
O5	0.80375 (7)	1.01841 (13)	0.40182 (6)	0.0266 (2)
N2	0.72024 (8)	0.86953 (15)	0.60787 (7)	0.0224 (2)
N4	0.55934 (7)	0.79191 (15)	0.48111 (7)	0.0232 (2)
H4	0.5168 (15)	0.752 (3)	0.5118 (15)	0.064 (6)*
N1	0.83843 (7)	0.98771 (15)	0.55706 (7)	0.0228 (2)
N3	0.59383 (8)	0.86753 (15)	0.36370 (7)	0.0228 (2)
C9	0.81589 (9)	0.93483 (17)	0.63069 (8)	0.0231 (3)
C11	0.67820 (9)	0.90751 (17)	0.44496 (8)	0.0216 (2)
C12	0.52493 (9)	0.79891 (18)	0.38801 (9)	0.0247 (3)
H12	0.4601 (12)	0.758 (2)	0.3432 (11)	0.027 (4)*
C10	0.65472 (9)	0.85802 (17)	0.51619 (8)	0.0214 (2)
C8	0.77546 (9)	0.97551 (17)	0.46063 (8)	0.0217 (2)
C15	0.94090 (9)	1.0539 (2)	0.58291 (9)	0.0281 (3)
H15A	0.9644 (16)	1.126 (3)	0.6409 (16)	0.056 (6)*
H15B	0.9400 (16)	1.136 (3)	0.5311 (15)	0.052 (6)*
H15C	0.9864 (18)	0.947 (3)	0.5919 (16)	0.064 (7)*
C13	0.69146 (10)	0.8075 (2)	0.68087 (9)	0.0281 (3)
H13A	0.7503 (14)	0.808 (3)	0.7393 (13)	0.037 (5)*
H13B	0.6421 (16)	0.902 (3)	0.6872 (15)	0.054 (6)*
H13C	0.6599 (16)	0.680 (3)	0.6668 (14)	0.050 (5)*
C14	0.58116 (10)	0.8956 (2)	0.26772 (9)	0.0282 (3)
H14A	0.6294 (14)	0.818 (3)	0.2556 (13)	0.044 (5)*
H14B	0.5940 (16)	1.030 (3)	0.2598 (14)	0.049 (5)*
H14C	0.5130 (16)	0.864 (3)	0.2249 (15)	0.050 (5)*
O4	0.16405 (7)	0.53913 (15)	0.37731 (6)	0.0297 (2)
H4A	0.2180 (16)	0.593 (3)	0.3738 (14)	0.047 (5)*
O2	0.45129 (6)	0.69157 (14)	0.56176 (6)	0.0275 (2)
O1	0.33645 (7)	0.67306 (15)	0.41487 (6)	0.0298 (2)
O3	0.40547 (7)	0.61533 (16)	0.69704 (6)	0.0311 (2)
H3	0.4443 (17)	0.645 (3)	0.6650 (16)	0.061 (6)*
C1	0.28984 (9)	0.58156 (17)	0.53489 (8)	0.0221 (2)
C6	0.19273 (9)	0.52601 (18)	0.47030 (9)	0.0237 (3)
C2	0.31443 (9)	0.56542 (18)	0.63089 (9)	0.0236 (3)
C5	0.12501 (9)	0.45492 (19)	0.50127 (9)	0.0268 (3)
H5	0.0609 (13)	0.417 (2)	0.4566 (12)	0.029 (4)*
C3	0.24597 (10)	0.49508 (19)	0.66139 (9)	0.0271 (3)
H3A	0.2650 (14)	0.486 (3)	0.7277 (13)	0.036 (5)*
C7	0.36193 (9)	0.65282 (18)	0.50043 (9)	0.0233 (3)
C4	0.15242 (10)	0.43917 (19)	0.59610 (10)	0.0281 (3)
H4B	0.1039 (13)	0.389 (2)	0.6177 (12)	0.033 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O6	0.0235 (4)	0.0408 (5)	0.0193 (4)	−0.0023 (4)	0.0045 (4)	0.0005 (4)
O5	0.0259 (4)	0.0328 (5)	0.0220 (4)	−0.0026 (4)	0.0112 (4)	0.0004 (3)
N2	0.0208 (5)	0.0277 (5)	0.0169 (5)	−0.0002 (4)	0.0065 (4)	0.0007 (4)
N4	0.0194 (5)	0.0273 (5)	0.0212 (5)	−0.0009 (4)	0.0072 (4)	0.0003 (4)
N1	0.0191 (5)	0.0276 (5)	0.0198 (5)	−0.0008 (4)	0.0065 (4)	−0.0002 (4)
N3	0.0202 (5)	0.0278 (5)	0.0178 (5)	−0.0002 (4)	0.0058 (4)	−0.0003 (4)
C9	0.0217 (6)	0.0245 (6)	0.0211 (6)	0.0018 (4)	0.0073 (5)	−0.0001 (4)
C11	0.0204 (6)	0.0240 (6)	0.0179 (5)	0.0009 (4)	0.0058 (4)	0.0002 (4)
C12	0.0202 (6)	0.0288 (6)	0.0225 (6)	−0.0006 (5)	0.0066 (5)	−0.0006 (5)
C10	0.0193 (5)	0.0232 (5)	0.0196 (6)	0.0011 (4)	0.0065 (4)	0.0000 (4)
C8	0.0212 (6)	0.0225 (5)	0.0198 (6)	0.0009 (4)	0.0073 (5)	0.0000 (4)
C15	0.0191 (6)	0.0375 (7)	0.0260 (6)	−0.0036 (5)	0.0081 (5)	−0.0008 (5)
C13	0.0253 (6)	0.0385 (7)	0.0203 (6)	−0.0020 (5)	0.0096 (5)	0.0017 (5)
C14	0.0266 (6)	0.0381 (7)	0.0177 (6)	−0.0017 (5)	0.0074 (5)	0.0012 (5)
O4	0.0249 (5)	0.0404 (5)	0.0206 (4)	−0.0012 (4)	0.0066 (4)	−0.0021 (4)
O2	0.0209 (4)	0.0368 (5)	0.0238 (4)	−0.0024 (4)	0.0088 (3)	0.0004 (4)
O1	0.0284 (5)	0.0407 (5)	0.0208 (4)	−0.0019 (4)	0.0109 (4)	0.0012 (4)
O3	0.0223 (4)	0.0476 (6)	0.0204 (4)	−0.0025 (4)	0.0064 (4)	−0.0009 (4)
C1	0.0204 (6)	0.0239 (6)	0.0215 (6)	0.0017 (4)	0.0084 (5)	0.0003 (4)
C6	0.0218 (6)	0.0247 (6)	0.0227 (6)	0.0029 (5)	0.0078 (5)	−0.0016 (5)
C2	0.0207 (6)	0.0267 (6)	0.0218 (6)	0.0031 (4)	0.0076 (5)	0.0002 (4)
C5	0.0205 (6)	0.0298 (6)	0.0286 (6)	−0.0002 (5)	0.0091 (5)	−0.0032 (5)
C3	0.0274 (6)	0.0320 (6)	0.0241 (6)	0.0033 (5)	0.0131 (5)	0.0020 (5)
C7	0.0230 (6)	0.0243 (6)	0.0226 (6)	0.0016 (4)	0.0097 (5)	−0.0004 (4)
C4	0.0256 (6)	0.0303 (6)	0.0321 (7)	0.0013 (5)	0.0160 (5)	0.0009 (5)

Geometric parameters (Å, º)

O6—C9	1.2191 (16)	C13—H13B	1.02 (2)
O5—C8	1.2151 (16)	C13—H13C	0.98 (2)
N2—C9	1.3838 (16)	C14—H14A	0.98 (2)
N2—C10	1.3636 (16)	C14—H14B	0.97 (2)
N2—C13	1.4631 (16)	C14—H14C	0.97 (2)
N4—H4	0.989 (15)	O4—H4A	0.91 (2)
N4—C12	1.3431 (16)	O4—C6	1.3541 (16)
N4—C10	1.3608 (16)	O2—C7	1.2940 (15)
N1—C9	1.3976 (16)	O1—C7	1.2530 (16)
N1—C8	1.4144 (15)	O3—H3	0.94 (2)
N1—C15	1.4700 (16)	O3—C2	1.3545 (15)
N3—C11	1.3882 (15)	C1—C6	1.4189 (17)
N3—C12	1.3282 (16)	C1—C2	1.4125 (17)
N3—C14	1.4677 (15)	C1—C7	1.4821 (17)
C11—C10	1.3641 (17)	C6—C5	1.3870 (18)
C11—C8	1.4369 (17)	C2—C3	1.3900 (18)
C12—H12	0.963 (16)	C5—H5	0.949 (18)
C15—H15A	0.97 (2)	C5—C4	1.3865 (19)
C15—H15B	1.00 (2)	C3—H3A	0.970 (19)
C15—H15C	0.97 (2)	C3—C4	1.3862 (19)
C13—H13A	0.964 (19)	C4—H4B	0.985 (18)
C9—N2—C13	120.37 (10)	N2—C13—H13B	110.8 (12)
C10—N2—C9	118.77 (10)	N2—C13—H13C	111.1 (12)
C10—N2—C13	120.82 (10)	H13A—C13—H13B	107.1 (16)
C12—N4—H4	122.1 (13)	H13A—C13—H13C	111.6 (16)
C12—N4—C10	106.04 (10)	H13B—C13—H13C	108.1 (17)
C10—N4—H4	131.8 (13)	N3—C14—H14A	110.6 (11)
C9—N1—C8	127.35 (10)	N3—C14—H14B	108.5 (12)
C9—N1—C15	116.09 (10)	N3—C14—H14C	109.2 (12)
C8—N1—C15	116.47 (10)	H14A—C14—H14B	107.8 (17)
C11—N3—C14	127.02 (10)	H14A—C14—H14C	111.6 (16)
C12—N3—C11	107.59 (10)	H14B—C14—H14C	109.1 (17)
C12—N3—C14	125.38 (11)	C6—O4—H4A	102.8 (13)
O6—C9—N2	121.39 (12)	C2—O3—H3	106.0 (14)
O6—C9—N1	121.55 (11)	C6—C1—C7	119.69 (11)
N2—C9—N1	117.06 (10)	C2—C1—C6	118.11 (11)
N3—C11—C8	131.80 (11)	C2—C1—C7	122.19 (11)
C10—C11—N3	105.73 (10)	O4—C6—C1	121.15 (11)
C10—C11—C8	122.38 (11)	O4—C6—C5	118.35 (11)
N4—C12—H12	126.2 (9)	C5—C6—C1	120.49 (12)
N3—C12—N4	110.91 (11)	O3—C2—C1	121.83 (11)
N3—C12—H12	122.8 (9)	O3—C2—C3	117.11 (11)
N2—C10—C11	123.53 (11)	C3—C2—C1	121.05 (12)
N4—C10—N2	126.72 (11)	C6—C5—H5	118.8 (10)
N4—C10—C11	109.72 (11)	C4—C5—C6	119.64 (12)
O5—C8—N1	122.17 (11)	C4—C5—H5	121.6 (10)
O5—C8—C11	126.98 (11)	C2—C3—H3A	119.1 (11)
N1—C8—C11	110.85 (10)	C4—C3—C2	119.10 (12)
N1—C15—H15A	109.6 (13)	C4—C3—H3A	121.8 (11)
N1—C15—H15B	107.6 (12)	O2—C7—C1	117.50 (11)
N1—C15—H15C	111.9 (14)	O1—C7—O2	121.91 (11)
H15A—C15—H15B	111.0 (17)	O1—C7—C1	120.59 (11)
H15A—C15—H15C	108.1 (19)	C5—C4—H4B	119.3 (10)
H15B—C15—H15C	108.7 (18)	C3—C4—C5	121.59 (12)
N2—C13—H13A	108.1 (11)	C3—C4—H4B	119.1 (10)
N3—C11—C10—N2	177.45 (11)	C13—N2—C9—O6	−1.19 (19)
N3—C11—C10—N4	−0.66 (14)	C13—N2—C9—N1	178.67 (11)
N3—C11—C8—O5	2.1 (2)	C13—N2—C10—N4	0.05 (19)
N3—C11—C8—N1	−177.84 (12)	C13—N2—C10—C11	−177.73 (12)
C9—N2—C10—N4	177.81 (11)	C14—N3—C11—C10	−179.96 (12)
C9—N2—C10—C11	0.03 (18)	C14—N3—C11—C8	−3.4 (2)
C9—N1—C8—O5	−176.99 (12)	C14—N3—C12—N4	−179.58 (11)
C9—N1—C8—C11	2.91 (17)	O4—C6—C5—C4	179.76 (12)
C11—N3—C12—N4	0.14 (14)	O3—C2—C3—C4	−178.92 (12)
C12—N4—C10—N2	−177.29 (12)	C1—C6—C5—C4	0.36 (19)
C12—N4—C10—C11	0.75 (14)	C1—C2—C3—C4	0.36 (19)
C12—N3—C11—C10	0.32 (14)	C6—C1—C2—O3	179.98 (11)
C12—N3—C11—C8	176.88 (13)	C6—C1—C2—C3	0.73 (18)
C10—N2—C9—O6	−178.96 (12)	C6—C1—C7—O2	−176.21 (11)
C10—N2—C9—N1	0.89 (17)	C6—C1—C7—O1	3.39 (18)
C10—N4—C12—N3	−0.55 (14)	C6—C5—C4—C3	0.8 (2)
C10—C11—C8—O5	178.14 (12)	C2—C1—C6—O4	179.52 (11)
C10—C11—C8—N1	−1.76 (17)	C2—C1—C6—C5	−1.10 (18)
C8—N1—C9—O6	177.25 (12)	C2—C1—C7—O2	3.42 (18)
C8—N1—C9—N2	−2.60 (18)	C2—C1—C7—O1	−176.98 (12)
C8—C11—C10—N2	0.49 (19)	C2—C3—C4—C5	−1.1 (2)
C8—C11—C10—N4	−177.62 (11)	C7—C1—C6—O4	−0.84 (18)
C15—N1—C9—O6	0.73 (18)	C7—C1—C6—C5	178.55 (11)
C15—N1—C9—N2	−179.12 (11)	C7—C1—C2—O3	0.35 (19)
C15—N1—C8—O5	−0.48 (18)	C7—C1—C2—C3	−178.90 (12)
C15—N1—C8—C11	179.42 (11)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N4—H4···O2	0.99 (2)	1.56 (2)	2.5441 (14)	179 (2)
O4—H4A···O1	0.91 (2)	1.69 (2)	2.5440 (13)	156.1 (19)
O3—H3···O2	0.94 (2)	1.72 (2)	2.5737 (13)	149 (2)

3,7-Dimethyl-2,6-dioxo-2,3,6,9-tetrahydro-1H-purin-7-ium 2,6-dihydroxybenzoate (tbr-26dhba-lag-cryst)

Crystal data

C7H9N4O2^+·C7H5O4^−	Dx = 1.551 Mg m^−3
Mr = 334.29	Melting point: 202 K
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54184 Å
a = 6.9579 (3) Å	Cell parameters from 5599 reflections
b = 16.5845 (6) Å	θ = 2.7–75.3°
c = 12.4718 (5) Å	µ = 1.06 mm^−1
β = 95.691 (4)°	T = 132 K
V = 1432.06 (10) Å^3	Plate, clear light colourless
Z = 4	0.34 × 0.24 × 0.08 mm
F(000) = 696

Data collection

Rigaku OD SuperNova Single source diffractometer with an Atlas detector	2970 independent reflections
Radiation source: micro-focus sealed X-ray tube, SuperNova (Cu) X-ray Source	2782 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.023
Detector resolution: 10.5357 pixels mm^-1	θmax = 76.5°, θmin = 4.5°
ω scans	h = −8→8
Absorption correction: multi-scan (CrysAlis PRO; Rigaku OD, 2019)	k = −20→20
Tmin = 0.649, Tmax = 1.000	l = −15→15
12055 measured reflections

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.055	All H-atom parameters refined
wR(F^2) = 0.147	w = 1/[σ^2(Fo^2) + (0.0629P)^2 + 1.3861P] where P = (Fo^2 + 2Fc^2)/3
S = 1.11	(Δ/σ)max < 0.001
2970 reflections	Δρmax = 0.51 e Å^−3
273 parameters	Δρmin = −0.24 e Å^−3
2 restraints

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O5	0.3845 (2)	0.28563 (9)	0.10648 (12)	0.0343 (4)
O6	0.4508 (2)	0.55891 (9)	0.10181 (12)	0.0315 (4)
O1	0.1578 (2)	0.43520 (9)	0.65282 (13)	0.0352 (4)
O2	0.2341 (2)	0.53812 (9)	0.55146 (12)	0.0327 (4)
O4	0.0791 (3)	0.45956 (11)	0.84553 (13)	0.0393 (4)
O3	0.2082 (3)	0.68358 (10)	0.61672 (14)	0.0394 (4)
N4	0.2886 (2)	0.42798 (11)	0.41420 (14)	0.0255 (4)
N1	0.4179 (3)	0.42264 (11)	0.10880 (14)	0.0265 (4)
N2	0.3665 (2)	0.50484 (10)	0.25777 (13)	0.0260 (4)
N3	0.3021 (3)	0.30658 (10)	0.34625 (14)	0.0289 (4)
C10	0.3345 (3)	0.43505 (12)	0.31106 (16)	0.0246 (4)
C11	0.3441 (3)	0.36048 (12)	0.26700 (16)	0.0260 (4)
C9	0.4144 (3)	0.49925 (13)	0.15252 (16)	0.0258 (4)
C8	0.3822 (3)	0.34889 (13)	0.15611 (16)	0.0261 (4)
C1	0.1489 (3)	0.56751 (13)	0.72638 (16)	0.0264 (4)
C7	0.1804 (3)	0.50870 (13)	0.64013 (16)	0.0267 (4)
C12	0.2705 (3)	0.34930 (13)	0.43346 (17)	0.0289 (4)
C2	0.1672 (3)	0.65122 (14)	0.71212 (18)	0.0310 (5)
C6	0.1026 (3)	0.53936 (14)	0.82804 (17)	0.0301 (5)
C13	0.3495 (4)	0.58438 (14)	0.30785 (19)	0.0336 (5)
C3	0.1432 (4)	0.70363 (15)	0.7959 (2)	0.0400 (6)
C5	0.0812 (3)	0.59200 (16)	0.91215 (19)	0.0368 (5)
C14	0.3021 (5)	0.21876 (14)	0.3388 (2)	0.0448 (6)
C4	0.1032 (4)	0.67333 (17)	0.8954 (2)	0.0425 (6)
H4B	0.077 (3)	0.7075 (14)	0.9512 (19)	0.022 (5)*
H1	0.454 (4)	0.4211 (16)	0.046 (2)	0.034 (7)*
H3	0.224 (5)	0.6347 (12)	0.575 (2)	0.057 (9)*
H3A	0.151 (4)	0.757 (2)	0.777 (2)	0.049 (8)*
H5	0.053 (4)	0.5737 (17)	0.980 (2)	0.040 (7)*
H14A	0.251 (5)	0.2049 (19)	0.262 (3)	0.056 (9)*
H13A	0.329 (4)	0.623 (2)	0.251 (3)	0.054 (9)*
H13B	0.232 (5)	0.5860 (19)	0.344 (3)	0.057 (9)*
H14B	0.225 (5)	0.198 (2)	0.398 (3)	0.064 (10)*
H4A	0.107 (5)	0.434 (2)	0.776 (3)	0.074 (11)*
H13C	0.457 (5)	0.595 (2)	0.356 (3)	0.061 (10)*
H4	0.273 (5)	0.473 (2)	0.459 (3)	0.081 (12)*
H14C	0.429 (6)	0.203 (2)	0.347 (3)	0.083 (13)*
H12	0.236 (4)	0.3280 (16)	0.5011 (13)	0.042 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O5	0.0441 (9)	0.0314 (8)	0.0282 (8)	−0.0026 (7)	0.0069 (6)	−0.0048 (6)
O6	0.0389 (8)	0.0325 (8)	0.0241 (7)	−0.0070 (6)	0.0073 (6)	0.0025 (6)
O1	0.0443 (9)	0.0289 (8)	0.0333 (8)	0.0019 (6)	0.0087 (7)	−0.0001 (6)
O2	0.0429 (9)	0.0325 (8)	0.0238 (7)	−0.0004 (6)	0.0095 (6)	−0.0014 (6)
O4	0.0461 (9)	0.0401 (9)	0.0335 (9)	0.0068 (7)	0.0136 (7)	0.0100 (7)
O3	0.0526 (10)	0.0293 (8)	0.0379 (9)	−0.0032 (7)	0.0133 (7)	0.0044 (7)
N4	0.0262 (8)	0.0295 (9)	0.0213 (8)	0.0000 (7)	0.0043 (6)	0.0003 (7)
N1	0.0284 (8)	0.0324 (9)	0.0192 (8)	−0.0043 (7)	0.0053 (6)	−0.0010 (7)
N2	0.0295 (8)	0.0266 (9)	0.0225 (8)	−0.0026 (7)	0.0051 (6)	0.0007 (6)
N3	0.0384 (10)	0.0250 (9)	0.0234 (8)	0.0010 (7)	0.0034 (7)	0.0003 (7)
C10	0.0203 (9)	0.0307 (10)	0.0227 (9)	0.0005 (7)	0.0022 (7)	0.0007 (7)
C11	0.0259 (9)	0.0296 (10)	0.0225 (10)	−0.0007 (8)	0.0028 (7)	0.0050 (8)
C9	0.0229 (9)	0.0318 (10)	0.0226 (9)	−0.0033 (7)	0.0009 (7)	−0.0001 (8)
C8	0.0254 (9)	0.0298 (10)	0.0229 (9)	−0.0017 (8)	0.0018 (7)	−0.0020 (8)
C1	0.0208 (9)	0.0329 (11)	0.0253 (10)	0.0005 (7)	0.0009 (7)	−0.0005 (8)
C7	0.0228 (9)	0.0325 (10)	0.0249 (10)	0.0014 (8)	0.0026 (7)	−0.0037 (8)
C12	0.0334 (11)	0.0317 (10)	0.0219 (10)	0.0010 (8)	0.0044 (8)	0.0014 (8)
C2	0.0279 (10)	0.0338 (11)	0.0316 (11)	−0.0023 (8)	0.0037 (8)	−0.0011 (9)
C6	0.0226 (9)	0.0413 (12)	0.0264 (10)	0.0040 (8)	0.0031 (7)	0.0036 (9)
C13	0.0415 (13)	0.0282 (11)	0.0324 (11)	0.0005 (9)	0.0110 (10)	−0.0020 (9)
C3	0.0395 (13)	0.0297 (12)	0.0520 (15)	−0.0028 (9)	0.0094 (11)	−0.0071 (10)
C5	0.0283 (11)	0.0547 (15)	0.0274 (11)	0.0026 (10)	0.0030 (8)	0.0006 (10)
C14	0.077 (2)	0.0237 (11)	0.0345 (13)	0.0023 (12)	0.0092 (13)	−0.0007 (9)
C4	0.0363 (12)	0.0553 (15)	0.0363 (12)	−0.0009 (11)	0.0056 (10)	−0.0185 (12)

Geometric parameters (Å, º)

O5—C8	1.219 (3)	C10—C11	1.358 (3)
O6—C9	1.215 (3)	C11—C8	1.447 (3)
O1—C7	1.241 (3)	C1—C7	1.484 (3)
O2—C7	1.297 (3)	C1—C2	1.407 (3)
O4—C6	1.354 (3)	C1—C6	1.418 (3)
O4—H4A	1.00 (4)	C12—H12	0.966 (10)
O3—C2	1.361 (3)	C2—C3	1.382 (3)
O3—H3	0.975 (10)	C6—C5	1.384 (3)
N4—C10	1.361 (3)	C13—H13A	0.95 (3)
N4—C12	1.335 (3)	C13—H13B	0.97 (3)
N4—H4	0.94 (4)	C13—H13C	0.93 (4)
N1—C9	1.384 (3)	C3—C4	1.391 (4)
N1—C8	1.391 (3)	C3—H3A	0.92 (3)
N1—H1	0.85 (3)	C5—C4	1.376 (4)
N2—C10	1.364 (3)	C5—H5	0.94 (3)
N2—C9	1.389 (3)	C14—H14A	1.02 (3)
N2—C13	1.469 (3)	C14—H14B	1.01 (4)
N3—C11	1.385 (3)	C14—H14C	0.92 (4)
N3—C12	1.334 (3)	C4—H4B	0.93 (2)
N3—C14	1.460 (3)
C6—O4—H4A	104 (2)	O2—C7—C1	116.52 (18)
C2—O3—H3	101 (2)	N4—C12—H12	123.4 (17)
C10—N4—H4	123 (2)	N3—C12—N4	110.26 (18)
C12—N4—C10	106.82 (17)	N3—C12—H12	126.3 (17)
C12—N4—H4	131 (2)	O3—C2—C1	121.9 (2)
C9—N1—C8	128.99 (17)	O3—C2—C3	117.7 (2)
C9—N1—H1	114.3 (18)	C3—C2—C1	120.4 (2)
C8—N1—H1	116.6 (18)	O4—C6—C1	120.51 (19)
C10—N2—C9	118.03 (17)	O4—C6—C5	118.1 (2)
C10—N2—C13	122.05 (17)	C5—C6—C1	121.4 (2)
C9—N2—C13	119.91 (17)	N2—C13—H13A	106.9 (19)
C11—N3—C14	126.65 (19)	N2—C13—H13B	109.3 (19)
C12—N3—C11	107.60 (17)	N2—C13—H13C	111 (2)
C12—N3—C14	125.67 (19)	H13A—C13—H13B	105 (3)
N4—C10—N2	126.78 (19)	H13A—C13—H13C	114 (3)
C11—C10—N4	109.25 (18)	H13B—C13—H13C	111 (3)
C11—C10—N2	123.97 (18)	C2—C3—C4	119.8 (2)
N3—C11—C8	131.92 (19)	C2—C3—H3A	113 (2)
C10—C11—N3	106.06 (17)	C4—C3—H3A	127 (2)
C10—C11—C8	121.94 (18)	C6—C5—H5	121.8 (17)
O6—C9—N1	122.00 (18)	C4—C5—C6	118.8 (2)
O6—C9—N2	121.32 (19)	C4—C5—H5	119.4 (17)
N1—C9—N2	116.69 (17)	N3—C14—H14A	106.5 (18)
O5—C8—N1	121.97 (19)	N3—C14—H14B	107 (2)
O5—C8—C11	127.8 (2)	N3—C14—H14C	106 (3)
N1—C8—C11	110.24 (17)	H14A—C14—H14B	116 (3)
C2—C1—C7	122.34 (19)	H14A—C14—H14C	106 (3)
C2—C1—C6	117.99 (19)	H14B—C14—H14C	114 (3)
C6—C1—C7	119.65 (19)	C3—C4—H4B	121.2 (15)
O1—C7—O2	121.94 (19)	C5—C4—C3	121.6 (2)
O1—C7—C1	121.54 (19)	C5—C4—H4B	116.9 (14)
O4—C6—C5—C4	−179.5 (2)	C7—C1—C6—O4	−2.7 (3)
O3—C2—C3—C4	−179.3 (2)	C7—C1—C6—C5	176.86 (19)
N4—C10—C11—N3	0.2 (2)	C12—N4—C10—N2	−179.93 (19)
N4—C10—C11—C8	177.29 (18)	C12—N4—C10—C11	0.1 (2)
N2—C10—C11—N3	−179.78 (18)	C12—N3—C11—C10	−0.4 (2)
N2—C10—C11—C8	−2.6 (3)	C12—N3—C11—C8	−177.1 (2)
N3—C11—C8—O5	0.3 (4)	C2—C1—C7—O1	−177.7 (2)
N3—C11—C8—N1	−179.8 (2)	C2—C1—C7—O2	2.8 (3)
C10—N4—C12—N3	−0.4 (2)	C2—C1—C6—O4	178.56 (19)
C10—N2—C9—O6	−177.57 (18)	C2—C1—C6—C5	−1.9 (3)
C10—N2—C9—N1	2.5 (3)	C2—C3—C4—C5	−1.9 (4)
C10—C11—C8—O5	−176.0 (2)	C6—C1—C7—O1	3.6 (3)
C10—C11—C8—N1	3.9 (3)	C6—C1—C7—O2	−175.85 (18)
C11—N3—C12—N4	0.5 (2)	C6—C1—C2—O3	−178.87 (19)
C9—N1—C8—O5	177.6 (2)	C6—C1—C2—C3	1.0 (3)
C9—N1—C8—C11	−2.2 (3)	C6—C5—C4—C3	1.0 (4)
C9—N2—C10—N4	179.22 (18)	C13—N2—C10—N4	−1.6 (3)
C9—N2—C10—C11	−0.9 (3)	C13—N2—C10—C11	178.3 (2)
C8—N1—C9—O6	179.2 (2)	C13—N2—C9—O6	3.2 (3)
C8—N1—C9—N2	−0.9 (3)	C13—N2—C9—N1	−176.72 (18)
C1—C2—C3—C4	0.9 (4)	C14—N3—C11—C10	−177.3 (2)
C1—C6—C5—C4	0.9 (3)	C14—N3—C11—C8	5.9 (4)
C7—C1—C2—O3	2.4 (3)	C14—N3—C12—N4	177.5 (2)
C7—C1—C2—C3	−177.7 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O6^i	0.85 (3)	2.04 (3)	2.881 (2)	169 (3)
O3—H3···O2	0.98 (1)	1.63 (2)	2.558 (2)	157 (3)
O4—H4A···O1	1.00 (4)	1.62 (4)	2.550 (2)	153 (3)
N4—H4···O2	0.94 (4)	1.62 (4)	2.557 (2)	169 (4)

Symmetry code: (i) −x+1, −y+1, −z.