Supramolecular synthesis and thermochemical

CrystEngComm PAPER

Cite this: CrystEngComm, 2016, 18, 6378

Supramolecular synthesis and thermochemical investigations of pharmaceutical inorganic isoniazid salts† Cristiane C. de Melo,ab Paulo de Sousa Carvalho Jr,a Luan F. Diniz,a Richard F. D'Vries,a Alejandro P. Ayalab and Javier Ellena*a In multiple-drug therapy, isoniazid (INH) is considered one of the most important antibiotics for the treatment of tuberculosis. Beyond its pharmacological importance, INH is also a versatile compound that can be combined with several other molecules to produce salts and co-crystals. In this study, novel salts of INH, obtained from the reaction with pharmaceutically accepted inorganic acids (HBr, HNO3 and H2SO4), were investigated. The reaction of INH with H2SO4, gives rise to two forms: an INH sulfate and an INH sulfate hemihydrate salt. The four salts feature a supramolecular assembly quite different from the one described for INH hydrochloride. INH hydrobromide and INH nitrate adopt a head-to-tail assembly, where the cations (INH+) are directly connected to each other. However, this is not the case for the sulfate forms, where the cations appear surrounded by the anions, being connected to them through their pyridinium

Received 26th April 2016, Accepted 5th July 2016 DOI: 10.1039/c6ce00969g www.rsc.org/crystengcomm

and hydrazide groups. Interestingly, an unexpected homodimer is observed in the INH sulfate salt. Hirshfeld surface analysis was used to highlight and quantify the contributions of the main interactions. The relative thermal stability of these salts was studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and hot-stage microscopy (HSM). Although the melting points of both sulfate forms are practically the same, the four INH salts have distinct thermal profiles.

1. Introduction The need for rational approaches for the improvement of the performance and quality of active pharmaceutical ingredients (APIs) has led to a close relationship between crystal engineering and pharmaceutical sciences.1,2 The understanding of intermolecular interactions, known as the essence of crystal engineering, and their application in supramolecular synthesis has led to noteworthy advances in the development of novel solid forms of APIs.1–3 As is well known, a crystal can be designed by strong and directional intermolecular interactions, and depending on the functional groups present in the molecule this can lead to the combination of APIs with several coformers.1,4 In order to promote salt or cocrystal formation, the coformers must be complementary to the API and preferably belong to the list of compounds generally recog-

nized as safe (GRAS) by the US FDA.1,5 From a pharmaceutical perspective, salts or cocrystals are designed with the specific purpose of modifying a property of the parent API, such as solubility, stability, bioavailability, hardness, etc.1,4 In this context, isoniazid (INH, Scheme 1), an isonicotinic acid-derived hydrazide, has been the subject of many scientific studies not only due to its therapeutic properties but also because of the ability of the pyridine group of INH to form a variety of cocrystals with carboxylic acids.6–9 Since its introduction in 1952, isoniazid has been considered, together with rifampicin (RMP), pyrazinamide (PZA) and ethambutol

a

Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560970, São Carlos, SP, Brazil. E-mail: [email protected] b Departamento de Física, Universidade Federal do Ceará, CP 6030, 60440-900, Fortaleza, CE, Brazil † Electronic supplementary information (ESI) available: X-ray crystallographic information files (CIFs), crystallographic data table, Ortep-3 diagrams, Hirshfeld surfaces and 2-D fingerprint plots, PXRD patterns. CCDC 1474924 and 1474926– 1474928. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ce00969g

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Scheme 1 Molecular structure of INH.

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(ETB), as the cornerstone of tuberculosis chemotherapy.10 Although the combination of these drugs in a single dosage pill is actually an example of rational fixed-dose drug combinations (FDCs), there are studies indicating that INH undergoes degradation due to drug–drug interactions.11–13 RMP can react directly with INH through transhydrazone formation, yielding isonicotinyl hydrazine as a product (see the ESI†).13 In an attempt to modify INH stability in FDC tablets, recently Swapna et al. reported a cocrystal screening of INH with several GRAS coformers (resorcinol, vanillic, ferulic and caffeic acid).11 However, in spite of the many cocrystals developed, there is still a lack of scientific reports about inorganic INH salts. A survey of the Cambridge Structural Database showed only two entries: the dihydrochloride and the hydrogen phosphate INH salts.14,15 Aiming to investigate a possible occurrence of isostructurality between them, we initially kept our focus on the preparation of the hydrobromide salt, but in sequence, the nitrate and sulfate salts of INH were also prepared. Interestingly, in the course of our work we obtained a hemihydrate form of the sulfate salt with two ionic pairs in the asymmetric unit besides a water molecule. Consequently, the differences and similarities between these salts were explored in this study, taking into account the main intermolecular interactions and the synthons formed.

2. Experimental 2.1 Preparation INH (12 mg, 0.088 mmol) was dissolved in 5 mL of ethanol and stirred at 50 °C. Next, a stoichiometric amount of an aqueous solution of hydrobromic acid and nitric acid was added to the drug solution for preparing INH hydrobromide and INH nitrate salts, respectively. INH sulfate salts were obtained by dissolving 10 mg of isoniazid in 10 mL of hot ethanol. To this solution was added concentrated sulfuric acid (98%) until white crystals were formed. The crystals were separated by filtration, with one portion dissolved in ethanol (70%) and another portion dissolved in deionized water. Both solutions were stirred for 30 min at 50 °C. Crystals of isoniazid sulfate and isoniazid sulfate hemihydrate were obtained by slow evaporation of the ethanol and water solutions, respectively. 2.2 Structure determination Data sets for the hydrobromide, nitrate and sulfate salts of INH were collected on an Enraf-Nonius Kappa-CCD diffractometer using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). The software COLLECT16 and the HKL package (Denzo, XDisplayF and Scalepack software)17 were applied for the acquisition, indexing, integration and scaling of Bragg reflections. A data set for INH sulfate hemihydrate was collected at 120 K (Cryostream, Oxford Cryosystems) on a Bruker D8 VENTURE diffractometer equipped with a PHOTON 100 CMOS detector system using Mo-Kα radiation. Unit cell deter-


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mination, data collection and integration were performed using SAINT and SADABS.18 All structures were solved by direct methods and refined by full-matrix least squares on F 2 using SHELXL-2013.19 The crystallographic software packages PARST95,20 PLATON,21 WinGX,22 ORTEP-3 (ref. 23) and MERCURY24 were used to analyze and prepare material for publication. The crystallographic information files were deposited in the Cambridge Structural Database under the codes CCDC 1474926 (INH hydrobromide), CCDC 1474927 (INH nitrate), CCDC 1474928 (INH sulfate) and CCDC 1474924 (INH sulfate hemihydrate). 2.3 Hirshfeld surfaces and derived fingerprint plots Hirshfeld surface analysis was used for the examination of the different types of interactions within the crystal structures of INH salts, showing the areas susceptible to weak and strong interactions. The 2-dimensional fingerprint plots derived from the 3-dimensional dnorm Hirshfeld surfaces were generated using Crystal Explorer 3.1.25 The color pattern on the surface identifies the areas involved in the interactions. In the dnorm surface representation, contacts shorter than the sum of the van der Waals radii are indicated as red areas, generally located in the acceptor (concave) and donor (convex) regions. White areas represent the contacts with a length close to the van der Waals sum and the blue areas represent the longer contacts. 2.4 Hot-stage microscopy (HSM) HSM experiments were carried out on a Linkam T95-PE device coupled to a Leica DM2500P optical microscope. Single crystals of INH salts were heated at a constant rate of 10 °C min−1 from 30 °C to 250 °C, with this process being stopped after the melting of the compound. Images were recorded by a CCD camera attached to the microscope at time intervals of 10 s via the software Linksys 32. 2.5 Thermal analysis Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-60 thermobalance. The samples were placed in open alumina pans under a N2 flow (100 mL min−1) and heated from 30 to 300 °C at a constant rate of 10 °C min−1. Differential scanning calorimetry (DSC) curves were obtained with a Shimadzu DSC-60 instrument. The samples were sealed in crimped aluminum pans and heated under a N2 flow (100 mL min−1) from 30 to 300 °C at a constant rate of 10 °C min−1. TGA and DSC experiments were carried out using approximately 1–2 mg of each compound. The curves were analyzed using the Shimadzu TA-60 software (version 2.2).

3. Results and discussion The ORTEP-3 type view of the asymmetric unit of INH salts is depicted in Fig. S1 (see the ESI†). The crystallographic data

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Table 1 Crystal data and refinement parameters of INH salts

Compound

INH hydrobromide

INH nitrate

INH sulfate

INH sulfate hemihydrate

Formula Formula weight (g mol−1) Wavelength (λ) Crystal system Space group Temperature (K) Unit cell a (Å) b (Å) c (Å) α (°) β (°) γ (°) Volume (Å3) Z Density (calculated) (mg m−3) Absorption coefficient (mm−1) Absorption correction θ range for data collection Index range h k l Measured data Unique data Symmetry factor (Rint) Completeness to θmax FĲ000) Parameters refined Goodness of fit on F 2 R [I >2σ(I)]

C6H8BrN3O 218.06 0.71073 Monoclinic P21/c 293(2) 6.6307(4) 16.3280Ĳ13) 7.4836(5) 90 92.183(7) 90 809.63(10) 4 1.789 5.022

C6H8N4O4 200.16 0.71073 Monoclinic Cc 293(2) 6.3630(13) 15.970(6) 8.043(2) 90 92.151(16) 90 816.7(4) 4 1.628 0.138

C6H9N3O5S 235.22 0.71073 Monoclinic P21/n 293(2) 7.3259(2) 6.3982(2) 19.5779(7) 90 98.2460Ĳ10) 90 908.18(5) 4 1.720 0.365

C12H20N6O11S2 488.46 0.71073 Monoclinic P21/c 120(2) 6.9562(3) 19.7321(8) 13.4459(6) 90 93.4175Ĳ13) 90 1842.31Ĳ14) 4 1.761 0.367

Gaussian, Tmin = 0.458, Tmax = 0.937 2.996° to 26.385° −8 to 8 −20 to 20 −8 to 9 5442 1654 0.0802 99.8% 432 100 0.979 R1 = 0.0448, wR2 = 0.1056 R1 = 0.086, wR2 = 0.125 0.504/−0.397

— 3.449° to 25.331° −7 to 7 −19 to −19 −9 to 9 1462 1454 0.0494 99.6% 416 128 1.055 R1 = 0.0495, wR2 = 0.1261 R1 = 0.0636, wR2 = 0.1374 0.161/−0.148

— 3.138° to 26.399° −9 to 9 −8 to 7 −24 to 24 1859 1859 0.101 99.8% 488 136 1.052 R1 = 0.0463, wR2 = 0.1207 R1 = 0.0636, wR2 = 0.1291 0.475/−0.418

— 2.562° to 26.709° −8 to 7 −22 to 23 −16 to 12 10 216 3830 0.055 98.7% 1018 282 1.059 R1 = 0.0366, wR2 = 0.0929 R1 = 0.0460, wR2 = 0.0986 0.4715/−0.6114

R (all data) Largest diff. peak/hole (e Å−3)

and the geometric parameters of the H-bonds present in the crystal structures are listed in Tables 1 and 2, respectively. A detailed description of the crystal structures is given below.

INH hydrobromide In contrast to INH dihydrochloride13 (Fig. 1(a)), which occurs as two polymorphs in the monoclinic space groups P21/n and P21/a with two Cl− anions, INH hydrobromide crystallized in the monoclinic space group P21/c, with one cation, INH+, and one bromide anion in the asymmetric unit. The INH molecule has two nitrogen atoms that can be protonated, the pyridine N2 and the amine N3 atoms. However, because the pKa value of the pyridine group (3.8) is higher than that of the amine atom (1.8), in the presence of a monoprotic acid, such as HBr, the pyridine is protonated first. In the protonated form, the pyridine N2 atom acts as a proton donor to the hydrazide group, connecting the INH+ molecules through the bifurcated N2+– H2⋯O3 and N2+–H2⋯N3 H-bonds (Fig. 1(b)). Hence, these head-to-tail interactions give rise to infinite 1-D chains par-

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¯02) (Fig. 1(c)). These chains stack into columns allel to (1 along the c-axis via π–π interactions involving opposite pyridinium rings (Ct⋯Ct = 3.745(2) Å) (Fig. 1(d)). The 3-D assembly is characterized by an intercalation of bromide anions and drug columns along [010]. The counterion in turn is involved in a hydrogen bond with the hydrazide N1 atom of the drug (N1–H1⋯Br−). It is worth noting that this assembly is quite different from that described for the INH hydrochloride. In the latter, the cations do not interact directly with each other; they are intercalated by chloride anions, being connected to them through their pyridinium and hydrazide groups in order to form a 1-D chain (Fig. 1(a)).

INH nitrate This salt crystallizes in the monoclinic space group Cc with one INH+ cation and one nitrate anion in the asymmetric unit. As expected, the pyridine N2 atom is protonated, serving as a hydrogen bond donor to the O1 and N3 atoms of the hydrazide group. The INH+ molecules are also arranged in a head-to-tail motif, forming 1-D chains


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Table 2 Geometric parameters of the H-bonds in the INH salts

Interaction

D⋯H (Å)

Isoniazid hydrobromide 1.04 N2+–H2⋯O1 1.04 N2+–H2⋯N3 0.91 N1–H1⋯Br1− Isoniazid nitrate 1.15 N2+–H2⋯O1 1.15 N2+–H2⋯N3− C5–H5⋯O1 0.93 0.90 N3–H3B⋯O2− 0.80 N1–H1⋯O3− − 0.94 N3–H3A⋯O4 0.94 N3–H3A⋯O2− 0.80 N1–H1⋯O4− Isoniazid sulfate 1.04 N2+–H2⋯O2− 0.94 N3+–H3A⋯O4− 0.94 N3+–H3B⋯O3− 0.87 N3+–H3C⋯O3− − 0.91 N1–H1⋯O4 0.93 C4–H4⋯O2− C6–H6⋯O1 0.93 Isoniazid sulfate hemihydrate 0.96 N2+–H2⋯O3− 0.84 N1–H1⋯O5′− 0.93 N3+–H3A⋯O3′− 0.92 N3+–H3C⋯O4− − 0.93 C6–H6⋯O2′ 0.93 C5–H5⋯O3′− 0.95 N3+–H3B⋯O2− 0.85 N2′+–H2′⋯OW 0.92 N3′+–H3′A⋯O2′− 0.92 N3′+–H3′C⋯O5− 0.92 N3’+–H3’B⋯O4′− 0.86 N1′+–H1′⋯O4− − 0.87 OW–HWB⋯O4′ 0.87 OW–HWA⋯O2′− 0.87 OW–HWA⋯O5′−

D⋯A (Å)

H⋯A (Å)

D–H⋯A (°)

Symmetry code

2.720(4) 3.001(5) 3.385(3)

1.90 2.10 2.50

133 144 166

x + 1, −y + 1/2, +z + 1/2 x + 1, −y + 1/2, +z + 1/2 −x + 1, −y, −z + 1

2.937(6) 2.873(6) 2.969(6) 3.265(6) 2.955(6) 3.193(6) 3.214(6) 3.149(6)

2.20 1.79 2.39 2.46 2.37 2.41 2.50 2.46

119 156 120 150 130 140 132 144

x − 1, −y, +z + 1/2 x − 1, −y, +z + 1/2 x − 1, −y, +z + 1/2 x + 1/2, −y + 1/2, +z + 1/2 x + 1/2, −y + 1/2, +z + 1/2 x + 1/2, −y + 1/2, +z − 1/2 x, y, z x, y, z

2.612(2) 2.854(2) 2.844(2) 2.752(2) 2.751(2) 3.109(3) 3.244(2)

1.58 2.09 1.93 1.92 1.95 2.22 2.38

169 137 163 159 145 160 155

x − 1/2, −y + 1/2, +z − 1/2 −x + 1/2, +y + 1/2, −z + 1/2 −x + 1/2 + 1, +y + 1/2, −z + 1/2 x, y, z x, y, z −x + 1/2, +y − 1/2, −z + 1/2 −x + 1, −y + 2, −z

2.618(2) 2.704(2) 2.729(2) 2.743(2) 3.152(2) 3.099(2) 2.747(2) 2.704(2) 2.894(2) 2.704(2) 2.861(2) 2.693(2) 2.800(2) 2.962(2) 2.740(2)

1.67 1.88 1.84 1.84 2.33 2.28 1.81 1.87 1.98 1.79 1.94 1.85 1.95 2.41 1.88

171 170 159 167 148 147 168 169 169 172 174 171 166 122 171

x, −y + 1/2, +z + 1/2 −x, +y + 1/2, −z + 1/2 −x, +y + 1/2, −z + 1/2 x − 1, +y, +z x, y, z x, y, z x, y, z −x + 1, −y + 1, −z + 1 x + 1, +y, +z −x + 1, +y − 1/2, −z + 1/2 x, y, z x, −y + 1/2, +z + 1/2 x, −y + 1/2, +z + 1/2 −x, +y + 1/2, −z + 1/2 −x, +y + 1/2, −z + 1/2

parallel to (102). However, in this salt an additional C– H⋯O H-bond is established (C5–H5⋯O1) (Fig. 2(a)). This interaction is not observed in INH hydrobromide because, in comparison with INH nitrate, the INH+ molecules are oriented at a different angle with respect to each other. The angle between the C5–H5 bond and the O1 atom of the adjacent cation is 120° for INH nitrate and 104° for INH hydrobromide, with the latter being too close to 90°, which is not favorable for a hydrogen bond. As in INH hydrobromide, these chains are also stacked in columns along the c-axis, but since the inter-ring distances are longer than 3.8 Å, the π–π interactions are slightly weaker (Ct⋯Ct = 4.077(3) Å). The resulting 3-D assembly follows the same pattern as that reported for INH hydrobromide, with columns of drug molecules intercalated by layers of anions (Fig. 2(b) and (d)). However, in this case, the counterion is involved in interactions with isoniazid molecules of both column sides via the bifurcated N3–H3A⋯O3− and N1–H1⋯O3− H-bonds. Additionally, the combination of these interactions with the N1–H1⋯O3− and N1–H1⋯O4− H-bonds produces two R22(7) ring motifs, each one involv-


ing the nitrate anion and the hydrazide group of both sides of each column of drug molecules (Fig. 2(c)).

INH sulfate The INH sulfate structure presents a 1 : 1 stoichiometry, with one dication (INH2+) and one sulfate anion in the asymmetric unit. The main difference between this salt and the hydrobromide and nitrate salts is that now both basic sites of the drug are protonated. This is because sulfuric acid is a strong diprotic acid able to donate two protons: one to the amide N3 atom and the other to the pyridine N2 atom. As a doubly protonated dication, the INH2+ molecules cannot assume the head-to-tail arrangement previously described here once both N3 and N2 atoms are now hydrogen bond acceptors. Unlike the hydrobromide and nitrate salts, the INH2+ molecules do not interact directly with each other via pyridinium–hydrazide H-bonds. Instead, the cations are related by an inversion center forming a centrosymmetric R22(10) homodimer (Fig. 3(c)) through the reciprocal C6– H6⋯O1 interactions (C⋯O = 3.244(2) Å, ∠ C–H⋯O = 155°).

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Fig. 1 1-D chain pattern of H-bonds for INH hydrochloride (a) and INH hydrobromide (b) salts. View of the crystallographic plane (1¯02) in the crystal structure of INH hydrobromide (c). INH hydrobromide packing highlighting the 2-D columns and the bromide channels along the c-axis (d).

Such dimers further assemble into 1-D chains parallel to ¯¯ (84 3) (Fig. 3(b)) via an R24(10) motif formed by pyridinium– sulfate H-bonds (N2+–H2⋯O2− and C4–H4⋯O2−). These chains, in turn, are arranged into columns along the a-axis through π–π interactions (Ct1⋯Ct2 = 3.6345(12) Å; Ct2⋯Ct1 = 3.7648(12) Å) involving pyridinium rings of opposite orientation and also via the N3+–H3A⋯O4− and N3+–H3B⋯O3− H-bonds (Fig. 3(d)). Since these columns are intercalated by sulfate layers, each chain interacts with both left and right anions through their pyridinium and hydrazide groups (Fig. 3(b)). Adjacent INH+ chains and consequently adjacent columns (Fig. 3(a)) show two directions of growth (dihedral angle between them of 59.37°), being connected to one another by the sulfate via the R22(7) motif (N3+–H3C⋯O3− and N1–H1⋯O4−) and the N2+–H2⋯O2− H-bond (Fig. 3(d)). In order to verify the occurrence of the R22(10) homodimer, we have performed a detailed search on the CSD database. A total of 86 entries were obtained among salts, cocrystals and

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metal complexes of INH and only one compound (ref. code LATLEZ) showed the spatial arrangement to form the mentioned homodimer (C⋯O = 3.144(3) Å, ∠ C–H⋯O = 136°). In this context, it is possible to affirm that INH sulfate is the second structure presenting a homodimer that is not formed via conventional hydrazide–hydrazide interactions (Fig. S6, ESI†).

INH sulfate hemihydrate In the course of our experiments for preparing INH sulfate, we found a hemihydrate form of this salt. INH sulfate hemihydrate exhibits two dications (A and B; see Fig. S1† for atom labels) and two sulfate anions in the asymmetric unit in addition to one water molecule. Hence, both sites of the isoniazid molecule are protonated. The two INH2+ molecules are independent and differ slightly with respect to the torsion angle φ (C6–C1–C2–N1), which assumes the value of −179.87Ĳ18)° in cation A and −176.37Ĳ18)° in B. In fact, the hydrogen bonds


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Fig. 2 1-D chain pattern of H-bonds for INH nitrate (a). Crystal packing of INH nitrate along the c- (b) and a-axis (c). Partial view of the packing along the b-axis, showing the main supramolecular interactions and motifs formed between the nitrates and the INH+ cations of adjacent columns (d).

play a fundamental role in the conformation of these molecules. The N1′ atom of cation B is hydrogen bonded to the O4 atom of the anion. In order to favor the N1′–H1′ ⋯O4 hydrogen bond, the N1′ atom is positioned out of the plane defined by the pyridinium ring, which does not occur in cation A, where the N1 atom lies almost in the same plane. With respect to the supramolecular assembly (Fig. 4(a)), this salt also assumes an arrangement that allows both cations A and B to interact with each other through π–π stacking interactions (CtA⋯CtB = 3.6709(12) Å; CtB⋯CtA = 3.5412(12) Å). In fact, this salt exhibits a packing similar to the one described for the INH sulfate form, with cations A and B almost exactly related by a pseudo-glide plane. However, because the sulfate is involved in C–H⋯O interactions with the C4, C5 and C6 atoms of cation A, the H-bonds required for the stabilization of the homodimer are not favored in this structure (Fig. S2, ESI†). As expected, each cation establishes a specific pattern of interactions. Besides the pyridinium–sulfate N2+–H2⋯O3 Hbond, cation A forms with the sulfate anion, via the N3+– H3B⋯O3′− and N1+–H1⋯O5′− interactions, the same R22(7)


motif reported for the INH sulfate form (Fig. 4(b)). Interestingly, in cation B, the pyridinium ring acts as a hydrogen bond donor to the oxygen atom of the water molecule instead of to the oxygen atoms of the counterion. Indeed, only the hydrazide group of cation B interacts with the anion. The water molecules in turn are connected to two sulfate anions, establishing an R44(12) motif that binds the two cations (Fig. 4(c)). At this point, the role of the water molecule is clear and is associated with the stabilization of cation B, which appears in this structure as the result of inclusion of the water molecule in the lattice. Even so, this salt also maintains the typical 3-D assembly described for INH sulfate, with the drug intercalated by layers composed of sulfate anions and water molecules parallel to (010) (Fig. 4(d)).

Thermal characterization The thermal profile of INH salts was obtained by a combination of DSC, TGA and HSM techniques. The DSC curve of INH hydrobromide shows an endothermic peak at 218.19 °C (Tonset = 200.62 °C) attributed to the sublimation process, since this event is accompanied by a gradual mass loss in the

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Fig. 3 Crystal packing of INH sulfate along the c-axis highlighting the crystallographic plane (a) and the sulfate layers parallel to (001) (b). H-bond 1-D chain pattern showing the homo- and heterodimers established between the sulfates and the INH+ cations (c). Partial view of the packing along the b-axis showing the π–π interactions established between the 1-D chains (d).

TGA curve up to about 190 °C (Fig. 5(a)). INH nitrate exhibits an endothermic peak at 185.75 °C (Tonset = 183.11 °C), which was attributed to the melting process. This finding is consistent with the TGA curve, which does not show any mass loss at this temperature. The other exothermic peak is associated with the evaporation of the sample, with this salt stable up to around 188 °C. Above this temperature, an abrupt mass loss is observed in the TGA curve (Fig. 5(b)) due to the evaporation process. The DSC and TGA curves of INH sulfate are presented in Fig. 5(c). The DSC curve exhibits an endothermic peak characteristic of a melting process at 204.05 °C (Tonset = 197.12 °C). This event is followed by an exothermic peak attributed to the first stage of thermal decomposition. These events are in agreement with the TGA curve, which shows that INH sul-

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fate is thermally stable up to 209 °C before it starts to lose mass. Beyond the endothermic melting peak at 206.20 °C (Tonset = 201.82 °C), an initial endothermic peak at 99.06 °C (Tonset = 95.29 °C) is observed in the DSC curve of INH sulfate hemihydrate. This peak corresponds to the loss of water from the crystalline lattice and is followed by a mass loss of about 3.2% in the TGA curve at 98–106 °C, which agrees very well with the theoretical value (3.7%). The additional exothermic DSC peak is associated to the first stage of the decomposition process. This event agrees with the gradual mass loss that occurs in the TGA curve after 207 °C (Fig. 5(d)). It is worth noting that above 150 °C, the DSC and TG curves of Fig. 5(c) and (d) are similar, suggesting that INH sulfate hemihydrate transforms into the anhydrous form reported


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Fig. 4 Crystal packing of INH sulfate hemihydrate along the c-axis (a) showing the main supramolecular interactions and motifs involving the two ionic pairs and the water molecule (phenyl H-atoms were omitted for clarity) (b and c). View of the water/sulfate layers parallel to (010) (d).

Fig. 5 DSC and TGA curves of INH salts: (a) hydrobromide, (b) nitrate, (c) sulfate and (d) sulfate hemihydrate.

here. Powder X-ray diffraction (PXRD) experiments have confirmed this transition (for more details see the ESI†).


In addition, the DSC curves of these salts did not show any other peaks that could be associated with any phase

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agree with the DSC and TGA results (Fig. 6). From the HSM images, it is possible to see that INH hydrobromide does not melt but sublimes at around 185 °C. The images also show the beginning of melting of the INH nitrate crystal (∼168 °C). The melting of INH sulfate and INH sulfate hemihydrate are seen practically at the same temperature (203–205 °C). For the latter salt, one can note a change in the crystal appearance at 100 °C due to the loss of water.

Hirshfeld surfaces

Fig. 6 Hot-stage microscopy images of hydrobromide (a), nitrate (b), sulfate (c) and sulfate hemihydrate (d) INH crystals.

transition. Furthermore, they also show a thermal behavior different from that of the starting compound INH (melting point ∼170–173 °C). In order to visualize these thermal events, HSM was also performed and, as expected, the images

The 3D Hirshfeld surfaces (HS) mapped with dnorm for the INH+ cations are similar in the four INH salts (Fig. S3†). The intermolecular interactions (closer than the sum of their van der Waals radii) can be visualized on these surfaces as red areas located on the hydrazide group (donor/receptor) and in the pyridine ring (donor). Nevertheless, the 2D fingerprint plots derived from the Hirshfeld surface for each salt are unique. In particular, the INH hydrobromide plot differs from the others (Fig. 7(I)) by the presence of three spikes projecting along the diagonal of the plot instead of the two spikes observed in the other salts. The sharp thin spike, spreading up to the shorter contact distance (minimum value of di + de) of 2.4 Å, is characteristic of the strong N1–H1⋯Br−

Fig. 7 2-D fingerprint plots of hydrobromide (I), nitrate (II), sulfate (III) and sulfate hemihydrate (IV) INH salts. Full fingerprint plots are shown in the first line and these are resolved into O⋯H/H⋯O, N⋯H/H⋯N and H⋯Br/Br⋯H (only for I) contacts. IVa and IVb denote the two cations in the asymmetric unit of IV.

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interaction and represents 16.0% of all molecular surfaces. The two remaining broad spikes are associated with the reciprocal O⋯H/H⋯O and N⋯H/H⋯N contacts, contributing 15.7% and 8.2%, respectively. The O⋯H/H⋯O contacts are quite strong and this is reflected in the fingerprint plot, where the spikes corresponding to these contacts appear to be noticeably longer than the ones corresponding to the N⋯H/H⋯N contacts. The O⋯H/H⋯O contacts are also dominant in the crystal packing of INH nitrate (Fig. 7(II)), contributing 37.6% to the total HS. These interactions are depicted as two short broad spikes (minimum di + de = 2.15 Å), the upper one corresponding to the N–H⋯O hydrogen bonds and the lower one to the O⋯H–N interactions. The N⋯H/H⋯N contacts, in turn, appear as a more prominent and longer pair of spikes (minimum di + de = 1.95 Å), contributing only 8.0% of all molecular surfaces. The 2-D fingerprint plots of both salts, INH sulfate and INH sulfate hemihydrate (cations A and B), are quite similar (Fig. 7(III and IVa, and IVb)), although some peculiarities are observed. All plots display two non-symmetrical sharp spikes corresponding to the O⋯H/H⋯O contacts that, as expected, have the largest contributions. The upper long spike is associated with the H-bond donor and the lower short spike with the acceptor. In these three plots, the value of the shortest contact (minimum value of di + de) is nearly identical and close to 1.60 Å. Therefore, careful inspection reveals significant differences in the distribution pattern of blue points in these plots. First, at higher values of (de, di) the plots of cation A and INH sulfate are more compact, indicating a more efficient packing of them in comparison with cation B, where the points are more diffuse. Second, the high density of blue points in the lower spike of cation A agrees with the fact that it has C–H⋯O interactions that are not observed in cation B. Concerning the reciprocal N⋯H contacts, they do not appear as a pair of spikes as in the hydrobromide and nitrate salts. In fact, they not only have the lowest contributions, but they also have blue points concentrated at upper values of

Fig. 8 Relative contributions of the various intermolecular contacts to the Hirshfeld surface areas of hydrobromide (I), nitrate (II), sulfate (III) and sulfate hemihydrate (IV) INH salts. IVa and IVb denote the two cations in the asymmetric unit of IV.


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(de, di), which shows that these contacts are practically absent in the INH sulfate forms. A graphical summary showing the relative contributions from the different intermolecular contacts to the total HS in the INH salts is depicted in Fig. 8. As could be observed, in all the cases the supramolecular structure is formed in major proportion for the O⋯H/H⋯O interactions, except for the INH hydrobromide where the H⋯Br and H⋯H interactions (Fig. S4, ESI†) have the largest contributions. In addition to the oxygen atom of the carbonyl group, INH hydrobromide has no other oxygen atoms available for interactions. This is not the case with the nitrate and sulfate salts. Both salts have counterions rich in oxygen atoms, which can explain the major proportion of the O⋯H/H⋯O interactions and consequently the reduction of the H⋯H contributions.

4. Conclusions In this study, four inorganic salts of the anti-tuberculosis drug isoniazid (INH) were prepared and investigated using single-crystal X-ray diffraction, Hirshfeld surface analysis, thermogravimetry, differential scanning calorimetry and hotstage microscopy. Both INH hydrobromide and INH nitrate adopt a supramolecular 1-D arrangement where the cations (INH+ molecules) are connected to each other via pyridinium–hydrazide H-bonds. In the INH sulfate salt, the cations form rather unexpected R22(10) homodimers. The sulfate anions, in turn, bridge these homodimers into a 1-D chain via an R24(10) motif formed by pyridinium–sulfate H-bonds. Interestingly, such a homodimer is not preserved in the INH sulfate hemihydrate structure, which comprises two ionic pairs besides a water molecule in the asymmetric unit. The water molecule is involved in the stabilization of the second cation, establishing with the sulfate anion an R22(10) motif that binds both cation A and cation B. Overall, columns of cations intercalated between layers of anions characterize the 3-D assembly of these four salts. The DSC and TGA results revealed that the sulfate salts have a higher melting point than the nitrate and hydrobromide salts, most probably because of the presence of much stronger anion–INH+ H-bonds. The inclusion of the water molecule in the crystal lattice of the hemihydrate salt seems to have no effect on its thermal stability. Because it has a melting point very close to that of the non-solvated form, we suggest that INH sulfate hemihydrate converts into its anhydrous salt previously reported here after the loss of the water molecule. Indeed, PXRD experiments confirmed this transition. After heating, the experimental PXRD pattern of the hemihydrate form changed significantly, yielding a good match with the simulated PXRD pattern of the anhydrous salt. These results not only show a good agreement with the HSM images but also show that a better comprehension of the supramolecular interactions can be crucial for the development of more stable INH solid forms.

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Acknowledgements We would like to thank CNPq (C.C.M. project No. 150076/ 2013-4 to J. E.), FAPESP (grant 12/05616-7 to P. S. C. Jr, grant 15/25694-0 to L. F. D.) and CAPES (to A. P. A. and R. F. D.) for the financial support.

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