A study of the crystal structures, supra­molecular patterns and Hirshfeld surfaces of bromide salts of hypoxanthine and xanthine

Sathya, U.; Nirmalram, J.S.; Gomathi, S.; Dhivya, D.; Jegan Jennifer, S.; Abdul Razak, I.

doi:10.1107/S2056989022005278

research communications

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ISSN: 2056-9890

Volume 78| Part 6| June 2022| Pages 652-659

https://doi.org/10.1107/S2056989022005278

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A study of the crystal structures, supramolecular patterns and Hirshfeld surfaces of bromide salts of hypoxanthine and xanthine

Udhayasuriyan Sathya,^a Jeyaraman Selvaraj Nirmalram,^a ^* Sundaramoorthy Gomathi,^b Durairaj Dhivya,^a Samson Jegan Jennifer ^c and Ibrahim Abdul Razak ^c

^aCentre for Research and Development, PRIST Deemed to be University, Thanjavur, 613 403, Tamil Nadu, India, ^bDepartment of Chemistry, Periyar Maniammai Institute of Science and Technology, Thanjavur 613 403, Tamil Nadu, India, and ^cX-ray Crystallography Unit, School of Physics, University Sains Malaysia, 11800, USM, Penang, Malaysia
^*Correspondence e-mail: nirmalramjs@gmail.com

Edited by V. Jancik, Universidad Nacional Autónoma de México, México (Received 7 March 2022; accepted 18 May 2022; online 20 May 2022)

Two new crystalline salts, namely, hypoxanthinium bromide monohydrate, C₅H₅N₄O⁺·Br⁻·H₂O (I) and xanthinium bromide monohydrate, C₅H₅N₄O₂⁺·Br⁻·H₂O (II), were synthesized and characterized by single-crystal X-ray diffraction technique and Hirshfeld surface analysis. The hypoxanthinium and xanthinium cations in salts I and II are both in the oxo-N(9)–H tautomeric form. The crystal packing of the two salts is governed predominantly by N–H⋯O, N–H⋯Br, C–H⋯Br and O–H⋯Br interactions described by R²₃(9) and R²₂(8) synthons. The crystal packing is also consolidated by carbonyl⋯π interactions between symmetry-related hypoxanthinium (HX⁺) cations in salt I and xanthinium cations (XA⁺) in salt II. The combination of all these interactions leads to the formation of wave- and staircase-like architectures in salts I and II, respectively. The largest contributions to the overall Hirshfeld surface are from Br⋯H/H⋯Br contacts (22.3% in I and 25.4% in II) .

Keywords: crystal structure; hydrogen bonding; hirshfeld surface analysis; hypoxanthine; xanthine.

CCDC references: 2170923; 2170922

1. Chemical context

Over the past several decades, non-covalent interactions have been found to play a prominent role in coordination chemistry, materials science and pharmaceutical science (Černý & Hobza, 2007 ; Desiraju, 2013 ; Perumalla & Sun, 2014 ). Understanding the role of non-covalent interactions is important in the context of crystal engineering (Aakeröy et al., 2010 ; Pogoda et al., 2018 ; Cavallo et al., 2016 ; Desiraju et al., 2013 ) in order to design solids with desired properties. When it comes to pharmaceutics, active pharmaceutical ingredients (APIs) are known to exist in different solid forms such as salts, co-crystals, solvates, polymorphs and amorphous solids (Aaltonen et al., 2009 ). The salt and co-crystal forms of APIs have improved their solubility and bioavailability when compared to pure APIs (Thackaberry, 2012 ; Xu, et al., 2014 ). Drugs with low solubility/bioavailability are usually converted to their salts or crystallized in their co-crystal/polymorphic/solvate forms to enhance their properties. Herein, we report two new salts of hypoxanthine (HX) and xanthine (XA).

Hypoxanthine (C₅H₄N₄O) [systematic name: 1,9-dihydro-purine-6-one] and xanthine (C₅H₄N₄O₂) [systematic name: 3,7-dihydro-purine-2,6-dione] are well-known purine-based nucleotides (Emel'yanenko et al., 2017 ) present in t-RNA and DNA in the form of the nucleoside inosine (Plekan et al., 2012 ). Purine derivatives are widely known for their therapeutic applications such as antagonization of the adenosine receptor, anti-inflammatory, antimicrobial, antioxidant, anti-tumour, anti-asthmatic and psycho-stimulant drug activity (Meskini et al., 1994 ; Burbiel et al., 2006 ). HX and XA are also found as intermediates in the biological degradation of nucleic acid to uric acid. Furthermore, HX is used as an indicator of hypoxia and it is known to inhibit the effect of several drugs (Dubler et al., 1987a ,b ). It is also used to destroy harmful agents such as cancer cells (Susithra et al., 2018 ). Purine-based derivatives of HX and XA bind with the DNA base pairs through weak hydrogen bonds (Latosińska et al., 2014 ; Rutledge et al., 2007 ). Additionally, hypoxanthine-guanine phosphoribosyl transferase plays an important role in activating antiviral drugs in the human body and xanthine has been used as a mild stimulant drug (Faheem et al., 2020 ).

The structure of hypoxanthine and xanthine consists of fused six-membered pyrimidine and five-membered imidazole rings. HX and XA can exist in two tautomeric forms, oxo-N(7)–H and oxo-N(9)–H (Plekan et al., 2012; Gulevskaya & Pozharskii, 1991 ), as shown below. So far, two polymorphic forms of HX (Schmalle et al., 1988 ; Yang & Xie, 2007 ) and a limited number of hypoxanthinium and xanthinium salts have been reported in the literature; hypoxanthinium nitrate monohydrate, hypoxanthinium chloride monohydrate (Cabaj et al., 2019 ; Schmalle et al., 1990 ; Sletten & Jensen, 1969 ), xanthinium nitrate monohydrate and xanthinium hydrogensulfate monohydrate (Sridhar, 2011 ).

In the hypoxanthinium salts, the hypoxanthine molecule is usually also protonated at the N7 position, resulting in the oxo-N(9)–H tautomer. Similarly, xanthinium nitrate monohydrate, xanthinium hydrogensulfate monohydrate (Sridhar, 2011) and xanthinium perchlorate dihydrate (Biradha et al., 2010 ) are also in the oxo-N(9)–H tautomeric form and are therefore protonated on the N7 position. Studies of non-covalent interactions involving hypoxanthine and xanthine bases with inorganic acids have increased because their hydrogen-bonding patterns are similar to those of purine bases (Maixner & Zachova, 1991 ; Sridhar, 2011; Kistenmancher & Shigematsu, 1974 ). In the current work, the crystal structures, supramolecular packing patterns and Hirshfeld surface analyses of hypoxanthinium bromide monohydrate (I) and xanthinium bromide monohydrate (II) are reported.

2. Structural commentary

Hypoxanthinium bromide monohydrate (I) crystallizes in the monoclinic space group P2₁/c with one hypoxanthinium cation (HX⁺), one bromide anion (Br⁻) and one water molecule in the asymmetric unit, as shown in Fig. 1. Here, the HX⁺ cation exists in the oxo-N(9)–H tautomeric form with the N7 atom of the purine ring protonated, as can be seen from the N—C bond distance [N7—C8 = 1.3219 (17) Å vs N9—C8 = 1.3419 (18) Å] and C—N—C bond angles [C5—N7—C8 = 107.98 (11)° and C4—N9—C8 = 108.32 (10)°]. Those values are similar to those in the crystal structure of hypoxanthinium chloride monohydrate [N7—C8 = 1.325 (2) Å and N9—C8 = 1.336 (2) Å, C5—N7—C8 = 107.35 (16)° and C4—N9—(C8 = 108.28 (15)°; Kalyanaraman et al., 2007 ; Sletten & Jensen, 1969]. The N3—C4—C5—N7 and N9—C4—C5—C6 torsion angles are 179.07 (12) and −179.58 (12)°, respectively. These values are similar to those observed in the crystal structure of the neutral hypoxanthine molecule (Schmalle et al., 1988; Yang & Xie, 2007). The HX⁺ cation, Br⁻ anion and the water molecule interact through N—H⋯Br, N—H⋯O and C—H⋯Br hydrogen bonds with donor–acceptor distances N⋯Br = 3.2419 (13) Å, N9⋯O6 = 2.7579 (14) Å and C8⋯Br1 = 3.4875 (15) Å (Table 1), forming an $[R_{3}^{2}]$ (9) motif. The water molecule present in the lattice prevents the formation of base pairs (Varani & McClain, 2000 ) between the HX⁺ cations.

Table 1
Hydrogen-bond geometry (Å, °) for I

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N9—H9⋯Br1ⁱ	0.85 (1)	3.08 (2)	3.5397 (12)	117 (2)
N9—H9⋯O6ⁱ	0.85 (1)	1.98 (2)	2.7579 (14)	153 (2)
N1—H1⋯Br1	0.84 (1)	2.41 (1)	3.2419 (12)	170 (2)
N7—H7⋯O1Wⁱⁱ	0.85 (1)	1.81 (2)	2.6401 (16)	165 (2)
O1W—H1W⋯N3ⁱⁱⁱ	0.86 (1)	2.08 (1)	2.9200 (16)	165 (2)
O1W—H2W⋯Br1	0.85 (1)	2.48 (1)	3.2894 (12)	161 (2)
C8—H8⋯Br1ⁱ	0.93	2.89	3.4875 (15)	123
Symmetry codes: (i) $[x+1, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (ii) $[-x-1, y-{\script{1\over 2}}, -z+{\script{3\over 2}}]$ ; (iii) .

Figure 1
ORTEP view of the molecular components of salts I and II, showing the atom-labelling scheme. Displacement ellipsoids are drawn at 50% probability level. Hydrogen bonds are shown as dashed lines.

Xanthinium bromide monohydrate (II) also crystallizes in the monoclinic space group P2₁/c with one xanthinium cation (XA⁺), one bromide anion (Br⁻) and one water molecule in the asymmetric unit (Fig. 1). The XA⁺ cation has the N7—C8 bond [1.312 (5) Å] shorter than N9–C8 one [1.344 (5) Å]. The C—N—C bond angles are C5—N7—C8 = 108.2 (3)° and C4—N9—C8 = 107.7 (3)° and, therefore, the cation can also be described as the oxo-N(9)–H tautomer. These values are similar to those in xanthinium perchlorate dihydrate [N7—C8 = 1.314 (3) Å, N9—C8 = 1.341 (3) Å, C5—N7—C8 = 108.3 (16)° and C4—N9—C8 = 107.58 (15)°; Biradha et al., 2010). The N3—C4—C5—N7 and N9—C4—C5—C6 torsion angles in II are 179.07 (12)° and −179.58 (12)°, respectively. Finally, the two symmetry-related XA⁺ cations in II form a base pair similar to that observed between guanine and uracil (Varani & McClain, 2000).

3. Supramolecular features

In I, the protonated HX⁺ cation interacts with another inversion-related HX⁺ and Br⁻ pair via N1—H1⋯Br1, C8—H8⋯Br1ⁱⁱ and N9—H9⋯O6ⁱⁱ hydrogen bonds (Table 1). These interactions lead to the formation of a nine-membered ring with $[R_{3}^{2}]$ (9) (type D) primary graph-set motif (Sletten & Jensen, 1969). Along with this, the HX⁺ cation interacts with another inversion-related HX⁺ cation and a water molecule through O1W—H1W⋯N3ⁱⁱⁱ and N7—H7⋯O1Wⁱⁱ hydrogen bonds. The combination of these interactions leads to the formation of an eleven-membered R₃³(11) (type I) ring motif. The interaction is very similar to the water-mediated base pairs observed in the crystal structure of hypoxanthinium chloride and the nucleobase pairs in DNA and RNA (Sletten & Jensen, 1969; Reddy et al., 2001 ; Brandl et al., 2000 ). Here the O1W atom of the water molecule acts as both a hydrogen-bond donor and a hydrogen-bond acceptor. The $[R_{3}^{2}]$ (9) and R₃³(11) ring motifs combine to form a supramolecular ribbon. Adjacent ribbons are connected through pairs of O1W—H2W⋯Br1 hydrogen bonds with R₆⁴(16) and R₆⁴(14) (types N and O motifs) ring motifs, respectively, through pairs of C8—H8⋯Br1ⁱ and N7—H7⋯O1Wⁱⁱ hydrogen bonds (Fig. 2). The combination of all these interactions leads to the formation of a wave-like supramolecular architecture that extends along the b-axis direction (Fig. 3). The crystal structure is further consolidated by carbonyl⋯π interactions (C6=O6 and π cloud of the imidazole (centroid Cg1) and pyridine (centroid Cg2) rings of the HX⁺ cation) between symmetry-related cations with C=O⋯Cg1^iv, C=O⋯Cg1^v, C=O⋯Cg2^iv and C=O⋯Cg2^v distances of 3.5796 (12), 3.2478 (12) Å, 3.3862 (12) and 3.4747 (12) Å, respectively, and angles of 101.58 (8), 91.45 (8), 105.03 (8) and 103.46 (8)°, respectively [symmetry codes: (iv) −1 + x, y, z; (v) x, $[{1\over 2}]$ − y, $[{1\over 2}]$ + z] (Fig. 4). Salt I is isomorphous with hypoxanthinium chloride monohydrate (Sletten & Jensen, 1969).

Figure 2
Hypoxanthinium and bromide ions in salt I forming ribbons together with water molecules through O—H⋯Br, N—H⋯Br and C—H⋯Br interaction. [Symmetry codes: (i) −1 − x, − $[{1\over 2}]$ + y, $[{3\over 2}]$ − z; (ii) 1 + x, $[{1\over 2}]$ − y, − $[{1\over 2}]$ + z; (iii) −x, 1 − y, 1 − z].

Figure 3
A view of three-dimensional wave-like supramolecular architecture along the b-axis direction.

Figure 4
A view of the C=O(carbonyl)⋯π interactions (dashed lines) between the HX⁺ cations in salt I. [Symmetry codes: (i) −1 + x, y, z; (ii) x, $[{1\over 2}]$ − y, $[{1\over 2}]$ + z].

In the crystal structure of salt II, the XA⁺ cation interacts with its inversion-related equivalent to form a dimer through a pair of N1—H1⋯O2ⁱ hydrogen bonds (Table 2) with an $[R_{2}^{2}]$ (8) graph-set motif (type C in the scheme above). The dimer is flanked on both sides by a water molecule (O1W), forming a pair of O1W—H2W⋯O2^iv and O1W—H1W⋯O6ⁱⁱ hydrogen bonds with an $[R_{3}^{2}]$ (8) graph-set motif (type H), leading to the formation of a tetrameric unit. The tetrameric unit is formed by an alternate arrangement of $[R_{2}^{2}]$ (8) and $[R_{3}^{2}]$ (8) ring motifs, which extend as DADA array (dimeric units held together by four hydrogen bonds between the self-complementary DADA arrays; D = donor and A = acceptor) along the ac plane. Neighbouring tetrameric units are then connected through two sets of $[R_{2}^{2}]$ (7) motifs (Jeffrey & Saenger, 1991 ) formed by N7—H7⋯O1W and O1W—H1W⋯O6ⁱⁱ hydrogen bonds and an $[R_{2}^{2}]$ (4) (type L) motif formed by a pair of O1W—H1W⋯O6ⁱⁱ interactions. The tetrameric units combine into a supramolecular ribbon extended along the ac plane (Fig. 5). Neighbouring perpendicular supramolecular ribbons are then interconnected through pairs of N3—H3⋯Br1ⁱⁱⁱ and N9—H9⋯Br1 hydrogen bonds with an R₈⁶(28) ring motif, which assembles them into a staircase-like supramolecular architecture as shown in Figs. 6 and 7. The crystal structure is further consolidated by carbonyl⋯π interactions between symmetry-related XA⁺ cations [C6=O6 and π cloud of the pyridine ring (centroid Cg2) of the XA⁺ unit) with C=O⋯Cg2^vi and C=O⋯Cg2^vii distances of 3.366 (3) and 3.477 (3) Å, respectively, and angles of 108.2 (2) and 118.7 (2)° [symmetry codes: (vi) 1 + x, y, z; (vii) 1 − x, 1 − y, 1 − z; Fig. 8).

Table 2
Hydrogen-bond geometry (Å, °) for II

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1⋯O2ⁱ	0.82 (2)	2.09 (2)	2.903 (4)	175 (4)
N3—H3⋯Br1ⁱⁱ	0.82 (2)	2.48 (2)	3.301 (3)	176 (4)
N7—H7⋯O1W	0.82 (2)	1.81 (2)	2.609 (4)	163 (4)
N9—H9⋯Br1	0.82 (2)	2.43 (2)	3.237 (3)	172 (4)
O1W—H1WA⋯O6ⁱⁱⁱ	0.86 (1)	1.95 (1)	2.802 (4)	171 (5)
O1W—H1WB⋯Br1^iv	0.86 (1)	3.03 (4)	3.490 (3)	115 (3)
O1W—H1WB⋯O2^v	0.86 (1)	2.05 (3)	2.816 (4)	149 (4)
Symmetry codes: (i) ; (ii) $[x, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (iii) ; (iv) $[x+1, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (v) .

Figure 5
Formation of a supramolecular ribbon with a DADA array in salt II via N—H⋯O and O—H⋯O hydrogen bonds between cations and water molecules.

Figure 6
Supramolecular ribbons connecting adjacent ribbons through N—H⋯Br interactions. [Symmetry codes: (i) 1 − x, 1 − y, 2 − z; (ii) 2 − x, 1 − y, 1 − z; (iii) x, $[{1\over 2}]$ − y, $[{1\over 2}]$ + z; (iv) 1 + x, y, −1 + z].

Figure 7
The formation of a three-dimensional supramolecular staircase structure along the ac plane.

Figure 8
A view of the C=O(carbonyl)⋯π interactions between XA⁺ cations in salt II. [Symmetry codes: (i) 1 + x, y, z; (ii) 1 − x, 1 − y, 1 − z].

4. Hirshfeld surface analysis

Hirshfeld surface analyses and their associated two-dimensional fingerprint plots (McKinnon et al., 2007 ; Spackman & Jayatilaka, 2009 ) were generated using Crystal Explorer 17.5 (Turner et al., 2017 ). The Hirshfeld surfaces of the title compounds mapped over d_norm feature several red spots in the regions of D–A (D = donor, A = acceptor) interactions (Cárdenas-Valenzuela et al., 2018 ; Atioğlu et al., 2018 ). In this regard, the contribution of the interatomic contacts to the d_norm surface map can help differentiate whether the contact is longer (blue) or shorter (red) than the sum of the van der Waals radii of the two interacting atoms. The Hirshfeld surfaces of salts I and II are shown in Fig. 9a and 10a, respectively and the hydrogen-bonding interactions between the hydrated ion pairs I and II and the respective neighbouring moieties are shown in Fig. 9b and 10b, respectively. The intense red spots on the Hirshfeld surface indicate the shortest interatomic distances corresponding to the hydrogen bonds. They are also clearly identified by the two long spikes in the fingerprint plots and can be quantified using the percentage distribution of the interacting types. Such analyses of the salts I and II are shown in Figs. 11 and 12 giving the following contributions: All (100%), O⋯H/H⋯O (I 19.7%, II 23.4%), N⋯H/H⋯N (I 13.5%, II 7.5%) C⋯H/H⋯C (I 6.4%, II 9.6%), H⋯H/H⋯H (I 23.4%, II 15.9%) and C⋯C/C⋯C (I 0.9%, II 0.1%) (Table 5), indicating that the most abundant contact is Br⋯H/H⋯Br with 22.3% in I and 25.4% in II, respectively.

Figure 9
(a) Hirshfeld surface mapped over d_norm for salt I. (b) Intermolecular interactions and the three-dimensional Hirshfeld surface for salt I.

Figure 10
(a) Hirshfeld surface mapped over d_norm for salt II. (b) Intermolecular interactions and the three-dimensional Hirshfeld surface for salt II.

Figure 11
Hirshfeld surface analysis and two-dimensional fingerprint plots for salt I plotted over d_norm, with interactions to neighbouring fragments shown as dashed lines.

Figure 12
Hirshfeld surface analysis and two-dimensional fingerprint plots for salt II plotted over d_norm, with interactions to neighbouring fragments shown as dashed lines.

5. Comparative analysis

The data obtained by comparative analysis of the crystal structures, supramolecular interactions, hydrogen-bonding motifs and packing patterns of structurally similar halide salts such as adeninium bromide, adeninium chloride, guaninium bromide, guaninium chloride and hypoxanthinium chloride (Maixner & Zachova, 1991; Sridhar, 2011; Kistenmancher & Shigematsu, 1974; Langer & Huml, 1978 ) are listed and compared in Table 3.

Table 3
Comparison of purine derivatives with hydrobromic acid and hydrochloric acid

	Adeninium bromide hemihydrate	Adeninium chloride monohydrate	Guaninium chloride monohydrate	Guaninium bromide monohydrate	Hypoxanthinium chloride monohydrate	Hypoxanthinium bromide monohydrate (I)	Xanthinium bromide monohydrate (II)
Cell parameters (a, b, c, β; Å, °)	9.018 (2), 4.845 (2), 19.693 (5), 112.8	8.771 (2), 4.834 (2), 19.46 (1), 114.25	4.591 (1), 9.886 (2), 18.985 (1), 99.62	4.8708 (7), 13.237 (3), 14.638 (2), 93.906 (10)	4.8295 (9), 17.7285 (22), 9.0077 (21), 94.59 (3)	4.8487 (4), 18.4455 (15), 9.0782 (7), 94.808 (1)	4.9225 (2), 22.7572 (17), 7.5601 (5) 103.003 (3)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P2/c	P2/c	P2₁/c	P2₁/c	P2₁/c	P2₁/c	P2₁/c
Protonation site	N1	N1	N7	N7	N7	N7	N9
Type of hydrogen bonding	N—H⋯O, N—H⋯Br, N—H⋯N, O—H⋯O, C—H⋯Br	N—H⋯O, N—H⋯Cl, N—H⋯N, O—H⋯Cl, C—H⋯Cl	N—H⋯O, N—H⋯Br, N—H⋯N, O—H⋯Br, C—H⋯Br	N—H⋯O, N—H⋯Cl, N—H⋯N, O—H⋯Cl, C—H⋯Cl	N—H⋯Cl, N—H⋯O, O—H⋯N, O—H⋯Cl, C—H⋯Cl	N—H⋯Br, N—H⋯O, O—H⋯N, O—H⋯Br, C—H⋯Br	N—H⋯O, N—H⋯Br, O—H⋯O
Type of stacking	–	–	C=O⋯π	C=O⋯π	C=O⋯π	C=O⋯π	C=O⋯π
Primary motif	$[R_{2}^{2}]$ (10)	$[R_{2}^{2}]$ (10)	$[R_{2}^{2}]$ (8)	$[R_{2}^{2}]$ (8)	$[R_{3}^{2}]$ (9)	$[R_{3}^{2}]$ (9)	$[R_{2}^{2}]$ (8)
Secondary motif	$[R_{3}^{2}]$ (7), R₄²(14)	$[R_{3}^{2}]$ (7), R₄²(14)	$[R_{3}^{2}]$ (7), $[R_{2}^{2}]$ (10), R₄³(11)	$[R_{3}^{2}]$ (7), $[R_{2}^{2}]$ (10), R₄³(11)	R₃³(11), R₆⁴(16), R₆⁴(14)	R₃³(11), R₆⁴(16), R₆⁴(14)	$[R_{2}^{2}]$ (7), $[R_{2}^{2}]$ (4)
Type of packing architecture	Ribbon	Ribbon	Ribbon	Ribbon	Wave	Wave	Staircase

Salt I has similar unit-cell parameters and packing patterns to the hypoxanthinium chloride salt. The molecular recognition between the hypoxanthine base and acid happens via N—H⋯O, C—H⋯Br/Cl and N—H⋯Br/Cl hydrogen-bond motifs with $[R_{3}^{2}]$ (9) (type D), R₃³(11) (type I), R₆⁴(16) (type N) and R₆⁴(14) (type O) graph-set motifs. Salt II forms base pairs via N—H⋯O hydrogen bonds described by $[R_{2}^{2}]$ (8) (type C), R₂³(8) (type H) (Wei, 1977 ; Maixner & Zachova, 1991), $[R_{2}^{2}]$ (7) (type F) and $[R_{2}^{2}]$ (4) (type L) graph-set motifs. Salt II cannot be compared with its chloride analogue since its crystal structure has not yet been reported.

A comparison between some related purine-based chloride and bromide salts revealed that type A, B and C hydrogen-bond motifs are predominant. The commonly observed motifs in purine based salts are shown in the scheme. A comparison of salts I and II with the reported crystal structures revealed that the bromide and chloride salts of I are isomorphous and therefore, one might predict, the unreported xanthinium chloride monohydrate could be isomorphous with its bromide salt II.

6. Database survey

A survey of the Cambridge Structural Database (CSD, version 5.43, update of March 2022; Groom et al., 2016 ) for reported structures of hypoxanthine and xanthine derivatives identified the hypoxanthine molecule (CSD refcodes GEBTUC and GETBUC01; Schmalle et al., 1988; Yang & Xie, 2007) and the following salts: hypoxanthinium nitrate monohydrate (BONKOE and BONKOE54; Cabaj et al., 2019; Schmalle et al., 1990), hypoxanthinium chloride monohydrate (HYPXCL and HYPXCL01; Sletten & Jensen, 1969; Kalyanaraman et al., 2007) as well as three xanthine salts, viz. xanthinium perchlorate monohydrate (VURMUR; Biradha et al., 2010), xanthinium nitrate monohydrate (YADJAQ; Sridhar, 2011) and xanthinium hydrogensulfate monohydrate (YADJEU; Sridhar, 2011). In all of the hypoxanthinium salts, the hypoxanthine molecule is protonated at the N7 position and interacts with the anion through N—H⋯Cl/O and C=O⋯π interactions. In the xanthinium salts, the xanthine molecules are protonated at the N7 position in xanthinium nitrate monohydrate and xanthinium hydrogensulfate monohydrate and at the N9 position in xanthinium perchlorate monohydrate. In all of the crystal structures, the xanthinium cation interacts with the anion through N—H⋯O, O—H⋯O and C=O⋯π interactions.

7. Synthesis and crystallization

A general method was used for the preparation and crystallization of the hypoxanthinium bromide monohydrate (I) and xanthinium bromide monohydrate (II) using the following quantities: 0.0340 mg (0.25mmol) of hypoxanthine for I and 0.0380 mg (0.25 mmol) of xanthine for II.

The indicated amount of the base was dissolved in 20 mL of distilled water and 2 mL of hydrobromic acid (5% in water) were added. The reaction mixture was heated to 358 K for 30 min using a water bath. The resulting solution was allowed to slowly evaporate at room temperature. After a few days, colourless plate-like crystals were obtained.

8. Refinement

Crystal data, data collection and structure refinement details for salts I and II are summarized in Table 4. All C-bound hydrogen atoms were placed in idealized positions and refined using a riding model, with C—H = 0.93 Å and U_iso(H) = 1.2U_eq (C). The H atoms of the water molecule were located in a difference-Fourier map and refined with the O—H distance restrained to 0.85–0.86 Å and with U_iso(H) = 1.5 U_eq(O). The hydrogen atoms bound to the nitrogen atoms in salts I and II were located in difference-Fourier maps and either refined freely (in I) or with the distance restraint N—H = 0.82 Å and with U_iso(H) = 1.2U_eq(N) (in II).

Table 4
Experimental details

	I	II
Crystal data
Chemical formula	C₅H₅N₄O⁺·Br⁻·H₂O	C₅H₅N₄O₂⁺·Br⁻·H₂O
M_r	235.06	251.06
Crystal system, space group	Monoclinic, P2₁/c	Monoclinic, P2₁/c
Temperature (K)	296	303
a, b, c (Å)	4.8487 (4), 18.4455 (15), 9.0782 (7)	4.9225 (2), 22.7572 (17), 7.5601 (5)
β (°)	94.808 (1)	103.003 (3)
V (Å³)	809.07 (11)	825.18 (9)
Z	4	4
Radiation type	Mo Kα	Mo Kα
μ (mm⁻¹)	5.05	4.96
Crystal size (mm)	0.46 × 0.26 × 0.21	0.55 × 0.37 × 0.31

Data collection
Diffractometer	Bruker APEXII CCD	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Bruker, 2016 )	Multi-scan (SADABS; Bruker, 2016)
T_min, T_max	0.403, 0.641	0.316, 0.561
No. of measured, independent and observed [I > 2σ(I)] reflections	17895, 2383, 2037	5810, 1855, 1418
R_int	0.028	0.045
(sin θ/λ)_max (Å⁻¹)	0.707	0.696

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.021, 0.056, 1.05	0.036, 0.080, 1.10
No. of reflections	2383	1855
No. of parameters	128	139
No. of restraints	6	9
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement	Only H-atom coordinates refined
Δρ_max, Δρ_min (e Å⁻³)	0.34, −0.29	0.42, −0.62
Computer programs: APEX2 and SAINT (Bruker, 2016), SHELXS97 (Sheldrick 2008 ), SHELXT2014/5 (Sheldrick, 2015a ), SHELXL2018/3 (Sheldrick, 2015b ), PLATON (Spek, 2020 ), Mercury (Macrae et al., 2020 ), POVRay (Cason, 2004 ) and publCIF (Westrip,2010 ).

Supporting information

CCDC references: 2170923; 2170922

Crystal structure: contains datablocks I, II. DOI: https://doi.org/10.1107/S2056989022005278/jq2017sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989022005278/jq2017Isup2.hkl

Structure factors: contains datablock II. DOI: https://doi.org/10.1107/S2056989022005278/jq2017IIsup3.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2056989022005278/jq2017Isup4.cml

Supporting information file. DOI: https://doi.org/10.1107/S2056989022005278/jq2017IIsup5.cml

checkCIF report

Computing details top

For both structures, data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016). Program(s) used to solve structure: SHELXS97 (Sheldrick 2008) for (I); SHELXT2014/5 (Sheldrick, 2015a) for (II). For both structures, program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2020), Mercury (Macrae et al., 2020) and POVRay (Cason, 2004); software used to prepare material for publication: PLATON (Spek, 2020) and publCIF (Westrip,2010).

6-Oxo-6,9-dihydro-1H-purin-7-ium bromide monohydrate (I) top

Crystal data top

C₅H₅N₄O⁺·Br⁻·H₂O	F(000) = 464
M_r = 235.06	D_x = 1.930 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 4.8487 (4) Å	Cell parameters from 2383 reflections
b = 18.4455 (15) Å	θ = 2.2–30.2°
c = 9.0782 (7) Å	µ = 5.05 mm⁻¹
β = 94.808 (1)°	T = 296 K
V = 809.07 (11) Å³	Plate, colourless
Z = 4	0.46 × 0.26 × 0.21 mm

Data collection top

Bruker APEXII CCD diffractometer	2037 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.028
Absorption correction: multi-scan (SADABS; Bruker, 2016)	θ_max = 30.2°, θ_min = 2.2°
T_min = 0.403, T_max = 0.641	h = −6→6
17895 measured reflections	k = −25→26
2383 independent reflections	l = −12→12

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: mixed
R[F² > 2σ(F²)] = 0.021	H atoms treated by a mixture of independent and constrained refinement
wR(F²) = 0.056	w = 1/[σ²(F_o²) + (0.0273P)² + 0.2516P] where P = (F_o² + 2F_c²)/3
S = 1.05	(Δ/σ)_max = 0.002
2383 reflections	Δρ_max = 0.34 e Å⁻³
128 parameters	Δρ_min = −0.29 e Å⁻³
6 restraints	Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0080 (9)

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq
Br1	−0.30053 (4)	0.47161 (2)	0.77208 (2)	0.04456 (8)
O6	−0.2120 (2)	0.27474 (6)	0.73221 (11)	0.0337 (2)
N9	0.4049 (2)	0.18412 (7)	0.42420 (13)	0.0278 (2)
H9	0.529 (3)	0.1816 (10)	0.3639 (18)	0.041 (5)*
N3	0.3696 (2)	0.31647 (6)	0.43388 (13)	0.0288 (2)
N1	0.0413 (3)	0.35239 (6)	0.59830 (14)	0.0293 (2)
H1	−0.035 (4)	0.3873 (9)	0.638 (2)	0.045 (5)*
N7	0.1015 (3)	0.15533 (6)	0.57823 (13)	0.0283 (2)
H7	0.000 (4)	0.1316 (10)	0.6329 (19)	0.044 (5)*
C5	0.1107 (3)	0.22987 (7)	0.57102 (14)	0.0232 (2)
C8	0.2792 (3)	0.12926 (8)	0.48894 (16)	0.0309 (3)
H8	0.312119	0.080303	0.473349	0.037*
C2	0.2315 (3)	0.36593 (8)	0.50019 (16)	0.0309 (3)
H2	0.266402	0.414166	0.478442	0.037*
O1W	−0.7539 (3)	0.60447 (7)	0.73814 (15)	0.0447 (3)
H1W	−0.653 (4)	0.6346 (10)	0.695 (2)	0.067*
H2W	−0.671 (4)	0.5639 (7)	0.741 (2)	0.067*
C4	0.3015 (3)	0.24822 (7)	0.47360 (13)	0.0235 (2)
C6	−0.0384 (3)	0.28405 (7)	0.64259 (14)	0.0245 (2)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.05176 (12)	0.02567 (9)	0.06016 (13)	0.00535 (6)	0.02789 (9)	−0.00045 (7)
O6	0.0347 (5)	0.0341 (5)	0.0352 (5)	0.0018 (4)	0.0205 (5)	−0.0004 (4)
N9	0.0272 (5)	0.0312 (6)	0.0267 (5)	0.0032 (4)	0.0122 (5)	−0.0009 (4)
N3	0.0292 (6)	0.0287 (6)	0.0299 (6)	−0.0017 (5)	0.0116 (5)	0.0026 (5)
N1	0.0325 (6)	0.0257 (5)	0.0316 (6)	0.0025 (5)	0.0133 (5)	−0.0016 (5)
N7	0.0311 (6)	0.0251 (5)	0.0301 (6)	−0.0029 (4)	0.0123 (5)	−0.0005 (4)
C5	0.0223 (6)	0.0259 (6)	0.0224 (6)	−0.0010 (5)	0.0072 (5)	−0.0009 (5)
C8	0.0344 (7)	0.0267 (6)	0.0328 (7)	0.0024 (5)	0.0105 (6)	−0.0019 (5)
C2	0.0333 (7)	0.0273 (6)	0.0334 (7)	−0.0011 (5)	0.0103 (6)	0.0033 (5)
O1W	0.0458 (7)	0.0321 (6)	0.0608 (8)	0.0075 (5)	0.0319 (6)	0.0077 (5)
C4	0.0216 (6)	0.0282 (6)	0.0215 (6)	0.0006 (5)	0.0066 (5)	−0.0008 (5)
C6	0.0234 (6)	0.0285 (6)	0.0223 (6)	0.0012 (5)	0.0060 (5)	−0.0014 (5)

Geometric parameters (Å, º) top

O6—C6	1.2308 (15)	N7—C8	1.3219 (17)
N9—C8	1.3419 (18)	N7—C5	1.3774 (17)
N9—C4	1.3741 (16)	N7—H7	0.849 (14)
N9—H9	0.847 (14)	C5—C4	1.3748 (17)
N3—C2	1.3078 (18)	C5—C6	1.4221 (17)
N3—C4	1.3579 (16)	C8—H8	0.9300
N1—C2	1.3581 (17)	C2—H2	0.9300
N1—C6	1.3879 (17)	O1W—H1W	0.857 (9)
N1—H1	0.840 (14)	O1W—H2W	0.848 (9)

C8—N9—C4	108.32 (10)	N7—C8—N9	109.73 (12)
C8—N9—H9	127.8 (13)	N7—C8—H8	125.1
C4—N9—H9	123.8 (13)	N9—C8—H8	125.1
C2—N3—C4	112.29 (11)	N3—C2—N1	125.15 (13)
C2—N1—C6	125.30 (12)	N3—C2—H2	117.4
C2—N1—H1	119.4 (14)	N1—C2—H2	117.4
C6—N1—H1	115.3 (14)	H1W—O1W—H2W	107.4 (17)
C8—N7—C5	107.98 (11)	N3—C4—N9	127.42 (11)
C8—N7—H7	127.6 (13)	N3—C4—C5	126.22 (12)
C5—N7—H7	124.4 (13)	N9—C4—C5	106.36 (11)
C4—C5—N7	107.61 (11)	O6—C6—N1	122.73 (12)
C4—C5—C6	121.08 (12)	O6—C6—C5	127.31 (13)
N7—C5—C6	131.31 (11)	N1—C6—C5	109.96 (11)

C8—N7—C5—C4	−0.02 (16)	N7—C5—C4—N3	179.07 (12)
C8—N7—C5—C6	179.24 (14)	C6—C5—C4—N3	−0.3 (2)
C5—N7—C8—N9	0.27 (17)	N7—C5—C4—N9	−0.23 (15)
C4—N9—C8—N7	−0.42 (17)	C6—C5—C4—N9	−179.58 (12)
C4—N3—C2—N1	0.2 (2)	C2—N1—C6—O6	179.92 (14)
C6—N1—C2—N3	−0.6 (2)	C2—N1—C6—C5	0.5 (2)
C2—N3—C4—N9	179.41 (14)	C4—C5—C6—O6	−179.46 (14)
C2—N3—C4—C5	0.3 (2)	N7—C5—C6—O6	1.4 (3)
C8—N9—C4—N3	−178.90 (14)	C4—C5—C6—N1	−0.12 (18)
C8—N9—C4—C5	0.39 (16)	N7—C5—C6—N1	−179.30 (14)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N9—H9···Br1ⁱ	0.85 (1)	3.08 (2)	3.5397 (12)	117 (2)
N9—H9···O6ⁱ	0.85 (1)	1.98 (2)	2.7579 (14)	153 (2)
N1—H1···Br1	0.84 (1)	2.41 (1)	3.2419 (12)	170 (2)
N7—H7···O1Wⁱⁱ	0.85 (1)	1.81 (2)	2.6401 (16)	165 (2)
O1W—H1W···N3ⁱⁱⁱ	0.86 (1)	2.08 (1)	2.9200 (16)	165 (2)
O1W—H2W···Br1	0.85 (1)	2.48 (1)	3.2894 (12)	161 (2)
C8—H8···Br1ⁱ	0.93	2.89	3.4875 (15)	123

Symmetry codes: (i) x+1, −y+1/2, z−1/2; (ii) −x−1, y−1/2, −z+3/2; (iii) −x, −y+1, −z+1.

2,6-Dioxo-2,3,6,9-tetrahydro-1H-purin-7-ium bromide monohydrate (II) top

Crystal data top

C₅H₅N₄O₂⁺·Br⁻·H₂O	F(000) = 496
M_r = 251.06	D_x = 2.021 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 4.9225 (2) Å	Cell parameters from 1418 reflections
b = 22.7572 (17) Å	θ = 2.9–29.6°
c = 7.5601 (5) Å	µ = 4.96 mm⁻¹
β = 103.003 (3)°	T = 303 K
V = 825.18 (9) Å³	Plate, colourless
Z = 4	0.55 × 0.37 × 0.31 mm

Data collection top

Bruker APEXII CCD diffractometer	1418 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.045
Absorption correction: multi-scan (SADABS; Bruker, 2016)	θ_max = 29.7°, θ_min = 2.9°
T_min = 0.316, T_max = 0.561	h = −6→6
5810 measured reflections	k = −30→30
1855 independent reflections	l = −9→9

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.036	Hydrogen site location: difference Fourier map
wR(F²) = 0.080	Only H-atom coordinates refined
S = 1.10	w = 1/[σ²(F_o²) + (0.0151P)² + 1.7175P] where P = (F_o² + 2F_c²)/3
1855 reflections	(Δ/σ)_max < 0.001
139 parameters	Δρ_max = 0.42 e Å⁻³
9 restraints	Δρ_min = −0.62 e Å⁻³

Special details top

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br1	−0.16454 (8)	0.20412 (2)	0.47569 (6)	0.03200 (14)
O6	0.8033 (5)	0.47823 (12)	0.6033 (4)	0.0318 (6)
C6	0.6301 (7)	0.44615 (16)	0.6465 (5)	0.0241 (8)
N1	0.5267 (7)	0.45642 (14)	0.7988 (4)	0.0273 (7)
H1	0.578 (8)	0.4867 (13)	0.855 (5)	0.033*
C2	0.3313 (8)	0.42399 (16)	0.8628 (5)	0.0254 (8)
O2	0.2541 (6)	0.43894 (13)	0.9992 (4)	0.0380 (7)
N3	0.2278 (6)	0.37489 (13)	0.7647 (4)	0.0251 (7)
H3	0.126 (7)	0.3545 (16)	0.812 (5)	0.030*
C4	0.3212 (7)	0.36166 (15)	0.6147 (5)	0.0238 (8)
C5	0.5108 (7)	0.39456 (16)	0.5538 (5)	0.0237 (8)
N7	0.5509 (7)	0.36852 (14)	0.3972 (4)	0.0273 (7)
H7	0.651 (8)	0.3804 (18)	0.332 (5)	0.033*
C8	0.3919 (8)	0.32168 (18)	0.3652 (6)	0.0304 (9)
H8	0.384 (9)	0.2947 (18)	0.265 (6)	0.036*
N9	0.2477 (7)	0.31597 (14)	0.4956 (5)	0.0279 (7)
H9	0.148 (7)	0.2876 (14)	0.503 (6)	0.033*
O1W	0.8947 (7)	0.42353 (14)	0.2374 (4)	0.0434 (8)
H1WA	0.983 (9)	0.4523 (16)	0.298 (6)	0.065*
H1WB	0.980 (9)	0.415 (2)	0.154 (5)	0.065*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.0307 (2)	0.0253 (2)	0.0413 (2)	−0.00152 (17)	0.01087 (16)	−0.00286 (18)
O6	0.0317 (15)	0.0309 (15)	0.0360 (16)	−0.0110 (12)	0.0147 (13)	−0.0028 (12)
C6	0.0231 (18)	0.0217 (18)	0.028 (2)	−0.0001 (14)	0.0057 (16)	0.0046 (15)
N1	0.0294 (17)	0.0253 (17)	0.0292 (19)	−0.0097 (14)	0.0111 (15)	−0.0046 (14)
C2	0.0255 (19)	0.0224 (19)	0.029 (2)	−0.0030 (15)	0.0079 (17)	0.0042 (16)
O2	0.0431 (17)	0.0432 (18)	0.0338 (17)	−0.0118 (14)	0.0216 (14)	−0.0079 (14)
N3	0.0246 (16)	0.0232 (16)	0.0305 (18)	−0.0041 (12)	0.0128 (14)	0.0050 (13)
C4	0.0236 (18)	0.0193 (17)	0.027 (2)	0.0011 (14)	0.0028 (15)	0.0039 (15)
C5	0.0231 (18)	0.0246 (18)	0.0232 (19)	−0.0018 (14)	0.0050 (15)	−0.0008 (15)
N7	0.0291 (17)	0.0286 (17)	0.0265 (18)	−0.0023 (14)	0.0109 (14)	0.0005 (14)
C8	0.035 (2)	0.028 (2)	0.028 (2)	−0.0013 (17)	0.0051 (18)	−0.0048 (17)
N9	0.0299 (17)	0.0186 (15)	0.0346 (19)	−0.0040 (13)	0.0061 (15)	0.0000 (14)
O1W	0.0484 (19)	0.0449 (19)	0.046 (2)	−0.0199 (15)	0.0290 (16)	−0.0172 (15)

Geometric parameters (Å, º) top

O6—C6	1.221 (4)	C4—N9	1.370 (5)
C6—N1	1.380 (5)	C5—N7	1.378 (5)
C6—C5	1.425 (5)	N7—C8	1.312 (5)
N1—C2	1.383 (5)	N7—H7	0.82 (2)
N1—H1	0.82 (2)	C8—N9	1.344 (5)
C2—O2	1.224 (5)	C8—H8	0.97 (4)
C2—N3	1.374 (5)	N9—H9	0.82 (2)
N3—C4	1.350 (5)	O1W—H1WA	0.857 (10)
N3—H3	0.82 (2)	O1W—H1WB	0.860 (10)
C4—C5	1.355 (5)

O6—C6—N1	122.2 (3)	C5—C4—N9	107.2 (3)
O6—C6—C5	126.6 (4)	C4—C5—N7	107.3 (3)
N1—C6—C5	111.2 (3)	C4—C5—C6	121.8 (3)
C6—N1—C2	128.1 (3)	N7—C5—C6	130.9 (3)
C6—N1—H1	116 (3)	C8—N7—C5	108.2 (3)
C2—N1—H1	115 (3)	C8—N7—H7	125 (3)
O2—C2—N3	122.2 (3)	C5—N7—H7	127 (3)
O2—C2—N1	121.2 (3)	N7—C8—N9	109.6 (4)
N3—C2—N1	116.6 (3)	N7—C8—H8	125 (3)
C4—N3—C2	118.7 (3)	N9—C8—H8	125 (3)
C4—N3—H3	126 (3)	C8—N9—C4	107.7 (3)
C2—N3—H3	115 (3)	C8—N9—H9	123 (3)
N3—C4—C5	123.6 (3)	C4—N9—H9	129 (3)
N3—C4—N9	129.2 (3)	H1WA—O1W—H1WB	107 (2)

O6—C6—N1—C2	179.8 (4)	N9—C4—C5—C6	179.6 (3)
C5—C6—N1—C2	−0.7 (5)	O6—C6—C5—C4	179.3 (4)
C6—N1—C2—O2	−178.6 (4)	N1—C6—C5—C4	−0.1 (5)
C6—N1—C2—N3	0.8 (6)	O6—C6—C5—N7	−1.2 (7)
O2—C2—N3—C4	179.3 (4)	N1—C6—C5—N7	179.3 (4)
N1—C2—N3—C4	−0.1 (5)	C4—C5—N7—C8	−0.1 (4)
C2—N3—C4—C5	−0.6 (5)	C6—C5—N7—C8	−179.6 (4)
C2—N3—C4—N9	−179.3 (4)	C5—N7—C8—N9	0.1 (5)
N3—C4—C5—N7	−178.8 (3)	N7—C8—N9—C4	0.0 (4)
N9—C4—C5—N7	0.1 (4)	N3—C4—N9—C8	178.7 (4)
N3—C4—C5—C6	0.8 (6)	C5—C4—N9—C8	−0.1 (4)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O2ⁱ	0.82 (2)	2.09 (2)	2.903 (4)	175 (4)
N3—H3···Br1ⁱⁱ	0.82 (2)	2.48 (2)	3.301 (3)	176 (4)
N7—H7···O1W	0.82 (2)	1.81 (2)	2.609 (4)	163 (4)
N9—H9···Br1	0.82 (2)	2.43 (2)	3.237 (3)	172 (4)
O1W—H1WA···O6ⁱⁱⁱ	0.86 (1)	1.95 (1)	2.802 (4)	171 (5)
O1W—H1WB···Br1^iv	0.86 (1)	3.03 (4)	3.490 (3)	115 (3)
O1W—H1WB···O2^v	0.86 (1)	2.05 (3)	2.816 (4)	149 (4)

Symmetry codes: (i) −x+1, −y+1, −z+2; (ii) x, −y+1/2, z+1/2; (iii) −x+2, −y+1, −z+1; (iv) x+1, −y+1/2, z−1/2; (v) x+1, y, z−1.

Comparison of purine derivatives with hydrobromic acid and hydrochloric acid top

	Adeninium bromide hemihydrate	Adeninium chloride monohydrate	Guaninium chloride monohydrate	Guaninium bromide monohydrate	Hypoxanthinium chloride monohydrate	Hypoxanthinium bromide monohydrate (I)	Xanthinium bromide monohydrate (II)
Cell parameters (a, b, c, β; Å, °)	9.018 (2), 4.845 (2), 19.693 (5), 112.8	8.771 (2), 4.834 (2), 19.46 (1), 114.25	4.591 (1), 9.886 (2), 18.985 (1), 99.62	4.8708 (7), 13.237 (3), 14.638 (2), 93.906 (10)	4.8295 (9), 17.7285 (22), 9.0077 (21), 94.59 (3)	4.8487 (4), 18.4455 (15), 9.0782 (7), 94.808 (1)	4.9225 (2), 22.7572 (17), 7.5601 (5) 103.003 (3)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P2/c	P2/c	P2₁/c	P2₁/c	P2₁/c	P2₁/c	P2₁/c
Protonation site	N1	N1	N7	N7	N7	N7	N9
Type of hydrogen bonding	N—H···O, N—H···Br, N—H···N, O—H···O, C—H···Br	N—H···O, N—H···Cl, N—H···N, O—H···Cl, C—H···Cl	N—H···O, N—H···Br, N—H···N, O—H···Br, C—H···Br	N—H···O, N—H···Cl, N—H···N, O—H···Cl, C—H···Cl	N—H···Cl, N—H···O, O—H···N, O—H···Cl, C—H···Cl	N—H···Br, N—H···O, O—H···N, O—H···Br, C—H···Br	N—H···O, N—H···Br, O—H···O
Type of stacking	–	–	C═O···π	C═O···π	C═O···π	C═O···π	C═O···π
Primary motif	R²₂(10)	R²₂(10)	R²₂(8)	R²₂(8)	R²₃(9)	R²₃(9)	R²₂(8)
Secondary motif	R²₃(7) R²₄(14)	R²₃(7), R²₄(14)	R²₃(7), R²₂(10), R³₄(11)	R²₃(7), R²₂(10), R³₄(11)	R³₃(11), R⁴₆(16), R⁴₆(14)	R³₃(11), R⁴₆(16), R⁴₆(14)	R²₂(7), R²₂(4)
Type of packing architecture	Ribbon	Ribbon	Ribbon	Ribbon	Wave	Wave	Staircase

Percentage of non-covalent interaction in supramolecular packing analyzed by Hirshfeld surface analysis top


CONTACT	SALT (I)	SALT (II)
H···Br /Br···H	22.3%	25.4%
O···H/H···O	19.7%	23.4%
H···N/N···H	13.5%	7.5%
C···H/H···C	6.4%	9.6%
H···H	23.4%	15.9%

Acknowledgements

Author contributions are as follows. Conceptualization, JSNR, SG; synthesis, US and DD; writing (review and editing of the manuscript) JSNR, SG and US; crystal-structure determination, SJJ and IAR.

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research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

A study of the crystal structures, supra­molecular patterns and Hirshfeld surfaces of bromide salts of hypoxanthine and xanthine

1. Chemical context

2. Structural commentary

3. Supra­molecular features

4. Hirshfeld surface analysis

5. Comparative analysis

6. Database survey

7. Synthesis and crystallization

8. Refinement

Supporting information

Acknowledgements

References

research communications

A study of the crystal structures, supramolecular patterns and Hirshfeld surfaces of bromide salts of hypoxanthine and xanthine

3. Supramolecular features