Belakavadi K Sagar1, Marisiddaiah Girisha1, Hemmige S Yathirajan1, Ravindranath S Rathore2, Christopher Glidewell3. 1. Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysuru 570 006, India. 2. Centre for Biological Sciences (Bioinformatics), School of Earth, Biological and Environmental Sciences, Central University of South Bihar, Patna 800 014, India. 3. School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, Scotland.
Abstract
In both 2-amino-4,4,7,7-tetra-methyl-4,5,6,7-tetra-hydro-1,3- benzo-thia-zol-3-ium benzoate, C11H19N2S+·C7H5O2-, (I), and 2-amino-4,4,7,7-tetra-methyl-4,5,6,7-tetra-hydro-1,3-benzo-thia-zol-3-ium picrate (2,4,6-tri-nitro-phenolate), C11H19N2S+·C6H2N3O7-, (II), the cations are conformationally chiral as the six-membered rings adopt half-chair conformations, which are disordered over two sets of atomic sites giving approximately enanti-omeric disorder. For both cations, the bond lengths indicate delocalization of the positive charge comparable to that in an amidinium cation. The bond lengths in the picrate anion in (II) are consistent with extensive delocalization of the negative charge into the ring and onto the nitro groups, in two of which the O atoms are disordered over two sets of sites. In (I), the ionic components are linked by N-H⋯O hydrogen bonds to form a chain of rings, and in (II), the N-H⋯O hydrogen bonds link the components into centrosymmetric four-ion aggregates containing seven hydrogen bonded rings of four different types.
In both 2-amino-4,4,7,7-tetra-methyl-4,5,6,7-tetra-hydro-1,3- benzo-thia-zol-3-ium benzoate, C11H19N2S+·C7H5O2-, (I), and 2-amino-4,4,7,7-tetra-methyl-4,5,6,7-tetra-hydro-1,3-benzo-thia-zol-3-ium picrate (2,4,6-tri-nitro-phenolate), C11H19N2S+·C6H2N3O7-, (II), the cations are conformationally chiral as the six-membered rings adopt half-chair conformations, which are disordered over two sets of atomic sites giving approximately enanti-omeric disorder. For both cations, the bond lengths indicate delocalization of the positive charge comparable to that in an amidiniumcation. The bond lengths in the picrate anion in (II) are consistent with extensive delocalization of the negative charge into the ring and onto the nitro groups, in two of which the O atoms are disordered over two sets of sites. In (I), the ioniccomponents are linked by N-H⋯O hydrogen bonds to form a chain of rings, and in (II), the N-H⋯O hydrogen bonds link the components into centrosymmetric four-ion aggregates containing seven hydrogen bonded rings of four different types.
Benzothiazoles are an important class of heterocycliccompounds which possess a wide spectrum of biological properties, including analgesic, anticonvulsant, antihelmintic, anti-inflammatory antimalarial, antimicrobial, antitubercular, and antitumour, activity, as well as antioxidant and fungicidal activity (Imramovský et al., 2013 ▸; Smita Revankar et al., 2014 ▸; Naga Raju et al., 2015 ▸; Ranga et al., 2015 ▸). In addition, substituted 2-aminothiazole derivatives are important as potent and selective humanadenosine A3 receptor antagonists (Jung et al., 2004 ▸). Prompted by the importance of benzothiazoles in general, we have now determined the molecular and supramolecular structures of two salts derived from a substituted benzothiazole, 2-amino-4,4,7,7-tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazole, namely 2-amino-4,4,7,7-tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazol-3-ium benzoate (I) and 2-amino-4,4,7,7- tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazol-3-ium picrate (2,4,6-trinitrophenolate) (II), which we report here. The compounds were prepared by acid–base reactions between the neutral benzothiazole and the appropriate acid in methanolic solution.
Structural commentary
Compound (I) consists of a reduced benzothiazolium cation in which protonation has occurred exclusively at atom N13, and a benzoate anion and the two ions within the selected asymmetric unit are linked by two fairly short and nearly linear N—H⋯O hydrogen bonds, forming an (8) motif (Fig. 1 ▸ and Table 1 ▸). In the cation, the six-membered ring is disordered over two sets of atomic sites with occupancies 0.721 (5) and 0.279 (5), and each disorder component adopts a half-chair conformation (Fig. 2 ▸). The ring-puckering parameters calculated for the atom sequence (Cx3A,Cx4,Cx5,Cx6,Cx7,Cx7A), where x = 1 for the major conformer and x = 2 for the minor form, of Q = 0.452 (5) Å, θ = 47.3 (8)° and φ = 146.1 (10)° when x = 1, with corresponding values Q = 0.453 (13) Å, θ = 138.5 (19)° and φ = 340 (3)° when x = 2. For an idealized half-chair form the puckering angles are θ = 50.8° and φ = (60k + 30)°, where k represents an integer. In each of (I) and (II), in fact, the cation exhibits no internal symmetry and hence is conformationally chiral: in each case the space group confirms the presence of equal numbers of the two conformational enantiomers. In the benzoate anion in (I), the carboxyl group makes a dihedral angle of 10.5 (2)° with the aryl ring, and the two C—O distances are identical within experimental uncertainty, 1.252 (3) and 1.255 (3) Å, consistent with the complete transfer of a proton from the acid component to atom N13, as deduced from difference maps and confirmed by the refinement.
Figure 1
The independent ionic components of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level, and the two N—H⋯O hydrogen bonds within the selected asymmetric unit are shown as dashed lines.
Table 1
Hydrogen-bond geometry (Å, °) for (I)
D—H⋯A
D—H
H⋯A
D⋯A
D—H⋯A
N12—H12A⋯O32
0.86
2.10
2.918 (8)
158
N12—H12B⋯O32i
0.86
1.97
2.785 (9)
158
N13—H13⋯O31
0.86
1.77
2.621 (10)
174
N22—H22A⋯O32
0.86
2.13
2.86 (2)
142
N22—H22B⋯O32i
0.86
2.13
2.92 (2)
152
N23—H23⋯O31
0.86
1.74
2.56 (3)
157
Symmetry code: (i) .
Figure 2
The disordered cation in compound (I), showing the approximately enantiomorphic nature of the two disorder components. For the sake of clarity the H atoms and most of the atom labels have been omitted: the major form is drawn as solid lines and the minor form as broken lines.
Compound (II) contains the same cation as (I) along with a picrate (2,4,6-trinitrophenolate) anion, and the two ions in the selected asymmetric unit are linked by a two-centre N—H⋯O hydrogen bond and a three-centre N—H⋯(O)2 hydrogen bond, generating two edge-fused rings of (6) and (6) types (Fig. 3 ▸ and Table 2 ▸).
Figure 3
The independent ionic components of compound (II), showing the atom-labelling scheme. For the sake of clarity, only the major disorder components are shown. Displacement ellipsoids are drawn at the 30% probability level, and the N—H⋯O hydrogen bonds within the selected asymmetric unit are shown as dashed lines.
Table 2
Hydrogen-bond geometry (Å, °) for (II)
D—H⋯A
D—H
H⋯A
D⋯A
D—H⋯A
N12—H12A⋯O32i
0.86
2.57
3.219 (10)
133
N12—H12A⋯O33i
0.86
2.31
3.039 (10)
142
N12—H12A⋯O42i
0.86
2.41
3.166 (16)
147
N12—H12A⋯O43i
0.86
2.58
3.197 (15)
129
N22—H22A⋯O32i
0.86
2.34
3.154 (14)
158
N22—H22A⋯O33i
0.86
2.40
3.143 (14)
146
N22—H22A⋯O42i
0.86
2.36
3.190 (19)
163
N22—H22A⋯O43i
0.86
2.46
3.197 (18)
144
N12—H12B⋯O31
0.86
2.11
2.855 (9)
145
N12—H12B⋯O32
0.86
2.20
2.870 (9)
134
N12—H12B⋯O42
0.86
2.30
2.932 (13)
131
N22—H22B⋯O31
0.86
1.97
2.704 (14)
142
N22—H22B⋯O32
0.86
2.14
2.768 (14)
130
N22—H22B⋯O42
0.86
2.16
2.730 (17)
123
N13—H13⋯O31
0.86
2.19
2.891 (14)
138
N23—H23⋯O31
0.86
2.15
2.81 (2)
134
Symmetry code: (i) .
The cation again exhibits conformational disorder over two sets of atomic sites having occupancies 0.575 (4) and 0.425 (4). For the major conformer, the ring-puckering parameters, calculated for the atom sequence (Cx3A,Cx4,Cx5,Cx6,Cx7,Cx7A) are Q = 0.444 (10) Å, θ = 41.9 (15)° and φ = 150 (2) when x = 1 and Q = 0.441 (14) Å, θ = 136 (2)° and φ = 328 (3)° when x = 2, so that the ring- puckering parameters are very similar to those found in compound (I). Thus in each compound the puckering amplitude for the two conformers are very similar, and the puckering angles, related approximately by θmin = (180 − θmaj) and φmin = (180 + φmaj), where min and maj refer to the minor and major components, indicate clearly the approximately enantiomorphic relationship between the two conformers (Fig. 2 ▸).In both compounds the bond distances C12—N12 and C12—N13 are nearly identical, 1.329 (6) and 1.323 (3) Å respectively in (I) and 1.312 (3) and 1.336 (9) Å in (II), indicative of significant delocalization of the positive charge into the amino group with significant contributions to the electronic structure from the forms (A) and (B), comparable to an amidiniumcation (see Scheme). This explains not only why the site of protonation is exclusively at the ring N atom, since protonation of the amino group would not permit any charge delocalization, but also the observation that the amino N atom does not act as a hydrogen-bond acceptor.In the picrate anion of (II), two of the three independent nitro groups adopt two different orientations and the occupancies for the two orientations bonded to atoms C32 and C36 are 0.769 (7) and 0.231 (7), and 0.789 (6) and 0.211 (6) respectively (Fig. 4 ▸). The major and minor conformations at C32 make dihedral angles of 17.9 (3) and 27.2 (7)° with the ring, with an angle of 44.9 (7)° between the two orientations, and the corresponding values for the nitro group at C36 are 12.0 (2), 39.0 (8) and 50.4 (8)°. By contrast, the fully ordered nitro group at C34 makes a dihedral angle of only 4.5 (2)° with the ring. The C—O distance, 1.241 (3), is short for its type [mean value (Allen et al., 1987 ▸) 1.362 Å, lower quartile value 1.353 Å], and the C—N distances, range 1.442 (3)–1.458 (3) Å, are all somewhat short for their type (mean value 1.468 Å, lower quartile value 1.460 Å): in addition, the bonds C31—C32 and C31—C36 are significantly longer than the other C—C distances in this ring. These observation, taken together, indicate that the quinonoid form (D), and its o-quinonoid isomers, and the ketonic form (E) are significant contributors to the overall electronic structure of the anion in addition to the classically delocalized form (C) (see Scheme).
Figure 4
The disordered anion in compound (II), showing the two orientations of two of the nitro groups: for the sake of clarity the H atoms have been omitted,
Supramolecular interactions
The major and minor conformers of the cation in (I) and those of both ions in (II) are involved in very similar patterns of hydrogen bonding (Tables 1 ▸ and 2 ▸), so that it is necessary to discuss only those formed by the major conformers. Because of the charge delocalization in both ions in each of (I) and (II), as noted above, all of the N—H⋯O interactions in both compounds can be regarded as charge-assisted hydrogen bonds (Gilli et al., 1994 ▸). In addition to the two N—H⋯O hydrogen bonds within the selected asymmetric unit of compound (I) (Fig. 1 ▸), the structure contains a third such interaction which links the cation-anion pairs which are related by the c-glide plane at y = 0.5 into a (4) (8)[(8)] chain of rings running parallel to the [001] direction (Fig. 5 ▸).
Figure 5
Part of the crystal structure of compound (I) showing the formation of a chain of rings running parallel to [001]. Hydrogen bonds are shown as dashed lines and for the sake of clarity the H atoms bonded to C atoms have been omitted.
In addition, the N—H⋯O hydrogen bonds within the selected asymmetric unit of (II) (Fig. 3 ▸), the structure contains one further three-centre N—H⋯(O)2 hydrogen bond, and the hydrogen bonds together generate a four-ion aggregate in which a centrosymmetric 4(8) ring is surrounded by three inversion-related pairs of rings, one each of (4), (6) and (6) types, so that, in total, there are seven hydrogen-bonded rings of four different types in the aggregate (Fig. 6 ▸). It is notable that only one of the nitro groups in (II) participates in the hydrogen bonding, and that both C—H⋯π(arene) and aromatic π–π stacking interactions are absent from both structures.
Figure 6
Part of the crystal structure of compound (II) showing the formation of a centrosymmetric four-ion aggregate. For the sake of clarity, only the major disorder components are shown, and the H atoms bonded to C atoms and the unit cell outline have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 − x, −y, 1 − z).
Database survey
It is of interest briefly to survey the structures of some related amino-substituted benzo-1,3-thiazoles. In the structure of 2-amino-6-nitrobenzo-1,3-thiazole (Glidewell et al., 2001 ▸), a combination of N—H⋯N and N—H⋯O hydrogen bonds generates a three-dimensional framework structure, while the monohydrate of the same benzothiazole, also forms a three-dimensional framework structure, but now built from a combination of N—H⋯N, N—H⋯O and O—H⋯O hydrogen bonds (Lynch, 2002 ▸): in neither of these structures does the amino N atom act as a hydrogen-bond acceptor, just as found here in the structures of (I) and (II). We note also that in trans-bis(2-amino-6-nitrobenzo-1,3-thiazole)dichloroplatinum(II), which crystallizes as a tetrakis(dimethylformamide) solvate (Lynch & Duckhouse, 2001 ▸), the benzothiazole ligand coordinates to the metalcentre via the ring N atom, rather than via the amino N atom. Finally in 2-amino-6-nitrobenzo-1.3-thiazol-3-ium hydrogen sulfate (Qian & Huang, 2011 ▸), the protonation of the benzothiazole component occurs exclusively at the ring N atoms and the ions are linked by a combination of N—H⋯O and O—H⋯O hydrogen bonds to form a sheet structure, again with the amino group acting as a double donor of hydrogen bonds, but not as an acceptor.
Synthesis and crystallization
2-Amino-4,4,7,7-tetramethyl-4,5,6,7-tetrahydro-1,3-benzothiazole (200 mg, 0.94 mmol) and the equivalent amount of the respective acid i.e. benzoic acid (119.4 mg, 0.94 mmol) for (I) and picric acid (229 mg, 0.94 mmol) for (II) were dissolved together in hot methanol. The resulting solutions were allowed to cool slowly to ambient temperature, and the crystalline products were collected by filtration and dried in air. Crystals suitable for single-crystal X-ray diffraction were selected directly from the samples as prepared; m.p. (I) 457 K, (II) 483 K.
Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. It was apparent from an early stage in the refinements that in both (I) and (II) the cation was disordered over two sets of atomic sights corresponding to two different conformations of the six-membered ring, and that two of the nitro groups in the anion of (II) were disordered, again over two sets of atomic sites corresponding to different orientations relative to the aryl ring. For the minor conformers of the cations, the bonded distances and the one-angle non-bonded distances were restrained to be the same as the corresponding distances in the major conformer, subject to s.u.s of 0.005 and 0.01 Å, respectively; similar restraints were applied to the minor conformations of the disorderednitro groups in the anion of (II). In addition, the anisotropic displacement parameters for pairs of atoms occupying essentially the same physical space were constrained to be identical. Subject to these conditions, the occupancies of the two cation conformations in (I) refined to 0.721 (5) and 0.279 (5), and those in (II) refined to 0.575 (4) and 0.425 (4), while those of the nitro groups in (II) bonded to C32 and C36 refined to 0.769 (7) and 0.231 (7), and 0.789 (6) and 0.211 (6) respectively.
Table 3
Experimental details
(I)
(II)
Crystal data
Chemical formula
C11H19N2S+·C7H5O2−
C11H19N2S+·C6H2N3O7−
Mr
332.45
439.45
Crystal system, space group
Monoclinic, Cc
Monoclinic, P21/n
Temperature (K)
296
296
a, b, c (Å)
10.6089 (3), 22.7141 (5), 8.8959 (2)
10.7928 (2), 6.9591 (1), 28.0176 (5)
β (°)
122.211 (1)
97.408 (1)
V (Å3)
1813.73 (8)
2086.79 (6)
Z
4
4
Radiation type
Cu Kα
Cu Kα
μ (mm−1)
1.67
1.82
Crystal size (mm)
0.20 × 0.20 × 0.12
0.30 × 0.25 × 0.20
Data collection
Diffractometer
Bruker Kappa APEXII
Bruker Kappa APEXII
Absorption correction
Multi-scan (SADABS; Bruker, 2012 ▸)
Multi-scan (SADABS; Bruker, 2012 ▸)
Tmin, Tmax
0.796, 0.819
0.696, 0.712
No. of measured, independent and observed [I > 2σ(I)] reflections
12953, 3209, 3138
40786, 4122, 3099
Rint
0.028
0.060
(sin θ/λ)max (Å−1)
0.619
0.618
Refinement
R[F2 > 2σ(F2)], wR(F2), S
0.028, 0.072, 1.05
0.053, 0.139, 1.07
No. of reflections
3209
4122
No. of parameters
260
336
No. of restraints
42
46
H-atom treatment
H-atom parameters constrained
H-atom parameters constrained
Δρmax, Δρmin (e Å−3)
0.14, −0.10
0.25, −0.23
Absolute structure
Flack x determined using 1373 quotients [(I+)−(I−)]/[(I+)+(I−)] (Parsons et al., 2013 ▸)
All H were treated as riding atoms in geometrically idealized positions with distances C—H = 0.93 Å (aromatic), 0.96 Å (CH3) or 0.97 Å (CH2) and N—H = 0.86 Å, and with U
iso(H) = kU
eq (C), where k = 1.5 for the methyl groups which were permitted to rotate but not to tilt and 1.2 for all other H atoms. One bad outlier reflection (39) was omitted from the final refinement of (I).The correct orientation of the structure of (I), relative to the polar axis direction, was established by means of the Flack x parameter (Flack, 1983 ▸), calculated (Parsons et al., 2013 ▸) using 1373 quotients of the type [(I
+)−(I
−)]/[(I
+)+(I
−)], and by means of the Hooft y parameter (Hooft et al., 2010 ▸): x = 0.061 (7) and y = 0.0561 (8): use of the TWIN/BASF procedure in SHELXL for the determination of the Flack x parameter gave a less well defined value, x = 0.053 (18). In the final analysis of variance for compound (II), there was a large value, 6.892, of K = [mean(F
2)/mean(F
c
2)] for the group of 433 very weak reflections having F
c/F
c(max) in the range 0 < F
c/F
c(max) < 0.006.Crystal structure: contains datablock(s) global, I, II. DOI: 10.1107/S2056989017011446/lh5849sup1.cifStructure factors: contains datablock(s) I. DOI: 10.1107/S2056989017011446/lh5849Isup2.hklStructure factors: contains datablock(s) II. DOI: 10.1107/S2056989017011446/lh5849IIsup3.hklClick here for additional data file.Supporting information file. DOI: 10.1107/S2056989017011446/lh5849Isup4.cmlClick here for additional data file.Supporting information file. DOI: 10.1107/S2056989017011446/lh5849IIsup5.cmlCCDC references: 1566446, 1566445Additional supporting information: crystallographic information; 3D view; checkCIF report
Absolute structure: Flack x determined using 1373 quotients
[(I+)-(I-)]/[(I+)+(I-)]
(Parsons et al., 2013)
Hydrogen site location: inferred from neighbouring sites
Absolute structure parameter: 0.061 (7)
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.
Hydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.053
H-atom parameters constrained
wR(F2) = 0.139
w = 1/[σ2(Fo2) + (0.0565P)2 + 0.8506P] where P = (Fo2 + 2Fc2)/3
S = 1.07
(Δ/σ)max < 0.001
4122 reflections
Δρmax = 0.25 e Å−3
336 parameters
Δρmin = −0.23 e Å−3
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.
Authors: Aleš Imramovský; Vladimír Pejchal; Šárka Štěpánková; Katarína Vorčáková; Josef Jampílek; Ján Vančo; Petr Šimůnek; Karel Královec; Lenka Brůčková; Jana Mandíková; František Trejtnar Journal: Bioorg Med Chem Date: 2013-02-01 Impact factor: 3.641