research communications
of 4-[(2-hydroxy-3-methoxybenzyl)amino]benzoic acid hemihydrate
aDepartment of Applied Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh UP 202 002, India, bDepartment of Chemistry, Langat Singh College, B. R. A. Bihar University, Muzaffarpur, Bihar 842 001, India, cDepartment of Chemistry, Indian Institute of Technology Kanpur 208016 UP, India, dOndokuz Mayıs University, Arts and Sciences Faculty, Department of Physics, 55139 Samsun, Turkey, and eNational Taras Shevchenko University, Department of Chemistry, Volodymyrska str., 64, 01601 Kyiv, Ukraine
*Correspondence e-mail: faizichemiitg@gmail.com, tiskenderov@ukr.net
In the crystal of the title vanilline derivative, 2C15H15NO4·H2O, the secondary amine molecule is accompanied by half equivalent of water. The molecule is non-planar, with torsion angle Caryl—CH2—NH—Caryl of −83.9 (2)°. In the crystal, the system of O—H⋯O hydrogen bonds, including bridging water molecules residing on crystallographic twofold axes, results in a two-dimensional layered structure. Within the layers, there are also weak N—H⋯π interactions involving the vanilline benzene ring.
Keywords: crystal structure; 2-hydroxy-3-methoxy-benzaldehyde; 4-aminobenzoic acid (PABA); secondary amine; hydrogen bonding; vanillin.
CCDC reference: 1888072
1. Chemical context
The title compound is obtained by reduction of reported (Kamaal et al., 2018) (E)-4-(2-hydroxy-3-methoxybenzylideneamino)benzoic acid with sodiumborohydride. The Schiff base is formed by condensation of 4-aminobenzoic acid with o-vanilline. Both p-aminobenzoic acid and o-vanilline have biological importance, for example as a bacterial cofactor involved in the synthesis of folic acid (Robinson, 1966). Another example is benzocaine, the ethyl ester of p-aminobenzoic acid, which is a local anaesthetic. The mechanism includes inhibiting voltage-dependent sodium channels on the nerve membrane, which results in stopping the signal propagation (Neumcke et al., 1981). The present work is also a part of an ongoing structural study of and secondary for their utilization in the synthesis of new organic compounds and application of excited-state proton transfer and fluorescent chemosensor (Faizi et al., 2016a,b, 2018a,b; Kumar et al., 2018; Mukherjee et al., 2018).
2. Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The title compound has two substituted aromatic rings at either end of the –CH2-NH– linkage [Caryl—CH2—NH—Caryl torsion angle = −83.9 (2)°]. The water solvent stabilizes the through hydrogen bonding. The secondary amine N atom has a practically planar trigonal configuration deviating by just 0.03 (1) Å from the mean plane of the adjacent atoms, and it is apparently conjugated with the adjacent benzene ring [the C—N bond length is 1.368 (2) Å]. For comparison, the reported C—N distance in of the ethyl 4-[(E)-(4-hydroxy-3-methoxybenzylidene)amino]benzoate Schiff base is 1.274 (2) Å (Ling et al., 2016) and in the zwitterion it is 1.312 Å (Kamaal et al., 2018). The C6—O2 bond of the hydroxyl group [1.371 (2)Å] and those of the acid moiety [O3—C15 = 1.224 (2) and O4—C15 = 1.317 (2) Å] are in the expected ranges. The C5—O1 bond length to the methoxy group is 1.372 (2) Å.
3. Supramolecular features
In the crystal, molecules are connected via O—H⋯O interactions forming layers in the ab plane (Table 1, Fig. 2). While the N—H group is not involved in traditional hydrogen-bonding interactions, there are intermolecular N—H⋯π interactions within the layers (Table 1, Fig. 3).
4. Database survey
A search through the Cambridge Structural Database (CSD, Version 5.39, update Aug 2018; Groom et al., 2016) gave nine hits for the secondary amine. There are only two examples of similar compounds in the literature: ethyl 4-{[(2-hydroxyphenyl)methyl]amino}benzoate, (I) (WEFQEG; Salman et al., 2017), and ethyl 4-[(3,5-di-tert-butyl-2-hydroxybenzyl)amino]benzoate, (II) (VABTAV;. Shakir et al., 2010). Other related structures based on benzylidene-phenyl-amine are reported as n-propyl 4-[2-(4,6-dimethoxypyrimidin-2-yloxy)benzylamino]benzoate, (III) (ILAGIL; Wu et al., 2003), and [4-(2-hydroxybenzylamino)benzoato-κO]triphenyltin(IV), (IV) (WENXAP; Jiang et al., 2006), There is also one very similar compound, viz. ethyl 4-[(2-hydroxybenzyl)amino]benzoate (Salman et al., 2017), in which the 3-methoxy group in the title compound is replaced by a hydrogen atom and the carboxylic acid is replaced by an ester. The torsion angle Caryl—CH2—NH—Caryl in the title compound [−83.9 (2)°] compares well to those in I (73.68°), II (77.38°) and IV (−87.28°) despite the difference in substituent groups.
5. Synthesis and crystallization
To a hot stirred solution of 4-aminobenzoic acid (PABA) (1.00 g, 7.2 mmol) in methanol (15 ml) was added vanillin (1.11 g, 7.2 mmol). The resultant mixture was then heated under reflux. After an hour, precipitates were formed. The reaction mixture was heated for about a further 30 minutes for the completion of the reaction, which was monitored through TLC. The reaction mixture was cooled to room temperature, filtered and washed with hot methanol. It was then dried in a vacuum to give (E)-4-(2-hydroxy-3-methoxybenzylideneamino)benzoic acid (1) in 78% yield.
Compound (1) (1.00 g, 3.7 mmol) was dissolved in 25 mL of methanol and reduced by addition of excess sodium borohydride (0.28 g, 7.4 mmol). The solution was stirred until the yellow colour disappeared. Then the solution was diluted with 8–10 times the volume of water and the pH was adjusted to 6 by addition of 12% HCl. The white precipitate was collected and dried in air. Colourless single crystals of the title compound, suitable for X-ray analysis, were obtained by slow evaporation of a methanol solution.
6. Refinement
Crystal data, data collection and structure . The N—H and O—H H atoms were located in difference-Fourier maps and freely refined, while the C-bound H atoms were included in calculated positions and treated as riding, with fixed C—H = 0.93 Å, and Uiso(H) = 1.2Ueq(C,N).
details are summarized in Table 2
|
Supporting information
CCDC reference: 1888072
https://doi.org/10.1107/S2056989018018455/ld2147sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989018018455/ld2147Isup2.hkl
Supporting information file. DOI: https://doi.org/10.1107/S2056989018018455/ld2147Isup3.cml
Data collection: APEX2 (Bruker, 2014); cell
SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).2C15H15NO4·H2O | F(000) = 1192 |
Mr = 564.57 | Dx = 1.436 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
a = 24.742 (3) Å | Cell parameters from 6409 reflections |
b = 5.5002 (6) Å | θ = 2.5–28.2° |
c = 19.387 (2) Å | µ = 0.11 mm−1 |
β = 98.292 (6)° | T = 296 K |
V = 2610.8 (5) Å3 | Prism, colorless |
Z = 4 | 0.45 × 0.34 × 0.14 mm |
Bruker APEXII CCD diffractometer | 2138 reflections with I > 2σ(I) |
φ and ω scans | Rint = 0.065 |
Absorption correction: multi-scan (SADABS; Bruker, 2014) | θmax = 26.0°, θmin = 2.9° |
h = −30→30 | |
16146 measured reflections | k = −6→6 |
2570 independent reflections | l = −23→23 |
Refinement on F2 | 1 restraint |
Least-squares matrix: full | Hydrogen site location: mixed |
R[F2 > 2σ(F2)] = 0.042 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.107 | w = 1/[σ2(Fo2) + (0.0366P)2 + 3.8398P] where P = (Fo2 + 2Fc2)/3 |
S = 1.05 | (Δ/σ)max < 0.001 |
2570 reflections | Δρmax = 0.26 e Å−3 |
203 parameters | Δρmin = −0.22 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. |
x | y | z | Uiso*/Ueq | ||
O2 | 0.95635 (5) | 0.7070 (2) | 0.67260 (6) | 0.0206 (3) | |
O5 | 0.500000 | 0.7903 (3) | 0.750000 | 0.0222 (4) | |
O1 | 0.97314 (5) | 1.0831 (2) | 0.59182 (6) | 0.0238 (3) | |
O3 | 0.56242 (5) | 0.2504 (2) | 0.65933 (7) | 0.0271 (3) | |
O4 | 0.58989 (5) | 0.6076 (3) | 0.70737 (8) | 0.0347 (4) | |
N1 | 0.81774 (6) | 0.2583 (3) | 0.63512 (8) | 0.0197 (3) | |
C14 | 0.74975 (6) | 0.5056 (3) | 0.68083 (8) | 0.0168 (3) | |
H14 | 0.775566 | 0.623501 | 0.696467 | 0.020* | |
C9 | 0.76511 (6) | 0.2985 (3) | 0.64627 (8) | 0.0158 (3) | |
C12 | 0.65625 (6) | 0.3621 (3) | 0.66873 (8) | 0.0174 (4) | |
C15 | 0.59858 (7) | 0.3981 (3) | 0.67792 (9) | 0.0194 (4) | |
C7 | 0.86973 (6) | 0.6259 (3) | 0.60621 (9) | 0.0177 (4) | |
C10 | 0.72476 (7) | 0.1239 (3) | 0.62307 (9) | 0.0181 (4) | |
H10 | 0.734254 | −0.014204 | 0.599877 | 0.022* | |
C13 | 0.69600 (7) | 0.5340 (3) | 0.69160 (8) | 0.0174 (3) | |
H13 | 0.686232 | 0.671711 | 0.714756 | 0.021* | |
C5 | 0.92543 (7) | 0.9565 (3) | 0.57298 (9) | 0.0187 (4) | |
C6 | 0.91728 (6) | 0.7641 (3) | 0.61739 (8) | 0.0175 (4) | |
C2 | 0.83105 (7) | 0.6781 (3) | 0.54820 (9) | 0.0206 (4) | |
H2 | 0.799081 | 0.587223 | 0.539822 | 0.025* | |
C8 | 0.86335 (6) | 0.4187 (3) | 0.65624 (9) | 0.0184 (4) | |
H8A | 0.896629 | 0.323016 | 0.662065 | 0.022* | |
H8B | 0.859392 | 0.487346 | 0.701326 | 0.022* | |
C4 | 0.88690 (7) | 1.0051 (3) | 0.51542 (9) | 0.0220 (4) | |
H4 | 0.892432 | 1.131150 | 0.485232 | 0.026* | |
C11 | 0.67166 (7) | 0.1554 (3) | 0.63432 (9) | 0.0182 (4) | |
H11 | 0.645659 | 0.037923 | 0.618889 | 0.022* | |
C3 | 0.83997 (7) | 0.8641 (3) | 0.50316 (9) | 0.0236 (4) | |
H3 | 0.814245 | 0.895045 | 0.464242 | 0.028* | |
C1 | 0.98495 (8) | 1.2765 (3) | 0.54726 (10) | 0.0262 (4) | |
H1A | 0.986748 | 1.213811 | 0.501435 | 0.039* | |
H1B | 1.019360 | 1.348797 | 0.565539 | 0.039* | |
H1C | 0.956674 | 1.397043 | 0.544826 | 0.039* | |
H5 | 0.5159 (12) | 0.891 (5) | 0.7772 (13) | 0.076 (10)* | |
H1 | 0.8246 (9) | 0.120 (4) | 0.6076 (12) | 0.038 (6)* | |
H2A | 0.9900 (10) | 0.771 (5) | 0.6651 (13) | 0.050 (7)* | |
H4A | 0.5536 (12) | 0.630 (6) | 0.7145 (15) | 0.073 (9)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O2 | 0.0122 (6) | 0.0235 (7) | 0.0262 (6) | −0.0025 (5) | 0.0029 (5) | 0.0043 (5) |
O5 | 0.0150 (8) | 0.0244 (10) | 0.0277 (10) | 0.000 | 0.0043 (7) | 0.000 |
O1 | 0.0199 (6) | 0.0212 (7) | 0.0313 (7) | −0.0052 (5) | 0.0067 (5) | 0.0063 (5) |
O3 | 0.0136 (6) | 0.0270 (7) | 0.0411 (8) | −0.0032 (5) | 0.0055 (5) | −0.0011 (6) |
O4 | 0.0173 (7) | 0.0366 (8) | 0.0524 (9) | 0.0000 (6) | 0.0127 (6) | −0.0187 (7) |
N1 | 0.0137 (7) | 0.0186 (7) | 0.0280 (8) | −0.0020 (6) | 0.0071 (6) | −0.0034 (6) |
C14 | 0.0150 (8) | 0.0163 (8) | 0.0191 (8) | −0.0038 (6) | 0.0020 (6) | −0.0007 (6) |
C9 | 0.0132 (7) | 0.0169 (8) | 0.0173 (8) | 0.0001 (6) | 0.0025 (6) | 0.0030 (6) |
C12 | 0.0148 (8) | 0.0192 (8) | 0.0184 (8) | 0.0004 (7) | 0.0037 (6) | 0.0024 (7) |
C15 | 0.0166 (8) | 0.0220 (9) | 0.0201 (8) | 0.0010 (7) | 0.0042 (6) | 0.0013 (7) |
C7 | 0.0156 (8) | 0.0184 (8) | 0.0207 (8) | 0.0012 (7) | 0.0084 (6) | −0.0023 (7) |
C10 | 0.0176 (8) | 0.0145 (8) | 0.0224 (8) | 0.0004 (7) | 0.0035 (7) | −0.0002 (7) |
C13 | 0.0176 (8) | 0.0172 (8) | 0.0178 (8) | 0.0012 (7) | 0.0039 (6) | −0.0011 (7) |
C5 | 0.0161 (8) | 0.0180 (8) | 0.0236 (9) | 0.0000 (7) | 0.0083 (7) | −0.0010 (7) |
C6 | 0.0149 (8) | 0.0187 (8) | 0.0198 (8) | 0.0030 (7) | 0.0057 (6) | −0.0020 (7) |
C2 | 0.0159 (8) | 0.0232 (9) | 0.0230 (9) | −0.0007 (7) | 0.0038 (7) | −0.0033 (7) |
C8 | 0.0116 (7) | 0.0196 (9) | 0.0246 (9) | −0.0012 (7) | 0.0043 (6) | 0.0003 (7) |
C4 | 0.0249 (9) | 0.0215 (9) | 0.0211 (9) | 0.0028 (7) | 0.0082 (7) | 0.0034 (7) |
C11 | 0.0157 (8) | 0.0150 (8) | 0.0239 (9) | −0.0040 (6) | 0.0026 (6) | 0.0009 (7) |
C3 | 0.0217 (9) | 0.0287 (10) | 0.0202 (9) | 0.0018 (8) | 0.0025 (7) | −0.0001 (7) |
C1 | 0.0290 (10) | 0.0180 (9) | 0.0351 (10) | −0.0029 (8) | 0.0159 (8) | 0.0043 (8) |
O2—C6 | 1.371 (2) | C7—C6 | 1.391 (2) |
O2—H2A | 0.93 (3) | C7—C2 | 1.398 (2) |
O5—H5 | 0.823 (17) | C7—C8 | 1.519 (2) |
O5—H5i | 0.823 (17) | C10—C11 | 1.374 (2) |
O1—C5 | 1.374 (2) | C10—H10 | 0.9300 |
O1—C1 | 1.427 (2) | C13—H13 | 0.9300 |
O3—C15 | 1.224 (2) | C5—C4 | 1.385 (2) |
O4—C15 | 1.317 (2) | C5—C6 | 1.397 (2) |
O4—H4A | 0.94 (3) | C2—C3 | 1.383 (3) |
N1—C9 | 1.368 (2) | C2—H2 | 0.9300 |
N1—C8 | 1.445 (2) | C8—H8A | 0.9700 |
N1—H1 | 0.96 (2) | C8—H8B | 0.9700 |
C14—C13 | 1.384 (2) | C4—C3 | 1.388 (3) |
C14—C9 | 1.401 (2) | C4—H4 | 0.9300 |
C14—H14 | 0.9300 | C11—H11 | 0.9300 |
C9—C10 | 1.411 (2) | C3—H3 | 0.9300 |
C12—C13 | 1.390 (2) | C1—H1A | 0.9600 |
C12—C11 | 1.399 (2) | C1—H1B | 0.9600 |
C12—C15 | 1.477 (2) | C1—H1C | 0.9600 |
C6—O2—H2A | 109.7 (15) | O1—C5—C6 | 114.50 (15) |
H5—O5—H5i | 96 (4) | C4—C5—C6 | 119.92 (16) |
C5—O1—C1 | 117.37 (14) | O2—C6—C7 | 118.78 (15) |
C15—O4—H4A | 113.6 (19) | O2—C6—C5 | 120.42 (15) |
C9—N1—C8 | 125.37 (15) | C7—C6—C5 | 120.79 (15) |
C9—N1—H1 | 117.7 (13) | C3—C2—C7 | 120.46 (16) |
C8—N1—H1 | 116.7 (13) | C3—C2—H2 | 119.8 |
C13—C14—C9 | 119.78 (15) | C7—C2—H2 | 119.8 |
C13—C14—H14 | 120.1 | N1—C8—C7 | 115.22 (14) |
C9—C14—H14 | 120.1 | N1—C8—H8A | 108.5 |
N1—C9—C14 | 122.47 (15) | C7—C8—H8A | 108.5 |
N1—C9—C10 | 119.03 (15) | N1—C8—H8B | 108.5 |
C14—C9—C10 | 118.50 (14) | C7—C8—H8B | 108.5 |
C13—C12—C11 | 118.44 (15) | H8A—C8—H8B | 107.5 |
C13—C12—C15 | 121.46 (15) | C5—C4—C3 | 119.45 (16) |
C11—C12—C15 | 120.06 (15) | C5—C4—H4 | 120.3 |
O3—C15—O4 | 123.39 (15) | C3—C4—H4 | 120.3 |
O3—C15—C12 | 123.62 (16) | C10—C11—C12 | 120.71 (15) |
O4—C15—C12 | 112.98 (15) | C10—C11—H11 | 119.6 |
C6—C7—C2 | 118.60 (16) | C12—C11—H11 | 119.6 |
C6—C7—C8 | 118.28 (15) | C2—C3—C4 | 120.72 (16) |
C2—C7—C8 | 123.08 (15) | C2—C3—H3 | 119.6 |
C11—C10—C9 | 120.84 (15) | C4—C3—H3 | 119.6 |
C11—C10—H10 | 119.6 | O1—C1—H1A | 109.5 |
C9—C10—H10 | 119.6 | O1—C1—H1B | 109.5 |
C14—C13—C12 | 121.73 (15) | H1A—C1—H1B | 109.5 |
C14—C13—H13 | 119.1 | O1—C1—H1C | 109.5 |
C12—C13—H13 | 119.1 | H1A—C1—H1C | 109.5 |
O1—C5—C4 | 125.57 (16) | H1B—C1—H1C | 109.5 |
C8—N1—C9—C14 | −0.4 (3) | C8—C7—C6—C5 | 179.87 (15) |
C8—N1—C9—C10 | −179.51 (15) | O1—C5—C6—O2 | −3.0 (2) |
C13—C14—C9—N1 | −178.88 (15) | C4—C5—C6—O2 | 176.85 (15) |
C13—C14—C9—C10 | 0.2 (2) | O1—C5—C6—C7 | 177.53 (14) |
C13—C12—C15—O3 | −179.11 (16) | C4—C5—C6—C7 | −2.7 (2) |
C11—C12—C15—O3 | 3.0 (3) | C6—C7—C2—C3 | −0.2 (2) |
C13—C12—C15—O4 | 1.8 (2) | C8—C7—C2—C3 | −177.81 (16) |
C11—C12—C15—O4 | −176.06 (15) | C9—N1—C8—C7 | −83.9 (2) |
N1—C9—C10—C11 | 178.86 (15) | C6—C7—C8—N1 | −170.01 (14) |
C14—C9—C10—C11 | −0.3 (2) | C2—C7—C8—N1 | 7.6 (2) |
C9—C14—C13—C12 | −0.3 (2) | O1—C5—C4—C3 | −179.06 (16) |
C11—C12—C13—C14 | 0.3 (2) | C6—C5—C4—C3 | 1.2 (2) |
C15—C12—C13—C14 | −177.56 (15) | C9—C10—C11—C12 | 0.4 (3) |
C1—O1—C5—C4 | −2.0 (2) | C13—C12—C11—C10 | −0.4 (2) |
C1—O1—C5—C6 | 177.78 (14) | C15—C12—C11—C10 | 177.54 (15) |
C2—C7—C6—O2 | −177.34 (14) | C7—C2—C3—C4 | −1.3 (3) |
C8—C7—C6—O2 | 0.3 (2) | C5—C4—C3—C2 | 0.8 (3) |
C2—C7—C6—C5 | 2.2 (2) |
Symmetry code: (i) −x+1, y, −z+3/2. |
Cg1 is the centroid of the C2–C7 ring. |
D—H···A | D—H | H···A | D···A | D—H···A |
O5—H5···O2ii | 0.82 (2) | 2.06 (2) | 2.8640 (19) | 164 (3) |
O2—H2A···O3iii | 0.93 (3) | 1.82 (3) | 2.6844 (17) | 154 (2) |
O4—H4A···O5 | 0.94 (3) | 1.81 (3) | 2.6776 (15) | 153 (3) |
N1—H1···Cg1iv | 0.96 (2) | 2.40 (2) | 3.3008 (18) | 157 (2) |
Symmetry codes: (ii) −x+3/2, y+1/2, −z+3/2; (iii) x+1/2, y+1/2, z; (iv) x, y−1, z. |
Acknowledgements
The authors thank the Department of Chemistry, L. S. College, B. R. A. Bihar University, and the Department of Applied Chemistry, Faculty of Engineering & Technology, Aligarh Muslim University, for providing laboratory facilities.
Funding information
The authors are grateful to the Department of Chemistry, National Taras Shevchenko University, for financial support.
References
Bruker (2014). APEX2, SAINT and SADABS. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Faizi, M. S. H., Alam, M. J., Haque, A., Ahmad, S., Shahid, M. & Ahmad, M. (2018a). J. Mol. Struct. 1156, 457–464. CrossRef CAS Google Scholar
Faizi, M. S. H., Ali, A. & Potaskalov, V. A. (2016a). Acta Cryst. E72, 1366–1369. Web of Science CrossRef IUCr Journals Google Scholar
Faizi, M. S. H., Dege, N. & Iskenderov, T. S. (2018b). Acta Cryst. E74, 410–413. CrossRef IUCr Journals Google Scholar
Faizi, M. S. H., Gupta, S., Mohan, V. K., Jain, K. V. & Sen, P. (2016b). Sens. Actuators B Chem. 222, 15–20. Web of Science CrossRef CAS Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CrossRef IUCr Journals Google Scholar
Jiang, H., Ma, J.-F. & Zhang, W.-L. (2006). Acta Cryst. E62, m2745–m2746. CrossRef IUCr Journals Google Scholar
Kamaal, S., Faizi, M. S. H., Ali, A., Ahmad, M. & Iskenderov, T. (2018). Acta Cryst. E74, 1847–1850. CrossRef IUCr Journals Google Scholar
Kumar, M., Kumar, A., Faizi, M. S. H., Kumar, S., Singh, M. K., Sahu, S. K., Kishor, S. & John, R. P. (2018). Sens. Actuators B Chem. 260, 888–899. CrossRef CAS Google Scholar
Ling, J., Kavuru, P., Wojtas, L. & Chadwick, K. (2016). Acta Cryst. E72, 951–954. Web of Science CrossRef IUCr Journals Google Scholar
Mukherjee, P., Das, A., Faizi, M. S. H. & Sen, P. (2018). ChemistrySelect, 3, 3787–3796. CrossRef CAS Google Scholar
Neumcke, B., Schwarz, W. & Stampfli, R. (1981). Pflugers Arch. 390, 230–236. CrossRef CAS PubMed Web of Science Google Scholar
Robinson, F. A. (1966). The Vitamin Co-factors of Enzyme Systems, pp. 541–662. London: Pergamon. Google Scholar
Salman, M., Abu-Yamin, A. A., Sarairah, I., Ibrahim, A. & Aldamen, M. A. (2017). Z. Kristallogr. 232, 631–632. CAS Google Scholar
Shakir, R. M., Ariffin, A. & Ng, S. W. (2010). Acta Cryst. E66, o2916. CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Wu, J., Zhang, P.-Z., Lu, L., Yu, Q.-S., Hu, X.-R & Gu, J.-M. (2003). Chin. J. Struct. Chem. 22, 613–616. CAS Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.