research communications
3-Methyl-1-{(E)-[1-(4-methylpyridin-2-yl)ethylidene]amino}thiourea: and Hirshfeld surface analysis
aDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan, Malaysia, bDepartment of Physics, Bhavan's Sheth R. A. College of Science, Ahmedabad, Gujarat 380 001, India, and cResearch Centre for Crystalline Materials, School of Science and Technology, Sunway University, 47500 Bandar Sunway, Selangor Darul Ehsan, Malaysia
*Correspondence e-mail: edwardt@sunway.edu.my
The title disubstituted thiourea derivative, C10H14N4S, features an almost planar imine (E configuration, C3N) core flanked by thiourea (CN2S) and methylpyridyl (C5N) residues (each plane has a r.m.s. deviation of the respective fitted atoms of 0.0066 Å). The dihedral angles between the core and the thiourea and pyridyl residues are 20.25 (8) and 7.60 (9)°, respectively, indicating twists in the molecule; the dihedral angle between the outer planes is 13.62 (7)°. There is an anti-disposition of the amine-N—H atoms which allows for the formation of an intramolecular amine-N—H⋯N(imine) hydrogen bond that closes an S(5) loop. In the crystal, amine-N—H⋯N(pyridyl) hydrogen bonds lead to zigzag (glide symmetry) supramolecular chains along the c-axis direction. These are connected into a supramolecular layer propagating in the bc plane by thioamide-N—H⋯S(thione) hydrogen bonds via eight-membered thioamide {⋯HNCS}2 synthons.
CCDC reference: 1818317
1. Chemical context
Thiosemicarbazones (TSCs) are thiourea derivatives that form versatile ligands containing mixed hard–soft, nitrogen–sulfur donor atoms. TSC and its derivatives have attracted considerable attention due to their promising biological applications, especially in the realm of anti-tumour (Hussein et al., 2015), anti-viral (Easmon et al., 1992), anti-malarial (Kumar et al., 2014), anti-fungal (Lobana et al., 2017), anti-bacterial (Khan & Asiri, 2018) and anti-parasitic (Njogu & Chibale, 2013) activities. Their biological potential has been found to be enhanced by the addition of at the terminal N-position (Liberta and West, 1992). In fact, a thiosemicarbazone drug, methisazone (N-methylisatin β-thiosemicarbazone) was reported as an anti-viral agent by McNeill in 1972 (McNeill, 1972) and field trials for methisazone as a prophylactic agent against smallpox were carried out in West Pakistan between 1964 and 1970 (Heiner et al., 1971). More recently, phase I and phase II clinical trials were conducted for triapine (3-aminopyridinecarbaldehyde thiosemicarbazone) in untreated patients with advanced-stage cervical cancer where triapine showed an inhibition of ribonucleotide reductase and thus enhanced the radiochemosensitivity by prolonging DNA repair time (Kunos & Sherertz, 2014). With this interest and as a part of on-going investigations on a series of thiosemicarbazone and their transition metal complexes, the title compound, namely the N-methyl thiosemicarbazone derived from 2-acetyl-4-methyl pyridine, (I), was synthesized. Herein, its crystal and molecular structures along with an analysis of its Hirshfeld surface and fingerprint plots are reported.
2. Structural commentary
The molecular structure of (I), Fig. 1, comprises three distinct almost planar residues, namely the thiourea (C1,N1,N2,S1), central imine (C3,C4,C5,N3) and methylpyridyl (N4,C5–C10) residues, coincidentally each with the r.m.s. deviation of the respective fitted atoms being 0.0066 Å. Twists in the molecule are apparent about the N2—N3 and C3—C5 bonds as seen in the values of the C1—N2—N3—C3 and C4—C3—C5—C9 torsion angles of −167.44 (13) and 171.34 (13)°, respectively. This is reflected in the dihedral angles between the mean planes through the central and each of the thiourea and methylpyridyl residues of 20.25 (8) and 7.60 (9)°, respectively; the dihedral angle between the outer planes is 13.62 (7)°. The configuration about the C3=N3 imine bond [1.2872 (19) Å] is E. The molecule in (I) features an anti-disposition of the amine-N—H atoms, which facilitates the formation of an intramolecular amine-N1—H⋯N3(imine) hydrogen bond to close an S(5) loop, Table 1. The methyl groups lie to opposite sides of the molecule and can also be described as being anti to one another.
3. Supramolecular features
The most prominent feature of the molecular packing is the formation of eight-membered thioamide {⋯HNCS}2 synthons owing to the formation of thioamide-N2—H⋯S1(thione) hydrogen bonds between centrosymmetrically related molecules, Table 1. These serve to link zigzag (glide symmetry) supramolecular chains, along the c-axis direction and sustained by amine-N1—H⋯N4(pyridyl) hydrogen bonds, into a supramolecular layer propagating in the bc plane, Fig. 2a. Additional stabilization of the layers is afforded by methyl-C—H⋯π(pyridyl) interactions, Table 1. Layers stack along the a axis without directional interactions between them, Fig. 2b.
4. Analysis of the Hirshfeld surfaces
The Hirshfeld surface calculations were performed according to recent work on a related organic molecule (Tan et al., 2017) and serve to provide more detailed information on the influence of intermolecular interactions in the crystal. The dominant N—H⋯S and N—H⋯N hydrogen-bonding interactions in the structure of (I) are viewed as bright-red spots near the respective donor and acceptor atoms on the Hirshfeld surfaces mapped over dnorm shown in Fig. 3. The diminutive red spots near the pyridyl-N4 and -H9 atoms indicate the presence of intermolecular C—H⋯N interactions. In addition to the above, the crystal also features comparatively weak intermolecular C—H⋯S interactions and short interatomic C⋯S/S⋯C contacts, Table 2, viewed as faint-red spots in Fig. 3. The Hirshfeld surfaces mapped over electrostatic potential shown in Fig. 4 represent the donors and acceptors of intermolecular interactions with blue and red regions corresponding to positive and negative electrostatic potentials, respectively.
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The overall two-dimensional fingerprint plot for (I), showing the key interatomic contacts, is illustrated in Fig. 5a; fingerprint plots delineated (McKinnon et al., 2007) into H⋯H, C⋯H/H⋯C, S⋯H/H⋯S and N⋯H/H⋯N contacts are shown in Fig. 5b–e. The percentage contributions from the different interatomic contacts to the Hirshfeld surface are summarized in Table 3. A spike at de + di ∼2.2 Å with the label `a' in the middle of the plot and those around it at de + di ∼2.2 and 2.4 Å, labelled with `b′ and `c′, in the plot of Fig. 5a indicate the presence of the short interatomic H2A⋯H4B contact (Table 2) and intermolecular N—H⋯N and N—H⋯S hydrogen bonds (Table 1), respectively. The significant contribution of 16.7% from C⋯H/H⋯C contacts to the Hirshfeld surface of (I) is the result of the short C3⋯H4A contact (Table 2) and C—H⋯π interaction (Table 1), viewed as a pair of very short peaks at de + di ∼2.8 Å and the parabolic distribution of points around de + di ∼2.9 Å, respectively. The points related to the most prominent interlayer contact, i.e. S1⋯H7 (Table 2), are merged within the plot delineated into S⋯H/H⋯S contacts (Fig. 5d) due to the presence of N—H⋯S hydrogen bonds. The contribution of 0.6% from C⋯S/S⋯C contacts to the Hirshfeld surfaces of (I) indicate the presence of the short C4⋯S1 contact listed in Table 2. The other interatomic contacts summarized in Table 3 having large interatomic separations have a negligible effect on the packing.
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5. Database survey
Reflecting the interest in this class of compounds, there are no fewer than 16 structures related to (I) included in the Cambridge Structural Database (Version 5.38; Groom et al., 2016), i.e. that are neutral and feature N1-bound alkyl or aryl group and a C3-bound pyridyl ring; the C4-bound methyl group is common to all structures. The most closely related structure to (I), i.e. with an unsubstituted 2-pyridyl ring at the C3-position, has been described three times, being originally reported in 1999 (Bermejo et al., 1999). Most structures feature N1-bound aryl rings, and all feature an anti-disposition of the N—H groups.
6. Synthesis and crystallization
All chemicals were of analytical grade and were used without any further purification. 2-Acetyl-4-methyl pyridine (0.68 g, 0.005 mol) in absolute ethanol (40 ml) was dissolved and added to 4-methyl-3-thiosemicarbazide (0.52 g, 0.005 mol) dissolved in absolute ethanol (40 ml). The mixture was then heated in a water bath for 10 mins with constant and vigorous stirring until the volume reduced to 30 ml. The product that formed was filtered off, washed with cold ethanol and dried in a desiccator over anhydrous silica gel. Brown platy crystals suitable for single crystal X-ray −1): 3274 ν(N—H), 1589 ν(C=N), 1118 ν(N—N), 1045 ν(C=S). MS (m/z): 222.
were obtained by recrystallization with absolute ethanol as solvent. M.pt: 468.8–470.1 K. IR (cm7. Refinement
Crystal data, data collection and structure . The carbon-bound H atoms were placed in calculated positions (C—H = 0.95–0.98 Å) and were included in the in the riding model approximation, with Uiso(H) set to 1.2–1.5Ueq(C). The nitrogen-bound H atoms were located in a difference-Fourier map but were refined with a distance restraint of N—H = 0.88±0.01 Å, and with Uiso(H) set to 1.2Ueq(N).
details are summarized in Table 4Supporting information
CCDC reference: 1818317
https://doi.org/10.1107/S2056989018001305/hb7729sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989018001305/hb7729Isup2.hkl
Data collection: CrysAlis PRO (Agilent, 2011); cell
CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).C10H14N4S | F(000) = 472 |
Mr = 222.31 | Dx = 1.309 Mg m−3 |
Monoclinic, P21/c | Cu Kα radiation, λ = 1.5418 Å |
a = 8.8108 (3) Å | Cell parameters from 9894 reflections |
b = 14.9044 (4) Å | θ = 3.0–71.2° |
c = 9.3583 (3) Å | µ = 2.33 mm−1 |
β = 113.391 (4)° | T = 100 K |
V = 1127.93 (7) Å3 | Plate, brown |
Z = 4 | 0.15 × 0.13 × 0.03 mm |
Rigaku Oxford Diffraction Gemini E diffractometer | 2184 independent reflections |
Radiation source: Enhance X-ray source | 1995 reflections with I > 2σ(I) |
Mirror monochromator | Rint = 0.034 |
Detector resolution: 16.1952 pixels mm-1 | θmax = 71.3°, θmin = 5.5° |
ω scan | h = −10→10 |
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011) | k = −18→17 |
Tmin = 0.852, Tmax = 1.000 | l = −11→11 |
21767 measured reflections |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Hydrogen site location: mixed |
R[F2 > 2σ(F2)] = 0.035 | H atoms treated by a mixture of independent and constrained refinement |
wR(F2) = 0.097 | w = 1/[σ2(Fo2) + (0.063P)2 + 0.4965P] where P = (Fo2 + 2Fc2)/3 |
S = 1.02 | (Δ/σ)max = 0.001 |
2184 reflections | Δρmax = 0.41 e Å−3 |
145 parameters | Δρmin = −0.23 e Å−3 |
2 restraints |
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 | ||
S1 | 1.01197 (4) | 0.43326 (2) | 0.80392 (4) | 0.01660 (14) | |
N1 | 0.78625 (16) | 0.52040 (9) | 0.56429 (14) | 0.0167 (3) | |
H1N | 0.728 (2) | 0.5689 (9) | 0.527 (2) | 0.020* | |
N2 | 0.85960 (15) | 0.58621 (8) | 0.80403 (14) | 0.0151 (3) | |
H2N | 0.908 (2) | 0.5811 (12) | 0.9041 (11) | 0.018* | |
N3 | 0.73527 (15) | 0.64746 (8) | 0.73280 (14) | 0.0151 (3) | |
N4 | 0.57917 (15) | 0.85680 (9) | 0.79338 (14) | 0.0169 (3) | |
C1 | 0.87633 (17) | 0.51659 (10) | 0.71665 (16) | 0.0142 (3) | |
C2 | 0.7888 (2) | 0.45156 (11) | 0.45507 (18) | 0.0206 (3) | |
H2A | 0.9018 | 0.4289 | 0.4860 | 0.031* | |
H2B | 0.7508 | 0.4772 | 0.3502 | 0.031* | |
H2C | 0.7155 | 0.4022 | 0.4554 | 0.031* | |
C3 | 0.73564 (17) | 0.72169 (10) | 0.80323 (16) | 0.0137 (3) | |
C4 | 0.86453 (18) | 0.75205 (10) | 0.95558 (17) | 0.0176 (3) | |
H4A | 0.8263 | 0.7403 | 1.0389 | 0.026* | |
H4B | 0.8841 | 0.8165 | 0.9508 | 0.026* | |
H4C | 0.9676 | 0.7192 | 0.9768 | 0.026* | |
C5 | 0.59425 (17) | 0.78267 (10) | 0.71764 (16) | 0.0144 (3) | |
C6 | 0.45183 (19) | 0.91141 (11) | 0.71570 (19) | 0.0201 (3) | |
H6 | 0.4387 | 0.9638 | 0.7675 | 0.024* | |
C7 | 0.33818 (18) | 0.89648 (11) | 0.56488 (18) | 0.0198 (3) | |
H7 | 0.2515 | 0.9381 | 0.5154 | 0.024* | |
C8 | 0.35303 (17) | 0.81950 (10) | 0.48712 (16) | 0.0168 (3) | |
C9 | 0.48424 (17) | 0.76211 (10) | 0.56638 (16) | 0.0153 (3) | |
H9 | 0.4989 | 0.7089 | 0.5175 | 0.018* | |
C10 | 0.23240 (19) | 0.79716 (11) | 0.32435 (18) | 0.0205 (3) | |
H10A | 0.2929 | 0.7874 | 0.2576 | 0.031* | |
H10B | 0.1546 | 0.8470 | 0.2830 | 0.031* | |
H10C | 0.1714 | 0.7426 | 0.3267 | 0.031* |
U11 | U22 | U33 | U12 | U13 | U23 | |
S1 | 0.0203 (2) | 0.0138 (2) | 0.0143 (2) | 0.00414 (13) | 0.00545 (15) | 0.00082 (13) |
N1 | 0.0217 (6) | 0.0131 (6) | 0.0134 (6) | 0.0032 (5) | 0.0050 (5) | 0.0000 (5) |
N2 | 0.0174 (6) | 0.0140 (6) | 0.0111 (6) | 0.0029 (5) | 0.0028 (5) | 0.0006 (5) |
N3 | 0.0158 (6) | 0.0148 (6) | 0.0144 (6) | 0.0029 (5) | 0.0057 (5) | 0.0017 (5) |
N4 | 0.0178 (6) | 0.0160 (7) | 0.0171 (6) | 0.0018 (5) | 0.0072 (5) | −0.0005 (5) |
C1 | 0.0152 (7) | 0.0125 (7) | 0.0161 (7) | −0.0017 (5) | 0.0073 (5) | 0.0008 (5) |
C2 | 0.0278 (8) | 0.0167 (8) | 0.0158 (7) | 0.0015 (6) | 0.0072 (6) | −0.0027 (6) |
C3 | 0.0158 (7) | 0.0143 (7) | 0.0126 (7) | 0.0000 (5) | 0.0073 (5) | 0.0008 (5) |
C4 | 0.0193 (7) | 0.0141 (7) | 0.0166 (7) | 0.0020 (6) | 0.0042 (6) | −0.0023 (6) |
C5 | 0.0161 (7) | 0.0134 (7) | 0.0156 (7) | −0.0002 (5) | 0.0082 (6) | 0.0013 (5) |
C6 | 0.0216 (7) | 0.0157 (8) | 0.0228 (8) | 0.0035 (6) | 0.0086 (6) | −0.0022 (6) |
C7 | 0.0174 (7) | 0.0185 (8) | 0.0223 (8) | 0.0043 (6) | 0.0067 (6) | 0.0022 (6) |
C8 | 0.0162 (7) | 0.0189 (8) | 0.0158 (7) | 0.0000 (6) | 0.0069 (6) | 0.0033 (6) |
C9 | 0.0182 (7) | 0.0143 (7) | 0.0149 (7) | 0.0005 (6) | 0.0081 (6) | 0.0000 (5) |
C10 | 0.0204 (7) | 0.0221 (8) | 0.0166 (7) | 0.0028 (6) | 0.0050 (6) | 0.0028 (6) |
S1—C1 | 1.6920 (15) | C4—H4A | 0.9800 |
N1—C1 | 1.3295 (18) | C4—H4B | 0.9800 |
N1—C2 | 1.4547 (19) | C4—H4C | 0.9800 |
N1—H1N | 0.873 (9) | C5—C9 | 1.396 (2) |
N2—C1 | 1.3649 (19) | C6—C7 | 1.386 (2) |
N2—N3 | 1.3776 (17) | C6—H6 | 0.9500 |
N2—H2N | 0.864 (9) | C7—C8 | 1.392 (2) |
N3—C3 | 1.2872 (19) | C7—H7 | 0.9500 |
N4—C5 | 1.3474 (19) | C8—C9 | 1.392 (2) |
N4—C6 | 1.342 (2) | C8—C10 | 1.508 (2) |
C2—H2A | 0.9800 | C9—H9 | 0.9500 |
C2—H2B | 0.9800 | C10—H10A | 0.9800 |
C2—H2C | 0.9800 | C10—H10B | 0.9800 |
C3—C5 | 1.4923 (19) | C10—H10C | 0.9800 |
C3—C4 | 1.4971 (19) | ||
C1—N1—C2 | 123.66 (13) | H4A—C4—H4C | 109.5 |
C1—N1—H1N | 117.9 (13) | H4B—C4—H4C | 109.5 |
C2—N1—H1N | 118.3 (13) | N4—C5—C9 | 122.66 (13) |
C1—N2—N3 | 117.92 (12) | N4—C5—C3 | 116.91 (12) |
C1—N2—H2N | 117.3 (12) | C9—C5—C3 | 120.42 (13) |
N3—N2—H2N | 122.4 (12) | N4—C6—C7 | 124.37 (14) |
C3—N3—N2 | 118.81 (12) | N4—C6—H6 | 117.8 |
C5—N4—C6 | 116.62 (12) | C7—C6—H6 | 117.8 |
N1—C1—N2 | 116.71 (13) | C6—C7—C8 | 118.98 (14) |
N1—C1—S1 | 123.75 (11) | C6—C7—H7 | 120.5 |
N2—C1—S1 | 119.50 (11) | C8—C7—H7 | 120.5 |
N1—C2—H2A | 109.5 | C7—C8—C9 | 117.28 (13) |
N1—C2—H2B | 109.5 | C7—C8—C10 | 122.34 (14) |
H2A—C2—H2B | 109.5 | C9—C8—C10 | 120.38 (14) |
N1—C2—H2C | 109.5 | C8—C9—C5 | 120.08 (14) |
H2A—C2—H2C | 109.5 | C8—C9—H9 | 120.0 |
H2B—C2—H2C | 109.5 | C5—C9—H9 | 120.0 |
N3—C3—C5 | 114.63 (12) | C8—C10—H10A | 109.5 |
N3—C3—C4 | 126.45 (13) | C8—C10—H10B | 109.5 |
C5—C3—C4 | 118.89 (12) | H10A—C10—H10B | 109.5 |
C3—C4—H4A | 109.5 | C8—C10—H10C | 109.5 |
C3—C4—H4B | 109.5 | H10A—C10—H10C | 109.5 |
H4A—C4—H4B | 109.5 | H10B—C10—H10C | 109.5 |
C3—C4—H4C | 109.5 | ||
C1—N2—N3—C3 | −167.44 (13) | N3—C3—C5—C9 | −6.6 (2) |
C2—N1—C1—N2 | −179.61 (13) | C4—C3—C5—C9 | 171.34 (13) |
C2—N1—C1—S1 | 2.5 (2) | C5—N4—C6—C7 | −0.5 (2) |
N3—N2—C1—N1 | 9.20 (19) | N4—C6—C7—C8 | 1.0 (2) |
N3—N2—C1—S1 | −172.80 (10) | C6—C7—C8—C9 | −0.8 (2) |
N2—N3—C3—C5 | −178.98 (11) | C6—C7—C8—C10 | 178.57 (15) |
N2—N3—C3—C4 | 3.2 (2) | C7—C8—C9—C5 | 0.1 (2) |
C6—N4—C5—C9 | −0.3 (2) | C10—C8—C9—C5 | −179.24 (13) |
C6—N4—C5—C3 | 179.94 (13) | N4—C5—C9—C8 | 0.4 (2) |
N3—C3—C5—N4 | 173.16 (12) | C3—C5—C9—C8 | −179.77 (13) |
C4—C3—C5—N4 | −8.85 (19) |
Cg1 is the centroid of the (N4,C5–C9) ring. |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1N···N3 | 0.88 (2) | 2.23 (2) | 2.6148 (18) | 106 (1) |
N1—H1N···N4i | 0.88 (2) | 2.33 (2) | 3.0714 (18) | 142 (1) |
N2—H2N···S1ii | 0.87 (1) | 2.55 (1) | 3.3955 (13) | 168 (2) |
C10—H10A···Cg1i | 0.98 | 2.89 | 3.7546 (18) | 147 |
Symmetry codes: (i) x, −y+3/2, z−1/2; (ii) −x+2, −y+1, −z+2. |
Contact | Distance | Symmetry operation |
H2A···H4B | 2.23 | 2 - x, -1/2 + y, 3/2 - z |
H9···N4 | 2.54 | x, 3/2 - y, -1/2 + z |
H7···S1 | 2.83 | -1 + x, 3/2 - y, -1/2 + z |
C3···H4A | 2.84 | x, 3/2 - y, -1/2 + z |
C4···S1 | 3.4545 (15) | 2 - x, 1 - y, 2 - z |
H···H | 50.4 |
C···H/H···C | 16.7 |
S···H/H···S | 16.5 |
N···H/H···N | 14.9 |
C···C | 0.7 |
C···S/S···C | 0.6 |
S···N/N···S | 0.2 |
Footnotes
‡Additional correspondence author, e-mail: thahira@upm.edu.my.
Acknowledgements
We thank the Department of Chemistry (Universiti Putra Malaysia; UPM) for access to facilities.
Funding information
This research was funded by UPM and the Malaysian Government under the Malaysian Fundamental Research Grant Scheme (FRGS No. 01–01-16-1833FR). LCL thanks the Malaysian government for the award of a MyBrain scholarship.
References
Agilent (2011). CrysAlis PRO. Agilent Technologies, Yarnton, England. Google Scholar
Bermejo, E., Carballo, R., Castiñeiras, A., Domínguez, R., Liberta, A. E., Maichle-Mössmer, C., Salberg, M. M. & West, D. X. (1999). Eur. J. Inorg. Chem. pp. 965–973. CrossRef Google Scholar
Brandenburg, K. (2006). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Easmon, J., Heinisch, G., Holzer, W. & Rosenwirth, B. (1992). J. Med. Chem. 35, 3288–3296. CrossRef PubMed CAS Google Scholar
Farrugia, L. J. (2012). J. Appl. Cryst. 45, 849–854. Web of Science CrossRef CAS IUCr Journals Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CSD CrossRef IUCr Journals Google Scholar
Heiner, G. G., Fatima, N., Russell, P. K., Haase, A. T., Ahmad, N., Mohammed, N., Thomas, D. B., Mack, T. M., Khan, M. M., Knatterud, G. L., Anthony, R. L., Mccrumb, F. R. Jr (1971). Amer. J. Epidem. 94, 435–449. CrossRef CAS Google Scholar
Hussein, M. A., Guan, T. S., Haque, R. A., Ahamed, M. B. K. & Majid, A. M. S. A. (2015). Spectrochim. Acta A Mol. Biomol. Spectrosc. 136, 1335–1348. CrossRef CAS PubMed Google Scholar
Khan, S. A. & Asiri, A. M. (2018). Int. J. Biol. Macromol. 107, 105–111. CrossRef CAS PubMed Google Scholar
Kumar, K., Schniper, S., González-Sarrías, A., Holder, A. A., Sanders, N., Sullivan, D., Jarrett, W. L., Davis, K., Bai, F., Seeram, N. P. & Kumar, V. (2014). Eur. J. Med. Chem. 86, 81–86. CrossRef CAS PubMed Google Scholar
Kunos, C. A. & Sherertz, T. M. (2014). Front. Oncol. https://www.frontiersin.org/article/10.3389/fonc.2014.00184. doi: 10.3389/fonc.2014.00184. Google Scholar
Liberta, A. E. & West, D. X. (1992). Biometals, 5, 121–126. CrossRef PubMed CAS Web of Science Google Scholar
Lobana, T. S., Indoria, S., Sood, H., Arora, D. S., Randhawa, B. S., Garcia-Santos, I., Smolinski, V. A. & Jasinski, J. P. (2017). Inorg. Chim. Acta, 461, 248–260. CrossRef CAS Google Scholar
McKinnon, J. J., Jayatilaka, D. & Spackman, M. A. (2007). Chem. Commun. pp. 3814–3816. Web of Science CrossRef Google Scholar
McNeill, T. A. (1972). Antimicrob. Agents Chemother. 1, 6–11. CrossRef CAS PubMed Google Scholar
Njogu, P. M. & Chibale, K. (2013). Curr. Med. Chem. 20, 1715–1742. CAS PubMed Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Tan, M. Y., Crouse, K. A., Ravoof, T. B. S. A., Jotani, M. M. & Tiekink, E. R. T. (2017). Acta Cryst. E73, o1001–o1008. Web of Science CSD CrossRef IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals Google Scholar
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