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1.  Crystal Structure Elucidation and Anticancer Studies of (-)-Pseudosemiglabrin: A Flavanone Isolated from the Aerial Parts of Tephrosia apollinea 
PLoS ONE  2014;9(3):e90806.
Tephrosia apollinea is a perennial shrublet widely distributed in Africa and is known to have medicinal properties. The current study describes the bio-assay (cytotoxicity) guided isolation of (-)-pseudosemiglabrin from the aerial parts of T. apollinea. The structural and stereochemical features have been described using spectral and x-ray crystallographic techniques. The cytotoxicity of isolated compound was evaluated against nine cancer cell lines. In addition, human fibroblast was used as a model cell line for normal cells. The results showed that (-)-pseudosemiglabrin exhibited dose-dependent antiproliferative effect on most of the tested cancer cell lines. Selectively, the compound showed significant inhibitory effect on the proliferation of leukemia, prostate and breast cancer cell lines. Further studies revealed that, the compound exhibited proapoptotic phenomenon of cytotoxicity. Interestingly, the compound did not display toxicity against the normal human fibroblast. It can be concluded that (-)-pseudosemiglabrin is worthy for further investigation as a potential chemotherapeutic agent.
doi:10.1371/journal.pone.0090806
PMCID: PMC3946547  PMID: 24608571
2.  Potential of silver against human colon cancer: (synthesis, characterization and crystal structures of xylyl (Ortho, meta, &Para) linked bis-benzimidazolium salts and Ag(I)-NHC complexes: In vitro anticancer studies) 
Background
Since the first successful synthesis of Ag(I)-N-heterocyclic carbene complex in 1993, this class of compounds has been extensively used for transmetallation reactions where the direct synthesis using other metal ions was either difficult or impossible. Initially, silver(I)-NHC complexes were tested for their catalytic potential but could not get fame because of lower potential compare to other competent compounds in this field; however, these compounds proved to have vital antimicrobial activities. These encouraging biomedical applications further convinced researchers to test these compounds against cancer. The current work has been carried out with this aim.
Results
N-ipropylbenzimidazole was synthesized by reaction of benzimidazole with ipropyl bromide. The subsequent treatment of the resulting N-alkylbenzimidazole with ortho/meta/para-(bromomethylene) benzene afforded corresponding bis-benzimidazolium bromides (5-7). The counter anion (Br-) of each salt was replaced by hexaflourophosphate (PF6-) for the ease of handling and further purification (8-10). Each salt (Ligand), in halide form, was further allowed to react with Ag2O with stirring at room temperature for a period of two days to synthesize dinuclear Ag(I)-NHC complexes (11-13). All synthesized compounds were characterized by spectroscopic techniques and microanalysis. Molecular structures of compounds 5, 9 &10 were established through single crystal x-ray diffraction technique. All the compounds were assessed for their anti-proliferation test on human colorectal cancer cell line (HCT 116). Results showed that the ligands (5-10) showed mild to negligible cytotoxicity on HCT 116 cells whereas respective silver complexes (11-13) exhibited dose dependent cytotoxicity towards the colon cancer cells with IC50 ranges between 9.7 to 44.5 μM. Interestingly, the complex 13 having para-xylyl spacer was found the most active (IC50 9.7 μM) that verifies our previously reported results.
Conclusions
All the bis-benzimidazolium salts (8-10) were found inactive whereas after bonding with silver cations, the Ag(I)-NHC complexes (11-13) showed a dose dependent cytotoxic activity. This proved that silver practice an important role in death of cancer cells. Also, the N-alkyl/aryl substitutions and ortho/metal/para xylyl units regulate the cytotoxicity.
doi:10.1186/1752-153X-7-27
PMCID: PMC3637141  PMID: 23391345
3.  Design, synthesis and structural studies of meta-xylyl linked bis-benzimidazolium salts: potential anticancer agents against ‘human colon cancer’ 
Background
Benzimidazole derivatives are structurally bioisosteres of naturally occurring nucleotides, which makes them compatible with biopolymers of living systems. This property gives benzimidazole a biological and clinical importance. In the last decade, this class of compounds has been reported to possess anti-allergic, anti-diabatic, anti-HIV, anti-hypertensive, anti-inflammatory, anti-mycobacterial, anti-oxidant, anti-protozoal, and anti-viral properties. The researchers are now interested to explore their potential as anti-cancer agents. In the present study, an effort was made to further explore this area of research. Furthermore, in order to increase the solubility and efficacy of these heterocycles, the interest is now shifted to the salts of these compounds. With this background, we planned to synthesize a series of meta-xylyl linked bis-benzimidazolium salts to assess their anti-proliferation efficacy on human colon cancer cell line (HCT 116).
Results
A number of N-alkylbenzimidazoles were synthesized by reactions of benzimidazole with alkyl halides (i-PrBr, PrBr, EthBr, Pent-2-ylBr, BuBr, BenzCl, HeptBr). The subsequent treatment of the resulting N-alkylbenzimidazoles with 1,3-(bromomethylene)benzene afforded corresponding bis-benzimidazolium salts. All synthesized compounds were characterized by spectroscopic techniques (Additional file 1: NMR & FT-IR) and microanalysis. Molecular structures of selected compounds were established through single crystal x-ray diffraction studies. All the compounds were assessed for their anti-proliferation test on human colorectal cancer cell line (HCT 116). Results showed that the compounds exhibited dose dependent cytotoxicity towards the colon cancer cells with IC50 ranges between 0.1 to 17.6 μM. The anti-proliferation activity of all compounds was more pronounced than that of standard reference drug 5-flourouracil (IC50 =19.2 μM).
Conclusions
All the synthesized bis-benzimidazolium salts showed potential anticancer activity. Out of them, some of these salts showed IC50 value as low as 0.1–0.2 μM. Based on the results it can be concluded that, the bis-benzimidazolium salts could probably be the potential source of chemotherapeutic drugs.
doi:10.1186/1752-153X-6-68
PMCID: PMC3542276  PMID: 22809051
4.  3,3′-Diethyl-1,1′-(1,4-phenyl­ene­dimethyl­ene)diimidazol-3-ium bis­(hexa­fluoro­phosphate) 
In the title mol­ecular salt, C18H24N4 2+·2PF6 −, the complete dication is generated by a crystallographic inversion centre. The central benzene ring makes a dihedral angle of 77.19 (9)° with each of the imidazole rings. In the crystal, C—H⋯F inter­actions link the cations and anions into layers lying parallel to the bc plane. The hexa­fluoro­phosphate anion is disordered over two sets of sites in a 0.520 (11):0.480 (11) ratio.
doi:10.1107/S1600536812028966
PMCID: PMC3394059  PMID: 22798924
5.  3,3′-Di-n-propyl-1,1′-(1,3-phenyl­ene­dimethyl­ene)di(1H-imidazol-3-ium) bis­(hexa­fluorophosphate) 
In the title compound, C20H28N4 2+·2PF6 −, the dihedral angles between the benzene ring and the imidazole rings are 70.18 (11) and 69.83 (11)°, while the imidazole rings form a dihedral angle of 40.52 (12)°. In the crystal, weak C—H⋯F inter­actions link the mol­ecules into a two-dimensional network parallel to (001). A π–π inter­action with a centroid–centroid distance of 3.601 (1) Å is also observed in the crystal structure.
doi:10.1107/S1600536812026955
PMCID: PMC3393961  PMID: 22798826
6.  3,3′-Dicyclo­pentyl-1,1′-(1,3-phenyl­enedimethyl­ene)dibenzimidazol-1-ium bis­(hexa­fluoro­phosphate) 
In the title compound, C32H36N4 2+·2PF6 −, the cation and the anions each have crystallographic twofold rotation symmetry. The benzimidazole ring is almost planar [r.m.s. deviation = 0.0161 (1) Å] and makes a dihedral angle of 5.77 (4)° with its symmetry-related component and a dihedral angle of 80.96 (5)° with the central benzene ring. The cyclo­pentyl ring adopts a half-chair conformation. In the crystal, mol­ecules are linked into a three-dimensional network through C—H⋯F hydrogen bonds. A C—H⋯π inter­action is also observed.
doi:10.1107/S160053681202274X
PMCID: PMC3379432  PMID: 22719630
7.  3,3′-[1,2-Phenyl­enebis(methyl­ene)]bis­(1-ethyl-1H-benzimidazol-1-ium) bis­(hexa­flourophosphate) 
In the title compound, C26H28N4 2+·2PF6 −, the complete cation is generated by a crystallographic twofold axis. The benz­imidazole ring is almost planar (r.m.s. deviation = 0.0207 Å) and makes dihedral angles of 50.12 (2)° with its symmetry-related component and 65.81 (2)° with the central benzene ring. In the crystal, mol­ecules are linked into a three-dimensional network by C—H⋯F inter­actions. A π–π inter­action with a centroid–centroid distance of 3.530 (1) Å is observed. Four F atoms of the hexa­fluoro­phosphate anion are disordered over two sets of sites in a 0.889 (6):0.111 (6) ratio.
doi:10.1107/S1600536812019344
PMCID: PMC3379238  PMID: 22719436
8.  3,3′-[1,2-Phenyl­enebis(methyl­ene)]bis­(1-octylbenzimidazolium) dibromide monohydrate 
In the title hydrated mol­ecular salt, C38H52N4 2+·2Br−·H2O, the central benzene ring of the dication makes dihedral angles of 89.47 (13) and 72.69 (12)° with the pendant benzimidazol-3-ium rings. The conformations of the octyl side chains are completely different. In the crystal, the components are linked by O—H⋯Br, C—H⋯Br and C—H⋯O hydrogen bonds into a two-dimensional network lying parallel to the ac plane. Aromatic π–π stacking inter­actions are also observed [shortest centroid-to-centroid separation = 3.5047 (16) Å].
doi:10.1107/S1600536812008331
PMCID: PMC3343909  PMID: 22589990
9.  3,3′-[1,4-Phenyl­enebis(methyl­ene)]­bis­(1-propyl­benzimidazolium) dichloride dihydrate 
The asymmetric unit of the title compound, C28H32N4 2+·2Cl−·2H2O, contains half of a 3,3′-[1,4-phenyl­enebis(methyl­ene)]bis­(1-propyl­benzimidazolium) cation, one chloride anion and one water mol­ecule. The complete cation is generated by a crystallographic inversion center. The central benzene ring forms a dihedral angle of 66.06 (11)° with its adjacent benzimidazolium ring system. In the crystal, the cations, anions and water mol­ecules are linked by O—H⋯Cl, C—H⋯O and C—H⋯Cl hydrogen bonds into a three-dimensional network. The crystal packing is further stabilized by π–π inter­actions, with centroid–centroid distances of 3.5561 (15) and 3.6708 (15) Å.
doi:10.1107/S1600536812007738
PMCID: PMC3297903  PMID: 22412706
10.  3,3′-[1,2-Phenyl­enebis(methyl­ene)]bis­(1-ethyl­benzimidazolium) dibromide 
In the title mol­ecular salt, C26H28N4 2+·2Br−, the central benzene ring makes dihedral angles of 76.75 (11) and 82.40 (10)° with the pendant benzimidazole rings. The corresponding angle between the benzimidazole rings is 57.03 (9)°. In the crystal, the cations and anions are linked via C—H⋯Br hydrogen bonds, forming sheets lying parallel to the bc plane. The crystal structure also features weak C—H⋯π inter­actions.
doi:10.1107/S1600536812002802
PMCID: PMC3297300  PMID: 22412490
11.  3,3′-[1,2-Phenyl­enebis(methyl­ene)]bis­(1-propyl­benzimidazolium) dibromide hemihydrate 
The asymmetric unit of the title compound, C28H32N4 2+·2Br−·0.5H2O, contains one 3,3′-[1,2-phenyl­enebis(methyl­ene)]bis­(1-propyl­benzimidazolium) cation, two bromide anions and one half-mol­ecule of water. In the crystal, the whole 3,3′-[1,2-phen­yl­enebis(methyl­ene)]bis­(1-propyl­benzimidazolium) cation and one of the bromide anions are each disordered over two positions with site-occupancy ratios of 0.751 (6):0.249 (6) and 0.680 (8):0.320 (8). For the major component of the disordered cation, the central benzene ring forms dihedral angles of 83.6 (5) and 88.2 (4)° with its adjacent imidazole rings, while these angles for the minor component are 89.2 (15) and 84.9 (13)°. In the crystal, the cations and anions are linked by C—H⋯Br hydrogen bonds into dimers and then stacked along the c axis. The crystal packing is further stabilized by C—H⋯π and π–π inter­actions [shortest centroid–centroid distance = 3.646 (4) Å].
doi:10.1107/S1600536812001596
PMCID: PMC3275219  PMID: 22347075
12.  3-(3-Cyano­benz­yl)-1-methyl-1H-imidazol-3-ium hexa­fluoro­phosphate 
In the title compound, C12H12N3 +·PF6 −, the hexa­fluoro­phosphate anion is disordered over two orientations with refined site occupancies of 0.8071 (17) and 0.1929 (17). The dihedral angle between the imidazole and benzene rings in the cation is 71.26 (7)°. In the crystal, the cations and anions are linked by C—H⋯F and C—H⋯N hydrogen bonds into a three-dimensional network.
doi:10.1107/S1600536812001882
PMCID: PMC3275236  PMID: 22347092
13.  2-[(2,4,4,6,6-Penta­chloro-1,3,5,2λ5,4λ5,6λ5-triaza­triphosphinin-2-yl)aza­nid­yl]pyridinium 
The title compound, C5H5Cl5N5P3, crystallizes as a zwitterion in which the pyridine N atom is protonated. An S(6) ring motif is formed via an intra­molecular C—H⋯N hydrogen bond. The triaza­triphosphinine ring adopts an envelope conformation, with one N atom displaced by 0.145 (1) Å from the other atoms. In the crystal, N—H⋯N and C—H⋯N hydrogen bonds link the mol­ecules into centrosymmetric dimers containing one R 2 2(7) ring motif and two R 2 2(8) ring motifs.
doi:10.1107/S160053681200013X
PMCID: PMC3274999  PMID: 22346944
14.  1,3-Bis(2-cyano­benz­yl)imidazolium bromide 
In the title salt, C19H15N4 +·Br−, the central imidazole ring makes dihedral angles of 83.1 (2) and 87.6 (2)° with the terminal benzene rings. The dihedral angle between the terminal benzene rings is 6.77 (19)°; the cyanide substituents have an anti orientation. In the crystal, the cations and anions are linked via C—H⋯N and C—H⋯Br hydrogen bonds, forming sheets lying parallel to the ac plane.
doi:10.1107/S1600536811048951
PMCID: PMC3239090  PMID: 22199938
15.  3,3′-Diallyl-1,1′-[o-phenyl­enebis(methyl­ene)]diimidazol-3-ium bis­(hexa­fluoro­phosphate) 
In the cation of the title mol­ecular salt, C20H24N4 2+·2PF6 −, the central benzene ring makes dihedral angles of 84.19 (7) and 79.10 (7)° with the pendant imidazole rings. In one of the hexa­fluoro­phosphate anions, the six F atoms are disordered over two sets of sites, with an occupancy ratio of 0.842 (3):0.158 (3). In the crystal, the cations and anions are linked by numerous C—H⋯F hydrogen bonds, thereby forming a three-dimensional network.
doi:10.1107/S160053681103474X
PMCID: PMC3200788  PMID: 22065513
16.  Bis(1-benzyl-3-methyl­imidazolium-κC 2)mercury(II) bis­(hexa­fluoridophosphate) 
The asymmetric unit of the title complex, [Hg(C11H12N2)2](PF6)2, consists of one bis­(1-benzyl-3-methyl­imidazolium)mercury(II) cation, one half of the cation and an additional HgII atom, which lies on an inversion centre, and three hexa­fluorido­phosphate anions. The HgII atoms exist in a linear coordination geometry [C—Hg—C = 178.9 (2) and 180°] formed by two carbene C atoms from the imidazole rings. In the crystal, the cations and anions are connected via C—H⋯F hydrogen bonds, forming a three-dimensional network.
doi:10.1107/S1600536811032235
PMCID: PMC3200814  PMID: 22064754
17.  2,4-Bis[(3-allyl­imidazolium-1-yl)meth­yl]mesitylene bis­(hexa­fluoridophosphate) 
In the title mol­ecular salt, C23H30N4 2+·2PF6 −, the central benzene ring of the cation makes dihedral angles of 89.80 (8) and 85.23 (7)° with the pendant imidazole rings. In the crystal, the cations and anions are linked by numerous C—H⋯F hydrogen bonds, thereby forming a three-dimensional network.
doi:10.1107/S1600536811027541
PMCID: PMC3213513  PMID: 22091092
18.  3,3′-Di-n-propyl-1,1′-[p-phenyl­enebis(methyl­ene)]diimidazolium dibromide 
The asymmetric unit of the title compound, C20H28N4 2+·2Br−, consists of half a 3,3′-di-n-propyl-1,1′-[p-phenyl­enenis(methyl­ene)]diimidazolium cation and a bromide anion. The cation is located on an inversion center and adopts an ⋯AAA⋯ trans conformation. In the crystal, the cation is linked to the anions via weak C—H⋯Br hydrogen bonds.
doi:10.1107/S1600536811026146
PMCID: PMC3212318  PMID: 22090975
19.  3,3′-[1,2-Phenyl­enebis(methyl­ene)]bis­(1-heptyl­benzimidazolium) dibromide monohydrate 
In the title salt, C36H48N4 2+·2Br−·H2O, the central benzene ring makes dihedral angles of 84.77 (9) and 69.92 (7)° with the adjacent imidazole rings. In the crystal, one of the heptyl groups is disordered over two sets of sites with an occupancy ratio of 0.474 (5):0.526 (5). In the crystal, the cations, anions and water mol­ecules are connected via inter­molecular O—H⋯Br, C—H⋯Br and C—H⋯O hydrogen bonds, forming a three-dimensional network.
doi:10.1107/S1600536811023476
PMCID: PMC3152130  PMID: 21837184
20.  3,3′-Dibenzyl-1,1′-(2,4,6-trimethyl-m-phenyl­enedimethyl­ene)diimidazol-3-ium dibromide 
In the title molecular salt, C31H34N4 2+·2Br−, the central benzene ring makes dihedral angles of 80.47 (12) and 82.78 (12)° with the adjacent imidazole rings. The dihedral angle between the two terminal phenyl rings is 79.16 (13)°. In the crystal, the cations and anions are linked via C—H⋯Br hydrogen bonds, forming supra­molecular chains along the c axis.
doi:10.1107/S1600536811005204
PMCID: PMC3051991  PMID: 21522396
21.  2,4-Bis[(3-butyl­imidazol-3-ium-1-yl)meth­yl]-1,3,5-trimethyl­benzene bis­(hexa­fluoro­phosphate) 
In the title molecular salt, C25H38N4 2+·2PF6 −, one of the butyl groups and four F atoms in the basal plane of one of the PF6 − octa­hedra are disordered over two sets of sites, with occupancy ratios of 0.704 (5):0.296 (5) and 0.71 (3):0.29 (3), respectively. The central benzene ring makes dihedral angles of 85.17 (12) and 81.97 (12)° with the terminal imidazole rings. In the crystal, cations and anions are linked together via inter­molecular C—H⋯F hydrogen bonds forming a three-dimensional network.
doi:10.1107/S1600536811003916
PMCID: PMC3052114  PMID: 21522326
22.  Potassium bis­[bis­(1-benzyl-3-methyl­imidazolium)silver(I)] tris­(hexa­fluoridophosphate) 
In the title compound, K[Ag(C11H12N2)2]2(PF6)3, the 12-coordinate potassium cation lies on a crystallographic twofold axis and one of the hexa­fluoro­phosphate anions is generated by symmetry. In the complex cation, the AgI ion is coordinated by two C atoms; the two imidazolium rings are orientated at a dihedral angle of 8.14 (14)°. In the 1-benzyl-3-methyl­imidazolium units, the dihedral angles between imidazolium and phenyl rings are 80.47 (15) and 76.53 (14)°. The F atoms of the general-position hexa­fluoro­phosphate anion are disordered over two sets of sites in a 0.767 (17):0.233 (17) ratio. In the crystal, the hexa­fluoro­phosphate anions link the cations into three-dimensional networks via inter­molecular C—H⋯F hydrogen bonds and are further consolidated by π–π stacking [centroid–centroid distances = 3.5518 (15) Å] inter­actions.
doi:10.1107/S1600536810051925
PMCID: PMC3050194  PMID: 21522610
23.  1,3-Bis[(3-allyl­imidazol-3-ium-1-yl)meth­yl]benzene bis­(hexa­fluoridophosphate) 
In the title compound, C20H24N4 2+·2PF6 −, the ethene and 3-allyl­imidazolium moieties of the cation are disordered over two positions with refined site occupancies of 0.664 (19):0.336 (19) and 0.784 (7):0.216 (7), respectively, whereas four F atoms of one hexa­fluoridophosphate anion and all atoms in the other hexa­fluoridophosphate anion are disordered over two positions with refined site occupancies of 0.764 (5):0.2365) and 0.847 (9):0.153 (9), respectively. The benzene ring is inclined at angles of 78.2 (3), 81.3 (4) and 73.9 (12)° with the 1H-imidazol-3-ium ring and the major and minor components of the disordered 1H-imidazol-3-ium ring, respectively. In the crystal, the hexa­fluoridophosphate anions link the cations into two-dimensional networks parallel to (001) via inter­molecular C—H⋯F hydrogen bonds. The crystal structure is further consolidated by π–π [centroid–centroid distance = 3.672 (3) Å] and C—H⋯π inter­actions.
doi:10.1107/S1600536810050683
PMCID: PMC3050142  PMID: 21522792
24.  3,5-Bis(3-butylimidazolium-1-ylmethyl)toluene bis(hexafluorophosphate) 
In the title compound [systematic name: 3,3′-Dibutyl-1,1′-(5-methyl-m-phenyl­enedimethyl­ene)diimidazol-1-ium bis­(hexa­fluoridophosphate)], C23H34N4 2+·2PF6 −, the imidazole rings are inclined at angles of 68.06 (7) and 75.05 (8)° with respect to the central benzene ring. In the crystal, mol­ecules are linked into one-dimensional columns along [010] via weak inter­molecular C—H⋯F hydrogen bonds. The crystal structure is further consolidated by weak C—H⋯π(arene) inter­actions. One of the n-butyl groups is disordered over two sites with refined occupancies of 0.694 (5) and 0.306 (5). In addition, four of the F atoms of one of the PF6 − cations are disordered over two sites with occupancies of 0.64 (3) and 0.36 (3).
doi:10.1107/S1600536810040055
PMCID: PMC3009333  PMID: 21588994
25.  Bis{1,4-bis­[(3-butyl­imidazolium-1-yl)meth­yl]benzene}­silver(I) bis­(hexa­fluoridophosphate) 
The asymmetric unit of the title complex, [Ag2(C22H30N4)2](PF6)2, consists of one AgI ion, one 1,4-bis­[(3-butyl­imidazolium-1-yl)meth­yl]benzene ligand and one discrete hexa­fluoridophosphate anion. The formula unit is generated by an inversion center. The unique AgI ion is coordinated by two C atoms of two heterocyclic carbene ligands in an essentially linear geometry. In the crystal structure, cations and anions are linked through weak C—H⋯F hydrogen bonds, forming a three-dimensional network.
doi:10.1107/S1600536810036524
PMCID: PMC2983204  PMID: 21587429

Results 1-25 (27)