tris-(Benzimidazol-2-yl-methyl)-amine as a versatile building block in Ru(II) polypyridyl chemistry

Article abstract of DOI:10.1007/s007060270006

tris-(Benzimidazol-2-yl-methyl)-amine, H₃ntb, was prepared and used in the synthesis of dinuclear Ru(II) polypyridyl and polynuclear Ru(II)-Co(III) complexes of the type [Ru₂(H₂ntb) (bpy)₄]³⁺, [Ru

Full text of DOI:10.1007/s007060270006

Monatshefte fuÈr Chemie 133, 59±69 (2002)
tris-(Benzimidazol-2-yl-methyl)-amine
as a Versatile Building Block in Ru(II)
Polypyridyl Chemistry
Ã
Lallan Mishra and Ragini Sinha
Department of Chemistry, Banaras Hindu University, Varanasi-221005, India
Summary. tris-(Benzimidazol-2-yl-methyl)-amine, H3ntb, was prepared and used in the synthesis of
dinuclear Ru(II) polypyridyl and polynuclear Ru(II)±Co(III) complexes of the type [Ru2(H2ntb)
(bpy)4]3 ‡ , [Ru2(Hntb)(phen)4]2 ‡ , [(Ru2(H2ntb)(bpy)4)2Co(en)2]9 ‡, and [(Ru2(Hntb)(phen)4)2
Co(en)2]7 ‡ (bpy ˆ 2,20-bipyridine, phen ˆ 1,10-phenanthroline, en ˆ 1,2-diaminoethane). The com-
plexes were characterized by elemental analysis as well as spectroscopic and redox data. The lumines-
cent properties of the complexes were also studied. The complexes showed signi®cant antitumour
and anti-HIV activities.
Keywords. tris-(Benzimidazol-2-yl-methyl)-amine; Polynuclear Ru(II) complexes; Polypyridyl
chemistry; Redox properties; Luminescence; Antitumour and anti-HIV activities.
Introduction
Multimetallic assemblies of de®ned architecture have been the object of studies
of energy and electron transfer processes [1], the construction of nanostructured
materials [2], and the fabrication of molecular devices [3]. In these context,
ruthenium polypyridyl complexes have played a key role due to their chemical
stability, redox properties, and favourable photophysical characteristics [4].
Among the various synthetic strategies that have been followed for the con-
struction of polynuclear complexes [5±10], the stepwise method based on the
preparation of mononuclear building units which are then linked in a desired
fashion has shown signi®cant advantages, especially with respect to polydentate
ligands.
Two types of bridging ligands (electron-poor and electron-rich) are generally
used to assemble polypyridyl ruthenium building blocks [11]. There are some
ligands which are electrically neutral but can be charged upon complex formation
by deprotonating one or more dissociable NHprotons present in the system.
Complexes derived from ligands of this type, e.g. 2,20-bis-(2-pyridyl)-benzimida-
zole [12], 2,6-bis-(20-pyridyl) benzimidazole [13], 2,20-bis-(2-benzimidazolyl)-
4,40-bipyridine [14], or 2,6-bis-(benzimidazolyl)-pyridine [15, 16], have received
Ã
Corresponding author. E-mail: lmishra@banaras.ernet.in

60
L. Mishra and R. Sinha
considerable attention. Additionally, compounds with head-to-tail bis-benzimida-
zole units (Hoechst 33258) and several of its derivatives have already been reported
[17] as anticancer drugs including tris-benzimidazole with an extended recognition
site [18].
Thus, in view of above reports, it was found worthwhile to synthesize tris-
(benzimidazol-2-yl-methyl)-amine (H3ntb) and to complex it with Ru(II) poly-
pyridyls giving dinuclear complexes which could then be linked with a bridging
unit (trans-[Co(en)2Cl2]Cl) to form oligometallic complexes.
Results and Discussion
The complexes were found to be thermally stable and soluble in acetone, ethanol,
methanol, acetonitrile, DMF, and DMSO. The molecular composition of the
complexes was determined on the basis of elemental analysis and MS data. Their
molar conductivities in solution (10À3 M CH3CN; Table 1) agreed well with the
charges present.
‡
The complexes (1) and (2) gave molecular ion peaks as [M±PFÀ6 ] and [M±
2PFÀ6 ]2 ‡ respectively whereas ES mass data for the complexes (3) and (4) gave
molecular ion peak at m/z 3950 and m/z 3848 respectively (Table 1).
During the condensation of H3ntb with Ru(II) polypyridyls, initially a
mononuclear complex leaving at least one benzimidazole unit uncoordinated
was expected. However, complexes of composition [Ru2(bpy)4(H2ntb)]3 ‡ and
[Ru2(phen)4(Hntb)]2 ‡ were obtained, excluding the probability of any benzimi-
dazole nitrogen to remain uncoordinated. Therefore, further coordination via the
amino nitrogen had to be expected.
Infrared spectra
IR peaks due to ꢀ(CˆN) observed at 1470 cmÀ1 in the spectrum of free ligand
shifted to 1460, 1440, 1460, and 1440 cmÀ1 in the spectra of the complexes 1±4,
respectively, indicating coordination with the metal ion. The sharp peak observed
at 839±842 cmÀ1 in the spectra of the complexes was assigned to ꢀ(PFÀ6 ).
To gain further information on the bonding mode, the1HNMR spectra of the
complexes were compared with those of the free ligand.

62
L. Mishra and R. Sinha
1H NMR spectra
1HNMR spectra of the ligand and complexes were recorded in DMSO±d6. Peak
1
1
assignments (see Experimental) were supported from H, HCOSY spectra for 1
and 3; the spectra of 3 and 4 turned out to be less complex than those of 1 and 2
which is consistent with earlier reports on Ru±Co complexes [19, 20]. The up®eld
shift of the NHprotons from 12.8 ppm in the free ligand to 3.41 and 5.53 ppm in
the spectra of 1 and 2 was also found to agree with the [21] literature.
The CH2 protons of the ligand, observed at ꢁ ˆ 4.6 ppm, did not shift signi-
®cantly in the spectra of 1 and 2; in the spectra of 3 and 4, however, the corre-
sponding ꢁ values are 2.55À2.48 and 3.53 ppm, respectively. The peaks at 2.18 and
at 2.10À2.03 ppm in the spectra of 3 and 4 were assigned to the CH2 and NH2
protons of ethylenediamine.
UV/Vis spectra
UV/Vis spectra of 10À5 M solutions of 1±4 in DMSO were recorded in the
region of 200±800 nm. Two intense peaks 284 and 277 nm in the spectrum of
the ligand were assigned to intraligand [19] charge-transfer transition; the
corresponding transitions in the spectra of 1±4 occurred at 285, 286, 291, and
276 nm, respectively. An additional shoulder at 376 nm in 3 and 360 nm in 4
Ã
was assigned to MLCT transitions from cobalt to ethylene diamine (ꢂ ). Other
peaks arising from trans-[Co(en)2Cl2]Cl (10À3M) were observed at 643 and
647 nm in the spectra of 3 and 4 and are consistent with transitions of cobalt as
reported in the literature [22]. An additional intense peak at 448±450 nm in the
Ã
UV/Vis spectra of 1±4 is supposed to arise from dꢂ(Ru)!ꢂ (ligand) transitions
[23].
1
The structures of the complexes as deduced from IR, HNMR, UV/Vis, FAB
MS, and elemental analyses are shown in Fig. 1.
Luminescence
Solutions of the complexes in CH3CN (10À6 M) excited at MLCT wave-
Ã
lengths (dꢂ(Ru) ! ꢂ (ligand) ca. 450 nm) emitted in the wavelength region of
533±612 nm. From 1 and 2 only one emission peak (612 and 597 nm, respectively)
was observed, whereas 3 and 4 showed two emissions each (535, 533 nm; 609,
584 nm). Representative luminescence spectra of the complexes 2 and 4 are given
in Figs. 2 and 3. The appearance of an extra emission in 3 and 4 originates either (i)
from the presence of an impurity or (ii) from an excitation level different from
those of Ru(II). The ®rst possibility was ruled out on the basis of the spectroscopic
data of the complexes. Furthermore, in view of an earlier report [24] it might be
considered that the second emission arises from another ruthenium centre.
However, these emissions have been reported to occur at longer wavelengths
(ca. 700 nm), whereas in the present case the emissions were observed at ca.
533±535 nm. Therefore, it might be inferred that the additional emissions arise
from the Co(en)2 unit.

Ru(II) Polypyridyl Chemistry
63
Fig. 1. Proposed structures of the complexes
Electrochemistry
The redox behaviour of the complexes has been studied in acetonitrile solution
using cyclic and differential pulse voltammetric techniques with tetrabutyl

64
L. Mishra and R. Sinha
Fig. 2. Room temperature luminescence spectra of [Ru2Hntb(phen)4]2 ‡ (2, ÐÐ) and MeCN (-- - -)
Fig. 3. Room temperature luminescence spectra of [(Ru2Hntb(phen)4)2Co(en)2]7 ‡ (4, ÐÐ) and
MeCN (- - - -)
ammonium perchlorate (TBAP) as supporting electrolyte; a representative volt-
ammogram is shown in Fig. 4.
Ligand oxidation was not observed in the region from 0 to ‡ 2 V. However,
complexes 1 and 2 showed a single reversible two-electron oxidation (RuIII/RuII)
at 1.34 and 1.35 V, whereas 3 and 4 exhibited a single reversible four-electron
oxidation at 1.37 and 1.36 V, indicating that in the former case two ruthenium
centres are oxidized (each by one electron), whereas in the latter reaction the four
ruthenium centres are oxidized by one electron each.
Additionally, H3ntb and its complexes exhibited three reversible reductive
processes. As reported earlier [20], a Co(III)±Co(II) reduction process could not be

Ru(II) Polypyridyl Chemistry
65
Fig. 4. Cyclic and differential pulse voltammograms of (a) [Ru2Hntb(phen)4]2 ‡ (2) and (b)
[(Ru2Hntb(phen)4)2Co(en)2]7 ‡ (4) in MeCN (10À3 M) using TBAP (10À1 M) as supporting electro-
‡
lyte and Fc/Fc as internal reference at a scan rate of 100 mV Á sÀ1
observed in present system, but Co(II)±Co(I) reduction was observed at a potential
overlapping with the reduction potential [25, 26] for bpy and phen ligands.
Therefore, these different redox processes could not be distinguished.
Cytotoxic and anti-HIVactivities
The cytotoxicity (IC50) of the free ligand and its Ru(II) polypyridyl complexes
tested on different tumour cells are shown in Table 2. Signi®cant activity of the free
ligand against HCT-8, IA9, HOS, KB, KB-VIN, MCF-7, A549, and U87-MG has

66
L. Mishra and R. Sinha
Table 2. Cytotoxicitya and anti-HIVb (IC50, mg/cm3) activity against different tumour cells and HIV
Compound Tumour cells
HCT-8c
HIV
IC50 (mg/cm3)
IA9
HOS
KB
KB-VIN MCF-7
A549
U87-MG
H3ntb
< 5.2 (52) 1.5
5
4.9
10
NA
NA
±
4.3
4.9
9.2
NA
NA
±
6.2
6.5
3.5
< 2.5 (57) > 20 (34)
1.99
2.16
2.33
21.0
1.95
500
1
4.2
7.5
2.3
3.8
NA
NA
±
4.8
3.9
5
> 20 (48)
> 20 (48)
17
2
19
> 20 (38)
3
NA
NA
NA
±
NA
NA
±
15.5
4
AZT
NA
±
> 20 (5.9) NA
±
±
a
NA ˆ not active (inhibition ꢁ 5% at 20 mg per cm3) during prescreen test, value in parentheses is the percent inhibition;
tumour/tissue type: HCT-8: ileoceca, IA9: ovarian, HOS: bone, KB: nasopharynx, MCF-7: breast, A549: lung, U87-MG:
b
glioblastoma; AZT ˆ azidothymidine; IC50 ˆ inhibitory concentration for 50% growth inhibition; EC50 ˆ effective
concentration for 50% growth inhibition; ED50(mg per cm3) is less than 50% at highest test concentration
c
been observed with ligands bearing similar functional groups [17]. As reported
[17], the inner ring nitrogen atom of a benzimidazole ring might coordinate with
DNA; therefore, the ligand containing three NHgroups was found to be very active.
Among the complexes 1 and 2, 1 was found to be more active since it contains two
NHgroups in addition to the ¯exibility associated with bipyridine as compared to
the rigid nature of 1,10-phenanthroline. Complexes 3 and 4 were inactive against
HCT-8, 1A9, HOS, KB, KB-VIN, and MCF-7, but showed some activity against
A-549 and U87-MG. The decrease in the activity of the Ru±Co complexes may be
due to their bulkier nature.
The anti-HIV activity of the complexes is also given in Table 2. Although the
free ligand showed slightly higher anti-HIVactivity as compared to its complexes 1
and 2, all of them were found to be very signi®cant anti-HIV active as compared to
standard AZT.
The steric effects of terminal ligands (polypyridyl) on the activity of the
complexes is quite apparent. Ru(II) complexes containing a 1,10-phenanthroline
ring as the terminal ligand were found to be less active compared to the complexes
bearing a 2,20-bipyridine ring [27, 28]. However, this surmise calls for deeper
investigation.
Experimental
Solvents and starting material were purchased from Sigma-Aldrich and used without further
puri®cation except for ethanol which was used after distillation. All reactions were carried out under a
nitrogen atmosphere. Aluminum oxide (Merck) was used for column chromatography. Commercial
tetrabutylammonium bromide (Fluka) was converted to pure tetrabutylammonium perchlorate (TBAP)
according to Ref. [29].
Microanalytical, FAB mass, and ES mass data were acquired with a Carlo Erba Elemental
Analyzer 1108, a JEOL SX-102/DA-6000 mass spectrometer, and a JEOL D-300(El/Cl) mass spec-
trometer at C.D.R.I., Lucknow, India. The results of elemental analyses agreed with the calculated
values within experimental error. IR (KBr pellets) and UV/Vis spectra (CH3CN) were recorded on
Jasco FTIR 5300 and Shimadzu UV-1601 spectrophotometers at the Department of Chemistry,
B.H.U., Varanasi, India. Luminescence spectra were measured in MeCN using a Perkin-Elmer 50B

Ru(II) Polypyridyl Chemistry
67
luminescence spectrometer at the University of Bristol, UK, whereas electrochemical measurements
were performed at the University of Hyderabad, India (working and auxiliary electrodes: platinum,
reference electrode: SCE, reference: ferrocene.
Synthesis of the ligand
tris-(Benzimidazol-2-yl-methyl)-amine (H3ntb) was prepared by a reported procedure [32].
o-Phenylenediamine (3.24 g, 0.03 mol) and tricarboxyamine (1.92 g, 0.01 mol) were dissolved in 4
N HCI and re¯uxed for 8 h. After cooling to room temperature, the content was diluted with water
(10 cm3) and neutralized with 10% aqueous Na2CO3 solution. The violet coloured solid thus
obtained was ®ltered and recrystallized from EtOHfollowed by puri®cation by column chromato-
graphy using alumina as adsorbent and CH3CN as eluent. Evaporation of the solvent in vacuo
yielded a solid which was recrystallized from CH3CN and Et2O to afford colourless shiny crystals.
Yield: 80% m.p.: 180 Æ 1ꢂC; MS: m/z ˆ 407; IR (KBr): ꢀ ˆ 3450 (ꢀNH), 1570 (ꢀar), 1470 (ꢀCˆN
)
0
0
cmÀ1; 1HNMR ( DMSO-d6, 90 MHz): ꢁ ˆ 12.8 (3H, s, NH), 7.8 (6H, d, PhH2,2 ), 7.5 (6H, t, PhH3,3 ),
4.6 (6H, s, CH2) ppm.
Synthesis of metal complexes
[Ru(bpy)2Cl2], [Ru(phen)2Cl2], and trans-[Co(en)2Cl2]Cl were prepared according to the literature
[22, 33].
[Ru2(H2ntb)(bpy)4][(PF6)3] (1; C69H62F18 N15P3Ru2) and [Ru2(Hntb)(phen)4]
[(PF6)2] (2; C72H51F12N15P2Ru2)
To an ethanolic solution (10 cm3) of Ru(bpy/phen)2Cl2 (1.04/1.06 g, 0.002 mol), H3ntb (0.407 g,
3
0.001 mol) in EtOH(5 cm ) was added under stirring. Stirring was continued for 2 h, followed by
heating at re¯ux for 24 h. Subsequently, precipitation was effected by addition of an excess of a
methanolic solution of NH4PF6. Light orange (1) or light brown (2) solids were obtained which were
puri®ed by column chromatography using alumina as adsorbent and MeCN:saturated aqueous
KNO3:water ˆ 7:1:0.5 as eluent [34]. The eluates were evaporated to dryness, and the residues
obtained were dissolved in a minimum volume of acetone followed by the addition of an excess of a
methanolic solution of NH4PF6. The solids were recrystallized from ethanol.
1: Yield: 60%; m.p.: >250ꢂC; MS: m/z ˆ 1378; IR (KBr): ꢀ ˆ 3428 (ꢀNH), 1640 (ꢀpy), 1541 (ꢀar),
0
1
1460 (ꢀCˆN); 860 ꢀꢀPF † cmÀ1; HNMR ( DMSO-d6, 90 MHz): ꢁ ˆ 8.84 (8H, d, H3,3 ), 8.17 (8H, t,
À
6
0
0
00
0
H
4,40), 7.74 (6H, d, PhH2,2 ), 7.57 (6H, t, H5,5 ‡ 2H, d, H66 ), 7.54 (6H, t, PhH3,3 ), 7.19 (2H, m,
0
H5,5 ), 7.17 (6H, d, H6,6 ), 4.20 (6H, s, CH2), 3.41 (2H, s, NH) ppm.
2: Yield: 55%; MS: m/z ˆ 1474; IR (KBr): ꢀ ˆ 2975 (ꢀNH), 1640 (ꢀpy), 1485 (ꢀar), 1440 (ꢀCˆN),
À1
1HNMR ( DMSO-d6, 90 MHz): ꢁ ˆ 9.40 (2H, d, H2,9), 9.06 (6H, d,0 H2,9),
À
860 ꢀꢀPF † cm
;
6
0
8.66 (8H, s, H5,6), 8.06 (8H, d, H4,7), 8.33 (8H, t, H3,8), 7.80 (6H, d, PhH2,2 ), 7.5 (6H, t PhH3,3 ), 5.53
(IH, s, NH), 4.30 (6H, s, CH2) ppm.
[(Ru2(H2ntb)(bpy)4)2Co(en)2][(PF6)9] (3; C132CoH120F54N34P9Ru4)
To a solution of trans-[Co(en)2Cl2]Cl (0.032 g, 0.125 mmol) in 10 cm3 DMSO, a solution of 1
(0.438 g, 0.25 mmol) in 5 cm3 DMSO was added under stirring; then, the mixture was re¯uxed for
24 h. After ®ltration, an excess of a methanolic solution of NH4PF6 was added. The crude greenish-
brown solid obtained was ®ltered and puri®ed as described for 1 and ®nally crystallized from EtOH.
Yield: 50%; m.p.: >250ꢂC; MS: m/z ˆ 3950; IR (KBr): ꢀ ˆ 3422 (ꢀNH), 1620 (ꢀpy), 1540 (ꢀar),
0
1460 (ꢀCˆN), 860 ꢀꢀPF † cmÀ1; 1HNMR ( DMSO-d6, 90 MHz): ꢁ ˆ 8.85 (16H, d, H3,3 )0, 8.83 (4H, d,
À
6
0
0
0
0
H
6,6 ), 8.17 (16H, t, H4,4 ), 8.16 (4H, t, H5,5 ), 7.74 (12H, d, PhH2,2 ), 7.73 (12H, d, H6,6 ), 7.55 (12H,

68
L. Mishra and R. Sinha
0
0
t, PhH3,3 ), 7.53 (12H, t, H5,5 ), 3.36 (4H, s, NH), 2.25 (12H, 5, CH2), 2.48 (8H, s, CHe2n), 2.18 (8H, s,
NHe2n) ppm.
[(Ru2(Hntb)(phen)4)2Co(en)2][(PF6)7] (4; C144CoH118F42N34P7 Ru4)
4 was prepared and puri®ed similar to the procedure employed for 3 except that 2 (0.267 g,
0.25 mmol) was used as an educt instead of 1.
Yield: 40%; m.p.: >250ꢂC; MS: m/z ˆ 3848; IR (KBr): ꢀ ˆ 3360 (ꢀNH) 1660 (ꢀpy), 1500 (ꢀar),
1440 (ꢀCˆN), 860 ꢀꢀPF † cmÀ1; 1HNMR ( DMSO-d6, 90 MHz): ꢁ ˆ 8.73 (16H, d, H2,9), 8.30 (16H, 8,
À
6
0
0
H
5,6), 8.00 (12H, m, H4,7), 7.73 (16H, m, PhH2,2 and H4,7), 7.50 (12H, m, PhH3,3 ), 7.33 (16H, m,
H3,8), 5.53 (2H, s, NH), 3.53 (20H, s, CH2 and CH2en), 2.18 (8H, s, NHe2n) ppm.
Assay of cytotoxic and anti-HIVactivities
The free ligand and its complexes were assayed for their cytotoxicity and anti-HIV activities at the
School of Pharmacy, University of North Carolina, USA adopting reported procedures [30]. Stock
cultures were grown in T-75 ¯asks containing 50 cm3 of RPMI-1640 medium with glutamine,
bicarbonate, and 5% fetal calf serum. Cells were dissociated with 0.25% trypsin and 3 mM 1,2-
cyclohexanediamine tetraacetic acid in NKT buffer (137 mM, NaCl, 5.4 mM KCl and 10 mM tris, pH
7.4). Experimental cultures were plotted in microtiter plates containing 0.2 cm3 of growth medium
per well at densities of 1000±200000 cells per well.
The assay of cytotoxicity of the free ligand and its complexes against different tumour cells was
performed in 96-well microtiter plates based on metabolic reduction of 3-(4,5-dimethyl-thiazol-2-
yl)-2,5-diphenyltetrazolium bromide (MTT ) by the mitochondrial dehydrogenase of viable cells to a
blue formazon product which was estimated spectrophotometrically.
The procedure used in the National Institute's test [31] was adopted to evaluate the activity
of free ligand and its complexes against HIV to detect agents acting at any stage of the virus
reproductive cycle. Compounds that degenerate or are rapidly metabolized in the culture conditions
may not show activity in the screen. All tests were compared with at least one positive (e.g. AZT
treated) control performed at the same time under identical conditions. In this procedure, the com-
pounds were initially dissolved in DMSO and then diluted (1:100) in a cell culture medium be-
fore preparing serial half-logarithmic dilutions. T4 lymphocytes (CEM cell line) were added, and
after a brief interval HIV-1 was added resulting into 1:200 ®nal dilution of the compound. Uninfected
cells served as basic controls. MTT was added to all wells, and cultures were incubated at 37ꢂC in
5% CO2 atmosphere for 6 days to allow the development of formazon colour by viable cells which
were then analyzed spectrophotometrically for quantitative formazon production.
Acknowledgements
Lallan Mishra gratefully acknowledges ®nancial assistance from U.G.C., New Delhi, India. The
authors are also grateful to Prof. H. Itokawa, University of North Carolina, USA for evaluating the
cytotoxicity and anti-HIV activities of the compounds, to Dr. M. D. Ward, University of Bristol, UK
for the luminescence measurement, and to Dr. B. G. Maiya, University of Hyderabad, India for
providing cyclic and differential pulse voltammetric data.
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Ru(II) Polypyridyl Chemistry
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Received May 9, 2001. Accepted (revised) June 7, 2001