Synthesis, characterization, interactions with DNA and bovine serum albumin (BSA), and antibacterial activity of cyclometalated iridium(III) complexes containing dithiocarbamate derivatives

Article abstract of DOI:10.1080/00958972.2014.945924

Three mononuclear cyclometalated iridium(III) complexes having dithiocarbamate ligands, [Ir^III(2-C₆H₄py)₂(L)] (where 2-C₆H₄py = 2-phenylpyridine; and L¹H = 4-MePipzcdtH, L^2<

Full text of DOI:10.1080/00958972.2014.945924

Journal of Coordination Chemistry, 2014
Vol. 67, No. 15, 2643–2660, http://dx.doi.org/10.1080/00958972.2014.945924
Synthesis, characterization, interactions with DNA and
bovine serum albumin (BSA), and antibacterial activity of
cyclometalated iridium(III) complexes containing
dithiocarbamate derivatives
TITAS MUKHERJEE†, MANJIRA MUKHERJEE†, BUDDHADEB SEN†,
SNEHASIS BANERJEE‡, GEETA HUNDAL§ and PABITRA CHATTOPADHYAY*†
†Department of Chemistry, Burdwan University, Burdwan, India
‡Department of Chemistry, Government College of Engineering and Leather Technology, Kolkata,
India
§Department of Chemistry, Guru Nanak Dev University, Amritsar, India
(
Received 1 March 2014; accepted 4 June 2014)
III
Three mononuclear cyclometalated iridium(III) complexes having dithiocarbamate ligands, [Ir (2-
1
2
C
6
H
4
py)
2
(L)] (where 2-C
6
H
4
py = 2-phenylpyridine; and L H = 4-MePipzcdtH, L H = MorphcdtH,
and L H = 4-BzPipercdtH for 1, 2, and 3, respectively), were synthesized from [Ir(2-
py) Cl] ·1/4CH Cl by displacing the two bridging chlorides with one dithiocarbamate ligand.
The complexes were characterized using physicochemical and spectroscopic tools along with struc-
3
C
6
H
4
2
2
2
2
2
6 4 2
tural analysis of [Ir(2-C H py) (L )] (2) by single crystal X-ray diffraction. Structural analysis of 2
showed a distorted octahedron in which the nitrogen donor of one 2-phenylpyridine and the carbon
donor of another 2-phenylpyridine are in axial positions, trans to one another. Electrochemical anal-
ysis by cyclic voltammetry showed the irreversible two-electron equivalent reduction voltammo-
grams of 1, 2, and 3 attributable to Ir(III) to Ir(I). Electronic characterizations of these complexes
are consistent with significant delocalization of the sulfur electron density onto the empty metal
d-orbital. The intercalative interaction of the complexes with calf thymus DNA was evaluated using
absorption, fluorescence quenching, and viscosity measurements. The binding affinities of these
complexes with bovine serum albumin were estimated in terms of quenching constants using the
Stern–Volmer equation. Study of antibacterial activities of the complexes by agar disk diffusion
against some species of pathogenic bacteria was also performed.
*
Corresponding author. Email: pabitracc@yahoo.com
2014 Taylor & Francis
©

2
644
T. Mukherjee et al.
Keywords: Iridium(III) complex; Dithiocarbamates; Crystal structure; Redox behavior with DFT;
Biological study
1. Introduction
The chemistry of iridium(III) complexes is important in organic light-emitting diodes, cata-
lysts, and anticancer agents. Iridium(III) complexes, particularly bis- and tris-cyclometalated
(
C^N) complexes of iridium(III) based on 2-phenylquinoline and its derivatives, have been
widely used as promising electroluminescent materials for OLED technology because cy-
6
clometalated iridium(III) complexes having a d -electron configuration show strong spin-
orbit coupling, efficient intersystem crossing from the singlet excited state to the triplet
manifold, and enhancement of the T -S transition, displaying efficient phosphorescent
1
0
emission at room temperature [1–10]. Several reports on the evolution of selective recogni-
2
+
tion for Hg with multi-signaling optical–electrochemical response by introducing sulfur to
these cyclometalated ligands are also in the literature [11–17] compared to the reports of
biological applications of iridium(III) complexes [18–28]. Among these biological applica-
tions, DNA-binding studies have been a focus to explore the intercalation characteristics of
iridium(III) complexes into DNA in quest of relatively inert iridium-based anticancer com-
plexes of higher potency, higher cancer cell selectivity, lower resistance, and reduced side
effects [25–28]. Study of interaction with DNA and bovine serum albumin (BSA), and anti-
bacterial activity of cyclometalated iridium(III) complexes bearing dithiocarbamate deriva-
tives, is still unexplored.
–
Dithiocarbamate (R NCS ) sulfurs maintain additional coordination capability due to the
2
2
availability of a pair of potentially bonding electrons, leading to applications in chemical
bonding, industry, biology, and biochemistry [29–35]. The nature of the heterocycle and the
metal ion attached to dithiocarbamate control the potential pharmacological and catalytic
efficiency of the metal complexes [36].
Herein, an account of the syntheses, characterizations, electrochemical behaviors, and bio-
logical properties of a new series of three mononuclear cyclometalated iridium(III) complexes
bearing dithiocarbamate derivatives has been described. This report deals with the chemistry of
III
[
(
Ir (2-C H py) (SS)] complexes, where 2-C H py = 2-phenylpyridine and SS = 4-MePipzcdt
6 4 2 6 4
1
2
3
L ) for 1, Morphcdt (L ) for 2, and 4-BzPipercdt (L ) for 3. The physico-chemical and spectro-
scopic properties of 1, 2, and 3 along with the detailed structural analyses of 2 have been
included. The intercalative mode of binding interactions of these complexes with calf thymus
DNA (CT-DNA) have been investigated to explore the biological activity of new complexes,
as pharmacological effectiveness depends on intercalative binding to DNA [37–40]. The asso-
ciation of 2 with BSA with spectroscopic tools has also been examined. Antibacterial activities
of 1, 2, and 3 against Escherichia coli, Vibrio cholerae, Streptococcus pneumoniae, and Bacil-
lus cereus have also been studied by agar disk diffusion.
2
2
. Experimental
.1. Materials and physical measurements
Iridium trichloride, 2-phenylpyridine, morpholine, and 4-benzylpiperidine (Aldrich) were
purchased and used without purification. [Ir(2-C H py) Cl] ·1/4CH Cl was prepared
6
4
2
2
2
2

Cyclometalated iridium(III) complexes
2645
following the reported procedure [41]. CT-DNA was obtained from Bangalore Genie, India.
Solvents used for spectroscopic studies and for synthesis were purified and dried by stan-
dard procedures before use. The organic moieties, 4-methylpiperazine-l-carbodithioic acid
1
2
(
4-MePipzcdtH, L H), morpholine-4-carbodithioic acid (MorphcdtH, L H), and 4-benzyl-
3
piperidine-l-carbodithioic acid (4-BzPipercdtH, L H) were obtained as solid products fol-
lowing our earlier report [42].
The Fourier transform infrared spectra of the ligand and complexes were recorded on a
−
1
Perkin Elmer FTIR model RX1 spectrometer using KBr pellets from 4000 to 300 cm .
Solution phase electronic spectra were recorded on a JASCO UV–vis/NIR spectrophotome-
ter model V-570 from 200 to 1100 nm. Elemental analyses were carried out on a Perkin
Elmer 2400 series-II CHNS Analyzer. Steady-state fluorescence emission and excitation
spectra were recorded with a Hitachi F-4500 FL spectrophotometer. Mass spectra of 1, 2,
TM
and 3 were recorded on Micromass Q-T of micro . NMR spectra of the ligands and com-
plexes have been recorded on a Bruker DPX-300. Solution conductivities were measured
using a Systronics Conductivity Meter 304 model and redox potentials by CHI620D poten-
−
3
−1
−4
−1
tiometer in DMF at ~10 M L . Stock solutions of protein (1.00 × 10 M L ) were pre-
pared by dissolving BSA in 0.05 M phosphate buffer at pH 7.4 and storing at 0–4 °C in the
dark. The concentration of BSA was determined from optical density measurements using
−
1
−1
molar absorptivity of ε280 = 44,720 M cm [43].
1
2
3
2
.2. Syntheses of [Ir(2-C H py) (L )] (1), [Ir(2-C H py(L )] (2) and [Ir(2-C H py) (L )]
6
4
2
6
4
6
4
2
(3)
The complexes have been synthesized with the help of a common procedure as stated
1
2
below. To solution of the ligand, L H (177 mg, 1.0 mM) for 1, or L H (164 mg, 1.0 mM)
3
for 2, or L H (253 mg, 1.0 mM) for 3 in DMSO-MeCN (v/v 1 : 1), the solution of [Ir(2-
C H py) Cl] ·1/4CH Cl (557 mg, 0.5 mM) in acetonitrile was added. The resulting solu-
6
4
2
2
2
2
tion was refluxed under nitrogen for 12 h and the color changed from faded yellow to
orange. On slow evaporation, a microcrystalline solid appeared, which was then subjected
to purification by TLC on a silica plate with C H –MeCN (v/v 3 : 1) as eluent; an orange
6
6
band separated, which was extracted with MeCN. Needle-shaped crystals of [Ir(2-
2
C6H4py) (L )] suitable for X-ray diffraction were grown from this solution on evaporation
2
at ambient temperature.
1
[
Ir(2-C H py) (L )] (1): [C H N IrS ]; Yield: 88%. Anal. Calcd: C, 49.76; H, 4.03;
6
4
2
28 27
4
2
−
1
N, 8.29; S, 9.49. Found: C, 49.03; H, 3.89; N, 8.76; S, 9.11. IR (cm ): νC=N, 1490; νa(SCS)
012, 996. ESI-MS (m/z): [M + Na ], 698.90(23% abundance); [M + H ], 676.898 (59.1%
abundance). Conductivity (Λ , M cm ) in DMF: 42. H NMR (δ, ppm in dmso-d ): 4.35
,
+
+
1
−
1
−1
1
o
6
(
m, 3H of N–CH ); 3.79 (m, 4H of S C–N(CH ) ); 3.15 (m, 4H of –N(CH ) ). Protons of
3 2 2 2 2 2
2
-C H py: C –H (8.84, d, 2H), C –H (8.01, d, 2H), C –H (7.69, d, 2H), C –H (7.68, m,
6 4 1 5 4 3
2
H), C –H, C –H, and C –H (7.43, m, 6H), C –H (7.27, m, 2H).
6
7
8
2
2
6 4 2 27 24 3 2
[Ir(2-C H py) (L )] (2): [C H N OIrS ]; Yield: 90%. Anal. Calcd: C, 48.92; H, 3.65;
−
1
N, 6.34; S, 9.67. Found: C, 48.01; H, 3.41; N, 6.97; S, 9.01. IR (cm ): νC=N, 1480;
νa(SCS), 1035, 1014. ESI-MS (m/z): [M + Na ], 685.85 (21% abundance); [M + H ], 663.858
+
+
−1
−1
1
(57.6% abundance). Conductivity (Λ , M cm ) in DMF: 35. H NMR (δ, ppm in dmso-
o
d ): 3.78–3.86 (m, 4H of S C–N(CH ) ); 3.65–3.67 (m, 4H of O(CH ) ). Protons of
6
2
2 2
2 2
2
-C H py: C –H (8.86, d, 2H), C –H (8.00, d, 2H), C –H (7.69, d, 2H), C –H (7.67, m,
6 4 1 5 4 3
2
H), C –H, C –H, and C –H (7.41, m, 6H), C –H (7.27, m, 2H).
6
7
8
2

2
646
T. Mukherjee et al.
3
[
Ir(2-C H py) (L )] (3): [C H N IrS ]; Yield: 83%. Anal. Calcd: C, 55.98; H, 4.29;
6
4
2
35 32
3
2
−
1
N, 5.60; S, 8.54. Found: C, 55.22; H, 4.11; N, 6.10; S, 8.08. IR (cm ): ν , 1496; νa(SCS)
042, 1025. ESI-MS (m/z): [M + Na ], 754.02 (16% abundance); [M + H ], 752.008 (34%
abundance). Conductivity (Λ , M cm ) in DMF: 45. H NMR (δ, ppm in dmso-d ):
,
C=N
+
+
1
−
1
−1
1
o
6
7
2
2
.19–7.29 (m, 5H of C H ); 3.91 (m, 4H of S C–N(CH ) ); 3.18 (t, 2H of CH , j = 6.2);
6 5 2 2 2 2
.21 (m, 1H of CH); 1.73–1.77 (m, 4H of (CH ) ). Protons of 2-C H py: C –H (8.83, d,
2
2
6
4
1
H), C –H (8.02, d, 2H), C –H (7.70, d, 2H), C –H (7.69, m, 2H), C –H, C –H, and C –H
5
4
3
6
7
8
(
7.43, m, 6H), C –H (7.28, m, 2H).
2
2
.3. X-ray crystallography
X-ray data of a suitable crystal of 2 were collected on a Bruker Apex-II CCD diffractometer
using Mo Kα (λ = 0.71069) radiation. The data were corrected for Lorentz and polarization
effects, and empirical absorption corrections were applied using SADABS from Bruker. A
total of 13,691 reflections were measured, out of which 4012 were independent and 2299
were observed [I > 2σ(I)] for theta (θ) 32°. The structure was solved by direct methods
2
using SIR-92 and refined by full-matrix least squares based on F using SHELX-97 [44].
The twofold axis passes through the metal ion, nitrogens of the piperazine ring and their
substituent carbons. Therefore, the asymmetric unit contains half the molecule. All non-
hydrogen atoms were refined anisotropically. The refinement showed rotational disorder in
the piperazine ring which could be resolved by splitting the two unique carbons into two
components and refining their sof and thermal parameters as free variables with restraints
over the bond distances. All hydrogens were fixed geometrically with their Uiso values 1.2
times the phenylene and methylene carbons, and 1.5 times the methyl carbons. All calcula-
tions were performed using the Wingx package [45, 46]. Important crystallographic parame-
ters are given in table 1.
Table 1. Crystal data and structure refinement for 2.
Empirical formula
27 3 2 2 2
C H24IrN O S , 0.5H O
Fw
678.81
Triclinic
P-1
Crystal system
Space group
a (Å)
9.7183(4)
b (Å)
c (Å)
11.4054(5)
12.3144(6)
100.737(2)
1274.16(10)
2
1.769
5.433
664
β (°)3
V (Å )
Z
−
3
DCalcd (g cm
)
−
1
μ (Mo Kα) (mm
F (0 0 0)
)
θ Range (°)
No. of reflns. collected
No. of indep. reflns.
1.77–27.00
19,902
5488
R
int
0.0224
5195
316
No. of reflns. (I > 2σ(I))
No. of refined parameters
Goodness-of-fit (F )
2
1.091
R
1
, wR
2
(I > 2σ(I))
0.0193, 0.0491
= 0.0211, 0.0500
R indices (all data)
R
1

Cyclometalated iridium(III) complexes
.4. DNA-binding experiments
2647
2
All the experiments involving CT-DNA were studied by spectrophotometric titration and
fluorescence quenching using ethidium bromide (EB) as a DNA scavenger in tris-buffer
solution, and performing the experiment by our method [47].
Concentrated stock solution of 2 was prepared by dissolving the compound in DMSO
and diluting with tris–HCl buffer to the required concentration for all experiments.
Absorption spectral titration experiment was performed by keeping constant the concen-
tration of 2 and varying the CT-DNA concentration. To eliminate the absorbance of
DNA itself, equal solution of CT-DNA was added both to 2 solution and to the refer-
ence solution.
In the EB fluorescence displacement experiment, 5 μL of EB tris–HCl solution (1.0 mM
−
1
L ) was added to 1.0 mL of DNA solution (at saturated binding levels), storing in the dark
for 2.0 h. Then solution of the compound was titrated into the DNA/EB mixture and diluted
in tris–HCl buffer to 5.0 mL to get the solution with the appropriate complex 1/CT-DNA
mole ratio. Before measurements, the mixture was shaken and incubated at room tempera-
ture for 30 min. The interaction of 2 with CT-DNA was investigated by absorption and
emission spectra along with viscometric measurements to discern between DNA intercala-
tion and groove binding.
2
.5. BSA-binding experiments
−
5
−1
Binding with BSA for 2 was done dissolving BSA in MilliQ water (1.0 × 10 M ) and
the stock solution of the complex was prepared in DMSO-H O (1 : 99 v/v) mixture at 1.0 ×
2
−
5
−1
1
0
M
concentration. Both the absorption and fluorescence quenching experiments (λex
=
280 nm) were performed by gradually increasing the complex concentration, keeping
fixed the concentration of BSA. All the experimental sets were carefully degassed purging
pure nitrogen for 5 min.
2.6. Antimicrobial screening
The biological activities of free dithiocarbamic acids and the iridium(III) derivatives of
dithiocarbamates (1, 2, and 3) have been studied for antibacterial activities by agar well dif-
fusion method [48, 49]. The antibacterial activities were done at 100 μg/mL concentration
in DMF using three pathogenic gram-negative bacteria (E. coli, V. cholerae, and S. pneumo-
niae) and one gram-positive pathogenic bacterium (B. cereus). DMF was used as a negative
control. The Petri dishes were incubated at 37 °C for 24 h. After incubation, the plates were
observed for inhibition zones. The diameter of the zone of inhibition was measured in mm.
2
.7. Theoretical calculation
To clarify the configurations and energy levels of 1, 2, and 3, DFT calculations were carried
out in G09W program using B3LYP/6-31G(d) calculation and correlation function as
implemented in the Gaussian program package, Gaussian 09. Thermal contribution to the
energetic properties was considered at 298.15 K and one atmosphere pressure.

2
648
. Results and discussion
.1. Synthesis and characterization of complexes
T. Mukherjee et al.
3
3
1
2
3
L H, L H, and L H were synthesized by the reaction between carbon disulfide and differ-
1
ent amines in ethanol, and later characterized by FTIR and H NMR spectra. Treatment of
these ligands with [Ir(2-C H py) Cl] ·1/4CH Cl under refluxing conditions in DMSO-
6
4
2
2
2
2
MeCN (v/v 1 : 1) having a nitrogen atmosphere resulted in cleavage of the chloro bridge
III
and formation of mononuclear iridium(III) complexes of the general formula [Ir (2-
C H py) (L)], which were obtained from column chromatography using acetonitrile as
6
4
2
orange microcrystalline solid on evaporation. Here, the dithiocarbamates are bidentate
monobasic ligands (viz. scheme 1). Complexes 1–3 are sparingly soluble in organic sol-
vents, except hexane, but fairly soluble in DMF and DMSO, and are stable in both the solid
state and solution in air. Spectroscopic and elemental analyses confirm the formulations of
−
3
the complexes. The molar conductivity of freshly prepared solution (~1 × 10 M concentra-
−
1
−1
−1
−1
−1
−1
tion) of 1 (Λ = 42 M cm ), 2 (Λ = 35 M cm ), and 3 (Λ = 45 M cm ) in DMF
M
M
M
are consistent with non-electrolytes. The magnetic susceptibility measurements are in agree-
ment with diamagnetic complexes.
3
.2. Structural description of 2
2
An ORTEP view of [Ir(2-C H py) (L )] (2) with atom-labeling scheme is illustrated in
6
4
2
figure 1, and the selected bond distances and angles are listed in table 2. The structural
2
Figure 1. ORTEP view of [Ir(2-C
6
H
4
py)
2
(L )] (2) with atom-labeling scheme of the crystallographically indepen-
dent part (Symmetry codes: (i) −x, y, z; (ii) x, −y, −z; (iii) −x, −y, −z).

Cyclometalated iridium(III) complexes
2649
Table 2. Coordination bond lengths (Å) and angles (°) for 2.
Bond lengths (Å)
Ir(1)–S(1)
Ir(1)–S(2)
Ir(1)–N(2)
2.490(8)
2.459(7)
2.047(2)
Ir(1)–N(3)
Ir(1)–C(16)
Ir(1)–C(27)
2.011(3)
2.006(3)
2.040(2)
Bond angles (°)
C(27)–Ir(1)–N(3)
C(27)–Ir(1)–S(1)
S(1)–Ir(1)–S(2)
S(2)–Ir(1)–N(2)
C(16)–Ir(1)–N(2)
C(16)–Ir(1)–N(3)
N(3)–Ir(1)–N(2)
80.38(11)
88.85(7)
71.16(2)
100.14(11)
80.27(11)
90.27(11)
94.18(11)
N(2)–Ir(1)–S(1)
N(3)–Ir(1)–S(1)
C(27)–Ir(1)–S(2)
C(27)–Ir(1)–N(2)
C(16)–Ir(1)–S(1)
C(16)–Ir(1)–C(27)
N(3)–Ir(1)–N(2)
97.17(8)
99.59(9)
95.28(7)
172.51(9)
169.99(8)
94.54(11)
94.18(11)
Notes: Symmetry transformations used to generate equivalent atoms: #1 −x + 1, y, −z + 3/2.
analysis shows that 2 resides on a P-1 site in the triclinic crystal system. The crystal struc-
ture of 2 is a distorted octahedron in which the nitrogen of one 2-phenylpyridine and the
carbon donor of another 2-phenylpyridyl ligand are in axial positions. Both Ir–C σ-bonds
are equal (1.994(4) Å) and significantly shorter than the Ir–N dative bond lengths of 2.039
(3) Å. The bond distance of Ir–C in 2 is comparable with the previously reported Ir–C
bonds in cyclometalated complexes (1.996(9) Å), but the Ir–N bond is slightly longer than
those (1.987(7) Å) [50]; both are comparable with reported values [51]. Both Ir–S distances
are also equal (2.4854(11) Å) and longer than those found in [Ir(Et NCS ) ] (2.364(3) Å)
2
2 3
[
51] due to the attachment of sulfur to carbon of more electronegativity than phosphorus.
The S–Ir–S angle of 71.00(5)° is smaller compared to the observed value of 79.40(1)° in
the previous report [51]. The structural analysis showed a highly disordered water O1 W
near the inversion center; it was refined with 0.5 sof. The hydrogens attached to it were
located from the difference Fourier and were initially refined with 0.5 sofs with their isotro-
pic thermal parameters fixed as 1.2 times that of the oxygen and O–H distance at 0.84(2) Å.
3
.3. IR spectra
−
1
The IR spectra of the ligands display intense stretches at 2850 and 1445–1430 cm from
1
−1
νC-H of νN–Me, and νC=N, respectively. The νC–H of ν
of L H for 1 (2915 cm ) is blue-
N–Me
−
1
shifted from νC–H of νN–Me of the free ligand (2850 cm ), indicating metal ligand coordina-
−
l
tion. The band around 1500 cm indicates double bond character of C–N bond in the
ligand frame, confirmed by the X-ray structure. This could be attributed to electron releas-
ing of the heterocyclic group towards sulfur, a feature that induces electron delocalization
over the carbon–nitrogen bond and the CS fragment. This is shown by the ν
shift to
C=N
2
−
1
−1
higher energies (ca. 1480–1496 cm ) with respect to the free acids (ca. 1445–1430 cm ),
and these bands lie between the stretching frequencies expected for a double C=N (1610–
−
l
−1
1
690 cm ) and single C–N bond (1250–1350 cm ). The blueshift of the C=N stretching
frequency on going from the free acids to their metal complexes gives support to the biden-
−
1
tate character [52] of the carbodithioic acid ligands. Two bands at 1042–996 cm (sepa-
−
1
rated by less than 20 cm ), assignable to the ν
6
and one band for the νs(SCS) at 708–
a(SCS)
−
1
78 cm of the complexes, suggest unsymmetrical chelating bidentate mode of coordina-
tion to iridium(III) [53]. The stretching due to νCOC (asym and sym), νN–Me, and νCCC
asym and sym) remain unchanged in spectra of the complexes from the free ligands,
excluding coordination to the metals via nitrogen and oxygen donors.
(

2
650
T. Mukherjee et al.
Table 3. UV–vis spectral data.
a
Compound
λmax (nm) (log ε)
1
2
3
435 (2.50), 390 (3.35), 359sh (3.95), 335 (3.92), 313 (3.97)
442 (2.60), 386 (3.35), 353sh (3.94), 314 (3.92)
438 (2.65), 382 (3.39), 352sh (3.90), 319 (3.96)
a
In DMSO solvent.
−
5
Figure 2. Emission spectra of 1, 2, and 3 at 10 M in deoxygenated acetonitrile solution at 298 K (λex = 364,
40, and 313 nm for 1, 2, and 3, respectively).
3
3
.4. Electronic and fluorescence spectra
The electronic spectral data of 1, 2, and 3 in DMSO have been tabulated in table 3
figure 2). The higher energy band at 385–390 nm for all three complexes with high extinc-
(
tion coefficients are due to coordinated carbon atom from piperidine moiety, C(σ)–Ir(III)
charge transfer transition. The other higher energy intense transitions at 352 and 359 nm are
due to n→π* and π→π* charge transfer transitions. Complexes 1, 2, and 3 display
relatively low-energy bands at 435, 442, and 438 nm, respectively, with low extinction
coefficient values assignable to spin-allowed metal-to-ligand charge transfer [dπ(Ir)→π*(2-
C H py)]. The characteristics of these transitions are very close to the previous reports for
6
4
[dπ(Ir)–π*(bipy)] [51, 54–56].
3
.5. Redox studies
The cyclic voltammograms (CV) of 1, 2, and 3 were recorded in DMF at room temperature.
A three-electrode cell, platinum, Ag/AgCl, and a platinum wire as a working, reference, and
auxiliary electrode, respectively, has been used for measurements. The CV of 1, 2, and 3 are
shown in figure 3, and the electrochemical data are tabulated in table 4. The complexes
exhibit an irreversible reductive response at E ≈ −0.766 to −0.810 V (versus Ag/AgCl)
1
/2
3
+
+
corresponding to Ir /Ir couple. Small differences in the ΔE values 665, 593, and 630 mV
p
for 1, 2, and 3, respectively, have been observed, increasing in the order 1 > 3 > 2. The trend

Cyclometalated iridium(III) complexes
2651
5
4
3
2
1
0
1
2
Black = 3, Red = 2, Blue = 1
-
-
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
E (V)
Figure 3. CV (scan rate 50 mV/s) of 1, 2, and 3 in DMF solution of 0.1 M TBAP using platinum working elec-
trode. Potentials vs. non-aqueous Ag/AgCl.
a
Table 4. Electrochemical data for 1, 2, and 3.
Complex
E
pc (V)
E
pa (V)
ΔE
p
(mV)
E1/2 (V)
1
2
3
−1.094
−1.143
−1.063
−0.464
−0.478
−0.470
665
593
630
−0.810
−0.766
−0.779
a
+
Potentials vs. non-aqueous Ag/Ag reference electrode, scan rate 50 mV/s, supporting electro-
lyte: tetra-N-butylammonium perchlorate (0.1 M).
of reduction potential values can be explained by the energy level diagram for the frontier π
MOs of complexes obtained from DFT calculation (viz. supporting information). This study
supports lowest cathodic potential value for 2 (−0.766 V) in accord with the calculated
LUMO of 2 is of lowest energy (LUMO (133) = −0.0473 eV), whereas the LUMO of the
highest energy (LUMO (137) = −0.0443 eV) is in agreement with the highest cathodic
potential (−0.810 V) for 1.
3
3
.6. DNA-binding study of 2
.6.1. Absorption spectral study. Electronic absorption spectroscopy is an effective
method to examine the binding modes of 2 with DNA. In general, binding of the compound
to the DNA helix shows an increase of the CT band for 2, due to strong intercalative inter-
actions between an aromatic chromophore of the compound and the base pairs of DNA [57,
5
4
8]. The absorption spectra of 2 in the absence and presence of CT-DNA are given in figure
. The extent of the hyperchromism in the absorption band is generally consistent with the
strength of intercalative binding [59, 60]. Figure 5 indicates that 2 interacts strongly with
5
−1
CT-DNA (K = 0.31 × 10 M ), and the observed spectral changes may be rationalized in
b
terms of intercalative binding [61]. In order to further illustrate the binding strength of 2
with CT-DNA, the intrinsic binding constant K was determined from the spectral titration
b
data using the equation [62]:

2
652
T. Mukherjee et al.
0
0
0
0
0
.4
.3
.2
.1
.0
f
a
300
400
500
600
700
Wavelength (nm)
−
4
Figure 4. Electronic spectral titration of 2 with CT-DNA at 269 nm in tris–HCl buffer; [2] = 1.09 × 10 ; [DNA]:
−
6
−6
−6
−6
−6
−1
(
a) 0.0, (b) 1.25 × 10 , (c) 2.50 × 10 , (d) 3.75 × 10 , (e) 5.00 × 10 , and (f) 6.25 × 10 M L . The arrow
indicates the increase of DNA concentration.
a f
Figure 5. Plot of [DNA]/(ε –ε ) vs. [DNA] for the absorption of CT-DNA with 2 in tris–HCl buffer.
½DNAꢀ=ðea ꢁ ef Þ ¼ ½DNAꢀ=ðeb ꢁ ef Þ þ 1=½Kb=ðeb ꢁ ef Þꢀ
(1)
where [DNA] is the concentration of DNA, ε , ε , and ε correspond to the extinction coeffi-
f
a
b
cient, respectively, for free 2, for each addition of DNA to 2 and for 2 in fully bound form.
A plot of [DNA]/(ε – ε ) versus [DNA], gives K , the intrinsic binding constant as the ratio
a
f
b
of slope to intercept. From the [DNA]/(ε – ε ) versus [DNA] plot (figure 6), the binding
a
f
5
−1
constant K for 2 was estimated to be 0.31 × 10 M (R = 0.99478 for five points). How-
b
ever, the binding constant K for 2 with CT-DNA was found to be lower than those
b
observed in reported intercalative complexes [35, 63, 64], but comparable with the partial
intercalant cobalt(II) complex of dimethylcantharate and 2,2′-bipyridine [65].

Cyclometalated iridium(III) complexes
2653
1
000
a
e
8
6
4
2
00
00
00
00
0
600
650
700
750
Wavelength(nm)
Figure 6. Emission spectra of the CT-DNA-EB system in tris–HCl buffer upon titration of 2. λex = 522 nm; [EB]
−
5
−1
−5
−5
−5
−5
=
(
9.6 × 10 M L , [DNA] = 1.25 × 10 ; [Complex]: (a) 0.0, (b) 1.36 × 10 , (c) 2.72 × 10 , (d) 4.08 × 10 , and
−5 −1
e) 5.44 × 10 M L . The arrow denotes the gradual increase of concentration of the complex.
3.6.2. Fluorescence quenching technique. Fluorescence intensity of EB bound to DNA at
6
12 nm shows a decreasing trend with increasing concentration of the compound. The
quenching of EB bound to DNA by the compound is in agreement with the linear Stern–
Volmer equation [66]:
I0=I ¼ 1 þ Ksv½Qꢀ
(2)
where I0 and I represent the fluorescence intensities in the absence and presence of
quencher, respectively. Ksv is a linear Stern–Volmer quenching constant and Q is the con-
centration of quencher. In the quenching plot in figure 6 of I /I versus 2, K is given by the
0
sv
4
ratio of the slope to intercept. The Ksv value for 2 is 0.39 × 10 (R = 0.99746 for five
points), suggesting a strong affinity of the compound to CT-DNA.
3
.6.3. Number of binding sites. Fluorescence quenching data were used to determine the
binding sites (n) for 2 with CT-DNA. Figure 7 shows the fluorescence spectra of EB-DNA
in the presence of different concentrations of 2. It can be seen that the fluorescence intensity
at 612 nm was used to estimate Ksv and n.
If it is assumed that there are similar and independent binding sites in EB-DNA, the rela-
tionship between the fluorescence intensity and the quencher medium can be deduced from
equation (3):
nQ þ B ꢁ! Qn. . .B
(3)
where B is the fluorophore, Q is the quencher, [nQ + B] is the postulated complex between
the fluorophore and n molecules of the quencher. The constant K is given by equation (4):
n
K ¼ ½Q . . .Bꢀ=½Qꢀ ꢂ ½Bꢀ
(4)
n
If the overall amount of biomolecules (bound or unbound with the quencher) is B , then
o
[
B ] = [Q …B] + [B], where B is the concentration of unbound biomolecules, and the
o
n

2
654
T. Mukherjee et al.
1
1
1
1
1
.65
.50
.35
.20
.05
2
4
6
8
[
Complex] x 105
Figure 7. Plot of I
mer quenching constant (Ksv) for 2 = 0.39 × 10 (R = 0.99746 for five points).
0
/I vs. 2 for titration of CT-DNA-EB system with 2 using a spectrofluorimeter; linear Stern–Vol-
4
relationship between the fluorescence intensity and the unbound biomolecule as [B]/[B ] = I/
o
I that is:
0
,
log½I0 ꢁ Iꢀ ¼ log K þ n log½Qꢀ
(5)
where n is the number of binding sites of 2 with CT-DNA, which can be determined from
the slope of log[(I − I)/I] versus log[Q], as shown in figure 8. The calculated value of the
0
number of binding sites (n) is 1.19 (R = 0.99862 for five points). The value of n approxi-
mately equals 1, and thus indicates the existence of just a single binding site in DNA for 2.
-
-
-
-
-
-
0.2
0.4
0.6
0.8
1.0
1.2
-4.8
-4.6
-4.4
-4.2
-4.0
log[Complex 2]
Figure 8. Plot of I
0
/I vs. [Complex] for titration of 2 with CT-DNA-EB system in tris–HCl buffer.

Cyclometalated iridium(III) complexes
2655
3.6.4. Viscometric measurements. To obtain direct evidence of DNA interaction with 2
and specifically to discriminate between DNA intercalation and groove-binding modes, vis-
cosity measurements were carried out. Upon binding, a DNA intercalator causes an increase
in the viscosity of the DNA double helix due to its insertion between the DNA base pairs
and consequently to lengthening of the DNA double helix. In contrast, a partial and/or non-
classical intercalation could bend (or kink) the DNA helix, reducing the effective length
and its viscosity [67]. The effect of 2 on viscosity of CT DNA is shown in figure 9. The
viscosity of DNA increased dramatically upon addition of 2 and is nearly linear at low con-
centration region. These results strongly indicate that the imine dithiocarbamate compound
intercalated deeply into the DNA base pairs.
3
3
.7. Binding experiments with BSA
.7.1. Absorption spectral technique. Absorption spectra of BSA in the absence and
presence of 2 at different concentrations were recorded in DMSO-H O medium (figure 10).
2
The spectra indicate a significant increase in the absorbance of BSA by increasing the con-
centration of the complex and are indicative of the fact that BSA adsorbs strongly the com-
plex on its surface. From these data, the apparent association constant (Kapp) of the
complex with BSA has been determined using the following equation [68]:
1
=ðAobs ꢁ A0Þ ¼ 1=ðAc ꢁ A0Þ þ 1=KappðAc ꢁ A0Þ½complexꢀ
where Aobs is the observed absorbance of the solution containing different concentrations of
the complex at 280 nm, A and A are the absorbances of BSA and the complex at 280 nm,
0
c
respectively, with a concentration of C , and K
represents the apparent association
0
app
constant. The enhancement of absorbance at 280 nm was due to absorption of the surface
complex, based on the linear relationship between 1/(Aobs − A ) versus reciprocal concentra-
0
tion of the complex with a slope equal to 1/Kapp(A − A ) and an intercept equal to
c
0
1
1
1
1
1
1
0
.17
.14
.11
.08
.05
.02
.99
n
n
0.0
2.0
4.0
6.0
8.0
10.0
[
Complex 2]/[DNA] x 10-2
Figure 9. Viscometric experiments of CT-DNA and 2 at 25 °C (50 mM tris-buffer); assays were performed in
2
triplicate. Slope: 1.999 ± 0.004; R = 0.99621 for nine points.

2
656
T. Mukherjee et al.
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
f
a
2
60
280
300
320
Wavelength(nm)
Figure 10. Absorption titration spectra of BSA in the presence of 2. Concentration range of complex is 0–6.25 ×
−
6
−1
10
M .
1
/(A − A ) (figure 11). The value of the apparent association constant (Kapp) determined
c 0
4
−1
from this plot is 8.4388 × 10 M (R = 0.99462 for five points).
3
.7.2. Fluorescence quenching. The effect of increasing the concentration of 2 on the
fluorescence emission spectra of BSA were studied and is presented in figure 12. With the
addition of complex, BSA fluorescence emission is quenched. The fluorescence quenching
is described by the Stern–Volmer relation [60]:
I =I ¼ 1 þ K ½complexꢀ
0
sv
8
6
4
2
2
4
6
8
10
(
1/[Complex])x105
_
Figure 11. Linear dependence of 1/A A
0
on the reciprocal concentration of 2.

Cyclometalated iridium(III) complexes
2657
6
5
4
3
2
1
000
000
000
000
000
000
0
3
2
2
.0
.5
.0
a
1.5
.0
e
1
1
2
3
4
5
6
[
Complex]x10
300
350
Wavelength(nm)
400
450
Figure 12. Fluorescence quenching titration of BSA varying the concentrations of 2, [Complex]: (a) 0.0, (b) 1 ×
−
5
−5
−5
−5
−1
10
, (c) 2 × 10 , (d) 3 × 10 , and (e) 4 × 10 M L ; inset shows the Stern–Volmer plot.
MeOH-MeCN
n
n
L H + [Ir(2-C H py) Cl ]
[(2-C H py) IrL ]
6 4 2
6
4
2
2 2
Reflux / 10 h
n = 1, Complex 1
n = 2, Complex 2
n = 3, Complex 3
S
3
1
6
8
4
5
2
N
SH
X
7
1
N
X= -N-CH3 ,
X= -O,
4-MePipzcdtH (L H)
2
MorphcdtH (L H)
2-C H py
6 4
3
X= -CH-CH Ph , 4-BzPipercdtH(L H)
2
Scheme 1. Synthetic strategy of iridium(III) complexes with the corresponding ligands.
where I and I represent the fluorescence intensities of BSA in the absence and presence
0
of quencher, respectively. Ksv is the linear Stern–Volmer quenching constant and [complex]
the molar concentration of the quencher. A linear plot (inset, figure 12) between I /I against
0
5
[complex] was obtained, and from the slope, we calculated Ksv as 5.31 × 10 (R = 0.98942
for five points).
3
.8. Antibacterial activity
Antibacterial activity of the dithiocarbamic acids (HL) and the corresponding complexes are
tabulated in table 5. Comparisons of the biological activity of the dithiocarbamates and their
iridium(III) derivatives with the standard antibiotic chloramphenicol at different

2
658
T. Mukherjee et al.
Table 5. Antibacterial data of free dithiocarbamic acids (LH) and 1, 2, and 3 (100 μg/mL).
Inhibition zone in mm
Compound for treatment
E. coli
V. cholerae
B. cereus
S. pneumoniae
1
L H
03
04
04
07
13
08
19
0
03
03
06
06
15
09
28
0
04
07
05
05
14
09
30
0
02
03
03
05
13
07
20
0
2
L H
3
L H
1
2
3
Chloramphenicol
DMF
concentrations have been carried out taking usual precautions. From this study, it may be
concluded that the iridium(III) complexes have higher activity than the ligand, but less effi-
cient than the antibiotics. The increased activity may be due to increase of the delocaliza-
tion of π-electrons over the whole chelate ring imparting increased lipophilic character to
the metal complexes. This higher lipophilicity of the complexes facilitates the penetration
ability into the bacterial cell membranes, and as result, it perturbs the respiration process of
the bacteria and slows down the further growth of the micro-organisms.
4
. Conclusion
Three neutral cyclometalated iridium(III) complexes bearing dithiocarbamate derivatives
1
2
3
formulated as [Ir(2-C H py) (L )] (1), [Ir(2-C H py) (L )] (2), and [Ir(2-C H py) (L )] (3)
6
4
2
6
4
2
6
4
2
have been synthesized and characterized by solid and solution phase spectroscopic studies,
including detailed structural analysis of 2 by single-crystal X-ray study. The interaction with
CT-DNA shows that these cyclometalated iridium(III) complexes are good intercalators to
CT-DNA with an adequate number of coordination sites. The viscometric measurement
shows that 2 interacts with CT-DNA as a intercalant. The antibacterial studies suggest that
all the complexes have higher activities than the free dithiocarbamic acids (LH) against four
pathogenic bacteria (E. coli, V. cholerae, S. pneumoniae, and B. cereus); among these three
complexes, 2 has more antibacterial effect.
Supplementary material
Crystallographic data for 2 have been deposited with the Cambridge Crystallographic Data
Center, CCDC No. 932588. Copies of this information are available on request at free of
charge from CCDC, 12 Union Road, Cambridge CB21EZ, UK (Fax: +44 1223 336 033;
E-mail: deposit@ccdc.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgements
Financial support from the Council of Scientific and Industrial Research (CSIR), New
Delhi, India is gratefully acknowledged.

Cyclometalated iridium(III) complexes
2659
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