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Organometallic Iridium(III) Cyclopentadienyl Anticancer Complexes Containing C,N-Chelating Ligands
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Author(s): Zhe Liu, Abraha Habtemariam, Ana M. Pizarro, Guy J.
Clarkson, and Peter J. Sadler
Article Title: Organometallic Iridium(III) Cyclopentadienyl Anticancer
Complexes Containing C,N-Chelating Ligands
Year of publication: 2011
Link to published article: http://dx.doi.org/10.1021/om2005468
Publisher statement: This document is the Accepted Manuscript
version of a Published Work that appeared in final form in
Organometallics, copyright © American Chemical Society after peer
review and technical editing by the publisher. To access the final
edited and published work see http://dx.doi.org/10.1021/om2005468
Organometallic Iridium(III) Cyclopentadienyl Anticancer Complexes
Containing C,N-Chelating Ligands
Zhe Liu, Abraha Habtemariam, Ana M. Pizarro, Guy J. Clarkson, and Peter J. Sadler*
Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4
7AL, U.K.
Abstract: Organometallic Ir(III) cyclopentadienyl complexes [(η5-Cpx)Ir(C^N)Cl],
Cpx
=
Cp*,
C^N
=
2-(p-tolyl)pyridine
(3),
(Cpxph),
2-phenylpyridine
=
2-phenylquinoline
(2),
Cpx = tetramethyl(phenyl)cyclopentadienyl
2-(2,4-difluorophenyl)pyridine
C^N
(1),
(4),
and
Cpx
=
tetramethyl(biphenyl)cyclopentadienyl (Cpxbiph), C^N = 2-phenylpyridine (5), have
been synthesized and characterized. The X-ray crystal structures of 2 and 5 have been
determined and show typical “piano-stool” geometry. All the complexes hydrolyzed
rapidly in aqueous solution (<5 min) even at 278 K. The pKa values of the aqua
adducts 1A−5A are in the range of 8.31−8.87, and follow the order 1A > 2A > 4A >
5A ≈ 3A. Hydroxo-bridged dimers {[(η5-Cpx)Ir]2(μ-OD)3}+ (Cpx = Cp*, 6; Cpxph, 7;
Cpxbiph, 8) are readily formed during pH titrations at ca. pH 8.7. Complexes 1 and 3−5
bind strongly to 9-ethylguanine (9-EtG), moderately strongly to 9-methyladenine
(9-MeA), and hence preferentially to 9-EtG when in competition with 9-MeA. The
extent of guanine and adenine binding to complex 2 was significantly lower for both
purines due to steric hindrance from the chelating ligand. All complexes showed
potent cytotoxicity, with IC50 values ranging from 6.5 μM to 0.7 μM towards A2780
human ovarian cancer cells. Potency toward these cancer cells increased with
additional phenyl substitution on Cp*: Cpxbiph > Cpxph > Cp*. Cpxbiph, with complex 5
exhibiting sub-micromolar activity (2× as active as cisplatin). These data demonstrate
how the aqueous chemistry, nucleobase binding and anticancer activity of C,N-bound
IrIII cyclopentadienyl complexes can be controlled and fine-tuned by the modification
1
of the chelating and cyclopentadienyl ligands.
*To whom correspondence should be addressed. E-mail: P.J.Sadler@warwick.ac.uk.
2
Introduction
Organometallic complexes offer enormous scope for the design of anticancer
candidates due to their versatile structures, potential redox features and wide range of
ligand substitution rates.1 For examples, titanocenyl, ferrocenyl and RuII arene
anticancer complexes are attracting current attention.2 However, there are only a
limited number of reported studies on the anticancer activity of organometallic
iridium complexes.3 In general iridium(III) complexes are usually thought to be
relatively inert.4
The negatively-charged pentamethylcyclopentadienyl ligand (Cp*) is often an
excellent stabilizing ligand for organometallic iridium(III) complexes. However, a
number of Cp* IrIII complexes of the type [(η5-C5Me5)Ir(XY)Cl]0/+, where XY =
N,N-bound 1,3,5-triaza-7-phosphatricyclo-[3.3.1.1]decane),3i ethylenediamine (en),
2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or N,O-bound picolinate (pico),3a
have been reported to be inactive towards A2780 human ovarian cancer cells.
However, the electronic and steric properties of the ligands can have a major effect on
the chemical and biological activities of transition metal complexes.5 Recently we
have demonstrated that introduction of phenyl or biphenyl substituents on Cp* can
improve cancer cell cytotoxicity of N,N-chelated IrIII complexes significantly.3a In
addition, we have found that replacement of the neutral N,N-bound chelating ligand
(bpy) by the negatively-charged C,N-bound 2-phenylpyridine (phpy) ligand in a Cp*
IrIII chlorido complex can switch on biological activity.3b This finding has led us to
make detailed investigations of Cp* IrIII complexes with C,N-bound ligands and to
study the influence of phenyl- or biphenyl-substituted Cp* on their chemical and
biological properties.
We report studies of the aqueous chemistry (hydrolysis, acidity of the resultant aqua
adducts), nucleobase binding and cancer cell toxicity of IrIII complexes containing
C,N-chelating ligands based on 2-phenylpyridine with electron-donating or
3
electron-withdrawing substituents, and with Cp* and substituted Cp* ligands, Chart 1.
The results suggest that this new class of organometalic Ir(III) complexes is well
suited for development as anticancer agents.
Chart 1. Iridium Cyclopentadienyl Complexes Studied in This Work
0/+
Cpx
Ir
N
Z
C
Cpx
Cp*
Cpxph
N
N
Cpxbiph
N
C^N
N
F
F
dfphpy
phpy
tpy
phq
Z=Cl
Z=D2O
Z=9-EtG
Z=9-MeA
Cp
x
C^N
1
1A
1G
1Ad
Cp*
tpy
2
2A
2G
2Ad
Cp*
phq
3
3A
3G
3Ad
Cp*
dfphpy
4
4A
4G
4Ad
Cp
xph
phpy
5
5A
5G
5Ad
Cp
xbiph
phpy
dimer
Cpx
6
Cp*
7
Cpxph
8
Cpxbiph
Cpx
Ir
D
O
D
O
+
Ir
Cpx
O
D
Experimental Section
Materials. 2-Phenylpyridine, 2-(2,4-difluorophenyl)pyridine, 2-(p-tolyl)pyridine,
2-phenylquinoline, 9-ethylguanine, and 9-methyladenine were purchased from
Sigma-Aldrich. Methanol was distilled over magnesium/iodine prior to use. Dimers
4
[(η5-C5Me5)IrCl2]2,6 [(η5-C5Me4C6H5)IrCl2]2,3a and [(η5-C5Me4C6H4C6H5)IrCl2]2,3a
were prepared according to reported methods.
Syntheses.
[(η5-C5Me5)Ir(tpy)Cl] (1). A solution of [(η5-C5Me5)IrCl2]2 (48 mg, 0.06 mmol),
2-(p-tolyl)pyridine (20 mg, 0.12 mmol) and sodium acetate (20 mg, 0.24 mmol) in
CH2Cl2 (15 mL) was stirred for 2 h at ambient temperature. The solution was filtered
through celite. The filtrate was evaporated to dryness on a rotary evaporator, and
washed with diethyl ether. The product was recrystallized from CHCl3/hexane. Yield:
45 mg (70%). 1H NMR (CDCl3): δ = 8.65 (d, 1H, J = 5.7 Hz), 7.75 (d, 1H, J = 8.3
Hz), 7.62 (m, 2H), 7.57 (d, 1H, J = 8.0 Hz), 7.03 (t, 1H, J = 6.3 Hz), 6.86 (d, 1H, J =
7.8 Hz), 1.68 (s, 15H). 13C NMR (CDCl3): δ = 151.29, 140.79, 136.74, 128.35,
123.27, 121.65, 118.43, 88.41, 77.36, 8.83. Anal. Calcd. for C22H25ClNIr (531.14): C,
49.75; H, 4.74; N, 2.64. Found: C, 49.66; H, 4.65; N, 2.68. MS: m/z 496 [M − Cl]+.
[(η5-C5Me5)Ir(phq)Cl] (2). The synthesis was performed as for 1 using
[(η5-C5Me5)IrCl2]2 (48 mg, 0.06 mmol), 2-phenylquinoline (25 mg, 0.12 mmol), and
sodium acetate (20 mg, 0.24 mmol). Yield: 43 mg (75%). 1H NMR (CDCl3): δ = 8.71
(d, 1H, J = 8.8 Hz), 8.02 (d, 1H, J = 8.7 Hz), 7.93 (d, 2H, J = 8.8 Hz), 7.77 (m, 2H),
7.69 (t, 1H, J = 8.1 Hz), 7.53 (t, 1H, J = 6.7 Hz), 7.24 (t, 1H, J = 7.8 Hz), 7.07 (t, 1H,
J = 7.7 Hz), 1.57 (s, 15H). 13C NMR (CDCl3): δ = 156.22, 137.76, 136.49, 131.36,
130.87, 130.33, 127.97, 126.34, 125.30, 122.11, 116.56, 89.11, 77.35, 76.71, 9.26.
Anal. Calcd. for C25H25ClNIr (567.13): C, 52.94; H, 4.44; N, 2.47. Found: C, 53.06;
H, 4.41; N, 2.42. MS: m/z 531 [M − Cl]+. Crystals suitable for X-ray diffraction were
obtained by slow evaporation of a methanol/diethyl ether solution at ambient
temperature.
[(η5-C5Me5)Ir(dfphpy)Cl] (3). The synthesis was performed as for 1 using
[(η5-C5Me5)IrCl2]2 (48 mg, 0.06 mmol), 2-(2,4-difluorophenyl)pyridine (23 mg, 0.12
mmol), and sodium acetate (20 mg, 0.24 mmol). Yield: 46 mg (70%). 1H NMR
(CDCl3): δ = 8.71 (d, 1H, J = 6.0 Hz), 8.19 (d, 1H, J = 8.8 Hz), 7.69 (t, 1H, J = 8.0
5
Hz), 7.31 (d, 1H, J = 8.8 Hz), 7.10 (t, 1H, J = 6.3 Hz), 6.49 (t, 1H, J = 9.5 Hz), 1.67 (s,
15H). 13C NMR (CDCl3): δ = 151.54, 143.53, 137.52, 122.7, 117.28, 98.20. 89.02,
77.35, 8.78. Anal. Calcd. for C21H21ClF2NIr (553.10): C, 45.60; H, 3.83; N, 2.53.
Found: C, 45.76; H, 3.71; N, 2.46. MS: m/z 517 [M − Cl]+.
[(η5-C5Me4C6H5)Ir(phpy)Cl] (4). A solution of [(η5-C5Me4C6H5)IrCl2]2 (46 mg,
0.05 mmol), 2-phenylpyridine (15 mg, 0.10 mmol) and sodium acetate (16 mg, 0.20
mmol) in CH2Cl2 (15 mL) was heated under reflux in an N2 atmosphere for 24 h. The
solution was filtered through celite. The filtrate was evaporated to dryness on a rotary
evaporator and washed with diethyl ether. The product was recrystallized from
CHCl3/hexane. Yield: 37 mg (57%). 1H NMR (MeOD-d4): δ = 8.60 (d, 1H, J = 5.3
Hz), 8.04 (d, 1H, J = 8.3 Hz), 7.84 (m, 2H), 7.65 (d, 1H, J = 7.8 Hz), 7.38 (m, 3H),
7.33 (m, 2H), 7.16 (t, 1H, J = 6.1 Hz), 7.13 (t, 1H, J = 7.2 Hz), 7.09 (t, 1H, J = 7.3
Hz), 1.85 (s, 3H), 1.74 (s, 3H),1.72 (s, 3H), 1.56 (s, 3H). 13C NMR (CDCl3): δ =
151.53, 137.13, 135.65, 131.14, 128.77, 127.33, 123.90, 122.20, 118.96, 77.35, 9.66.
Anal. Calcd. for C26H25ClNIr (579.16): C, 53.92; H, 4.35; N, 2.42. Found: C, 53.77;
H, 4.31; N, 2.41. MS: m/z 543 [M − Cl]+.
[(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5). The synthesis was performed as for 4 using
[(η5-C5Me4C6H4C6H5)IrCl2]2 (53 mg, 0.05 mmol), 2-phenylpyridine (15 mg, 0.10
mmol) and sodium acetate (16 mg, 0.20 mmol). Yield: 37 mg (57%). 1H NMR
(CDCl3): δ = 8.51 (d, 1H, J = 5.3 Hz), 7.81 (d, 1H, J = 7.3 Hz), 7.72 (m, 2H), 7.64 (m,
5H), 7.51 (m, 4H), 7.37 (d, 1H, J = 7.6 Hz), 7.16 (t, 1H, J = 7.3 Hz), 7.05 (t, 1H, J =
6.0 Hz), 6.94 (t, 1H, J = 7.3 Hz), 1.92 (s, 3H), 1.82 (s, 3H),1.79 (s, 3H), 1.67 (s, 3H).
13
C NMR (CDCl3): δ = 151.45, 139.90, 136.98, 135.57, 131.05, 129.01, 127.22,
123.93, 122.51, 118.94, 77.34, 9.88. Anal. Calcd. for C32H29ClNIr (655.25): C, 58.66;
H, 4.46; N, 2.14. Found: C, 58.46; H, 4.35; N, 2.18. MS: m/z 619 [M − Cl]+.Crystals
suitable for X-ray diffraction were obtained by slow evaporation of a
methanol/diethyl ether solution at ambient temperature.
Methods and Instrumentation.
6
X-ray crystallography. All diffraction data were obtained on an Oxford
Diffraction Gemini four-circle system with a Ruby CCD area detector using Mo Kα
radiation. Absorption corrections were applied using ABSPACK.7 The crystals were
mounted in oil and held at 100(2) K with the Oxford Cryosystem Cobra. The
structures were solved by direct methods using SHELXS (TREF)8 with additional
light atoms found by Fourier methods. Complexes 2 and 5 were refined against F2
using SHELXL9, and hydrogen atoms were added at calculated positions and refined
riding on their parent atoms.
X-ray crystallographic data for complexes 2 and 5 have been deposited in the
Cambridge Crystallographic Data Centre under the accession numbers CCDC 829525
and 829524, respectively.
NMR Spectroscopy. 1H NMR spectra were acquired in 5 mm NMR tubes at 298 K
(unless stated otherwise) on either Bruker DPX 400 (1H = 400.03 MHz) or AVA 600
(1H = 600.13 MHz) spectrometers. 1H NMR chemical shifts were internally
referenced to CHCl3 (7.26 ppm) for chloroform-d1, CHD2OD (3.33 ppm) for
methanol-d4 or to 1,4-dioxane (3.75 ppm) for aqueous solutions. All data processing
was carried out using XWIN-NMR version 3.6 (Bruker UK Ltd.).
Mass Spectrometry. Electrospray ionization mass spectra (ESI-MS) were obtained
on a Bruker Esquire 2000 Ion Trap Spectrometer. Samples were prepared in 50%
CH3CN and 50% H2O (v/v). The mass spectra were recorded with a scan range of m/z
50–1000 for positive ions.
Elemental Analysis. CHN elemental analyses were carried out on a CE-440
elemental analyzer by Exeter Analytical (UK) Ltd.
pH Measurement. pH* values (pH meter reading without correction for the effect
of deuterium on the glass electrode) of NMR samples in D2O were measured at ca.
298 K directly in the NMR tube, before and after recording the NMR spectra, using a
Corning 240 pH meter equipped with a micro combination electrode calibrated with
Aldrich buffer solutions of pH 4, 7 and 10.
7
Determination of pKa Values. To generate the aqua complexes, chlorido
complexes were dissolved in D2O and 0.98 mol equiv of AgNO3 were added. The
solution was stirred for 24 h at 298 K, and AgCl was removed by filtration. For
determinations of pKa* values (pKa values for solutions in D2O), the pH* values of
solutions of the aqua complexes in this study were varied from ca. pH* 2 to 10 by the
addition of dilute NaOD and DClO4, and 1H NMR spectra were recorded. The
chemical shifts of the chelating ligand protons and/or of the methyl protons of Cpx
were plotted against pH*. The pH* titration curves were fitted to the
Henderson-Hasselbalch equation, with the assumption that the observed chemical
shifts are weighted averages according to the populations of the protonated and
deprotonated species. These pKa* values can be converted to pKa values by use of the
equation pKa = 0.929pKa* + 0.42 as suggested by Krezel and Bal10 for comparison
with related values in the literature.
Interactions with Nucleobases. The reaction of complexes 1–5 (ca. 1 mM) with
nucleobases typically involved addition of a solution containing 1 mol equiv of
nucleobase in D2O to an equilibrium solution of complexes 1–5 in 20% MeOD-d4/80%
D2O (v/v). 1H NMR spectra of these solutions were recorded at 310 K after various
time intervals.
Cytotoxicity. The A2780 human ovarian cancer cell line was obtained from the
ECACC (European Collection of Animal Cell Cultures, Salisbury, UK). The cells
were maintained in RPMI 1640 media (supplemented with 10% fetal calf serum, 1%
L-glutamine, and 1% penicillin/streptomycin). All cells were grown at 310 K in an
humidified atmosphere containing 5% CO2. Stock solutions of the IrIII complexes
were firstly prepared in DMSO to assist dissolution (maximum final DMSO
concentration 2%), and then diluted into 0.9% saline and medium (1:1). After plating
5000 A2780 cells per well on day 1, IrIII complexes were added to the cancer cells on
day 3 at concentrations ranging from 0.05 μM to 50 μM. Cells were exposed to the
complexes for 24 h, washed with PBS, supplied with fresh medium, and allowed to
8
grow for three doubling times (72 h). Protein content (proportional to cell survival)
was then measured using the sulforhodamine B (SRB) assay.11 The standard errors are
based on two independent experiments carried out in triplicate.
Results
Five IrIII half-sandwich complexes of the type [(η5-Cpx)Ir(C^N)Cl], where Cpx is
pentamethylcyclopentadienyl Cp*, or its phenyl (Cpxph) or biphenyl (Cpxbiph)
derivatives, and the C,N-chelating ligands are 2-(p-tolyl)pyridine (tpy, 1),
2-phenylquinoline
(phq,
2),
2-(2,4-difluorophenyl)pyridine
(dfphpy,
3),
or
2-phenylpyridine (phpy, 4 and 5), were synthesized in good yields by reaction of the
chelating ligand with the appropriate dimer [(η5-Cpx)IrCl2]2 in CH2Cl2. All the
synthesized complexes were fully characterized by
1
H NMR and
13
C NMR
spectroscopy, ESI-MS and CHN elemental analysis. All the complexes in this study
are chiral, but no attempt was made to resolve them and racemates are used in the
following studies.
X-ray Crystal Structures. The X-ray crystal structures of [(η5-C5Me5)Ir(phq)Cl]
(2) and [(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5) were determined. The structures and
atom numbering schemes are shown in Figure 1. Crystallographic data are shown in
Table 1, and selected bond lengths and angles are listed in Table 2. Complexes 2 and
5 adopt the expected half-sandwich pseudo-octahedral “three-leg piano-stool”
geometry with the iridium bound to a η5-cyclopentadienyl ligand (Ir to ring centroid
distances of 1.829 and 1.825 Å, respectively). The Ir−Cl bond distances are 2.3989(16)
and 2.3886(8) Å for 2 and 5, respectively. The Ir−C(chelating ligand) bond lengths in
2 and 5 [2.045(6) and 2.057(3) Å, respectively] are significantly shorter than the Ir−N
bond lengths [2.128(5) and 2.080(3) Å, respectively]. For complex 5, the twist angle
between the bound cyclopentadienyl ring and the central phenyl ring is 48.93°, while
9
that between the bound ring and the terminal phenyl ring is 28.50°. The phenyl rings
are twisted by 21.55°. No intermolecular π-ring stacking in the unit cell is observed in
the two crystal structures.
Figure 1. X-ray crystal structures and atom numbering schemes for complexes
[(η5-C5Me5)Ir(phq)Cl] (2) and [(η5-C5Me4C6H4C6H5)Ir(2-phpy)Cl] (5).
Table
1.
Crystallographic
Data
for
[(η5-C5Me5)Ir(phq)Cl]
(2)
and
[(η5-C5Me4C6H4C6H5)Ir(2-phpy)Cl] (5)
2
5
formula
C25H25ClIrN
C32H29ClIrN
MW
567.11
655.21
cryst color
orange block
orange block
cryst size (mm)
0.40 × 0.40 × 0.10
0.22 × 0.18 × 0.12
λ (Å)
0.71073
0.71073
temp (K)
100
100
cryst syst
orthorhombic
monoclinic
space group
P2(1)2(1)2(1)
P2(1)/n
a (Å)
8.05090(13)
10.0094(4)
b (Å)
15.9169(3)
22.9497(6)
c (Å)
16.0586(3)
11.2128(4)
α (°)
90
90
β (°)
90
103.473(3)
90
90
2057.84(6)
2504.84(14)
γ (°)
3
vol(Å )
10
Z
4
4
1.830
1.737
abs coeff (mm )
6.628
5.459
F(000)
1104
1288
θ range (deg)
3.11 to 30.45
3.10 to 29.33
index ranges
−11 ≤ h ≤ 6, −20 ≤ k ≤ 22,
−22 ≤ l ≤ 11
−13 ≤ h ≤ 12, −31 ≤ k ≤ 31,
−14 ≤ l ≤ 15
reflections collected
10123
23877
independent reflections
5496 [R(int) = 0.0266]
6211 [R(int) = 0.0542]
data/restraints/params
5496/0/253
6211/0/320
final R indices [I > 2σ(I)]
R1 = 0.0368, wR2 = 0.0856
R1 = 0.0293, wR2 = 0.0680
R indices (all data)
R1 = 0.0407, wR2 = 0.0886
R1 = 0.0377, wR2 = 0.0699
GOF
1.053
0.999
largest diff peak and hole
(e Å−3)
6.037 and −1.728
1.754 and −2.149
−3
density(calc) (Mg·m )
−1
Table 2. Selected Bond Lengths (Å) and Angles (deg) for [(η5-C5Me5)Ir(phq)Cl] (2)
and [(η5-C5Me4C6H4C6H5)Ir(2-phpy)Cl] (5)
Bond(s)
Ir−C(Cpx)
Ir−C(centroid)
Ir−C
Ir−N
Ir−Cl
Ir−Ir
C−Ir−N
C−Ir−Cl
N−Ir−Cl
2
2.139(6)
2.157(6)
2.167(5)
2.243(6)
2.304(6)
1.829
2.045(6)
2.128(5)
2.3989(16)
77.4(2)
89.66(17)
87.23(13)
5
2.151(3)
2.163(3)
2.183(3)
2.240(3)
2.243(3)
1.825
2.057(3)
2.080(3)
2.3886(8)
78.27(13)
88.20(9)
86.34(8)
Hydrolysis Studies. The hydrolysis of complexes 1–5 in 20% MeOD-d4/80% D2O
(v/v) was monitored by 1H NMR spectroscopy at different temperatures. The presence
of methanol ensured the solubility of the complexes. All these IrIII complexes undergo
rapid hydrolysis. Equilibrium was reached by the time the first 1H NMR spectrum was
11
acquired (~5 min) even at 278 K. At equilibrium 20%–50% of complex 1–5 was in
the hydrolyzed form, based on 1H NMR peak integrals.
To confirm the hydrolysis of the complexes, NaCl was added to equilibrium
solutions containing the chlorido complexes and their aqua adducts to final
concentrations of 4, 23 and 104 mM NaCl, mimicking the chloride concentrations in
cell nucleus, cell cytoplasm and blood plasma, respectively.12 1H NMR spectra were
then recorded within 10 min of the Cl− additions at 298 K. Upon addition of NaCl, the
1
H NMR peaks corresponding to the chlorido complexes increased in intensity whilst
peaks for the aqua adducts decreased, Figures 2 and S1. These data confirm the
formation of the aqua adducts and the reversibility of the process. On the basis of 1H
NMR data, anation of aqua complexes 1A−5A was almost complete in 104 mM [Cl−]
or in 23 mM [Cl−], and ca. 5% of aqua complex was observed at 4 mM [Cl−] after 10
min with no further change after 24 h.
Figure 2. Confirmation of hydrolysis of IrIII complex [(η5-C5Me5)Ir(tpy)Cl] (1). (A)
1
H NMR spectrum of an equilibrium solution of 1 (1 mM) in 20% MeOD-d4/80%
D2O (v/v) at 298 K. (B) 1H NMR spectrum recorded 10 min after addition of NaCl
(final concentration, 4 mM) to the equilibrium solution of 1. Complex 1A corresponds
to the aqua complex [(η5-C5Me5)Ir(tpy)(D2O)] +. The peaks for the chlorido complex 1
increased in intensity while peaks for the aqua complex 1A decreased (ca. 5% 1A
12
remaining) upon addition of NaCl.
pKa* Determination. Changes in the 1H NMR chemical shifts of the methyl
protons of Cp* or protons of the coordinated chelating ligands in aqua complexes
1A−5A and [(η5-C5Me5)Ir(phpy)(D2O)]+ (for comparison), were followed with
change in pH* over a range of 2−10 (Figure S2). 1H NMR peaks assigned to aqua
complexes gradually shifted to high field with increase in pH*. The resulting pH
titration curves were fitted to the Henderson-Hasselbalch equation, from which the
pKa* values of the coordinated water were determined. This gave pKa values between
8.31 and 8.87 (Table 3), with the Cpxbiph aqua complex 5A and fluoro-substituted
phenylpyridine Cp* complex 3A having the lowest pKa values (8.31 and 8.32,
respectively) and the methyl-substituted phenylpyridine complex 1A having the
highest (8.87).
Table 3. pKa* and pKa Valuesa for the Deprotonation of the Coordinated D2O in
Complexes 1A−5A and [(η5-C5Me5)Ir(phpy)(D2O)]+
Aqua Complex
pKa*
pKa
5
+
9.10
8.87
[(η -C5Me5)Ir(tpy)(D2O)] (1A)
5
+
8.86
8.65
[(η -C5Me5)Ir(phq)(D2O)] (2A)
5
+
8.51
8.32
[(η -C5Me5)Ir(dfphpy)(D2O)] (3A)
5
+
8.64
8.45
[(η -C5Me4C6H5)Ir(phpy)(D2O)] (4A)
5
+
8.50
8.31
[(η -C5Me4C6H4C6H5)Ir(phpy)(D2O)] (5A)
5
+
[(η -C5Me5)Ir(phpy)(D2O)]
8.97
8.75
a
pKa values calculated from pKa* according to Krezel and Bal.10
During the pH titrations for aqua complexes 1A−5A, the appearance of a new set of
peaks was detected with increasing pH* (>8.7). The new peaks are attributable to the
free C,N-chelating ligands and to the hydroxo-bridged dimers {[(η5-Cpx)Ir]2(μ-OD)3}+
(Cpx = Cp*, 6; Cpxph, 7; Cpxbiph, 8, see Chart 1). The 1H NMR peaks for
hydroxo-bridged dimers 6−8 increased in intensity with increase in pH*. For complex
13
[(η5-C5Me5)Ir(dfphpy)(D2O)]+ (3A), the amount of dimer 6 increased from 23% at
pH* 9.0 to 50% at pH* 9.6, Figure 3. ESI-MS studies on the diluted sample (0.2 mM)
gave
a
major
peak
at
m/z
709.2,
consistent
with
the
presence
of
{[(η5-C5Me5)Ir]2(μ-OD)3}+ (calcd m/z 710.2).
Figure 3. Methyl region of 1H NMR spectra from the pH titration of the aqua
complex [(η5-C5Me5)Ir(dfphpy)(D2O)]+ (3A), showing an increase in intensity of the
peak for the hydroxo-bridged dimer {[(η5-C5Me5)Ir]2(μ-OD)3}+ (6) with increase in
pH*.
Interactions with Nucleobases. Since DNA is a potential target for transition
metal
anticancer
drugs,13
nucleobase
binding
reactions
of
complexes
[(η5-C5Me5)Ir(tpy)Cl] (1), [(η5-C5Me5)Ir(phq)Cl] (2), [(η5-C5Me5)Ir(dfphpy)Cl] (3),
[(η5-C5Me4C6H5)Ir(phpy)Cl] (4) and [(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5), with
9-ethylguanine (9-EtG) and 9-methyladenine (9-MeA) were investigated. Solutions of
1−5 (ca. 1 mM, containing an equilibrium mixture of 1−5 and their respective aqua
adducts 1A−5A) and 1 mol equivalent of 9-EtG or 9-MeA in 20% MeOD-d4/80%
14
D2O (v/v) were prepared, and 1H NMR spectra were recorded at different time
intervals at 310 K. The percentages of nucleobase adducts formed by the complexes
after 24 h reaction, based on 1H NMR peak integrals, are shown in Table S1 and
Figure 4.
Figure 4. Bar chart showing the extent of binding of complexes 1−5 (ca. 1 mM in 20%
MeOD-d4/80% D2O) to the nucleobases 9-EtG and 9-MeA at equilibrium (24 h),
based on 1H NMR peak integrals.
In the 1H NMR spectrum of a solution containing 4 (0.8 mM) and 1 mol equiv
9-ethylguanine (20% MeOD-d4/80% D2O, pH* 7.4, 310 K), one set of new peaks
assignable to the 9-EtG adduct 4G appeared, showing that 100% of 4 had reacted
after 10 min (Figure S3). A significant change in chemical shift from 8.62 ppm for the
chlorido complex 4 to 9.28 ppm for 9-EtG adduct 4G for the CH=N (phpy ligand)
proton was observed. A new 9-EtG H8 peak appeared at 7.44 ppm (singlet), shifted
by 0.34 ppm to high field relative to that of free 9-EtG. After 24 h, no further change
was observed. The ESI-MS of an equilibrium solution contained a major peak at m/z
723.2,
confirming
the
formation
of
the
9-EtG
adduct
4G,
[(η5-C5Me4C6H5)Ir(phpy)(9-EtG)]+ (calcd m/z 722.9). Similarly, complexes 1, 3 and 5
also formed 9-EtG adducts to the extent of 100% after 24 h. Only complex 2
containing 2-phenylquinoline showed less strong binding to 9-EtG (45%, Figure 4 and
Table S1).
15
Complexes 3−5 formed moderately strong 9-MeA adducts (35−63% at equilibrium
after 24 h, Table S1). Only complex 1 [(η5-C5Me5)Ir(tpy)Cl] showed an exceptionally
high affinity for 9-MeA, with 90% adduct formation after 24 h. Complex 2 containing
2-phenylquinoline formed almost no 9-MeA adduct (<5%). Except for 2, two adenine
nucleobase adducts were formed in the reactions between complexes 1, 3−5 with
9-MeA, most likely through iridium binding to N1 or N7 of adenine in a ratio
typically of 1:5.
Competition reactions between equi-molar amounts of 9-EtG and 9-MeA and
complexes 1, and 3−5 (ca. 1 mM) in 20% MeOD-d4/80% D2O (v/v, pH* ca. 7.4) at
310 K were investigated. In the competitive experiment, 9-EtG adducts 3G, 4G or 5G
were observed as the only product for complexes 3−5, and as 80% 9-EtG adduct for
complex 1 (20% 9-MeA adduct), Table S1. The 1H NMR spectra for the competition
reaction for 3 are shown in Figure 5.
Figure 5. Low field region of the 1H NMR spectra for the competitive reaction
between 9-EtG and 9-MeA and complex 3 [(η5-C5Me5)Ir(dfphpy)Cl]. (A) Equilibrium
solution of 3 (1.0 mM) in 20% MeOD-d4/80% D2O (v/v, pH* 7.4) at 310 K,
containing both the chlorido complex 3 and its aqua adduct 3A. (B) 10 min after
addition of equimolar amounts of 9-EtG and 9-MeA, showing the complete formation
of 9-EtG adduct 3G.
16
Figure 6. Low field region of the 1H NMR spectra for the reaction between 9-EtG and
complex 4 [(η5-C5Me4C6H5)Ir(phpy)Cl] at 310 K. (A) Equilibrium solution of 4 (0.8
mM) in 20% MeOD-d4/80% D2O (v/v, pH* 7.4), containing both the chlorido
complex 4 and its aqua adduct 4A. (B) 10 min after addition of NaCl (final
concentration, 4 mM) to the equilibrium solution. (C) The complete formation of
9-EtG adduct 4G after addition of 1 mol equiv 9-EtG.
Reactions
of
complexes
[(η5-C5Me4C6H5)Ir(phpy)Cl]
(4)
and
[(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5) with 9-EtG were also investigated in the
presence of 4 mM NaCl, Figure 6. A solution of 4 (ca. 0.8 mM) in 20% MeOD-d4/80%
D2O (v/v) containing an equilibrium mixture of 4 and aqua adduct 4A was prepared,
and its 1H NMR spectrum was recorded, Figure 6A. Then NaCl was added to give a 4
mM [Cl−] solution. The aqua adduct 4A was suppressed to ca. 5%, Figure 6B.
Addition of ca. 1 mol equivalent of 9-EtG resulted in complete formation of 9-EtG
adduct after 24 h at 310 K. Complex 5 also formed a guanine adduct quantitatively in
the presence of 4 mM [Cl−].
Cytotoxicity. The cytotoxicity of complexes 1−5 towards A2780 human ovarian
cancer cells was investigated, Table 4. The IC50 value (concentration at which 50% of
the cell growth is inhibited) for Cp* IrIII complexes 1−3 is comparable with that of
17
cisplatin. Complexes 4 and 5 containing Cpxph or Cpxbiph were even more potent,
especially complex 5 with an IC50 value of 0.7 μM (ca. twice as active as cisplatin).
Overall, the cytotoxic potency increases with phenyl substitution on Cp*: Cpxbiph >
Cpxph > Cp*, Table 4 and Figure S4.
Table 4. Inhibition of Growth of A2780 Human Ovarian Cancer Cells by Complexes
1−5 and Comparison with Cisplatin
Complex
IC50 a (μM)
[(η5-C5Me5)Ir(tpy)Cl] (1)
[(η5-C5Me5)Ir(phq)Cl] (2)
[(η5-C5Me5)Ir(dfphpy)Cl] (3)
[(η5-C5Me4C6H5)Ir(phpy)Cl] (4)
[(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5)
[(η5-C5Me5)Ir(phpy)Cl] b
Cisplatin
3.28±0.14
2.55±0.03
6.53±0.50
2.14±0.50
0.70 ±0.04
10.78 ±1.72
1.19±0.12
a
Drug-treatment period was 24 h. b Ref 3b.
Discussion
X-ray Crystal Structures. A search of the Cambridge Crystallographic Database
showed that the crystal structure of complex 5 is only the second example to be
reported of a metal complex containing the Cpxbiph ligand. The only other example is
the
bipyridine
complex
[(η5-C5Me4C6H4C6H5)Ir(bpy)Cl]PF6
(where
bpy
=
bipyridine).3a The two complexes are structurally very similar. In complex 5, the
chelating ligand is closer to the IrIII center than in [(η5-C5Me4C6H4C6H5)Ir(bpy)Cl]PF6
since the Ir−C bond length [2.057(3) Å] in complex 5 is significantly shorter than the
Ir−N bond length [2.091(5) Å]3a in the latter complex. The short Ir–C(phenylpyridine)
distance causes a slight elongation of the Ir–cyclopentadienyl (centroid) bond with
distance of 1.825 Å in complex 5 (Table 2), compared to 1.787 Å in complex
18
[(η5-C5Me4C6H4C6H5)Ir(bpy)Cl]PF6. The Ir–Cl bond lengths are similar in the two
complexes, with distances of 2.3886(8) Å (Table 2) and 2.3840(14) Å,3a respectively.
This behavior is similar to that observed for the C,N-chelated complex
[(η5-C5Me5)Ir(phpy)Cl]14
when
compared
to
the
N,N-chelated
complex
[(η5-C5Me5)Ir(bpy)Cl]Cl.3b
The bond lengths and bond angles in complexes [(η5-C5Me5)Ir(phq)Cl] (2) and
[(η5-C5Me5)Ir(phpy)Cl]14 are similar, except that the Ir−N bond for 2 [2.128(5) Å] is
longer than that of [(η5-C5Me5)Ir(phpy)Cl] [2.080(2) Å], implying weaker σ donation
from N to the iridium center due to the electron-withdrawing phenyl ring in the
quinoline moiety.
Hydrolysis and pKa of Aqua Adducts. Since M–OH2 (M = metal) aqua
complexes are often more reactive than the equivalent chlorido complexes,3a,12,15
hydrolysis of the M–Cl bonds can represent an activation step for transition metal
anticancer complexes.16 There are only a few previous studies of the aquation
(substitution of Cl by H2O) of organometallic IrIII complexes.3a,4a,17 In the work
reported here all the [(η5-Cpx)Ir(C^N)Cl] complexes 1−5 hydrolyzed too rapidly for
determination of their hydrolysis rates by NMR, even for the biphenyl substituted
Cpxbiph complex 5 [(η5-C5Me4C6H4C6H5)Ir(2-phpy)Cl]. We have previously reported
half-lives for the hydrolysis of some Cpxph or Cpxbiph IrIII complexes containing
N,N-bound 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) chelating ligands of
ca. 4 min at 310 K.3a The electron-donor methyl groups on the Cp ring and the
negatively-charged C,N-chelating ligands may together contribute to the fast
hydrolysis observed for the complexes reported here since the increased effective
charge on the Ir center may facilitate chloride loss. Fast hydrolysis rates also have
been reported for some hexamethylbenzene RuII complexes,18 acetylacetonate RuII
and OsII complexes,19 and picolinate IrIII complexes.3a These results illustrate that IrIII
complexes are not always inert and that Ir−Cl bonds can be labile.
At chloride concentrations typical of blood plasma (104 mM) and cell cytoplasm
19
(23 mM), complexes 1−5 were found to be almost all present in solution as the
relatively unreactive chlorido species, and only about 5% of complexes 1−5 was
present as the reactive aqua species at a chloride concentration of 4 mM close to that
of the cell nucleus. These data suggest that aquation is suppressed almost totally at the
saline concentration in blood. However, complexes 1−5 might be activated by
aquation in the cell nucleus. Another possibility is that the complexes might react by
direct substitution of chloride by nucleobases (DNA).
The aqua complexes [(η5-C5Me5)Ir(tpy)(D2O)]+ (1A), [(η5-C5Me5)Ir(phq)(D2O)]+
(2A), [(η5-C5Me5)Ir(dfphpy)(D2O)]+ (3A), [(η5-C5Me4C6H5)Ir(phpy)(D2O)]+ (4A),
[(η5-C5Me4C6H4C6H5)Ir(phpy)(D2O)]+ (5A), and [(η5-C5Me5)Ir(phpy)(D2O)]+, have
similar pKa values, ranging from 8.31 to 8.87 (Table 3). Although the substituents on
the Cp* ring and the 2-phenylpyridine chelating ligand do not significantly affect the
acidity of the bound water, the observed trend caused by these substituents is clear,
following the order 1A > 2A > 4A > 5A ≈ 3A. The presence of phenyl and biphenyl
substituents on the Cp* ring lower the pKa value by ca. 0.3 to 0.4 units, consistent
with the electron-withdrawing properties of these groups. Substitution of
cyclometalated
2-phenylpyridine
by
the
fluorinated
chelating
ligand
2-(2,4-difluorophenyl)pyridine leads to a decrease in pKa by 0.4 units. However,
replacing cyclometalated 2-phenylpyridine by 2-(p-tolyl)pyridine or by the more
π-delocalized 2-phenylquinoline, has little effect on the pKa value (ca. 0.1 unit)
However,
the
aqua
complexes
[(η5-C5Me5)Ir(phpy)(D2O)]+,
[(η5-C5Me4C6H5)Ir(phpy)(D2O)]+ (4A) and [(η5-C5Me4C6H4C6H5)Ir(phpy)(D2O)]+ (5A)
containing the C,N-chelated 2-phenylpyridine ligand, have significantly higher pKa
values (average 1.9 units higher) than those for the structurally similar Ir III Cpx
analogues bearing the N,N-bound 2,2'-bipyridine (bpy) ligand.3a Therefore, the
replacement of the neutral bpy ligand by the anionic phpy ligand plays a significant
role in decreasing the acidity of the aqua complexes, consistent with previous
reports.3a,19a The high pKa values of 1A−5A thus ensure that most of the hydrolyzed
20
complexes would be present in the active aqua form at physiological pH.
The pH titration also showed that additional species were formed for all complexes
studied here above pH 8.7 (Figure 3), indicating the formation of the hydroxo-bridged
dimers {[(η5-Cpx)Ir]2(μ-OD)3}+ (6−8, Chart 1). However, this does not occur when the
chelating ligand is N,N-chelated bipyridine ligand.3a Clearly the Ir−C bond shows less
stability with respect to the formation of dimers 6−8 than the Ir−N bond. It seems
likely that the mechanism of formation of 6−8 involves initial cleavage of the Ir−C
bond, followed by Ir−N bond breakage. Some OsII and RuII complexes containing
N,O-bound or O,O-bound ligands have been reported to form hydroxo-bridged dimers
readily during their hydrolysis, resulting in their inactivity toward cancer cell lines. 20
In this work, no hydroxo-bridged dimers of 6−8 were observed during hydrolysis, and
all the complexes are active toward A2780 human ovarian cancer cells. This indicates
that the C,N-chelated IrIII complexes are stable in aqueous solution and the dimers
6−8 formed only appear at high pH and have no negative effect on their cytotoxicity
at physiological pH.
Interactions with Nucleobases. Interaction with DNA is often associated with the
cytotoxicity of metal anticancer drugs.13 In this study, the interactions with model
nucloebases, 9-EtG and 9-MeA, and with complexes 1−5 were investigated (Figure 4
and Table S1). All the C,N-chelated IrIII complexes, except 2, showed an
exceptionally high nucleobase affinity with 100% guanine adduct formation for 9-EtG,
which
may
contribute
to
their
high
cytotoxicity.
Complexes
[(η5-C5Me4C6H5)Ir(phpy)Cl] (4) and [(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5) still bind
strongly to guanine in the presence of 4 mM [Cl−] (typically the chloride
concentration in cell nucleus) (Figure 6), which suggests that this class of iridium
complexes might interact with DNA in the cell nucleus. Compared with N,N-chelated
IrIII complexes,3a complexes containing a C,N-bound chelating ligand bind more
significantly to 9-EtG, which may be due to their inherent advantage of having higher
pKa values. Under similar pH conditions, hydrolyzed C,N- complexes are more likely
21
to be present as the reactive aqua form compared to the N,N-chelated complexes.
Complexes 1−5 bind more weakly to adenine compared to guanine. The
competition between 9-EtG and 9-MeA for the C,N-bound IrIII complexes give rise to
9-EtG adducts as the only product for 3−5 and as major product for 1 (Figure 4 and
Table S1), confirming that binding to guanine is stronger than to adenine. This may
due to the steric hindrance caused by the NH2 group at the 6-position of the adenine
ring. In addition, guanine is usually considered to be a stronger electron donor than
adenine.21 The widely used anticancer drug in clinical, cisplatin, also prefers guanine
over adenine.22 Organometallic RuII, OsII and IrIII complexes containing
N,O-chelating ligands or O,O-chelating ligands such as picolinate and acetylacetonate,
which possess oxygen as an H-bond acceptor for adenine C6NH2, also bind to both
guanine and adenine residues.3a,19a,20b,23
IrIII cyclopentadienyl complexes containing a neutral N,N-chelating ligand (phen,
bpy, or ethylenediamine) bind selectively to 9-EtG, but not to adenine.3a The
formation of adenine adducts in this work may be due to the interaction between NH2
of 9-MeA and negatively-charged carbons on the C,N-chelating ligand.3b Thus
complexes 1−3 containing different substituents on the phenylpyridine bind to 9-MeA
differently. The electron donating methyl group on the phenyl ring in complex
[(η5-C5Me5)Ir(tpy)Cl] (1) increases electron density and may facilitate the interaction
with 9-MeA (>90%). In contrast, only 35% of complex [(η5-C5Me5)Ir(dfphpy)Cl] (3)
formed 9-MeA adducts which may be due to the presence of the electron-withdrawing
fluoro group. Complex 2 [(η5-C5Me5)Ir(phq)Cl] has the lowest affinity for both model
nucleobases among the five complexes, with 45% and <5% binding to 9-EtG and
9-MeA, respectively. The weaker binding is most likely due to the steric hindrance
caused by the quinoline ligand.
Cytotoxicity. We have reported that some Cp* IrIII complexes containing N,N-, or
N,O-chelating ligands are inactive toward A2780 human ovarian cancer cells.3a
However, all Cp* complexes [(η5-C5Me5)Ir(tpy)Cl] (1), [(η5-C5Me5)Ir(phq)Cl] (2),
22
and [(η5-C5Me5)Ir(dfphpy)Cl] (3) studied here showed promising activity toward the
human ovarian A2780 cancer cell line with IC50 values ranging from 2.5−6.5 μM
(Table 4), close to the value we reported recently for [(η5-C5Me5)Ir(phpy)Cl].3b Thus
the introduction of C,N-chelating ligands is an effective method for switching on the
cancer cell cytotoxicity of Cp* Ir III complexes. The strong binding of Ir to
nucleobases, especially to guanine bases, may provide an important contribution
towards the cytotoxicity. Also the neutral C,N- complexes display a more
hydrophobic character than the positively-charged N,N- complexes3b and therefore
possess enhanced cellular uptake which may also contribute to the cytotoxicity. The
introduction of substituents on the phenylpyridine ring enhanced the cytotoxicity of
[(η5-C5Me5)Ir(phpy)Cl],
Table
4.
Particularly
active
is
the
complex
[(η5-C5Me5)Ir(phq)Cl] (2), which is 4 times as potent as [(η5-C5Me5)Ir(phpy)Cl]. The
ability of the phq ligand to intercalate into DNA24 may contribute to this enhanced
potency.
The
introduction
of
phenyl
or
biphenyl
substituents
onto
the
tetramethylcyclopentadienyl ring to give complexes [(η5-C5Me4C6H5)Ir(phpy)Cl] (4)
and [(η5-C5Me4C6H4C6H5)Ir(phpy)Cl] (5), results in a dramatic increase in
cytotoxicity compared to the parent Cp* complex [(η5-C5Me5)Ir(phpy)Cl], Table 4
and Figure S4. The activity of RuII arene complexes also increases with the size of the
coordinated arene.25 This suggests that these phenyl groups may play a crucial role in
the mechanism of action of these phenylpyridine complexes. First, the phenyl or
biphenyl ring increases the hydrophobicity of the molecule, which may assist with
passage across cell membranes. In addition, the extended phenyl rings can intercalate
into DNA, thus causing distortion of DNA structure. We have reported that the
intercalative ability of 1,10-phenanthroline IrIII chlorido complexes increases in the
order of Cpxbiph > Cpxph > Cp*.3a Complexes 4 and 5 containing phenyl or biphenyl
substitutions may interact with DNA by a dual mode: nucleobase binding to iridium
accompanied by intercalation of the phenyl groups, which is a different mechanism of
23
action from that of cisplatin.
Conclusions
Iridium-based anticancer agents, including organometallic iridium complexes, are
currently attracting attention as potential anticancer agents with novel mechanisms of
action. We have studied here the effects of changing the Cpx ligand and
negatively-charged C,N-chelating ligand of IrIII cyclopentadienyl complexes of the
type [(η5-Cpx)Ir(C^N)Cl] on the hydrolysis of the chlorido complex, acidity of the
aqua adduct, nucleobase binding, and cancer cell cytotoxicity.
All the complexes undergo rapid hydrolysis (<5 min at 278 K) due to the strongly
electron-donating methyl group and negatively-charged C,N-chelating ligand.
However the complexes are likely to be present in their unhydrolyzed forms in the
extracellular fluid and cell cytoplasm (typically 104 and 23 mM [Cl−]), whereas they
are likely to be activated by hydrolysis in the cell nucleus ([Cl−] ca. 4 mM). Generally,
the aqua adducts of the C,N- complexes studied here possess low acidity, with pKa
values 1.9 units higher than N,N- analogues, which ensures that the active aqua
adduct is the major form after hydrolysis at physiological pH, and may contribute to
the strong binding to guanine. The substituents on both the Cp* ring and on
2-phenylpyridine can fine-tune the acidity of aqua adduts according to their electronic
effects. Hydroxo-bridged dimers {[(η5-Cpx)Ir]2(μ-OD)3}+ (6–8) are observed at high
pH (>8.7), which suggests the stability of C,N- complexes is not as high as N,Nanalogues under strongly basic conditions.
Complexes 1 and 3–5 show strong binding to the nucleobase guanine, even in the
presence of 4 mM [Cl−]. Unlike the IrIII complexes containing N,N-chelating
ligands,3a the C,N- complexes can also bind to adenine. However, they show a strong
preference for binding to guanine over adenine. Complex 2 displayed the lowest
extent of nucleobase binding among the complexes studied due to steric hindrance
from the chelating ligand 2-phenylquinoline.
24
All the C,N- complexes showed very promising anticancer activity toward A2780
human ovarian cancer cells. The Cp* complexes possess activity comparable to
cisplatin. The introduction of phenyl or biphenyl substituent significantly improved
their cytotoxicity, especially for the Cpxbiph complex 5 which has submicromolar
activity against A2780 cancer cells. The strong binding to guanine bases and
hydrophobicity may contribute to their high activity. This study shows that desirable
features can be introduced into this class of complexes to optimize their design as
anticancer agents.
Acknowledgements.
Z.L. was supported by a University of Warwick Research Scholarship (WPRS). We
thank the ERC (grant no. 247450 for P.J.S.), EPSRC, ORSAS, ERDF and AWM for
Science City funding, and members of COST Action D39 for stimulating discussions.
Supporting Information Available: Details of the extent of nucleobase binding
(Table S1), aqueous chemistry (Figures S1 and S2), nucleobase studies (Figure S3),
IC50 comparison (Figure S4). This material is available free of charge via the Internet
at http://pubs.acs.org. X-ray crystallographic data in CIF format are available from the
Cambridge Crystallographic Data Centre (http://www.ccdc.cam.ac.uk).
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