Structural evidence for the facile chelate-ring opening reactions of novel platinum(II)-pyridine carboxamide complexes

Article abstract of DOI:10.1039/b108378n

The design of Pt(II) complexes with novel structural features is of great relevance in the search for new anticancer agents with innovative chemical and biological properties. In this work we have synthesised and structurally characterised three novel Pt(

Full text of DOI:10.1039/b108378n

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Structural evidence for the facile chelate-ring opening reactions of
novel platinum(II)–pyridine carboxamide complexes†
a
a,b
a
a
c
c
Junyong Zhang, Qin Liu, Chunying Duan, Ying Shao, Jian Ding, Zehong Miao,
a
a
Xiao-Zeng You and Zijian Guo*
a
State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute,
Nanjing University, 210093 Nanjing, P.R. China. E-mail: zguo@netra.nju.edu.cn
Department of Food Sciences and Engineering, Nanjing College of Economics,
b
c
210003 Nanjing, P.R. China
Division of Anti-tumor Pharmacology, State Key Laboratory for New Drug Research,
Shanghai Institute of Materia Medica, Shanghai Institutes of Biological Sciences,
Chinese Academy of Sciences, Shanghai 200031, P.R. China
Received 14th September 2001, Accepted 14th November 2001
First published as an Advance Article on the web 21st January 2002
The design of Pt() complexes with novel structural features is of great relevance in the search for new anticancer
agents with innovative chemical and biological properties. In this work we have synthesised and structurally
characterised three novel Pt() complexes with a pyridine carboxamide ligand (HL). The X-ray structural analysis
shows that complex 1, [Pt(L-N, N’) ] crystallises in space group C2, while complex 2, [Pt(L-N,N’)(L-N)Cl] crystallises
2
1
in space group P2 . ESMS and H NMR data show that one of the two ligands in complex 1 undergoes a fast
1
Ϫ
ring-opening reaction to give complex 2 in the presence of Cl . Complex 2 demonstrated notable activity against
human and murine leukemia cells (HL-60 and P388), but rather weak potency against lung adenocarcinoma
A-549. However, complex 1 was found to be basically inactive against the above cell lines. This work provides
new examples in the design of platinum compounds that can be activated in vivo by biological anions.
1
2–16
Introduction
Farrell et al.
have examined a series of platinum() com-
plexes with N-containing heterocyclic ligands such as pyridine,
thiazole and quinoline and found some of them to possess
potent anticancer activities.
Platinum() complexes with a [PtN ] structure are generally
considered anticancer inactive since they cannot readily bind to
DNA due to the high thermodynamic stability of the Pt–N
The search for novel platinum and other metal-based anti-
cancer drugs with better efficacy and lower toxicity has been
going on since the discovery of the anti-proliferation activity
of cisplatin in the 1960s. It is expected that new metallo-
therapeutic agents will be discovered through the efforts of
chemists from the medicinal inorganic chemistry field.
Platinum compounds are currently amongst the most widely
used drugs for the treatment of cancer. However, their severe
toxicity such as nephrotoxicity and neurotoxicity, coupled with
drug resistance which develops within patients after initial
1
4
2
–4
17
bonds. For example, a half-life of ca. 23 years has been
estimated for direct NH exchange in [Pt(NH ) ] at 25 ЊC in
2ϩ
5,6
3
18
3 4
aqueous NH solution. However, recent examples have illus-
3
trated that the Pt–N bond can become labile and be displaced
by Cl when a phosphorus atom (part of a ligand with a strong
trans effect) is introduced at the trans position to the nitrogen
The chelate ring-opened Pt() aminophosphine
complexes bind rapidly and strongly to thymine and uracil
under physiological conditions. In this work we have syn-
thesised three Pt() complexes with a pyridine carboxamide
chelating ligand, two of them have been structurally charac-
terized by X-ray diffraction. Our data demonstrate that the
presence of a bulky group on the amide nitrogen facilitates the
transformation of the bis-chelate complex to its ring-opened
counterpart in solution.
Ϫ
5
treatments, have greatly limited their wider clinical application.
Although several new-generation platinum-based drugs that
have reduced toxicity have recently been introduced into clinical
usage, patients develop cross-resistance to them due to their
structural similarity to cisplatin. Therefore, compounds with
novel structural features are required which could potentially
provide different biological properties.
Among the new design strategies developed, the incorpor-
ation of pyridine-derived ligands into platinum() complexes
has resulted in the discovery of ZD0473 (former JM473 or
19–21
atoms.
22
7,8
AM473), cis-[PtCl (NH )(2-MePy)], which is under phase
2
3
9
I clinical evaluation. The steric hindrance of the 2-methyl-
pyridine ligand in ZD0473 dramatically slows down the
hydrolysis rate of the chlorine in the cis position and the reac-
tivity towards cellular thiols such as glutathione. Recent data
show that ZD0473 forms a specific adduct with duplex DNA.
Experimental
Solvents such as pyridine, ethanol and acetone were all ana-
lytical reagents and used as received. K [PtCl ] and 4-methyl-
aniline were purchased from the First Reagent Factory of
Shanghai, and 2-pyridinecarboxylic acid from Sigma. K [PtCl ]
was synthesized by the reduction of K [PtCl ] using hydrazine
following the standard procedure.
The infrared spectra were recorded on a Bruker VECTOR22
spectrometer as KBr pellets (4000–500 cm ). The H and 2D
10
11
2
6
2
4
†
Electronic supplementary information (ESI) available: the aromatic
2
6
region of the 2D COSY NMR spectrum of HL (in CDCl , 298 K);
3
23
calculated mass spectra of peaks 1, 5, 6Ј, 9, 11, 12; cytotoxic activity of
cisplatin against selected tumor lines. See http://www.rsc.org/suppdata/
dt/b1/b108378n/
Ϫ1
1
DOI: 10.1039/b108378n
J. Chem. Soc., Dalton Trans., 2002, 591–597
This journal is © The Royal Society of Chemistry 2002
591

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COSY NMR spectra were recorded on a Bruker DRX-500
spectrometer using standard pulse sequences. Elementary
analysis was performed on a Perkin-Elmer 240C analytical
instrument.
Electrospray mass spectra were recorded using an LCQ elec-
tron spray mass spectrometer (ESMS, Finnigan) by loading
Crystallography
Table 1 summarizes the crystal data, data collection, structural
solution and refinement parameters for complexes 1 and 2. The
crystal of 1 was an orange block that was mounted on a glass
fiber and used for data collection. Cell constants and an orient-
ation matrix for data collection were obtained by least-squares
refinement of diffraction data from 34 reflections in the range
1
.0 µl of solution into the injection valve of the LCQ unit and
then injecting into the mobile phase solution (50% of aqueous
methanol) that was carried through the electrospray interface
7
.10 < θ < 14.2 on a Siemens P4 four-circle diffractometer. Data
were collected at 293 K using monochromated Mo-Kα radi-
Ϫ1
into the mass analyzer at a rate of 200 µl min . The voltage
ation and the ω–2θ scan technique with a variable scan speed
employed at the electrospray needles was 5 kV, and the capillary
was heated to 200 ЊC. A maximum ion injection time of 200 ms
along with 10 scans was set. Positive and negative ion mass
spectra were obtained. The predicted isotope distribution pat-
terns for each of the complexes were calculated using the Isopro
Ϫ1
5
.0–50.0 min in ω and corrected for Lorentz and polarization
effects. An empirical absorption correction was made (ψ-scan).
The structure was solved by direct-methods which revealed the
position of all non-hydrogen atoms and refined on F by a
full-matrix least-squares procedure using anisotropic displace-
ment parameters. All the hydrogen atoms were placed in their
2
24
3
.0 program.
calculated positions (C–H = 0.93 Å, N–H = 0.86 Å) with Uiso
.2 Ueq.
A pale yellow prism of 2 of approximate dimensions 0.15 ×
=
Preparations
1
N-(4-Methylphenyl)-2-pyridinecarboxamide (HL). This ligand
was prepared following a reported procedure for a carboxamide
0.10 × 0.10 mm was used for data collection and cell parameter
determination on an Enraf-Nonius CAD-4 diffractometer with
the ω–2θ scan mode. The data were also corrected for Lorentz
and polarization effects and an absorption correction was
made using the ψ-scan method. The structure was solved by
Patterson methods and completed by iterative cycles of least-
squares refinement and ∆F-syntheses. H-Atoms were located in
their calculated positions and treated as riding on the atoms to
which they are attached. Only the Pt and Cl atoms were refined
anisotropically. The occupancies for all the atoms are 50% due
25,26
compound.
A mixture of 2-pyridinecarboxylic acid (1.231 g,
1
0 mmol) and 4-methylaniline (1.072 g, 10 mmol) in 10 ml of
pyridine was heated with stirring to 100 ЊC in an oil bath. Then,
triphenylphosphite (3.1 g, 10 mmol) was added slowly to the
mixture. The mixture was further stirred for 3 h at 100 ЊC, then
cooled to room temperature. The solvent was removed by
rotary evaporation to give a pale brown solution from which
colorless crystals were obtained. Yield: 70% [Found: C: 73.47;
H: 5.67; N: 13.19. Calc. for C H N O: C: 73.58; H: 5.66;
1
3
12
2
Ϫ1
Ϫ1
Ϫ3
N: 13.21%]. IR: νNH: 3320 cm , ν : 1675 cm .
to their disorder. The density of crystal 2 was 0.792 mg mm
CO
which is close to the value obtained from the density deter-
Ϫ3
mination experiment (0.823 mg mm ). All calculations were
[
PtL ] (1), [PtL Cl] (2) and [PtLCl] (3). These three com-
2 2
27
carried out using the SHELXTL program.
plexes were prepared as follows. An aqueous solution of
K PtCl (0.1 mmol, 41.5 mg, 2 ml) was mixed with an acetone
CCDC reference numbers 170947 and 170948.
See http://www.rsc.org/suppdata/dt/b1/b108378n/ for crystal-
lographic data in CIF or other electronic format.
2
4
solution of HL in 1 : 2 molar ratio (0.2 mmol, 42.5 mg, 2 ml).
The mixture was then refluxed for 2 h. A yellowish precipitate
formed upon cooling to room temperature, and this was col-
lected by filtration. The yellow precipitate (38.4 mg) was
recrystallised from the mixed solvents acetone and water, and
orange crystals suitable for X-ray determination were obtained
and identified as complex 1 and yellow crystals as complex 2
and they were mechanically separated [Found: 1, C: 50.43
Cytotoxicity assay
Tumor cell lines were grown in RPMI-1640 medium sup-
Ϫ1
plemented with 10% (vol/vol) calf serum, 2 mmol L glut-
Ϫ1
amine, 100 U mL penicillin (U = 1 unit of activity), and
Ϫ1
1
00 µg mL streptomycin (GIBCO, Grand Island, NY) at
H: 3.60, N: 9.75. Calc. for C H N O Pt: C: 50.52, H: 3.56,
26
22
4
2
37 ЊC under 5% CO . Cells in 100 µL culture medium were
2
N: 9.68%. 2, C: 47.70, H: 3.57, N: 8.61. Calc. for C H -
26
23
Ϫ1
seeded into 96-well plates (Falcon, CA).
For murine leukemia P-388 and human leukemia HL-60
ClN O Pt: C: 47.73, H: 3.51, N: 8.56%]. IR: 1, ν : ≈3447 cm ,
4
2
NH
Ϫ1
Ϫ1
Ϫ1
Ϫ1
νCO: 1633 cm ; 2, ν : ≈3447 cm , ν : 1676 cm , 1650 cm .
NH
CO
cells, the microculture MTT [3-(4,5-dimethylthiazol-2-yl)-
The mother-liquor, which contains 2 molar equivalents of KCl
formed during the reaction, was left for slow evaporation,
giving complex 3 [Found: C: 32.6, H: 2.57, N: 5.79. Calc. for
28
2
,5-diphenyl-2H-tetrazolium bromide] assay was conducted
as follows. The cells were treated in triplicate with grade
concentrations of complexes 1, 2 and the reference drug cis-
C H Cl N OPt: C: 32.64, H: 2.53, N: 5.86%].
13
12
2
2
platin at 37 ЊC for 48 h. A 20 µL aliquot of MTT solution
Ϫ1
(
5 mg mL ) was added directly to all the appropriate wells.
Density determination
The culture was then incubated for 4 h. A 50 µl aliquot of 50%
SDS [CH (CH ) OSO Na]–5% isobutyl alcohol–0.01moL
3
2
11
3
The density of one of the crystals of 2 was determined as
Ϫ1
Ϫ3
mL hydrochloride solution was added. After the plates
were incubated overnight, the optical densities were read on a
plate reader (model VERSA Max, Molecular Devices) at
0
.823 mg mm using buoyancy methods from the mixed
solvents water and ethanol, in which 2 is nearly insoluble.
5
70 nm.
Ring-opening experiments in the presence of LiCl followed by
H NMR and ESMS
For human hepatoma BEL-7402 and lung adenocarcinoma
1
A-549 cells, the growth inhibition was analysed by the sulfo-
29
For NMR experiments, solutions of LiCl in CD OD (50 mM)
rhodamine B(SRB) assay. Simply, following the treatment
with complexes 1 and 2 for 72 h, the cell cultures were fixed with
10% trichloroacetic acid and incubated for 60 min at 4 ЊC.
Then, the plates were washed and dried. SRB solution
(0.4% wt/vol in 1% acetic acid) was added and the culture was
incubated for an additional 15 min. After the plates were
washed and dried, bound stain was solubilized with Tris buf-
fer, and the optical densities were read on the same plate
reader at 515 nm. The growth inhibitory rate of treated cells
was calculated by (ODcontrol Ϫ ODtest)/ODcontrol × 100%.
3
and complex 1 (6.25 mM) in CD OD were prepared separately.
3
Immediately after the mixing of 0.1 ml LiCl with 0.4 ml com-
1
plex 1, H NMR spectra were recorded and monitored for 48 h.
Samples for ESMS experiments were prepared as follows.
Ϫ1
Complex 1 was dissolved in CHCl to make a 2 mg ml sol-
3
Ϫ1
ution and then diluted with CH OH to 1 mg ml for the
3
measurement. The ESMS spectra of complex 1 and LiCl in
CDCl /CD OD were recorded immediately after the addition
3
3
of LiCl and followed for 3 days.
5
92 J. Chem. Soc., Dalton Trans., 2002, 591–597

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Table 1 Crystal data and structure refinement for complexes 1 and 2
1
2
Empirical formula
Formula weight
T /K
Crystal size/mm
Crystal habit, colour
Crystal system
Space group
a/Å
C H N O Pt
617.57
293(2)
C H ClN O Pt
654.02
293(2)
0.15 × 0.10 × 0.10
Prism, yellow
Monoclinic
P21
2
6
22
4
2
26 23
4
2
0.30 × 0.15 × 0.15
Block, orange
Monoclinic
C2
13.7781(11)
12.8683(15)
6.2537(13)
95.229(11)
1104.2(3)
9.459(5)
b/Å
c/Å
13.394(5)
10.898(5)
96.82(5)
1370.9(11)
1
0.792
2.621
β/Њ
V/Å
3
Z
2
Ϫ3
Calculated density/Mg m
1.857
6.386
Ϫ1
µ/mm
Reflections collected
Independent reflections
Goodness-of-fit on F
1264
2675
1115 (Rint = 0.0290)
1.149
2517 (Rint = 0.0186)
1.154
2
Final R indices [I > 2σ(I )]
Largest difference peak, hole/e Å
R1 = 0.0599, wR2 = 0.1420
1.889, Ϫ0.838
R1 = 0.0508, wR2 = 0.1410
1.168, Ϫ0.524
Ϫ3
Scheme 1
Results and discussion
Scheme 1 outlines the reaction of K [PtCl ] with HL in 1 : 2
molar ratio that gives complexes 1, 2 and 3. The complexes were
separated mechanically and characterized by IR, H NMR,
be seen from Fig. 1, Pt() in 1 adopted a distorted square-
planar geometry coordinated by two equivalent pyridine nitro-
gen atoms [N(1) and N(1A)] and two amide N atoms [N(8) and
N(8A)] from two ligands arranged in a cis configuration. The
2
4
1
mean deviation from the PtN plane [Pt(1), N(1), N(8), N(1A)
4
ESMS and X-ray diffraction.
and N(8A)] is 0.003 Å. The approximately square-planar
coordination of the Pt() atom generates two five-numbered
rings. The torsion angles Pt(1)–N(1)–C(6)–C(7) and Pt(1)–
N(8)–C(7)–C(6) are Ϫ1.1Њ and 14.4Њ, respectively. As can be
noted the bond length of Pt(1)–N(8) is in the expected region
for a Pt()–amide bond, while the Pt(1)–N(1) bond length
Crystallography
The molecular structure and numbering scheme for complex 1
is shown in Fig. 1. Selected bond lengths and angles for com-
plexes 1 and 2 are listed in Tables 2 and 3, respectively. As can
J. Chem. Soc., Dalton Trans., 2002, 591–597
593

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Table 2 Selected bond lengths (Å) and angles (Њ) for 1
Pt(1)–N(1)
N(8)–C(9)
C(7)–C(6)
N(1)–C(2)
C(3)–C(4)
C(5)–C(6)
2.081(6)
1.453(7)
1.474(11)
1.348(6)
1.395(5)
1.395(5)
Pt(1)–N(8)
N(8)–C(7)
C(7)–O(7)
C(2)–C(3)
C(4)–C(5)
C(6)–N(1)
2.021(7)
1.346(15)
1.242(15)
1.393(6)
1.393(5)
1.351(6)
N(8)–Pt(1)–N(8A)
N(8A)–Pt(1)–N(1)
C(9)–N(8)–C(7)
C(9)–N(8)–Pt(1)
N(8)–C(7)–C(6)
92.6(4)
172.2(3)
115.2(7)
128.4(6)
113.5(9)
N(8)–Pt(1)–N(1)
N(1A)–Pt(1)–N(1)
O(7)–C(7)–C(6)
C(2)–N(1)–Pt(1)
Pt(1)–N(8)–C(7)
79.5(2)
108.3(3)
114.1(10)
122.8(4)
115.4(6)
Symmetry code: A Ϫx ϩ 2, y, Ϫz ϩ 1.
a
Table 3 Selected bond lengths (Å) and angles (Њ) for 2
Pt(1)–N(1)
Pt(1)–N(1Ј)
N(1)–C(2)
C(2)–C(3)
C(4)–C(5)
C(6)–C(7)
N(8)–C(9)
C(6Ј)–C(7Ј)
2.080(7)
2.056(7)
Pt(1)–N(8Ј)
Pt(1)–Cl(1)
N(1)–C(6)
C(3)–C(4)
C(5)–C(6)
C(7)–N(8)
C(7)–O(7)
C(7Ј)–N(8Ј)
1.968(9)
2.336(3)
1.299(12)
1.388(13)
1.415(15)
1.534(11)
1.404(10)
1.483(12)
1.287(10)
1.375(14)
1.346(12)
1.330(10)
1.261(10)
1.273(11)
Fig. 2 An ORTEP view of complex 2.
instead of 2. Fig. 2 shows the molecular structure and number-
ing scheme of complex 2 in which the Pt() is coordinated in a
square planar geometry composed of three nitrogen atoms and
a chloride anion. It is notable that in complex 2 the two ligands
are trans to each other (i.e. N1 is trans to N1Ј). This suggests
that a cis–trans isomerization reaction has occurred during the
conversion from complex 1 to 2. The mean deviation from the
N(8Ј)–Pt(1)–N(1Ј)
N(1Ј)–Pt(1)–N(1)
N(1Ј)–Pt(1)–Cl(1)
C(9)–N(8)–C(7)
C(9Ј)–N(8Ј)–C(7Ј)
C(7Ј)–N(8Ј)–Pt(1)
C(9Ј)–N(8Ј)–Pt(1)
79.4(3)
176.6(3)
96.3(2)
126.7(7)
117.1(8)
120.1(7)
122.6(6)
N(8Ј)–Pt(1)–N(1)
N(8Ј)–Pt(1)–Cl(1)
N(1)–Pt(1)–Cl(1)
O(7)–C(7)–C(6)
O(7Ј)–C(7Ј)–C(6Ј)
Pt(1)–N(1Ј)–C(2Ј)
N(8Ј)–C(7Ј)–C(6Ј)
97.2(3)
175.4(2)
87.1(2)
118.7(7)
114.4(8)
127.6(6)
112.1(8)
PtN Cl plane [Pt(1), N(1), N(8Ј), N(1Ј) and Cl(1)] is 0.0177 Å.
3
a
Primes are used to distinguish atoms belonging to two different lig-
The coordination of the Pt() atom also generates a five-
membered ring and the torsion angles Pt(1)–N(8Ј)–C(7Ј)–C(6Ј)
and Pt(1)–N(1Ј)–C(6Ј)–C(7Ј) are 3.9(10)Њ and 9.7(9)Њ, respect-
ively. The Pt(1)–N(8Ј) and Pt(1)–N(1Ј) bond lengths are shorter
than the corresponding Pt–N bonds in 1 and come close to the
average value. The Pt–Cl bond in 2 is similar to the normal
ands.
Ϫ
value expected for Cl trans to a pyridine N [2.336(3) Å]
and the N(8Ј)–Pt(1)–Cl(1) angle is in agreement with linear
coordination [175.4(2)Њ].
The ring-closed ligand in complex 2 adopts a configuration in
which the N(1Ј) atom is cis to N(8Ј). The torsion angle N(8Ј)–
C(7Ј)–C(6Ј)–N(1Ј) is Ϫ9.2(11)Њ. Rotation of the amide group by
1
80Њ about the C(7)–C(6) bond places the amide nitrogen atom
and pyridine nitrogen on different sides, thus giving a ring-
opened ligand in which the torsion angle N(8)–C(7)–C(6)–N(1)
is Ϫ135.7(8)Њ. Deprotonation of the ring-closed ligand makes
the N(8Ј)–C(7Ј) bond shorter than that of N(8)–C(7) in the
ring-opened ligand, indicating some double-bond character in
N(8Ј)–C(7Ј). The ring orientation in complex 2 can be defined
as follows. Labeling the phenyl ring in the ring-closed ligand as
I [C(9Ј)–C(14Ј)], the pyridyl ring in the ring-opened ligand as II
[
N(1)–C(6)], the phenyl ring in the ring-opened one as III [C(9)–
2
7
Fig. 1 An ORTEP view of complex 1 (symmetry code A Ϫx ϩ 2, y,
Ϫz ϩ 1).
C(14)], and the best metal coordination plane C (PtN Cl), it is
3
found that the ligands are oriented differently. The dihedral
angles between best planes C–I, C–II, C–III, I–II, I–III and
II–III are 62.84(3)Њ, 64.47(2)Њ, 61.02(2)Њ, 38.58(4)Њ, 78.39(2)Њ
and 69.47(2)Њ, respectively.
(
2.081(6) Å) is 0.06–0.1 Å longer than the normal values found
for Pt–N. This may be due to the different electron-donating
abilities of amide N and pyridine N atoms. The C(9)–N(8)
30
bond distance is longer than that in [Cu(bpb)(H O)] [1.398(3)
2
ESMS Studies
31
Å] and Ni(bpb) [1.412(3) Å] (bpb = 1,2–bis(2–pyridine-
32
carboxamido)benzene). The phenyl rings are not in the PtN4
The ESMS spectra of 1, 2 and 3 were recorded in CH Cl/
3
plane. The dihedral angle between the phenyl ring I [C(9)–
MeOH solution. Fig. 3a shows the ESMS spectrum of complex
1. As can be seen, there are four major species and the highest
C(14)] and the PtN plane (identified as C) is 64.6(4)Њ and the
4
angle between the two phenyl rings is 8.1(3)Њ. This conform-
ation may be stabilized by the π–π stacking interactions
between the two phenyl rings, with the shortest interplanar
atom–atom separation being ca. 3.04 Å [C(9) ؒ ؒ ؒ C (9A)].
Since the density of complex 2 determined by the buoyancy
peak (peak 1) has an m/z value of 618.3 and can be assigned to
ϩ
positively charged [PtL ] (Fig. 3b). The m/z value and isotopic
2
distribution of peak 2 (719.8) corresponded to complex [PtL -
2
ϩ
ϩ
Na CH OH] , peak 3 (1235.7) to [Pt L ] and peak 4 (1256.9)
to [Pt L Na] . The calculated molecular mass and the iso-
3
3
2
4
ϩ
2
4
Ϫ3
method was 0.823 mg mm , the value of Z was confined to 1
topic distribution of these peaks match perfectly with the
5
94
J. Chem. Soc., Dalton Trans., 2002, 591–597

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Table 4 Observed and calculated molecular masses of the complexes
1
, 2, 3 and of compound 1 ϩ LiCl after a few days
a
Peak
Compound
Observed mass
Calculated mass
ϩ
1
2
3
4
5
6
6
7
8
9
0
1
2
[PtL2]
617.3–622.2
718.7–722.7
1232.8–1239.7
1255.0–1262.1
617.3–622.3
675.1–681.1
653.1–659.1
1269.0–1277.9
1326.9–1335.9
475.5–480.1
640.7–644.6
654.7–658.7
475.3–481.1
617.6
718.6
1234.1
1257.1
617.6
676.0
654.0
1270.6
1330.0
478.3
640.6
655.0
478.3
ϩ
[PtL Na CH OH]
2
3
3
ϩ
[Pt L ]
2
4
ϩ
[Pt L Na]
2
4
ϩ
[PtL2]
[PtL ClNa]
ϩ
2
ϩ
Ј
[PtL Cl Ϫ H]
2
ϩ
[Pt L Cl]
2
4
ϩ
[Pt L Cl Na]
2
4
2
Ϫ
[PtLCl2]
[PtL Na]
2
ϩ
[PtL Cl]
ϩ
1
1
1
2
Ϫ
[PtLCl2]
a
The peaks are separated by 1 m/z in the mass region indicated.
ϩ
ϩ
to [PtL ClNa] (peak 6, 676.1), [Pt L Cl] (peak 7, 1271.7) and
2
2
4
ϩ
[
Pt L Cl Na] (peak 8, 1329.9), respectively. The expanded
2 4 2
spectrum of peak 6Ј is shown in Fig. 4c. These data suggest that
in solution complex 2 can easily undergo ring-closure to give
Fig. 3 (a) The ESMS spectrum of complex 1 in CHCl /CH OH. (b)
3
3
ϩ
ϩ
complex 1 ([PtL ] ).
The expanded spectrum of peak 1 which is assigned to [PtL ] .
2
2
Fig. 5a shows the ESMS spectrum of complex 3. The major
corresponding formulae (see Fig. S2). The data suggest that
complex 1 in solution was the same as in the solid state and that
both ligands exist in the chelated form.
The ESMS spectrum of complex 2 is shown in Fig. 4a. There
Fig. 5 (a) The ESMS spectrum of complex 3 in CHCl /CH OH. (b)
3
3
Ϫ
The expanded spectrum of peak 9 ([PtLCl ] ).
2
peak detected (peak 9, 477.3, Fig. 5b) can be assigned to neg-
Ϫ
atively charged [PtLCl ] . The ESMS data of the complexes are
2
summarised in Table 4, together with the calculated values.
Fig. 6a shows the ESMS spectrum of a mixture of complex 1
and LiCl, which was recorded immediately after mixing (about
2
6
min). As can be seen, the highest peak has a m/z value of
56.7 (peak 11, Fig. 6b) and can be assigned to positively
ϩ
charged [PtL Cl] , which is similar to peak 6Ј (Fig. 4c) although
2
peak 6Ј is less resolved. However, the peak (618.3) representing
the double ring-closed complex 1 has nearly disappeared, and a
ϩ
small amount of [PtL Na] (peak 10, 641.6) exists in the spec-
2
trum (Fig. 6a). This suggests that complex 1 undergoes a facile
chelate-ring opening reaction in the presence of excess Cl to
give complex 2 and the process is completed within a few
minutes. About 2 days after mixing, complex 3 appeared in the
ESMS spectrum. Fig. 7a shows the spectrum of the mixture
recorded 3 days after mixing. The highest peak has a m/z value
Fig. 4 (a) The ESMS spectrum of complex 2 in CHCl /CH OH. (b)
3
3
Ϫ
ϩ
The expanded spectrum of peak 5 ([PtL ] ). (c) The expanded spectrum
2
ϩ
ϩ
of peak 6Ј ([PtL Cl–H] ) and 6 ([PtL ClNa] ).
2
2
are 5 major species detected in the spectrum, among them, the
highest peak (peak 5) has an m/z value of 618.3 (Fig. 4b)
ϩ
which can also be assigned to positively charged [PtL ] , which
of 477.2 (peak 12, Fig. 7b) and can be attributed to negatively
2
Ϫ
is identical to peak 1 in Fig. 3b. The other peaks can be assigned
charged [PtLCl ] , which is identical to peak 9 (Fig. 5b). This
2
J. Chem. Soc., Dalton Trans., 2002, 591–597
595

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1
Table 5 H NMR chemical shifts of complex 2
1
H resonance
Chemical shift (ppm)
2
3
4
5
8.79d
7.22t
7.78t
7.72d
7.78t
7.22t
1
1
0 ϩ 14
1 ϩ 13
2
3
4
5
Ј
Ј
Ј
Ј
9.31d
7.58t
8.13t
7.93d
6.84d, 6.81d
1
0Ј ϩ 11Ј ϩ 13Ј ϩ 14Ј
1
of the H signals are extremely overlapped, detailed assign-
Ϫ
ments cannot be made. The addition of Cl to a fresh solution
of complex 1 gives complex 2 as shown by ESMS, however, the
1
process is too fast to be followed by H NMR. Fig. 8 shows a
Fig. 6 (a) The ESMS spectrum of complex 1 ϩ LiCl in CHCl3/
CH OH immediately after mixing (about 2 min). (b) The expanded
3
ϩ
spectrum of peak 11 ([PtL Cl] ).
2
Fig. 8 The 2D COSY NMR spectrum of complex 1 recorded 48 h
after the addition of LiCl (50 mM). The peaks labeled * may
correspond to the formation of complex 3 and free ligand.
Fig. 7 (a) The ESMS spectrum of complex 1 ϩ LiCl in CHCl3/
CH OH 3 days after mixing. (b) The expanded spectrum of peak 12
3
Ϫ
(
[PtLCl ] ).
2D COSY NMR spectrum for 1 recorded 48 h after the add-
2
ition of LiCl (in a mixture of CD OD/CDCl in 1 : 4 ratio). The
3
3
suggests that the ring-opened ligand in complex 2 can be
further replaced by Cl to give rise to complex 3 instead of
double ring-opened complexes. This is also confirmed by NMR
spectroscopy.
major peaks can be assigned to complex 2 and their chemical
shifts are listed in Table 5 (see Scheme 1 for the numbering of
the positions). It is noted that small peaks started to appear in
the spectrum (Fig. 8) and they are labeled with an asterisk.
These peaks increased in intensity with time and became
dominant species after 1 week. These are tentatively assigned
to complex 3 and free ligand (Scheme 1) as identified by the
ESMS.
Ϫ
NMR spectroscopy
1
The H NMR spectrum of free HL shows 6 aromatic signals at
8
.68d, 7.58m, 7.99t, 8.26d, 7.66d, and 7.22d ppm. These can be
assigned to the proton resonances of 2, 3, 4, 5, 10 ϩ 14, 11 ϩ
3, respectively, according to the connectivities in the 2D COSY
In vitro antitumor activity
1
spectrum (Fig. S1) (see Scheme 1 for the numbering of the
positions in HL and complex 2).
Complexes 1 and 2 demonstrated different antitumor activity
in vitro. Fig. 9 shows the inhibitory rate of complexes 1 and 2
against HL-60, BEL-7402, P388 and A-549 tumor cell lines. As
can be seen, complex 1 was found to be inactive against the
above cell lines, while complex 2 demonstrated notable activity
against those cell lines. Complex 2 is effective at a concentration
Due to the fast conversion of complex 2 into 1 in the absence
Ϫ
1
of Cl as observed by ESMS, the H NMR spectra of com-
plexes 1 and 2 in CDCl /CD OD are identical; two sets of
3
3
signals are observed, suggesting the inequivalency of the two
ligands. Because the ring-closure of complex 2 may give the
trans isomer of complex 1 (complex 4, Scheme 1) and as some
Ϫ5
Ϫ1
of 10 mol l against HL-60, BEL-7402 and A-549, tumor cell
lines and the antitumor activity decreases to zero after the
5
96
J. Chem. Soc., Dalton Trans., 2002, 591–597

View Article Online
Natural Science Foundation of China for the Distinguished
Young Scientists Fund (Grant No. 29925102) and the Major
State Basic Research Development Program (Grant No.
G200007750). One of us (Z. G.) is grateful for the support from
the Cheung Kong Scholar Program. The authors would like to
thank Mr Yong-Jiang Liu and Mr Hua-Qin Wang (Nanjing
University) for providing the X-ray structural determinations,
and Mr Chenghui Xu and Ms Weiyi Yang for conducting the
cytotoxicity assays.
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Ϫ8
Ϫ1
concentration of complex 2 was diluted to 10 mol l . In
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rates of complex 2 were 81.7, 47.7, 99.5 and 87.5%, respectively,
whereas those of complex 1 were all 0.0%, at the same concen-
1
Ϫ5
Ϫ1
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Summary
3
In this work we have synthesised three novel Pt() complexes
with a pyridine carboxamide ligand HL. The formation of
complex 1, 2 or 3 is dependent on the absence or presence of
chloride (Scheme 1). Two of the three complexes have been
structurally characterised. In complex 1 the two ligands che-
lated to Pt() are in a cis configuration, while in complex 2 one
chelate ring is opened and the two ligands are oriented in a trans
conformation.
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2
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2
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2
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Ϫ
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2
can undergo ring-closure to give complex 1 in the absence of
Ϫ
Ϫ
Cl . Complex 2 can react further with Cl to give complex 3,
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2
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3
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2
demonstrated similar activity to cisplatin against HL-60 cell
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controlled by a biological anion.
0 J. Y. Zhang, Q. Liu, Y. Xu, Y. Zhang, X. Z. You and Z. J. Guo, Acta
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1
We are grateful for financial support from the National
J. Chem. Soc., Dalton Trans., 2002, 591–597
597