Synthesis, characterization, and cytotoxicity of platinum(IV) carbamate complexes

Article abstract of DOI:10.1021/ic2000816

The synthesis, characterization, and cytotoxicity of eight new platinum(IV) complexes having the general formula cis,cis,trans-[Pt(NH₃) ₂Cl₂(O₂CNHR)₂] are reported, where R = tert-butyl (4), cyclopentyl (5), cyclohexyl (6), phenyl (7), p-tolyl (8), p-anisole (9), 4-fluorophenyl (10), or 1-naphthyl (11). These compounds were synthesized by reacting organic isocyanates with the platinum(IV) complex cis,cis,trans-[Pt(NH₃)₂Cl₂(OH)₂]. The electrochemistry of the compounds was investigated by cyclic voltammetry. The aryl carbamate complexes 7-11 exhibit reduction peak potentials near -720 mV vs Ag/AgCl, whereas the alkyl carbamate complexes display reduction peak potentials between -820 and -850 mV vs Ag/AgCl. The cyclic voltammograms of cis,cis,trans-[Pt(NH₃)₂Cl₂(O ₂CCH₃)₂] (1), cis,cis,trans-[Pt(NH ₃)₂Cl₂(O₂CCF₃) ₂] (2), and cis-[Pt(NH₃)₂Cl₄] (3) were measured for comparison. Density functional theory studies were undertaken to investigate the electronic structures of 1-11 and to determine their adiabatic electron affinities. A linear correlation (R² = 0.887) between computed adiabatic electron affinities and measured reduction peak potentials was discovered. The biological activity of 4-11 and, for comparison, cisplatin was evaluated in human lung cancer A549 and normal MRC-5 cells by the MTT assay. The compounds exhibit comparable or slightly better activity than cisplatin against the A549 cells. In MRC-5 cells, all are equally or slightly less cytotoxic than cisplatin, except for 4 and 5, which are more toxic.

Full text of DOI:10.1021/ic2000816

ARTICLE
pubs.acs.org/IC
Synthesis, Characterization, and Cytotoxicity of Platinum(IV)
Carbamate Complexes
Justin J. Wilson and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S Supporting Information
b
ABSTRACT: The synthesis, characterization, and cytotoxicity
of eight new platinum(IV) complexes having the general
formula cis,cis,trans-[Pt(NH3)2Cl2(O2CNHR)2] are reported,
where R = tert-butyl (4), cyclopentyl (5), cyclohexyl (6), phenyl
(7), p-tolyl (8), p-anisole (9), 4-fluorophenyl (10), or 1-naphthyl
(11). These compounds were synthesized by reacting organic
isocyanates with the platinum(IV) complex cis,cis,trans-[Pt-
(NH3)2Cl2(OH)2]. The electrochemistry of the compounds
was investigated by cyclic voltammetry. The aryl carbamate
complexes 7-11 exhibit reduction peak potentials near -720
mV vs Ag/AgCl, whereas the alkyl carbamate complexes display reduction peak potentials between -820 and -850 mV vs Ag/
AgCl. The cyclic voltammograms of cis,cis,trans-[Pt(NH3)2Cl2(O2CCH3)2] (1), cis,cis,trans-[Pt(NH3)2Cl2(O2CCF3)2] (2), and
cis-[Pt(NH3)2Cl4] (3) were measured for comparison. Density functional theory studies were undertaken to investigate the
electronic structures of 1-11 and to determine their adiabatic electron affinities. A linear correlation (R2 = 0.887) between
computed adiabatic electron affinities and measured reduction peak potentials was discovered. The biological activity of 4-11 and,
for comparison, cisplatin was evaluated in human lung cancer A549 and normal MRC-5 cells by the MTT assay. The compounds
exhibit comparable or slightly better activity than cisplatin against the A549 cells. In MRC-5 cells, all are equally or slightly less
cytotoxic than cisplatin, except for 4 and 5, which are more toxic.
’ INTRODUCTION
to platinum(II), must occur before aquation and DNA binding,
however (Scheme 1).
The simple coordination compound cis-diamminedichloro-
platinum(II), or cisplatin, is an effective anticancer drug that has
been used in the clinic since 1978.1 Its success has given rise to the
second-generation platinum drugs carboplatin and oxaliplatin.
These three platinum(II) complexes are believed to operate by a
similar mechanism. Aquation of the leaving groups, chloride for
cisplatin and carboxylate and oxalate for carboplatin and oxali-
platin, respectively, generates reactive cis-diam(m)ineplatinum
cations, which react readily with the purine nucleobases in
DNA.2,3 Structural distortions in DNA induced by platinum
binding4-7 trigger multiple cellular responses that ultimately lead
to cell death.8,9
Despite the clinical success of these compounds, the require-
ment for intravenous administration and associated long-term
toxic side effects10-12 diminish the quality of life for patients.
Platinum anticancer complexes in the 4þ oxidation state have
shown considerable promise both for oral administration and for
reduction of systematic toxicity.13-15 The orally administered
platinum(IV) complex satraplatin progressed as far as phase III in
clinical trials.16 The increased stability of these complexes, due to
their low-spin d6 electronic configuration, aids in their survival of
the acidic environment of the gastrointestinal tract before being
absorbed into the bloodstream. They operate by a mechanism
similar to that of the first- and second-generation platinum(II)
analogues. An activation step, namely, reduction from platinum(IV)
In addition to their kinetic stability, another favorable property
of platinum(IV) complexes relative to their platinum(II) coun-
terparts is the presence of two additional coordination sites that
can be modified to alter their pharmacokinetic properties. By
varying the two axial ligands, one can predictably alter the redox
potential17-19 and lipophilicity20-22 of the platinum(IV) com-
plex while leaving the DNA-binding cis-diammineplatinum moi-
ety unaltered. Furthermore, the axial coordination positions
serve as binding sites for other biologically active ligands, which
may have synergistic effects with platinum therapy, as demon-
strated by us23-25 and by others.26,27 The ability to tether
platinum(IV) complexes via the axial ligands to various nanode-
livery devices for increased cellular uptake and selectivity28-35 is
another advantage. The design of new platinum(IV) anticancer
complexes, however, is limited by the current synthetic
methodology.36-42 Most of the newly tested platinum(IV)
complexes bear either chloro, hydroxo, or carboxylato axial
ligands. The development of new synthetic methodologies for
accessing the platinum(IV) manifold can expand the range of
complexes having novel properties.
Received: January 13, 2011
Published: March 01, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
Scheme 1
resulting yellow solution was filtered through Celite. Pentane (∼10 mL)
was layered on top of the THF solution, and the mixture was kept at -
40 °C for 1 h to afford pale-yellow microcrystals of 2. These microcrystals
were collected by vacuum filtration and washed with pentane before being
dried in vacuo. Yield: 0.127 g (56%). Mp: 194-196 °C. 1H NMR (400
MHz): δ6.63 (br s, 6H). 13C{1H} NMR (100 MHz): δ161.8 (q, 2JCF = 37
1
Hz), 111.7 (q, JCF = 288 Hz). 19F{1H} NMR (377 MHz): δ -73.6.
195Pt{1H} NMR (86 MHz): δ 1182. IR (KBr, cm-1): 3426 m br, 3280 s,
3232 s, 3197 s, 3075 m, 1722 vs, 1559 w, 1382 m, 1331 m, 1212 s, 1162 vs,
1034 w, 859 w, 781 m, 739 m, 524 w. Anal. Calcd for 2, C4H6Cl2F6N2O4Pt:
C, 9.13; H, 1.15; N, 5.32. Found: C, 9.38; H, 1.21; N, 5.31.
The synthesis of platinum(IV) complexes with axial methyl,
ethyl, and isopropyl carbamato ligands was described over
10 years ago.36 Since then, their biological properties have only
rarely been explored,43 and further investigations of the scope of
this synthetic methodology have not been pursued. In the
present work, we report both a modification and an expansion
of this approach through the synthesis of eight new platinum(IV)
complexes of both alkyl and aryl carbamates as well as a brief
investigation of their biological activity. Computational density
functional theory (DFT) studies were undertaken to gain a
deeper understanding of the electronic structure of these new com-
plexes. The results presented here indicate that platinum(IV)
carbamates are a promising new class of anticancer drug
candidates.
General Synthesis of cis,cis,trans-[Pt(NH3)2Cl2(O2CNHR)2].
To a suspension of cis,cis,trans-[Pt(NH3)2Cl2(OH)2] (0.20 g, 0.60
mmol) in 1 mL of DMF was added a 1 mL DMF solution containing
4 mol equiv of the isocyanate. The resulting mixture was stirred for 12 h
at room temperature, resulting in the formation of a homogeneous
solution. The solution was filtered, and the desired product was
precipitated by the addition of diethyl ether. The solid was collected
by either filtration or centrifugation. To remove residual DMF, the solid
was suspended in water for 30 min, isolated by centrifugation, resus-
pended in ethanol, isolated by centrifugation, resuspended in diethyl
ether, isolated by centrifugation, and finally dried under vacuum.
Compound 4. R = tert-butyl. White solid. Yield: 0.153 g (48%). Mp:
1
238-245 °C (dec). H NMR (400 MHz): δ 6.65 (br, 6H), 6.05 (br,
’ EXPERIMENTAL SECTION
2H), 1.17 (s, 18H). 13C{1H} NMR 100 MHz): δ 162.8, 49.4, 29.3.
195Pt{1H} NMR (86 MHz): δ 1276. IR (KBr, cm-1): 3387 vs, 3301 m,
3220 s, 2975 m, 2931 w, 1640 vs, 1629 vs, 1505 vs, 1462 m, 1393 w,
1366 m, 1281 vs, 1211 s, 1079 m, 943 m, 788 w, 727 w, 645 w, 569 w,
434 w. ESI-MS (negative-ion mode): m/z 531.1 [M-]. Anal. Calcd for 4,
C10H26Cl2N4O4Pt: C, 22.56; H, 4.92; N, 10.52. Found: C, 23.14; H,
4.83; N, 10.65.
General Considerations. All reactions were carried out under
normal atmospheric conditions. Solvents were used as received without
additional drying or purification. All isocyanates were used as received
from commercial vendors. The compounds cis,cis,trans-[Pt(NH3)2-
Cl2(OH)2], cis,cis,trans-[Pt(NH3)2Cl2(O2CCH3)2] (1), and cis-[Pt-
(NH3)2Cl4] (3) were synthesized as previously described44,45 using
cisplatin purchased from Strem Chemicals, Inc., as the starting material.
Physical Measurements. NMR measurements were recorded on
a Bruker DPX-400 spectrometer in the MIT Department of Chemistry
Instrumentation Facility at 20 °C with deuterated dimethyl sulfoxide
(DMSO-d6) as the solvent. All NMR chemical shifts (δ) are reported in
Compound 5. R = cyclopentyl. White solid. Yield: 0.240 g (72%).
Mp: 208-214 °C (dec). 1H NMR (400 MHz): δ 6.67 (br, 6H), 6.55
(br, 2H), 3.78-3.71 (m, 2H), 1.68-1.34 (m, 16H). 13C{1H} NMR
(400 MHz): δ 163.4, 52.7, 32.5, 23.3. 195Pt{1H} NMR (86 MHz): δ
1274 (major), 1262 (minor). IR (KBr, cm-1): 3402 s, 3354 vs, 3243 vs,
2959 s, 2869 m, 1629 vs, 1509 vs, 1358 m, 1297 s, 1252 s, 1099 w,
1037 w, 1008 w, 953 w, 782 w, 581 w. ESI-MS (negative-ion mode): m/z
555.0 [M-]. Anal. Calcd for 5, C12H26Cl2N4O4Pt: C, 25.91; H, 4.71; N,
10.07. Found: C, 25.76; H, 4.66; N, 10.29.
Compound 6. R = cyclohexyl. White solid. Yield: 0.287 g (81%).
Mp: 228-233 °C (dec). 1H NMR (400 MHz): δ 6.67 (br, 6H), 6.47
(br, 2H), 3.19 (br, 2H), 1.71-1.50 (m, 10H), 1.22-1.02 (m, 10H).
13C{1H} NMR (100 MHz): δ 163.0, 50.0, 33.1, 25.3, 24.9. 195Pt{1H}
NMR (86 MHz): δ 1276 (major), 1263 (minor). IR (KBr, cm-1):
3375 vs, 3304 m, 3239 s, 2931 s, 2853 m, 1628 vs, 1500 vs, 1244 s,
1035 m, 931 w, 783 w, 724 w, 706 w, 587 w. ESI-MS (negative-ion
mode): m/z 583.0 [M - H]-. Anal. Calcd for 6, C14H30Cl2N4O4Pt: C,
28.77; H, 5.17; N, 9.59. Found: C, 28.89; H, 5.20; N, 9.44.
Compound 7. R = phenyl. Yellow solid. Yield: 0.214 g (60%). Mp:
171-176 °C (dec). 1H NMR (400 MHz): δ 9.12 (br, 2H), 7.47 (d, 4H),
7.18 (t, 4H), 6.86 (t, 2H), 6.79 (br, 6H). 13C{1H} NMR (100 MHz): δ
160.7, 140.7, 128.3, 121.0, 118.0. 195Pt{1H} NMR (86 MHz): δ 1265. IR
(KBr, cm-1): 3243 s br, 1654 vs, 1595 s, 1514 s, 1438 s, 1393 s, 1315 s,
1227 s, 1045 m, 1025 m, 754 m, 691 m. ESI-MS (negative-ion mode):
m/z 570.9 [M - H]-. Anal. Calcd for 7, C14H18Cl2N4O4Pt: C, 29.38;
H, 3.17; N, 9.79. Found: C, 29.56; H, 3.07; N, 9.67.
Compound 8. R = p-tolyl. Pale-orange solid. Yield: 0.229 g (64%).
Mp: 149-151 °C (dec). 1H NMR (400 MHz): δ 9.02 (br, 2H), 7.36 (d,
4H), 6.98 (d, 4H), 6.79 (br, 6H), 2.20 (s, 6H). 13C{1H} NMR (100
MHz): δ 160.8, 138.2, 129.7, 128.7, 118.1, 20.4. 195Pt{1H} NMR (86
MHz): δ 1264. IR (KBr, cm-1): 3404 s, 3353 s, 3217 br vs, 2924 w, 1663
vs, 1635 s, 1592 m, 1522 vs, 1502 m, 1404 m, 1314 s, 1289 s, 1250 m,
1223 vs, 1041 s, 817 m, 776 m, 740 w, 654 w, 582 w, 510 w. ESI-MS
1
parts per million (ppm) and referenced as described below. H and
13C{1H} NMR spectra were referenced internally to residual solvent
peaks, and chemical shifts are expressed relative to tetramethylsilane,
SiMe4 (δ = 0 ppm). 195Pt{1H} and 19F{1H} NMR spectra were
referenced externally using standards of K2PtCl4 in D2O (δ = -1628
ppm) and trifluorotoluene (δ = -63.72 ppm), respectively. Fourier
transform infrared spectra were recorded with a ThermoNicolet Avatar
360 spectrophotometer running the OMNIC software. Samples were
prepared as KBr disks. Cyclic voltammograms were obtained at room
temperature using a VersaSTAT3 potentiostat from Princeton Applied
Research accompanied by the V3 Studio software. A three-electrode
system was used comprising glassy carbon as the working electrode, a Pt
wire as the auxiliary electrode, and Ag/AgCl (aqueous, saturated NaCl)
as the reference electrode. Samples were prepared as 2 mM solutions
in N,N-dimethylformamide (DMF) with 0.1 M (n-Bu4N)PF6 as the
supporting electrolyte. Reported values are peak potentials of the
irreversible reduction event at a scan rate of 100 mV s-1. Under the
3
conditions described here, the reversible ferrocene/ferrocenium
redox couple was consistently found between 0.54 and 0.55 V vs
Ag/AgCl. Electrospray ionization mass spectrometry (ESI-MS) mea-
surements were acquired on an Agilent Technologies 1100 series
LC-MSD trap.
Synthesis of cis,cis,trans-[Pt(NH3)2Cl2(O2CCF3)2] (2). The
compound cis,cis,trans-[Pt(NH3)2Cl2(OH)2] (0.144 g, 0.429 mmol)
was suspended in 2 mL of trifluoroacetic anhydride. The mixture was
stirred for 1 h at room temperature, open to air, at which point the
volatile anhydride had evaporated, leaving a white residue. A 2 mL
volume of tetrahydrofuran (THF) was added to the residue, and the
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Inorganic Chemistry
ARTICLE
Table 1. Summary of the X-ray Crystallographic Information and Data Collection Parameters for 4-7
4 3DMSO
5 2DMF
6 2DMSO
7 DMSO 0.5H2O
3
3
3
3
3
formula
fw
C
16H44Cl2N4O7PtS3
C18H40Cl2N6O6Pt
702.55
C18H42Cl2N4O6PtS2
740.67
C16H25Cl2N4O5.5PtS
659.45
766.72
space group
a, Å
P1
P1
Pbcn
P21/c
8.9361(10)
10.5917(12)
16.762(2)
89.100(2)
89.454(2)
72.258(2)
1510.8(3)
2
9.0048(9)
12.6877(13)
13.2385(14)
116.484(2)
93.069(2)
92.288(2)
1348.4(2)
2
25.5442(17)
10.7982(7)
20.7211(13)
11.5530(5)
31.3292(13)
13.5070(6)
b, Å
c, Å
R, deg
β, deg
γ, deg
V, Å3
Z
109.7380(10)
5715.5(6)
8
4601.6(3)
8
F
calcd, g cm-3
1.685
1.730
1.721
1.904
3
T, °C
-173(2)
5.066
-173(2)
5.444
-173(2)
5.281
-173(2)
6.458
μ(Mo KR), mm-1
θ range, deg
total no. of data
no. of unique data
no. of parameters
completeness to θ (%)
R1a (%)
2.02-28.25
30 171
1.72-29.73
29 870
1.59-29.18
118 407
7718
1.73-25.11
75 374
8189
7386
7585
337
304
323
546
98.9
98.7
100.0
99.8
2.91
2.38
4.27
3.44
wR2b (%)
5.50
5.00
5.66
4.96
GOFc
1.040
1.039
1.021
1.044
max, min peaks, e Å-3
1.749, -1.229
1.879, -2.044
1.672, -0.787
1.264, -0.733
3
2 2
a R1 = ∑ Fo| - |Fc /∑|Fo|. b wR2 = {∑[w(Fo2 - Fc2)2]/∑[w(Fo ) ]}1/2. c GOF = {∑[w(Fo2 - Fc2)2]/(n - p)}1/2, where n is the number of data and p is
the number of refined parameters.
Table 2. Summary of the X-ray Crystallographic Information and Data Collection Parameters for 8-11
8 2DMSO 0.74H2O
9 acetone
10 3.5DMF
11 3DMF
3
3
3
3
3
formula
fw
C
20H35Cl2N4O6.74PtS2
C19H28Cl2N4O7Pt
690.44
C24.5H40.5Cl2F2N7.5O7.5Pt
864.13
C31H43Cl2N7O7Pt
891.71
769.55
space group
a, Å
P1
P21/c
P21
P1
11.6514(7)
14.7016(8)
18.4270(11)
68.7800(10)
78.9840(10)
84.3020(10)
2886.6(3)
4
18.9166(10)
9.5089(5)
13.7632(7)
18.0923(7)
7.0254(3)
11.7178(14)
12.0190(15)
13.2152(16)
87.170(2)
77.548(2)
75.264(2)
1757.6(4)
2
b, Å
c, Å
26.4980(11)
R, deg
β, deg
γ, deg
V, Å3
Z
92.5790(10)
104.0310(10)
2473.2(2)
4
3267.6(2)
4
F
calcd, g cm-3
1.771
1.854
1.757
1.685
3
T, °C
-173(2)
5.234
-173(2)
5.935
-163(2)
4.524
-173(2)
4.199
μ(Mo KR), mm-1
θ range, deg
total no. of data
no. of unique data
no. of parameters
completeness to θ (%)
R1a (%)
1.49-29.32
60 845
2.16-28.74
51 159
6409
1.24-28.66
68 723
16 640
903
1.58-28.40
35 819
15 609
8716
706
304
457
98.7
99.7
99.8
98.8
4.36
2.82
2.83
2.27
wR2b (%)
8.05
4.83
5.70
4.21
GOFc
1.029
1.030
1.039
1.037
max, min peaks, e Å-3
3.119, -2.625
2.131, -0.468
1.221, -1.191
0.942, -0.712
3
a R1 = ∑ Fo| - |Fc /∑|Fo|. b wR2 = {∑[w(Fo2 - Fc2)2]/∑[w(Fo ) ]}1/2. c GOF = {∑[w(Fo2 - Fc2)2]/(n - p)}1/2, where n is the number of data and p is
2 2
the number of refined parameters.
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Inorganic Chemistry
ARTICLE
Scheme 2
Table 3. 195Pt and Selected 1H NMR Shifts for 4-11 in
DMSO-d6 at 20 °C
(dec). 1H NMR (400 MHz): δ 9.01 (s, 2H), 8.19-8.17 (m, 2H), 7.88-
7.85 (m, 2H), 7.67 (d, 2H), 7.62 (d, 2H), 7.50-7.40 (m, 6H), 6.82 (br,
6H). 13C{1H} NMR (100 MHz): δ 161.9, 135.5, 133.7, 127.9, 127.5,
125.7, 125.6, 125.2, 123.5, 123.4, 120.6. 195Pt{1H} NMR (86 MHz): δ
1269. IR (KBr, cm-1): 3407 s, 3355 s, 3211 s br, 3063 m, 1654 vs, 1566
m, 1490 s, 1410 s, 1341 s, 1247 s, 1101 w, 999 w, 791 m, 772 s, 544 w.
ESI-MS (negative-ion mode): m/z 669.8 [M - H]-. Anal. Calcd for 11,
C22H22Cl2N4O4Pt: C, 39.30; H, 3.30; N, 8.33. Found: C, 39.44; H, 3.29;
N, 8.39.
δ 1H, carbamate
compound
δ
195Pt, ppm
NH, ppm
δ 1H, NH3, ppm
4
1276
6.05
6.55
6.47
9.12
9.02
8.98
9.21
9.01
6.65
6.67
6.67
6.79
6.79
6.77
6.78
6.82
5
1275 (major), 1262 (minor)
6
1276 (major), 1263 (minor)
7
1265
1264
1265
1265
1269
X-ray Crystallographic Studies. Single crystals were mounted in
Paratone oil on a cryoloop and frozen under a 110 or 100 K KRYO-
FLEX nitrogen cold stream. Data were collected on a Bruker APEX
CCD X-ray diffractometer with graphite-monochromated Mo KR
radiation (λ = 0.710 73 Å) controlled by the APEX2 software package.46
Absorption corrections were applied using SADABS.47 The structures
were solved using direct methods and refined on F2 with the SHELXTL-
97 software package.48,49 Structures were checked for higher symmetry
using PLATON.50 All non-hydrogen atoms were located and refined
anisotropically. Unless otherwise stated, hydrogen atoms were placed in
idealized locations and given isotropic thermal parameters equivalent to
either 1.5 (terminal CH3 or NH3 hydrogen atoms) or 1.2 times the
thermal parameter of the atom to which they were attached. Structure
refinement was carried out using established strategies.51 Specific details
regarding crystal growth and refinement are described in the Supporting
Information (SI), and parameters are shown in Tables 1, 2, and S1 in the
SI. Selected bond lengths and angles for 2 are listed in Table S2 in the SI.
Theoretical Calculations. DFT calculations were performed
using the Gaussian 03 (revision D01) software package.52 Geometry
optimizations, frequency calculations, and molecular orbital generations
were all carried out using the B3LYP functional.53,54 For the light atoms
(carbon, hydrogen, chlorine, nitrogen, oxygen, and fluorine), the
6-31þþG(d,p) basis set55 was used, and for platinum, the LANL2DZ
basis set and effective core potential56 were used. No solvation models
were employed; the results described for all complexes are in the gas
phase. Frequency calculations were carried out on all optimized geo-
metries to verify the absence of imaginary values. To determine adiabatic
electron affinities, an additional set of geometry optimizations and
energy calculations were performed for the analogous monoanionic
platinum(III) complexes with a doublet spin state. The difference in
energy between the platinum(III) anion and neutral platinum(IV)
8
9
10
11
(negative-ion mode): m/z 599.0 [M - H]-, 1199.0 [2M - H]-. Anal.
Calcd for 8, C16H22Cl2N4O4Pt: C, 32.01; H, 3.69; N, 9.31. Found: C,
32.14; H, 3.75; N, 9.51.
Compound 9. R = p-anisole. Pale-orange solid. Yield: 0.190 g
(50%). Mp: 115-117 °C (dec). 1H NMR (400 MHz): δ 8.98 (br, 2H),
7.37 (d, 4H), 6.77 (br d, 10H), 3.68 (s, 6H). 13C{1H} NMR (100 MHz):
δ 160.9, 153.8, 134.0, 119.5, 113.5, 55.1. 195Pt{1H} NMR (86 MHz): δ
1265. IR (KBr, cm-1): 3223 s br, 2835 w, 1647 vs, 1515 vs, 1410 m, 1298
s, 1223 vs, 1178 m, 1029 s, 827 m, 777 w, 739 w, 652 m, 583 w, 527 w.
ESI-MS (negative-ion mode): m/z 630.9 [M]-. Anal. Calcd for 9,
C16H22Cl2N4O4Pt: C, 30.39; H, 3.51; N, 8.86. Found: C, 30.45; H, 3.48;
N, 8.73.
Compound 10. R = 4-fluorophenyl. Pale-yellow solid. Yield: 0.237
1
g (65%). Mp: 208-210 °C (dec). H NMR (400 MHz): δ 9.21 (br,
2H), 7.48-7.45 (m, 4H), 7.02 (app t, 4H), 6.78 (br, 6H). 13C{1H}
NMR (100 MHz): δ 160.7, 156.9 (d, 1JCF = 236 Hz), 137.1, 119.5, 114.7
2
(d, JCF = 22.0 Hz). 19F{1H} NMR (377 MHz): δ -125.2 (s, 2F).
195Pt{1H} NMR (86 MHz): δ 1265. IR (KBr, cm-1): 3367 s, 3238 s br,
1657 vs, 1512 vs, 1407 s, 1389 m, 1305 m, 1259 s, 1213 vs, 1157 w, 1101
w, 1037 s, 832 s, 653 m, 584 w, 513 w. ESI-MS (negative-ion mode): m/z
606.9 [M - H]-, 1216.0 [2M - H]-. Anal. Calcd for 10,
C14H16Cl2F2N4O4Pt: C, 27.64; H, 2.65; N, 9.21. Found: C, 27.42; H,
2.50; N, 9.02.
Compound 11. R = 1-naphthyl. Pale-orange solid. Yield: 0.093 g
(46%, using 0.30 mmol of the starting material). Mp: 152-157 °C
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Figure 1. Variable-temperature 195Pt (left) and 1H (right) NMR spectra of 5 in DMSO-d6.
independent experiments, each of which consisted of three replicates per
concentration level. Dilutions of the platinum(IV) compounds in
growth medium were prepared from concentrated solutions (10-
20 mM) in DMSO. Cisplatin was diluted from a phosphate-buffered
saline solution (2 mM).
Chart 1
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The synthesis of platinum(IV)
carbamate complexes was accomplished by treating cis,cis,
trans-[Pt(NH3)2Cl2(OH)2] with the desired isocyanate in a
DMF solution (Scheme 2). Because the starting complex, cis,
cis,trans-[Pt(NH3)2Cl2(OH)2], is largely insoluble in DMF, the
progress of the reaction was monitored visually by observing
conversion of the reaction mixture from a suspension to a
homogeneous solution. The synthesis of the analogous methyl,
ethyl, and isopropyl carbamate complexes has been reported
previously.36 In this prior study, the authors prepared these
complexes by suspending cis,cis,trans-[Pt(NH3)2Cl2(OH)2] in
neat isocyanate with no additional solvent. In cases where
isocyanate is expensive or toxic, the use of only a slight excess,
as demonstrated in the present work, is a clear advantage.
Platinum(IV) carbamates are either white (4-6) or pale-
yellow to pale-orange (7-11) solids. Compounds 7-11 are the
first reported platinum(IV) complexes bearing aryl carbamate
ligands and thus extend the scope of the chemistry beyond simple
alkyl isocyanates. Compounds 4-11 exhibit good solubility in
DMF and DMSO, moderate solubility in THF, acetonitrile, and
acetone, and poor solubility in water and halogenated organic
solvents. In solution, the aryl carbamate complexes, 7-11,
decompose to dark-brown solutions when exposed to ambient
light over the course of several hours. The alkyl carbamate
complexes, 4-6, remain stable in solution even in the presence
of light. The aromatic substituents of 7-11 most likely play a role
in the photodecomposition of the complexes. In the absence of
light, all of the compounds are stable in solution.
complexes is the adiabatic electron affinity of the latter. The geometry
optimization of the one-electron-reduced analogue of 3 gave rise to a
structure with an imaginary frequency. Hence, its adiabatic electron
affinity was not computed. Atomic coordinates, energies, and the lowest
frequency of all optimized structures are provided in Tables S3-S23 in
the SI.
Cell Lines and Culture Conditions. Human A549 (lung
carcinoma) and human MRC-5 (normal lung fibroblasts) were grown
as adherent monolayers in a growth medium consisting of Dulbecco’s
Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine
serum and 1% penicillin/streptomycin. The cultures were grown in
25 cm2 flasks in an incubator at 37 °C with a humidified atmosphere
composed of 5% CO2.
Cytotoxicity Assays. The colorimetric MTT assay was used to
determine the cytotoxicity of cisplatin and compounds 4-11. Trypsi-
nized A549 and MRC-5 cells were seeded into a 96-well plate at cell
densities of 1500 and 2500 cells/well, respectively, in 200 μL of growth
medium and were incubated for 24 h. The medium was then removed,
and 200 μL of new growth medium containing various concentrations of
the platinum complexes was added. After 72 h, the medium was
removed, 200 μL of a 0.8 mg/mL solution of MTT in DMEM was
added, and the plate was incubated for an additional 4 h. The DMEM/
MTT mixture was aspirated, and 200 μL of a mixture of 90% DMSO and
10% glycine buffer, pH 10.5, was added to dissolve the purple formazan
crystals. The absorbance of the plates was read at 570 nm. Absorbance
values were normalized to the platinum-free control wells and plotted as
[Pt] versus % viability. IC50 values were extrapolated from the resulting
curves. The reported IC50 values are the averages from at least three
Characterization of compounds 4-11 was accomplished by
NMR spectroscopy, IR spectroscopy, mass spectrometry, ele-
mental analysis, and X-ray crystallography (vide infra). Elemental
analyses of the complexes are in good agreement with expected
values, and electrospray ionization mass spectrometry gave rise
to the expected [M - H]- signals, further validating the
molecular formulas of these compounds. The IR spectra
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Table 4. Selected Interatomic Lengths (Å) and Angles (deg) for 4-11a
4
5
6
7b
8b
9
10b
11
Pt1-Cl1
Pt1-Cl2
Pt1-N1
Pt1-N2
Pt1-O1
Pt1-O3
2.3145(7)
2.3326(8)
2.037(2)
2.3324(6)
2.3170(6)
2.0423(19)
2.038(2)
2.3075(7)
2.3151(7)
2.042(2)
2.041(2)
2.026(2)
2.001(2)
2.3272(10)
2.3202(10)
1.993(3)
1.995(3)
1.993(3)
1.995(3)
2.3367(9)
2.3324(9)
2.034(3)
2.033(3)
2.000(3)
2.002(3)
2.3187(6)
2.3271(6)
2.029(2)
2.3201(11)
2.3127(11)
2.054(4)
2.3176(6)
2.3126(6)
2.0299(17)
2.0349(18)
2.0065(13)
2.0161(13)
2.037(2)
2.038(2)
2.048(3)
2.0117(19)
2.0062(19)
2.0088(16)
1.9970(16)
2.0230(18)
2.0113(18)
2.0209(18)
1.9936(19)
N1-Pt1-N2
Cl1-Pt1-Cl2
O1-Pt1-N1
O1-Pt1-Cl2
O3-Pt1-N2
O3-Pt1-Cl1
O1-Pt1-O3
90.00(10)
92.57(3)
96.54(9)
90.12(6)
94.89(9)
87.04(6)
174.06(8)
90.02(8)
91.99(2)
86.31(7)
87.53(5)
89.31(8)
88.30(5)
174.22(7)
93.17(10)
91.48(3)
97.67(10)
90.47(6)
93.21(9)
89.64(6)
174.17(9)
92.21(13)
92.68(4)
89.81(14)
94.64(3)
91.70(9)
90.90(2)
91.27(8)
88.69(6)
91.94(8)
87.92(5)
174.20(7)
93.49(10)
89.24(3)
92.42(7)
91.33(2)
91.81(6)
88.74(4)
96.96(6)
84.12(4)
173.33(6)
96.66(12)
86.18(8)
91.99(13)
88.04(9)
91.90(14)
89.58(10)
91.40(14)
87.67(9)
93.15(12)
88.14(8)
92.70(13)
87.07(9)
174.94(11)
168.70(11)
174.72(8)
a The numbers in parentheses are the estimated standard deviations of the last significant figures. Atoms are labeled as indicated in Figures 2 and 3. b Two
molecules per asymmetric unit are present in the crystal lattice. The parameters shown here are only for one of those molecules.
Figure 2. Molecular structures of 4-7. Ellipsoids are drawn at the 50% probability level.
displayed characteristic CdO stretching frequencies ranging
from 1647 to 1663 cm-1 for the aryl carbamate complexes and
from 1628 to 1629 cm-1 for the alkyl carbamate complexes. All
complexes also display N-H stretching frequencies derived from
the ammine ligands, which appear as a broad series of bands near
ammine coordination to a platinum(IV) center; protons of
ammines coordinated to platinum(II) centers typically lie farther
upfield, between 3 and 5 ppm.57
The 195Pt NMR spectra of 7-11 display a single resonance in
the range 1264-1269 ppm. Given that the known window for
195Pt NMR shifts is >15 000 ppm, the small variance in chemical
shifts among these complexes indicates that the peripheral substit-
uents of the aryl rings have little effect on the magnetic environment
of the platinum nucleus. The alkyl carbamate complex, 4, exhibits a
single 195Pt NMR resonance in DMSO-d6 at 1276 ppm. These
chemical shifts are in the range expected for platinum(IV)
complexes58,59 and are close to related platinum(IV) alkyl
and aryl carboxylate complexes, which fall between 1000 and
1300 ppm. Although both the 1H and 13C NMR spectra of 5 and
6 are consistent with the presence of a single species in solution,
the 195Pt NMR spectra at 20 °C in DMSO-d6 display two
resonances at approximately 1276 and 1262 ppm in relative
3200 cm-1
.
The 1H, 13C, and 19F (for 10) NMR spectra of the complexes
1
display all expected resonances. 195Pt and selected H NMR
chemical shifts are summarized in Table 3. The signal corre-
sponding to the NH proton of the carbamate ligands is observed
between 8.97 and 9.21 ppm for the aryl carbamate complexes,
7-11, and between 6.05 and 6.55 ppm for the alkyl carbamate
complexes, 4-6. This 3 ppm shift reflects significant deshielding
of the NH carbamate resonance relative to the alkyl substituents
by the aryl substituents. The proton resonances of the coordi-
nated ammine ligands appear in all complexes as broad peaks
ranging from 6.67 to 6.82 ppm. These values are consistent with
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Figure 3. Molecular structures of 8-11. Ellipsoids are drawn at the 50% probability level.
Figure 4. Three intramolecular hydrogen-bonding motifs observed in the crystal structures of 4-11. These three examples are compounds 8 (left), 7
(middle), and 5 (right).
intensities of approximately 2:1. As the temperature of the
NMR sample is increased, the two resonances eventually
coalesce (Figure 1 and S1 in the SI), between 50 and 65 °C.
Consistent with the known temperature dependence of 195Pt
NMR chemical shifts,60 the peaks are shifted downfield at
higher temperatures as well.
resonance of the carbamate ligand exhibits a significant tempera-
ture dependence, shifting upfield by 0.5 ppm at 80 °C. A small
peak in the spectrum near 5.8 ppm broadens into the baseline at
35 °C. This peak most likely corresponds to the NH resonance of
1
a minor conformational isomer. The aliphatic region of the H
NMR spectrum is unaffected by changes in temperature. The
broad peak of the coordinated NH3 protons is only slightly
affected by an increase in temperature; a small upfield shift occurs
1
This fluxional process could also be monitored by H NMR
spectroscopy, as shown in Figures 1 and S1 in the SI. The NH
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Table 5. Peak Reduction Potentials for 1-11 Measured by
why only two of these isomers are observed by 195Pt NMR
spectroscopy, but it is possible that two of the isomers have very
similar chemical shifts and are therefore not resolved as distinct
peaks. Another possibility is that one of the conformational
isomers is significantly less stable than the other two and never
accumulates in a high enough concentration to be observed
under equilibrium conditions. The use of variable-temperature
195Pt NMR spectroscopy to distinguish between chemically
similar stereoisomers has been reported before.61-64 Our find-
ings here similarly validate this method as a valuable tool for
distinguishing isomers that could not otherwise be discerned by
the more commonly used 1H and 13C NMR spectroscopy.
X-ray Crystal Structures. Complexes 4-11 were all charac-
terized by X-ray crystallography and are the first such structurally
characterized platinum carbamate complexes. Compound 2 was
also crystallographically characterized. Information about this
structure and the crystal growth conditions for 2 can be found in
the Supporting Information (Figure S2 and Tables S1 and S2).
Relevant bond distances and angles for the platinum carbamate
complexes are listed in Table 4, and the structures are shown in
Figures 2 and 3. The bond distances are typical; the Pt-Cl bond
lengths are close to 2.3 Å and the Pt-O/N distances are ∼2.0 Å.
All complexes display the expected octahedral coordination
geometry for platinum(IV). In addition, the structures all have
the same stereochemistry as that of the starting platinum(IV)
hydroxo compound (cis,cis,trans), which is retained upon for-
mation of the carbamate ligands.
Cyclic Voltammetry in DMF
compound
Ep, V vs Ag/AgCl
1
-0.72 (-0.64)a,b
-0.35 (0.01)a,c
-0.45 (-0.26)a,b
-0.85
2
3
4
5
-0.85
6
-0.82
7
-0.73
8
-0.71
9
-0.72
10
11
-0.66
-0.63
a Values in parentheses are for those measured in aqueous solution.
b Reference 66. c This work (Figure S3 in the SI).
A common feature among the eight complexes is the presence
of intramolecular hydrogen bonding between the oxygen atom of
the axial carbamate ligands and the equatorial ammine ligands.
Three different geometries are observed for this interaction, as
shown in Figure 4. In the first geometry, the oxygen atoms lie in a
plane bisecting the N-Pt-N angle and interact with both
coordinated ammines equally. This situation occurs for 6, both
molecules in the asymmetric unit of 7, one of the molecules in the
asymmetric unit of 8, and 11. The geometry in which both
of the oxygen atoms are twisted to opposite sides and hydrogen
bond with the different ammines is observed only in the case
of 5. In the final case, one of the oxygen atoms is twisted to the
side and interacts with only one of the coordinated ammines.
This geometry is the most common hydrogen-bonding motif for
this class of compounds and occurs in the remaining structures.
As discussed above, different conformational isomers, reflect-
ing alternative orientations of the substituents about the carba-
mate C-N bond, exist (Chart 1). The most commonly observed
isomer is that with a syn/syn ligand orientation, observed for 5, 6,
both molecules in the asymmetric unit of 7, both molecules in the
asymmetric unit of 8, and one of the molecules in the asymmetric
unit of 10. The anti/syn isomer occurs in 4, 9, and the other
molecule in the asymmetric unit of 10. The naphthyl carbamate
complex 11 crystallized exclusively as the anti/anti isomer. The
occurrence of all three possible isomers throughout the crystal
structures of 4-11 reflects a small energy difference between
isomers in solution, as observed by NMR spectroscopy.
Figure 5. Cyclic voltammograms of 1-11. Data obtained for 2 mM
solutions of the complexes in DMF with 0.1 M (n-Bu4N)PF6 as the
supporting electrolyte. The scan rate was 100 mV s-1
.
3
and shoulders due to coupling to 14N (I = 1) become visible. The
Cyclic Voltammetry. The biological activity of platinum(IV)
complexes is mediated by their redox chemistry. In most cases,
platinum(IV) complexes, unlike their platinum(II) progeny, do
not bind directly to DNA or other biological nucleophiles.65 The
redox potential of platinum(IV) complexes is therefore believed
to be an important factor in their efficacy as antitumor agents.
With this possibility in mind, we studied redox potentials of
1-11 by cyclic voltammetry.
1
changes in the 195Pt and H NMR spectra as a function of
the temperature are fully reversible; after increasing the tem-
perature to 80 °C, the original spectra can be obtained
at 20 °C.
As shown in Chart 1, three possible conformational isomers
exist for the complexes depending on the orientation of substit-
uents about the C-N bond of the carbamate ligand. It is not clear
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Figure 6. Qualitative molecular orbital diagrams for 1 (left), 2 (middle), and 3 (right). The x-, y-, and z-axes are defined as the bisector of the Cleq-Pt-
Cleq and NH3-Pt-NH3 angles, the bisector of the Cleq-Pt-NH3 angles, and the line comprising the Pt atom and its two axial ligands, respectively.
Because of the limited aqueous solubility of 4-11, the cyclic
voltammograms were recorded in DMF using 0.1 M (n-Bu4N)PF6
as the supporting electrolyte. For comparison, the electrochemical
properties of compounds 1-3 were also investigated by cyclic
voltammetry in the same solvent and electrolyte system. As expected
for the platinum(IV)/platinum(II) redox couple, all of the com-
pounds exhibit a single irreversible reduction event in the potential
window of þ0.4 to -1.2 V vs Ag/AgCl. The peak potentials (Ep)
by up to 300 mV. Even though the potentials measured in DMF are
shifted significantly from those measured in aqueous solution, a
comparison of peak potentials for the carbamate complexes 4-11
to those for 1-3 is valuable for understandingtherelativestabilityof
the complexes in the biological milieu.
Compounds 7-9 display nearly identical peak potentials near
-720 mV. This similarity indicates that electron-donating
groups in the para position of the aryl carbamate ligands have
little effect on the redox potentials of these compounds. Notably,
the peak potentials of these compounds are indistinguishable
from that of 1. Thus, the aryl carbamate ligands confer the same
degree of stabilization to the 4þ oxidation state as the acetate
ligand. Compound 10 exhibits a higher peak potential at -660
mV. This higher potential isattributedtothe electron-withdrawing
fluorine atom on the aromatic ring of the carbamate ligand, which
favors reduction. The least negative peak potential of the series,
at -630 mV, is displayed by the naphthyl carbamate complex 11.
The reason for 11 having a peak potential approximately 100 mV
more positive than those of 7-9 is not entirely clear. A possible
explanation is that the increased steric bulk from the large naphthyl
group favors ligand dissociation and consequently reduction. The
alkyl carbamates 4-6 have peak potentials between -820 and -
850 mV. The alkyl substituents on the carbamate ligand stabilize
the 4þ oxidation state by about 100 mV relative to the aryl
substituents. This observation indicates that alkyl carbamates are
stronger electron donors and are more capable of stabilizing the
electron-poor 4þ oxidation state than aryl carbamates.
for these processes obtained at a scan rate of 100 mV s-1 are
3
reported in Table 5, and the corresponding cyclic voltammograms
are shown in Figure 5.
For a given set of equatorial ligands on a platinum(IV) center,
the reduction potential in water predictably changes as the axial
ligands are varied.18,66 Trifluoracetate ligands produce the most
easily reduced (highest redox potential) platinum(IV) com-
plexes, followed by chloride ligands, and then acetate ligands in
that order. Axial hydroxo ligands, although not investigated here,
give rise to platinum(IV) complexes that are even more difficult
to reduce than those with acetate ligands. For example, the
precursor complex cis,cis,trans-[Pt(NH3)2Cl2(OH)2] is reported
to have a very negative peak potential at -880 mV in aqueous
media.66 In moving from water to DMF, we find here that this
general trend still exists for the chloride and carboxylate ligands,
as Ep(2) > Ep(3) > Ep(1). The peak potentials themselves, how-
ever, are shifted significantly from those measured in water. In water,
the measured peak potentials of 1 and 3 have been determined by
others to be -640 and -260 mV, respectively,66 and the peak
potential of 8 was measured to be 10 mV (Figure S3 in the SI). In
DMF, these potentials are -720 (1), -350 (2), and -450 (3) mV,
indicating that this solvent change decreases the reduction potential
Theoretical Calculations. Geometry optimizations were car-
ried out for 1-11 at the DFT B3LYP theoretical level. For the
carbamate complexes, 4-11, geometry optimizations were
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Figure 7. Qualitative molecular orbital diagram for 6 (left), 7 (middle), and 11 (right). The x-, y-, and z-axes are defined as the bisector of the Cleq-Pt-
Cleq and NH3-Pt-NH3 angles, the bisector of the Cleq-Pt-NH3 angles, and the line comprising the Pt atom and its two axial ligands, respectively.
Table 6. Computed Adiabatic Electron Affinities and the Difference in Computed Interatomic Distances between the Neutral
Platinum(IV) Complex and the Anionic Platinum(III) Complexa
Δd, Å
compound
electron affinity, eV
Pt-Cl1
Pt-Cl2
Pt-N1
Pt-N2
Pt-O1
Pt-O2
1
2.754
3.791
2.576
2.570
2.321
2.683
2.631
2.901
3.129
3.060
0.030 21
0.024 79
0.026 95
0.027 19
0.360 70
0.337 39
0.338 78
0.024 54
0.024 12
0.023 05
0.030 05
0.025 53
0.026 98
0.027 18
0.036 82
0.031 96
0.032 34
0.024 54
0.024 19
0.023 06
-0.014 39
-0.021 39
-0.008 36
-0.007 91
-0.011 19
-0.013 19
-0.012 87
-0.007 84
-0.008 97
-0.007 90
-0.014 25
-0.020 78
-0.008 32
-0.007 90
0.336 53
0.397 07
0.408 72
0.395 33
0.394 20
0.028 40
0.035 18
0.034 67
0.390 23
0.391 58
0.390 17
0.397 01
0.411 75
0.396 31
0.394 22
0.029 18
0.035 07
0.034 58
0.390 24
0.391 65
0.390 28
2
4
5
6
7
0.333 28
8
0.333 66
9
-0.007 87
-0.008 84
-0.007 92
10
11
a Atoms are labeled as shown in Figures 2 and 3.
computed only for the anti/anti isomers (Chart 1). Although no
symmetry restraints were placed on the geometry optimizations,
all optimized structures attained nearly perfect C2v symmetry,
with the principal 2-fold axis bisecting the Cl-Pt-Cl and
NH3-Pt-NH3 angles in the equatorial plane. In this configura-
tion, intramolecular hydrogen bonding occurs between the
oxygen atom of the carbamate ligands and both of the coordi-
nated ammine ligands, as observed experimentally in several of
the crystal structures.
The frontier molecular orbitals (FMOs) of 1-3 are shown in a
qualitative molecular orbital diagram in Figure 6. The expected
two-over-three d-orbital splitting for a nearly octahedral transi-
tion-metal complex is predicted by these computations. The
LUMO and LUMOþ1 are nearly degenerate and are dz2 and dxy
σ* in character. The HOMO to HOMO-3 are also close in
energy. These orbitals are dx2-y2, dxz, and dyz π*, and Cl 3p
nonbonding in character. The HOMO-LUMO gaps of 1-3 are
4.16, 3.97, and 3.70 eV, respectively. These values correspond to
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The gas-phase adiabatic electron affinities of 1, 2, and 4-11
were computed by optimizing the geometry of the one-electron-
reduced platinum(III) species as a doublet anion and then
subtracting its total energy from the neutral platinum(IV)
species. The results are summarized in Table 6. The optimized
geometries of the platinum(III) species give rise to stationary
points on the potential energy surface, as evidenced by the lack of
imaginary frequencies, except in the case of 3. A comparison of
the geometrical parameters of the calculated platinum(IV) and
platinum(III) structures is presented in Table 6. Upon reduction
to platinum(III), two mutually trans bonds are elongated sig-
nificantly, whereas the other bonds are only altered slightly. This
result is expected, for the addition of another electron to the
closed-shell platinum(IV) species would require population of an
antibonding orbital. For most cases, the axial platinum-carbamate
bond is lengthened by approximately 0.4 Å in the platinum(III)
complex, which is consistent with the conventional view of
platinum(IV) reduction depicted in Scheme 1. For 6-8, however,
significant bond elongation occurs for the equatorial, mutually
trans platinum-chloride and platinum-ammine bonds. These
bonds are elongated by approximately 0.34 Å. This result suggests
that in some cases elimination of the equatorial ligands may be a
viable reductive pathway of platinum(IV) and is consistent with
experimental observation of this pathway by others.67-69 The
computed gas-phase adiabatic electron affinities correlate wellwith
the observed peak potentials for reduction. In Figure 8, the peak
potentials are plotted as a function of the electron affinity. The
relationship is roughly linear (R2 = 0.887), and as expected, larger
electron affinities correlate with more positive peak potentials.
Biological Properties. The cytotoxicities of compounds 4-11
and cisplatin against human lung carcinoma (A549) and human
normal lung (MRC-5) cells were measured by the MTT assay.
The results are shown in Table 7, and dose-response curves
can be found in Figures S4 and S5 in the SI. In A549 cells, most
of the platinum(IV) carbamates exhibit a level of cytotoxicity
that is similar to that of cisplatin. The IC50 values range from 3
to 6.7 μM, compared to an IC50 of 7.0 μM for cisplatin.
Compounds 4 and 5, bearing tert-butyl and cyclopentyl carba-
mate ligands, showed greater cytoxicity than the other com-
pounds, with the IC50 values being 1.0 and 0.6 μM, respectively.
In the noncancerous lung fibroblasts (MRC-5), all of the
complexes, except for 4 and 5, were slightly less cytotoxic than
cisplatin. The IC50 of cisplatin in this cell line was 4.3 μM, and
those of 6-11 ranged from 4.8 to 16.0 μM. The cytotoxicities
of 4 and 5 in the lung fibroblasts were marked by IC50 values
of 2.8 and 2.3 μM, indicating that they are approximately a
factor of 2 less cytotoxic in the healthy cells compared to the
cancerous cells, as observed for the other carbamate complexes.
The results here demonstrate that, like the platinum(IV)
complexes bearing axial chloro, hydroxo, and acetato ligands,
this newly synthesized class of platinum(IV) complexes bearing
axial carbamato ligands also contains viable anticancer drug
candidates.
Figure 8. Plot of the computed adiabatic electron affinity versus the
experimentally measured reduction peak potentials. The black line is the
least-squares fit to the data.
Table 7. Cytotoxicities of Cisplatin and 4-11 in A549 and
MRC-5 Cells
IC50 (μM)a
compound
A549
MRC-5
4
1.0 ( 0.3
0.6 ( 0.3
6.7 ( 2.9
6.7 ( 2.1
3.0 ( 1.0
5.3 ( 2.5
3.7 ( 1.2
4.3 ( 1.5
7.0 ( 2.6
2.8 ( 0.8
2.3 ( 0.7
12.7 ( 3.5
16.0 ( 6.6
5.1 ( 1.8
9.3 ( 3.2
7.6 ( 3.1
4.8 ( 0.3
4.3 ( 1.3
5
6
7
8
9
10
11
cisplatin
a IC50 values, the concentrations of platinum where cell growth is
inhibited by 50% compared to controls run in the absence of added
complexes, measured by the MTT assay following a 72 h exposure.
Values are the average of at least three independent experiments, and the
reported errors are the corresponding standard deviations.
the magnitude of the d-orbital splitting and reflect the ordering of
the acetate, trifluoroacetate, and chloride ligands in the spectro-
chemical series.
The FMOs of 6, 7, and 11 are shown in the qualitative
molecular orbital diagram in Figure 7. The FMOs of the other
alkyl carbamate complexes, 4 and 5, are qualitatively similar to
those of 6, as are the FMOs of the other aryl carbamate
complexes, 8-10, to complex 7. Like 1-3, the LUMO and
LUMOþ1 of the carbamate complexes are dz2 and dxy σ* in
character. The HOMOs, however, are ligand-localized π
orbitals. The presence of these orbitals leads to smaller
HOMO-LUMO gaps for the aryl carbamate complexes,
which range from 2.52 to 2.99 eV, compared to those of 1-
3. This result is consistent with the observation that 7-11 are
sensitive to light. The smaller HOMO-LUMO gap may
render dissociative excited states accessible by the energy
provided by visible light.
’ CONCLUSIONS
The syntheses of eight new platinum(IV) carbamate com-
plexes are described. The general reactivity of both aryl and alkyl
isocyanates with the important platinum(IV) synthon cis,cis,
trans-[Pt(NH3)2Cl2(OH)2] provides a valuable synthetic meth-
odology for the design of new platinum(IV) complexes with
novel properties. It also offers a route for the functionalization of
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ARTICLE
isocyanate-bearing nanomaterials through the formation of a
platinum-carbamate bond. In contrast to the more common
platinum(IV) carboxylate complexes, platinum(IV) carbamates
adopt different isomeric forms depending on the rotational
orientation of the ligand, as revealed by NMR spectroscopy
and X-ray crystallography, and their ligand-based orbitals are
intermediate in energy between those of the empty and filled
metal d orbitals, as determined by DFT calculations. Electro-
chemical studies of platinum(IV) carbamates display redox
potentials similar to those of platinum(IV) acetates, such as
satraplatin. This result suggests that platinum(IV) carbamate
complexes should exhibit stability properties in biological
milieu similar to those of the acetate complexes. The fact that
platinum(IV) carbamates display cytotoxicities similar to or
better than that of cisplatin reveals their therapeutic potential.
Finally, DFT computational analyses indicate that the redox
potential of platinum(IV) complexes can be correlated with
their computed electron affinities, corroborating experimen-
tally deduced reduction pathways.
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’ AUTHOR INFORMATION
(30) Rieter, W. J.; Pott, K. M.; Taylor, K. M. L.; Lin, W. J. Am. Chem.
Soc. 2008, 130, 11584–11585.
(31) Dhar, S.; Daniel, W. L.; Giljohann, D. A.; Mirkin, C. A.; Lippard,
S. J. J. Am. Chem. Soc. 2009, 131, 14652–14653.
Corresponding Author
*E-mail: lippard@mit.edu.
’ ACKNOWLEDGMENT
(32) Taylor-Pashow, K. M. L.; Rocca, J. D.; Xie, Z.; Tran, S.; Lin, W.
This work was supported by Grant CA034992 from the
National Cancer Institute. Spectroscopic instrumentation at
the MIT DCIF is maintained with funding from NIH Grant
1S10RR13886-01.
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