Synthesis and reactions of platinum(IV) complexes with sodium ascorbate

Article abstract of DOI:10.1016/S0020-1693(02)01058-7

The platinum(IV) complexes [PtCl₂(OH)₂(N^∩N)] (N^∩N=en, N,N-dmen, N,N′-dmen) were prepared by oxidation of [PtCl₂(N^∩N)] with hydrogen peroxide. The complexes were characterized by multinuclea

Full text of DOI:10.1016/S0020-1693(02)01058-7

Inorganica Chimica Acta 340 (2002) 65ꢀ69
/
www.elsevier.com/locate/ica
Synthesis and reactions of platinum(IV) complexes with sodium
ascorbate
Malcolm J. Arendse a, Gordon K. Anderson b,*, Raquel N. Majola a, Nigam P. Rath b
a Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
b Department of Chemistry, University of Missouriꢀ
/St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
Received 7 January 2002; accepted 3 May 2002
Abstract
The platinum(IV) complexes [PtCl2(OH)2(NSN)] (NSNꢁ
/
en, N,N-dmen, N,N?-dmen) were prepared by oxidation of
[PtCl2(NSN)] with hydrogen peroxide. The complexes were characterized by multinuclear NMR and infrared spectroscopy, as
well as microanalysis. The reactions of these compounds with sodium ascorbate were monitored spectroscopically. Reduction of the
platinum(IV) complexes by sodium ascorbate occurred only slowly. An oxalatoplatinum(IV) complex [Pt(C2O4)Cl(OH)(N,N-
dmen)]×/H2O was isolated from the reaction of [PtCl2(OH)2(N,N-dmen)] and sodium ascorbate and characterized by an X-ray
diffraction study.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Platinum complexes; Antitumor activity; Ascorbic acid; Oxalate; Oxidation
1. Introduction
OH, Cl, OCOCH3, OCOCF3; L2ꢁ
/
en, dach or Lꢁ
/
NH2Pri) was dependent on the nature of the axial
ligands and the amine ligands [6]. For the ethylenedia-
mine complexes, the reduction rates and reduction
Platinum(IV) complexes, such as iproplatin,
[PtCl2(OH)2(NH2Pri)2], are currently used as antitumor
drugs in chemotherapy, where their higher water
solubility makes oral administration possible. These
platinum(IV) compounds have less severe side effects
than cisplatin, cis-[PtCl2(NH3)2], and they are active
against certain tumor cells that are resistant to cisplatin
potentials were found to increase in the order OHB
/
OCOCH3BClBOCOCF3, which follows the order of
/
/
increasing electron-withdrawing abilities of the ligands.
Reduction of the ethylenediamine complexes was slower
than the corresponding reactions of the diaminocyclo-
hexane or isopropylamine complexes. Hambley and
coworkers earlier investigated the electrochemical re-
[1ꢀ4]. Platinum(IV) complexes generally undergo ligand
/
substitution more slowly than their platinum(II) ana-
logues [5] and, since platinum(II) species are known to
bind to DNA, it is believed that platinum(IV) complexes
serve as prodrugs for platinum(II) derivatives. Thus,
they must be reduced in vivo before DNA binding
occurs. A number of cellular reductants could achieve
this, including cysteine, the sulfhydryl protein, glu-
tathione and ascorbic acid.
duction of platinum(IV) complexes [Pt(en)Cl2Y2] (Yꢁ
/
Cl, OH, RCOO) and found that the tetrachloro com-
plexes were reduced more readily than the complexes
with hydroxo or carboxylato axial ligands [7]. We have
investigated the effect of N-methyl substituents on the
rate of ascorbic acid reduction of ethylenediamineplati-
num(IV) complexes, and the results of this study are
presented here.
A recent study focused on the reduction of platinu-
m(IV) complexes by ascorbic acid. Choi and coworkers
found that the rate of reduction of [PtX2Cl2L2] (Xꢁ
/
2. Experimental
* Corresponding author. Tel.: ꢂ
5342
E-mail address: ganderson@umsl.edu (G.K. Anderson).
/
1-314-516-5311; fax: ꢂ1-314-516-
/
K2PtCl4, deuterium oxide, N,N-dmen and N,N?-
dmen were purchased from Sigma Aldrich. [PtI2(N,N-
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 5 8 - 7

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M.J. Arendse et al. / Inorganica Chimica Acta 340 (2002) 65ꢀ69
/
3
2.76 (s, JPtH
3
26 Hz, 6H, CH3), 2.95 (dd, JHH
dmen)] and [PtI2(N,N?-dmen)] were prepared as de-
scribed elsewhere [8]. Infrared spectra were recorded on
a Perkin Elmer 1600 FTIR spectrometer as nujol mulls.
ꢁ
/
ꢁ
/12, 3
3
Hz, 2H, CH2), 3.02 (dd, JHH
ꢁ
/
12, 3 Hz, 2H, CH2).
195Pt{1H} NMR: d 770 (s).
UVꢀ
/
Vis spectra were recorded on a GBC UVꢀVis 920
/
[PtCl2(OH)2(N,N?-dmen)]: yield 0.17 g, 87%. Anal.
Calc. for C4H16Cl2N2O2Pt: C, 12.37; H, 3.61; N, 7.22.
1
spectrophotometer. H NMR spectra were recorded on
a Varian Gemini 200 MHz spectrometer. Chemical
shifts were referenced relative to D2O at 4.80 ppm.
195Pt{1H} NMR spectra were recorded on a Varian
Unity 300 MHz spectrometer. Chemical shifts were
Found: C, 11.81; H, 3.38; N, 6.73%. 1H NMR (D2O): d
3
2.59 (s, JPtH
ꢁ32 Hz, 6H, CH3), 2.85 (s, 4H, CH2).
/
195Pt{1H} NMR: d 782 (s).
referenced to an external sample of K2PtCl4 at ꢃ1624
/
ppm relative to H2PtCl6 at 0 ppm. Microanalyses were
performed by Atlantic Microlab, Norcross, GA.
2.4. Reactions of [PtCl2(OH)2(N,N-dmen)]with
sodium ascorbate
2.1. Crystallographic measurements and structure
resolution
A solution of [PtCl2(OH)2(N,N-dmen)] (0.029 g,
0.075 mmol) and sodium ascorbate (0.15 g, 0.75
mmol) in D2O (2.0 ml) was allowed to stir at ambient
temperature for 48 h. Yellow crystals, which were
suitable for an X-ray diffraction study, separated on
standing.
Crystallographic measurements were performed on a
Bruker SMART CCD detector system single crystal X-ray
diffractometer at a temperature of 213(2) K as described
elsewhere [9]. Structure solution and refinement were
carried out using the SHELXTL-PLUS (5.03) software
package [10].
2.5. Reactions of [PtCl2(OH)2(N,N-dmen)] with
sodium oxalate
2.2. Preparation of [PtCl2(NSN)]
[PtI2(NSN)] (1.1 g, 2.0 mmol) was suspended in water
(40 ml). Silver nitrate (0.68 g, 4.0 mmol) in H2O (6 ml)
was then added to the [PtI2(NSN)] suspension. The
reaction mixture was then stirred for 2 h at 50 8C in a
flask protected from light. After this time the precipitate
of AgI was removed by filtration. The filtrate was
collected and excess NaCl (1.2 g, 20 mmol) in H2O (6
ml) was added to the filtrate. The reaction mixture was
allowed to stir for 30 min at 50 8C. The yellow solution
was then cooled in the refrigerator for 2 days and yellow
crystals were obtained.
[PtCl2(OH)2(N,N-dmen)] (0.10 g, 0.25 mmol) was
dissolved in water (10 ml) and sodium oxalate (0.04 g,
0.25 mmol) was added to the solution. The reaction
mixture was then allowed to stir at room temperature
1
for 72 h and monitored by H NMR spectroscopy.
2.6. Reactions of [PtCl2(OH)2(N,N-dmen)] with
sodium oxalate in presence of platinum(II)
[PtCl2(N,N-dmen)]: yield 0.25 g, 69%. Anal. Calc. for
C4H12Cl2N2Pt: C, 13.56; H, 3.39; N, 7.91. Found: C,
[PtCl2(OH)2(N,N-dmen)] (0.01 g, 0.025 mmol) and
sodium oxalate (0.004 g, 0.025 mmol) were dissolved in
D2O (0.5 ml). [PtCl2(N,N-dmen)] (0.001 g, 0.0025
mmol) was added to the solution and the reaction
mixture was allowed to stir at room temperature for 7
1
13.59; H, 3.37; N, 7.89%. H NMR (D2O) d 2.89 (s,
3JPtH
ꢁ30 Hz, 6H, CH3); 2.70 (s, 4H, CH2).
/
[PtCl2(N,N?-dmen)]: yield 0.26 g, 72%. Anal. Calc. for
C4H12Cl2N2Pt: C, 13.56; H, 3.39; N, 7.91. Found: C,
1
days. H NMR spectra of the solution were recorded
periodically.
1
13.62; H, 3.49; N, 7.78%. H NMR (D2O) d 2.66 (s,
3JPtH
ꢁ42 Hz, 6H, CH3); 2.88 (s, 4H, CH2).
/
2.3. Preparation of [PtCl2(OH)2(NSN)]
2.7. Reaction of [PtCl2(OH)2(N,N-dmen)] with oxalic
acid
A solution of [PtCl2(NSN)] (0.18 g, 0.50 mmol) in
water (10 ml) was treated with a tenfold excess of 3%
H2O2 (11 ml) at 50 8C for 2 h. The solvent was removed
under reduced pressure and the product was collected as
a yellow solid, which was washed with ether and dried in
vacuo.
[PtCl2(OH)2(N,N-dmen)]: yield 0.16 g, 82%. Anal.
Calc. for C4H16Cl2N2O2Pt: C, 12.37; H, 3.61; N, 7.22.
Found: C, 12.30; H, 3.66; N, 6.69%. 1H NMR (D2O): d
[PtCl2(OH)2(N,N-dmen)] (0.11 g, 0.28 mmol) was
dissolved in water (10 ml) and oxalic acid (0.061 g, 0.50
mmol) was added to the solution. The reaction mixture
was stirred at room temperature for 4 days in a flask
protected from light. A pale yellow precipitate was
obtained and the precipitate was collected by filtration.
A pale yellow solid was obtained.

M.J. Arendse et al. / Inorganica Chimica Acta 340 (2002) 65ꢀ
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69
67
3. Results and discussion
due to [PtCl2(N,N-dmen)] and dehydroascorbic acid
(4.20, 4.29 and 4.61 ppm) were detectable. In addition,
3.1. Preparation of platinum(IV) complexes
low intensity signals were observed at 3.75 (m), 4.03 (dt,
3JHH
3
6, 2 Hz), and 4.53 ppm (d, JHH
ꢁ
/
ꢁ2 Hz), which
/
Platinum(IV) complexes containing diamine ligands
may be prepared by oxidation of the corresponding
platinum(II) precursors using hydrogen peroxide or
chlorine. The platinum(II) complexes [PtCl2(NSN)]
could be due to a platinum(IV) ascorbate complex.
When the solution was allowed to stand for an extended
period of time, yellow crystals of the oxalatoplatinum-
(IV) complex [Pt(C2O4)Cl(OH)(N,N-dmen)].H2O sepa-
rated.
(NSNꢁ
en, N,N-dmen, N,N?-dmen) were prepared in
/
good yields by reaction of [PtI2(NSN)] with 2 equiv. of
AgNO3 in aqueous solution [7,11], followed by addition
of excess sodium chloride. Further treatment with excess
hydrogen peroxide in water at 50 8C for 1 h gave the
platinum(IV) derivatives, [PtCl2(OH)2(NSN)].
3.3. Crystal structure of Pt(C2O4)Cl(OH)(N,N-
dmen)]×/H2O
Crystal data for [Pt(C2O4)Cl(OH)(N,N-dmen)]×
/H2O
The 1H NMR spectrum of [PtCl2(N,N-dmen)] ex-
hibits a singlet resonance at 2.89 ppm with 195Pt
and refinement parameters are listed in Table 1. The
complex crystallized in the centrosymmetric space group
P21/n. Its molecular structure is shown in Fig. 1. It is the
first example of a neutral platinum(IV) oxalate complex
to be characterized crystallographically (vide infra). The
complex consists of a platinum atom coordinated by a
distorted octahedral array of bidentate oxalate and
N,N-dmen ligands, one chloride and one hydroxide.
The chloride and hydroxide are mutually cis, the former
lying trans to the unsubstituted nitrogen of the diamine,
and the hydroxide trans to one arm of the oxalate. The
complex exists as two enantiomers, both of which are
present in the solid state structure. Selected bond
satellites (3JPtH
ꢁ36 Hz) due to the N-methyl groups,
/
and a broad resonance at 2.70 ppm due to the none-
quivalent CH2 hydrogens of the ligand backbone. Upon
oxidation to [PtCl2(OH)2(N,N-dmen)], the N-methyl
signal shifts slightly to lower frequency (2.76 ppm),
and the coupling to 195Pt is reduced to 26 Hz as expected
when the complex geometry changes from square planar
to octahedral. The methylene hydrogens give rise to
complex multiplets at 2.98 and 3.02 ppm. The 1H NMR
spectrum of [PtCl2(N,N?-dmen)] exhibits a singlet re-
sonance at 2.66 ppm with 195Pt satellites (3JPtH
ꢁ42 Hz)
/
due to the protons of the N-methyl groups, and a singlet
at 2.88 ppm due to the methylene hydrogens of the
ligand backbone. In the 1H NMR spectrum of
[PtCl2(OH)2(N,N?-dmen)], the N-methyl resonance oc-
curs at lower frequency (2.59 ppm) and the coupling to
195Pt is reduced to 30 Hz. The methylene protons give
rise to a singlet at 2.85 ppm. The 195Pt NMR spectra of
distances and angles are listed in Table 2. The Ptꢀ
/
O
bond distances involving the oxalate ligands are essen-
˚
tially identical, being about 0.03 A longer than the Ptꢀ
/
˚
OH distance of 1.987(4) A is
OH distance. The Ptꢀ
similar to those observed in other hydroxoplatinum(IV)
complexes containing diamine ligands [12,13]. The Ptꢀ
/
/
N
˚
bond trans to Cl (2.065(5) A) is slightly longer than that
[PtCl2(OH)2(N,N-dmen)]
and
[PtCl2(OH)2(N,N?-
˚
trans to oxalate (2.034(5) A), reflecting the higher trans-
dmen)] exhibit signals at 770 and 782 ppm, respectively.
These chemical shifts are typical of platinum(IV) com-
plexes containing diamine ligands [1,2].
˚
N distance (2.050 A)
influence of chloride. The mean Ptꢀ
/
Table 1
Crystal
data
and
structure
refinement
parameters
for
3.2. Reactions of platinum(IV) complexes
[Pt(C2O4)Cl(OH)(N,N-dmen)]×H2O
Reactions of [PtCl2(OH)2(NSN)] (NSNꢁ
en, N,N-
dmen, N,N?-dmen) with a 10-fold excess of ascorbic
acid in aqueous solution at pH 7 were monitored by
/
Formula
Mw
C6H15ClN2O6Pt
441.74
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/n
UVꢀVis spectroscopy. The platinum(IV) complexes
/
˚
a (A)
7.0205(8)
11.9868(13)
13.6133(15)
98.335(4)
1133.5(2)
4
exhibit strong absorptions between 330 and 350 nm,
and these should disappear when the complexes are
reduced [6]. The spectra showed no detectable change
over 2 h, however, indicating that the complexes were
not reduced rapidly under these conditions. In order to
gain some insight into what might take place during
these reactions, [PtCl2(OH)2(N,N-dmen)] was treated
with sodium ascorbate in D2O and the reaction was
˚
b (A)
˚
c (A)
b (8)
3
˚
V (A )
Z
rcalc (g cmꢃ3
)
2.589
12.630
832
m(Mo Ka) (mmꢃ1
F(000)
)
R1 (F ꢀ2s)
wR2 (all data)
Rint
0.0266
0.0678
0.026
1
monitored by H NMR spectroscopy over 48 h. In the
early stages of the reaction only signals due to the
reactants could be observed, but after 24 h resonances

68
M.J. Arendse et al. / Inorganica Chimica Acta 340 (2002) 65ꢀ69
/
accompany oxidation of the ascorbate ligand. Its for-
mation might suggest that oxidative degradation of the
ascorbate complex had occurred without reduction of
platinum(IV), although other mechanisms may be
envisaged.
The reaction of [PtCl2(OH)2(N,N-dmen)] with so-
dium ascorbate is likely to proceed by initial displace-
ment of one of the axial hydroxo groups by ascorbate.
Such monodentate ascorbate complexes have been
proposed previously as intermediates in the reduction
of platinum(IV) complexes [17]. At this stage intramo-
lecular degradation of ascorbate could take place.
Alternatively, oxidation of the ascorbate ligand could
produce the observed dehydroascorbic acid and water,
simultaneously generating the platinum(II) complex
[PtCl2(N,N-dmen)]. Dehydroascorbic acid is known to
react slowly to form oxalic and threonic acids [18,19],
and reaction of the platinum(IV) precursor with the
oxalic acid thus produced could generate the isolated
product. In any case, reaction of [PtCl2(OH)2(N,N-
dmen)] with sodium ascorbate is either very slow or
involves an unfavorable equilibrium, such that the
platinum(IV) complex dominates the NMR spectra.
Only after an extended period of time was the oxalato-
platinum(IV) complex formed, and then only in low
yield.
Fig. 1. Molecular structure of [Pt(C2O4)Cl(OH)(N,N-dmen)]×H2O
/
showing the atom-numbering scheme.
Table 2
˚
Selected bond lengths (A) and bond angles (8) for
[Pt(C2O4)(OH)Cl(N,N-dmen)]×H2O
Bond lengths
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
/
O(1)
O(2)
O(3)
N(1)
N(2)
C(l)
1.987(4)
2.018(4)
2.022(4)
2.065(5)
2.034(4)
2.3151(13)
1.283(7)
O(3)ꢀ
O(4)ꢀ
O(5)ꢀ
C(1)ꢀ
N(1)ꢀ
N(1)ꢀ
N(2)ꢀ
/
C(2)
C(1)
C(2)
1.298(7)
1.226(7)
1.224(7)
1.532(8)
1.497(7)
1.496(8)
1.494(8)
/
/
/
/
/
/C(2)
/
/
C(3)
C(5)
C(4)
/
/
O(2)ꢀ
/
C(1)
/
Bond angles
O(1)ꢀ
O(1)ꢀ
O(2)ꢀ
O(1)ꢀ
O(2)ꢀ
O(3)ꢀ
O(1)ꢀ
/
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
/
O(2)
O(3)
O(3)
N(2)
N(2)
N(2)
N(1)
93.81(17)
176.49(16)
82.72(15)
90.86(17)
91.77(18)
88.67(16)
87.83(18)
O(2)ꢀ
O(3)ꢀ
N(2)ꢀ
O(1)ꢀ
O(2)ꢀ
O(3)ꢀ
N(1)ꢀ
/
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
Ptꢀ
/
N(1)
176.47(16)
95.60(18)
85.07(19)
91.19(11)
89.16(11)
89.34(11)
93.94(13)
/
/
/
/N(1)
We investigated the reaction of [PtCl2(OH)2(N,N-
dmen)] with sodium oxalate, as well as with oxalic acid,
to try to establish how the oxalatoplatinum(IV) complex
may be produced. 1H NMR studies of the reaction
between [PtCl2(OH)2(N,N-dmen)] and Na2C2O4 sug-
gested that no reaction had occurred after 4 days. The
1H NMR spectrum of the reaction mixture indicated the
presence of the starting material only. Similarly, no
/
/
/
/
N(1)
/
/
/
/Cl
/
/
/
/Cl
/
/
/
/Cl
/
/
/
/
Cl
1
is within the normal range observed for diamineplati-
num(IV) complexes [12,13]. The bond angles about
platinum range from 82.72 to 93.948. The smallest
angles are those associated with the two 5-membered
change was observed in the H NMR spectrum of the
reaction between [PtCl2(OH)2(N,N-dmen)] and excess
oxalic acid after monitoring for 4 days (although an
unidentified pale yellow solid precipitated from the
reaction mixture).
rings (Oꢀ
/
Ptꢀ
/
Oꢁ
/
82.72(15)8; Nꢀ
/
Ptꢀ
/
Nꢁ85.07(19)8),
/
and the largest (95.60(18)8) is that between O3 and
N1, the bulky N(CH3)2 group. The bis(oxalato)-
In order to establish whether the presence of a
platinum(II) complex might accelerate the production
of the oxalatoplatinum(IV) complex, we monitored the
reaction between [PtCl2(OH)2(N,N-dmen)] and sodium
oxalate in the presence of a catalytic amount of
[PtCl2(N,N-dmen)] by 1H NMR spectroscopy. It has
been shown that substitution reactions of platinum(IV)
complexes can be accelerated by the presence of
platinum(II) species that promote ligand substitution
through the formation of bridged platinum(II)-
platinum(IV) intermediates [20]. However, in this case
the only effect seemed to be to produce more of the
platinum(II) complex.
platinum(IV) dianions trans-[Pt(C2O4)2X2]2ꢃ (Xꢁ
/
Cl,
Br, I, OH) have also been characterized by X-ray
diffraction [14,15]. In these, the PtꢀO distances range
/
˚
from 1.99 to 2.02 A, the chloro complex, for example,
exhibiting bonds that span this range of distances. In
˚
[Pt(C2O4)2(OH)2]2ꢃ the Ptꢀ
OH distances are 1.992(7) A
[12], identical to that in the present complex.
/
3.4. Formation of oxalatoplatinum(IV) complexes
We have shown that platinum(II) ascorbate com-
plexes may be oxidized in air to give the corresponding
oxalates [16]. Formation of the platinum(IV) oxalate
[Pt(C2O4)Cl(OH)(N,N-dmen)] was surprising, however,
because reduction of platinum(IV) was expected to
Since direct addition of oxalate (or oxalic acid) to
[PtCl2(OH)2(N,N-dmen)], with or without added plati-
num(II) complex, does not appear to give oxalatoplati-
num(IV) derivatives, it may be that the latter are indeed

M.J. Arendse et al. / Inorganica Chimica Acta 340 (2002) 65ꢀ
/
69
69
formed in the reaction of [PtCl2(OH)2(N,N-dmen)] with
ascorbate by intramolecular degradation of coordinated
ascorbate.
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4. Conclusions
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achieved in high yields (ꢀ80%). Reduction of the
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New York, 1973.
/
platinum(IV) complexes to platinum(II) by sodium
ascorbate appears to be slow. The reactions between
[PtCl2(OH)2(NSN)] and sodium oxalate or oxalic acid
did not produce oxalatoplatinum(IV) species, whereas
the addition of a catalytic amount of [PtCl2(NSN)]
appeared to enhance the rate of reduction of platinum-
(IV) to platinum(II).
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5. Supplementary material
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Chem. Soc. 16 (1995) 981.
Tables of bond lengths and angles (S1), anisotropic
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coordinates and isotropic displacement parameters
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This work was supported through a grant from the
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9508228), a University of MissouriꢀSt. Louis Research
/
Award and a grant from the University of the Western
Cape Linkage Program. Thanks are expressed to NSF
for grants for the purchase of NMR and X-ray
instrumentation.