Oxidative degradation of the ascorbate anion in the presence of platinum and palladium. Formation and structures of platinum and palladium oxalate complexes

Article abstract of DOI:10.1016/S0277-5387(01)00843-9

The reactions of [Pt(NO₃)₂(dppm)] (dppm = bis(diphenylphosphino)methane) and cis-[Pt(NO₃)₂(PEt₃)₂] with sodium ascorbate are described. Complexes containing O,O-coordinated ascorbate ligand

Full text of DOI:10.1016/S0277-5387(01)00843-9

Polyhedron 20 (2001) 2495–2503
www.elsevier.com/locate/poly
Oxidative degradation of the ascorbate anion in the presence of
platinum and palladium. Formation and structures of platinum
and palladium oxalate complexes
Malcolm J. Arendse 1, Gordon K. Anderson *, Nigam P. Rath
Department of Chemistry, Uni6ersity of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
Received 20 February 2001; accepted 16 May 2001
Abstract
The reactions of [Pt(NO3)2(dppm)] (dppm=bis(diphenylphosphino)methane) and cis-[Pt(NO3)2(PEt3)2] with sodium ascorbate
are described. Complexes containing O,O-coordinated ascorbate ligands are formed initially, but on standing further oxidation
and cleavage of the ligand occur to produce the corresponding oxalate complexes. The reactions were monitored by NMR
spectroscopy, and reactions of [Pt(NO3)2(dppm)] with oxalic acid or calcium threonate also produced [Pt(C2O4)(dppm)]. Reactions
of [PtMe(Me2CO)(dppe)]+ or [PdMe(Me2CO)(PSP)]+ (PSP=dppe, dppp) with sodium ascorbate result in cleavage of the MꢀC
bond and oxidation of ascorbate to again produce metal oxalate derivatives. The solid state structures of
[Pt(C2O4)(dppm)]·Me2CO, [Pd(C2O4)(dppe)]·H2O and [Pd2(m-C2O4)(dppp)2][BF4]2·2Me2CO, determined by X-ray crystallography,
are described. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Oxidative degradation; Platinum; Palladium; Oxalates
metal ion has a stable lower valence form, it will not
participate in the electron transfer scheme and will thus
1. Introduction
not function as a catalyst [3]. Since then, other studies
The oxidation of vitamin C (L-ascorbic acid) to ox-
alic acid in aqueous solution is known to be catalyzed
by metal ions [1]. This process is believed to involve
oxidation of ascorbic acid to dehydroascorbic acid,
followed by hydrolysis that leads to opening of the
lactone ring to produce 2,3-diketogulonic acid. Cleav-
age and further oxidation produce oxalic acid and
L
-threonic acid (Scheme 1) [1,2]. Martell and Khan
performed pioneering work on the catalytic effects of
iron(III) and copper(II), and their chelate complexes,
on the oxidation of ascorbic acid [3,4]. The rate-deter-
mining step in their proposed mechanism involves elec-
tron transfer from the ascorbate monoanion (HAsc−)
to the metal ion. The authors stated that unless the
* Corresponding author. Tel.: +1-314-516-5311; fax: +1-314-516-
5342.
E-mail address: ganderson@umsl.edu (G.K. Anderson).
1 Current address: Department of Chemistry, University of the
Western Cape, Private Bag XI7, Bellville 7535, South Africa.
Scheme 1. Oxidation and hydrolysis of L-ascorbic acid.
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00843-9

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M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
have shown that the reaction of ascorbic acid with
dioxygen in aqueous pyridine is catalyzed by cobalt(II),
nickel(II), manganese(II) and zinc(II) [4–6]. In all these
cases the role of the metal ion in the process is not a
redox function, but it serves to increase the rate of
radical formation. The proposed mechanism involves
the formation of monomeric metal–dioxygen com-
plexes. The dimeric m-peroxo ruthenium ascorbate com-
plex [Ru2Cl2(HAsc)2(OH2)2(m-O2)] was proposed as an
intermediate in the catalytic oxidation of ascorbic acid
by ruthenium(III) [7]. All these studies have focused on
the kinetic aspects of the oxidation of ascorbic acid to
dehydroascorbic acid, but not on the fate of the latter.
There are few data in the literature on the further
degradation of ascorbic acid to oxalic acid in the
presence of metal ions.
Scheme 2. Outline for the synthesis of monodentate ascorbate com-
plexes.
[MClMeL2] (M=Pt or Pd, L2=dppe or dppp) as
precursors, these having the advantage of possessing
strong MꢀP and MꢀC bonds, as well as being easily
prepared. Our aim was to prepare monodentate ascor-
bate complexes of the type [MMe(HAsc)L2] (M=Pt or
Pd; L2=dppe or dppp) from the precursor complexes
[MClMeL2] as outlined in Scheme 2. Treatment of these
complexes with 1 equiv. of AgBF4 in acetone solution
would produce cationic species with a weakly coordi-
nated solvent molecule, which could be easily replaced
by ascorbate on treatment of the cationic intermediate
with sodium ascorbate.
Recently, studies of reactions between L-ascorbic acid
and cobalt(II) and gadolinium(III) in aqueous solution
have shown that the metal ascorbate complexes that are
formed initially slowly decompose to generate oxalate
complexes, which precipitate from solution [8]. We have
recently reported the synthesis and characterization of a
series of diamine- and diphosphine-platinum ascorbate
complexes [9]. In the diphosphine case, species of the
type [Pt(Asc-O2,O3)(PSP)]·H2O (PSP=dppm, dppe,
dppp) were formed, whereas a number of complexes
were identified for the diamine derivatives. These in-
cluded [Pt(Asc-O2,O3)(NSN)], monodentate ascorbate
species of the form [Pt(Asc-O2)2(NSN)], and the ther-
modynamically stable [Pt(Asc-C2,O3)(NSN)] [9].
Whereas the dppe and dppp ascorbate derivatives
proved to be stable towards further reaction, the dppm
complex slowly underwent oxidative degradation to the
corresponding oxalate complex. This paper describes
our attempts to determine the nature of this process.
Oxalate complexes of the type [M(C2O4)L2] (M=Pd,
Pt; L=tertiary phosphine or L2=diphosphine) are
well known, and have been prepared previously from
reaction of the corresponding carbonates with oxalic
acid [10–12], or by treatment of [MCl2L2] with
Ag2C2O4 [13,14].
We have also attempted to synthesize and isolate
monodentate ascorbate complexes. In addition to their
implication in the reactions of the diamineplatinum
species mentioned above [9], monodentate ascorbate
species have been implicated as intermediates in the
reduction of diamineplatinum(IV) complexes used in
chemotherapy [15]. We reasoned that an appropriate
approach to the synthesis of monodentate ascorbate
complexes would be to block three of the four coordi-
nation sites of the square planar palladium(II) or plat-
inum(II) center to prevent the formation of ascorbate
chelates. (Platinum(IV) species used in chemotherapy
are generally believed to be reduced to the divalent
form prior to binding to DNA.) We chose to use the
platinum and palladium diphosphine complexes
2. Results and discussion
2.1. Reactions of cis-[Pt(NO3)2(PEt3)2] and
[Pt(NO3)2(dppm)] with sodium ascorbate
We have previously reported the preparation and
characterization of platinum(II) ascorbate complexes
containing diamine or diphosphine ligands [9]. The
ascorbate groups in the diamine complexes were ini-
tially coordinated through O2 and O3, but the com-
plexes isomerized slowly to the more thermo-
dynamically stable C,O-chelates. In contrast, the
diphosphine compounds were isolated as the O,O-
bound chelates, [Pt(Asc-O2,O3)(PSP)]·H2O (PSP=
dppe, dppp), but attempts to crystallize these from a
variety of solvents were unsuccessful. Crystallization
from solvents such as CHCl3 or CH2Cl2 resulted in
cleavage of the ascorbate moiety and production of the
dichloroplatinum(II) complexes [PtCl2(PSP)]. Crystal-
lization from other solvents was also complicated by
the susceptibility of the complexes to oxidation [9]. We
have also found that reaction of the monodentate phos-
phine complex cis-[Pt(NO3)2(PEt3)2] with sodium ascor-
bate results in formation of the ascorbate chelate
[Pt(Asc-O2,O3)(PEt3)2], as determined by 31P{1H}
NMR analysis of the reaction mixture (l(P) 5.6 d,
2JPP=22 Hz, 1JPt–P=3300 Hz; 6.2 d, 2JPP=22 Hz,
1JPt–P=3472 Hz). Elemental analysis of the product
obtained from this reaction, however, indicated that
partial oxidation to the corresponding oxalate complex,
[Pt(C2O4)(PEt3)2], had occurred (the analytical data
were consistent with a 1:2 mixture of the ascorbate and
oxalate complexes).

M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
2497
The reaction between [Pt(NO3)2(dppm)] and sodium
ascorbate in the presence of air also produced the
oxalate complex [Pt(C2O4)(dppm)]. This reaction was
monitored by NMR spectroscopy over a period of 48 h.
The 31P{1H} NMR spectrum of the reaction mixture of
[Pt(NO3)2(dppm)] and sodium ascorbate in wet acetone
immediately after mixing revealed the presence of two
species (Fig. 1). Two overlapping signals were observed
at −61.7 ppm (1JPt–P=2962 and 3009 Hz) due to
[Pt(Asc-O2,O3)(dppm)], while a single peak at −66.0
ppm (1JPt–P=3401 Hz) was due to unreacted
[Pt(NO3)2(dppm)]. As the reaction proceeded, the peak
at −66.0 ppm disappeared and resonances due to two
new species were observed. The pair of doublets cen-
tered at −58.0 ppm (2JPP=81 Hz) was due to an
unsymmetrical species with a chelating dppm ligand,
and the singlet at −63.5 ppm (1JPtP=3085 Hz) was
due to [Pt(C2O4)(dppm)]. After 27 h the peak due to the
ascorbate complex had diminished, whereas the signal
due to the oxalate complex had increased in intensity.
After 48 h the ascorbate complex had been converted
completely to the oxalate complex, and the peaks due
to the unsymmetrical intermediate had disappeared.
Crystals of [Pt(C2O4)(dppm)]·Me2CO separated from
the reaction mixture upon standing.
and [Pt(C2O4)(dppm)] was formed in each case. When
[Pt(NO3)2(dppm)] was treated with 1 equiv. of calcium
threonate, complete conversion to the oxalate complex
was apparent after 21 h. The reaction between
[Pt(NO3)2(dppm)] and oxalic acid in a water–acetone
mixture was 50% complete after 5 h, and [Pt(C2O4)-
(dppm)] was again formed quantitatively after 21 h. If
threonate and/or oxalate ions were produced by cleav-
age of dehydroascorbic acid during the course of the
reaction of [Pt(NO3)2(dppm)] with ascorbate ion, they
could certainly account for the formation of
[Pt(C2O4)(dppm)]. Oxidation of dehydroascorbic acid
itself was slow, however, compared with the reactions
of the platinum complexes. When a solution of dehy-
droascorbic acid in wet acetone was allowed to stand at
ambient temperature, virtually no reaction occurred
over a period of 7 days. Thus, uncatalyzed degradation
of dehydroascorbic acid is too slow to account for the
formation of [Pt(C2O4)(dppm)], and some continued
interaction of the ligand with platinum appears likely to
account for the oxidation of ascorbate.
2.2. Reactions of methylplatinum and methylpalladium
complexes with sodium ascorbate
We have also carried out reactions of [Pt(NO3)2-
(dppm)] with calcium threonate and with oxalic acid,
The nitrate complex [Pt(NO3)Me(dppe)] (l(P) 35.4,
1
1JPtP=4538 Hz; l(P) 50.6, JPtP=1792 Hz) was pre-
pared by treatment of [PtClMe(dppe)] with 1 equiv. of
AgNO3 in CH2Cl2–MeOH solution, followed by filtra-
tion and evaporation of the solvents. In acetone-d6
solution, the nitrate was displaced by solvent, resulting
in a modest change in the 31P NMR parameters (l(P)
1
1
34.6, JPtP=3914 Hz; l(P) 50.5, JPtP=1724 Hz). The
reaction between [Pt(NO3)Me(dppe)] and sodium ascor-
bate produced a mixture of products. The major species
was believed to be the monodentate ascorbate complex
[Pt(HAsc-O3)Me(dppe)] (l(P) 36.4, 1JPtP=4436 Hz;
1
l(P) 50.8, JPtP=1844 Hz), whereas the minor product
exhibited two signals at 30.9 and 32.9 ppm which were
assigned to the chelate complex [Pt(Asc-O2,O3)(dppe)]
[9]. The reaction was slow and oxidative degradation of
the complexes occurred on standing in solution, so it
was not possible to separate the products.
In order to increase the reaction rates, we turned our
attention to the corresponding palladium complexes.
Treatment of [PdClMe(dppe)] with 1 equiv. of AgBF4
in acetone solution produced the cationic complex
[PdMe(Me2CO)(dppe)]+. After filtration, the 31P{1H}
NMR spectrum of the filtrate consisted of a doublet at
62.6 ppm (2JPP=25 Hz) due to the P atom trans to Me,
and a broad doublet at 38.6 ppm (2JPP=25 Hz) due to
the P atom trans to acetone. The broadness of the latter
signal is likely to be due to rapid, reversible dissociation
of solvent molecules in solution. This assignment is
consistent with that for the analogous platinum com-
plex described above, where the signal due to the P
Fig. 1. 31P{1H} NMR spectra of reaction mixtures of
[Pt(NO3)2(dppm)] and sodium ascorbate in acetone-d6 solution after 2
h (top), 21 h, 27 h and 48 h.

2498
M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
atom trans to Me was observed at higher frequency
than that trans to acetone.
the dimeric derivative would be favored, whereas with
excess oxalate [Pd(C2O4)(PSP)] would be the major
product. Indeed, treatment of the dinuclear oxalate
complexes [Pd2(m-C2O4)(PSP)2][BF4]2 with excess
K2C2O4 resulted in bridge cleavage and formation of
the mononuclear complexes.
An acetone solution of [PdMe(Me2CO)(dppe)]BF4
was treated with a solution of sodium ascorbate in
D2O, and the reaction was monitored by 31P{1H} NMR
spectroscopy. During the course of the reaction several
unidentified intermediates were formed, but after 60 h
the oxalate complex [Pd(C2O4)(dppe)] was produced
almost quantitatively. The first new species to be
formed may have been the monodentate ascorbate
complex, [Pd(HAsc-O3)Me(dppe)], but we were unable
to isolate this or any of the other intermediates. In-
stead, crystals of the oxalate complex [Pd(C2O4)(dppe)]·
H2O separated from an aliquot of the reaction mixture
when it was allowed to stand in an NMR tube. The
solid state structure of the oxalate complex was deter-
mined by X-ray crystallography. The 31P{1H} NMR
spectrum of this complex recorded in CDCl3 solution
contained a single peak at 58.0 ppm. Another aliquot of
the reaction mixture produced crystals of a dinuclear
oxalate-bridged complex [Pd2(m-C2O4)(dppe)2][BF4]2,
whose 31P{1H} NMR spectrum in CDCl3 solution con-
sisted of a singlet at 70.3 ppm.
2.3. Crystal structure of [Pt(C2O4)(dppm)]·Me2CO
Crystals of [Pt(C2O4)(dppm)]·Me2CO were isolated
by slow evaporation of the mixture obtained from the
reaction of [Pt(NO3)2(dppm)] with sodium ascorbate in
acetone solution. The molecular structure of the com-
plex is shown in Fig. 2. Selected bond distances and
angles are listed in Table 1. The coordination geometry
around platinum is planar, the sum of the angles about
the metal being 360.05°, but the presence of two
strained rings results in two smaller and two larger
angles. The O1–Pt–O2 angle of 81.42(10)° is similar to
the corresponding angle of 81.4(3)° in [Pt(C2O4)-
(PMe3)2] [16]. The small P1–Pt–P2 angle of
In a manner similar to that for the dppe case, [Pd-
ClMe(dppp)] was converted to [PdMe(Me2CO)-
(dppp)]BF4 by treatment with AgBF4 in acetone solu-
tion. After removal of the precipitate of AgCl, the pale
yellow filtrate was treated with a solution of sodium
ascorbate in D2O, and a green solution was obtained.
Again, several unidentified complexes were detected by
31P{1H} NMR spectroscopy, in addition to the signals
due to the starting material. Crystals of the oxalate-
bridged dimer [Pd2(m-C2O4)2(dppp)2][BF4]2 separated
from an aliquot of the reaction mixture in an NMR
tube. The structure of the dimer was determined by
X-ray crystallography. The 31P{1H} NMR spectrum of
[Pd2(m-C2O4)(dppp)2][BF4]2 in CDCl3 solution showed a
singlet at 19.8 ppm. Evaporation of the reaction mix-
ture after 48 h produced a yellow solid, which was
identified by elemental analysis as [Pd(C2O4)(dppp)]·
2H2O. The 31P{1H} NMR spectrum of this complex
dissolved in CDCl3 solution consisted of a singlet at
15.7 ppm.
The methyl group present originally in [PdClMe-
(PSP)] (PSP=dppe or dppp) was lost during the
course of the reaction with AgBF4 and NaHAsc. Some
HBF4 may be formed (since a small amount of water
would be present in the acetone, then D2O was added
deliberately), which could cleave the PdꢀC bond to
produce [Pd(OH2)2(PSP)]2+. Oxidative degradation of
ascorbate to oxalate is likely to occur in the coordina-
tion sphere of palladium by a mechanism similar to
that operating in the platinum case described earlier.
Then, depending on the palladium/oxalate ratio, either
the monomeric oxalate complex or the oxalate-bridged
dimer would be formed. With a deficiency of oxalate,
Fig. 2. The molecular structure of [Pt(C2O4)(dppm)]. The acetone
molecule has been omitted for clarity.
Table 1
,
Selected bond distances (A) and angles (°) for [Pt(C2O4)(dppm)]·
Me2CO
Bond distances
Pt–O1
Pt–O2
Pt–P1
Pt–P2
O2–C27
O1–C26
O3–C26
O4–C27
C26–C27
Bond angles
O1–Pt–O2
O1–Pt–P1
O2–Pt–P2
P1–Pt–P2
P1–C25–P2
O3–C26–O1
O4–C27–O2
O1–C26–C2
O2–C27–C1
O3–C26–C2
O4–C27–C1
2.073(3)
2.075(3)
2.2170(9)
2.2232(10)
1.295(5)
1.302(5)
1.215(5)
1.219(5)
1.571(5)
81.42(10)
102.30(8)
102.84(8)
73.49(4)
91.81(16)
124.8(3)
124.3(4)
115.5(3)
116.3(3)
119.7(3)
119.4(3)

M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
2499
73.49(4)° in the present case is due to the strained
four-membered chelate ring. The larger P1–Pt–O1 and
P2–Pt–O2 angles of 102.30(8) and 102.84(8)° compen-
sate for these angles being reduced from 90°. The
P–Pt–O angles are substantially larger than the mean
P–Pt–O angles of 90.7° in [Pt(C2O4)(PMe3)2] [16] and
95.8° in [Pt(C2O4)(dcpe)]·MeCN [17]. The mean Pt–O
O–H···O angles are indicative of strong hydrogen
bonding interactions.
2.5. Crystal structure of
[Pd2(v-C2O4)(dppp)2][BF4]2·2Me2CO
The molecular structure of the cation in [Pd2(m-
C2O4)(dppp)2][BF4]2·2Me2CO is shown in Fig. 4. The
molecule is a dinuclear oxalate-bridged complex with a
center of inversion located at the midpoint of the CꢀC
bond of the bridging oxalate ligand. The only other
structure of a dinuclear oxalate-bridged palladium
complex is that of [(Bu2S)(Ph)Pd(m-C2O4)Pd(Ph)(SBu2)]
[19]. The simple binuclear geometry involving symmet-
ric, in-plane bridging with a single oxalate ion binding
through two oxygen atoms to each of two metal centers
has been observed in the chemistry of nickel(II),
iron(II), manganese(II), cobalt(II), zinc(II) and cop-
per(II) [20–28]. Table 3 lists some selected bond dis-
tances and angles for one half of the [Pd2(m-C2O4)-
(dppp)2]2+ cation. The planar oxalate bridge ensures
that the two coordination planes around palladium are
,
distance of 2.074 A is similar to those in [Pt(C2O4)-
,
(PMe3)2] (2.065 A) and [Pt(C2O4)(dcpe)]·MeCN (2.063
,
A). As expected, the coordinated C–O distances of
,
1.295(5) and 1.302(5) A are longer than the uncoordi-
,
nated C–O distances of 1.219(5) and 1.215(5) A, where
the latter are formally double bonds. The coordinated
C–O distances are similar to the corresponding dis-
,
tances of 1.30(1) A in [Pt(C2O4)(PMe3)2] [16] and
,
1.29(1) A in [Pt(C2O4)(dcpe)]·MeCN [17]. Intermolecu-
lar hydrogen bonding occurs between the uncoordi-
nated oxygen atom O3 of the oxalate ligand and two
hydrogen atoms on the phenyl groups of another
,
molecule. The O3···H21 distance is 2.554 A and the
O3–H21–C21 angle is 167.8°, while the O3···H4 dis-
,
tance is 2.488 A and the O3–H4–C4 angle is 160.4°.
,
There is also hydrogen bonding between the oxygen
atom of the acetone molecule and one of the methylene
hydrogen atoms of a dppm ligand. The O1S···H25
also co-planar with a Pd–Pd distance of 5.416 A. This
is in contrast to the m-hydroxo dimer [Pd2(m-OH)2-
(dppp)2][BF4]2 where the dihedral angle between the
,
distance is 2.342 A and the O1S–H25–C25 angle is
131.8°.
2.4. Crystal structure of [Pd(C2O4)(dppe)]·H2O
Crystals of [Pd(C2O4)(dppe)]·H2O were obtained by
slow evaporation of an acetone solution of the product
obtained from the reaction of [PdMe(Me2CO)(dppe)]-
BF4 with sodium ascorbate. The molecular structure of
[Pd(C2O4)(dppe)]·H2O is shown in Fig. 3. Selected bond
distances and angles are listed in Table 2. The coordi-
nation geometry around palladium is perfectly planar
as in the platinum case above, the sum of the angles
around palladium being 360.01°. Here P(1)–Pd–P(2),
O(1)–Pd–O(2), O(1)–Pd–P(2) and O(2)–Pd–P(2) an-
gles are 84.52(5), 81.78(12), 99.26(9) and 94.45(10)°,
respectively. The mean Pd–O and Pd–P distances of
Fig. 3. Molecular structure of [Pd(C2O4)(dppe)]·H2O.
,
2.061(3) and 2.229(13) A agree well with the corre-
sponding distances in [Pd(C2O4)(PEt3)2] [18]. The C–O
distances in the oxalate ligand are also similar to those
in [Pd(C2O4)(PEt3)2]. As above, the coordinated C–O
Table 2
,
Selected bond distances (A) and angles (°) for [Pd(C2O4)(dppe)]·H2O
Bond distances
Pd–O(1)
Pd–O(2)
Pd–P(1)
Pd–P(2)
O(2)–C(2)
O(1)–C(1)
O(3)–C(1)
O(4)–C(2)
C(1)–C(2)
Bond angles
,
,
distances of 1.282(5) A and 1.294(5) A are longer than
the uncoordinated C–O distances of 1.212(5) and
2.068(3)
2.053(3)
2.2217(13)
2.2357(13)
1.282(5)
1.294(5)
1.212(5)
1.214(5)
1.561(7)
O(1)–Pd–O(2)
O(1)–Pd–P(2)
O(2)–Pd–P(2)
P(1)–Pd–P(2)
O(3)–C–O(1)
O(4)–C–O(2)
O(1)–C(1)–C(2)
O(2)–C(2)–C(1)
O(3)–C(1)–C(2)
O(4)–C(2)–C(1)
81.78(12)
99.26(9)
94.45(10)
84.52(5)
123.8(5)
124.8(5)
116.1(4)
116.3(4)
120.1(4)
118.9(5)
,
1.214(5) A. Intermolecular hydrogen bonding occurs in
this case between the hydrogens of the water molecule
and the uncoordinated oxygen atoms of two different
molecules of [Pd(C2O4)(dppe)]. The O(5S)–H(5A)···
O(3) angle is 173.1°, while the H(5A)···O(3) distance is
,
2.060 A. The O(5S)–H(5B)···O(3) angle is 159.0° while
,
the H(5B)···O(4%) distance is 2.031A. The H···O dis-
,
tances of 2.031 and 2.060 A, as well as the near linear

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M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
and PEt3 compounds undergo degradation of the ascor-
bate ligands to produce the corresponding oxalates, and
the solid state structure of [Pt(C2O4)(dppm)]·Me2CO
has been determined by X-ray crystallography. The
available evidence suggests that oxidative cleavage of
the ascorbate probably occurs within the coordination
sphere of the metal. Such coordination and oxidative
cleavage of ascorbate represents one possible fate of
platinum compounds used in cancer therapy. In the
palladium systems, the initially formed monodentate
ascorbate complexes [PdMe(HAsc-O3)(PSP)] undergo
PdꢀC bond cleavage and oxidation of the ascorbate
ligand to again produce oxalate complexes, which may
be isolated as monomeric or dimeric derivatives
[Pd(C2O4)(PSP)] or [Pd2(m-C2O4)(PSP)2]2+. Further
studies on the mechanism of oxidative degradation of
ascorbate ligands in the presence of metal ions are in
progress.
Fig. 4. Molecular structure of the [Pd2(m-C2O4)(dppp)2]2+ cation.
The BF4− ions and acetone molecules have been omitted for clarity.
Table 3
,
Selected bond distances (A) and angles (°) for the [Pd2(m-
C2O4)(dppp)2]2+ cation
4. Experimental
Bond distances
Pd(1)–O(1)
Pd(1)–O(2)
Pd(1)–P(1)
Pd(1)–P(2)
O(1)–C(1)
Bond angles
2.1022(16) O(1)–Pd(1)–O(2)
2.0993(17) O(1)–Pd(1)–P(1)
80.68(6)
93.31(5)
93.10(5)
Silver nitrate, sodium ascorbate and calcium
threonate were purchased from Aldrich. [Pt(NO3)2-
(dppm)] and [PdClMe(cod)] were prepared as described
previously [9,30]. NMR spectra were recorded on a
Varian Unity plus 300 spectrometer at 300.0 MHz for
2.2280(7)
2.2239(6)
1.257(3)
O(2)–Pd(1)–P(2)
P(1)–Pd(1)–P(2)
92.85(3)
C(1)–O(1)–Pd(1)
C(1)c1–O(2)–Pd(1)
O(2)c1–C(1)–O(1)
111.56(15)
111.61(15)
124.4(2)
O(2)–C(1)c1 1.255(3)
C(1)–C(1)c1 1.548(5)
1H and 121.4 MHz for 31P, respectively. H chemical
1
O(2)c1–C(1)–C(1)c1 118.0(3)
shifts were measured relative to the residual solvent
signal, while 31P chemical shifts were measured relative
to external H3PO4.
O(1)c1–C(1)–C(1)c1 117.7(3)
two coordination planes is 33.3(8)° with a shorter Pd–
4.1. Preparation of cis-[Pt(NO3)2(PEt3)2]
,
Pd distance of 3.098(2) A [29]. In [Pd2(m-C2O4)-
(dppp)2]2+ the O(1)–Pd(1)–O(2), O(1)–Pd(1)–P(1),
P(1)–Pd(1)–P(2) and O(2)–Pd(1)–P(2) angles are
80.68(6)°, 93.31(3)°, 82.85(3)° and 93.10(5)°, respec-
tively, and the sum of the angles around palladium is
359.94°. The mean Pd–O distance is slightly longer
than the corresponding distances in [Pd(C2O4)(dppe)]
and [Pd(C2O4)(PEt3)2] [18]. In the bridging oxalate lig-
and the C–O distances are nearly identical (1.255(3)
An aqueous solution (5 ml) of silver nitrate (0.34 g,
2.0 mmol) was added to a suspension of [PtCl2(PEt3)2]
(0.50 g, 1.0 mmol) in acetone (40 ml) in a flask pro-
tected from light. The reaction mixture was stirred at
room temperature (r.t.) for 24 h. After removing the
AgCl by filtration through Celite, the filtrate was evap-
orated to dryness to leave a yellow solid (0.40 g, 72%).
Anal. Calc. for C12H30N2O6P2Pt: C, 25.95; H, 5.44; N,
5.04. Found: C, 26.00; H, 5.49, N, 4.96%. 31P{1H}
,
and 1.257(3) A), as expected because the negative
charge is delocalized over the four CꢀO bonds. Similar
features were observed in the structure of
[(Bu2S)(Ph)Pd(m-C2O4)Pd(Ph)(SBu2)] [19] with C–O
1
NMR (acetone-d6): l(P) 7.3 (s, JPtP=3730 Hz).
,
distances of 1.253(3) and 1.247(3) A.
4.2. Reaction of cis-[Pt(NO3)2(PEt3)2] with sodium
ascorbate
3. Summary
[Pt(NO3)2(PEt3)2] (0.28 g, 0.50 mmol) in acetone (10
ml) was added dropwise to a solution of sodium ascor-
bate (0.20 g, 1.0 mmol) at r.t. under dinitrogen. After
20 h, the 31P{1H} NMR spectrum of an aliquot taken
from the reaction mixture showed two doublets at l(P)
The reactions of [Pt(NO3)2(PSP)] (PSP=dppm,
dppe, dppp) or cis-[Pt(NO3)2(PEt3)2] with sodium
ascorbate produce the ascorbate complexes [Pt(Asc-
O2,O3)(PSP)] or [Pt(Asc-O2,O3)(PEt3)2]. The dppe and
dppp derivatives are stable in solution, but the dppm
1
5.61 (2JPP=22 Hz, JPtP=3300 Hz) and 6.17 (2JPP=22
Hz, 1JPtP=3472 Hz), due to [Pt(Asc-O2,O3)(PEt3)2].

M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
2501
The solvent was removed in vacuo to produce a yellow
crystalline solid (0.10 g, 33%). H NMR (acetone-d6):
4.7. Preparation of [PdMe(Me2CO)(dppe)]BF4
1
l(H) 1.16 (12H, t, CH3), 3.22 (18H, m, CH2), 3.64 (2H,
A solution of AgBF4 (0.25 g, 1.32 mmol) in acetone
(20 ml) was added to a stirred solution of [Pd-
ClMe(dppe)] (0.56 g, 1.0 mmol) in acetone (20 ml)
under dinitrogen in the absence of light. The color of
the solution changed from yellow to a milky white due
to formation of AgCl. After 3 h the now brown reac-
tion mixture was filtered through Celite and the yellow
filtrate was reduced to 10 ml. The 31P{1H} NMR
spectrum of the filtrate (acetone-d6 added) exhibited
3
m, H6, H6’), 3.89 (1H, t, JHH=6 Hz, H5), 4.29 (1H,
3
d, JHH=6 Hz, H4). 31P{1H} NMR (acetone-d6): l(P)
2
1
2
5.6 (d, JPP=22 Hz, JPtP=3300 Hz), 6.2 (d, JPP=22
1
Hz, JPtP=3472 Hz).
4.3. Reaction of [Pt(NO3)2(dppm)] with sodium
ascorbate
2
resonances at 62.5 (d, JPP=25 Hz) and 39.0 ppm (br).
A mixture of [Pt(NO3)2(dppm)] (0.39 g, 0.5 mmol)
and sodium ascorbate (0.20 g, 1.0 mmol) in acetone (20
ml) was stirred at r.t. in the presence of air. The
reaction was monitored by NMR spectroscopy over a
period of 48 h. After 60 h, a cream-colored precipitate
was formed, which was collected by filtration and iden-
tified as [Pt(C2O4)(dppm)]. 31P{1H} NMR (CDCl3):
4.8. Formation of [Pd(C2O4)(dppe)]·H2O and
[Pd2(v-C2O4)(dppe)2][BF4]2
A solution of sodium ascorbate (0.20 g, 1.0 mmol) in
D2O (1 ml) was added under N2 to the yellow filtrate of
[PdMe(Me2CO)(dppe)]BF4 formed above. The solution
darkened immediately, but turned yellow again after 30
min. 31P NMR (acetone-d6): l(P) 63.3 (d, 2JPP=25
Hz), 61.1 (d, 2JPP=25 Hz). Yellow crystals of
[Pd(C2O4)(dppe)]·H2O suitable for X-ray diffraction
formed in the NMR tube after 3 days. 1H NMR
1
l(P) −63.5 (s, JPtP=3087 Hz). Crystals of [Pt(C2O4)-
(dppm)]·Me2CO were obtained by slow evaporation of
an acetone solution.
4.4. Reaction of [Pt(NO3)2(dppm)] with calcium
threonate
2
(CDCl3): l(H) 2.66 (d, 4H, JPH=20 Hz, CH2), 7.48–
7.85 (m, 20H, C6H5). A second yellow solid was iso-
lated after 60 h by removing the solvent in vacuo.
31P{1H} NMR (CDCl3): l(P) 58.0 (s). Crystals of
[Pd2(m-C2O4)(dppe)2][BF4]2 separated from this solution
on standing in the NMR tube.
A solution of calcium threonate (0.083 g, 0.25 mmol)
in D2O (10 ml) was added dropwise to a solution of
[Pt(NO3)2(dppm)] (0.193 g, 0.25 mmol) in acetone (20
ml) under an atmosphere of dinitrogen. The reaction
mixture was stirred at r.t. for 21 h. An aliquot of the
reaction mixture was transferred to a NMR tube and
analyzed by 31P{1H} NMR spectroscopy. It exhibited a
single resonance at −64.0 ppm (1JPtP=3040 Hz).
4.9. Reaction of [PdMe(Me2CO)(dppe)]BF4 with
dehydroascorbic acid
Silver tetrafluoroborate (0.01 g, 0.05 mmol) was
added to a solution of [PdClMe(dppe)] (0.028 g, 0.05
mmol) in acetone (20 ml), and the mixture was allowed
to stir for 2 h in the dark at r.t. It was filtered through
Celite to remove the precipitate of AgCl, and the pale
yellow filtrate was placed under dinitrogen. A solution
of dehydroascorbic acid (0.009 g, 0.05 mmol) in water
(5 ml) was added dropwise. An aliquot of the resulting
dark yellow reaction mixture was taken and transferred
to an NMR tube containing a few drops of acetone-d6.
4.5. Reaction of [Pt(NO3)2(dppm)] with oxalic acid
A mixture of [Pt(NO3)2(dppm)] (0.39 g, 0.5 mmol) in
acetone (20 ml) and oxalic acid (0.045g, 0.5 mmol) in
water (5 ml) was stirred at r.t. After 5 h a white solid
was collected by filtration (0.10 g, 50%). 31P{1H} NMR
1
(acetone-d6): l(P) −63.5 (s, JPtP=3040 Hz).
4.6. Preparation of [PdClMe(dppe)]
2
31P{1H} NMR: l(P) 34.2 (d, JPP=25 Hz), 58.0 (s),
2
A CH2Cl2 solution (20 ml) of dppe (0.80 g, 2.0 mmol)
was added dropwise to a stirred CH2Cl2 solution (40
ml) of [PdClMe(cod)] (0.53 g, 2.0 mmol) under dinitro-
gen at r.t. After 1.5 h the solvent was removed under
reduced pressure, and the resulting off-white solid was
62.5 (d, JPP=25 Hz).
4.10. Reaction of [PdMe(Me2CO)(dppe)]BF4 with
calcium threonate
1
washed with ether and dried in vacuo (0.95 g, 86%). H
A solution of [PdMe(Me2CO)(dppe)]BF4 was pre-
pared as above, and a solution of calcium threonate
(0.017 g, 0.06 mmol) in water (15 ml) was added
dropwise while stirring under dinitrogen. An aliquot of
the dark yellow reaction mixture was taken and trans-
ferred to an NMR tube containing a few drops of
NMR (CDCl3): l(H) 0.79 (dd, 3JPH=7, 3 Hz, 3H,
CH3), 2.37 (m, 4H, CH2), 7.40–7.65 (m, 20H, C6H5).
2
31P{1H} NMR (CDCl3): l(P) 31.5 (d, JPP=24 Hz),
59.2 (d, 2JPP=24 Hz). 31P{1H} NMR (acetone-d6):
2
2
l(P) 32.5 (d, JPP=25 Hz), 60.7 (d, JPP=25 Hz).

2502
M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
2
acetone-d6. 31P{1H} NMR: l(P) 34.2 (d, JPP=25 Hz),
58.0 (s), 62.5 (d, JPP=25 Hz).
4.13. Con6ersion of [Pd2(v-C2O4)(dppe)2][BF4]2 to
[Pd(C2O4)(dppe)]
2
A solution of [Pd2(m-C2O4)(dppe)2][BF4]2 in acetone-
d6 (1 ml) was placed in an NMR tube. Excess K2C2O4
in D2O (1 ml) was added and the NMR spectrum of the
reaction mixture was recorded. The peak originally
observed at 72.5 ppm disappeared, and a new reso-
nance was found at 62.5 ppm (s).
4.11. Reaction of [PdMe(Me2CO)(dppe)]BF4 with
oxalic acid
A solution of [PdMe(Me2CO)(dppe)]BF4 was pre-
pared as above, and excess oxalic acid in water (15 ml)
was added dropwise. An aliquot of the dark yellow
reaction mixture was taken after 2 h and transferred to
an NMR tube containing a few drops of acetone-d6.
31P{1H} NMR: l(P) 58.0 (s).
4.14. Preparation of [PdClMe(dppp)]
A solution of dppp (0.82 g, 2.0 mmol) in CH2Cl2 (20
ml) was added dropwise to a stirred CH2Cl2 solution
(40 ml) of [PdClMe(cod)] (0.53 g, 2.0 mmol) under
dinitrogen at r.t. After 1.5 h the solvent was removed
and the resulting white solid was washed with ether and
4.12. Preparation of [Pd2(v-C2O4)(dppe)2][BF4]2·2H2O
A solution of AgBF4 (0.13 g, 0.50 mmol) in acetone
(15 ml) was added to a solution of [PdClMe(dppe)]
(0.28 g, 0.50 mmol) in CH2Cl2 (15 ml), and the reaction
mixture was stirred in the dark for 2 h at r.t. The
solution was filtered through Celite to remove the white
precipitate of AgCl. Anhydrous oxalic acid (0.045 g,
0.50 mmol) was added to the yellow filtrate and the
solution was stirred for 1 h at room temperature. A
yellow precipitate formed which was collected by
filtration (0.177 g, 50%). Anal. Calc. for
C54H52B2F8O6P4Pd2: C, 49.60; H, 3.98. Found: C,
1
dried in vacuo (1.14 g, 100%). H NMR (CDCl3): l(H)
3
0.74 (dd, JPH=7, 3 Hz, 3H, CH3), 2.32 (m, 4H, CH2),
3.44 (m, 2H, CH2), 7.30–7.60 (m, 20H, C6H5). 31P{1H}
NMR (CDCl3): l(P) 6.8 (d, 2JPP=51 Hz), 27.0 (d,
2JPP=51 Hz).
4.15. Preparation of [PdMe(Me2CO)(dppp)]BF4
A mixture of [PdClMe(dppp)] (0.28 g, 0.51 mmol)
and AgBF4 (0.097 g, 0.51 mmol) in acetone (30 ml) was
stirred in the dark for 3 h. The precipitate of AgCl was
removed by filtration through Celite, and the filtrate
was placed under N2. 31P NMR (acetone-d6 added):
49.71; H, 3.83%. IR (Nujol): w(CO) 1608 cm−1 1H
.
NMR (CD3OD): l(H) 2.92 (d, 8H, 2JPH=24 Hz,
CH2), 7.70 (m, 40H, C6H5). 31P{1H} NMR (CDCl3):
l(P) 70.9 (s).
2
2
l(P) −3.7 (d, JPP=51 Hz), 30.7 (d, JPP=51 Hz).
Table 4
Crystal data and structure refinement for [Pt(C2O4)(dppm)]·Me2CO, [Pd(C2O4)(dppe)]·H2O and [Pd2(m-C2O4)(dppp)2][BF4]2·2Me2CO
[Pt(C2O4)(dppm)]·Me2CO
[Pd(C2O4)(dppe)]·H2O
[Pd2(m-C2O4)(dppp)2][BF4]2·2Me2CO
Crystal size (mm3)
Crystal system
Space group, Z
0.34×0.21×0.04
triclinic
0.38×0.38×0.33
orthorhombic
Pbca, 8
0.34×0.26×0.04
triclinic
(
(
P1, 2
P1, 2
Unit cell parameters
,
a (A)
9.9717(11)
11.0885(12)
13.6966(15)
76.749(7)
85.525(7)
76.231(7)
1431.3(3)
1.683
180(2)
5.051
1.94–26.46
24382
14.7148(3)
18.8486(3)
19.6469(3)
90
90
90
5449.13(16)
1.489
298(2)
0.834
2.04–25.00
99394
10.4624(1)
12.7079(2)
14.4698(2)
110.083(1)
102.728(1)
106.447(1)
1621.67(4)
1.449
213(2)
0.724
1.85–26.40
29734
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Density (g cm−3) (calculated)
Temperature (K)
Absorption coefficient (mm−1
Range (°)
)
Reflections collected
Independent reflections (Rint
)
5863 (0.045)
343
0.0240, 0.0652
0.0273, 0.0704
1.142
4806 (0.12)
333
0.0572, 0.1010
0.1003, 0.1142
1.102
6614 (0.05)
381
0.0323, 0.0685
0.0481, 0.0740
1.021
Least-squares parameters
R(F), Rw(F2) (F2\2.0|(F2))
R(F), Rw(F2) (all data)
S(F2)

M.J. Arendse et al. / Polyhedron 20 (2001) 2495–2503
2503
4.16. Reaction of [PdMe(Me2CO)(dppp)]BF4 with
References
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Acknowledgements
This work was supported through a grant from the
National Science Foundation (grant number CHE-
9508228), a University of Missouri-St. Louis Research
Award, and a grant from the University of Missouri/
University of the Western Cape Linkage Program.
Thanks are expressed to NSF for grants for the pur-
chase of NMR and X-ray instrumentation.
[32] Siemens Analytical X-Ray Division, Madison, WI, 1995.