Amine-bridged binuclear complexes involving [Pd(ethylenediamine)(H2O)2]2+, 4,4'-bipiperidine and DNA constituents

Article abstract of DOI:10.1080/00958972.2015.1035263

The complex formation equilibria in the reaction of [Pd(en)(H₂O)₂]²⁺ with 4,4'-bipiperidine (Bip) and DNA constituents such as inosine, inosine-5'-monophosphate, uracil, uridine, thymine, and thymidine were investigated at

Full text of DOI:10.1080/00958972.2015.1035263

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Amine-bridged binuclear complexes
involving [Pd(ethylenediamine)
(H2O)2]2+, 4,4′-bipiperidine and DNA
constituents
Perihan A. Khalaf Allaa, Mohamed M. Shoukryab & Rudi van Eldikcd
a Faculty of Science, Department of Chemistry, University of
Cairo, Cairo, Egypt
b Faculty of Science, Department of Chemistry, Islamic University,
Madina, Kingdom of Saudi Arabia
c Department of Chemistry and Pharmacy, University of Erlangen-
Nuremberg, Erlangen, Germany
d Faculty of Chemistry, Jagiellonian University, Krakow, Poland
Accepted author version posted online: 30 Mar 2015.Published
online: 29 Apr 2015.
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To cite this article: Perihan A. Khalaf Alla, Mohamed M. Shoukry & Rudi van Eldik (2015): Amine-
bridged binuclear complexes involving [Pd(ethylenediamine)(H2O)2]2+, 4,4′-bipiperidine and DNA
constituents, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2015.1035263
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Journal of Coordination Chemistry, 2015
http://dx.doi.org/10.1080/00958972.2015.1035263
Amine-bridged binuclear complexes involving [Pd
(ethylenediamine)(H2O)2]2+, 4,4′-bipiperidine and DNA
constituents
PERIHAN A. KHALAF ALLA†, MOHAMED M. SHOUKRY*†‡ and
RUDI VAN ELDIK§¶
†Faculty of Science, Department of Chemistry, University of Cairo, Cairo, Egypt
‡Faculty of Science, Department of Chemistry, Islamic University, Madina, Kingdom of Saudi Arabia
§Department of Chemistry and Pharmacy, University of Erlangen-Nuremberg, Erlangen, Germany
¶Faculty of Chemistry, Jagiellonian University, Krakow, Poland
(Received 9 January 2015; accepted 18 March 2015)
+2
Pd
+2
Pd
(en)
HN
NH
(en)
(100)
OH2
OH2
Ino
+
Pd
+2
Pd
HN
(en)
(en)
NH
(110)
OH2
Ino
Ino
+
Pd
+
Pd
HN
(en)
NH
(en)
(120)
Ino
Ino
Binuclear complexes involving 4,4′-bipiperidine linkage of two Pd(en)2+ species were investigated.
The binding of DNA constituents to the binuclear complex was studied.
The complex formation equilibria in the reaction of [Pd(en)(H2O)2]2+ with 4,4′-bipiperidine (Bip)
and DNA constituents such as inosine, inosine-5′-monophosphate, uracil, uridine, thymine, and thy-
midine were investigated at 25 °C and 0.1 M ionic strength. The [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+
complex and its hydrolyzed species were formed. Substitution of coordinated water by inosine (Ino)
as DNA constituent formed the complexes [(Ino)(en)Pd(Bip)Pd(en)(H2O)]3+ and [(Ino)(en)Pd(Bip)
Pd(en)(Ino)]2+. The formation of the binuclear complexes was further supported by spectral
*Corresponding author. Email: shoukrymm@hotmail.com
© 2015 Taylor & Francis

2
P.A. Khalaf Alla et al.
measurements. The formation constants of all possible mono- and binuclear complexes were
determined and their speciation diagrams were evaluated.
Keywords: Palladium(II) complexes; 4,4′-bipiperidine; DNA constituents; Binuclear complexes;
Equilibrium constants
1. Introduction
Platinum complexes such as cisplatin, carboplatin, and oxaliplatin are used for treatment of
different types of cancer [1, 2]. Drawbacks during treatment, such as vomiting, drug resis-
tance, nephrotoxicity, ototoxicity, neurotoxicity, and cardiotoxicity, encouraged researchers
to design new classes of platinum complexes with improved anti-tumor properties. The
recent generation of anti-tumor complexes such as orally active polynuclear Pt(II) com-
plexes are now being tested in preclinical trials [3, 4]. These polynuclear Pt(II) complexes
consist of either two or three platinum centers that are linked through a flexible bridge such
as an aliphatic chain [5–7] or a rigid bridge that consists, for instance, of 4,4′-bipiperidine
molecules [8]. The reason for increasing interest in multinuclear complexes is their ability
to form DNA adducts that differ significantly from those formed by cisplatin and related
complexes [9], resulting in a completely different anti-tumor behavior. Some of the dinu-
clear platinum(II) complexes are highly effective in vitro in cisplatin-resistant cell lines as
well as in several tumor cells [10, 11].
An apparent advantage of these types of complexes is the high charge (+4) compared to
the neutral mononuclear complexes, resulting in good solubility, efficient electrostatic
interaction with the poly-anionic DNA (the major pharmacological target of platinating
agents), and fast uptake [12]. Understanding of the chemical equilibria of these binuclear
complexes is of special concern for pharmaceutical and biomedical research.
Pd(II) and Pt(II) complexes have the same general structures and thermodynamic proper-
ties. However, the complexes formed by Pd(II) are five orders of magnitude more reactive
than their platinum counterparts. Therefore, Pd(II) complexes are good models for the
analogous Pt(II) complexes in solution. Current research in our laboratories focused on the
complex formation equilibria of (diamine)PdCl2 [13–19] and those of dinuclear palladium
(II) [20–22] complexes with bio-relevant ligands.
The present investigation involved the interaction of [Pd(en)(H2O)2]2+ with DNA
constituents and 4,4′-bipiperidine. Also, the binuclear complex involving 4,4′-bipiperidine
linking two Pd(en)2+ units was investigated. The binding of DNA constituents to the latter
complex were studied using potentiometric and spectrophotometric techniques.
2. Experimental
2.1. Materials
K2PdCl4, ethylenediamine (en), and 4,4′-bipiperidine 2HCl (Bip) were obtained from
Aldrich Chemical Co. The DNA constituents (inosine, inosine-5′-monophosphate (IMP),
thymine, thymidine, uracil, and uridine) were provided by Sigma Chemical Co. For equilib-
rium studies, [Pd(en)Cl2] was converted into the diaqua complex by treating it with two
equivalents of AgNO3 as described before [15]. All solutions were prepared in deionized
water. The structural formulas of the investigated ligands are given in scheme 1.

Binuclear complexes involving DNA
3
2.2. Synthesis
Pd(en)Cl2 was prepared by dissolving K2PdCl4 (2.82 mmol) in 10 mL water under stirring.
The clear solution of [PdCl4]2− was filtered and ethylenediamine (2.82 mmol), dissolved in
10 mL H2O, was added dropwise to the stirred solution. The pH was adjusted to 2–3 by
addition of HCl and/or NaOH. A yellowish-orange precipitate of Pd(en)Cl2 was formed and
stirred for a further 30 min at 50 °C. After filtering off the precipitate, it was thoroughly
washed with H2O, ethanol, and diethyl ether. A yellow powder was obtained. Anal Calcd
for C2H8N2PdCl2: C, 10.11; H, 3.37; N, 10.55. Found: C, 10.0; H, 4.25; N, 10.3%. Pd(en)
Cl2 was converted into the diaqua complex [Pd(en)(H2O)2](NO3)2 by stirring the chloride
complex with two equivalents of AgNO3 overnight, and removing the AgCl precipitate by
filtration through a 0.1 μm pore membrane filter. Great care was taken to ensure that the
resulting solution was free of Ag+ ions and that the chloride complex had been converted
into the aqua species. The filtrate was diluted to the desired volume in a standard volumetric
flask.
2.3. Potentiometric measurements
Potentiometric titrations were performed with a Metrohm 686 titroprocessor equipped with
a 665 Dosimat. The titroprocessor and electrode were calibrated with standard buffer solu-
tions, prepared according to NBS specification [23]. All titrations were carried out at 25.0
0.1 °C in purified nitrogen atmosphere using a titration vessel described previously [24].
The acid dissociation constants of the ligands were determined by titrating 0.05 mmol
samples of each with standard NaOH solutions. Ligands were converted into their
protonated form with standard HNO3 solutions. The acid dissociation constants of the
coordinated water molecules in [Pd(en)(H2O)2]2+ were determined by titrating 0.05 mmol
of complex with standard 0.05 M NaOH solution.
2.3.1. Preparation of potentiometric samples for mononuclear complexes. The forma-
tion constants of the complexes were determined by titrating solution mixtures of [Pd(en)
(H2O)2]2+ (0.05 mmol) and the ligand in the concentration ratio of 1 : 2 (Pd : ligand) for
the DNA constituents.
2.3.2. Preparation of potentiometric samples for binuclear complexes. The formation
constants of the binuclear complexes of Bip were determined by titrating a solution mixture
of 0.05 mmol of [Pd(en)(H2O)2]2+ and Bip in a concentration ratio of 2 : 1 (Pd : Bip). The
formation constants of the binuclear DNA complexes were determined by titrating solution
mixtures of 0.05 mmol of [Pd(en)(H2O)2]2+, Bip, and DNA constituents in a concentration
ratio of 2 : 1 : 2 (Pd:Bip:DNA constituent). The each titrated solution mixtures had a
volume of 40 mL and the titrations were carried out at 25 °C and 0.1 M ionic strength
(adjusted with NaNO3). A standard 0.05 M NaOH solution was used as titrant. The pH
meter readings were converted to hydrogen ion concentration by titrating a standard HNO3
solution (0.01 M), the ionic strength of which was adjusted to 0.1 M with NaNO3, with a
standard NaOH (0.05 M) solution at 25 °C. The pH values were plotted against p[H]
values. The relationship pH–p[H] = 0.05 was observed.

4
P.A. Khalaf Alla et al.
The species formed were characterized by the general equilibrium
pM þ qL þ rHꢀðMÞpðLÞqðHÞr
for which the formation constants are given by
½ðMÞpðLÞqðHÞrꢀ
bpqr
¼
[M]p [L]q[H]r
where M represents the [Pd(en)(H2O)2]2+ ion concentration, L stands for Bip or DNA, and
H for the proton. In case of the binuclear complex with DNA constituents, M, L, and H
stand for [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+, DNA constituent, and proton, respectively. The
calculations were performed using the computer program MINIQUAD-75 [25]. The stoi-
chiometry and stability constants of the complexes formed were determined by trying vari-
ous possible composition models for the systems studied. The model selected was that
which gave the best statistical fit and was chemically consistent with the magnitudes of
various residuals, as described elsewhere [25]. Tables 1–3 list the stability constants
together with their standard deviations and the sum of the squares of the residuals derived
from the MINIQUAD output. The concentration distribution diagrams were obtained with
the program SPECIES (Pettit, L., Personal Communication, University of Leeds, 1993)
under the experimental condition used.
2.4. Spectrophotometric measurements
Spectrophotometric measurements on Pd(en)–Bip complexes were performed by recording
the UV–visible spectra of solutions (A–D), where (A) 2 × 10−4 M of Pd(en)(H2O)22+; (B)
2 × 10−4
M M M of NaOH; (C)
of Pd(en)(H2O)22+ + 1 × 10−4 of Bip + 2 × 10−4
2 × 10−4 M of Pd(en)(H2O)22+ + 1 × 10−4 M of Bip + 4 × 10−4 M of NaOH; and (D)
1 × 10−4 M of Bip. The spectral measurements on binuclear complex of thymine, taken as
example for DNA, were performed by recording the UV–visible spectra of solutions (E–H),
where (E) 2 × 10−4 M of [Pd(en)(H2O)2]2+, 1 × 10−4 M of Bip and 2 × 10−4 M of NaOH;
(F) 2 × 10−4 M of [Pd(en)(H2O)2]2+, 1 × 10−4 M of Bip, 1 × 10−4 M of thymine and
3 × 10−4 M of NaOH; (G) 2 × 10−4 M of [Pd(en)(H2O)2]2+, 1 × 10−4 M of Bip, 2 × 10−4
M
of thymine and 4 × 10−4 M of NaOH; and (H) 2 × 10−4 M of thymine. Under these prevail-
ing experimental conditions and after neutralization of the hydrogen ions released during
complex formation, it was assumed that the complexes were completely formed. In each
mixture, the volume was increased to 10 mL by addition of deionized water and the ionic
strength was kept constant at 0.1 M NaNO3.
3. Results and discussion
3.1. Acid–base equilibria of the ligands
The acid dissociation constants of the ligands were determined in a solution of constant
ionic strength of 0.1 M (NaNO3) at 25 °C. The results obtained are in good agreement with
the literature data [26].

Binuclear complexes involving DNA
5
Table 1. Formation constants for complexes of [Pd(en)(H2O)2]2+ with DNA
constituents at 25 °C and 0.1 M ionic strength.
M L Ha
log10 βb
pKa
c
System
Pd(en)-OH
1 0 ꢁ1
1 0 ꢁ2
2 0 ꢁ2
−6.11 (0.03)
−15.35 (0.05)
−8.76 (0.09)
6.11
9.24
Inosine
0 1 1
1 1 0
1 2 0
1 1 1
8.80 (0.03)
6.83 (0.04)
11.26 (0.04)
12.10 (0.05)
8.80
5.27
Inosine-5′-
monophosphate
0 1 1
0 1 2
1 1 0
1 2 0
1 1 1
9.02 (0.02)
15.24 (0.03)
8.76(0.03)
12.31 (0.04)
15.26 (0.03)
9.02
6.22
6.50
9.18
Uracil
0 1 1
1 1 0
1 2 0
9.18 (0.01)
8.35 (0.01)
14.88 (0.02)
Thymine
Thymidine
Uridine
0 1 1
1 1 0
1 2 0
9.65 (0.01)
8.56 (0.01)
15.14 (0.02)
9.65
9.54
9.01
0 1 1
1 1 0
1 2 0
9.54 (0.02)
8.84 (0.08)
14.69 (0.08)
0 1 1
1 1 0
1 2 0
9.01 (0.01)
8.70 (0.02)
14.37 (0.03)
aM, L and H are the stoichiometric coefficients corresponding to Pd(en), DNA constituent and
H+, respectively.
blog β of Pd(en)–DNA complexes. Standard deviations are given in parentheses; sum of square
of residuals are less than 5e−7
.
cThe pKa of the protonated species (log β111 – log β110).
3.2. Complex formation equilibria of [Pd(en)(H2O)2]2+
The reaction of [Pd(en)(H2O)2]2+ with some DNA constituents was previously studied [27].
Its complex formation equilibria were investigated under the experimental condition used
for the formation of binuclear complexes.
[Pd(en)(H2O)2]2+ may undergo deprotonation. Its acid–base chemistry was characterized
by fitting the potentiometric data to various acid–base models. The best-fit model was con-
sistent with the formation of three species: 10-1, 10-2, and 20-2. The first two species, 10-1
and 10-2, are due to deprotonation of the two coordinated water molecules, as given in
equations (1) and (2). The third species, 20-2, is the dimeric di-μ-hydroxy-bridged complex
of two 10-1 species according to equation (3). The dimeric species 20-2 was detected by
El-Sherif [28] for a related system.
pKa1
2þ
½Pd(en)(H2O)2ꢀ ꢀ ½Pd(en)(H2OÞðOHÞꢀþ þ Hþ
(1)
100
10ꢁ1

6
P.A. Khalaf Alla et al.
pKa2
½Pd(en)(H2O)ðOHÞꢀþ ꢀ ½Pd(en)ðOHÞ2ꢀ þ Hþ
(2)
10ꢁ1
10ꢁ2
½Pd(en)ðH2OÞðOHÞꢀþ þ ½Pd(en)ðH2OÞðOHÞꢀþlog Kdimer
ꢀ
½PdðenÞðOHÞ2PdðenÞꢀ2þ þ 2H2O (3)
10ꢁ1
10ꢁ1
20ꢁ2
The pKa1 and pKa2 values for [Pd(en)(H2O)2]2+ are 6.11 and 9.24, respectively. The equilib-
rium constant for the dimerization reaction (3) can be calculated by equation (4) [29] as
logKdimer = 3.46.
logKdimer ¼ log b20ꢁ2 ꢁ 2log b10ꢁ1
(4)
DNA constituents such as uracil, uridine, thymine, and thymidine have basic nitrogen
donors (N3) in the measurable pH range [30, 31] and as a consequence form 1 : 1 and 1 : 2
complexes with Pd(en)2+ species. As a result of the high pKa values of pyrimidines
(pKa > 9), complex formation predominates above pH 8.5. The thymine complex is more
stable than that of uracil, probably due to the higher basicity of the N3 site of thymine
resulting from the inductive effect of the extra electron-donating methyl group. Cytosine
undergoes N3 protonation under mild acidic conditions. The value obtained for its protona-
tion constant is pKa 4.65. The lower values of the stability constants of its complexes, table
3, reflect the difference in the basicity of the donor site.
Inosine and nucleotides such as inosine-5′-monophosphate form the monoprotonated
complex, in addition to formation of 1 : 1 and 1 : 2 complexes. The pKa value of the proto-
nated inosine complex is 5.27. This value corresponds to N1H. The lowering of this value
with respect to that of free inosine (pKa = 8.80) is due to acidification upon complex forma-
tion [32, 33]. The IMP complex is more stable than that of inosine. This may be accounted
for on the basis of different coulombic forces operating between the ions resulting from the
negatively charged phosphate group. Hydrogen bonding between the phosphate group and
the exocyclic amine is also thought to contribute to the increased stability. Such hydrogen
bonding was reported previously for similar systems [34, 35].
14
12
10
pH
8
6
4
2
-
pd(en) + Bipiperidine
o calculated
0
1
2
3
4
Moles of base added per mole of Bip
Figure 1. Potentiometric titration curve for 0.05 mmols of [Pd(en)(H2O)2]2+ and 0.025 mmols of Bip at 25 °C
and 0.1 M NaNO3.

Binuclear complexes involving DNA
7
Table 2. Complex formation constants for the complexes formed
between [Pd(en)(H2O)2]2+ and bipiperidine at 25 °C and 0.1 M ionic
strength.
M L Ha
log10 βb
pKa
c
System
4,4′-Bipiperidine
0 1 1
0 1 2
1 1 0
1 1 1
2 1 0
10.96 (0.02)
21.12 (0.01)
13.31 (0.07)
19.66 (0.04)
20.00 (0.09)
10.96
10.16
6.35
aM, L and H are the stoichiometric coefficients corresponding to Pd(en), Bip and
H+, respectively.
blog10 β of Pd(en)–Bip complexes. Standard deviations are given in parentheses;
sum of square of residuals are less than 5e−7
.
cThe pKa of the ligand or the protonated complex.
3.3. Complex formation equilibria of the binuclear Pd(en)2+complex with
4,4′-bipipridine
The titration curve of a solution mixture of 4,4′-bipiperidine and Pd(en)(H2O)2]2+ in the
ratio 1 : 2, figure 1, shows a sharp inflection at a = 1 (a is number of mole of base added
per mole of 4,4′-bipiperidine), corresponding to the complete formation of the [(H2O)(en)Pd
(Bip)Pd(en)(H2O)]4+ complex with a formation constant of log β210 = 20.00, see scheme 2
and table 2.
Beyond a = 1, the binuclear complex is subjected to deprotonation. In this region, the
titration data were fitted considering the formation of the hydrolyzed species with stoichio-
metric coefficients of 10-1 and 10-2 as given in scheme 3. The pKa1 of the bridged complex
[(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+ is significantly higher than that of Pd(en)(H2O)2 and in
2+
fair agreement with its pKa2 value. This may be due to the electron donation of 4,4′-bip-
iperidine in coordination with the PdII center. This is expected to decrease the electrophilic-
ity of the PdII ion, to weaken the Pd–OH2 bond and consequently increase the pKa value.
Also, the pKa1 and pKa2 values of the bridged complex [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+
are nearly equal. This indicates that the two Pd(en) units behave as two separated species,
i.e. the Bip bridge does not allow the two Pd(en) units to communicate electronically with
each other. This can be explained on the basis of the absence of π-conjugation in the Bip
bridge.
100
210
100
80
60
40
20
0
110
111
4
2
3
5
6
7
8
9
10
11
12
pH
Figure 2. Concentration distribution of various species as a function of pH in the [Pd(en)(H2O)2]2+-4,4′-
bipiperidine system.

8
P.A. Khalaf Alla et al.
Table 3. Formation constants for the binuclear complexes of [(H2O)(en)Pd(Bip)Pd(en)
(H2O)]4+ with some DNA constituents at 25 °C and 0.1 M ionic strength.
M L Ha
log10 βb
pKa
c
System
[(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+
1 0 ꢁ1
1 0 ꢁ2
−9.64 (0.04)
−19.03 (0.02)
9.64
9.39
Inosine
0 1 1
1 1 0
1 2 0
8.80 (0.02)
5.40 (0.03)
9.87 (0.05)
8.80
Inosine-5′-monophosphate
0 1 1
0 1 2
1 1 0
1 2 0
9.02 (0.02)
15.24( 0.03)
5.52(0.02)
9.02
6.22
10.00 (0.04)
Uracil
0 1 1
1 1 0
1 2 0
9.18 (0.01)
5.98 (0.04)
10.94 (0.06)
9.18
9.65
9.54
9.01
Thymine
Thymidine
Uridine
0 1 1
1 1 0
1 2 0
9.65 (0.01)
6.16 (0.03)
11.38 (0.03)
0 1 1
1 1 0
1 2 0
9.54( 0.02)
5.25 (0.05)
10.49 (0.02)
0 1 1
1 1 0
1 2 0
9.01 (0.01)
5.04 (0.02)
10.15 (0.03)
aM, L and H are the stoichiometric coefficients corresponding to [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+
,
DNA constituent, and H+ respectively.
blog10 β of binuclear complexes, Standard deviations are given in parentheses; sum of square of
residuals are less than 5e−7
.
cthe pKa of the ligands or the aqua complexes.
The speciation diagram for the Pd(en)-bipiperidine system is given in figure 2. The binu-
clear complex, [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+ (210), starts to form at low pH and on
increasing pH its concentration increases. It is the predominant species up to pH 7.2 and
reaches a maximum concentration of 98.13% at pH 7.6.
100
100
120
10-2
80
60
110
40
20
10-1
0
2
3
4
5
6
7
8
9
10
11
12
pH
Figure 3. Concentration distribution of various species as a function of pH in the [(H2O)(en)Pd-Bip-Pd(en)
(H2O)]4+-inosine system.

Binuclear complexes involving DNA
9
3.4. Complex formation equilibria of the binuclear Pd(en)2+complex involving
4,4′-bipiperidine and some selected DNA constituents
The complex formation between [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+ and inosine, taken as an
example for a DNA constituent, showed the formation of 1 : 1 and 1 : 2 complexes, as
given in scheme 4. The stability constant of the DNA complexes is of the order
thymine > uracil > inosine. The thymidine complex is more stable than that of uridine, see
table 3. This may be explained as a result of the difference in the basicity of the donor atom
as reflected by the pKa values.
The speciation diagram of the [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+-inosine complex is
given in figure 3. The 1 : 1 complex starts to form at pH 4 and on increasing pH its concen-
tration increases and reaches a maximum concentration of 59.11% at pH 7. The 1 : 2 com-
plex attains a maximum degree of formation of 78.17% at pH 10. The hydrolyzed species
are formed above pH 9.0. From a biological point of view, it is interesting to note that the
DNA complex predominates in the physiological pH range, and the reaction of the binu-
clear complex with DNA is quite feasible.
2+
Spectral bands for Pd(en)(H2O)2 and its 4,4′-bipiperidine complex were compared.
They are quite different in terms of the position of the maximum wavelength and molar
absorptivity, see figure 4. The spectrum of the [Pd(en)(H2O)2]2+complex (mixture A) shows
an absorption maximum at 340 nm. The spectrum obtained for the [(H2O)(en)Pd(Bip)Pd
(en)(H2O)]4+ complex (mixture B) exhibits a band at 322 nm. This band is further shifted
to 316 nm by adding 2 × 10−4 M NaOH for the formation of [(OH)(en)Pd(Bip)Pd(en)
(OH)]2+ (mixture C). There is no UV absorption for free Bip in this region (mixture D).
2+
Spectral bands of the Pd(en)(H2O)2 complexes involving 4,4′-bipiperidine and thymine
were compared. The spectrum obtained for [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+ (mixture E)
occurs at 322 nm. The spectrum obtained for [(H2O)(en)Pd(Bip)Pd(en)(thymine)]3+ (mix-
ture F) exhibits a shoulder at 322 nm. The spectra of the binuclear complexes [(H2O)(en)Pd
(Bip)Pd(en)(thymine)]3+ (mixture F) and [(thymine)(en)Pd(Bip)-Pd(en)(thymine)]2+
(mixture G) show an isosbestic point at 322 nm, indicating an equilibrium between these
complexes. Further information on the binuclear complex formation may require further
studies such as mono and polynuclear NMR measurements.
0.35
0.3
0.25
0.2
C
B
A
D
250
350
Wavelength (nm)
450
Figure 4. The electronic spectra of Pd(en)–Bip complexes. Compositions of the solution mixtures A, B, C and D
are given in the Experimental Section.

10
P.A. Khalaf Alla et al.
O
O
N
7
6
N
7
HN
1
HN
1
O
P
9
N
3
N
9
N
3
O
OH
N
O
O
O
O
OH
OH
OH
OH
Inosine
Inosine-5´-monophosphate
O
O
O
CH3
CH3
4
4
4
HN
3
HN
HN
3
3
2
2
2
1
1
N
1
N
H
O
O
O
N
OH
O
H
Thymine
Uracil
Thymidine
OH
O
4
HN
3
2
1
O
N
OH
O
OH
OH
Uridine
Scheme 1. Structural formulas of the DNA constituents.
+2
+
HN
(en) Pd
NH2
(111)
OH2
- H+
+2
HN
Pd
NH
(en)
(110)
OH2
HN
2+
(en)(H O)
Pd
2
2
+2
+2
Pd (en)
Pd
NH
(en)
(210)
OH2
OH2
Scheme 2. Complex formation equilibria of Pd(en)–Bip complexes.

Binuclear complexes involving DNA
11
+2
+2
HN
NH
(en)
Pd
(en) Pd
(100)
OH2
OH2
-H+
+
+2
HN
HN
(en)
(en)
Pd
Pd
NH
NH
(10-1)
-H+
OH
OH2
+
+
(en)
(en)
Pd
Pd
(10-2)
OH
OH
Scheme 3. Acid–base equilibria of [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+
.
+2
Pd
+2
Pd
(en)
HN
NH
(en)
(100)
OH2
OH2
Ino
+
Pd
+2
Pd
HN
(en)
(en)
(en)
NH
(110)
OH2
Ino
Ino
+
Pd
+
Pd
HN
NH
(en)
(120)
Ino
Ino
Scheme 4. Complex formation equilibria of the [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+-inosine complex.
4. Conclusions
The present investigation describes complex formation equilibria of [Pd(en)(H2O)2]2+with
selected DNA constituents and 4,4′-bipiperidine. The results indicate that the formation of
binuclear complexes and the reaction with DNA constituents is feasible. The data support

12
P.A. Khalaf Alla et al.
the biological significance of di- and trinuclear platinum(II) complexes having potent anti-
tumor activity [36]. In this study, [Pd(en)(H2O)2]2+ forms the dihydroxo-bridged dimer (20-
2), as reported for most Pd-diamine complexes. The stability constant of the Pd(en)-inosine
complex is lower than that for the Pd(pic)–inosine complex, where pic = 2-picolylamine
[18]. This is attributed to the π-acceptor properties of the pyridyl group of pic, which leads
to an increase in the electrophilicity of Pd(II) and consequently increases the stability con-
stants of its complex. Therefore, the structure of the diamine has an effect on the stability
of the DNA adduct.
The formation of the binuclear complex through bridging with 4,4′-bipiperidine decreases
the electrophilicity of the PdII centers. This in turn leads to an increase in the pKa of the
diaqua-bridged complex and a decrease in the stability constants of the binuclear DNA
complexes. The Bip bridge does not allow the two Pd(en) units to communicate electroni-
cally with each other. This is evidenced by the similarity of the pKa values of the diaqua-
bridged complex.
It is interesting to compare the results of this study with those of the previously reported
results for similar binuclear Pd(II) complexes. The stability constant of the binuclear inosine
complex formed with [(H2O)(NH3)2Pd(Bip)Pd(NH3)2(H2O)]4+ is log10 K = 5.51, as
reported before [20], in good agreement with that of the binuclear inosine complex formed
with [(H2O)(en)Pd(Bip)Pd(en)(H2O)]4+ (log10 K = 5.40), as reported in the present study.
Disclosure statement
No potential conflict of interest was reported by the authors.
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