Palladium phosphine complexes from 8-(thio)-theophylline, 8-(methylthio)-theophylline and 8-(benzylthio)-theophylline

Article abstract of DOI:10.1016/S0020-1693(00)00177-8

A series of palladium(II) complexes have been synthesized and characterized to examine the coordination chemistry of several 8-thio-purine derivatives: 8-(thio)-theophylline (8-TTH₂), 8-(benzylthio)-theophylline (8-BzTTH) and 8-(methylthio)-theophylline (8-MTTH). All of the complexes have been studied by IR and multinuclear (¹H, ³¹P, ¹³C NMR) spectroscopy. A representative example of a Pd complex, [cis-Pd(8-BzTT)₂(PPh₃)₂], has been authenticated by X-ray crystallography. In this complex square-planar coordination about palladium occurs through two deprotonated N7 purine atoms cis to each other, and two triphenylphosphine phosphorus atoms. In this paper, we present and discuss details of the synthesis, characterization and structural properties of the novel complexes. The synthesis and the characterization of 8-(benzylthio)-theophylline (8-BzTTH), a new organic ligands based on the molecular skeleton of 8-thio-purine, and a new procedure to obtain 8-(methylthio)-theophylline (8-MTTH) are also presented. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00177-8

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 307 (2000) 125–130
Note
Palladium phosphine complexes from 8-(thio)-theophylline,
8-(methylthio)-theophylline and 8-(benzylthio)-theophylline
Antonio Romerosa a,*, Carlos Lo´pez-Magan˜a a, Mustapha Saoud a, Sonia Man˜as a,
Enrique Colacio b, Jose´ Sua´rez-Varela b
a Departamento de Qu´ımica F´ısica, Bioqu´ımica y Qu´ımica Inorga´nica, Facultad de Ciencias Experimentales, Uni6ersidad de Almer´ıa,
04071 Almer´ıa, Spain
b Departamento de Qu´ımica Inorga´nica. Facultad de Ciencias, Uni6ersidad de Granada, A6da, Fuente Nue6a s/n, 18071 Granada, Spain
Received 1 June 1999; accepted 22 September 1999
Abstract
A series of palladium(II) complexes have been synthesized and characterized to examine the coordination chemistry of several
8-thio-purine derivatives: 8-(thio)-theophylline (8-TTH2), 8-(benzylthio)-theophylline (8-BzTTH) and 8-(methylthio)-theophylline
(8-MTTH). All of the complexes have been studied by IR and multinuclear (1H, 31P, 13C NMR) spectroscopy. A representative
example of a Pd complex, [cis-Pd(8-BzTT)2(PPh3)2], has been authenticated by X-ray crystallography. In this complex square-pla-
nar coordination about palladium occurs through two deprotonated N7 purine atoms cis to each other, and two triphenylphos-
phine phosphorus atoms. In this paper, we present and discuss details of the synthesis, characterization and structural properties
of the novel complexes. The synthesis and the characterization of 8-(benzylthio)-theophylline (8-BzTTH), a new organic ligands
based on the molecular skeleton of 8-thio-purine, and a new procedure to obtain 8-(methylthio)-theophylline (8-MTTH) are also
presented. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Palladium complexes; Purine base complexes
bioinorganic chemistry due to the central role that these
compounds have in the chemistry of nucleic acid
1. Introduction
derivatives [1]. Purines are multidentate ligands exhibit-
ing a variety of potential donor groups and conse-
and pyrimidine bases are of paramount importance in
Studies on the interactions of metal ions with purine
quently behave as effective ligands for a wide range of
metal ions. In fact, in addition to the N donor atoms,
and to the carboxylate group, it has recently been
shown that purine bases can even bind through C
atoms. In particular, it has been demonstrated that the
C(8) site in caffeine can efficiently coordinate to
[Ru(NH3)3Cl2] [2a], [Os(NH3)5] [2b] and 9-(2-(2-
aminoethylamino)ethyl)adenine [3] with [RuCl2-
(DMSO)2]. In order to study this very rare kind of
interaction between purine bases and metals, that could
be of biological relevance, the synthesis of new exam-
Scheme 1.
ples of complexes exhibiting the C8–metal interaction
is of interest and largely desirable. In this regard, our
* Corresponding author. Tel./fax: +34-950-21 5008.
group has recently shown a novel route to C8
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00177-8

126
A. Romerosa et al. / Inorganica Chimica Acta 307 (2000) 125–130
coordination of theophylline under mild conditions [4]
by previous coordination of 8-(methylthio)-theophylline
(8-MTTH), a 8-thio-purine base similar to those found
in retinal rod photoreceptors [5], to palladium.
In order to obtain more information about the chem-
ical properties of 8-thio purine derivatives we have
studied the reactivity of [cis-PdCl2(PPh3)2] with 8-
(thio)-theophylline (8-TTH2), 8-(methylthio)-theophyl-
line (8-MTTH) and 8-(benzylthio)-theophylline (8-
BzTTH) (Scheme 1).
atoms (except phenyl hydrogen) were placed at calcu-
lated positions and then were refined isotropically.
The final R(Fo) value was 0.0987 [R(Fo)all
0.2302, wR(Fo)=0.1726, wR(Fo)all data=0.2235].
=
data
The structure was solved using the SIR97 program [7].
In the final refinement the SHELX-96 program was used
[8] and in the graphic representation the ORTEP pro-
gram was used [9].
2.2. Preparation
2.2.1. Synthesis of 8-MTTH (1) and 8-BzTTH (2)
The two ligands 1 and 2 were prepared using a
similar procedure. CH3Br (0.5 g, 5.3 mmol) or
PhCH2Br (0.9 g, 5.3 mmol) (8-MTTH or 8-BzTTH,
respectively) was added to a suspension of KTTH in 20
ml of EtOH, which is generated in situ by reaction of
TTH2 (1 g, 5 mmol) with 1 equiv. of KOH (0.28 g, 5
mmol). After 6 h refluxing, the precipitated obtained
was filtered out, washed with H2O (2×2 ml), EtOH
(2×2 ml) and finally air dried.
2. Experimental
All reagents were of analytical grade and were used
without further purification. [cis-PdCl2(PPh3)2] [4] and
the ligand 8-TTH2 were prepared as described previ-
ously [6]. The ligand 8-MTTH was synthesized as de-
scribed in the literature [6] or with the improved
procedure described below. Deuterated solvents for
NMR measurements were dried over molecular sieves
1
(0.4 nm). H, 13C{1H} and 31P{1H} NMR spectra were
(2) 8-BzTTH.
recorded on a Bruker AVANCE DRX 300 spectrome-
ter. Peak positions are relative to tetramethylsilane (1H,
13C{1H}) and were calibrated with respect to the resid-
ual protonated solvent (1H) or to the solvent resonance
(13C). The 31P{1H} NMR spectra are given with respect
to external 85% H3PO4 in D2O with downfield values
taken as positive. Infrared spectra were recorded (on
KBr discs) on a IR-ATI Mattson Infinity Series. Ele-
mental analysis (C, H, N, S) were performed on a
Fisons Instruments EA 1108 elemental analyser.
Yield (1): 76%.
Yield (2): 83%; m.p. 277–278°C.
2.3. Synthesis of the complex
DMSO·[PdCl(8-TTH)2(DMSO)(PPh3)] (3)
Reaction of 8-TTH2 (0.1 g, 0.5 mmol) with KOH
(0.028 g, 0.5 mmol) and [cis-PdCl2(PPh3)2] (0.35 g, 0.5
mmol) in 20 ml of EtOH at reflux yielded a red–orange
precipitate, which was filtered out, washed with two
portions of cold EtOH and air dried. By slow evapora-
tion of a solution of this red powder in DMSO, red
microcrystals were obtained, which were filtered out,
washed with two portions (2 ml) of EtOH and air dried.
Yield: 44% from 8-TTH2.
2.1. X-ray crystallography
The crystal used for X-ray work (0.25×0.15×0.11
mm) was obtained from a solution of [cis-Pd(8-
BzTT)2(PPh3)2] (5) in CHCl3 by slow evaporation of the
solvent. Data were collected at 294 K on a Stoe–
Siemens AED diffractometer, using graphite-
2.4. Synthesis of the complexes [PdCl(8-BzTT)(PPh3)2]
(4) and [Pd(8-BzTT)2(PPh3)2] (5)
,
monochromatized Mo Ka radiation (u=0.71069 A).
The unit cell parameters were obtained from least-
squares refinement of 25 well-centered reflections (5B
qB15°). The data were collected by the ꢀ−2q scan
mode (1.52B2qB19.96°) and were corrected for
Lorentz and polarization effects and for absorption.
The compound crystallizes in the monoclinic system,
In a similar procedure to that described above for
compound 3, the reaction of 8-BzTTH, KOH and
[cis-PdCl2(PPh3)2] in 1:1:1 and 2:2:1 molar ratios, re-
spectively, gave rise to red–orange (4) and orange (5)
precipitates.
Yield (4): 45%.
Yield (5): 60%.
space group P21/c, with a=13.434(5), b=20.783(5),
3
,
,
c=21.091(5) A, i=92.96(5)°, U=5881(3) A , Z=4,
Dc=1.348 g cm−3 and v=0.495 mm−1. 5438 unique
reflections were collected (2qmax=40, h=0–12, k=0–
19, l= −20 to 20). Due to the poor statistics of the
weak reflections, 2385 reflections with ꢀFꢀ\2|(F) were
included in the calculations.
2.5. Synthesis of the complex [Pd(8-MTT)2(PPh3)2] (6)
As reported above for 5, the reaction of 8-MTTH
with KOH and [cis-PdCl2(PPh3)2] in a 2:2:1 molar ratio
in refluxed ethanol gave 6 as an orange powder.
Yield: 36% (Tables 2–5).
In the final refinement, all purine non-hydrogen
atoms were refined anisotropically and purine hydrogen

A. Romerosa et al. / Inorganica Chimica Acta 307 (2000) 125–130
127
Table 1
Selected bond distances (A) and angles (°) for (8-BzTT)2Pd(PPh3)2 (5)
Compound 8-MTTH was authenticated by compari-
son of its spectroscopic data with those reported in the
literature [6].
,
Pd1ꢀN7B
Pd1ꢀN7A
Pd1ꢀP1
2.043(16)
2.060(15)
2.319(6)
2.326(6)
1.74(2)
1.779(18)
1.75(2)
1.82(2)
1.46(2)
1.42(3)
1.41(2)
1.23(2)
1.37(3)
1.45(2)
1.36(2)
1.20(2)
1.46(3)
1.35(3)
1.38(3)
1.35(2)
1.36(2)
1.37(2)
C10AꢀC11A
C1BꢀN1B
N1BꢀC2B
N1BꢀC6B
C2BꢀO2B
C2BꢀN3B
C3BꢀN3B
N3BꢀC4B
C6BꢀO6B
C6BꢀC5B
C4BꢀN9B
C4BꢀC5B
C5BꢀN7B
N7BꢀC8B
C8BꢀN9B
C10BꢀC11B
1.54(2)
1.43(2)
1.40(2)
1.42(2)
1.22(2)
1.38(3)
1.47(2)
1.41(3)
1.24(2)
1.39(3)
1.32(2)
1.38(3)
1.39(2)
1.37(2)
1.36(2)
1.48(2)
NMR experiments alone provide an unequivocal as-
signment of the structure proposed for 8-BzTTH. Key
points of the NMR analysis (1H NMR) are: a reso-
nance at l 13.38 ppm, typical of a NꢀH group [4], the
absence of any SꢀH resonance and the presence of a
singlet that can safely be ascribed to a SꢀCH2ꢀ. A
perusal of all of these points suggest that the ligand is
produced by substitution on the 8-TTH2 sulfur atom
with a benzyl group. The 13C NMR spectrum is in
agreement with the formula proposed. In particularly,
the C-8 resonance arises (148.7 ppm) in a closer posi-
tion to that in 8-MTTH (153.0 ppm) than to that in
8-TTH2 (163.8 ppm). This result suggests that there is a
strong influence of the nature of the substituent on the
electronic behaviour of the imidazolic ring of the purine
base.
Pd1ꢀP2
S1ꢀC8A
S1ꢀC10A
S2ꢀC8B
S2ꢀC10B
C1AꢀN1A
N1AꢀC2A
N1AꢀC6A
C2AꢀO2A
C2AꢀN3A
C3AꢀN3A
N3AꢀC4A
C6AꢀO6A
C6AꢀC5A
C4AꢀN9A
C4AꢀC5A
C5AꢀN7A
N7AꢀC8A
C8AꢀN9A
3.2. Palladium complexes (3, 4, 5, 6)
N7BꢀPd1ꢀN7A
N7BꢀPd1ꢀP1
N7AꢀPd1ꢀP1
N7BꢀPd1ꢀP2
N7AꢀPd1ꢀP2
P1ꢀPd1ꢀP2
87.1(6)
169.2(5)
88.0(4)
87.4(5)
168.2(5)
99.1(2)
The ligands 8-TTH2, 8-MTTH and 8-BzTTH react in
EtOH with KOH and [cis-PdCl2(PPh3)2] to afford a
variety of palladium complexes according to the molar
ratio between reactants and the particular ligand.
Noticeably, the reaction of 8-TTH2 with KOH and
[cis-PdCl2(PPh3)2] leads to the formation of a red pre-
cipitate (compound 3a) irrespective of the stoichiometry
adopted. The IR spectrum shows a characteristic
3. Results and discussion
3.1. Ligands
w(NꢀH) purine broad absorption (2780–3250 cm−1
)
and significant coordinated purine and PPh3 absorption
signals. This red precipitate is practically insoluble in
most of the organic and inorganic solvents and only in
hot DMSO it exhibits a solubility sufficient to run
NMR spectra.
The ligands 8-(methylthio)-theophylline (8-MTTH)
(1) and 8-(benzylthio)-theophylline (8-BzTTH) (2) were
obtained by reaction of 8-TTH2 with KOH and BrCH3
with BrCH2Ph in a 1:1:1 molar ratio.
Table 2
Analytical results for the complexes
Found (calc.) (%)
C
H
N
S
2
3
4
5
6
C
14H14N4O2S1
55.8 (55.6)
45.5 (45.14)
62.3 (62.0)
62.4 (62.3)
57.3 (57.7)
4.5 (4.6)
4.7 (4.44)
4.7 (4.5)
4.5 (4.5)
4.2 (4.5)
17.9 (18.5)
6.8 (7.26)
5.5 (5.8)
10.1 (9.1)
10.1 (10.4)
10.0 (10.6)
12.8 (12.5)
3.1 (3.3)
4.9 (5.2)
5.4 (5.9)
C29H34N4O4S3ClPPd
C50H43N4O2SClP2Pd
C64H56N8O4S2P2Pd
C52H48N8O4S2P2Pd
Table 3
1H parameters (l in ppm, J in Hz. All spectra recorded in CDCl3, 3 in DMSO-d6)
N1ꢀCH3
N3ꢀCH3
N7ꢀH
SꢀCH3
SꢀCH2
Phꢀ
PPh
2
3
4
5
6
3.24
3.14
3.16
3.04
3.25
3.46
3.52
3.25
3.23
3.34
13.38
11.58
4.49(s)
7.42–7.25
7.29–7.70
7.67–7.23
7.41–7.18
4.15
7.10
6.94–7.08
5.20 (AB, J=13.39)
2.29

128
A. Romerosa et al. / Inorganica Chimica Acta 307 (2000) 125–130
Table 4
13C{1H} parameters (l in ppm. All spectra recorded in CDCl3)
C2
C4
C5
C6
C8
N1ꢀCH3
N3ꢀCH3
SꢀCH3
SꢀCH2
Ph
a
8-TTH2
1
149.9
150.0
153.8
139.3
142.2
137.7
103.5
107.6
108.3
151.4
151.0
151.4
163.8
153.0
148.7
27.7
27.5
28.2
31.0
29.6
30.3
14.0
2
36.1
127.9–129.4
3 b
4
154.8
155.0
138.5
142.2
114.0
114.7
151.5
150.8
150.2
148.4
27.6
27.6
29.1
28.8
36.3
40.8
134.8–126.7
135.9–126.7
5
6 b
a Included for comparision.
b No soluble enough.
At room temperature the 31P{1H} NMR spectrum
features two distinct resonances, which transform in a
single resonance (23.70 ppm) at 60°C. The H NMR
spectra reveal only one ligand is coordinated to the
palladium by substitution of a chloride atom. In this
case, the two PPh3 groups could be trans to each other
because the 31P NMR spectrum exhibits only a single
resonance with a chemical shift similar to those re-
ported for trans-PPh3 palladium complexes [10]. In the
synthetic reaction of that compound a cis to trans
isomerization occurs, as previously reported for [trans-
PdCl(8-MTT)(PPh3)2] [4]. Moreover, when a 1:2 metal-
to-ligand molar ratio is used a complex resulting from
the substitution of two chloride atoms by two 8-BzTT−
is obtained. In such a case, the 1H NMR spectrum
shows that SꢀCH2ꢀ protons of both purine molecules
arise as an AB system, which points out two protons
that are magnetically different to each other in both
molecules. Finally, the PPh3 resonance in the 31P NMR
suggests a cis-disposition of the phosphines, as confi-
rmed by X-ray crystallography.
The reactivity of 8-MTTH is similar to that reported
above for 8-BzTTH. The crystal structure of [trans-
PdCl(8-MTT)(PPh3)] in which only a single chloride
atom is substituted has been published previously [4].
Nevertheless, a different complex where both chloride
ligands have been replaced (compound 6) can be ob-
tained by reacting 8-MTTH with KOH and [cis-
Cl2Pd(PPh3)2] in a 2:2:1 molar ratio in refluxed EtOH.
In this case, the 31P NMR spectrum suggests a trans
disposition of the phosphines.
1
spectrum at this temperature shows a broad signal at
11.53 ppm, which is assigned to the HꢀN7 proton. At
lower temperature, when two equilibrating species are
1
present in solution the H NMR spectra shows a single
resonance at 11.58 ppm, suggesting that the purine
ligands are coordinated to the metal in the same way.
Resonances due to ꢀCH3 purine groups match with this
supposition. Finally, by slow evaporation of a solution
of the red precipitate in DMSO, a red microcrystal
(compound 3) was obtained, which displayed the same
spectroscopic behaviour.
The IR spectra of compound 3 shows bands of free
and coordinated DMSO molecules (w(SꢁO)=1070,
1022 cm−1). This finding is supported by elemental
analysis of this recrystallised compound, which is in
accord with Pd(8-TTH)(DMSO)2(PPh3)Cl composition.
These data suggest the composition [Pd(8-
TTH)(DMSO)(PPh3)Cl] DMSO for this compound,
with the ligand being coordinated to the metal through
the sulfur atom.
Unfortunately, the X-ray structure of the red precipi-
tate 3a could not be determined, although we can
hypothesize from the experimental data that
tryphenylphosphine, a ligand and likely a chloride atom
are coordinated to the metal. However, we cannot
exclude a priori that this compound has a polymeric
nature with a chloride atom acting as bridging ligand
between two single units of Pd(8-TTH)(PPh3).
On the other hand, compounds 4 and 5 are obtained
when 1 mol of 8-BzTTH reacts with 1 and 1/2 mol of
[cis-Cl2Pd(PPh3)2]. All of the obtained complexes do
not show any absorbance assignable to the N7ꢀH
stretching vibrations in the infrared spectra. The ab-
Table 5
31P{1H} parameters (l in ppm. All spectra recorded in CDCl3, 3 in
DMSO-d6)
l (T=r.t.)
l (T=60°C)
1
sence of the N7ꢀH resonance in the H NMR spectra,
3
23.77 (29%)
26.98 (71%)
21.49
23.76 (100%)
compared with the free ligand 8-BzTTH, suggests that
the ligand coordination site is N7 atom. When a 1:1
molar ratio of 8-BzTTH and [cis-Cl2Pd(PPh3)2] was
used to carry out the reaction, the elemental analysis of
the final compound and the integration of the 1H NMR
4
5
6
30.94
14.95

A. Romerosa et al. / Inorganica Chimica Acta 307 (2000) 125–130
129
triphenylphosphine ligands and the phenyl groups of
the purine ligands.
Finally, bond distances and angles are similar to
those found in other N(7)-bonded purine complexes
[4,11], and there are no significant interactions between
molecules.
4. Conclusions
The reactivity in basic media (KOH in EtOH) of a
series of thio-theophilline derivatives towards [cis-
PdCl2(PPh3)2] suggests that these compounds present a
different behaviour depending on whether or not the
sulfur purine atom is substituted. 8-(Thio)-theophylline
(8-TTH2) reacts only through the sulphur S8 atom with
the N7 atom not being coordinated. Unlike TTH2,
8-(methylthio)-theophylline (8-MTTH) and 8-(benz-
ylthio)-theophylline (8-BzTTH) react only through the
N7 purine atom. Cleavage of the C8ꢀS bond is never
observed. The complexes obtained by reaction with
[cis-PdCl2(PPh3)3] have different stoichiometry depend-
ing on the molar ratio of the reactants. In reactions
with 8-BzTTH (1:1 molar ratio) and 8-MTTH (in any
ratio) [4] cis–trans isomerization of the ligands around
metal is observed.
Fig. 1. Molecular diagram of the Complex (8-BzTT)2Pd(PPh3)2 (5).
Triphenylphosphine phenyl groups are not included for clarity.
3.3. Crystal and molecular structure of
(8-BzTT)2Pd(PPh3)2 (5)
In order to confirm the structural composition of the
palladium derivatives and to collect novel geometrical
information on this class of compounds, an X-ray
analysis has been carried out on a single crystal of
[cis-Pd(8-BzTT)2(PPh3)2]. An ORTEP drawing of the
complex is shown in Fig. 1 with the atomic numbering
scheme. A list of selected bond distances and angles
appear in Table 1.
Acknowledgements
We acknowledge the help given by Maurizio Pe-
ruzzini (ISSECC-CNR, Firenze). This work was sup-
ported by the Programa Hispano-Marroqu´ı (AECI,
MAE).
In this compound, the palladium(II) atom is coordi-
nated by the phosphorus atoms of two triphenylphos-
phine molecules and through the N7 atoms of two
8-BzTT− anions. As anticipated by NMR analysis, the
two triphenylphosphine groups are cis to each other
and trans to the two purine ligands. The coordination
polyhedron of the metal atom adopts a marked dis-
torted square-planar geometry (P2ꢀPdꢀP1=99.1(2)°,
N7AꢀPdꢀN7B=87.1(6)°) likely caused by the steric
repulsion between the two triphenylphosphine ligands.
The two purine bases show a relative disposition in
which their SꢀCH2ꢀPh groups point in opposite direc-
tions. The two purine molecules are practically planar
(the angle between imidazolic and pyrimidine mean
planes being 3.1(6) and 3.1(9)° for A and B purine
molecules, respectively), while the exocyclic groups do
not show any significant deviation from the purine
plane. The two purines are twisted away from the metal
coordination mean plane (the angles between the mean
planes of A and B purines, and with the Pd mean plane
are 76.7(7), 72.5(6) and 76.9(6)°, respectively) in order
to minimize the steric repulsion between the bulky
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