A new route for the synthesis of bis(pyridine dicarboxylato)bis(triphenylphosphine) complexes of ruthenium(II) and X-ray structural characterisation of the biologically active trans-[Ru(PPh3)2(L1H)2] (L1H2 = pyridine 2,3-dicarboxylic acid)

Article abstract of DOI:10.1016/S0277-5387(01)00736-7

Ruthenium(II) complexes of the general formula Ru(LH)₂(PPh₃)₂, (LH₂ = pyridine 2,3-; 2,4-; 2,5- and 2,6-dicarboxylic acid) are synthesised by a new method. All the pyridine dicarboxylic acids are found to behave as bidentate, monobasic chelating donors, with one carboxyl group remaining idle. The electrochemical behaviour of the compounds is explored with the help of cyclic voltammetry. The antibacterial activities of these compounds were examined against Escherichia coli in nutrient broth in order to check their potential for antitumour activity. Single-crystal X-ray analysis of the complex involving pyridine 2,3-dicarboxylic acid (L¹H₂) revealed that the coordination environment consists of a centrosymmetric, axially elongated N₂P₂O₂ octahedron with a pair of coordinated L¹H ligands occupying the equatorial plane.

Full text of DOI:10.1016/S0277-5387(01)00736-7

Polyhedron 20 (2001) 975–980
www.elsevier.nl/locate/poly
A new route for the synthesis of
bis(pyridine dicarboxylato)bis(triphenylphosphine)
complexes of ruthenium(II) and X-ray structural characterisation
of the biologically active trans-[Ru(PPh3)2(L1H)2]
(L1H2=pyridine 2,3-dicarboxylic acid)
Parbati Sengupta a, Saktiprosad Ghosh a,*, Thomas C.W. Mak b
a Department of Inorganic Chemistry, Indian Association for the Culti6ation of Science, Jada6pur, Calcutta 700 032, India
b Department of Chemistry, Chinese Uni6ersity of Hong Kong, Shatin, New Territories, Hong Kong SAR, People’s Republic of China
Received 28 September 2000; accepted 31 January 2001
Abstract
Ruthenium(II) complexes of the general formula Ru(LH)2(PPh3)2, (LH2=pyridine 2,3-; 2,4-; 2,5- and 2,6-dicarboxylic acid) are
synthesised by a new method. All the pyridine dicarboxylic acids are found to behave as bidentate, monobasic chelating donors,
with one carboxyl group remaining idle. The electrochemical behaviour of the compounds is explored with the help of cyclic
voltammetry. The antibacterial activities of these compounds were examined against Escherichia coli in nutrient broth in order to
check their potential for antitumour activity. Single-crystal X-ray analysis of the complex involving pyridine 2,3-dicarboxylic acid
(L1H2) revealed that the coordination environment consists of a centrosymmetric, axially elongated N2P2O2 octahedron with a
pair of coordinated L1H ligands occupying the equatorial plane. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Ruthenium complexes; Pyridine dicarboxylic acids; X-ray structures; Antibacterial activity
1. Introduction
along with its 2,4- and 2,5-isomers, is found to inhibit
the enzyme GA 2b-hydroxylase and its mechanistically
related enzyme proline 4-hydroxylase [23]. Dipicolinic
acid complexes of iron are found to play the role of
electron carriers in some model biological systems [24–
26] and are recognised as specific molecular tools in
DNA cleavage [27]. The heat resistance property of the
spores of several gram-positive eubacteria during
sporulation is attributed to the presence of the calcium
salt of DPA [28].
Ru(III) complexes with the proposed formulae
Na[Ru(DPA)2]·2H2O and [Ru(DPA)(L)] (LH=picol-
inic acid, nicotinic acid, isonicotinic acid, glycine, or-
thoamino phenol) have been reported [10], but none of
them has its strucuture uneqivocally established. More-
over, no report on Ru(II) complexes of the other pyri-
dine dicarboxylic acids is available till now. The present
work describes the synthesis of ruthenium(II) com-
plexes of the general formula Ru(LH)2(PPh3)2 (LH2=
pyridine-2,3-, 2,4-, 2,5- and 2,6-dicarboxylic acids).
They were characterised by elemental analysis, spectro-
Pyridine-2,6-dicarboxylic acid or dipicolinic acid
(DPAH2) forms stable chelates with simple metal ions
and oxometal cations [1–6] and can display widely
varying coordination demeanour, functioning as a
bidentate [7–10], tridentate [11–13], meridian [14,15] or
bridging ligand [16]. Along with picolinic acid, dipicol-
inic acid can stabilise unfamiliar oxidation states [17,18]
and is found to be useful in corrosion inhibition [19]
and decontamination of nuclear reactors [20]. The other
isomeric pyridine dicarboxylic acids like pyridine 2,3-,
2,4- and 2,5- dicarboxylic acids behave like picolinic
acid and act as chelating bidentate NꢀO donors, the
second carboxyl group remaining idle.
A very important characteristic of these ligands is
their diverse biological activity [21,22]. Dipicolinic acid,
* Corresponding author. Tel: +91-33-4734971, ext. 3372; fax:
+91-33-4732805.
E-mail address: icspg@mahendra.iacs.res.in (S. Ghosh).
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00736-7

976
P. Sengupta et al. / Polyhedron 20 (2001) 975–980
scopic techniques (UV–Vis, IR, NMR), measurement
of magnetic susceptibility at room temperature and
conductance in solution. Their chemical reactivity and
electrochemical behaviour were also explored. The
molecular structure of Ru(L1H)2(PPh3)2 (L1H2=pyri-
dine 2,3-dicarboxylic acid) was determined by single-
crystal X-ray crystallography.
icals were of reagent grade and used without further
purification. Tetraethylammonium perchlorate (TEAP)
used for electrochemical work was prepared as reported
in the literature [30].
2.2. Physical measurements
As all four pyridine dicarboxylic acids (2,3-, 2,4-, 2,5-
and 2,6- designated as L1H2, L2H2, L3H2, L4H2, respec-
tively) behave as a bidentate chelating NꢀO donor like
picolinic acid, an octahedral complex of the general
formula [Ru(LH)2(PPh3)2] (where LH2=a pyridine di-
carboxylic acid) can exist in five geometrical isomers
I–V shown below.
Elemental analyses were performed with a Perkin–
Elmer 240 CHNS/O analyser. IR and electronic spectra
were recorded on a Perkin–Elmer 783 spectrophotome-
ter (as KBr pellets) and on a Shimadzu UV–Vis record-
ing
spectrophotometer,
respectively.
Solution
conductance was measured on a Systronics direct read-
ing conductivity meter (Model 304) and magnetic sus-
ceptibility was measured with a PAR vibrating sample
magnetometer using Hg[Co(SCN)4] as the calibrant.
Electrochemical data were collected with a BAS CV-27
electrochemical analyser and a BAS Model X–Y
recorder at 298 K. Cyclic voltammetry experiments
were carried out with a platinum working electrode,
platinum auxiliary electrode and Ag ꢀ AgCl reference
1
electrode. The H NMR spectra was recorded with a
Bruker 300-MHz NMR spectrometer relative to SiMe4,
and 31P NMR with a UNITY-400 NMR spectrometer
relative to external 85% H3PO4 using (CD3)2SO as
solvent.
In order to identify the actual form in which the
complex [Ru(LH)2(PPh3)2] exists, we tried to prepare
good single crystals of all the four complexes but
succeeded only in the case of the pyridine 2,3-dicar-
boxylic acid complex. X-ray crystallography enabled us
to establish that the isomer I was produced in the
synthetic procedure adopted in this study. 31P NMR
spectra of this complex exhibit only one signal at l 45.5
ppm. The spectra of the other [Ru(LH)2(PPh3)2] com-
plexes also exhibit a similar characteristic feature (single
signal l ꢀ45.5 ppm) that is consistent with the trans
disposition of the two triphenylphosphines groups.
However, whether each of them exists as the form I or
II cannot be ascertained in the absence of X-ray struc-
tural data for the other complexes. We have explored
various methods to prepare good single crystals of these
three complexes, but so far success has eluded us.
2.3. Synthesis of complexes
2.3.1. Ru(PPh3)2(L1H)2 (1) (L1H2=pyridine
2,3-dicarboxylic acid)
Preparation of Ag-salt of pyridine 2,3-dicarboxylic
acid. Pyridine 2,3-dicarboxylic acid (167 mg, 1 mmol)
was dissolved in hot water and solid NaHCO3 added to
it pinch by pinch until the pH of the solution is ꢀ5.
An aqueous solution of AgNO3 (169 mg, 1 mmol) was
added to it and stirred for about 3 h. The silver salt was
separated out, filtered and washed with water. It was
dried in vacuum.
To the methanolic (60 ml) suspension of the Ag-salt
of pyridine 2,3-dicarboxylic acid, Ru(PPh3)3Cl2 (960
mg, 1 mmol) was added and the reaction mixture was
refluxed for 6 h and filtered hot. The AgCl residue was
rejected. The resultant deep violet solution deposited
shiny blue crystals of 1 on standing, which were filtered
washed with a little benzene and then with methanol
and dried in vacuum over fused CaCl2. Yield: 82%.
Anal. Found: C, 62.4; H, 4.02; N, 2.81. Calc. for
RuC50H38N2P2O8: C, 62.6; H, 3.96; N, 2.92%. IR (KBr
pellet, cm−1): 3420b, 3090, 2930b, 1715, 1635, 1475b,
1420, 1355, 750, 685, 630, 585, 490 (b=broad). 1H
NMR spectral data (DMSO-d6, 300 MHz, 22°C l
ppm): 17.6 (br, s, 1H), 7.0–8.6 (m, 36H) (aromatic). 31P
(DMSO-d6, 400 MHz, 22°C, l ppm): 45.5 (s, 2P).
2. Experimental
2.1. Materials
Commercial ruthenium trichloride, RuCl3·xH2O,
purchased from Arora Matthey (Calcutta, India) was
processed by repeated evaporation to dryness with
conc. HCl. Ru(PPh3)3Cl2 was prepared using a previ-
ously published procedure [29]. All four pyridine dicar-
boxylic acids (pyridine 2,3-, 2,4-, 2,5- and
2,6-dicarboxylic acids) used as ligands were purchased
from Aldrich, and AgNO3 from BDH. All other chem-

P. Sengupta et al. / Polyhedron 20 (2001) 975–980
977
2.3.2. Ru(PPh3)2(L2H)2 (2) (L2H2=pyridine 2,4-
dicarboxylic acid)
2.3.4. Ru(PPh3)2(L4H)2 (4) (L4H2=pyridine
2,6-dicarboxylic acid):
The compound was prepared as a crystalline dark
brown solid by the same procedure as described above
through the Ag-salt method using pyridine 2,4-dicar-
boxylic acid instead of pyridine 2,3-dicarboxylic acid.
Yield: 65%. Anal. Found: C, 62.32; H, 4.0; N, 2.8. Calc.
for RuC50H38N2P2O8: C, 62.6; H, 3.96; N, 2.92%. IR
(KBr pellet, cm−1): 3470b, 3050, 2680b, 1720, 1645,
1475, 1415, 1365, 1330, 750, 695, 610, 535, 505 (b=
The compound was prepared as an orange crystalline
solid by the same procedure as described above using
the respective acid in place of pyridine 2,3-dicarboxylic
acid. Yield: 72%. Anal. Found: C, 61.86; H, 4.05; N,
2.91. Calc. for RuC50H38N2P2O8: C, 62.6; H, 3.96; N,
2.92%. IR (KBr pellet, cm−1): 3570, 3440b, 3060, 2630b,
1725, 1620, 1475, 1430, 1350, 740, 695, 605, 535, 520
(b=broad). 1H NMR data: (DMSO-d6, 300 MHz,
22°C, l ppm): 17.1 (br, s, 1H), 6.9–7.8 (m, 36H). 31P
(DMSO-d6, 400 MHz, 22°C, l ppm): 45.5 (s, 2P).
1
broad). H NMR spectral data (DMSO-d6, 300 MHz,
22°C, l ppm): 17.4 (br, s, 1H), 6.9–8.4 (m, 36H). 31P
(DMSO-d6, 400 MHz, 22°C, l ppm): 46.2 (s, 2P).
2.4. X-ray crystallography
2.3.3. Ru(PPh3)2(L3H)2 (3) (L3H2=pyridine
2,5-dicarboxylic acid)
Dark blue prismatic crystals of 1 were isolated by
slow cooling of the mother liquor obtained by refluxing
the silver salt of pyridine 2,3-dicarboxylic acid with
Ru(PPh3)3Cl2 in methanol medium after filtering out
the precipitated AgCl. A single crystal of dimensions
0.26×0.20×0.18 mm3 was chosen for diffraction
study. As the compound crystallised in the monoclinic
space group P21/c with Z=2, the molecule is located
at a centre of symmetry. Intensity data were collected at
296 Kon a Bruker Smart CCD area detector system
using graphite-monochromatised Mo Ka radiation
The compound was prepared as a crystalline dark
reddish–brown solid by the same procedure as de-
scribed above using pyridine 2,5-dicarboxylic acid in
place of pyridine 2,3-dicarboxylic acid. Yield: 78%.
Anal. Found: C, 61.92; H, 4.1; N, 2.85. Calc. for
RuC50H38N2P2O8: C, 62.6; H, 3.96; N, 2.92%. IR (KBr
pellet, cm−1): 3440b, 3060, 2620b, 1730, 1625, 1480,
1435, 1390, 1330, 755, 705, 615, 530, 508, 500 (b=
1
broad). H NMR spectral data (DMSO-d6, 300 MHz,
22°C, l ppm): 16.9 (br, s, 1H), 6.8–8.6 (m, 36H). 31P
(DMSO-d6, 400 MHz, 22°C, l ppm): 45.2 (s, 2P).
,
(u=0.71073 A). The intensities were corrected for em-
pirical absorption effects using the SADABS program
(Rint=0.0233) [31]. The structure was solved by the
direct method. All non-hydrogen atoms were refined
anisotropically by full-matrix least-squares, with a rid-
ing model for the hydrogen atoms, using the SHELXTL
package [32]. The crystal data and refinement parame-
ters are summarised in Table 1.
Table 1
Crystal data and structure refinement for [Ru(PPh3)2(L1H)2] (1)
6
Empirical formula
Formula weight
Wavelength (A)
Crystal system
Space group
C50H38N2O8P2Ru
478.92
,
0.71073
monoclinic
P21/c (No. 14)
Unit cell dimensions
,
a (A)
10.8275(5)
15.7721(7)
12.2599(6)
90
94.610(1)
90
2086.9(2)
2
3. Results and discussion
,
b (A)
,
c (A)
3.1. Crystal structure of Ru(PPh3)2(L1H)2 (1)
(L1H2=Pyridine 2,3-dicarboxylic acid)
h (°)
i (°)
k (°)
3
,
Volume (A )
Z
The molecule is centrosymmetric, and its trans-octa-
hedral Ru(II) centre is coordinated by a pair of
monodeprotonated pyridine 2,3-dicarboxylic acid lig-
ands (L1H) acting in a bidentate chelating manner in
the equatorial plane, and the PPh3 groups fill up the
two axial positions. Each L1H ligand is chelated to the
Ru(II) centre through its pyridyl nitrogen atom and one
oxygen atom of the a-carboxylate group while the other
carboxyl group remains idle. Fig. 1 shows the ORTEP
[33] plot of 1, and selected bond lengths and bond
angles are given in Table 2. The bite angle of the L1H
ligand at the Ru(II) centre is 76.64(5)°. The equatorial
Dcalc (Mg m−3
)
1.524
0.513
980
Absorption coefficient (mm−1
F(000)
)
Crystal size (mm)
u Range for data collection
Index ranges
0.26×0.20×0.18
1.89–28.30°
−125h514, −205k521,
−115l516
Reflections collected
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
14 889
full-matrix least-squares on F2
5190/0/362
1.019
R1=0.0264, wR2=0.0673
R1=0.0385, wR2=0.0727
,
,
RuꢀN [2.069(1) A], equatorial RuꢀO [2.070 (1) A] and
Largest difference peak and hole 0.649 and −0.254
(e A
−3
,
axial RuꢀP [2.4168(4) A] bond lengths indicate an
,
)
axially elongated octahedral N2P2O2 coordination envi-

978
P. Sengupta et al. / Polyhedron 20 (2001) 975–980
ronment around the Ru(II) centre. The hydrogen atom
of the free carboxylic acid group is found to be hydro-
gen bonded with the carbonyl oxygen of the adjacent
coordinated carboxylate moiety. The CꢁO bond of the
,
coordinated carboxylate moiety [1.253(2) A] is longer
,
than the corresponding CꢁO bond [1.198(3) A] in the
free carboxyl group. Also, the CꢀO bond length in the
coordinated carboxylate [1.267(2) A] is shorter than the
corresponding bond length in the free carboxylic group
,
,
[1.282(3) A]. Such a difference is definitely due to the
participation of the carboxylate moiety in coordination
to Ru(II).
3.2. Infrared spectra
The IR spectra of the free ligands (LH2) exhibit the
w(CꢀO) stretch of the free ꢀCOOH group in the 1700–
1730 cm−1 range [18,34]. The presence of the ꢀCOOH
group involved in the intramolecular hydrogen bonding
is evident by a broad and irregularly shaped band in the
3340–2500 cm−1 region. In the IR spectra of the com-
plexes the w(CꢀO) bands observed in the spectra of the
free ligand are found to decrease in intensity signifi-
cantly and new bands appeared in the 1650–1610 and
1390–1350 cm−1 regions corresponding to the was(CꢀO)
and ws(CꢀO) modes of the coordinated carboxyl group.
The considerable difference between was and ws indicates
strong coordination of the ꢀCOO− oxygen to the Ru(II)
acceptor centre [35–37]. The presence of a rather weak
band in the 1730–1710 cm−1 region suggests that both
the carboxyl groups of the pyridine dicarboxylic acids
are not involved in coordination. This is corroborated
by the fact that each of the Ru(LH)2(PPh3)2 complexes
liberates CO2 when put in a saturated aqueous solution
of NaHCO3. However, we failed to isolate the corre-
sponding sodium salt, Na2[Ru(L)2(PPh3)2]. Coordina-
tion from the pyridine nitrogen is indicated by the red
shift of the pyridine ring in plane and out of plane de-
formation vibrations observed near 630–600 and 430–
400 by 15–20 cm−1 [38,39]. The characteristic bands of
coordinated PPh3 are present in all the complexes.
Fig. 1. ORTEP plot of the molecular structure of Ru(L1H)2(PPh3)2 (1).
Table 2
1
,
Selected bond lengths (A) and bond angles (°) for [Ru(PPh3)2(L H)2]
(1)
Bond lengths
Ru(1)ꢀN(1)
Ru(1)ꢀP(1)
Ru(1)ꢀO(4)
2.069(1)
2.4168(4)
2.070 (1)
O(3)ꢀC(25)
O(1)ꢀC(24)
O(4)ꢀC(25)
O(2)ꢀC(24)
1.253(2)
1.198(3)
1.267(2)
1.282(3)
Bond angles
N(1)ꢀRu(1)ꢀO(4) 76.64(5)
C(13)ꢀP(1)ꢀRu(1)
C(7)ꢀP(1)ꢀRu(1)
C(23)ꢀN(1)ꢀRu(1)
C(19)ꢀN(1)ꢀRu(1)
113.02(5)
120.88(6)
117.3 (1)
123.2(1)
N(1)ꢀRu(1)ꢀP(1)
O(4)ꢀRu(1)ꢀP(1)
89.02(4)
95.11(3)
C(1)ꢀP(1)ꢀRu(1) 111.58(6)
Table 3
Electronic spectra and cyclic voltammetric data
Complexes
Absorption in DMF maxima
Oxidation b
E1/2 (V) c, DEp
(mV) d
a
(nm); m×10−3 (M−1 cm−1
)
3.3. Magnetic moment and conductance
1
2
3
4
305, 8235;
358, 5650
306, 4034;
356, 6530
311, 6873;
353, 5778
329, 3916;
380, 7355
0.49, 40
0.43, 50
0.45, 60
0.37, 60
All these complexes are diamagnetic and hence con-
tain the Ru(II) acceptor centre. Measurement of con-
ductance in DMF solution indicates the non-electrolyte
nature of all the compounds.
3.4. Electronic spectra
a Electronic spectral data (absorption in DMF).
b Cyclic voltammetric data (298 K).
In the absence of any d–d transition, the electronic
spectra of such low spin d6 complexes are usually domi-
nated by metal to ligand charge transfers in the visible
region [40–42]. All the complexes exhibit two well-re-
solved MLCT transitions around 350–380 nm (Band-I)
and 300–330 nm (Band-II) (Table 3).
c Conditions: solvent, DMF; supporting electrolyte, TEAP (0.1 M);
working electrode, platinum; reference electrode, Ag ꢀ AgCl; solute
concentration, 10−3 M; scan rate, 0.4 V s−1; and temperature, 298 K.
E1/2 is calculated as the average of anodic (Epa) and cathodic (Epc
peak potential.
)
d DEp=Epa−Epc

P. Sengupta et al. / Polyhedron 20 (2001) 975–980
979
3.5. Electrochemistry
The value of MIC indicates that the antibacterial
activity of compound 1 is very high, that of 2 is quite
good and those of 3 and 4 are rather low. We intend to
pursue further studies on compound 1 and to examine
it for possible antitumour activity against selected tu-
mour models both in vitro and in vivo.
As the complexes are fairly soluble only in DMF, the
electron transfer behaviour of the complexes was
studied in DMF solution by cyclic voltammetry and the
data are presented in Table 3. A reversible ruthenium
(II)–ruthenium (III) couple is observed in the range
0.37–0.49 V. The one-electron nature of the couple is
substantiated by comparing its current height with
those of the standard ferrocene–ferrocinium couple
under exactly the same experimental conditions. The
DEp values lie around 60 mV and do not change with
scan rate, and the ipa/ipc ratio is nearly 1, indicating the
reversible nature of all the complexes.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 148937 for [Ru(PPh3)2(L1H)2]
(1). Copies of this information may be obtained free of
charge from The Director, CCDC, 12, Union Road,
Cambridge, CB2 1EZ, UK (fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
3.6. Study of biological acti6ity
Inhibition of cell division (associated with cell elon-
gation) is a characteristic property of anticarcinogenic
compounds like cis-platin and its analogues [43,44]. In
one of our previous works we selected some ruthenium
complexes as potential antitumour agents due to their
antibacterial activity against Escherichia coli, studied
their antitumour activity on two tumour models (Mca
mammary carcinoma and TLX5 lymphoma) trans-
planted on CBA mice and got quite encouraging results
[45,46]. The ruthenium complexes reported in this work
were also tested against E. coli (10 536) to find out
whether they can significantly inhibit the bacterial
growth with the final objective of testing them against
selected tumour models. Activities of the four com-
plexes against E. coli (10 536) were examined in a
nutrient broth. A culture was grown from a single
colony of E. coli in the nutrient broth (Difco) kept in
an Erlenmeyer flask and incubated at 37°C in a B.O.D.
incubator shaker. Standard solutions of all the four
complexes were prepared in a 1:1 DMSO–ethanol mix-
ture and requisite quantities of these solution were
added to 10 ml of the medium kept in 20ml sterilised
Acknowledgements
We are grateful to Profe`ssor A.K. Guha, Head of the
Department of Biological Chemistry, I.A.C.S., for the
antibacterial activity data. We are also indebted to Dr
1
P.K. Bhattacharyya, IICB, Calcutta, for H NMR data
and Dr A.C. Kunwar, NMR Centre, IICT, Hyderabad,
for 31P NMR data. We thank Dr S.K. Chattopadhyay,
B.E. College, Howrah, for fruitful discussions. A part
of this work is supported by the Hong Kong Research
Grants Council (Earmarked grant CUHK 4022/98P).
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[Ru(PPh3)2(L1H)2] (1)
[Ru(PPh3)2(L2H)2] (2)
[Ru(PPh3)2(L3H)2] (3)
[Ru(PPh3)2(L4H)2] (4)
20
100
200
250
0.0214
0.107
0.214
0.268
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