Synthesis and characterisation of some ruthenium(II) complexes of α-N heterocyclic carboxylic acids - x-ray structures of cis-[Ru(PPh3)2(L1)2] ·2CH3OH and cis-[Ru(PPh3)2(L3H)2] (L1H = pyridine 2-carboxylic acid and L3H2 = imidazole 4,5-dicarboxylic acid)

Article abstract of DOI:10.1016/S0277-5387(01)00946-9

Synthesis and characterisation of ruthenium(II) complexes of several α-N heterocyclic carboxylic acids of the general formula [Ru(PPh₃)₂(L)₂] are reported [LH = pyridine 2-carboxylic acid (L¹H), pyrazine 2-carboxylic acid (L²H), imidazole 4,5-dicarboxylic acid (L³H₂) and pyrazine 2,3-dicarboxylic acid (L⁴H₂)]. All the acids behaved as bidentate N-O chelating donors, the second carboxyl group of the dicarboxylic acids remaining free. Electrochemical behaviour of the complexes was explored by cyclic voltammetry. Single-crystal X-ray analysis of the two complexes cis-[Ru(PPh₃)₂(L¹)₂] ·2CH₃OH and cis-[Ru(PPh₃)₂(L³H)₂] led to the elucidation of the structures and showed that in both the complexes the two bulky PPh₃ groups were cis to each other.

Full text of DOI:10.1016/S0277-5387(01)00946-9

Polyhedron 20 (2001) 3349–3354
www.elsevier.com/locate/poly
Synthesis and characterisation of some ruthenium(II) complexes of
a-N heterocyclic carboxylic acids—X-ray structures of
cis-[Ru(PPh3)2(L1)2]·2CH3OH and cis-[Ru(PPh3)2(L3H)2]
(L1H=pyridine 2-carboxylic acid and L3H2=imidazole
4,5-dicarboxylic acid)
Parbati Sengupta a, Rupam Dinda a, Saktiprosad Ghosh a,*, W.S. Sheldrick b
a Department of Inorganic Chemistry, Indian Association for the Culti6ation of Science, Jada6pur, Calcutta 700 032, India
b Lehrstuhl fu¨r Analytische Chemie, Ruhr Uni6ersita¨t Bochum, D-44780 Bochum, Germany
Received 19 June 2001; accepted 26 September 2001
Abstract
Synthesis and characterisation of ruthenium(II) complexes of several a-N heterocyclic carboxylic acids of the general formula
[Ru(PPh3)2(L)2] are reported [LH=pyridine 2-carboxylic acid (L1H), pyrazine 2-carboxylic acid (L2H), imidazole 4,5-dicarboxylic
acid (L3H2) and pyrazine 2,3-dicarboxylic acid (L4H2)]. All the acids behaved as bidentate N–O chelating donors, the second
carboxyl group of the dicarboxylic acids remaining free. Electrochemical behaviour of the complexes was explored by cyclic
voltammetry. Single-crystal X-ray analysis of the two complexes cis-[Ru(PPh3)2(L1)2]·2CH3OH and cis-[Ru(PPh3)2(L3H)2] led to
the elucidation of the structures and showed that in both the complexes the two bulky PPh3 groups were cis to each other. © 2001
Elsevier Science Ltd. All rights reserved.
Keywords: Ruthenium(II) complexes; a-N heterocyclic carboxylic acid; X-ray structures; Electrochemistry
[18] and hence has biological implications. It is well
known that pyridine and pyrazine rings are, in general,
1. Introduction
good p-acceptors and can stabilise the ruthenium(II)
acceptor centre, which is a well-recognised p-donor
species [19–23] whereas imidazole ring is a poorer
p-acceptor, a better p-donor and tend to stabilise ruthe-
nium(III) more than ruthenium(II) [2,24]. Hence imida-
zole carboxylic acids are expected to have less
stabilising effect on the ruthenium(II) acceptor centre
compared to the corresponding pyridine and pyrazine
carboxylic acid ligands. This trend might be reflected in
the values of the ruthenium(II)/ruthenium(III) oxida-
tion couples of the complexes reported in the present
study.
In continuation of our studies of metal complexes of
such a-N heterocyclic mono and dicarboxylic acids we
report here the synthesis and characterisation of ruthe-
nium(II) complexes of pyridine 2-carboxylic acid (L1H),
pyrazine 2-carboxylic acid (L2H), imidazole 4,5-dicar-
a-N heterocyclic carboxylic acids like pyridine 2-car-
boxylic acid, pyrazine 2-carboxylic acid, imidazole 4,5-
dicarboxylic acid and pyrazine 2,3-dicarboxylic acid are
recognised as efficient N–O donors exhibiting diverse
mode of coordination [1–3]. Pyridine 2-carboxylic acid,
generally known as picolinic acid, is an efficient biden-
tate N–O donor forming five-membered chelates with
simple metal ions [1,4–8] as well as with oxometal
cations [9–13]. Though the coordination behaviour of
picolinic acid towards ruthenium(II) and ruthenium(III)
has been reported previously [14–17], there is still room
for further exploration, specially after the recognition
of the fact that picolinic acid is a tryptophan metabolite
* Corresponding author. Tel.: +91-33-473-4971/3372; fax: +91-
33-473-2805.
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)00946-9

3350
P. Sengupta et al. / Polyhedron 20 (2001) 3349–3354
boxylic acid (L3H2) and pyrazine 2,3-dicarboxylic acid
(L4H2) [25,26] along with the structural characterisation
of cis-[Ru(PPh3)2(L1)2]·2CH3OH and cis-[Ru(PPh3)2-
(L3H)2].
To the methanolic (60 ml) suspension of the Ag salt
of silver picolinate Ru(PPh3)3Cl2 (960 mg, 1 mmol) was
added and the reaction mixture was refluxed for 5 h
and filtered hot. AgCl residue was rejected. The resul-
tant orange solution deposited shiny orange crystals of
1 on standing. They were filtered, washed with little
benzene and cold methanol and dried in vacuum over
fused CaCl2. Yield: 73%. Anal. Calc. for RuC50H46-
N2P2O6: C, 64.2; H, 4.92; N, 2.99. Found: C, 63.89; H,
4.81; N, 2.90%. IR (KBr pellet, cm−1): 3540b, 3436b,
3400, 1670, 1631, 1596, 1483, 1433, 1348, 756, 697, 640,
532, 450 (b=broad).
2. Experimental
2.1. Materials
Commercial ruthenium trichloride, RuCl3·xH2O,
purchased from Arora Matthey (Calcutta, India) was
processed by repeated evaporation to dryness with con-
centrated HCl. Ru(PPh3)3Cl2 was prepared using a
previously published procedure [27]. All four carboxylic
acids (pyridine 2-carboxylic, pyrazine 2-carboxylic, imi-
dazole 4,5-dicarboxylic and pyrazine 2,3-dicarboxylic
acids) used as ligands were purchased from Aldrich and
AgNO3 from BDH. All other chemicals were of reagent
grade and used without further purification. Tetraethy-
lammonium perchlorate (TEAP) used for electrochemi-
cal work was prepared as reported in the literature [28].
2.3.2. Ru(PPh3)2(L2)2 (2) (L2H=pyrazine 2-carboxylic
acid)
The compound was prepared as a crystalline brown
solid by the same procedure as described above through
the Ag salt of pyrazine 2-carboxylic acid instead of
pyridine 2-carboxylic acid. Yield: 65%. Anal. Calc. for
RuC46H36N4P2O4: C, 63.3; H, 4.13; N, 6.42. Found: C,
63.23; H, 4.09; N, 6.38%. IR (KBr pellet, cm−1): 3346b,
3055b, 1647, 1578, 1482, 1433, 1339, 1320, 790, 702,
647, 521, 453 (b=broad).
2.2. Physical measurements
2.3.3. Ru(PPh3)2(L3H)2 (3) (L3H2=imidazole
4,5-dicarboxylic acid)
Elemental analysis was performed with a Perkin–
Elmer 240 CHNS/O analyser. IR and electronic spectra
were recorded in a Perkin–Elmer 783 spectrophotome-
ter (as KBr pellets) and on Shimadzu UV–Vis record-
The compound was obtained as yellow crystals using
the same procedure as described above taking imidazole
4,5-dicarboxylic acid in place of pyridine 2-carboxylic
acid. Yield: 78%. Anal. Calc. for RuC46H36N4P2O8: C,
59.0; H, 3.84; N, 5.98. Found C, 58.7; H, 3.72; N,
5.89%. IR (KBr pellet, cm−1): 3360b, 3060, 2620b, 1720,
1627, 1500, 1481, 1435, 1375, 1315, 750, 698, 629, 530,
425 (b=broad).
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 disc working elec-
trode, platinum auxiliary electrode and Ag ꢀ AgCl refer-
ence electrode.
2.3.4. Ru(PPh3)2(L4H)2 (4) (L4H2=pyrazine
2,3-dicarboxylic acid)
The compound was prepared as reddish-brown crys-
talline solid by the same procedure as described above
replacing pyridine 2-carboxylic acid by pyrazine 2,3-di-
carboxylic acid. Yield: 72%. Anal. Calc. for
RuC48H36N4P2O8: C, 60.0; H, 3.75; N, 5.83. Found: C,
59.3; H, 3.67; N, 5.82%. IR (KBr pellet, cm−1): 3320b,
3053, 2923b, 1715, 1627, 1481, 1434, 1333, 744, 695,
635, 524, 445 (b=broad).
2.3. Synthesis of complexes
2.3.1. Ru(PPh3)2(L1)2·2CH3OH (1) (L1H=pyridine
2-carboxylic acid)
2.3.5. [Ru(L1)2(ophen)]·CH3OH (5) and
2.3.1.1. Preparation of Ag salt of pyridine 2-carboxylic
acid. Pyridine 2-carboxylic acid (123 mg, 1 mmol) was
dissolved in water and solid NaHCO3 (84 mg, 1 mmol)
was added in small portions when the pH of the
solution became ꢀ5. Aqueous solution of AgNO3 (169
mg, 1 mmol) was then added and stirred for about 3 h.
Silver picolinate separated out and was filtered and
washed with water. It was dried in vacuum over fused
CaCl2.
[Ru(L1)2(bipy)]·CH3OH (6)
Ru(PPh3)2(L1)2 (94 mg, 0.1 mmol) was dissolved in
20 ml methanol and 0.1 mmol of o-phenanthroline (18
mg) or 2,2%-bipyridne (16 mg) was added to it. The
mixture was refluxed for 4 h and concentrated to a
small volume in a rotary evaporator. Orange crystalline
compound separated out. It was filtered and washed
with ether and finally dried in a CaCl2 desiccator.
Yield: 52%. Anal. Calc. for RuC25H20N4O5 (5): C,

P. Sengupta et al. / Polyhedron 20 (2001) 3349–3354
3351
53.85; H, 3.59; N, 10.05. Found: C, 53.5; H, 3.45; N,
9.89%. IR (KBr pellet, cm−1): 3368b, 1633, 1594, 1425,
1408, 1346, 663, 558, 454 (b=broad). Anal. Calc. for
RuC23H20N4O5 (6): C, 51.78; H, 3.75; N, 10.50. Found:
C, 51.2; H, 3.70; N, 10.48%. IR (KBr pellet, cm−1):
3339b, 1635, 1592, 1465, 1446, 1338, 659, 635, 557, 445
(b=broad).
crystallographic data and refinement parameters for 1
and 3 are summarised in Table 1. Intensity data were
collected at 203(2) and 293(2) K, respectively on a
Smart CD diffractometer using graphite monochroma-
,
tised Mo Ka radiation (u=0.71073 A). The intensities
were corrected for Lorentz and polarisation effects and
for absorption using the ABSCOR program [29]. The
structure was solved by the Patterson method. All
non-hydrogen atoms were refined anisotropically by
full-matrix least-squares, with a riding model for hydro-
gen atoms, using the SHELXTL PLUS-PC version [30].
2.4. X-ray crystallography
Single crystals of cis-[Ru(PPh3)2(L1)2]·2CH3OH (1)
and cis-[Ru(PPh3)2(L3H)2] (3) were obtained on allow-
ing the hot filtrate (after precipitating out AgCl) to cool
slowly to room temperature. An orange crystal of di-
mension 0.44×0.40×0.32 mm for 1 and a yellow
needle shaped crystal with dimension 0.40×0.25×0.20
mm for 3 were selected for X-ray diffraction studies.
3. Results and discussion
3.1. Crystal structures of
cis-[Ru(PPh3)2(L1)2]·2CH3OH (1) and
cis-[Ru(PPh3)2(L3H)2] (3)
(
Compound 1 crystallised in the triclinic space group P1
and 3 in the monoclinic space group P2(1)/n. The
A complex of the formula [Ru(PPh3)2(L)2] (L=L1,
L2, L3H or L4H) can exist in the following isomeric
forms I–V.
Table 1
Crystal data and structure refinement parameters for cis-
[Ru(PPh3)2(L1)2]·2CH3OH (1) and cis-[Ru(PPh3)2(L3H)2] (3)
1
3
Empirical formula
Formula weight
C50H46N2O6P2Ru
933.90
C46H43N4O8.5P2Ru
950.85
,
Wavelength (A)
0.71073
triclinic
0.71073
monoclinic
P2(1)/n
Crystal system
Space group
(
P1
Unit cell dimensions
,
a (A)
10.355(11)
12.152(10)
17.255(14)
86.99(5)
88.29(8)
89.16(5)
2167(3)
2
13.0803(9)
11.9546(8)
29.963(2)
90
102.387(1)
90
4576.2(5)
4
1.380
,
b (A)
Structure determination of complexes 1 and 3 by
single crystal diffraction technique clearly showed that
both are the isomer represented by V, with the cis
disposition of the two PPh3 groups. In both the com-
plexes ruthenium(II) acceptor centre is present in a
distorted octahedral N2P2O2 geometry, coordinated to
a pair of pyridine 2-carboxylic acid or imidazole 4,5-di-
carboxylic acid. The two ligands are in two mutually
perpendicular planes each of which contains one
triphenylphosphine and carboxylate oxygen of the
other ligand. The phosphorous and nitrogen atoms as
well as the two carboxylate oxygens are situated trans
to each other. Figs. 1 and 2 are the ORTEP plots of 1
and 3, respectively, and the relevant bond lengths and
bond angles are given in Tables 2 and 3, respectively. In
complex 1 the asymmetric unit contains two methanol
molecules. For one of them the carbon atom is disor-
dered and only one possible site (C 100, S.O.F=0.5)
could be located even after repeated trials. Quality of
the crystals obtained was not ideal and consequently
the crystallographic reliability indices R1 (0.1197) and
wR2 (0.3088) are rather high. However, the basic struc-
ture is correct and so important chemical properties of
the complex can be accounted for in terms of its
structural features but the metric values should not be
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc (Mg m−3
Absorption
)
1.431
0.489
0.469
coefficient (mm−1
)
F(000)
964
1956
0.40×0.25×0.20
1.60–25.09
Crystal size (mm)
q Range for data
collection (°)
0.44×0.40×0.32
2.27–25.00
Index ranges
−125h511,
−145k514,
−205l520
−155h515,
−135k514,
−355l527
23 480
Reflections collected 7595
Data/restraints/
parameters
7227/34/490
8122/0/592
Goodness-of-fit on
1.049
1.037
F2
Final R indices
[I\2|(I)]
R indices (all data)
R1=0.1197,
wR2=0.3088
R1=0.2217,
wR2=0.3516
5.095 and −0.926
R1=0.0355,
wR2=0.0851
R1=0.0451,
wR2=0.0906
0.657 and −0.496
Largest difference
peak and hole
−3
,
(e A
)

3352
P. Sengupta et al. / Polyhedron 20 (2001) 3349–3354
Table 2
,
Selected bond lengths (A) and bond angles (°) for cis-[Ru(PPh3)2-
(L1)2]·2CH3OH (1)
Bond lengths
Ru(1)ꢀN(81)
Ru(1)ꢀO(79)
Ru(1)ꢀO(89)
Ru(1)ꢀN(71)
Ru(1)ꢀP(1)
Ru(1)ꢀP(2)
N(71)ꢀC(76)
N(71)ꢀC(72)
2.060(10) C(76)ꢀC(77)
1.472(18)
1.266(14)
1.306(15)
1.327(15)
1.410(16)
1.474(16)
1.227(15)
1.285(16)
2.065(8)
2.097(9)
2.113(9)
2.319(4)
2.321(4)
C(77)ꢀO(79)
C(77)ꢀO(78)
N(81)ꢀC(86)
N(81)ꢀC(82)
C(86)ꢀC(87)
1.305(17) C(87)ꢀO(89)
1.362(16) C(87)ꢀO(88)
Bond angles
N(81)ꢀRu(1)ꢀO(79)
N(81)ꢀRu(1)ꢀO(89)
O(79)ꢀRu(1)ꢀO(89) 165.9(4)
O(79)ꢀRu(1)ꢀN(71)
O(89)ꢀRu(1)ꢀN(71)
N(81)ꢀRu(1)ꢀP(1)
O(79)ꢀRu(1)ꢀP(1)
O(89)ꢀRu(1)ꢀP(1)
N(71)ꢀRu(1)ꢀP(1)
92.9(3)
77.5(3)
N(81)ꢀRu(1)ꢀP(2) 175.0(3)
O(79)ꢀRu(1)ꢀP(2) 86.9(3)
O(89)ꢀRu(1)ꢀP(2) 101.8(3)
77.4(4)
91.6(3)
88.7(3)
101.3(3)
88.9(2)
174.7(3)
N(71)ꢀRu(1)ꢀP(2)
P(1)ꢀRu(1)ꢀP(2)
88.8(3)
96.26(12)
C(76)ꢀN(71)ꢀC(72) 121.6(10)
C(86)ꢀN(81)ꢀC(82) 121.3(10)
Fig. 1. ORTEP plot of cis-[Ru(PPh3)2(L1)2]·2CH3OH (1).
C(21)ꢀP(1)ꢀC(31)
C(51)ꢀP(2)ꢀC(61)
97.4(6)
98.7(6)
Table 3
,
Selected bond lengths (A) and bond angles (°) for cis-
[Ru(PPh3)2(L3H)2] (3)
Bond lengths
Ru(1)ꢀN(11)
Ru(1)ꢀN(21)
Ru(1)ꢀO(23)
Ru(1)ꢀO(13)
Ru(1)ꢀP(2)
2.090(2)
2.094(2)
2.1191(18)
2.1485(17)
2.3447(7)
2.3458(7)
1.370(3)
1.307(3)
1.366(4)
1.341(3)
1.468(4)
1.256(3)
1.279(3)
C(12)ꢀC(14)
O(11)ꢀC(14)
O(12)ꢀC(14)
N(11)ꢀC(13)
N(11)ꢀC(11)
N(12)ꢀC(11)
N(12)ꢀC(12)
C(13)ꢀC(15)
O(14)ꢀC(15)
O(13)ꢀC(15)
C(22)ꢀC(24)
O(21)ꢀC(24)
O(22)ꢀC(24)
1.481(4)
1.212(4)
1.306(4)
1.367(3)
1.322(3)
1.340(3)
1.362(4)
1.468(4)
1.252(3)
1.286(3)
1.474(4)
1.199(4)
1.319(4)
Ru(1)ꢀP(1)
N(21)ꢀC(23)
N(21)ꢀC(21)
N(22)ꢀC(22)
N(22)ꢀC(21)
C(23)ꢀC(25)
O(24)ꢀC(25)
O(23)ꢀC(25)
Fig. 2. ORTEP plot of cis-[Ru(PPh3)2(L3H)2] (3).
Bond angles
N(11)ꢀRu(1)ꢀO(13) 77.82(7)
C(11)ꢀN(11)ꢀC(13) 106.6(2)
C(11)ꢀN(12)ꢀC(12) 108.8(2)
C(21)ꢀN(21)ꢀC(23) 106.6(2)
C(21)ꢀN(22)ꢀC(22) 107.9(2)
N(11)ꢀC(13)ꢀC(15) 117.5(2)
N(21)ꢀC(23)ꢀC(25) 117.9(2)
O(13)ꢀRu(1)ꢀP(1)
P(1)ꢀRu(1)ꢀO(23)
103.95(5)
87.54(5)
stressed and the data are utilised only to describe the
main features. Bite angles [O(89)ꢀRu(1)ꢀN(81) and
O(79)ꢀRu(1)ꢀN(71)] of the two bidentate chelating N–
O donor picolinate ions are practically the same
[77.5(3) and 77.4(4)°, respectively] indicating identical
binding of the two picolinate ligands which is quite
expected. The rather small bite angle is responsible for
the appreciable distortion from the octahedral environ-
ment. The P(1)ꢀRu(1)ꢀP(2) angle [96.26(12)°] is greater
than 90° and may be attributed to the steric interactions
between the two bulky PPh3 molecules situated cis to
each other. The sum of the all four angles contained in
the equatorial plane P(1)ꢀO(79)ꢀN(71)ꢀO(89) [O(89)ꢀ
Ru(1)ꢀP(1)=88.9(2)°; O(89)ꢀRu(1)ꢀN(71)=91.6(3)°;
O(23)ꢀRu(1)ꢀN(11) 89.07(8)
P(2)ꢀRu(1)ꢀP(1) 101.46(2)
N(21)ꢀRu(1)ꢀO(13) 91.49(7)
points to the fact that the ruthenium(II) centre is
somewhat displaced from the equatorial plane. Though
both of the two RuꢀN bonds are situated trans to the
two phosphorous atoms, their lengths are found to be
,
unequal [Ru(1)ꢀN(81)=2.060(10)
A
and Ru(1)ꢀ
,
N(71)=2.113(9) A]. Summing up the appropriate bond
lengths [P(2)ꢀRu(1)+Ru(1)ꢀN(81)=2.321+2.060=
,
4.381 A; P(1)ꢀRu(1)+Ru(1)ꢀN(71)=2.319+2.113=
,
4.432
A;
O(79)ꢀRu(1)+Ru(1)ꢀO(89)=2.065+
O(79)ꢀRu(1)ꢀP(1)=101.3(3)°
and
N(71)ꢀRu(1)ꢀ
,
2.097=4.162 A] it can be clearly seen that the length of
O(79)=77.4(4)°] is 359.2°, which is less than 360° and
the all three axes of the coordination octahedron

P. Sengupta et al. / Polyhedron 20 (2001) 3349–3354
3353
around the ruthenium(II) acceptor centre are different.
This, along with the displacement of the ruthenium(II)
centre from the equatorial plane, points to the fact that
the complex cis-[Ru(PPh3)2(L1)2]·2CH3OH showed dis-
torted octahedral structure which suggests that it might
be chemically active to a significant extent. This is
substantiated by exploring the reactivity of this com-
pound towards two neutral bidentate ligands, 2,2%-
bipyridine and o-phenanthroline, leading to the
isolation of two new compounds, 5 and 6.
putting the complexes in a saturated aqueous solution
of NaHCO3 proved the existence of free carboxyl
groups. Coordination from the pyridine nitrogen is
indicated by the red shift of the pyridine ring in-plane
and out-of-plane deformation vibrations (observed near
630–600 and 430–400 cm−1) by 15–20 cm−1 [36,37].
Characteristic band of coordinated PPh3 is observed in
the spectra of 1–4 which is absent in the spectra of
complexes 5 and 6.
In complex 3, each ligand is attached to the rutheniu-
m(II) centre through its tertiary nitrogen of the imida-
zole moiety and the carboxylate oxygen of the adjacent
carboxyl group-the other carboxyl group remaining
idle. Sum of all the four angles contained in the equato-
rial [N(11)ꢀO(23)ꢀP(1)ꢀO(13)] plane [N(11)ꢀRuꢀ
O(13)=77.82(7)°; O(13)ꢀRuꢀP(1)=103.95(5)°; O(23)ꢀ
RuꢀP(1)=87.54(5)°; N(11)ꢀRuꢀO(23)=89.07(8)°] is
358.38°, which is a little less than 360° and indicates
that the ruthenium(II) centre is marginally displaced
from the equatorial plane. The RuꢀN(21) and
3.3. Magnetic moment and conductance
All the complexes are diamagnetic and contain ruthe-
nium(II) centre. The complexes behave as non-elec-
trolytes in DMF solution.
3.4. Electronic spectra
As is usual for ruthenium(II) complexes the elec-
tronic spectra are dominated by metal to ligand charge
trasfer in the visible region. Spectra of these complexes
in DMF exhibit two MLCT transitions around 360–
400 (Band-I) and 460–480 nm (Band-II) region [38–40]
(Table 4).
,
RuꢀN(11) bonds [2.094(2) and 2.090(2) A] are slightly
longer than usual due to the trans influence of the
coordinated PPh3 groups. The CꢁO bond of the coordi-
,
nated carboxylate moiety [1.252(3) A] is slightly longer
,
than the CꢁO of the free carboxyl group [1.212(4) A].
3.5. Electrochemistry
Also the CꢀO bond length in the coordinated carboxy-
late moiety [1.286(3) A] is shorter than the correspond-
ing bond length of the free carboxyl group [1.306(4) A].
,
Electron transfer behaviour of all the complexes was
examined in DMF solution by cyclic voltammetry and
the results are presented in Table 4. A reversible ruthe-
nium(II)/ruthenium(III) oxidation (ipa/ipc:1) is ob-
served in the 0.50–0.60 V region for all the complexes.
,
Such difference is indicative of strong coordination of
the carboxylate moiety to the ruthenium(II) centre. The
large P(1)ꢀRuꢀP(2) angle [101.46(2)°] is consistent with
the cis disposition of the two bulky PPh3 groups.
3.2. Infrared spectra
Table 4
Electronic spectra and cyclic voltammetric data (298 K)
The w(CꢀO) stretch of the ꢀCOOH group of the free
ligands lies in the 1700–1730 cm−1 range [31,32]. The
involvement of the free ꢀCOOH group in intramolecu-
lar hydrogen bonding in the complexes is reflected in
the presence of an irregularly shaped band in the
3400–2500 cm−1 region. Bands at 3540 and 3436 cm−1
in 1 are due to w(OꢀH) are indicative of the presence of
methanol in the crystal lattice. In the IR spectra of the
complexes the w(CꢀO) bands observed in the spectra of
the free ligand are found to decrease significantly and
appeared in the 1650–1610 and 1390–1350 cm−1 re-
gion corresponding to the was(CꢀO) and ws(CꢀO) modes
of the coordinated carboxylate moiety. This consider-
able difference between was and ws is indicative of strong
coordination of the carboxylate oxygen to the rutheniu-
m(II) acceptor centre [33–35]. The presence of a
medium intensity bands in the 1730–1710 cm−1 region
in the IR spectra of 3 and 4 suggests that both the
carboxyl groups of the dicarboxylic acids are not in-
volved in coordination. Liberation of CO2 gas on
Complex
Absorption in DMF maxima,
Oxidation E1/2
(nm), (m×10−3, M−1 cm−1
)
(V) a, (DEp, mV) b
1
2
3
4
5
6
266 (9400), 306s (4034),
366 (6530), 467 (721)
211 (55 008), 261 (13 479),
382 (4573), 479 (915)
266 (20 192), 277 (15 943),
392 (3359), 467 (3770)
268 (19 157), 275 (16 105),
365 (8056), 480 (1230)
265 (39 550), 390s (12 750),
442 (7200), 461 (7300)
264 (11 571), 392 (21 226),
430 (3602), 481 (2758)
+0.67 (60)
+0.60 (40)
+0.55 (60)
+0.61 (60)
+0.71 (50)
+0.67 (60)
s: shoulder.
a 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.
b DEp=Epa−Epc
.

3354
P. Sengupta et al. / Polyhedron 20 (2001) 3349–3354
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The one-electron nature of the couple is established by
comparing its current height with that of the standard
ferrocene/ferrocinium couple under exactly identical
experimental conditions. DEp values lie around 60 mV
and do not vary with the scan rate. The values of the
ruthenium(II)/ruthenium(III) oxidation couples are
found to follow the expected trend. E1/2 values for the
complexes containing pyridine 2-, pyrazine 2-carboxylic
acid and pyrazine 2,3-dicarboxylic acid are very near to
each other and are higher than that containing
imidazole 4,5-dicarboxylic acid.
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4. Supplementary material
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Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 153386 and 164529 for com-
pounds cis-Ru(PPh3)2(L1)2·2CH3OH (1) and cis-
Ru(PPh3)2(L3H)2 (3), respectively. 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).
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Acknowledgements
We thank Dr S.K. Chattopadhyay, B.E. College,
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