Synthesis, spectroscopic and electrochemical properties of some heteroleptic tris-chelates of ruthenium (II) involving 2,2′-bipyridine (bpy) and N-(aryl) pyridine-2-aldimine(L): X-ray crystal structures of [Ru(bpy)(L2)2](ClO4)2 ? H 2O and 3-N(4-tolyl) imidazo [1,5a] pyridinium perchlorate

Article abstract of DOI:10.1016/j.poly.2004.11.011

Four ruthenium (II) complexes of the formulae [Ru(bpy)(L₁) ₂](ClO₄)₂ ? H₂O (A1), [Ru(bpy)(L₂)₂](ClO₄)₂ ? H ₂O (A2), [Ru(bpy)(L₃)₂](ClO₄) ₂ ? H₂O (A3), [Ru(bpy)(L₄) ₂](ClO₄)₂ ? H₂O (A4) (where L₁ = N-(2-pyridylmethylene) phenyl amine, L₂ = N-(2-pyridylmethylene) 4-(methyl) phenyl amine, L₃ = N-(2-pyridylmethylene) 4-(chloro) phenyl amine, L₄ = N-(2-pyridylmethylene) 4-(fluoro) phenyl amine and bpy = 2,2′-bipyridyl) have been synthesized. In addition to these ruthenium complexes, we have also been able to isolate four imidazopyridinium perchlorate compounds B1-B4 from the same reactions. The X-ray crystal structures of one representative ruthenium complex (A2) and the imidazopyridinium perchlorate compound (B2) have been determined. The Ru(II) center in the complex is coordinated by six N donors with a distorted octahedral geometry. The imine ligands (L) act as bidentate N,N donors.

Full text of DOI:10.1016/j.poly.2004.11.011

Polyhedron 24 (2005) 201–208
www.elsevier.com/locate/poly
Synthesis, spectroscopic and electrochemical properties of some
heteroleptic tris-chelates of ruthenium (II) involving
2,20-bipyridine (bpy) and N-(aryl) pyridine-2-aldimine(L):
X-ray crystal structures of [Ru(bpy)(L2)2](ClO4)2 Æ H2O and
3-N(4-tolyl) imidazo [1,5a] pyridinium perchlorate
a
Dipankar Mishra , Subhendu Naskar , Bibhutosh Adhikary ,
a
a
b
Raymond J. Butcher , Shyamal Kumar Chattopadhyay
a,
*
a
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
b
Department of Chemistry, University of Howard, 525 College Street, NW, Washington, DC 20059, USA
Received 28 September 2004; accepted 1 November 2004
Abstract
Four ruthenium (II) complexes of the formulae [Ru(bpy)(L1)2](ClO4)2 Æ H2O (A1), [Ru(bpy)(L2)2](ClO4)2 Æ H2O (A2),
[Ru(bpy)(L3)2](ClO4)2 Æ H2O (A3), [Ru(bpy)(L4)2](ClO4)2 Æ H2O (A4) (where L1 = N-(2-pyridylmethylene) phenyl amine, L2 =
N-(2-pyridylmethylene) 4-(methyl) phenyl amine, L3 = N-(2-pyridylmethylene) 4-(chloro) phenyl amine, L4 = N-(2-pyridylmethyl-
ene) 4-(fluoro) phenyl amine and bpy = 2,20-bipyridyl) have been synthesized. In addition to these ruthenium complexes, we have
also been able to isolate four imidazopyridinium perchlorate compounds B1–B4 from the same reactions. The X-ray crystal struc-
tures of one representative ruthenium complex (A2) and the imidazopyridinium perchlorate compound (B2) have been determined.
The Ru(II) center in the complex is coordinated by six N donors with a distorted octahedral geometry. The imine ligands (L) act as
bidentate N,N donors.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium (II) complexes; Bidentate N,N donors; Imidazopyridinium perchlorate; Synthesis; X-ray structures
1. Introduction
switches [20,21]. A number of these compounds are also
reported to exhibit an interesting non-linear optical re-
sponse [22,23]. The a–a0 diimine ligands, most com-
monly investigated in these studies, are those where
both the donor nitrogen atoms are part of heterocyclic
rings like pyridine, pyrimidine and pyrazine. Compara-
tively less attention has been devoted [15,16,30] to sys-
tems where at least one nitrogen atom is not part of a
heterocyclic ring. In this paper, we report on the synthe-
sis and spectroscopic properties of four heteroleptic
ruthenium (II) complexes (A1–A4), where Ru(II) is
coordinated by one bipyridyl and two N-(aryl) pyri-
dine-2-aldimine ligands (L). We also report here the
Chemistry of ruthenium with a–a0 diimine ligands is
an area of unabated interest during the past two decades
[1–16] because of the interesting photophysical and pho-
tochemical characteristics of these compounds and their
possible applications in energy research [7,17], in elec-
troluminescent devices [18,19], as well as sensors and
*
Corresponding author. Tel.: +91 33 2668 4561; fax: +91 33 2668
4564/2916.
E-mail address: shch20@hotmail.com (S.K. Chattopadhyay).
0277-5387/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2004.11.011

202
D. Mishra et al. / Polyhedron 24 (2005) 201–208
synthesis and spectroscopic properties of four 3-N(aryl)
imidazo [1,5a] pyridinium compounds (B1–B4), which
were obtained as side products during the synthesis of
the ruthenium complexes.
a saturated methanolic solution (2 ml) of NaClO4 was
added and filtered. The filtrate was evaporated to dry-
ness and the crude product was washed with water
and dried over fused CaCl2. The crude mass was then
dissolved in chloroform and purified by column chroma-
tography using a silica gel (60–120 mesh) column. A
pink band was eluted with CHCl3–CH3CN (4:1), fol-
lowed by a brown band eluted with CHCl3–CH3CN
(3:2). Evaporation of the pink fraction led to isolation
of B2, whereas A2 was obtained from the brown frac-
tion. The pink product B2 was recrystallized from etha-
nol–isopropanol (1:1) (yield 70–75 mg, 34–36%). The
brown product was recrystallized from dichloromethane
to acetonitrile (1:2) solution (yield 175–180 mg, 31%).
The elemental analyses data of all the compounds are
in good agreement with the calculated values (Calc. val-
ues are in parentheses). RuC34H30N6O9Cl2 (A1): C,
48.87 (48.92); H, 3.52 (3.59); N, 9.98 (10.07)%.
RuC36H34N6O9Cl2 (A2): C, 49.67 (49.88); H, 3.77
(3.92); N, 9.69 (9.70)%; RuC34H28N6O9Cl4 (A3): C,
44.88 (44.98); H, 2.98 (3.08); N, 9.19 (9.26)%; RuC34H28-
N6O9F2Cl2 (A4): C, 46.53 (46.68); H, 3.13 (3.20); N, 9.54
(9.61)%; C13H11N2O4Cl (B1): C, 52.89 (52.97); H, 3.66
(3.73); N, 9.43 (9.50)%; C14H13N2O4Cl (B2): C, 54.41
(54.43); H, 4.17 (4.21); N, 8.97 (9.07)%; C13H10N2O4Cl2
(B3): C, 47.37 (47.41); H, 2.99 (3.03); N, 8.46 (8.51)%;
C13H10N2O4FCl (B4): C, 49.86 (49.92); H, 3.18 (3.20);
N, 8.89 (8.96)%.
2. Experimental
2.1. Materials
Commercial ruthenium trichloride (RuCl3 Æ xH2O)
purchased from Arora Matthey (Kolkata, India) was
processed by repeated evaporation to dryness with conc.
HCl. ꢀRu(bpy)Cl3ꢁ was prepared by the published proce-
dure [24]. Acetonitrile used for spectroscopic and elec-
trochemical studies was purified according to
a
reported method [25]. Tetraethyl ammonium perchlo-
rate (TEAP), used for the electrochemical work, was
prepared as reported in the literature [25]. All other
chemicals were of analytical grade and used without fur-
ther purification. The ligands (L) were prepared by
reacting pyridine-2-aldehyde with the appropriate aryl
amine using the published procedure [8].
2.2. Physical measurements
Elemental analyses were performed on a Perkin–El-
mer 240 C, H, N analyzer. UV–Vis spectra were re-
corded using
a
JASCO7850 spectrophotometer.
Luminescence spectra were obtained using a Perkin–El-
mer LS55 fluorescence spectrophotometer. Magnetic
susceptibilities were measured with a PAR model 155
vibrating sample magnetometer with Hg[Co(SCN)4] as
the calibrant. Cyclic voltammetry and differential pulse
voltammetry experiments were carried out using a
PAR Versastat-II instrument driven by E-chem soft-
ware. A three-electrode configuration with a Pt working
and auxiliary electrodes, Ag/AgCl reference electrode
and TEAP as the supporting electrolyte were used.
The potentials were calibrated against the ferrocene/ferr-
ocenium couple (0.44 V versus Ag/AgCl reference).
2.4. X-ray crystallography
For both A2 and B2, data were collected on a Bruker
smart CCD area detector diffractometer at 93(2) K,
using graphite monochromated Mo Ka radiation. The
data were solved by direct methods [26] and refined by
a least square program [27] using SHELX 97. The non-
hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were placed at
calculated positions and refined as riding atoms using
isotropic displacement parameters. The refinement
converged with residuals summarized in Table 1. Cor-
rections for Lorentz polarization effects and semiempir-
ical absorption were applied.
2.3. Synthesis of the complexes
CAUTION! Perchlorate salts of organic and metallo-
organic species are potentially explosive, and though we
have not encountered any problems in our work, neverthe-
less they should be handled with care.
2.4.1. Description of the structures
A2: The molecular structure of A2 is given in Fig. 1.
In this complex, Ru(II) is in an octahedral N6 donor
environment with the average trans angles being.
ꢀ175ꢁ, where as cis angles lie between 78ꢁ and 101ꢁ.
The complexes A1–A4 and the organic products B1–
B4 reported in this work were synthesized by a general
procedure in almost similar yields; therefore, details
are given for A2 and B2 as a representative example.
To a 30 ml methanolic solution of L2 (1 mmol, 182
mg) solid ꢀRu(bpy)Cl3ꢁ (0.5 mmol, 181.75 mg) was added
and the reaction mixture was heated to reflux for 4 h.
The solution was allowed to come to room temperature;
˚
The average Ru–N distance is 2.0602 A (Table 2), which
is quite comparable to that observed [28] for [Ru-
(bpy)3]2+. The average Ru–N distance (2.0585A) ob-
˚
served for the Ru(II)-bpy fragment is also similar to that
of the Ru(II)–(L2)2 fragment, where it has a value of
˚
˚
2.061 A. The Ru–Nimine distances (2.057 A) within the
Ru(II)–(L2)2 fragment are shorter than the Ru–Npy

D. Mishra et al. / Polyhedron 24 (2005) 201–208
203
Table 1
Crystallographic data for A2 and B2
A2
B2
Empirical formula
Formula weight
Temperature (K)
C36H34Cl2N6O9Ru
866.66
93(2)
C14H13N2O4Cl
308.71
93(2)
˚
Wavelength (A)
0.71073
triclinic
0.71073
orthorhombic
Pbca
Crystal system
Space group
Unit cell dimensions
ꢀ
P1
˚
a (A)
11.5049(11)
13.2695(13)
10.0776(7)
7.7626(5)
˚
b (A)
˚
c (A)
13.4267(12)
98.453(2)
34.819(3)
90
a (ꢁ)
b (ꢁ)
c (ꢁ)
99.652(2)
113.044(2)
1807.7(3)
90
90
3
˚
Volume (A )
2723.8(3)
8
1.489
Z
2
1.592
Density (calc.) (g cmꢁ3
)
Absorption coefficient (mmꢁ1
F(0 0 0)
Crystal size (mm)
)
0.646
884
0.256
1272
0.05 · 0.30 · 0.86
ꢁ14 6 h 6 14, ꢁ16 6 k 6 17, ꢁ18 6 l 6 17
13 617
0.45 · 0.5 · 0.99
Index ranges
ꢁ13 6 h 6 13, ꢁ10 6 k 6 9, ꢁ46 6 l 6 46
Reflections collected
Independent reflections
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2 r(I)]
R indices (all data)
19 578
3494 [Rint = 0.0634]
semi-empirical
8631 [Rint = 0.0290]
semi-empirical
full-matrix least-squares on F2
8631/0/516
1.053
full-matrix least-squares on F2
3494/0/191
1.090
R1 = 0.0407, wR2 = 0.0857
R1 = 0.0542, wR2 = 0.0907
0.728 and ꢁ0.719
R1 = 0.0586, wR2 = 0.1460
R1 = 0.0696, wR2 = 0.1526
0.794 and ꢁ0.433
ꢁ3
˚
Largest difference peak and hole (e A
)
˚
distances (the average is 2.065 A). The fact that the Ru–
Nimine distances are shorter than the Ru–Npy distances
of Ru-bpy as well as the Ru(II)–(L2)2 fragment is prob-
ably due the greater back bonding acceptor capability of
the imine moiety compare to the pyridine moiety. The
bite angle of the imine ligand (78.02(9)ꢁ (Table 2)) is also
very similar to that of bpy (78.61(9)ꢁ) and agrees well
with the values reported earlier [29,30]. One of the per-
chlorate ions is disordered in the lattice.
B2: The ORTEP diagram along with the atom num-
bering scheme for the compound B2 is shown in Fig. 2.
The bond distances within the imidazo pyridine frag-
ment clearly indicate that there is extensive delocaliza-
tion within the imidazole ring and the C5–C4 and N1–
C1 part of the pyridine ring. However the C4–C3 and
C1–C2 bonds are more like double bonds. The aryl ring
attached to the N2 atom is twisted by about 46ꢁ from
the plane of the imidazopyridine fragment (Table 3).
Fig. 1. ORTEP diagram and atom numbering scheme for the cation of
A2, showing 50% probability ellipsoids.
Fig. 2. ORTEP diagram and atom numbering scheme for the
compound B2, showing 50% probability ellipsoids.

204
D. Mishra et al. / Polyhedron 24 (2005) 201–208
Table 2
˚
N
Selected bond distances (A) and bond angles (ꢁ) for A2
N
CH
CH
reflux in MeOH,
NaClO4 soln.
Bond distances
Ru–N(2)
N
N
Ru
R
Ru(bpy)Cl3 +
N
N
(ClO4)2
2.053(2)
2.057(2)
2.057(2)
Ru–N(2A)
Ru–N(1)
Ru–N(1A)
2.059(2)
2.064(2)
2.071(2)
C
N
N
H
Ru–N(2B)
Ru–N(1B)
R
( L )
R
(A)
Bond angles
+
N(2)–Ru–N(2B)
N(2)–Ru–N(1B)
N(2B)–Ru–N(1B)
N(2)–Ru–N(2A)
N(2B)–Ru–N(2A)
N(1B)–Ru–N(2A)
N(2)–Ru–N(1)
96.38(9)
174.03(9)
78.02(9)
85.82(9)
177.80(9)
99.79(9)
78.61(9)
82.64(8)
N(1B)–Ru–N(1)
N(2A)–Ru–N(1)
N(2)–Ru–N(1A)
N(2B)–Ru–N(1A)
N(1B)–Ru–N(1A)
N(2A)–Ru–N(1A)
N(1)–Ru–N(1A)
98.46(9)
97.99(8)
94.23(9)
101.60(8)
88.97(9)
78.02(9)
172.11(9)
R
N
ClO4
N
(B)
B
1
L
A
R = H
R = Me
R = Cl
R = F
L1
L2
L3
L4
1
N(2B)–Ru–N(1)
2
3
4
2
3
Table 3
4
˚
Selected bond distances (A) and bond angles (ꢁ) for B2
Scheme 1.
C(1)–C(2)
C(1)–N(1)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(5)–N(1)
C(6)–N(2)
C(7)–N(1)
C(7)–N(2)
C(8)–C(9)
C(8)–C(14)
C(8)–N(2)
C(9)–C(10)
C(10)–C(11)
C(11)–C(13)
1.339(3)
1.390(3)
1.439(3)
1.349(3)
1.419(3)
1.367(3)
1.409(3)
1.379(3)
1.337(3)
1.341(3)
1.386(3)
1.392(3)
1.429(3)
1.391(3)
1.397(3)
1.392(3)
C(2)–C(1)–N(1)
C(1)–C(2)–C(3)
C(4)–C(3)–C(2)
C(3)–C(4)–C(5)
C(6)–C(5)–N(1)
C(6)–C(5)–C(4)
N(1)–C(5)–C(4)
C(5)–C(6)–N(2)
N(1)–C(7)–N(2)
C(9)–C(8)–N(2)
C(14)–C(8)–N(2)
C(7)–N(2)–C(6)
C(7)–N(2)–C(8)
C(6)–N(2)–C(8)
C(7)–N(1)–C(1)
C(7)–N(1)–C(5)
C(1)–N(1)–C(5)
118.4(2)
121.3(2)
120.7(2)
119.1(2)
oxidizes the solvent methanol to formaldehyde, which
then reacts with the schiff base in the presence of ꢀRu(b-
py)Cl3ꢁ to produce B, as described in Scheme 2.
106.09(18)
135.5(2)
118.4(2)
The 3-N(aryl) imidazo [1,5a] pyridinium compounds
(B1–B4) are deep pink in colour due to a strong absorp-
tion at 520–535 nm. The solvatochromism (Fig. 3)
exhibited by this band clearly suggests it is a charge
transfer transition. It was also observed that for a given
solvent the energy of the longest wavelength band fol-
lowed the order B3 < B1 = B2 < B4. The same trend
was followed in the E0 value of the first oxidation peak
for the compounds (Table 7). In fact, a plot of energy of
the transition (cmꢁ1) versus E0 value yields a good linear
relationship (R = 0.99; supplementary Figure. 1). How-
ever, this trend is different from that observed for the
HOMO–LUMO gap for these compounds (in the gas-
eous phase) as calculated by the PM3 method [33a,b],
which follows the order B3 < B4 ꢂ B1 < B2. This indi-
cates that solvation plays a very important role in deter-
mining the energy of the frontier orbitals of these
compounds, which is quite expected because of their
charged structures and appreciable ground state dipole
moment (Table 5). Interestingly, the transition energy
follows a roughly linear relationship (Table 6, supple-
mentary Figure. 2) with the change in dipole moment
(Dlge = le ꢁ lg) between the ground state (lg) and the
first excited singlet state (le), assuming the excited state
has the same geometry as the ground state i.e., a
Franck–Condon excited state. It was also observed that
the intensity of the first charge transfer transition fol-
lows a linear correlation (Table 6, supplementary
Figure 3) with Dl2ge. All the four compounds B1–B4 ex-
hibit luminescence, with a strong emission at ꢀ380 nm
(excited at 280 nm or 240 nm) at room temperature
(Table 4). These compounds with a strong charge trans-
fer transition near the visible region, appreciable dipole
moment in the ground state, moderate value of Dl, as
106.97(18)
107.50(18)
119.03(19)
119.67(19)
110.14(18)
124.94(18)
124.92(18)
128.70(19)
109.30(18)
121.98(19)
3. Results and discussion
Reaction of ꢀRu(bpy)Cl3ꢁ with L in refluxing metha-
nol produces a mixture of the Ru(II) complex A and
the imidazopyridine derivative B, both of which can be
separated by column chromatography and on recrystal-
lization, crystalline compounds can be isolated for both
species. Crystal structure determinations were made for
A2 and B2 as representative members of A and B,
respectively.
The imidazopyridinium derivatives B1 to B4: The for-
mation of the imidazopyridinium derivatives in the reac-
tion described in Scheme 1 is very interesting. Though
imidazopyridine compounds and some of their simple
salts are well known [31,32], the 3-N(aryl) imidazopyrid-
inium derivatives are rare. Thus a simple single step syn-
thesis of these compounds starting from readily
available laboratory chemicals, as described above, is
attractive. It has been reported [31] that imidazopyridine
is formed in the reaction of 2-picolylamine with formic
acid in presence of POCl3. Probably, a similar mecha-
nism is operative here. The ꢀRu(bpy)Cl3ꢁ (where the oxi-
dation state of ruthenium is closer to +4 than +3) first

D. Mishra et al. / Polyhedron 24 (2005) 201–208
205
H
H
C
H+
C
R
N
N
R
..
:
N
N
+
H
H
C
H2
C
O
OH
-OH-
H
C
H
C
-H+
+
N
R
+
N
R
N
+
N
C
C
H
H
H
Scheme 2.
5
10
15
20
25
30
35
40
Table 5
HOMO and LUMO energies and calculated dipole moments of the
imidazopyridinium derivatives
19260 --- for B1 (R=0.94)
--- for B2 (R=0.96)
19260
19080
18900
18720
18540
18360
--- for B3 (R=0.96)
--- for B4 (R=0.80)
Compounds
B1
B2
B3
B4
19080
18900
18720
18540
18360
EHOMO (eV)
ELUMO (eV)
ꢁ13.09 ꢁ13.23 ꢁ12.77 ꢁ13.17
ꢁ4.99
8.10
2.70
4.29
1.59
2.53
ꢁ5.05
8.18
3.70
ꢁ5.16
7.61
7.15
ꢁ5.11
8.06
6.53
ELUMO (eV) ꢁ EHOMO (eV)
lg (debye)
le (debye)
1.42
ꢁ2.28
5.20
2.06
ꢁ5.09
25.91
8.36
1.83
Dlge (i.e., le ꢁ lg) (debye)
2
)
(Dlge
3.349
well as excellent activity against K1 b-lactamase produc-
ing Klebsiella aerogenes 1082 E strain [34].
5
10
15
20
25
30
35
40
D
The redox behaviors of the pink compounds (B) have
been investigated using cyclic voltammetry (CV), as well
as differential pulse voltammetry (DPV) (Table 7). Both
B1 and B2 exhibit one quasi reversible oxidative re-
sponse at around 1.02 V; in the DPV one sharp peak
is obtained at the same potential demonstrating that
only one redox couple is involved. However for B3
and B4 no resolvable peak was observed in the CV,
Fig. 3. Plot of energy of CT transition vs. Dielectric constant (D) for
compounds B.
well as stable excited state, are good candidates for
screening of their possible non linear optical properties
[23]. Besides, the imidazopyridinium cephalosporins
are known to exhibit good Gram positive activity, as
Table 4
UV–Vis spectra and luminescence spectral data
Compounds
k
max/nm (e/lit molꢁ1 cmꢁ1 a
)
k
em/nmb
A1
A2
A3
A4
B1
B2
B3
B4
a
631 (641), 481 (5938), 440 (5792), 388c (1833), 283 (21 994), 235 (13 196)
631 (525), 482 (6472), 438 (6220), 315c(13 448), 282 (23 534), 231c (19 331)
631 (924), 485 (8375), 440 (7883), 390c (3941), 282 (39 413), 240c (24 633)
631 (957), 481 (10 209), 437 (9444), 387c (3190), 283 (36 499), 237 (21 695)
525 (886), 285 (6203), 270 (5907), 232 (12 582)
733
725
710
722
384
382
378
383
525 (1824), 310c (6566), 282 (12 787), 245 (23 345)
535 (3499), 288 (7607), 265c (7150), 237c (10 345)
520 (759), 285 (7290), 270 (6683), 232 (13 001)
Acetonitrile solution.
Qualitative spectra recorded in acetonitrile solutions at room temp., for A1, A2, A3, A4 the excitation wavelength was 480nm and for B1, B2, B3
b
and B4 it was 285, 280, 265 and 285 nm, respectively.
c
Shoulder.

206
D. Mishra et al. / Polyhedron 24 (2005) 201–208
Table 6
Correlation between the lowest energy absorption band and Dlge for
the imidazopyridinium derivatives
about 0.2 eV lower in energy than that of bpy, the first
reduction is assigned to electron addition to the p* orbi-
tal of coordinated L. In fact the EHMO calculation
shows that LUMO and LUMO + 1 have large contribu-
tions from Lp* orbitals and are separated from each
other by only 0.03 eV, so that the first two electron
reduction is probably due to simultaneous electron addi-
tion to LUMO and LUMO + 1. The second and third
reductions are probably due to addition of electrons to
LUMO + 2 and LUMO + 3, where the former is mainly
based on bpy, and the later have contributions from all
the three ligands (see supplementary Figure 5 for pic-
tures of the HOMOs and LUMOs).
Solvent
Energy (cmꢁ1) vs. Dlge
Molar extinction
2
coefficient vs. (Dlge
)
Slope Intercept Ra
Slope Intercept Ra
Dichloromethane 64.19 18 819
0.79 231.52 394.27 0.99
Acetone
Ethanol
Acetonitrile
a
46.21 18 879
74.68 18 999
59.89 19 063
0.71
0.79
59.65 824.40
94.45 521.39
0.99
0.98
0.95
0.87 108.27 740.82
R = correlation coefficient.
Table 7
Cyclic voltammetry and differential pulse voltammetry (DPV) data
The visible region of the electronic spectra of the
complexes consists of a weak transition at ꢀ630 nm
and a broad envelope from 600 to 375 nm consisting
of two peaks at ꢀ480 and ꢀ440 nm and a poorly re-
solved shoulder around 380 nm. The weak intensity of
the first transition suggests that it is probably a charge
transfer transition to a 3MLCT state [4,35,36]. The
bands near 480nm and 440nm are charge transfer tran-
Compounds
E0/V (DEp/mV) in MeCN solution
A1
1.45 (71), ꢁ0.86 (208), ꢁ1.61 (CV)/ꢁ1.46 (DPV),
ꢁ1.74 (DPV)
A2
A3
A4
1.45 (90), ꢁ0.86 (217), ꢁ1.54 (CV)/ꢁ1.41 (DPV),
ꢁ1.69 (DPV)
1.49 (92), ꢁ0.83 (145), ꢁ1.50 (CV)/ꢁ1.41 (DPV),
ꢁ1.57 (DPV)
1.48 (105), ꢁ0.82 (132), ꢁ1.56 (CV)/ꢁ1.42 (DPV),
1
sitions to MLCT states. Under the C2 symmetry of the
ꢁ1.70 (DPV)
1.02 (145)
1.02 (286)
complex, the metal dp orbitals will transform as a + 2b.
The lowest energy p* orbital of bpy will transform as b
while the lowest energy p* orbitals of the two imine li-
gands will transform as a + b. Thus nine MLCT transi-
tions are possible in principle, six of them are charge
transfer from metal to imine ligand Lp* orbitals, while
three of them are charge transfer from metal to bpy p*
orbital. Overlapping of these bands is expected to give
rise to a broad envelope. As mentioned earlier, the li-
gand Lp* orbitals are of lower energy than the bpy p*
orbital, so the MLCT transition to the later orbital is ex-
pected to be of higher energy than those involving the
Lp* orbital. Earlier workers [15] have assigned the peak
around 480 nm to that involving the Lp* orbital, while
the peak near 440 nm to that involving the bpy p* orbi-
tal. However the comparable intensity of both the bands
precludes such an assignment, as only one bpy ligand is
present, compared to two L ligands. Moreover it was
pointed out by Lever [37,38] that for a MLCT transition
involving the bpy p* orbital, a molar extinction coeffi-
cient of ꢀ4000 Mꢁ1 cmꢁ1 per bpy ligand is expected.
As the molar extinction coefficients of the transition
near 440nm for the present complexes are much larger
than that predicted by Lever, so the band ꢀ440 nm as
well as that ꢀ480 nm are assigned to MLCT transitions
involving the Lp* orbital, while the shoulder around
380nm is probably due to that involving the bpy p* orbi-
tal. This is also supported by the large difference of the
observed reduction potential in the complexes between
the L based reduction and bpy based reduction. The
other higher energy transitions are probably intraligand
in nature. An EHMO calculation on the compound A2
supports this conclusion. Since the actual symmetry of
the molecule in this case is less than C2 (it is C1), all
B1
B2
B3
B4
1.07, 0.91 (DPV)
1.26, 1.06 (DPV)
but in the DPV two well-defined oxidation peaks are ob-
served at 0.91 and 1.07 V for B3 and at 1.06 and 1.26 V
for B4.
The ruthenium complexes A1 to A4: The heteroleptic
tris chelate complexes are brown in colour with a strong
absorption around 480nm. For tris-chelate complexes
such as these three isomers are possible [16]. However
determination of the X-ray crystal structure of one rep-
resentative complex A2, shows that in the isolated com-
plex only one isomer is present in the solid state where
the two Npy of the two ligands (L) are cis to each other,
whereas the two Nimine are trans to each other, which
may be called a cis–trans isomer. The similarity of spec-
troscopic, electrochemical and chromatographic behav-
iors of A1, A3 and A4 with that of A2 led us to
believe that in these complexes the same isomer also
predominates.
The redox behavior of the complexes was studied by
cyclic voltammetry (CV) and differential pulse voltam-
metry (DPV). All the complexes undergo a quasi-revers-
ible oxidation at ꢀ1.3 to 1.4 V which may be assigned to
Ru(II) to Ru(III) oxidation. In the reductive side all the
complexes show a two-electron quasi-reversible reduc-
tion at ꢁ0.8 V and two one electron quasi-reversible
reductions near ꢁ1.4 to ꢁ1.7 V. The last two peaks
are not resolved in the CV, but they can be well identi-
fied in the DPV (supplementary Figure 4). As our PM3
calculations show that the LUMO of the ligand L is

D. Mishra et al. / Polyhedron 24 (2005) 201–208
207
the orbitals are of ꢀaꢁ symmetry. The three d orbitals are
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