Chemistry of Azoimidazoles: Synthesis, Spectral Characterization, Electrochemical Studies, and X-ray Crystal Structures of Isomeric Dichloro Bis[1-alkyl-2-(arylazo)imidazole] Complexes of Ruthenium(II)

Article abstract of DOI:10.1021/ic970446p

Several new ligands, azoimidazoles belonging to the class 1-methyl-2-(arylazo)imidazoles (L₁ (3)) and 1-benzyl-2-(arylazo)imidazoles (L₂ (4)) (R = H (a). Me (b), OMe (c), Cl (d), NO₂ (e)) have been synthesized and reacted with RuCl₃ in ethanol under refluxing conditions. Two isomers of the composition RuL₂Cl₂, green (i) and blue (iii), are chromatographically separated. The green isomer is quantitatively transformed to the blue isomer on refluxing in a high boiling solvent. The isomeric structures have been confirmed by X-ray crystallography. Crystal data are as follows. Green complex C₃₈H₃₄Cl₂N₈Ru (6a): crystal system monoclinie; space group C2/c; a = 15.680(8) A?; b = 22.766(14) A?; c = 11.473(5) A?; β = 119.27(4)°; V = 3573(3) A?³ Z = 4; R = 3.59percent; R_w = 4.38percent. Blue complex C₂₂H₂₄Cl₂N₈Ru (7b): crystal system monoclinic; space group P2₁/n, a = 9.547(6) A?; b = 22.554(14) A?; c = 11.748(8) A?; β = 99.07(5)°; V = 2498(3) A?³; Z = 4; R = 3.15percent; R_w = 4.5percent. With reference to the pairs of Cl, N(imidazole), and N(azo) bound to Ru, the green isomer (6a) has a trans-cis-cis configuration and the blue isomer (7b) is cis-trans-cis. In both structures the Ru-N(azo) distances are relatively shorter than Ru-N(imidazole), indicating stronger bonding in the former and the presence of a Ru-L π-interaction that is localized in the Ru-azo fragment. The isomer configuration is supported by IR and ¹H NMR data. The compounds exhibit t₂(Ru) → π*(L) MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple in the green complexes (5. 6) at 0.6-0.7 V and in the blue complexes at 0.7-0.8 V versus SCE and two successive azo reductions. The difference in the first metal and ligand redox potentials is linearly correlated with ν_CT (t₂(Ru) → π*(L).

Full text of DOI:10.1021/ic970446p

1672
Inorg. Chem. 1998, 37, 1672-1678
Ar ticles
Chemistry of Azoimidazoles: Synthesis, Spectral Characterization, Electrochemical Studies,
and X-ray Crystal Structures of Isomeric Dichloro Bis[1-alkyl-2-(arylazo)imidazole]
Complexes of Ruthenium(II)
Tarun Kumar Misra,† Debasis Das,† Chittaranjan Sinha,*,† Prasanta Ghosh,‡ and
Chandan Kumar Pal‡
Department of Chemistry, The University of Burdwan, Burdwan 713 104, India, and Department of
Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India
ReceiVed April 16, 1997
Several new ligands, azoimidazoles belonging to the class 1-methyl-2-(arylazo)imidazoles (L1 (3)) and 1-benzyl-
2-(arylazo)imidazoles (L2 (4)) (R ) H (a), Me (b), OMe (c), Cl (d), NO2 (e)) have been synthesized and reacted
with RuCl3 in ethanol under refluxing conditions. Two isomers of the composition RuL2Cl2, green (i) and blue
(iii), are chromatographically separated. The green isomer is quantitatively transformed to the blue isomer on
refluxing in a high boiling solvent. The isomeric structures have been confirmed by X-ray crystallography. Crystal
data are as follows. Green complex C38H34Cl2N8Ru (6a): crystal system monoclinic; space group C2/c; a )
15.680(8) Å; b ) 22.766(14) Å; c ) 11.473(5) Å; â ) 119.27(4)°; V ) 3573(3) Å3 Z ) 4; R ) 3.59%; Rw )
4.38%. Blue complex C22H24Cl2N8Ru (7b): crystal system monoclinic; space group P21/n, a ) 9.547(6) Å; b )
22.554(14) Å; c ) 11.748(8) Å; â ) 99.07(5)°; V ) 2498(3) Å3; Z ) 4; R ) 3.15%; Rw ) 4.51%. With reference
to the pairs of Cl, N(imidazole), and N(azo) bound to Ru, the green isomer (6a) has a trans-cis-cis configuration
and the blue isomer (7b) is cis-trans-cis. In both structures the Ru-N(azo) distances are relatively shorter
than Ru-N(imidazole), indicating stronger bonding in the former and the presence of a Ru-L π-interaction that
1
is localized in the Ru-azo fragment. The isomer configuration is supported by IR and H NMR data. The
compounds exhibit t2(Ru) f π*(L) MLCT transitions in the visible region. Redox studies show the Ru(III)/
Ru(II) couple in the green complexes (5, 6) at 0.6-0.7 V and in the blue complexes at 0.7-0.8 V Versus SCE
and two successive azo reductions. The difference in the first metal and ligand redox potentials is linearly correlated
with νCT (t2(Ru) f π*(L).
Introduction
range of oxidation states, varieties of reactivities of the
complexes, directional electron and energy transfer, light-to-
electrical energy conversion, photophysical and photochemical
The ruthenium chemistry of unsaturated nitrogenous ligands
has developed1-10 in recent times, primarily due to the wide
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† The University of Burdwan.
‡ Indian Association for the Cultivation of Science.
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S0020-1669(97)00446-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/31/1998

Chemistry of Azoimidazoles
Inorganic Chemistry, Vol. 37, No. 8, 1998 1673
properties, DNA intercalation, and ability to serve as building
blocks in supramolecular arrays. The major work has grown
around N,N-chelating pyridine bases and related species.1-10
The number of heteroatoms, ring size, and the substituents in
the heterocycle ring significantly modify the π-acidity and regu-
late the physical and chemical properties of the compounds.11
In search for other N-heterocycles, imidazole is chosen at
first because it is a ubiquitious ligand in chemical and biological
molecules and appears as such in biomolecules such as proteins
and nucleic acids.12-15 Recently the design of molecular
architectures with imidazole has aroused interest in understand-
ing biomolecular interactions with metal ions in biology and
providing models for the active sides of metalloproteins.12-15
In comparison to the progress in the chemistry of ruthenium-
pyridine/its derivatives, ruthenium-imidazole and derivatives
chemistry has been very slow.15 Ruthenium-imidazole com-
plexes are of interest for their antitumor activities.15
Chart 1
Ligands consisting of one pyridine ring with a pendent
nitrogen donor from an azo function, known as (arylazo)-
pyridines (aap, 1) (Chart 1), have been employed very recently
in the development of transition metal coordination chemis-
try.7,8,16 Due to unsymmetric N-donor sites in the azo imine
function, -NdN-CdN-, isomeric complexes in ruthenium
and osmium have been extensively studied.4-8 But a similar
chemistry of (arylazo)imidazoles (2) is scarce in the literature.17
Besides, the azo group is one of the potential functional units18
which may be photochromatic, pH-responsive, redox active, and
mediate electronic communications between photoredox active
groups. With this background we have initiated research on
ruthenium chelates of (arylazo)imidazoles. The exobidentate19
behavior of the imidazole group is restricted by N-alkylation,20
giving a new series of ligands 1-alkyl-2-(arylazo)imidazoles (L,
3/4) which behave as N,N-chelating systems. In this first report
we describe the synthesis, spectra, redox properties, and single-
crystal X-ray structures of the two isomers of the ruthenium-
(II) complexes RuL2Cl2 (where L is 1-methyl-2-(arylazo)-
imidazole (L1, 3) and 1-benzyl-2-(arylazo)imidazole (L2, 4).
Results and Discussion
A. Ligands and Complexes. 1-Methyl-2-(arylazo)imida-
zoles (L1, 3) (Chart 1) and 1-benzyl-2-(arylazo)imidazoles (L2,
4) are used as ligands. 2-(Arylazo)imidazoles (2) are synthe-
sized by coupling aryldiazonium ions with imidazole in aqueous
sodium carbonate solution (pH 7) and purified by the reported
method.21 The alkylation is carried out by adding alkyl halide
in dry THF solution to the corresponding 2-(arylazo)imidazole
in the presence of sodium hydride.20 The ligands are new and
act as N,N-chelating molecules. The donor centers are abbrevi-
ated as N(imidazole), N, and N(azo), N′. The atom-numbering
scheme is shown in the structures (3/4).
An ethanolic solution of L reacts with RuCl3 under dinitrogen
and affords the complexes RuL2Cl2 via spontaneous reductive
chelation. From the cooled reaction mixture, dark colored
crystals are collected in high yield. On chromatographic
separation (see below), major green (isomer (i) (5/6)) and minor
blue (isomer (iii) (7/8)) products are obtained (see Chart 2).
From the mother liquor major blue isomer (iii) is isolated. Even
when L is used in excess of 2 mol, only RuL2Cl2 is isolated
from this reaction.
(10) Lincoln, P.; Norde´n, B. J. Chem. Soc., Chem. Commun. 1996, 2145.
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T.; Kaino, T.; Kubota, K.; Sasaki, S. J. Am. Chem. Soc. 1996, 118,
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Trans. 1996, 2101. Halz, R. C.; Gobena, F. T. Polyhedron 1996, 15,
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1993, 126, 71. (b) Matthews, C.; Clegg, W.; Elsegood, M. R. J.; Leese,
T. A.; Thorp, D.; Thornton, P.; Lockhart, J. C. J. Chem. Soc., Dalton
Trans. 1996, 1531. (c) Rajan, R.; Rajaram, R.; Nair, B. U.; Ramasami,
T.; Mondal, S. K. J. Chem. Soc., Dalton Trans. 1996, 2019. (d)
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M. M.; Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. J. Am.
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F.; He, F. Y.; Huang, X. Y. Polyhedron 1996, 15, 3659. (f) Luneau,
D.; Risoan, G.; Rey, P.; Grand, A.; Caneschi, A.; Gatteschi, D.;
Laugier, G. Inorg. Chem. 1993, 32, 5616.
The pseudooctahedral dichloro species of type RuL2Cl2,
considering the unsymmetric bidentate chelation, 1-alkyl-2-
(arylazo)imidazole (NN′), can in principle occur in five geo-
Mukhopadhyay, A.; Goswami, S.; Ray, S.; Chakravorty, A. Inorg.
Chem. 1984, 23, 4633-4639. Bandyopadhyay, P.; Bandyopadhyay,
D.; Chakravorty, A.; Cotton, F. A.; Falvello, L. R.; Han, S. J. Am.
Chem. Soc. 1983, 105, 6327.
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117, 313. Cingolani, A.; Lorenzolti, A.; Leonesi, D.; Bondati, F. Gazz.
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681.
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LaMonica, G. J. Chem. Soc., Dalton Trans. 1995, 1671. Shyu, H. L.;
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172.
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Chakravorty, A. J. Chem. Soc., Dalton Trans. 1992, 1883. Datta, D.;
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1674 Inorganic Chemistry, Vol. 37, No. 8, 1998
Misra et al.
Chart 2
Green Isomer, 6a. The two atomic groups Ru, Cl(1), N(1a),
N(4), Cl(1a) and Ru, Cl(1), N(1), Cl(1a), N(4a) separately
constitute two excellent planes (mean deviation 0.03 Å and
dihedral angle 76.5°) and are orthogonal to the third molecular
plane Ru, N(1), N(4), N(4a), N(1a) (mean deviation 0.02 Å).
The N(1)-Ru-N(4) angle is 177.5(1),° which is distorted from
linearity by 2.5°. This deviation originates undoubtedly from
the acute (76.4(1)°) chelate bite angle. The chelate ring is
deviated from the mean plane by 0.05 Å and inclined at an
angle 4.1° with RuN4 plane. The phenylazo plane makes an
angle 50.8° with the chelated azoimine fragment and suggests
stereochemically nonequivalent C-H functions. Two cis-
phenylazo planes are also not parallel, and the dihedral angle
is 11.1°. The trans-chlorine angle is 174.8(1)° and is cor-
roborated with a distorted octahedral structure.
Figure 1. ORTEP plot and atom-labeling scheme for the trans-cis-
cis compound (6a). All non-hydrogen atoms are represented by their
40% probability ellipsoids.
The Ru-N′ (N(azo): N(4) or N(4a)) bond (2.016(3) Å) is
shorter than the Ru-N (N(imidazole): N(1) or N(1a)) (2.063(3)
Å) bond distance by 0.05 Å. The shortening may be due to
greater π-back-bonding, dπ(Ru) f π*(azo).7,8 The N-N
distance is not available in the free ligand. However, the data
available in some free azo ligands suggest that it is nearly 1.25
Å.8b In the complex the N-N distance is 1.229(5) Å. The
coordination can lead to a decrease in the N-N bond order
due to both σ-donor and π-acceptor character of the ligandssthe
latter character having more pronounced effect and possibly
being the reason for elongation. The selected bond parameters
are listed in Table 1.
Blue Isomer, 7b. The two atomic groups Ru, Cl(1), N(5),
N(4), N(1) (mean deviation 0.01 Å) and Ru, Cl(2), N(5), N(8)
(mean deviation 0.03 Å) constitute two orthogonal (dihedral
angle 91.1°) planes. On the other hand, the planarity of the
atomic group Ru, Cl(1), Cl(2), N(4), N(8) is not good (mean
deviation 0.2 Å) due to relatively large deviations of N(4) and
N(8) from the best plane in opposite directions. The trans-
angles around the Ru center in the planes range from 169.6(1)
to 176.9(1)° indicating distortions from the rectilinear geometry.
The chelate angles extended by azoimidazoles are 76.7(1) and
76.9(1)° and are considerably deviated from the ideal geometry.
As a consequence of the constraint of the bite angle, the ligands
are bent back from the coordinated chlorides. It is remarkable
that most of the distortions away from the octahedral positions
arise out of the small bite angle and are associated with the azo
nitrogens whereas the imidazole nitrogens occupy nearly axial
positions [N(1)-Ru-N(5) ) 176.9(1)°]. Each chelate ring is
a good plane with no atom deviating by more than 0.06 Å. The
dihedral angles between the chelate ring and the corresponding
p-tolyl ring are 53.4 and 56.7°. The cis-chloro angle of 91.1-
(1)° is very nearly the ideal octahedral angle and is comparable
to reported values.2c,7f Two chelate planes are deviated from
orthogonality (dihedral angle 95.8°) possibly due to steric
interaction.
Figure 2. ORTEP plot and atom-labeling scheme for the cis-trans-
cis compound (7b). All non-hydrogen atoms are represented by their
40% probability ellipsoids.
metrically isomeric forms.6,7h Two (i, ii) and three (iii-v) of
these respectively have the RuCl2 group in trans and cis
geometries. If the coordinating pairs of Cl, N, and N′ are
considered in that order, the configurations of these five are
trans-cis-cis (i), trans-trans-trans (ii), cis-trans-cis (iii),
cis-cis-cis (iv), and cis-cis-trans (v). Of these, two isomers,
the green, trans-cis-cis (i) (5/6) and the blue, cis-trans-cis
(iii) (7/8) are isolated and characterized by spectroscopic data.
A definitive assignment based on the three-dimensional X-ray
structure determination of these two isomers is reported here.
The green isomer is quantitatively converted to blue isomer iii
on refluxing in a high boiling solvent like, 1,2-dichlorobenzene.
B. Molecular Structures. Views of the molecular units of
green 6a and blue 7b are given in Figures 1and 2, respectively.
The coordination around ruthenium is approximately octahedral.
The atomic arrangement in 6a involves sequentially two trans-
chlorines, cis-N(imidazole)s (N), and cis-N(azo)s (N′) and
corresponds to a trans-cis-cis configuration. Similarly, the
arrangement in 7b is cis-trans-cis.
The N-N distance is comparatively longer than that of isomer
6a by ∼0.01 Å. The Ru-N(imidazole) (2.033(4), 2.043(4) Å)
distance is longer than Ru-N(azo) (1.991(4), 1.987(4) Å) and

Chemistry of Azoimidazoles
Inorganic Chemistry, Vol. 37, No. 8, 1998 1675
Table 1. Selected Bond Distances (Å) and Angles (deg) and Their
Estimated Standard Deviations for (i) 6a and (ii) 7b
of the complexes. The alkylation of azoimidazoles was sup-
ported by the disappearance of δN-H at ∼ 10.4 ppm and the
appearance of an alkyl signal in the aliphatic region.20 The
N-Me and methylenic N-CH2 Ph appear as singlets at 4.00
ppm for 3 and 5.6 ppm for 4, respectively. The imidazolic 4-
and 5-H protons appear at 7.2-7.3 and 7.1-7.2 ppm, respec-
tively. 2-Aryl protons (7-11-H) appear at 7-8.2 ppm, and the
signal movement is in accordance with the inductive and
electronic effect of the substituents.26
The N-Me signal appears at 4.2 and 4.1 ppm in the green
(5) and blue (7) complexes, respectively. The methylene
N-CH2 Ph appears as a singlet at 5.7 ppm in the green
complexes (6) while it gives AB type quartets at 5.4 and 5.7
ppm in the blue complexes (8). The geminal coupling constant25
is 16 Hz. There are, however, neither neighboring chiral centers
nor any appreciable bond rotational barriers, but there does exist
a distorted coordination around the metal. This overall distortion
leads to the molecular dissymmetry. The appearance of single
Ar-CH3 and Ar-OCH3 signals in the complexes is also
supported by earlier work on ruthenium(II) and osmium(II)
(arylazo)pyridine complexes.7g,8 The ligand binding to ruthe-
nium(II) is supported by downfield shifting of N-R signals by
0.1-0.2 ppm. The aryl protons 8,10- (8′, 10′-) H resonate
symmetrically at a single position, and 7- (7′-) and 11- (11′-) H
resonate asymmetrically indicative of a magnetically different
environment. The 11- (11′-) H is assigned as stereochemically
nearer than 7- (7′-) H to the metal center. In the blue complexes
(7, 8) the steric effect induces more distortion, which is reflected
in the greater separation in signal resonances of 11- (11′-) H
and 7- (7′-) H. The separation is less than 20 Hz in green and
greater than 50 Hz in the blue isomers.
(i) Trans-Cis-Cis: (C38H34N8Cl2Ru (6a))
Distances
Ru-N(1)
Ru-N(4)
Ru-Cl(1)
2.063(3)
2.016(3)
2.382(2)
C(1)-N(1)
C(1)-N(3)
N(3)-N(4)
1.325(5)
1.369(5)
1.299(5)
Angles
76.4(1)
86.5(1)
109.9(2)
120.2(2)
101.4(2)
92.2(1)
N(1)-Ru-N(4)
N(1)-Ru-Cl(1A)
Ru-N(1)-C(1)
Ru-N(4)-N(3)
N(1)-Ru-N(1A)
N(4)-Ru-Cl(1)
N(4A)-Ru-Cl(1)
Cl(1)-Ru-Cl(1A)
N(4)-Ru-N(4A)
N(4)-N(3)-C(1)
Ru-N(4)-C(11)
N(1)-Ru-Cl(1)
N(1)-Ru-N(4A)
174.8(1)
105.7(2)
109.3(3)
129.5(3)
90.3(1)
177.5(1)
91.0(1)
(ii) Cis-Trans-Cis (C22H24N8Cl2Ru (7b))
Distances
Ru-N(1)
Ru-N(5)
Ru-N(4)
Ru-N(8)
Ru-Cl(1)
Ru-Cl(2)
N(3)-N(4)
N(7)-N(8)
2.033(4)
2.043(4)
1.991(4)
1.987(4)
2.392(2)
2.407(2)
1.307(5)
1.301(5)
N(1)-C(1)
N(3)-C(1)
N(4)-C(5)
N(5)-C(12)
N(7)-C(12)
N(8)-C(16)
N(2)-C(4)
N(6)-C(15)
1.330(6)
1.351(5)
1.426(5)
1.331(6)
1.358(6)
1.438(6)
1.469(7)
1.451(7)
Angles
N(1)-Ru-N(4)
N(5)-Ru-N(8)
N(1)-Ru-Cl(1)
N(1)-Ru-Cl(2)
N(4)-Ru-Cl(1)
N(4)-Ru-Cl(2)
N(1)-Ru-N(8)
N(4)-Ru-N(5)
N(8)-Ru-Cl(2)
Ru-N(8)-C(16)
76.9(1)
76.7(1)
Ru-N(8)-N(7)
N(8)-N(7)-C(12)
N(7)-C(12)-N(5)
C(12)-N(5)-Ru
Ru-N(4)-N(3)
N(4)-N(3)-C(1)
N(3)-C(1)-N(1)
Ru-N(1)-C(1)
N(1)-Ru-N(5)
Ru-N(4)-C(5)
120.6(3)
108.9(4)
122.5(4)
109.6(3)
120.3(3)
109.2(4)
122.6(4)
110.4(3)
176.9(1)
126.0(3)
92.7(1)
89.1(1)
169.6(1)
90.0(1)
101.0(1)
101.0(1)
169.7(1)
126.7(3)
D. Redox Studies and Correlation with Electronic Spec-
tra. The redox behavior of the complexes in acetonitrile
solution was examined cyclic voltammetrically at a platinum
disk working electrode and the potentials are reported with
reference to the SCE. The voltamograms display metal oxida-
tions at the positive side and the ligand reductions at the negative
side to the SCE. The results are in Table 2, and a representative
voltamogram is shown in Figure 3.
In the potential range +0.5 to +2.0 V at the scan rate 50
mV s-1 in acetonitrile one reversible to quasireversible (peak-
to-peak separation ∆Ep ) 60-80 mV) oxidative response7,8 is
observed corresponding to the couple (1).
is the indication of an M-L π-interaction that is localized in
the M-azo fragment. The Ru-Cl distance is comparable with
the reported values.7f The bond parameters are given in Table
1.
C. Spectra. Sharp intense single bands at 1400-1415 cm-1
corresponding to νNdN in the ligands (L) are shifted to 1270-
1320 cm-1 in the complexes. The low-energy shifting is an
indication of N-coordination and may be attributed to Ru(dπ)
f π* charge transitions.6-8 Green RuL2Cl2 display an intense,
sharp, single stretch at 325-340 cm-1 assignable to νRu-Cl, and
the blue isomers exhibit two similar stretches at 310-320 cm-1
in agreement with the cis-RuCl2 configuration.
RuL2Cl2+ + e h RuL2Cl2
(1)
UV-visible spectral studies of the complexes exhibit absorp-
tions at 250-900 nm (Table 2). The allowed transitions at
nearly 400 nm are probably of ligand origin and are not
considered further. The green complexes (5, 6) show an intense
(ꢀ ∼ 104) band in the region 675 ( 15 nm and is assigned to
the t2(Ru) f π*(L) transition (MLCT), where the π* level has
large azo character.4-8 The blue isomers (7/8) have an intense
(ꢀ ∼ 104) absorption at 590 ( 10 nm with a weak shoulder (ꢀ
∼ 103) at 870 ( 10 nm. The more intense band at higher energy
is believed to be the spin-allowed singlet-singlet transition
while the weaker band at lower energy could be the corre-
sponding singlet-triplet transion made particularly, allowed by
the strong spin-orbit coupling in ruthenium.23 Other transition
probabilities like low-symmetry splitting of the metal level and
the presence of more than one interacting ligand (each contrib-
uting one π* orbital) are excluded due to a small extinction
coefficient.24
The data (Table 1) reveal that the complexes 7 and 8 exhibit
higher redox potentials by 0.1-0.2 V than that of green
complexes 5 and 6. In 7 and 8 two azo functions are
cis-oriented and back-bonding interactions may in turn occur
with two different dπ orbitals, and in 5 and 6 the trans-oriented
azo will compete for the same dπ orbital. This may lead to an
increase in effective charge on ruthenium in 7 and 8 and may
shift the ruthenium(III)/ruthenium(II) couple to more positive
values than for 5 and 6. The potential is sensitive to the
(22) Lockhart, J. C.; Rushton, D. J. J. Chem. Soc., Dalton Trans. 1991,
2633.
(23) (a) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (b)
Decurtines, S.; Felix, F.; Ferguson, J.; Gudel, H. U.; Ludi, A. J. Am.
Chem. Soc. 1980, 102, 4102.
(24) Pankuch, B. J.; Lacky, D. E.; Crosby, G. A. J. Phys. Chem. 1989, 84,
2061. Ceulemans, A.; Vanquickenborne, L. G. J. Am. Chem. Soc. 1981,
103, 2238.
(25) (a) Chattopadhyay, S.; Sinha, C.; Basu, P.; Chakravorty, A. J.
Organomet. Chem. 1991, 414, 421. (b) Sinha, C. Polyhedron 1993,
12, 2363.
The 1H NMR spectra of the ligands and complexes are
compared to determine the binding mode and stereochemistry

1676 Inorganic Chemistry, Vol. 37, No. 8, 1998
Misra et al.
Table 2. UV-Vis Spectrala and Cyclic Voltammetric Datab-d
Ru(III)/Ru(II)
azo-/azo
-EL , V (∆Ep, V)
ν
max, nm (10-3ꢀ, M-1 cm-1
)
0
compd
ν
CT (eV)
EM0, V (∆Ep, V)
(∆E°, (V)
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
7a
7b
7c
7d
7e
8a
8b
8c
8d
8e
664 (12.11)
662 (12.11)
661 (7.86)
669 (10.56)
680 (9.07)
660 (12.53)
668 (10.73)
661 (13.75)
674 (11.74)
685 (10.50)
865 (1.25)f
858 (0.97)f
856 (1.02)f
868 (1.20)f
864 (1.00)f
870 (1.10)f
861 (1.36)f
860 (1.25)f
866 (1.43)f
866 (1.88)f
395 (10.57)
414 (12.31)
450 (11.98)
386 (13.33)
400 (8.86)
409 (10.06)
416 (10.03)
444 (15.88)
405 (10.72)
404 (9.89)
588 (10.87)
590 (11.15)
588 (10.95)
592 (9.97)
610 (9.73)
580 (9.97)
593 (10.38)
590 (10.22)
594 (10.24)
604 (10.76)
333 (13.69)
1.869
1.875
1.877
1.855
1.825
1.88
1.858
1.875
1.841
1.812
2.110
2.103
2.111
2.100
2.034
2.140
2.093
2.103
2.089
2.055
0.614 (0.072)
0.581 (0.065)
0.560 (0.068)
0.666 (0.074)
0.764 (0.070)
0.643 (0.080)e
0.611 (0.075)e
0.591 (0.065)e
0.698 (0.070)e
0.802 (0.070)e
0.777 (0.080)
0.746 (0.070)
0.723 (0.075)
0.805 (0.075)
0.891 (0.080)
0.803 (0.078)
0.759 (0.085)
0.742 (0.080)
0.827 (0.085)
0.920 (0.083)
0.834 (0.101)
0.872 (0.082)
0.901 (0.110)
0.753 (0.120)
0.610 (0.084)
0.788 (0.076)
0.844 (0.110)
0.878 (0.110)
0.685 (0.120)
0.498 (0.090)
0.828 (0.120)
0.868 (0.110)
0.919 (0.095)
0.782 (0.122)
0.618 (0.110)
0.798 (0.114)
0.841 (0.103)
0.874 (0.102)
0.749 (0.117)
0.619 (0.104)
1.448
1.453
1.461
1.419
1.374
1.431
1.455
1.469
1.383
1.300
1.605
1.614
1.642
1.587
1.509
1.601
1.600
1.616
1.576
1.539
342 (15.38)
348 (12.24)
346 (15.48)
345 (11.21)
340 (13.92)
344 (13.31)
349 (13.31)
339 (14.67)
348 (12.59)
376 (15.85)
382 (22.15)
380 (18.29)
376 (20.32)
377 (25.02)
377 (20.12)
382 (22.14)
385 (21.41)
377 (21.67)
375 (23.14)
343 (17.01)
340 (24.39)
338 (21.18)
335 (23.49)
340 (28.90)
338 (24.31)
335 (25.01)
340 (23.15)
335 (24.19)
346 (27.58)
a In CH3CN. b Meaning and units of the symbols are the same as in text. c Solvent is CH3CN. d Supporting electrolyte TBAP(0.01 M), solute
concentration 10-3 M; scan rate is 0.05 V s-1. eQuasireversible Ru(IV)/Ru(III): Epa ) 1.142 (6a), 1.125 (6b), 1.115 (6c), 1.152 (6d), and 1.449 (6e)
f
V. Shoulder.
quasireversible second response (∆Ep ) 90-120 mV) may
correspond to the ruthenium(IV)/ruthenium(III) couple (2).
+
RuL2Cl22+ + e h RuL2Cl2
(2)
The ruthenium(III)/ruthenium(II) redox potential of the
present examples falls between the potentials of bipyridine27
and azopyridine complexes.7 This is corroborated with the
π-acidity order11 of the ligands such as bipyridine < azoimid-
azole < azopyridine. The azoimidazole ligands thus stabilize
(with respect to oxidation) ruthenium(II) better than bipyridine
but less effectively than azopyridines.
In the potential range 0.0 to -1.5 V reductive responses are
observed under similar conditions. The reduced species appears
to be less stable, and on scan reversal, multiple anodic responses
are observed. The reduction may be due to the azo function
analogous to the azopyridine systems. The LUMO of L can
accommodate up to two electrons and the reduction may be
represented by (3). A decrease in the σ-donor capacity of the
-NdN- y+e
\
z [-N‚‚‚N-]- y+e
\
z [-NsN-]2-
(3)
s
substituent in the ligand frame increases both the ruthenium-
(III)/ruthenium(II) and the first bound-ligand reduction poten-
tials.4 The two potentials correlate linearly with the slope less
than unity. The observation may be rationalized as follows. A
decrease in σ-donor capacity stabilizes ligand π orbitals and
increases the effective charge on ruthenium, and this in turn
stabilizes metal dπ-orbitals indirectly. Subsequently, dπ* back-
bonding further destabilizes ligand π* orbitals and stabilizes
the metal dπ-orbitals.28 The extent of perturbation is found to
be more pronounced in the ligand π* orbitals (slope of EM0 vs
EL0 plot is less than unity), which may be due to direct bonding
Figure 3. 3. Cyclic voltammograms in acetonitrile (0.1 M TBAP) at
a platinum working electrode. The solute concentration and scan rate
are ∼10-3 M and 50 mV S-1, respectively: (a) trans-cis-cis (5d)
(‚‚‚) cis-trans-cis (7d) (- - -); (b) trans-cis-cis (6b).
substituents both at the aryl ring and imidazole N(1)-position.
The green complexes (6) exhibit two successive oxidative
responses in which the first one refers to couple (1) and the
(26) (a) Chattopadhyay, P.; Sinha, C. Polyhedron, 1994, 13, 2689. (b)
Jackman, L. M. Applications of Nuclear Magnetic Resonance Spec-
troscopy in Organic Chemistry; Pergamon Press: New York, 1959.
(27) Johnson, E. C.; Sullivan, B. P.; Salmon, D. J.; Adeyemi, S. A.; Meyer,
T. J. Inorg. Chem. 1978, 17, 2211.
(28) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conard, D. Inorg. Chem. 1983,
22, 1617.

Chemistry of Azoimidazoles
Inorganic Chemistry, Vol. 37, No. 8, 1998 1677
Table 3. Comparison of Spectral and Redox Properties of Ru(aap)2Cl2 and RuL2Cl2
Ru(aap)2Cl2
RuL2Cl2
green (trans-trans-trans)
blue (cis-trans-cis)
green (trans-cis-cis)
blue (cis-trans-cis)
Ru(III)/Ru(II) couple, E°298 (V)
MLCT bands, λmax (nm)
0.90-0.95
635
1.10
580
0.6-0.7
665
0.7-0.9
590
of the substituent in the ligand frame.29 The difference in two
Table 4. Crystallographic Data for 6a and 7b
successive redox properties positive and negative to SCE (∆E°
6a
7b
0
) EM0(1) - EL (3)) may be correlated with the MLCT transition.
empirical formula
fw
cryst system
space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
C38H34Cl2N8Ru
774.7
monoclinic
C2/c
15.680(8)
22.766(14)
11.473(5)
119.27(4)
3573(3)
4
C22H24Cl2N8Ru
572.5
monoclinic
P21/n
9.547(6)
22.554(12)
11.748(8)
99.07(5)
2498(3)
4
The electronic excitation may be considered as intramolecular
redox process, and the energy νCT of the lowest MLCT transition
is expected to be linearly related with ∆E° (Table 1). The least-
squares plot of νCT (in eV) against ∆E° (V) corresponds to eq
4. A similar correlation has been observed for bpy30 and
((arylazo)pyridine)ruthenium complexes.4
νCT ) 1.19 ∆E° + 0.18
(4)
T, K
λ, Å
295
0.710 73
1.440
6.2
0.8832
222
3.59
4.38
1.27
295
0.710 73
1.522
E. Comparison with Azopyridine Analogues. (Arylazo)-
pyridines (1) and (arylazo)imidazoles (3/4) belong to the azo
imine, -NdN-CdN-, family of heterocyclic nitrogenous
compounds. The steric and electronic properties of the imida-
zole ring may be regulated by N(1)-substitution, and the
appearance of the ring as such in biomolecules make the
azoimidazoles more versatile ligands than azopyridines. The
present class of molecules have intermediate π-acidity in
comparison with bipyridine and azopyridines. Thus the elec-
tronic and redox properties of ruthenium(II) complexes lie
between Ru(bpy)2Cl2 and Ru(aap)2Cl2. The green isomer in
Ru(aap)2Cl2 is suggested to have the trans-trans-trans
configuration,4,7j while the RuL2Cl2 is trans-cis-cis as is
established by three-dimensional X-ray structure determination.
Two isomers of the cis-RuCl2 configuration are structurally
known7f as cis-trans-cis and cis-cis-cis in Ru(aap)2Cl2, but
in the present case we are able to isolate one isomer (cis-trans-
cis) only. The spectral and redox properties of the isomers
compared in the Table 3. The data reveal that the ruthenium-
(III)/ruthenium(II) couple is shifted to more positive potential
and the MLCT bands are blue shifted on going from azoimid-
azole to azopyridine-ruthenium(II) complexes. This is because
of the decreased π-acidity of the five-membered heterocycle
imidazole compared to the six-membered pyridine.11
π
calcd, g cm-3
µ(Mo KR), cm-1
8.7
transm coeffa
params refined
298
R,b %
3.15
4.51
1.51
c
Rw
%
GOFd
a Maximum value normalized to 1. b R ) Σ ||Fo| - |Fc||Σ/|Fo|. c RW
) [Σw(|Fo| - |Fc|)2/Σw|Fo| ]1/2; w-1 ) σ2(|Fo|) + g|Fo|;2 g ) 0.00030
2
for 6a and g ) 0.00050 for 7b. d The goodness-of-fit is defined as
[w(|Fo| - |Fc|)2/(n0 - nv)]1/2, where n0 and nv denote the numbers of
data and variables, respectively.
Synthesis of Ligands. The 1-alkylated-2-(arylazo)imidazoles (3/4)
were synthesized from the corresponding 2-(arylazo)imidazoles (2) by
reacting the latter with the respective alkyl halide in the presence of
sodium hydride in dry THF. A representative case is detailed below.
Available information on the N-alkylation of imidazole served as a
guide for setting experimental conditions.22
1-Methyl-2-(p-methylphenyl)azo)imidazole (3b). To a dry THF
solution (15 mL) of 2-((p-methylphenyl)azo)imidazole (2b) (1 g, 5.4
mM) was added NaH (50% paraffin) (0.30 g) in small portions, and
the mixture was stirred under cold conditions with an ice bath for 0.5
h. Methyl iodide (0.8 g, 5.6 mM) was added slowly through a pressure
equalizing system under stirring conditions for a period of 1 h and
then allowed to warm for another 1 h. The solution was evaporated to
dryness, extracted with chloroform, and washed with NaOH solution
(10%, 10 mL, ×3) and finally by distilled water (20 mL, ×3). The
chloroform extract was chromatographed over silica gel column (45
× 1 mL) prepared in benzene. The elution was performed by 19:1
benzene-acetonitrile. On slow evaporation in air, an orange crystalline
product was obtained, which was then dried over P4O10 under Vacuum.
The N(1)-benzylated derivatives (4) were synthesized similarly by using
benzyl bromide. The yields and melting points (°C) are as follows:
3a (40%, 100 ( 2), 3b (45%, 116 ( 1), 3c (35%, gum), 3d (39%, 160
( 1), 3e (50%, decomposed above 250 °C), 4a (45%, 94 ( 2), 4b
(50%, 150 ( 1), 4c (30%, gum), 4d (42%, 162 ( 1), 4e (50%, 176 (
1).
Preparation of Complexes. Dichlorobis [1-methyl-2-(phenylazo)-
imidazole]ruthenium(II), Ru(HaaiMe)2Cl2 (5a, 7a). Nitrogen gas was
passed through a brown solution of RuCl3‚3H2O (0.13 g, 0.5 mM) in
20 mL of ethanol. Then 0.2 g (1.08 mM) of 3a in 5 mL of ethanol
was added. The mixture was refluxed under nitrogen with magnetic
stirring for 10 h. During this period the solution color turned to green
to bluish green with a dark precipitate. The solution was cooled to
room temperature, filtered, and washed thoroughly with water, ethanol,
and finally diethyl ether. It was then dried in a vacuum desiccator
over P4O10. The dried product was dissolved in a small volume of
CH2Cl2 and was subjected to chromatography on a silica gel (60-120
Experimental Section
Materials. 2-(Arylazo)imidazoles were prepared by the reported
procedure.21 Ruthenium trichloride was treated before use as earlier.7
The purification of acetonitrile and preparation of tetrabutylammonium
perchlorate (TBAP) for electrochemical work were done as before.7
Dinitrogen was purified by bubbling through an alkaline pyrogallol
solution. All other chemicals and solvents were of reagent grade and
were used without further purification. Commercially available SRL
silica gel (60-120 mesh) was used for column chromatography.
Instrumentation. Spectroscopic data were obtained with use of
following instruments: UV-vis spectra, Shimadzu 160A; IR spectra
(KBr disk, 4000-200 cm-1), Perkin-Elmer 783 spectrophotometers;
1H NMR spectra, JEOL 100 and Brucker 200 MHz FTNMR spec-
trometers. Electrochemical measurements were done with use of
computer-controlled PAR model 270 VERSTAT electrochemical
instruments, using a platinum disk working electrode. All results were
collected at 298 K and are referenced to the saturated calomel electrode
(SCE) in acetonitrile. The reported potentials are uncorrected for
junction potential.
(29) Elliott, C. M.; Hersheuhart, E. J. J. Am. Chem. Soc. 1982, 104, 7519.
(30) (a) Dodsworth, E. S.; Lever, A. B. P. Chem. Phys. Lett. 1986, 124,
152. (b) Ghosh, P.; Chakravorty, A. Inorg. Chem. 1984, 23, 2242.

1678 Inorganic Chemistry, Vol. 37, No. 8, 1998
Misra et al.
mesh) column (30 × 1 mL). A green band was eluted by 9:1 benzene-
acetonitrile. A blue band was eluted further by 2:1 benzene-
acetonitrile. Crystals were obtained by complete evaporation of the
eluted solution at room temperature. The yields were as follows: green
product (5a), 0.16 g, 59.0%; blue product (7a), 0.03 g, 11.1%. The
bluish green solution was evaporated and subjected to chromatography,
and a small portion of the green part with a major blue product was
obtained. A pink mass was remained at the top of the column.
All data were corrected for Lorentz polarization, and an empirical
absorption correction was done on the basis of azimuthal scans.31 Of
the2595 (6a) and 3705 (7b) unique reflections 2006 and 2711 with I
> 3σ(I) were used for the respective structure solutions. Data reduction
and all calculations related to structure solution were carried out on a
Micro VAX II computer with the SHELXTL PLUS Programs.32 The
position of the metal atom in trans-cis-cis-6a and cis-trans-cis-
7b were determined by direct and Patterson heavy atom methods,
respectively. All non-hydrogen atoms wee located from subsequent
difference Fourier maps. The hydrogen atoms were included at
calculated positions with fixed thermal parameters (U ) 0.08 Å2).
Other complexes were prepared by following the identical procedure,
and the yields were varied 50-65% for the green isomers (5, 6) and
10-20% for the blue isomers (7, 8).
Isomer Conversion. Green f Blue. This is a thermal isomer-
ization process. A representative case is given here. The green isomer
Ru(MeaaiMe)2Cl2 (5b) (0.2 g, 0.35 mM) was suspended in 1,2-
dichlorobenzene (15 mL) and heated to reflux for 4 h (conversion was
tested by TLC). The solution was filtered, and the residue was washed
with diethyl ether. It was dissolved in CH2Cl2 and chromatographed
as before. A very small green band was eluted by 9.:1 benzene-
acetonitrile followed by a deep blue band in 2:1 benzene-acetonitrile.
The solution was evaporated in air slowly, and dark shining blue crystals
were deposited. The yield was 0.17 g (85%).
Acknowledgment. We are thankful to the Council of
Scientific and Industrial Research and Department of Science
and Technology, New Delhi, for financial support. The Uni-
versity Grants Commission provided a fellowship to D.D.
Crystallography was performed at the National Single Crystal
Diffractometer Facility, Department of Inorganic Chemistry,
Indian Association for the Cultivation of Science. Our sincere
thanks are due to Dr. B. B. De, University of Nottingham,
1
Nottingham, U.K., for recording some H NMR spectra.
X-ray Crystal Structure and Analysis. Crystals suitable for X-ray
work were grown by slow diffusion of hexane into dichloromethane
solution at 298 K (crystal sizes: trans-cis-cis (6a), 0.38 × 0.25 ×
0.30 mm3; cis-trans-cis (7b), 0.20 × 0.25 × 0.32 mm3). Data were
collected on a Siemens R3m/v diffractometer with graphite-monochro-
mated Mo KR radiation (λ ) 0.710 73 Å) at 295 K. Crystal data and
data collection parameters are listed in Table 4. Unit cell parameters
are determined by the least-squares fit of 30 machine-centered
reflections (2θ ) 15-28°). Systematic absence led to the identification
of space groups C2/c for 6a and P21/n for 7b. Data were collected by
the ω scan method over the 2θ range 3-45°. Two check reflections
were measured after every 98 reflections during data collections to
monitor crystal stability and showed no significant intensity reduction.
Supporting Information Available: Tables SI-SXII, listing
anisotropic displacement coefficients, complete bond distances and
angles, H coordinates and U values, atom coordinates and equivalent
isotropic displacement coefficients, and the elemental (C, H, N) analysis,
IR, and 1H NMR data for all the ligands and complexes, and UV-vis
1
and H NMR spectra of 6b and 8b (14 pages). Ordering information
is given on any current masthead page.
IC970446P
(31) North, A. C. T.; Phillips, D. C.; Mahtews, F. S. Acta Crystallogr.
1968, A24, 351.
(32) Sheldrick, G. M. SHELXTL-PLUS 88, Structure Determination
Software Programs; Nicolet Instrument Corp.: Madison, WI, 1988.