Chemistry of azopyrimidines. Part II. Synthesis, spectra, electrochemistry and X-ray crystal structures of isomeric dichloro bis[2-(arylazo)pyrimidine] complexes of ruthenium(II)

Article abstract of DOI:10.1016/S0277-5387(99)00189-8

2-(Arylazo)pyrimidines (aapm, 3) are new N,N′- chelating ligands in the azoimine family and were reacted with RuCl₃ in ethanol under refluxing conditions. Three isomers of the composition Ru(aapm)₂Cl₂ have been chromatographically separated and are established as having trans-cis-cis (tcc), cis-trans-cis (ctc) and cis-cis-cis (ccc) configurations with reference to the order of coordination pairs as Cl; N(pyrimidine), N and N(azo), N′. Two of the three isomeric structures have been confirmed by X-ray crystallography. In both of these structures, the Ru-N(azo) distances are relatively shorter than those of Ru-N(pyrimidine), indicating stronger bonding in the former and the presence of a Ru-(aapm) π-interaction that is localised in the Ru-azo fragment. The isomer configuration is supported by IR and ¹H NMR data. The complexes exhibit t₂(Ru)→π*(aapm) MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple in the green complexes [tcc-Ru(aapm)₂Cl₂] at 1.1-1.2 V and in the blue complexes [ctc- and ccc-Ru(aapm)₂Cl₂] at 1.2-1.4 V versus saturated calomel electrode (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*(aapm)]. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00189-8

Polyhedron 18 (1999) 2869–2878
www.elsevier.nl/locate/poly
Chemistry of azopyrimidines. Part II. Synthesis, spectra, electrochemistry
and X-ray crystal structures of isomeric dichloro bis[2-(arylazo)pyrimidine]
complexes of ruthenium(II)q
Prasanta K. Santraa, Tarun K. Misraa, Debasis Dasa, Chittaranjan Sinhaa ,
*
,
Alexandra M.Z. Slawinb, J. Derek Woollinsb
aDepartment of Chemistry, The University of Burdwan, Burdwan 713104, India
bLoughborough University, Loughborough, LE11 3TU, UK
Received 23 March 1999; accepted 5 July 1999
Abstract
2-(Arylazo)pyrimidines (aapm, 3) are new N,N9- chelating ligands in the azoimine family and were reacted with RuCl3 in ethanol under
refluxing conditions. Three isomers of the composition Ru(aapm)2Cl2 have been chromatographically separated and are established as
having trans–cis–cis (tcc), cis–trans–cis (ctc) and cis–cis–cis (ccc) configurations with reference to the order of coordination pairs as Cl;
N(pyrimidine), N and N(azo), N9. Two of the three isomeric structures have been confirmed by X-ray crystallography. In both of these
structures, the Ru–N(azo) distances are relatively shorter than those of Ru–N(pyrimidine), indicating stronger bonding in the former and
1
the presence of a Ru-(aapm) p-interaction that is localised in the Ru-azo fragment. The isomer configuration is supported by IR and H
NMR data. The complexes exhibit t2(Ru)→p*(aapm) MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II)
couple in the green complexes [tcc-Ru(aapm)2Cl2] at 1.1–1.2 V and in the blue complexes [ctc- and ccc-Ru(aapm)2Cl2] at 1.2–1.4 V
versus saturated calomel electrode (SCE) and two successive azo reductions. The difference in the first metal and ligand redox potentials
is linearly correlated with nCT [t2(Ru)→p*(aapm)].
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Azopyrimidines; High potential Ru(II) complexes; Isomers; Structures
1. Introduction
member of the azoimine family, 2-(arylazo)pyrimidine
(aapm, 3). Pyrimidine was chosen because of its higher
p-acidity than that found in conventional widely used
pyridine bases [33–37] and also due to its biochemical
importance [38–44]. Pyrimidine is an active component
[38–44] of antibiotics, antimicrobials, anticonvulsants,
antispasmatics, antineoplastics (e.g. bleomycin) and antidi-
abetogenics, and many derivatives have been used in seed
dressings, crop-disease control and veterinary drugs etc. In
coordination chemistry, the meta-related nitrogen in py-
rimidine has played an important role for connecting
different metals, transmitting antiferromagnetic interac-
tions and for obtaining magnetic systems of high nuclearity
[45–52]. Because of the peripheral N heteroatom, the
excited state of the pyrimidine moiety undergoes protona-
tion in aqueous solution and the complexes exhibit proton-
dependent photophysics and photochemistry [53]. Recent-
ly, the design of molecular architecture with pyrimidine
and bipyrimidine has aroused interest in the fields of
coordination, bioinorganic and magnetochemistry [53–62]
The p-acidity of the azoimine group, –N=N–C=N–, and
its ability to stabilise the low valent metal redox state have
encouraged us to study its chemistry and to design a
molecular architecture with this function [1–32]. An imine
group in a heterocyclic backbone with a pendant azo arm,
i.e., arylazoheterocycles, have been extensively studied
with respect to 2-(arylazo)pyridines (aap, 1) [1–22]. The
ruthenium complexes of these ligands are known in some
detail [1–19]. Progress in the azoimine chemistry of
azopyridines has encouraged us to explore the chemistry of
this function in other heterocycles. Our first successful
design was 2-(arylazo)imidazoles (2) [23–32].
To follow our interest in azoimine compounds with
better p-acidity than that of aap (1), we synthesised a third
qFor part I, see Polyhedron 18 (1999) 1909–1915.
*Corresponding author. Tel.: 191-0342-60810; fax: 191-0342-64452.
E-mail address: bdnuvlib@giascl01.vsnl.net.in (C. Sinha)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00189-8
1999 Elsevier Science Ltd. All rights reserved.

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P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
on account of their interactions with metal ions. In this
report, we describe the synthesis, spectral studies, redox
properties and single crystal X-ray structures of
ruthenium(II) complexes of 2-(arylazo)pyrimidines.
with benzene and collected. Evaporation of solvent gave a
gummy mass of the ligand. The yield was 1.2 g (31%).
2-( p-Tollylazo)pyrimidine [p-tapm (3b)] (28%,
gummy) and 2-( p-chlorophenylazo)pyrimidine ( p-clpapm,
3c) (40%, semi solid) were prepared similarly.
2. Experimental
2.4. Preparation of complexes tcc- and ctc-
dichlorobis[2-(phenylazo)pyrimidine]ruthenium(II),
Ru(papm)2Cl2 (green tcc, 4a; blue ctc, 4b)
2.1. Materials, general procedure and measurements
2-Aminopyrimidine was purchased from Aldrich.
Ruthenium trichloride was digested three times with conc.
HCl and evaporated to dryness before use, as described
previously [31]. Nitrosoaromatics were prepared using
known procedures [23–26]. The purification of acetonitrile
and the preparation of tetrabutylammonium perchlorate
(TBAP) for electrochemical work were carried out as
before [31]. Dinitrogen was purified by bubbling it through
an alkaline pyrogallol solution. All other chemicals and
solvents were of reagent grade and were used without
further purification. Commercially available alumina (neu-
tral) and silica gel (SRL) were used for column chroma-
tography. Spectroscopic data were obtained using the
following instruments: UV–Vis spectra, Shimadzu
UV160A; IR spectra (KBr disk, 4000–200 cm21), FTIR
JASCO model 420; 1H NMR spectra, Bruker (AC) 300
MHz FTNMR spectrometer. Electrochemical measure-
ments were performed using computer-controlled PAR
model 270 VERSASTAT Electrochemical instruments
with Pt-bead and GC-electrodes. All measurements were
carried out under a dinitrogen environment at 298 K with
reference to saturated calomel electrode (SCE) in acetoni-
trile. The reported potentials are uncorrected for junction
potential.
Nitrogen gas was bubbled through a brown solution of
RuCl3?3H2O (0.13 g, 0.5 mmol) in 20 cm3 of ethanol.
Then, 0.22 g (1.2 mmol) of papm in 5 cm3 of ethanol was
added. The mixture was refluxed under nitrogen, with
magnetic stirring, for 10 h. During this period, the solution
turned green, then bluish green with a dark precipitate. The
solution was cooled to room temperature, filtered, and the
solid was washed thoroughly with water, ethanol and
finally with Et2O. It was then dried in a vacuum desiccator
over P4O10. The dried product was dissolved in a small
volume of CH2Cl2 and was chromatographed on an
alumina (neutral) column (3031 cm). A blue band was
eluted using 9:1(v/v) benzene–acetonitrile. Crystals were
obtained by complete evaporation of the eluted solution at
room temperature. The yields were 0.125 g (47%) of green
product and 0.052 g (19%) of the blue product.
2.5. Preparation of ccc-Ru(papm)2Cl2 (4c)
A 0.1-g (0.38 mmol) amount of RuCl3?3H2O and 0.15 g
(0.82 mmol) of 2-(arylazo)pyrimidine in ethanol (30 cm3)
were refluxed under a dinitrogen atmosphere for 3 h. The
mixture was cooled and the precipitate was filtered and
washed with pet-spirit (60–808C). The solid was then
suspended in 60 cm3 of toluene and refluxed for 2 h. The
product was filtered hot and the solid was washed with
pet-spirit (60–808C). It was then dissolved in hot acetoni-
trile, and filtered and poured into three volumes of water.
Then the mixture was allowed to settle overnight. The
product was filtered and dried in vacuo over P4O10. It was
then dissolved in a minimum volume of CH2Cl2 and
chromatographed as earlier. The blue band was eluted
using a 4:1 (v/v) benzene–acetonitrile mixture. Crystals
were obtained by complete evaporation of the eluted
solution at room temperature. The yield of ccc-Ru(papm)
2Cl2 was 0.068 g (68%).
2.2. Preparation of ligands
2-(Arylazo)pyrimidines (3) were synthesised by con-
densing 2-aminopyrimidine with nitrosoaromatics. A rep-
resentative case is detailed below. The available infor-
mation on 2-(arylazo)pyridine [20,21] served as a guide for
setting up experimental conditions.
2.3. 2-(Phenylazo)pyrimidine (papm, 3a)
To a nitrogen-flushed solution of 2-aminopyrimidine
(2.0 g, 0.021 mol) in dry benzene (30 cm3) with sodium (1
g) was added nitrosobenzene (2.5 g, 0.023 mol) in small
portions for 2 h under refluxing conditions. The reaction
was continued for 3 h and cooled to room temperature. The
dark red solution obtained was then filtered and the filtrate
was washed with water (3320 cm3). It was then evapo-
rated to reduce the volume to 5 cm and chromatographed
on a silica gel (60–120 mesh) column (3031 cm) prepared
in pet-spirit (60–808C). A light yellow band was eluted
with pet-spirit and rejected. A red band was then eluted
2.6. Isomer conversion, tcc (4a) (green)→ctc (4b) (blue)
Isomer conversion was carried out thermally. One
representative case is detailed below. The green tcc-
Ru(papm)2Cl2 (0.1 g, 0.19 mmol) was suspended in
xylene (50 cm3) and heated to reflux for 8 h (conversion
was tested by TLC). The solution was filtered and the
residue was washed with Et2O. It was dissolved in a

P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
2871
minimum volume CH2Cl2 and subjected to chromatog-
raphy as before. A very small green band was eluted
slowly using 19:1 (v/v) benzene–acetonitrile along with a
blue tail followed by a deep blue band that was eluted
using 9:1 (v/v) benzene–acetonitrile. This blue solution
was evaporated in air at room temperature and dark
shining crystals were deposited. The yield was 0.082 g
(82%). A similar conversion of (4a)→(4c) was carried out
on boiling toluene and the yield was 78%.
puters. In both cases, hydrogen atoms were included in
calculated positions and refined with isotropic thermal
parameters, which were |1.2 [aromatic (H)] times the
equivalent isotropic thermal parameters of their parent
carbon atoms.
3. Results and discussion
3.1. Synthesis
2.7. X-ray crystallography
2-(Arylazo)pyrimidines (aapm, 3) were synthesised by
condensing nitrosoaromatics (ArNO) with 2-amino-
pyrimidine in the presence of metallic sodium in dry
benzene under refluxing conditions for 12 h. Purification
was carried out by chromatography. The ligands are new
and act as N,N9- bidentate chelating molecules. The donor
centres are abbreviated as N(1)(pyrimidine), N and N(azo),
N9, and the atom-numbering scheme is shown in the
structure 3 (see Scheme 1). Ethanolic solutions of aapm
react with RuCl3 under reflux (over N2) to afford the
complex Ru(aapm)2Cl2 via spontaneous reductive chela-
tion. Even when the ligand is used in excess of 2 mol
equivalents, only Ru(aapm)2Cl2 is isolated from this
reaction. The complexes were isolated in high yield from
the reaction mixture. One isomer (green, isomer A) is
sparingly soluble in ethanol and was isolated by filtration
from the reaction mixture. The blue filtrate was evaporated
and chromatographed on an alumina column. A blue
isomer (isomer B) and a deep-blue isomer (isomer C) were
eluted using 9:1 and 4:1 (v/v) benzene–acetonitrile mix-
tures, respectively. The elution trend may distinguish
between the complexes based on polarity difference be-
tween them. Thus, isomer B is less polar than isomer C.
The reaction of Ru(DMSO)4Cl2 and aapm in acetone
yields isomer A as the major product.
Crystals that were suitable for X-ray work were grown
by slow diffusion of hexane into a dichloromethane
solution at 298 K (crystal size: tcc-Ru(papm)2Cl2 (4a),
0.1530.1530.30 mm3; ccc-Ru(papm)2Cl2 (4c), 0.143
0.1730.19 mm3). Data were collected on a Siemens
SMART CCD diffractometer with graphite monochromat-
˚
ised Mo Ka radiation (l50.71073 A) at 295 K. Data on
the crystals and the collection parameters are listed in
Table 1. The data were corrected for absorption effects by
an empirical method using azimuthal scan data. Systematic
absences led to the identification of space groups Pbca
(orthorhombic crystal system) and P21 /C (monoclinic
crystal system), respectively. Of the 17 278 (tcc) and
10 621 (ccc) collected reflections, 3065 and 3684 with
I.2s(I) were used for structure solution, respectively. In
both cases, the structures were solved by the conventional
heavy-atom method and were refined by the full-matrix
least-squares method on all F20 data using the SHELXTL
5.03 package on Silicon Graphics Indigo-R4000 com-
Table 1
Summarised crystallographic data for complexes 4a and 4c
Crystal parameters
4a
4c
The ligands have unsymmetric bidentate N,N9- donor
centres. The pseudo-octahedral dichloro species,
Ru(aapm)2Cl2, can, in principle, occur in five geometrical-
ly isomeric forms. Two (i and ii) of these have the RuCl2
group in the trans geometry and the other three (iii–v)
have the RuCl2 group in the cis geometry. Considering the
coordinating pairs in the order of Cl, Cl; N, N; N9, N9
where N represents N(pyrimidine) and N9 represents
N(azo), the configurations of these five are trans–cis–cis
[tcc, (i)], trans–trans–trans [ttt, (ii)], cis–trans–cis [ctc,
(iii)], cis–cis–cis [ccc, (iv)] and cis–cis–trans [cct, (v)].
Isomer A (green complex) is almost quantitatively
converted to isomer B on boiling in a suspension of
xylene, while isomer C is produced when thermal trans-
formation is carried out in a suspension of toluene. In the
present work, three isomers were isolated that were green
(4a–6a), blue (4b–6b) and deep blue (4c–6c) and these
were characterised by spectral studies (vide infra) as tcc-,
ctc- and ccc-Ru(aapm)2Cl2, respectively. Two of these
three isomers, i.e., tcc-Ru(aapm)2Cl2 (4a) are ccc-
Formula
M
C20H16Cl2N8Ru
540.38
Orthorhombic
Pbca
13.0541(2)
10.5734(2)
31.1053(6)
90
4293.35(13)
0.71073
1.672
8
298
1.005
281
0.0291
0.0897
0.826
C20H16Cl2N8Ru 0.75(CH2Cl2)
604.07
Monoclinic
P21 /c
14.9548(6)
7.9177(3)
21.6196(9)
91.520(1)
2558.4(2)
0.71073
1.568
4
298
1.004
308
0.0386
0.1107
1.034
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/8
3
˚
V/A
˚
l/A
rcalcd /mg m23
Z
T/K
m(Mo Ka)/mm21
Refined parameters
Ra
wRb
GOFc
a R5S[F0 2Fc]/SF0.
b wR5[Sw(F02 2Fc2)/SwF04]1 / 2
14.3394P], P5(F20 12Fc2)/3.
,
w51/[s2(F20)1(0.0473P)21
c GOF is defined as [w(F0 2Fc)/(n0 2nv)]1 / 2, where n0 and nv denote
the numbers of data and variables, respectively.

2872
P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
Scheme 1.
Ru(aapm)2Cl2 (4c), have been characterised by X-ray
crystallographic methods. Crystallographic data for com-
plexes 4a and 4c are shown in Table 1.
3.2. Molecular structures
The crystal structures of the green (4a; isomer A) and
blue (4c; isomer C) isomers are shown in Figs. 1 and 2,
and bond parameters are listed in Tables 2 and 3, respec-
tively. The coordination around ruthenium is approximate-
ly octahedral. The atomic arrangement in 4a involves
sequentially two trans-chlorine, cis-pyrimidine-N(N) and
cis-azo-N(N9) and corresponds to the trans–cis–cis con-
figuration. Similarly, the arrangement in 4b is cis–cis–cis.
3.3. Green isomer, 4a
The two atomic groups Ru, Cl(1), N(1), Cl(2), N(27)
and Ru, Cl(1), N(21), Cl(2), N(7) separately constitute
˚
two excellent planes (mean deviation, 0.07 A) and are
orthogonal to the third molecular plane, Ru, N(1), N(7),
˚
N(27), N(21) (deviation, 0.08 A). The Ru atom is shifted
Fig. 1. Crystal structure and atom-labelling scheme for tcc-Ru(papm)2Cl2
(4a).
˚
by 0.04 A from the centre of gravity of the third plane

P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
2873
Fig. 2. Crystal structure and atom-labelling scheme for ccc-Ru(papm)2Cl2 (4c).
towards Cl(1). The N(1)–Ru–N(27) angle is 177.63(12)8;
this deviation originates undoubtedly from the acute
[(75.78(12)8] chelate bite angle [31]. The chelate ring, Ru,
Table 2
˚
Selected bond distances (A) and angles (8) for complex 4a, along with
their estimated standard deviations
˚
N(1), C(2), N(2), N(7), is planar (deviation, 0.04 A) while
Ru, N(21), C(21), C(22), N(12), N(27) is distorted to a
somewhat greater extent (deviation, 0.13 A) and the
dihedral angle is 1698. The trans-chlorine angle is
169.99(4)8. The structure of the molecule is shown in Fig.
1 and the bond parameters are listed in Table 2.
Distances
˚
Ru–N(1)
Ru–N(7)
Ru–Cl(1)
N(2)–N(7)
2.093(3)
2.000(3)
2.363(1)
1.299(5)
Ru–N(21)
Ru–N(27)
Ru–Cl(2)
2.072(3)
2.005(3)
2.401(1)
1.306(5)
N(22)–N(27)
Angles
The Ru–N9[N(azo): N(7) or N(27)] bond distance
N(1)–Ru–N(7)
N(1)–Ru–Cl(1)
N(1)–Ru–Cl(2)
Ru–N(7)–N(2)
N(7)–Ru–N(27)
N(7)–Ru–Cl(1)
N(7)–Ru–Cl(2)
N(1)–Ru–N(27)
N(1)–Ru–N(21)
75.64(13)
87.75(9)
86.03(9)
121.2(3)
105.53(13)
94.49(9)
91.57(9)
177.63(12)
102.95(12)
N(21)–Ru–N(7)
N(21)–Ru–N(27)
N(21)–Ru–Cl(1)
N(21)–Ru–Cl(2)
Cl(1)–Ru–Cl(2)
N(27)–Ru–Cl(1)
N(27)–Ru–Cl(2)
Ru–N(27)–N(22)
176.75(12)
75.78(12)
88.35(9)
85.40(9)
169.99(4)
94.19(9)
91.86(9)
117.6(2)
˚
(average, 2.00 A) is shorter than the Ru–N[N(pyrimidine):
˚
N(1) or N(21)] (average, 2.08 A) bond distance by |0.08
˚
A. The shortening may be due to greater p-backbonding,
d(Ru)→p*(azo). The N–N distance is not available for this
free ligand; however, the data available in some free azo
˚
ligands suggest that it is ca. 1.25 A [20,21]. In the
˚
complex, the N–N distance is 1.299(5) A. The coordina-
tion can lead to a decrease in the N–N bond order due to
both the s-donor and p-acceptor character of the ligands,
with the latter character having a more pronounced effect,
which may be the reason for elongation (supported by
spectral data). The pendant phenylazo rings are planar and
are inclined at angles of 41 and 358 relative to the chelated
azoimine fragment, suggesting stereochemically nonequi-
valent C–H functions. Two cis-phenylazo planes are semi-
parallel and are inclined at an angle of 228. The minimum
Table 3
˚
Selected bond distances (A) and angles (8) for complex 4c, along with
their estimated standard deviations
Distances
Ru–N(1)
Ru–N(21)
Ru–N(7)
Ru–N(27)
2.047(3)
2.027(3)
1.971(3)
2.014(3)
Ru–Cl(1)
Ru–Cl(2)
N(2)–N(7)
N(22)–N(27)
2.394(1)
2.401(1)
1.286(5)
1.295(5)
˚
distance between the centroid of phenyl rings is 3.716 A.
Angles
N(1)–Ru–N(7)
N(1)–Ru–Cl(1)
N(1)–Ru–Cl(2)
N(7)–Ru–Cl(1)
N(7)–Ru–Cl(2)
Ru–N(7)–N(2)
N(7)–Ru–N(27)
N(7)–Ru–N(21)
Cl(1)–Ru–Cl(2)
76.50(14)
94.62(11)
84.91(11)
170.58(10)
87.38(11)
121.00(3)
102.18(13)
99.85(14)
88.72(4)
N(1)–Ru–N(21)
N(21)–Ru–N(27)
N(21)–Ru–Cl(1)
N(21)–Ru–Cl(2)
N(27)–Ru–Cl(1)
N(27)–Ru–Cl(2)
Ru–N(27)–N(22)
N(1)–Ru–N(27)
96.27(14)
77.01(13)
84.06(10)
172.75(9)
87.02(9)
102.04(10)
117.6(3)
172.91(14)
3.4. Blue isomer, 4c
The atomic groups Ru, Cl(2), N(27), N(21) (mean
˚
deviation, 0.08 A) and Ru, Cl(1), N(27), N(7), N(1) (mean
˚
deviation, 0.09 A) constitute two almost orthogonal (di-
hedral angle, 888) planes. The third plane, Ru, Cl(1),
N(21), N(7), Cl(2) (mean deviation, 0.08 A), is less
orthogonal (dihedral angles, 98.78 and 82.68, respectively,
˚

2874
P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
with earlier planes). The structure is shown in Fig. 2 and
the bond parameters are listed in Table 3. The trans angles
around the ruthenium centre in the planes range from
170.58 to 172.918, indicating distortion from rectilinear
geometry. The chelate angles extended by azopyrimidine
are 76.5(1)8 and 77.0(1)8 and are considerably deviated
from the ideal octahedral geometry. As a consequence of
the constraint of the bite angle, the ligands are bent back
from the coordinated chlorides. The chelate rings, Ru,
ly, centred primarily on the azo group. Thus, the transitions
in the complexes at around 400 nm are probably of ligand
origin and were not considered further. The green com-
plexes (4a–6a) show an intense (e|104) band in the region
62365 nm, along with a weak shoulder at around 72664
nm (e|103), and are assigned to the t2(Ru)→ p*(L)
(MLCT) transition, where the p* orbital has a large azo
character [31]. Blue complexes exist in two isomeric forms
and both exhibit highly intense (e|104) MLCT transitions
at higher energies compared to the green complex: the
main band is observed in the range 580–595 nm while a
weak shoulder (e|103) appears in the range 760–830 nm.
The energy of the MLCT transition is symmetry-dependent
[63–65]. The X-ray structural study (vide supra) confirms
that isomer A belongs to tcc-Ru(aapm)2Cl2 (C2 symmetry)
and that isomer C belongs to ccc-Ru(aapm)2Cl2 (C1
symmetry) configurations, respectively. It is evident from
IR data that isomer B has a cis-RuCl2 configuration, thus,
it may be of either the cct or ctc type (both belong to C2
symmetry). The less symmetric ccc isomer should exhibit
a stronger interaction compared to the other isomers. On
comparing our results with results reported for similar
systems [6–19,31], ruthenium(II)–arylazopyridine com-
˚
N(1), C(2), N(2), N(7) (mean deviation, 0.05 A) and Ru,
˚
N(21), C(22), N(22), N(27) (mean deviation, 0.08 A), are
planar and the dihedral angle is 738. It is noticeable that
most of the distortions away from octahedral positions
arising out of the larger angular distortions are associated
with the azo nitrogens rather than the pyrimidine nitrogens
[N(7)–Ru–N(27),
102.18(13)8;
N(1)–Ru–N(21),
96.27(14)8]. The cis-chloro angle of 88.72 (4)8 is very
nearly the ideal octahedral angle and is comparable to
reported values [31]. The chelate rings are planar (mean
˚
deviation, 0.06 A) and are inclined at an angle of 738. The
N–N distance is shorter than that in its green counterpart.
˚
The Ru–N(pyrimidine) distances (2.047, 2.027 A) are
˚
longer than those of Ru–N(azo) (1.971, 2.014 A) and this
plexes, we may conclude that isomer
B is ctc-
is an indication of an M–L p-interaction that is localised
in the M–azo fragment. The Ru–Cl distance is comparable
with the reported values [20,21,31]. The pendant phenyl
rings are planar and are inclined at an acute angle with the
respective chelate rings. The dihedral angles between the
chelate ring and the corresponding phenyl ring are 51.6
and 23.28.
Ru(aapm)2Cl2. However, we have no structural evidence to
support this. The ccc-Ru(aapm)2Cl2 isomer exhibits a
transition at a higher energy (|10 nm) than that of the ctc
isomer. The more intense band at the higher energy is
believed to be the spin-allowed singlet–singlet transition
and the weaker band at lower energy could be the
corresponding singlet–triplet transition mode, which is
allowed by the strong spin–orbit coupling in ruthenium
[31].
3.5. Spectra
1
The H NMR spectra of the ligands and complexes were
The green complexes exhibit a single, intense, sharp
stretching frequency in the range 325–300 cm21 that can
be assigned to n(Ru–Cl) and a band in the region 235–220
cm21 that is due to n(Ru–N)(pyrimidine). The blue
complexes exhibit two Ru–Cl stretches in the ranges
330–300 and 280–250 cm21, supporting the cis-RuCl2
configuration. All of the complexes exhibit intense bands
at 1580 and 1400 cm21, corresponding to C=N and N=N
stretchings, respectively. Heterocyclic ring deformations
are observed at the usual positions [38–44], i.e., 1540,
1480, 1290, 1190, 1164, 1090, 1010, 965 cm21 etc. The
compared to determine the binding mode and stereo-
chemistry of the complexes. The proton-numbering pattern
is shown in aapm (3) and are assigned on the basis of
spin–spin interaction and changes therein on substitution.
The spectrum of the ligand is cleanly divided into two
parts (Table 5). The downfield portion is due to the
pyrimidine protons (4-6-H) [38–44] and the upfield signals
refer to azoaryl protons (8-12-H). Aryl protons are affected
by substitution; 9- and 11-H are mostly perturbed due to
changes in the electronic properties of the substituents in
the 10-position. The proton’s movement is corroborated
with the electromeric effect of the group. In the complexes,
4- and 6-H are downfield-shifted whereas 5-H either
remains unshifted or moves upfield relative to the free
ligand values. The Ar-Me signal has been particularly
useful in determining isomer configuration. The tcc- and
ctc-Ru( p-tapm)2Cl2 have C2-symmetry and a single Ar-
Me signal while ccc-Ru( p-tapm)2Cl2 is C1-symmetric and
is expected to exhibit two -Me signals of equal intensities
(Fig. 3). This is indeed observed. The aryl protons 9,11-
(99,119-)H in Ru(aapm)2Cl2 resonate symmetrically at a
ligands (3) exhibit a sharp band at 1420–1440 cm21
,
corresponding to N=N, and C=N appears as a single sharp,
intense stretch at around 1600 cm21. In the complexes,
n(N=N) is red shifted by 25–30 cm21, which is a good
indication of N-coordination.
UV–visible spectral studies of the complexes reveal
absorptions within the range 200–900 nm (Table 4). The
free ligands exhibit transitions at 360–370 nm (e|20,000)
and 430–450 nm (e|500), which are due to intraligand
charge transfer transitions, p→p* and n→p*, respective-

P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
2875
Table 4
Analyticala, UV–Vis spectralb and voltammetric datac–e
Compound
Analysis (%)
C
lmax, nm (1023e, M21 cm21
)
Ru(III)/Ru(II)
Ligand reduction
EM1 / 2(DE, mV)
2EL1 / 2, V (DEp, mV)
1.14(130), 1.42(170)
1.19(140), 1.37(180)
1.09(150), 1.30(180)
H
N
3a papm
65.18
(65.21)
66.68
(66.66)
54.90
4.30
(4.35)
5.00
(5.05)
3.18
30.39
(30.43)
28.26
(28.28)
25.65
431(0.42), 355(19.85), 225(7.46)
446(0.51), 360(26.72), 224(8.98)
447(0.26), 370(12.20), 224(3.85)
3b p-tapm
3c p-Clpapm
(54.92)
44.46
(44.44)
44.40
(44.44)
44.47
(44.44)
46.50
(46.48)
46.47
(46.48)
46.50
(46.48)
39.41
(39.40)
39.42
(3.20)
2.94
(2.96)
2.97
(2.96)
3.00
(2.96)
3.51
(3.52)
3.53
(3.52)
3.51
(3.52)
2.32
(2.30)
2.28
(25.63)
20.71
(20.74)
20.73
(20.74)
20.73
(20.74)
19.70
(19.72)
19.74
(19.72)
19.71
(19.72)
18.41
(18.39)
18.38
4a tcc-Ru(papm)2Cl2
4b ctc-Ru(papm)2Cl2
4c ccc-Ru(papm)2Cl2
5a tcc-Ru( p-tapm)2Cl2
5b ctc-Ru( p-tapm)2Cl2
5c ccc-Ru( p-tapm)2Cl2
6a tcc-Ru( p-Clpapm)2Cl2
6b ctc-Ru( p-Clpapm)2Cl2
6c ccc-Ru( p-Clpapm)2Cl2
723(5.44), 28(10.80), 433(11.13),
251(17.69)
1.08(77)
1.24(80)
1.31(63)
1.00(85)
1.18(80)
1.21(88)
1.17(100)
1.34(102)
1.40(81)
0.48(120), 0.69(170),
1.21e
830(0.82)f, 592(7.71), 403(8.73),
372(13.63), 251(14.34)
0.40(87), 0.61(104),
1.32e
809(0.82)f, 575(7.70), 416(8.96),
372(15.43), 252(17.49)
0.41(76), 0.81(80),
1.38e
724(4.81), 625(11.46), 448(12.04),
372(15.69), 257(17.75)
0.54(80), 1.06(67),
1.43e
838(0.79)f, 595(8.05), 425(11.35)
375(16.82), 250(18.20)
0.53(90), 0.78(120),
1.31e
820(1.08)f, 587(9.47), 437(13.15),
397(21.62), 252(20.13)
0.46(90), 1.10(66),
1.42e
732(4.84), 623(10.51), 441(12.10),
391(15.65), 251(22.49)
0.41(100), 0.64(120),
1.14e
842(0.86)f, 588(8.33), 406(12.90)
368(18.67), 252(17.32)
0.35(90), 0.54(110),
(39.40)
39.39
(39.40)
(2.30)
2.29
(2.30)
(18.39)
18.41
(18.39)
1.20e
760(1.23)f, 580(6.17), 428(8.34),
388(12.22), 254(15.14)
0.34(88), 0.51(90)
1.25e
a Calculated values are given in parentheses.
b In CH3CN.
c Meaning and units of the symbols are the same as in text.
d Solvent in CH3CN. Supporting electrolyte, TBAP (0.01 M); solute concentration, 1023 M; scan rate, 0.05 V s21
.
e Cathodic peak potential Epc, V.
f Shoulder.
Table 5
1H NMR data for the compoundsa [d/ppm, (J/Hz)]
Compound
4-Hb
5-Hc
6-Hb
8-Hb
9-H
10-H
7.58d
11-H
12-Hb
Ar-Me
2.43
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
8.24 (7.0)
8.20 (7.4)
8.30 (7.8)
8.92 (7.5)
9.06 (9.0)
9.17 (9.0)
8.85 (8.4)
8.97 (8.1)
9.00 (8.4)
8.92 (7.8)
9.08 (8.4)
9.19 (9.0)
8.10d
8.24 (7.0)
8.20 (7.4)
8.30 (7.8)
9.15 (9.0)
9.88 (9.0)
9.95 (9.0)
9.02 (9.0)
9.75 (9.0)
9.83 (8.4)
9.22 (9.0)
9.90 (8.4)
9.98 (9.0)
7.83 (9.0)
7.80 (9.0)
7.88 (8.0)
7.59 (7.5)
7.66 (7.5)
7.71 (7.5)
7.52 (7.5)
7.60 (8.4)
7.69 (7.5)
7.58 (7.8)
7.68 (7.8)
7.74 (7.5)
7.45c (8.0)
7.31 (8.0)
7.54 (8.0)
7.00 (7.8)
7.18 (7.5)
7.23 (8.0)
6.65 (7.5)
6.82 (8.4)
6.91 (7.5)
7.06 (7.5)
7.21 (7.8)
7.25 (7.8)
7.45c (8.0)
7.31 (8.0)
7.54 (8.0)
7.00 (7.8)
7.15 (7.5)
7.23 (8.0)
6.65 (7.5)
6.82 (8.4)
6.91 (7.5)
7.66 (7.5)
7.21 (7.8)
7.25 (7.8)
7.83 (9.0)
7.80 (9.0)
7.88 (8.0)
7.69 (7.5)
7.82 (7.5)
9.98 (7.5)
7.61 (7.5)
7.77 (8.4)
7.92 (7.5)
7.68 (7.8)
7.84 (7.8)
7.96 (7.5)
8.10d
8.14d
7.78 (8.0)
7.85 (7.5)
7.88 (8.4)
7.69 (8.4)
7.75 (8.1)
7.81 (7.5)
7.77 (8.4)
7.88 (8.4)
7.91 (7.8)
7.24d
7.39d
7.41d
2.48
2.53
2.65, 2.52
a Solvent in CDCl3.
b Doublet.
c Triplet.
d Multiplet.

2876
P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
Fig. 4. Cyclic voltammogram in MeCN (0.1 M TBAP) at a Pt-bead
working electrode. The solute concentration and scan rate are 1023 M and
50 mV s21, respectively. (i) tcc-Ru(papm)2Cl2 (4a) (green) (———); (ii)
ctc-Ru(papm)2Cl2 (4b) (blue) (- - -) and (iii) ccc-Ru(papm)2Cl2 (4c)
(deep blue) (– ? –).
tions are cis oriented and a back-bonding interaction may
occur with two different dp-orbitals, which may lead to an
increase in the effective charge on the ruthenium centre.
The ccc isomer shows a higher Ru(III)/Ru(II) couple
(0.07–0.1 V) than that of the ctc isomer. This may be due
to reduced symmetry (C1-symmetry) in ccc isomer relative
to ctc (C2-symmetry) isomer, which may lead to a better
Ru–L interaction. This is also supported by electronic
spectral data.
The Ru(III)/Ru(II) redox potential of the present exam-
ples is the highest amongst the known complexes of the
azoimine system. The higher p-acidity of azoimine com-
pared to bipyridine is reflected by the higher potential of
the former [20,21]. Ru(aai)2Cl2 shows the couple in the
range 0.6–0.9 V; while arylazopyridine complexes,
Ru(aap)2Cl2, are exhibited the same at 0.9–1.2 V. The
present complexes Ru(aapm)2Cl2 exhibit the highest po-
tential (1.1–1.4 V) in this family. This is in accord with
the p-acidity order [36,37] of the ligands: bipyridine,
azoimidazole,azopyridine,azopyrimidine. The azo-
pyrimidine ligands thus stabilise (with respect to oxidation)
ruthenium (II) better than the azopyridines.
In the potential range 0.0–1.8 V, reductive responses are
observed under similar conditions using a glassy carbon
working electrode. The reduced species appears to be less
stable and, on scan reversal, multiple anodic responses are
observed. The reduction responses were compared with the
results for free ligands. The free ligand displays two
quasireversible one-electron cyclic voltammetric responses
with peak-to-peak separation in the range 130–180 mV,
corresponding to the couples in Eq. (2):
Fig. 3. Ar-Me signal of ccc-Ru( p-tapm)2Cl2 (5c) in CDCl3 showing
inequivalence character.
single position, while 8-(89-)H and 12-(129-)H resonate
asymmetrically, which is indicative of a magnetically
different environment. The 12-(129-)H is stereochemically
nearer than 8-(89-)H to the metal centre. In blue complexes
(ctc- and ccc-isomers), the steric effect induces more
distortion and is reflected from greater separation in signal
resonance of 12-(129-)H and 8-(89-)H. The separation
follows the order ccc.ctc.tcc and average values are 75,
48 and 30 Hz, respectively.
3.6. Electrochemistry and correlation with electronic
spectra
The electrochemical behaviour of the complexes in
MeCN was examined by cyclic voltammetry. The volt-
ammogram displayed metal oxidations on the positive side
and the ligand reductions on the negative with respect to
SCE. The results are given in Table 4 and a representative
voltammogram is shown in Fig. 4.
In the potential range 10.5 to 12.0 V at a scan rate 50
mV s21 in acetonitrile, one reversible-to-quasireversible
(peak-to-peak separation DEp, 60–100 mV) oxidative
response is observed corresponding to the couple (1) at the
platinum disc electrode surface.
Ru (aapm)2Cl12 1 e2áRu (aapm)2Cl2
(1)
2
2
e
e
aapmáaapm2áaapm22
(2)
The data (Table 4) reveal that the blue complexes (ctc
and ccc) exhibit higher potentials by 0.1–0.2 V than the
green complexes. In the blue isomers, the two azo func-
The formal potentials of these couples are 21.2 and
21.4 V, respectively, which are less than that of the

P.K. Santra et al. / Polyhedron 18 (1999) 2869–2878
2877
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positive and negative to SCE [DE05EM0 (2)2EL0 (3)] may
be correlated with MLCT transitions. The electronic
excitation may be considered as an intramolecular redox
process and the energy, nCT, of the lowest MLCT transition
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nCT 5 0.962 DE0 1 0.491
(3)
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Supplementary data
Supplementary data are available from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK, on request, quoting the deposition numbers
CCDC 116289 (for 4a) and CCDC 116290 (for 4c).
Atomic coordinates (SHELXTL 5.03 program system;
Siemens Analytical X-ray instruments, Madison, WI, 1995)
and isotopic thermal parameters are listed in Tables SI–
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thankful to the Council of Scientific and Industrial Re-
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