μ-Halo bridged binuclear ruthenium(III) complexes featuring pyridazine ligands: Synthesis, structure, spectral and electrochemical properties

Article abstract of DOI:10.1016/j.poly.2011.12.035

The reaction of [RuX₃(AsPh₃)₃] and N,N-disubstituted pyridazine-3,6-diamine in benzene under reflux afford novel binuclear, halo-bridged, ruthenium(III) complexes with the general formula [Ru₂(AsPh₃)<

Full text of DOI:10.1016/j.poly.2011.12.035

Polyhedron 34 (2012) 136–142
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Polyhedron
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l-Halo bridged binuclear ruthenium(III) complexes featuring pyridazine
ligands: Synthesis, structure, spectral and electrochemical properties
⇑
Devaraj Pandiarajan, Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of [RuX3(AsPh3)3] and N,N-disubstituted pyridazine-3,6-diamine in benzene under reflux
afford novel binuclear, halo-bridged, ruthenium(III) complexes with the general formula
[Ru2(AsPh3)2(l-X)2X4(L)] (X = Cl or Br; L = N,N-disubstituted pyridazine-3,6-diamine). All the complexes
Received 16 November 2011
Accepted 19 December 2011
Available online 11 January 2012
have been characterized by elemental analysis, spectral (UV–Vis, EPR) and electrochemical methods
(CV, DPV). The electronic spectra of the complexes indicate the presence of d–d and intense LMCT tran-
sitions in the visible region. The molecular structure of one of the complexes (2) has been determined by
X-ray crystallography, which indicates the bridging coordination of the pyridazine in these binuclear
complexes. In complex (2) each metal has a distorted octahedral NCl4As coordination sphere constituted
by the two exogenous bridging chlorides. The complexes are paramagnetic (low spin, d5) in nature and all
the complexes at 77 K show rhombic distortion around the ruthenium ion with three different ‘g’ values
Keywords:
Binuclear ruthenium(III)
Pyridazine ligand
Crystal structure
Redox behavior
(gx – gy – gz). All the complexes exhibit two quasi-reversible one electron reductive responses (RuIII
–
RuIII/RuII–RuIII; RuII–RuIII/RuII–RuII) within the E1/2 range of ꢀ0.68 to ꢀ0.78 V and ꢀ1.25 to ꢀ1.33 V,
respectively, versus SCE.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
pKa values (2.24 for the parent pyridazine versus 5.25 for pyridine)
[18].
Renewed interest in pyridazine chemistry arises from the po-
tential that they have shown in the formation of polynuclear coor-
dination assemblies with diverse topologies. Generally transition
metal complexes of pyridazine containing ligands have produced
a large number of metal complexes with a wide range of interest-
ing magnetic, electrochemical and structural properties [1–4]. As
The two vicinal nitrogen donor atoms in the pyridazine and
phthalazine heterocycles allow, in principle, the coordination of
two metal centers in close proximity and provide an intramolecu-
lar pathway for spin exchange interactions, which are exclusively
antiferromagnetic in nature [19–22]. Furthermore, the orientation
of the nitrogen lone pairs allows bridging coordination on suitable
metal fragments. Often they are also bridged by other groups such
as hydroxide [23,24], halogen [25], sulfate [26], nitrate [15], iodate
[27] or azide [28]. However, in non-macrocyclic ligands, two addi-
tional donor sites attached to the aromatic ring on both sides of the
nitrogen atoms (3,6-positions) or secondary bridging ligands seem
to be a prerequisite to accommodate two or more metal atoms in
elaborate structures [1,14,29,30]. Thus, pyridazines without adja-
cent coordination sites act mostly as bidentate ligands [31]. These
structural, magnetic and electrochemical studies followed on from
earlier work by Blake and co-workers, who were the first to look at
the magnetic exchange between metal ions bridged by pyridazine-
and phthalazine-containing ligands [32–35].
p
-delocalized ligands with a low p⁄ level and strong
p-accepting
character, pyridazine ligands have been widely utilized in the con-
struction of polymetallic complexes. Pyridazine units have been
incorporated into numerous systems including acyclic ligands,
[5,6] coordination polymers [7], cryptands [8–10] and also macro-
cycles [11–14]. The diazine, N–N, fragment found in the pyridazine
and phthalazine entities, are the common focus of these binucleat-
ing ligands. Ligands of the type OXP, DCPZ and PTP, PPD and DPPN
(Fig. 1) generate six-membered chelate rings. Although there have
been several studies involving binucleating ligands which contain
the 1,2-diazine link [15–17], reports on complexes in which pyrid-
azine itself acts as a bridging ligand remain relatively rare. The
electron-deficient aromatic system, containing two potential N-
donor atoms, leads to a decrease the electron-donor abilities in
comparison to pyridines, which is reflected by significantly lower
The earliest studies relevant to our work are those by Thompson
and co-workers, who have been the key players in the development
of magneto structural data on pyridazine and phthalazine-bridged
acyclic dicopper(II) complexes [25,36,37]. Brooker et al. developed
a methodology concerning the synthesis and structural character-
ization of transition metal complexes of Schiff-base macrocycles
⇑
Corresponding author. Fax: +91 431 2407045/2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.12.035

D. Pandiarajan, R. Ramesh / Polyhedron 34 (2012) 136–142
137
ligands containing a
l
-chloro bridge between the two metal atoms
R = -CH=NOH
OXP
DCPZ
[43]. Further, Zanetti and co-workers reported Schiff base ligands
containing pyridazine ligands with first-row transition metals
[44]. Sun and co-workers described the synthesis and characteriza-
tion of 3,6-dipyridyl substituted pyridazine biruthenium complexes
and the catalytic activity of these complexes towards oxidation of
water to molecular oxygen evolution [45]. These observations
clearly indicate that our pyridazine-containing ligands are, as one
= -COO
R
R
N
=
S
PTP
N
N
N
N
=
=
PPD
might expect on the basis of their
p-acceptor properties, good at
stabilizing low oxidation states of transition metal ions. Hence,
we turned our attention towards pyridazine ligands modified with
two arms, based on their earlier reports and their catenation
property.
N
DPPN
We have long been involved in the synthesis and catalytic appli-
cations of new ruthenium(III) complexes with different ligand
systems, such as arylazophenols and Schiff bases [46–48]. The
Fig. 1. Dinucleating diazine ligands.
present work stems from our interest to design novel
dibridged binuclear pyridazine ruthenium complexes of the type
[Ru2(AsPh3)2( -X)2X4(L)], involving triphenylarsine as an ancillary
functionality. The molecular structure of complex 2 has been
established by single-crystal X-ray analysis, in combination with
electronic and EPR spectra. The relative stabilities of the oxidation
and reduction states are examined electrochemically.
l-halo
l
Cl
Cl
NH2
HN
Xylene
Reflux
N
N
N
N
+
R
HN
2. Experimental
R = -H, -CH3, -OCH3
2.1. Materials
Scheme 1. Synthesis of the ligands.
All reagents and solvents were of high purity grade and were
purchased from commercial sources and used as received unless
noted otherwise. Commercial RuCl3ꢁ3H2O was purchased from
Loba-Chemie Pvt. Ltd. The solvents were purified and dried accord-
ing to standard procedures. 3,6-Dichloropyridazine and the aniline
derivatives were obtained from Aldrich. The supporting electrolyte,
tetrabutyl ammonium perchlorate (n-Bu4NClO4), was dried in vac-
uum prior to use. The starting precursor complexes, [RuCl3(AsPh3)3]
[49] and [RuBr3(AsPh3)3] [50], were prepared according to the
reported procedure.
R
R
NH
X
X
HN
HN
Ru AsPh3
X
[RuX3(AsPh3)3]
Benzene
N
N
N
N
X
Ru AsPh3
NH
X
X
R
R
2.2. Physical measurements
R = -H
R = -CH3
L1
L2
X = Cl (1)
X = Cl (2)
X = Cl (3)
X = Br (4)
X = Br (5)
X = Br (6)
Microanalytical (C, H, N) data were obtained with a Perkin-
Elmer model 240C elemental analyzer. The solution electrical
conductivity was obtained using a Systronic 305 conductivity
bridge. 1H NMR spectra were recorded with a Bruker 400 MHz spec-
trometer. All 1H and 13C NMR spectra were taken in pure deuterated
DMSO solvent. Chemical shifts (d) are given in ppm and refer to TMS
as an internal standard. The EPR measurements were carried out
with an X-band (9.5 GHz) JEOL FA200 spectrometer. Electrochemi-
cal measurements were performed in a conventional two-compart-
ment cell with a polished glassy carbon working electrode, Pt wire
as a counter electrode and a saturate calomel electrode (SCE) as
the reference electrode. All the electrochemical measurements
were carried out with CHI model 650B (Austin, TX) electrochemical
workstation. The supporting electrolyte was 0.1 M [(n-C4H9)4N]
(ClO4) (TBAP), and the solute concentration was ca. 10ꢀ3 M. The
half-wave potential Eo 298 was set equal to 0.5(Epa + Epc), where
Epa and Epc are the anodic and cathodic cyclic voltammetric peak
potentials, respectively.
R = -OCH3 L3
R = -H
L1
L2
R = -CH3
R = -OCH3 L3
Scheme 2. Formation of the binuclear ruthenium pyridazine complexes.
containing pyridazine head units [13,5,38]. Escuer et al. reported
the synthesis, structure and magnetic behavior of two new nicke-
l(II) and cobalt(II) binuclear complexes with 1,4-dicarboxylatopy-
ridazine and their MO calculations of the super exchange
pathway through the pyridazine bridge [39].
Further, doubly cyclometallated palladium(II) acetate complexes
containing 3,6-diphenylpyridazine have been described [40]. Supra-
molecular networks assembled from zinc and manganese binuclear
complexes with pyridazine-3,6-dicarboxylate have been reported
[41]. A novel class of luminescent tricarbonyl rhenium(I) complexes
containing bridging 1,2-diazine ligands has also been described [42].
Mashima and co-workers synthesized the binuclear palladium
complexes of 3,6-bis(imino(2,6-diisopropylphenyl))pyridazine
2.3. Synthesis of the ligands
2.3.1. N,N0-Diphenyl-pyridazine-3,6-diamine (L1)
A mixture of 3,6-dichloropyridazine (0.5 g, 3.356 mmol) and
aniline (0.625 g, 6.71 mmol) in 40 cm3 of xylene was heated in

138
D. Pandiarajan, R. Ramesh / Polyhedron 34 (2012) 136–142
Table 1
Analytical data for the binuclear ruthenium(III) pyridazine complexes.
Complexes
Empirical formula
Color
M.p.
Calculated (found) (%)
C
H
N
1
2
3
4
5
6
C
C
C
C
C
C
52H44As2Cl6N4Ru2
54H48As2Cl6N4Ru2
54H48As2Cl6N4O2Ru2
52H44As2Br6N4Ru2
54H48As2Br6N4Ru2
54H48As2Br6N4O2Ru2
Green
Green
Green
Green
Green
Green
195–198
212–216
210–216
218–223
198–204
221–225
48.43(48.39)
49.22(49.20)
48.05(48.10)
40.13(40.18)
40.94(40.88)
40.13(40.11)
3.44(3.40)
3.67(3.71)
3.58(3.62)
2.85(2.80)
3.05(3.09)
2.99(2.95)
4.34(4.30)
4.25(4.21)
4.15(4.19)
3.60(3.63)
3.54(3.57)
3.47(3.51)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
270
370
470
570
670
770
Wavelength (nm)
Fig. 2. Electronic absorption spectrum of [Ru2(AsPh3)2(l-Cl)2Cl4(L2)] (2) in CH3CN.
Fig. 3. ORTEP diagram of [Ru2(AsPh3)2(l-Cl)2Cl4(L2)] showing 50% probability
thermal ellipsoids.
Table 2
Electronic spectra of the binuclear ruthenium(III) pyridazine complexes
2.3.3. N,N0-Bis-(4-methoxy-phenyl)-pyridazine-3,6-diamine (L3)
Yield: 0.982 g, 91%. M.p. 232 °C. 1H NMR (400 MHz, DMSO-d6,
Me4Si) d: 3.77 (6H, s, OCH3), 6.91 (4H, d, Ar-H), 7.04 (2H, s, py-H),
7.67 (4H, d, Ar-H), 8.70 (2H, br s, NH). 13C NMR (100 MHz, DMSO-
d6, Me4Si) d: 55.12 (OCH3), 113.85, 119.05, 119.66, 135.21, 152.07,
153.15 (aromatic carbons).
Complexes
kmax (nm) e
(dm3/mol/cm)
471(6133)b
1
2
3
4
5
6
648(4587)a
631(1968)a
635(1819)a
651(2673)a
647(1974)a
661(2662)a
287(9642)c
302(18,825)c
287(6584)c
280(6558)c
288(6283)c
285(6587)c
462(2968)b
458(2292)b
452(4873)b
465(2968)b
462(4683)b
a
b
c
d–d Transition.
LMCT.
Ligand centered transition (LCT).
2.4. Synthesis of the complexes
2.4.1. General procedure
A
degassed benzene solution of [RuCl3(AsPh3)3] or
an oil bath at 150 °C with continuous stirring for 6 h. The oil bath
was removed and after the mixture was cooled briefly, 30 cm3 of
water was added cautiously with stirring, followed by the addition
of 0.5 g solid sodium hydroxide. To the mixture, cooled to room
temperature (RT), 50 cm3 dichloromethane was added and the
suspension was stirred until the yellow lumps that had formed
were loosened to a yellow powder. The powder was filtered,
washed with water and diethyl ether, dried in air and then recrys-
tallized from hot acetone. Yield: 0.859 g, 97%. M.p. 241 °C. 1H NMR
(400 MHz, DMSO-d6, Me4Si) d: 7.12 (2H, t, Ar-H), 7.39 (4H, t, Ar-H),
7.53 (4H, d, Ar-H), 7.67 (2H, s, py-H), 10.46 (2H, br s, NH). 13C NMR
(100 MHz, DMSO-d6, Me4Si) d: 120.83, 124.18, 126.72, 129.25,
137.72, 149.12 (aromatic carbons).
[RuBr3(PPh3)] (2 mmol) was taken in a clean 100 ml round bottom
flask. The N,N-disubstituted pyridazine-3,6-diamine ligands
(1 mmol) were added to the above solution. The mixture was
heated at reflux for 8 h. The initial brown red color of the solution
gradually changed to green. The solvent was then evaporated un-
der high vacuum at low temperature, which followed by petroleum
ether (60–80 °C) workup gave a green colored solid. The solid mass
thus obtained was purified using a silica column (60–120 mesh)
chromatography. On elution with chloroform and methanol sol-
vents, the green band of the expected product was isolated. Single
crystals suitable for X-ray diffraction analysis were grown from
mixture of chloroform–acetonitrile solution at room temperature.
2.5. X-ray crystallography
2.3.2. N,N0-Di-p-tolyl-pyridazine-3,6-diamine (L2)
Yield: 0.916 g, 92%. M.p. 228 °C. 1H NMR (400 MHz, DMSO-d6,
Me4Si) d: 2.23 (6H, s, CH3), 7.08 (4H, d, Ar-H), 7.15 (2H, s, py-H),
7.57 (4H, d, Ar-H), 9.09 (2H, br s, NH). 13C NMR (100 MHz,
DMSO-d6, Me4Si) d: 20.34 (CH3), 118.13, 118.9, 120, 121, 129,
129.3, 138.5, 151.6 (aromatic carbons).
Single crystals of [Ru2(AsPh3)2(l-Cl)2Cl4(L2)].CH3CN (2) were
grown by slow evaporation of a chloroform–acetonitrile solution
at room temperature. A crystal of dimensions 0.30 ꢂ 0.26 ꢂ
0.22 mm was selected and the data were collected. The crystal was
mounted on the top of a glass fiber, and transferred to a Stoe IPDS

D. Pandiarajan, R. Ramesh / Polyhedron 34 (2012) 136–142
139
Table 3
Table 4
Crystal data and structure refinement for the binuclear ruthenium(III) pyridazine
Selected bond angles (°) and bond lengths (Å) for complex 2.
complex (2).
Bond length
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system, space group
Unit cell dimensions
a (Å)
C56H49As2Cl6N5Ru2
1356.68
296(2)
As(1)–Ru(1)
Cl(6)–Ru(1)
Cl(5)–Ru(1)
N(2)–Ru(1)
Cl(2)–Ru(1)
Cl(1)–Ru(1)
N(1)–N(2)
2.4423(12)
2.392(3)
2.383(3)
2.185(7)
2.307(3)
2.327(3)
1.368(10)
As(2)–Ru(2)
Cl(6)–Ru(2)
Cl(5)–Ru(2)
N(1)–Ru(2)
Cl(3)–Ru(2)
Cl(4)–Ru(2)
2.4453(13)
2.381(3)
2.375(3)
2.212(7)
2.346(3)
2.316(3)
0.71073
triclinic, P1
ꢀ
10.7679(6)
b (Å)
c (Å)
16.6809(11)
17.7289(10)
Bond angles
N(2)–Ru(1)–As(1)
Cl(2)–Ru(1)–Cl(6)
Cl(1)–Ru(1)–Cl(5)
Cl(6)–Ru(1)–As(1)
Cl(5)–Ru(1)–As(1)
Cl(1)–Ru(1)–As(1)
Cl(2)–Ru(1)–As(1)
N(2)–Ru(1)–Cl(1)
N(2)–Ru(1)–Cl(2)
N(2)–Ru(1)–Cl(5)
N(2)–Ru(1)–Cl(6)
Cl(1)–Ru(1)–Cl(6)
Cl(2)–Ru(1)–Cl(5)
Cl(5)–Ru(1)–Cl(6)
Cl(2)–Ru(1)–Cl(1)
N(1)–N(2)–Ru(1)
Ru(2)–Cl(5)–Ru(1)
175.0(2)
171.53(10)
169.83(10)
98.35(7)
95.01(7)
85.38(7)
85.55(8)
94.3(2)
89.6(2)
86.2(2)
86.7(2)
87.02(10)
89.32(10)
82.87(9)
100.84(10)
116.7(5)
88.52(8)
N(1)–Ru(2)–As(2)
Cl(4)–Ru(2)–Cl(6)
Cl(3)–Ru(2)–Cl(5)
Cl(6)–Ru(2)–As(2)
Cl(5)–Ru(2)–As(2)
Cl(3)–Ru(2)–As(2)
Cl(4)–Ru(2)–As(2)
N(1)–Ru(2)–Cl(3)
N(1)–Ru(2)–Cl(4)
N(1)–Ru(2)–Cl(5)
N(1)–Ru(2)–Cl(6)
Cl(3)–Ru(2)–Cl(6)
Cl(4)–Ru(2)–Cl(5)
Cl(5)–Ru(2)–Cl(6)
Cl(4)–Ru(2)–Cl(3)
N(2)–N(1)–Ru(2)
Ru(2)–Cl(6)–Ru(1)
178.5(2)
171.36(11)
172.79(11)
93.35(7)
96.15(7)
85.38(9)
86.85(9)
93.2(2)
93.0(2)
85.3(2)
87.0(2)
89.61(11)
88.12(11)
83.27(9)
99.00(12)
116.0(5)
88.16(9)
a
b (°)
(°)
90.624(4)
102.277(4)
108.369(4)
2942.6(3)
2, 1.531
1.941
1352
c
(°)
Volume (Å3)
Z, Calc. density (mg/m3)
Absorption coefficient (mmꢀ1
F (000)
)
Crystal size (mm)
Theta range for data collection
Limiting indices
0.30 ꢂ 0.26 ꢂ 0.22
1.18–23.67°
ꢀ12 6 h 6 12, ꢀ18 6 k 6 18,
ꢀ19 6 l 6 19
Reflections collected/unique
Completeness to 2h
Absorption correction
Maximum and minimum
transmission
44122/8789 (Rint = 0.1596)
23.67, 98.9%
semi-empirical from equivalents
0.6747 and 0.5936
Refinement method
full-matrix least-squares on F2
8789/0/643
1.038
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0773, wR2 = 0.1845
R1 = 0.1367, wR2 = 0.2064
1.717 and ꢀ1.051
Table 5
EPR data of the binuclear ruthenium(III) pyridazine complexes.
Largest diff. peak and hole (e Åꢀ3
)
Complexes
gx
gy
gz
hg⁄i
1
2
3
4
5
6
2.54
2.49
2.49
2.46
2.45
2.55
2.07
2.18
2.22
2.23
2.06
2.04
1.85
1.94
1.96
1.93
1.89
1.85
2.17
2.22
2.24
2.21
2.15
2.16
diffractometer using monochromated Mo
Ka radiation (kI =
0.71073). Corrections were made for Lorentz and polarization ef-
fects as well as for absorption (numerical). The structures were
solved and refined by full-matrix least-squares techniques on F2
using the SHELX 97 program [51,52]. The absorption corrections were
done by the multiscan technique. All data were correctedfor Lorentz
and polarization effects, and the non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in the refinement
process as per the riding model.
hg⁄i = [1/3 gx2 + 1/3 gy2 + 1/3 gz
]
.
2
1/2
3. Results and discussion
3.1. Synthesis of the N,N-disubstituted pyridazine-3,6-diamine ligands
Ligands of this sort have been synthesized by the very effective
nucleophilic displacement of chlorine from 3,6-dichloropyridazine
by p-substituted aniline in refluxing xylene, which followed by
subsequent workup gave the N,N-disubstituted pyridazine-3,6-
diamine as a free flowing solid in high isolated yields (Scheme 1)
[53]. The p-methyl substituted ligand is obtained as a yellow solid,
the p-methoxy derivative as a green solid and the aniline deriva-
tive as an orange solid. All the ligands are characterized by 1H
and 13C {1H} NMR in DMSO-d6.
3.2. Synthesis of the complexes
All the complexes are synthesized by refluxing a benzene solu-
tion of 1 mole of the ligand with 2 moles of [Ru(AsPh3)3X3] (X = Cl,
Br) under a nitrogen atmosphere for 8 h (Scheme 2). Coordination
was immediate, as a change of color from reddish brown to a green
color was apparent. The solvent was then removed under high
vacuum at low temperature followed by dichloromethane and
petroleum ether workup to give a green colored solid. The solid
residue thus obtained was purified using a silica gel column by
Fig. 4. EPR spectrum of [Ru2(AsPh3)2(l-Cl)2Cl4(L1)] at 77 K.
elution with chloroform and methanol, giving a green colored band
corresponding to the desired product. All the synthesized com-
plexes were found to be air stable in both the solid and the liquid
states at room temperature and were non-hygroscopic in nature.
The synthesized binuclear ruthenium(III) pyridazine complexes

140
D. Pandiarajan, R. Ramesh / Polyhedron 34 (2012) 136–142
Table 6
Redox potentials for the binuclear ruthenium(III) pyridazine complexes.a
Complexes
Ru(III)–Ru(III)/Ru(II)–Ru(III)
Ru(II)–Ru(III)/Ru(II)–Ru(II)
EPc (V)
EPa (V)
E1/2 (V)b
D
E (mV)c
EPc (V)
EPa (V)
E1/2 (V)b
D
E (mV)c
1
2
3
4
5
6
ꢀ0.79
ꢀ0.78
ꢀ0.79
ꢀ0.81
ꢀ0.73
ꢀ0.75
ꢀ0.68
ꢀ0.64
ꢀ0.67
ꢀ0.66
ꢀ0.63
ꢀ0.62
ꢀ0.74
ꢀ0.71
ꢀ0.73
ꢀ0.74
ꢀ0.68
ꢀ0.68
110
137
117
150
105
132
ꢀ1.44
ꢀ1.41
ꢀ1.45
ꢀ1.48
ꢀ1.37
ꢀ1.42
ꢀ1.19
ꢀ1.19
ꢀ1.20
ꢀ1.16
ꢀ1.12
ꢀ1.15
ꢀ1.32
ꢀ1.30
ꢀ1.33
ꢀ1.32
ꢀ1.25
ꢀ1.29
249
219
250
200
251
272
a
Potential data in volts vs. Fc+/Fc are from single scan cyclic voltammograms recorded at 25 °C in a 0.1 M acetonitrile solution of NBu4ClO4 (0.1 M); scan rate: 50 mV sꢀ1
.
Detailed experimental conditions are given in Section 2.
b
E1/2 = 1/2(Epa + Epc).
c
E = Epa ꢀ Epc denotes the potential difference between the anodic and cathodic potentials, respectively.
3.4. Description of the structure
The molecular structure of one of the complexes (2) has been
determined by single crystal X-ray analysis, and the ORTEP view
is shown in Fig. 3. A summary of single crystal X-ray structure
refinement and selected bond lengths and bond angles are given
in Tables
3 and 4, respectively. The complex [Ru2(AsPh3)2
(l-Cl)2Cl4(L2)] (2) was crystallized with CH3CN as the solvent for
crystallization. The structure consists of a binuclear ruthenium
complex bridged by chlorides and both ruthenium atoms are in a
distorted octahedral coordination environment. Each ruthenium
contains two terminal chlorides, two bridged chloro ligands, one
triphenylarsine and one of the nitrogen atoms from the pyridazine
ligand. As expected, the Ru2(l-Cl)2 unit is not planar due to the
Fig. 5. Cyclic voltammograms of 5 in CH3CN/0.1 M Bu4NClO4 versus SCE at 298 K
(scan rate 50 mV sꢀ1).
presence of the pyridazine ring. The angle between the planes con-
taining Cl(5)–Ru(1)–Cl(6) is 82.87(9)° and Cl(5)–Ru(2)–Cl(6) is
83.27(9)°. The terminal chlorine atoms are trans to the bridging
chlorine atoms and the AsPh3 molecules coordinate through As
atoms at the trans sites of the pyridazine nitrogens. The Ru–N(pyr-
idazine) distances, N(2)–Ru(1) 2.185(7) and N(1)–Ru(2) 2.212(7) Å
(Table 2) are similar to those reported for ruthenium complexes
containing the same coordinating atoms [55]. A major perturbation
of the structure is observed due to the bridging coordination nature
of the pyridazine molecule. Since the lone pairs of two nitrogen
atoms effectively interact with each metal atom, the two ruthe-
nium-centered octahedra may bend toward the bridging ligand.
The Ru(III)–As distances, As(1)–Ru(1) 2.4423(12) and As(2)–Ru(2)
2.4453(13) Å, are comparatively equal. The Ru–Cl (bridging)
distances are longer than the Ru–Cl (terminal) distances. These
distances are comparable with the distances reported for other
structurally characterized diruthenium(III) complexes containing
bridging and terminal chlorides [56,57].
are soluble in acetonitrile, DMF and DMSO producing intense green
colored solutions and they are insoluble in alkanes.
X-ray quality crystals of the complex were grown by slow evap-
oration of chloroform and acetonitrile solutions to give a pyrida-
zine-coordinated diruthenium bridging complex. Elemental
analyses data assigned to the molecular formula [Ru2(AsPh3)2
(l-X)2X4(L)] are presented in Table 1.
3.3. Electronic spectra
Absorption spectral studies are the easiest means of character-
izing ruthenium (III) complexes. Electronic spectra of all the com-
plexes have been recorded in dry acetonitrile solution in the range
200–800 nm. The ground state of ruthenium(III) in an octahedral
2
environment is T2g, arising from the t5 configuration, and the
2g
first excited doublet levels in the order of increasing energy are
3.5. EPR spectra
2
2A2g and T1g, arising from the t42ge1 configuration. Hence, two
g
bands corresponding to 2T2g ? 2A2g and 2T2g ? 2T1g are possible.
A selected spectrum of complex 2 is shown in Fig. 2 and the
spectral data of all the complexes are given in Table 2. All the com-
plexes display three intense absorptions in the region 725–255 nm.
The absorption spectra of the binuclear ruthenium complexes ex-
hibit very intense bands below 400 nm assigned to ligand-centered
EPR measurements of all the binuclear ruthenium complexes
clearly demonstrate the presence of a rhombic ligand field compo-
nent, as one might expect from the nature of the ligands around
the metal ion. The EPR spectral profiles of the binuclear ruthe-
nium(III) complexes (1–6) in frozen dichloromethane solutions
are summarized in Table 5 and a representative spectrum is shown
in Fig. 4. All the complexes show rhombic spectra with three distinct
‘g’ values (gx – gy – gz; gx = 2.45–2.55; gy = 2.04–2.22; gz = 1.85–
1.96) in decreasing order of magnitude. Such a rhombic distortion
(LC)
p ?
p⁄ transitions, and moderately intense bands located in
the visible region from 486 to 435 nm that are ascribed to ligand
to metal charge-transfer (LMCT) transitions taking place from the
filled ligand (HOMO) orbital to the singly-occupied ruthenium t2
can be divided into axial (D) and rhombic (V) components. The axial
orbital (HOMO).
A
broad band appearing between 553 and
distortion splits the t2 level into ‘a’ and ‘e’ and the rhombic compo-
nent again splits ‘e’ into non-degenerate components of the three
major resonances, two are lower and one is higher than the field
ꢃ330 mT. The observed rhombicity of the spectrum is understand-
able in terms of the gross molecular symmetry of these complexes
724 nm is a characteristic band for a d–d transition. The pattern
of the electronic spectra of all the complexes indicates the presence
of an octahedral environment around ruthenium(III) that is similar
to that of other ruthenium(III) octahedral complexes [54].

D. Pandiarajan, R. Ramesh / Polyhedron 34 (2012) 136–142
141
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-1.8
-1.3
-0.8
-0.3
0.2
potential m/V
Fig. 6. Differential pulse voltammogram for the complex [Ru2(AsPh3)2(
l-Cl)2Cl4(L2)] (5) in 0.1 M Bu4NClO4 in acetonitrile solution at 25 °C. The concentration of the
complex = 1 ꢂ 10ꢀ3; the potential scan rate = 2 mV sꢀ1
.
complex. The second reduction response, observed at E1/2 = ꢀ1.25
containing non-equivalent axes [58,47]. The presence of rhombic
distortion is apparent in the splitting of the perpendicular reso-
nance into two spaced components (gx and gy). The EPR spectrum
in Fig. 4 clearly contains a strong half-field resonance near 1300 G.
That is a spin-forbidden transition, characteristic of dinuclear com-
plexes with one unpaired electron on each ruthenium(III) atom
(S = ½). Overall the position of the lines and the nature of the EPR
spectra of the complexes are characteristics of low-spin ruthe-
nium(III) octahedral complexes [56]. Hence, the results from the
EPR spectral analysis indicate that these binuclear ruthenium(III)
complexes are significantly distorted from the ideal octahedral
geometry, as observed in the crystal structure of complex 2.
to ꢀ1.33 V (
DE = 219–272 mV), corresponds to the ruthenium(II)
complex. A similar behavior has been recently observed for chloro
bridged binuclear ruthenium(III) complexes [56].
Further, the differential pulse voltammograms of complex 5
reveals a feature associated with individual reduction steps on
the negative side of the potential window, corresponding to the
two pairs of peaks as observed in the cyclic voltammograms
(Fig. 6).
4. Conclusion
The present work details the first investigation of new binuclear
ruthenium(III) complexes bearing a pyridazine molecule. The
bridging coordination nature of the pyridazine in these complexes
was confirmed by a single crystal X-ray study and reveals the pres-
ence of a distorted octahedral geometry around ruthenium. Both
ruthenium ions in the complexes are in the low spin state and
display rhombic EPR spectra corresponding to the distorted octa-
hedral geometry around the ruthenium(III) ion. All the complexes
show two successive one electron quasi-reversible reductions
(RuIIIRuIII/RuIIIRuII and RuIIIRuII/RuIIRuII).
3.6. Electron transfer properties
The electrochemical properties of all the binuclear ruthe-
nium(III) complexes were studied in an oxygen free acetonitrile
solution by cyclic voltammetry using a glassy carbon electrode
and the potentials are expressed with reference to SCE. All the
binuclear complexes (1 ꢂ 10ꢀ3 M) are electroactive and exhibits
two metal centered reductions in the potential range ꢀ0.62 to
ꢀ1.9 V (0.05 M n-Bu4NClO4 as the supporting electrolyte). The
complexes display two successive Nernstian one electron quasi-
reversible reductive responses (RuIIIRuIII ꢀ RuIIIRuII ꢀ RuIIRuII)
on the negative potential of the cyclic voltammogram at a scan rate
of 50 mVsꢀ1. The potentials are summarized in Table 6 and a rep-
resentative voltammogram (for 5) is shown in Fig. 5. The redox
processes are quasi-reversible in nature, characterized by a rather
Acknowledgments
Financial support by the Council of Scientific and Industrial
Research (CSIR) New Delhi [Grant 01(2156)/07/EMR-II] is grate-
fully acknowledged. One of the authors, S.D.P., thanks CSIR for
the award of Senior Research Fellowship (SRF). We thank DST-FIST,
UGC-SAP India for X-ray, EPR and NMR facilities, School of Chem-
istry, Bharathidasan University, Tiruchirappalli.
large peak to peak separation (DE = 105–272 mV). A comparison of
the current heights (ipa) with that of the standard ferrocene/ferro-
cenium couple under identical experimental conditions reveals the
processes to be one electron reductions [59,60].
eꢀ
eꢀ
RuIII—RuIII ꢀ RuII—RuIII ꢀ RuII—RuII
Appendix A. Supplementary data
E1=2
E1=2
The first reduction process takes place in the potential range E1/
2 = ꢀ0.68 to ꢀ0.74 V ( E = 105–137 mV) and is attributed to the
reduction of ruthenium(III) to the corresponding mixed-valence
CCDC 796684 contains the supplementary crystallographic data
for complex 2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
D

142
D. Pandiarajan, R. Ramesh / Polyhedron 34 (2012) 136–142
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