3,6-Bis(2′-pyridyl)pyridazine (L) and its deprotonated form (L H +)- as ligands for {(acac)(2)Run+} or {(bpy)2Rum+}: Investigation of mixed valency in [{(acac)(2)Ru}(2)(μ-L - H+)]0 and [{(bpy)2Ru}2(μ-L - H+)]4+ by spectroelectrochemistry and EPR

Article abstract of DOI:10.1039/b417530a

Crystallographically characterised 3,6-bis(2′-pyridyl)pyridazine (L) forms complexes with {(acac)₂Ru} or {(bpy)₂Ru ²⁺} via one pyridyl-N/pyridazyl-N chelate site in mononuclear Ru ^II complexes (acac)₂Ru(L), 1, and [(bpy) ₂Ru(L)](ClO₄)₂, [3](ClO₄) ₂. Coordination of a second metal complex fragment is accompanied by deprotonation at the pyridazyl-C⁵ carbon {L → (L - H ⁺)^-} to yield cyclometallated, asymmetrically bridged dinuclear complexes [(acac)₂Ru^III(μ-L - H ⁺)Ru^III(acac)₂](ClO₄), [2](ClO ₄), and [(bpy)₂Ru^II(μ-L - H ⁺)Ru^II(bpy)₂](ClO₄)₃, [4](ClO₄)₃. The different electronic characteristics of the co-ligands, σ donating acac^- and π accepting bpy, cause a wide variation in metal redox potentials which facilitates the isolation of the diruthenium(III) form in [2](ClO₄) with antiferromagnetically coupled Ru^III centres (J = -11.5 cm^-1) and of a luminescent diruthenium(II) species in [4](ClO₄)₃. The electrogenerated mixed-valent Ru^IIRu^III states 2 and [4]⁴⁺ with comproportionation constants K_c > 10 ⁸ are assumed to be localised with the Ru^III ion bonded via the negatively charged pyridyl-N/pyridazyl-C⁵ chelate site of the bridging (L - H⁺)^- ligand. In spectroelectrochemical experiments they show similar intervalence charge transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g₁ - g ₃ ≈ 0.5) in the EPR spectra. However, the individual g tensor components are distinctly higher for the π acceptor ligated system ⁴⁺, signifying stabilised metal d orbitals. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417530a

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F U L L P A P E R
3,6-Bis(2ꢀ-pyridyl)pyridazine (L) and its deprotonated form (L −
H+)− as ligands for {(acac)2Run+} or {(bpy)2Rum+}: investigation
of mixed valency in [{(acac)2Ru}2(l-L − H+)]0 and
[{(bpy)2Ru}2(l-L − H+)]4+ by spectroelectrochemistry and EPR†
Sandeep Ghumaan,a Biprajit Sarkar,b Srikanta Patra,a Kumar Parimal,a Joris van Slageren,c
Jan Fiedler,d Wolfgang Kaim*b and Goutam Kumar Lahiri*a
a Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai, 400076,
India. E-mail: lahiri@chem.iitb.ac.in
b Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55, D-70550,
Stuttgart, Germany. E-mail: kaim@iac.uni-stuttgart.de
c 1. Physikalisches Institut, Universita¨t Stuttgart, Pfaffenwaldring 57, D-70550, Stuttgart,
Germany
d J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejsˇkova 3, CZ-18223, Prague, Czech Republic
Received 17th November 2004, Accepted 14th December 2004
First published as an Advance Article on the web 17th January 2005
Crystallographically characterised 3,6-bis(2ꢀ-pyridyl)pyridazine (L) forms complexes with {(acac)2Ru} or
{(bpy)2Ru2+} via one pyridyl-N/pyridazyl-N chelate site in mononuclear RuII complexes (acac)2Ru(L), 1, and
[(bpy)2Ru(L)](ClO4)2, [3](ClO4)2. Coordination of a second metal complex fragment is accompanied by deprotonation
at the pyridazyl-C5 carbon {L → (L − H+)−} to yield cyclometallated, asymmetrically bridged dinuclear complexes
[(acac)2RuIII(l-L − H+)RuIII(acac)2](ClO4), [2](ClO4), and [(bpy)2RuII(l-L − H+)RuII(bpy)2](ClO4)3, [4](ClO4)3. The
different electronic characteristics of the co-ligands, r donating acac− and p accepting bpy, cause a wide variation in
metal redox potentials which facilitates the isolation of the diruthenium(III) form in [2](ClO4) with
antiferromagnetically coupled RuIII centres (J = −11.5 cm−1) and of a luminescent diruthenium(II) species in
[4](ClO4)3. The electrogenerated mixed-valent RuIIRuIII states 2 and [4]4+ with comproportionation constants Kc >
108 are assumed to be localised with the RuIII ion bonded via the negatively charged pyridyl-N/pyridazyl-C5 chelate
site of the bridging (L − H+)− ligand. In spectroelectrochemical experiments they show similar intervalence charge
transfer bands of moderate intensity around 1300 nm and comparable g anisotropies (g1 − g3 ≈ 0.5) in the EPR
spectra. However, the individual g tensor components are distinctly higher for the p acceptor ligated system [4]4+,
signifying stabilised metal d orbitals.
Ni,11 Pd,
Pt,
Cu
and Ag,15 under different aspects.
9a,e,12
9b,e,12,13
9f ,14
Introduction
However, diruthenium species of L and the efficiency of L as
an electronic mediator between metal termini have not been
investigated so far. We therefore, synthesized diruthenium(III)
and diruthenium(II) complexes of L, viz., [(acac)2RuIII(l-
L − H+)RuIII(acac)2](ClO4), [2](ClO4), and [(bpy)2RuII(l-L −
H+)RuII(bpy)2](ClO4)3, [4](ClO4)3, along with their mononu-
clear counterparts (acac)2RuII(L), 1, and [(bpy)2RuII(L)](ClO4)2,
[3](ClO4)2, respectively. Though the ligand L functions as a
neutral N,Nꢀ-donor in the mononuclear compounds 1 and
[3](ClO4)2, it bridges the metal complex fragments via chemically
different N,Nꢀ and N,C− donor sets in the dinuclear complexes
[2](ClO4) and [4](ClO4)3. It may be noted that the study of
diruthenium complexes based on asymmetric bridging ligands
The design of polynuclear metal complexes which exhibit strong
intermetallic electronic coupling in the mixed-valent states via
the mediation of suitably bridging functionalities has generated
considerable research interest in recent years.1 This has been
primarily due to the relevance for biological processes,2 for
molecular electronics,3 and for theoretical studies of electron
transfer kinetics.4 In this context we have recently been involved
in the design of diruthenium complexes encompassing varying
combinations of bridging and ancillary functionalities. The
purpose of these efforts has been to scrutinize the effectivity
of selective ligand environments for the extent of intermetallic
electronic communication in the mixed-valent state(s).5 The
combinations between the bipyridine-like bis-chelating bridging
ligand 3,6-bis(2ꢀ-pyridyl)pyridazine (bppn, L) and ancillary
ligands such as p-acidic 2,2ꢀ-bipyridine (bpy) or electron-rich
acetylacetonate (acac−) have been chosen for the present work.
The potentially bridging ligand L has been extensively
5n,16
(with inequivalent metal coordination sites) is less common.
The present report describes the synthesis of 1, [2](ClO4),
[3](ClO4)2 and [4](ClO4)3, and the studies of the mixed-valence
properties of the diruthenium(II/III) complexes {[2] and [4]4+}
by detailed spectroelectrochemical and EPR investigations. In
addition we report the magnetic interaction in the paramag-
netic diruthenium(III) complex [2](ClO4) and the luminescence
properties of [4](ClO4)3.
studied in synthesizing a wide range of mononuclear and
polynuclear complexes of Cr,6 Mo,6,7 W, 6 Rh,
Ir,9 Ru,10
8,10c
† Electronic supplementary information (ESI) available: Fig. S1: 1H
NMR spectrum of NC5H4C4H2N2NC5H4 (L) in CDCl3. Fig. S2: Elec-
trospray mass spectra of 1 in CH2Cl2 and [2](ClO4), [3](ClO4)2, [4](ClO4)3
in CH3CN. Fig. S3: 1H NM◦R spectrum of 1 in (CD3)2SO. Table S1: Bond
Results and discussion
The ligand bppn (L) was prepared according to the reported
procedure,17 its crystal structure is shown in Fig. 1. The bond
distances and angles (Table S1, ESI†) are in the expected
˚
distances (A) and angles ( ) for L. See http://www.rsc.org/suppdata/
dt/b4/b417530a/
7 0 6
D a l t o n T r a n s . , 2 0 0 5 , 7 0 6 – 7 1 2
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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The complexes showed satisfactory microanalyses. Com-
pound 1 is neutral while [2](ClO4), [3](ClO4)2 and [4](ClO4)3
exhibited 1 : 1, 1 : 2 and 1 : 3 conductivities, respectively (see
Experimental section). The formation of the four complexes was
confirmed by their electrospray mass spectral data. Complexes 1,
[2](ClO4), [3](ClO4)2 and [4](ClO4)3 exhibited molecular masses
m/z at 534.03, 831.88, 747.16 and 1256.28 (Fig. S2, ESI†),
respectively, corresponding to 1+ (calculated molecular weight,
+
+
533.55), [2]+ (831.83), {[3](ClO4)} (747.04) and {[4](ClO4)2}
Fig. 1 Crystal structure of bppn (L).
(1259.04), respectively.
The complexes 1, [3](ClO4)2 and [4](ClO4)3 are diamagnetic
whereas [2](ClO4) showed l = 2.45 lB at 300 K (see later). In
the former, the ruthenium ions are stabilised in the +II state
whereas [2](ClO4) has the unpaired electrons associated with the
low-spin RuIII ions, antiferromagnetically coupled at 300 K. The
stabilisation of the +III states in chemically nonequivalent
[RuIIIO4N2]+ and [RuIIIO4NC]− sites of [2](ClO4) suggests that the
formation of the strongly p-donating C− donor centre in (L −
H+)− stabilises not only the ruthenium(III) ion directly linked
to it but also facilitates the oxidation of the other ruthenium
centre coordinated by the s-trans oriented bridging ligand. On
the other hand, the neutral form L in the mononuclear derivative
1 stabilises the RuII state of the same [RuO4N2] site.
range18 with an approximate s-trans/s-trans-conformation
of the rings (Fig. 1). The NMR spectrum of L is shown in Fig. S1
(ESI†). The complexation reactions of L were carried out
with the ruthenium precursors RuII(acac)2(CH3CN)2 and [RuII-
(bpy)2(EtOH)2]2+. The reactions of L with Ru(acac)2(CH3CN)2
or [Ru(bpy)2(EtOH)2]2+ in a 1 : 2 molar ratio in EtOH
under a dinitrogen atmosphere, followed by chromatography
using alumina, yielded mononuclear (acac)2RuII(L), 1, with
dinuclear [(acac)2RuIII(l-L − H+)RuIII(acac)2](ClO4), [2](ClO4),
or [(bpy)2RuII(L)](ClO4)2, [3](ClO4)2, with [(bpy)2RuII(l-L −
H+)RuII(bpy)2](ClO4)3, [4](ClO4)3, respectively {(L − H+)− orig-
inates via the deprotonation of pyridazyl-C5 in L} (Scheme 1).
It should be noted that the monomeric bipyridine complex
The 1H NMR spectrum of 1 in (CD3)2SO exhibits ten distinct
aromatic signals which includes six doublets [d/ppm (J/Hz):
8.14 (8.7); 8.44 (7.8); 8.51 (7.8); 8.63 (9.0); 8.67 (6.6); 8.80
(6.0)] and four triplets [d/ppm (J/Hz): 7.43 (6.0, 6.0); 7.57
(6.0, 6.0); 7.74 (6, 8.1); 8.07 (4.5, 3.3)]. The expected four CH3
and two CH protons of acac− appear as six distinct singlets at
1.50/1.54/2.15/2.19 and 5.26/5.37 ppm, respectively (Fig. S3,
ESI†). The corresponding dinuclear species [2](ClO4) failed to
show a conventional 1H NMR spectrum due to its paramagnetic
nature. The bipyridine-containing complexes [3](ClO4)2 and
[4](ClO4)3 display complicated 1H NMR spectra due to the
overlapping of >25 signals with similar chemical shifts in the
aromatic region.
The magnetic susceptibility of [2](ClO4) as a function of
temperature was recorded between 6 to 300 K. It exhibits a
magnetic moment (per dinuclear unit) of 2.45 lB at 300 K
which drops to 0.5 lB at 6 K. On lowering the temperature, the
magnetic susceptibility first increases up to a maximum at 20 K,
then decreases (Fig. 2). The maximum in the susceptibility is a
10c
[3](PF6)2 was reported earlier. The electronic spectrum and
cyclic voltammograms of [3](ClO4)2 are well in agreement with
the reported results (see later). In the mononuclear complexes
1 and [3](ClO4)2, L is bonded to the metal ion by neutral N,Nꢀ
donors whereas the dinuclear species [2](ClO4) and [4](ClO4)3
have the two ruthenium ions linked to the deprotonated bridging
ligand (L − H+)− in its s-cis/s-trans orientation via neutral N,Nꢀ
and anionic N,C− donor sets (cyclometallation). The alternative,
the binding of two metal ions via the available two N,Nꢀ donor
sets of L in the s-cis/s-cis orientation seems to be unlikely from
the steric point of view, particularly for essentially octahedrally
coordinated metal ions as in [2](ClO4) and [4](ClO4)3. Thus, in
10c,8a
9c
other dinuclear complexes of L involving Ru, Rh
or Ir,
similar deprotonation reactions with s-trans orientation and
coordinating N,Nꢀ and N,C− donor sets were reported. However,
for tetra- or penta-coordinated metal complexes in dinuclear
systems the s-cis/s-cis geometry of bridging L with two N,Nꢀ
9a–c,e,f ,12,14a–c,e,15
donor atoms has also been observed.
Scheme 1
D a l t o n T r a n s . , 2 0 0 5 , 7 0 6 – 7 1 2
7 0 7

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Fig. 2 Plot of magnetic susceptibility vs. temperature for [2](ClO4).
The solid line results from least-squares fit using equation (1) and the
parameters from the text.
signature of moderate antiferromagnetic exchange interaction.
The model used to fit the magnetic data considers a general
isotropic spin exchange Hamiltonian H = −2JS1·S2 where S1 =
1
S2 = 2 , using the van Vleck equation (eqn. (1)):19
Ng2b2
1
vM
=
(1)
3kT 1 + (1/3) exp(−2J/kT)
The obtained parameters are: g = 2.06 0.05, J = −11.5
2 cm−1, TIP ≈ 1 × 10−3 cm3 mol−1. The J value indicates in-
tramolecular antiferromagnetic coupling between the unpaired
electrons of the RuIII centres in each dinuclear molecule.
5c
The quasi-reversible RuIII ꢀ RuII couple (I) and an irreversible
RuIII → RuIV oxidation (II) of 1 appear at 0.022 and at
Epa = 1.53 V vs. SCE in CH3CN, respectively (Fig. 3(a),
Table 1). The one-electron nature of couple I was confirmed
by constant-potential coulometry whereas the same for the irre-
versible process II was established by comparing its differential
pulse voltammetric current height with that of couple I. The
RuIII ꢀ RuII couple for the corresponding mononuclear bipyri-
dine complex, [3](ClO4)2, appears at 1.33 V (reported value:
Fig. 3 Cyclic voltammograms (—) and differential pulse voltammo-
grams (---) of (a) 1, (b) [2](ClO4) and (c) [4](ClO4)3 in CH3CN.
RuIIRuII), respectively (Fig. 3(b), Table 1). The successive metal
based couples for the analogous bipyridine complex [4](ClO4)3,
RuIIRuII ꢀ RuIIRuIII (I) and RuIIRuIII ꢀ RuIIIRuIII (II) appear
at 0.77 and 1.25 V, respectively (Fig. 3(c), Table 1). Besides,
the compound also shows four bpy based reductions at −1.33
(III), −1.55 (IV), −1.68 (V) and −1.88 V (VI). Thus, moving
from an electron-rich acetylacetonate environment in [2](ClO4)
to the p-acidic bipyridine ancillary ligands in [4](ClO4)3, a
substantial stabilisation of the RuII state has taken place. The
potentials of the first RuIII/RuII and RuIV/RuIII redox processes
for the dinuclear complex [2](ClO4) and of the first RuIII/RuII
potential for [4](ClO4)3 are appreciably lower than those of
the corresponding mononuclear complexes 1 and [3](ClO4)2.
This justifies the identification of the first oxidation waves in
the dinuclear systems with the RuO4NC− site, the additional
destabilisation of the RuII states in the dinuclear complexes arises
10c
1.35 V ). Thus, a potential shift of ∼1.3 V has taken place
on moving from acac− to bpy ancillary ligands, as expected
from their difference in electronic nature. The appearance of the
RuIII ꢀ RuII couples of [Ru(acac)2(bpy)]+ and [Ru(bpy)3]2+ at
−0.05 V20 and 1.29 V,21 respectively, implies a slightly greater
ligand field strength of L relative to bpy. Complex [3](ClO4)2
exhibits three reversible reductions at −1.15, −1.56 and −1.81 V
10c
which match well with reported values. The dinuclear complex
[2](ClO4) exhibits two irreversible oxidation processes at Epa
1.10 (I) and 1.49 V (II) and two reversible reduction couples at
=
−0.16 (III) and −0.66 V (IV) which are assigned as successive
metal based oxidations (RuIIIRuIII → RuIIIRuIV and RuIIIRuIV
→
RuIVRuIV) and reductions (RuIIIRuIII ꢀ RuIIIRuII and RuIIIRuII ꢀ
Table 1 Redox potentials for complexesa
Compound
1
Couple
E◦298/V (DEp/mV)
RuIII ꢀ RuII (I)
0.022 (70)
RuIII → RuIV (II)
1.53b
[2](ClO4)
RuIIIRuIII → RuIIIRuIV (I)c
RuIIIRuIV → RuIVRuIV (II)
RuIIIRuIII ꢀ RuIIIRuII (III)d
RuIIIRuII ꢀ RuIIRuII (IV)
RuIII ꢀ RuII
1.1b
1.49b
−0.16 (90)
−0.66 (90)
[3](ClO4)2
[4](ClO4)3
1.33 (90)
bpy reductions
−1.15 (70), −1.56 (90), −1.81 (100)
RuIIRuII ꢀ RuIIRuIII (I)c
RuIIRuIII ꢀ RuIIIRuIII (II)
bpy reductions (III–VI)
0.77 (60)
1.25 (90)
−1.33 (60), −1.55 (60), −1.68 (60), −1.88 (60)
a Measured in CH3CN/(Et4N)ClO4, potentials E/V vs. SCE. b pa (irreversible process). c First oxidation at the cyclometallated site. d First reduction
E
at the non-cyclometallated site.
7 0 8
D a l t o n T r a n s . , 2 0 0 5 , 7 0 6 – 7 1 2

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Table 2 UV-Vis-NIR Data of 2n [n = 1, 0, −1] and 4n+ [n = 1, 2, 3, 4, 5] from spectroelectrochemistrya
Complex
kmax/nm (e/dm3 mol−1 cm−1
)
[2]+
2
298 (28800), 398 (9200), 521 (6690)
294 (26000), 415 (sh), 532 (9600), 685 (sh), 1295 (1800)
285 (24400), 310 (sh), 445 (14450), 525 (sh), 610 (sh), 750 (2900)
245 (37400), 310 (45500), 665 (sh)
248 (29500), 288 (49000), 435 (8670), 1335 (2830)
245 (25500), 291 (64000), 342 (sh), 469 (15180)
246 (29600), 295 (53500), 367 (19350), 395 (sh), 513 (12400), 830 (2400)
245 (27200), 295 (42600), 365 (27000), 500 (sh), 535 (14050), 865 (3090)
[2]−
[4]5+
[4]4+
[4]3+
[4]2+
[4]+
a Measurements in CH3CN/0.1 mol dm−3 Bu4NPF6 (OTTLE spectroelectrochemistry).
obviously from the strong r-donor effect of the carbanion centre
in the cyclometallated arrangement.
6690 dm3 mol−1 cm−1), in addition to intense ligand based
transitions in the UV region. In the one-electron reduced species
2 (RuIIIRuII), the charge-transfer transition is slightly red-shifted
to 532 nm (e = 9600 dm3 mol−1 cm−1) with a substantial increase
in intensity. Moreover, the mixed-valent RuIIIRuII species 2
displays a low-energy intervalence charge-transfer (IVCT) band
at 1295 nm (e = 1800 dm3 mol−1 cm−1) (Fig. 4(a)). The width
at half height (Dm1/2) was measured at 2790 cm−1. Following
the above arguments we attribute this IVCT band to a largely
localised transition from [RuIIO4N2] to [RuIIIO4NC−]. On further
one-electron reduction to the RuIIRuII state in [2]−, the IVCT
band disappears and a strong MLCT absorption appears at
445 nm (e = 14500 dm3 mol−1 cm−1) with shoulders at the lower
energy side at 525 and 610 nm and an additional weak band at
750 nm (Fig. 4(b)). The pronounced asymmetry is responsible
for the various MLCT absorptions of 2.
The 500 and 480 mV separations between the two successive
RuIII/RuII couples for [2](ClO4) and [4](ClO4)3, respectively,
are a result of combined effects of the built-in donor centre
asymmetry around the metal ions and of the bridging ligand
mediated intermetallic coupling. The resulting compropor-
tionation constant (Kc) values of the mixed-valent RuIIIRuII
states are 3 × 108 and 1.4 × 108, respectively [using the
relation RTlnKc = nF(DE)22]. The related diruthenium com-
plexes involving 3,6-substituted tetrazine based bridging lig-
ands, 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) and 3,6-bis(3,5-
dimethylpyrazolyl)-1,2,4,5-tetrazine (bpytz) exhibited much
more pronounced effects on the Kc values as a function of the
ancillary ligands: Kc = 3 × 108 (bpy)23 and 1 × 1013 (acac)24 or
107.6 (bpy) and 1013.9 (acac), respectively, in the mixed-valent
RuIIIRuII states. The fact that no such bpy/acac-based shift has
been observed with the present bridging ligand, (L − H+)−,
suggests that the asymmetry effect is primarily responsible for
the fairly large Kc values.
5o
5a
The RuII based intense MLCT band for [4]3+appears at 469 nm
(e = 15180 dm3 mol−1 cm−1). On oxidation to the mixed-valent
RuIIRuIII state in [4]4+, the MLCT band is blue shifted to 435 nm
(e = 8670 dm3 mol−1 cm−1) with appreciably lowered intensity
due to a diminished number of RuII centres. In addition, an IVCT
band appears at 1335 nm (e = 2830 dm3 mol−1 cm−1), with a width
at half height (Dm1/2) of 2660 cm−1 (Fig. 5(a)). This absorption
is attributed to a localised transition from the [RuIIN6] centre to
the [RuIIIN5C−] site, in agreement with the similar IVCT features
for 2. On further oxidation to the isovalent RuIIIRuIII state [4]5+,
the RuII based MLCT and IVCT bands disappear, only a weak
band remains at 665 nm (Fig. 5(b)).
UV-Vis-NIR spectroelectrochemical experiments for [2]n+
(n = −1, 0, 1) and [4]n+ (n = 1, 2, 3, 4, 5) were performed
in acetonitrile solution at 298 K using an OTTLE cell. Spectral
data are listed in Table 2 and the spectra are shown in Figs. 4
and 5. In agreement with the assignment of redox processes
the native RuIIIRuIII species [2]+ exhibits a moderately intense
(L − H+)− → RuIII based LMCT transition at 521 nm (e =
On one-electron reduction to [4]2+, the RuII → bpy MLCT
transition was found to be slightly red-shifted to 513 nm (e =
12400 dm3 mol−1 cm−1) with a slight drop in intensity. This
is a consequence of placing an electron in the LUMO (which
thus becomes a singly occupied MO, SOMO).25 In addition to
that the reduction results in a moderately intense low-energy
absorption at 830 nm (e = 2400 dm3 mol−1 cm−1) (Fig. 5(c))
which corresponds to an internal transition associated with the
bpy radical anion, p(SOMO) → p*(LUMO + 1). On further
reduction to [4]+, the MLCT transition and the low energy
radical anion transition are further red-shifted to 535 and
865 nm (Fig. 5(d)), respectively. These are consistent with the
second reduction associated with a second bpy co-ligand.
On excitation at the lowest energy MLCT band at 469 nm, the
dinuclear complex [4](ClO4)3 containing bipyridine co-ligands
displays a strong emission at 664 nm (quantum yield of φ = 0.14)
at 77 K in 4 : 1 EtOH–MeOH glass with vibrational fine structure
characteristic of emission from a 3MLCT excited state (Fig. 6).
The origin of the emission band at 469 nm was confirmed via
its excitation spectrum (Fig. 6, inset). The quantum yield of
[4](ClO4)3 is less than that for [Ru(bpy)3]2+ (φ = 0.34)26 but
much greater than that of the monomeric species [3](PF6)2
(kem = 630 nm in 1 : 1 1,2-dichloroethane–dichloromethane,
10c
φ = 0.03).
The in situ oxidised mononuclear complexes [1]+ and [3]3+
display EPR signals in frozen CH3CN at 4 K with a rhombic
{[1]+: g1 = 2.311, g2 = 2.214, g3 = 1.852} and an axial g
Fig. 4 UV-VIS-NIR spectroelectrochemistry for the conversions (a)
[2]+ → 2 and (b) 2→ [2]− in CH3CN/Bu4NPF6.
D a l t o n T r a n s . , 2 0 0 5 , 7 0 6 – 7 1 2
7 0 9

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Fig. 5 UV-VIS-NIR spectroelectrochemistry for the conversions of (a) [4]3+ → [4]4+, (b) [4]4+ → [4]5+, (c) [4]3+ → [4]2+ and (d) [4]2+ → [4]+ in
CH3CN/Bu4NPF6.
Fig. 6 Emission spectrum of [4](ClO4)3 in EtOH–MeOH (4 : 1) at 77 K
(kexcitation = 469 nm). Inset shows the corresponding excitation spectrum
(kexcitation = 664 nm).
tensor component splitting {[3]3+: g1,2 = 2.631, g3 = 2.173},
respectively (Fig. 7(a)). Both systems have a distorted octahe-
dral arrangement around the ruthenium(III) ion (low-spin 4d5
configuration),27 however, the higher g values for the p acceptor
ligated system [3]3+ signify stabilised metal d-orbitals. The one-
electron reduced species [3]+ shows a free radical-type EPR
signal at g = 1.997, characteristic of bpy•− bound to RuII.23
The paramagnetic RuIIIRuIII state in [2](ClO4) did not exhibit
any EPR signal even at 4 K in frozen CH3CN solution. This EPR
silence is in agreement with the relatively large antiferromagnetic
spin–spin coupling as described above. The in situ generated
RuIIIRuII reduced species 2 shows a rhombic EPR spectrum in
Fig. 7 X-Band EPR spectra of (a) 1+, (b) 2 and (c) [4]4+ in
acetonitrile at 4 K with g1 = 2.399, g2 = 2.259 and g3 = 1.798
CH3CN/0.1 mol dm−3 Bu4NPF6 at 4 K (* = cavity signal).
(Fig. 7(b)). The g anisotropy (g1 − g3) and the average g factor
(<g>) are calculated at 0.601 and 2.167, respectively, the corre-
sponding values for the mononuclear analogue [1]+ are 0.459 and
2.135. In comparison, the bipyridine analogue [4]4+ (RuIIRuIII)
exhibits a rhombic signal with g1 = 2.748, g2 = 2.468 and g3 =
2.176 leading to g1 − g3 = 0.572 and <g> = 2.475 (Fig. 7(c)), the
corresponding values are 0.458 and 2.488 for the mononuclear
[3]3+. The higher g anisotropy for both dinuclear systems in
relation to corresponding mononuclear species reflects increased
participation of the metals at the singly occupied MO, suggesting
strong ruthenium(III)/carbanion interaction and, perhaps, a
partial valence delocalisation. Again, however, the individual g
tensor components are distinctly higher for [4]4+, in accordance
with stabilised metal d orbitals in that p acceptor ligated system.
Reduction to [4]2+ produced an unresolved EPR signal at g =
2.0017, typical for a bpy•−/RuII situation.23
In conclusion, both the intervalence charge transfer (IVCT)
bands in the near infrared and the g anisotropies g1 − g3,
obtained from EPR are comparable for mixed-valent 2 and [4]4+
because these values reflect energy differences and suggest a
similar qualitative description of the frontier orbitals situation
in these mixed-valent species. As compared to the electronically
7 1 0
D a l t o n T r a n s . , 2 0 0 5 , 7 0 6 – 7 1 2

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rich acac− species 2, the individual g tensor components are
distinctly higher for the p acceptor (bpy) ligated system [4]4+
as are the redox potentials. Though different homovalent
forms were isolated as [2]+ (RuIIIRuIII) and [4]3+ (RuIIRuII),
the RuIIRuIII mixed-valent intermediates are easily generated
through reduction and oxidation, respectively. The asymmetric
coordination of the metal centres by (l-L − H+)− is assumed
to be the main cause of the fairly high comproportionation
constants Kc > 108.
solution was added to it and the mixture kept at 0 ◦C overnight.
Filtration yielded a precipitate which was washed with ice-cold
water followed by cold ethanol, and dried under vacuum. It was
then purified using a neutral alumina column. Initially, a red
compound corresponding to Ru(acac)3 was eluted by CH2Cl2–
CH3CN (25:1), followed by a brown compound with CH2Cl2–
CH3CN (5 : 1) corresponding to 1. With CH2Cl2–CH3CN (1 : 1),
a purple compound corresponding to [2](ClO4) was eluted.
Evaporation of solvent under reduced pressure yielded pure
complexes 1 and [2](ClO4), respectively.
Anal. Calc. for 1: C, 53.92; H, 4.53; N, 10.49. Found: C, 53.67;
H, 4.47; N, 10.03%. kmax/nm (e/dm3 mol−1 cm−1) in CH3CN at
298 K: 526 (11760), 428 (11800), 388 (12530), 304 (32220), 278
(35960). Yield: 30% (42 mg).
Anal. Calc. for [2](ClO4): C, 43.78; H, 4.00; N, 6.01. Found:
C, 43.52; H, 3.97; N, 5.98%. Conductivity: KM/X−1 cm2 mol−1
in acetonitrile at 298 K: 115. Yield: 55% (67 mg).
Experimental
Materials
The precursor compounds Ru(acac)2(CH3CN)2,28 cis-
[Ru(bpy)2Cl2]·2H2O21 and 3,6-bis(2ꢀ-pyridyl)pyridazine (L)17
were prepared according to the reported procedures. Other
chemicals and solvents were reagent grade and used as received.
For spectroscopic and electrochemical studies HPLC grade
solvents were used.
Synthesis of [(bpy)2Ru(L)](ClO4)2 [3](ClO4)2 and [(bpy)2Ru(l-
L − H+)Ru(bpy)2](ClO4)3 [4](ClO4)3. The starting complex
cis-[Ru(bpy)2Cl2]·2H2O (100 mg, 0.21 mmol) and AgClO4
(108.6 mg, 0.52 mmol) were taken in 15 cm3 absolute ethanol
and the mixture was refluxed for 2 h with stirring. The initial
violet solution changed to orange–red; it was then cooled and
filtered through a sintered glass funnel. The ligand L (24 mg,
0.10 mmol) was then added to the above solution containing
[Ru(bpy)2(EtOH)2]2+. The resulting mixture was heated to reflux
for 20 h under dinitrogen atmosphere. The initial orange-red
solution gradually changed to◦brown. The reaction mixture was
reduced to 5 cm3 and kept at 0 C overnight. The precipitate was
filtered and washed with ethanol. The solid mass thus obtained
was purified by using a neutral alumina column. Initially, an
orange compound corresponding to [3](ClO4)2 was eluted with
CH2Cl2–CH3CN (1.5 : 1). With CH2Cl2–CH3CN (1 : 3), a brown
compound corresponding to [4](ClO4)3 was then separated.
Evaporation of solvent under reduced pressure yielded the pure
complexes [3](ClO4)2 and [4](ClO4)3, respectively.
Anal. Calc. for [3](ClO4)2: C, 48.23; H, 3.10; N, 13.24. Found:
C, 48.53; H, 3.99; N, 12.82%. KM/X−1 cm2 mol−1 in acetonitrile
at 298 K: 264. kmax/nm (e/dm3 mol−1 cm−1) in CH3CN at 298
K: 480 (2430), 434 (4400), 286 (37100), 206 (30340). Yield: 25%
(44 mg).
Anal. Calc. for [4](ClO4)3: C, 47.68; H, 3.11; N, 12.36. Found:
C, 47.28; H, 3.01; N, 12.23%. KM/X−1 cm2 mol−1 in acetonitrile
at 298 K: 342. Yield: 50% (70 mg).
Physical measurements
UV-Vis-NIR spectroelectrochemical studies were performed in
CH3CN/0.1 mol dm−3 Bu4NPF6 at 298 K using an optically
transparent thin layer electrode (OTTLE) cell29 mounted in
the sample compartment of a Bruins Instruments Omega 10
spectrophotometer. FTIR spectra were taken on a Nicolet
spectrophotometer with samples prepared as KBr pellets. So-
lution electrical conductivity was checked using a Systronic
1
305 conductivity bridge. H NMR spectra were obtained with
a 300 MHz Varian FT spectrometer. The EPR measurements
were made in a two-electrode capillary tube30 with an X-band
(9.5 GHz) Bruker system ESP300, equipped with a Bruker
ER035M gaussmeter and a HP 5350B microwave counter. Cyclic
voltammetric, differential pulse voltammetric and coulometric
measurements were carried out using a PAR model 273A
electrochemistry system. Platinum wire working and auxiliary
electrodes and an aqueous saturated calomel reference elec-
trode (SCE) were used in a three-electrode configuration. The
supporting electrolyte was [NEt4]ClO4/0.1 mol dm−3 and the
solute concentration was ca. 10−3 mol dm−3. The half-wave
potential E◦ was set equal to 0.5(Epa + Epc), where Epa and
298
E
pc are anodic and cathodic cyclic voltammetric peak potentials,
respectively. A platinum wire-gauze working electrode was used
in coulometric experiments. The elemental analysis was carried
out with a Perkin-Elmer 240C elemental analyser. Electrospray
mass spectra were recorded on a Micromass Q–ToF mass
spectrometer. Emission experiments were made using a Perkin-
Elmer LS 55 spectrometer fitted with a cryostat and the
quantum yield (φ) was determined by following a previously
described method.31 The magnetic susceptibility of [2](ClO4)
as a function of temperature was recorded from 6 to 300 K
using a 0.1 T applied field on a Quantum Design MPMS
XL7 SQUID magnetometer. The data were corrected for the
diamagnetic contributions to the magnetic susceptibility using
Pascal’s constants, for the diamagnetic contribution of the
sample holder, and for temperature independent paramagnetism
(TIP).
Crystallography
Single crystals of bppn (L) were grown by slow diffusion of a
dichloromethane solution into hexane, followed by slow evapo-
ration. X-ray data of L were collected on a PC-controlled Enraf-
Nonius CAD-4 (MACH-3) single-crystal X-ray diffractometer
using Mo-Karadiation. The structure was solved and refined by
full-matrix least-squares on F2 using SHELX-97 (SHELXTL).32
Hydrogen atoms were included in the refinement process as per
the riding model.
Crystal data for bppn: C14H10N4, M = 234.26, monoclinic,
space group Pn, a = 5.7084(5), b = 6.5938(4), c = 15.3455(10)
◦
3
˚
˚
A, b = 91.693(6) , V = 577.35(7) A , T = 293(2) K, Z = 2,
l = 0.085 mm−1, e data (Rint) = 1122 (0.000), R1 (I > 2r(I)) =
0.0322, wR2 (all data) = 0.0888.
CAUTION! Perchlorate salts of metal complexes are generally
explosive. Care should be taken while handling such complexes.
CCDC reference number 256270.
Synthesis of (acac)2RuII(L) 1 and [(acac)2Ru(l-L − H+)Ru-
(acac)2](ClO4) [2](ClO4). The starting complex Ru(acac)2-
(CH3CN)2 (100 mg, 0.26 mmol), and the ligand L (30 mg,
0.13 mmol) were dissolved in 20 cm3 of ethanol and the mixture
was heated to reflux for 6h under a dinitrogen atmosphere.
The initial orange solution gradually changed to purple. The
solvent of the reaction mixture was evaporated to dryness under
reduced pressure. The solid mass thus obtained was dissolved in
minimum volume of acetonitrile, then excess aqueous NaClO4
See http://www.rsc.org/suppdata/dt/b4/b417530a/ for cry-
stallographic data in CIF or other electronic format.
Acknowledgements
Financial support received from the Department of Science
and Technology, New Delhi (India), the Council of Scientific
and Industrial research, New Delhi (India), the DAAD, the
DFG and the FCI (Germany) is gratefully acknowledged.
D a l t o n T r a n s . , 2 0 0 5 , 7 0 6 – 7 1 2
7 1 1

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Special acknowledgment is made to the Sophisticated Analytical
Instrument Facility, Indian Institute of Technology, Bombay,
for providing the NMR facility. X-Ray structural studies were
carried out at the National Single Crystal Diffractometer
Facility, Indian Institute of Technology, Bombay.
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