Ruthenium terpyridine complexes incorporating azo-imine based ancillary ligands. Synthesis, crystal structure, spectroelectrochemical properties and solution reactivities

Article abstract of DOI:10.1039/b005789o

Ruthenium terpyridine complexes of the type [Ru^II(trpy)(L)(X)][ClO₄]_n 1-15 [trpy = 2,2′ :6,2″-terpyridine; L = NC₅H₄N=NC₆H₄(R), R = H, m-Me, m-Cl, p-Me orp-Cl; X = Cl^- n = 1 (1-5); H₂O, n = 2 (6-10) or OH^-, n = 1 (11-15)] have been synthesized. The single crystal structures of 1 and 6 were determined. Complex 4 develops in two possible isomeric forms whereas all other complexes stabilise preferentially in one isomeric form. The complexes exhibit strong MLCT bands near 500 nm and ligand based transitions in the UV region. 1-5 exhibit moderately strong emissions at 77 K near 600 nm. The chloro (1-5) and the hydroxo (11-15) complexes display ruthenium(m)-ruthenium(ii) couples and three to four successive one-electron ligand based reductions. The aqua-complexes (6-10) exhibit a reversible 2e^-/2H⁺ single-step oxidation process in the pH range 1-5.5 corresponding to the [Ru^II(trpy)(L)(H₂O)]²⁺-[Ru ^Iv(trpy)(L)(O)]²⁺ couple and the potential decreases linearly with increase in pH. The chemical oxidations of 6-10 by an excess of Ce^IV in 0.5 M H₂SO₄ also lead to the formation of corresponding [Ru^IV(trpy)(L)(O)]²⁺. The oxo-complexes are stable only in the presence of an excess of Ce^IV, otherwise they catalyse the oxidation of water to dioxygen and convert back into the parent aqua-species. The pseudo first order rate constant of the process [Ru^IV(trpy)(L)(O)]²⁺ → [Ru^II(trpy)(L)(H₂O)]²⁺ has been determined. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b005789o

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Ruthenium terpyridine complexes incorporating azo-imine based
ancillary ligands. Synthesis, crystal structure, spectroelectro-
chemical properties and solution reactivities†
Biplab Mondal, Mrinalini G. Walawalkar and Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai-400076, India
Received 18th July 2000, Accepted 19th September 2000
First published as an Advance Article on the web 30th October 2000
Ruthenium terpyridine complexes of the type [RuII(trpy)(L)(X)][ClO4]n 1–15 [trpy = 2,2Ј:6,2Љ-terpyridine;
L = NC H N᎐NC H (R), R = H, m-Me, m-Cl, p-Me or p-Cl; X = ClϪ, n = 1 (1–5); H O, n = 2 (6–10) or OHϪ,
᎐
5
4
6
4
2
n = 1 (11–15)] have been synthesized. The single crystal structures of 1 and 6 were determined. Complex 4
develops in two possible isomeric forms whereas all other complexes stabilise preferentially in one isomeric form.
The complexes exhibit strong MLCT bands near 500 nm and ligand based transitions in the UV region. 1–5
exhibit moderately strong emissions at 77 K near 600 nm. The chloro (1–5) and the hydroxo (11–15) complexes
display ruthenium()–ruthenium() couples and three to four successive one-electron ligand based reductions.
The aqua-complexes (6–10) exhibit a reversible 2eϪ/2Hϩ single-step oxidation process in the pH range 1–5.5
corresponding to the [RuII(trpy)(L)(H2O)]2ϩ–[RuIV(trpy)(L)(O)]2ϩ couple and the potential decreases linearly
with increase in pH. The chemical oxidations of 6–10 by an excess of CeIV in 0.5 M H2SO4 also lead to the
formation of corresponding [RuIV(trpy)(L)(O)]2ϩ. The oxo-complexes are stable only in the presence of an
excess of CeIV, otherwise they catalyse the oxidation of water to dioxygen and convert back into the parent
aqua-species. The pseudo first order rate constant of the process [RuIV(trpy)(L)(O)]2ϩ → [RuII(trpy)(L)(H2O)]2ϩ
has been determined.
Introduction
The development of a new class of photo-redox active
ruthenium polypyridine complexes has been the subject matter
of continuous research activity.1 Strong metal to ligand charge-
transfer transitions, facile electron-transfer properties and long
3
lived MLCT excited states of this class of complexes make
them effective for designing photochemical and electrochemical
devices.2 In this direction a variety of ruthenium monoterpyrid-
ine complexes of the type [Ru(trpy)(L)] (trpy = 2,2Ј:6Ј,2Љ-
terpyridine) incorporating different kinds of ancillary ligands
(L) have been synthesized in recent years.3 The recent observ-
ations of the significant role of ancillary groups (L) in modulat-
ing the properties of this class of complexes4 have initiated the
Scheme 1 (i) NEt3, LiCl, 1:1 EtOH–water, heat.
idea of introducing a strong π-acidic azo-imine function
{NC H N᎐NC H (R)} in the [Ru(trpy)] core. This indeed leads
᎐
5
4
6
4
to the stepwise syntheses of a series of complexes: [RuII(trpy)-
synthesized from [Ru(trpy)Cl3] and L in the presence of NEt3
and LiCl in refluxing 1:1 EtOH–water medium (Scheme 1).
The complexes have been isolated as their perchlorate salts and
purified by using a silica gel column. The reactions also lead to
the formation of ttt- and ctc-[RuII(L)2Cl2] [ttt, trans-trans-trans;
ctc, cis-trans-cis with respect to the chlorides, pyridine(Np) and
azo(Na) nitrogens respectively5] as minor by-products (yield:
ttt, 5–7%; ctc, 8–10%). These are separated from the desired
complexes (1–5) by chromatographic techniques (Experimental
section).
(L)Cl]ϩ → [RuII(trpy)(L)(H2O)]2ϩ → [RuII(trpy)(L)(OH)]ϩ
and [RuII(trpy)(L)(H2O)]2ϩ → [RuIV(trpy)(L)(O)]2ϩ
.
Here
the chemically generated oxo-complexes are found to be active
catalysts for the facile oxidation of water to dioxygen. Herein
we report the synthetic aspects, detailed spectroelectrochemical
properties of the complexes and the crystal structures of two
members of the series (chloro- and aqua-species).
Results and discussion
Synthesis
The unsymmetrical nature of L leads to the possibility of two
isomers, A and B, of the complexes 1–5. However, in practice
A group of five azo-imine based ligands L1–L5 have been used
for the present study. They differ with respect to the location
and electronic nature of the ‘R’ groups present in the pendant
phenyl ring. The complexes [RuII(trpy)(L)Cl]ϩ 1–5 have been
† Electronic supplementary information (ESI) available: micro-
analytical and conductivity data for compounds 1–15. See http://
www.rsc.org/suppdata/dt/b0/b005789o/
DOI: 10.1039/b005789o
J. Chem. Soc., Dalton Trans., 2000, 4209–4217
This journal is © The Royal Society of Chemistry 2000
4209

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Table 1 Selected bond distances (Å) and angles (Њ) and their standard
only one has been systematically obtained for 1–3 and 5 (see
NMR). The crystal structure of 1 establishes the presence of
the isomeric form B (see later). Since the spectral features of
the other complexes (2, 3, 5) are akin to those of 1 (see later),
we therefore logically assume that they also exist in the same
isomeric form, B.
deviations for [Ru(trpy)(L1)Cl]ClO4
[ClO4]2ؒH2O 6
1
and [Ru(trpy)(L1)(H2O)]-
1
6
Ru–N(1)
Ru–N(2)
Ru–N(3)
Ru–N(4)
Ru–N(6)
Ru–Cl
2.070(2)
1.973(2)
2.075(2)
2.055(2)
1.966(2)
2.408(9)
1.304(3)
Ru–N(1)
2.069(6)
1.967(5)
2.069(6)
2.051(5)
1.944(5)
2.140(5)
1.274(7)
1
In the case of complex 4 the solution H NMR study indi-
Ru–N(2)
Ru–N(3)
Ru–N(4)
Ru–N(6)
Ru–O(30)
N(5)–N(6)
cates the presence of an intimate mixture of both isomers A and
B in a ratio of 1:3 (see NMR part). All our attempts to separate
the isomers either by column chromatography or by using TLC
plates have failed; however, we have succeeded in separating
them by a crystallisation technique. Two types of single crystals
have been obtained from the bulk; a rectangular type corre-
sponds to isomer B and a triangular type A (see later). Crystals
of specific types were separated carefully and used for further
characterisation. Unfortunately they are weakly diffracting
and not suitable for structure determination. Efforts to obtain
X-ray quality single crystals are in progress. In order to avoid
complications due to the presence of isomers in the bulk of 4,
only the crystals corresponding to isomer B were used for
further studies.
N(5)–N(6)
N(6)–Ru–N(2)
N(6)–Ru–N(4)
N(2)–Ru–N(4)
N(6)–Ru–N(1)
N(2)–Ru–N(1)
N(4)–Ru–N(1)
N(6)–Ru–N(3)
N(2)–Ru–N(3)
N(4)–Ru–N(3)
N(1)–Ru–N(3)
N(6)–Ru–Cl(1)
N(2)–Ru–Cl(1)
N(4)–Ru–Cl(1)
N(1)–Ru–Cl(1)
N(3)–Ru–Cl(1)
101.29(10)
76.55(10)
176.13(10)
87.40(9)
79.27(10)
103.73(10)
96.88(9)
79.56(11)
97.44(10)
158.82(10)
170.86(7)
87.01(7)
N(6)–Ru–N(2)
N(6)–Ru–N(4)
N(2)–Ru–N(4)
N(6)–Ru–N(1)
N(2)–Ru–N(1)
N(4)–Ru–N(1)
N(6)–Ru–N(3)
N(2)–Ru–N(3)
N(4)–Ru–N(3)
N(1)–Ru–N(3)
N(6)–Ru–O(30)
N(2)–Ru–O(30)
N(4)–Ru–O(30)
N(1)–Ru–O(30)
N(3)–Ru–O(30)
96.79(2)
76.2(2)
172.9(2)
90.4(2)
79.6(2)
99.6(2)
96.4(2)
78.8(2)
102.4(2)
157.9(2)
174.4(2)
88.3(2)
98.8(2)
87.9(2)
87.1(2)
Aqua complexes 6–10 have been synthesized in pure state
from the corresponding chloro derivatives 1–5 by using an
excess of AgNO3 in refluxing water medium, eqn. (1). They
95.38(8)
90.43(7)
88.34(7)
water
[RuII(trpy)(L)Cl]ϩ ϩ AgNO3
heat
1–5
[RuII(trpy)(L)(H2O)]2ϩ ϩ AgCl (1)
6–10
were isolated as their monohydrated perchlorate salts. The con-
version of chloro to aqua species has been authenticated by the
single crystal structure of 6 (see later). It shows that the iso-
meric structure B of the parent chloro derivative 1 has been
retained in the aqua product 6. The isomeric purity of all the
aqua complexes has been confirmed by the 1H NMR technique.
Hydroxo complexes 11–15 have been prepared from the aqua
derivatives 6–10 in the presence of alkali as shown in eqn. (2).
NaOH, water
[RuII(trpy)(L)(H2O)]2ϩ
heat
6–10
[RuII(trpy)(L)(OH)]ϩ (2)
11–15
Fig. 1 An ORTEP6 plot for [RuII(trpy)(L1)Cl]ClO4 1.
The monocationic complexes have been isolated as their mono-
hydrated perchlorate salts. The aqua → hydroxo conversion
is also found to be stereoretentive in nature.
The oxo complexes [RuIV(trpy)(L)(O)]2ϩ can be generated in
solution by chemical oxidation of [RuII(trpy)(L)(H2O)]2ϩ 6–10
using an excess of Ce4ϩ ion in 0.5 M H2SO4. However, they
catalyse the oxidation of water to dioxygen and subsequently
reduce back to the parent aqua species, a cycle which is repeated
on sequential addition of Ce4ϩ (see later). This prevented
isolation of the oxo-derivatives in the solid state.
The complexes 1–15 are diamagnetic. In acetonitrile solution
1–5 and 11–15 show 1:1 conductivity whereas 6–10 behave as
1:2 conductors in water. The complexes exhibit satisfactory
elemental analyses (ESI).
Crystal structures of [Ru(trpy)(L1)Cl]ClO4 1 and [Ru(trpy)(L1)-
(H2O)][ClO4]2ؒH2O 6
Single crystal structures of the complexes [Ru(trpy)(L1)Cl]ClO4
1 and [Ru(trpy)(L1)(H2O)][ClO4]2ؒH2O 6 are shown in Figs. 1
and 2. Selected bond distances and bond angles are given in
Table 1. The RuN5Cl and RuN5O co-ordination spheres in 1
and 6 respectively are distorted octahedral as can be seen from
the angles subtended at the metal ions (Table 1). Terpyridine
Fig. 2 An ORTEP plot for [RuII(trpy)(L1)(H2O)][ClO4]2ؒH2O 6.
ligand is co-ordinated in the expected meridional fashion with
the ligand L1 in cis orientation.7 The chloride or the water
molecule is trans to the azo nitrogen (Na) of L1 (structure B).
4210
J. Chem. Soc., Dalton Trans., 2000, 4209–4217

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The single crystal of 6 contains water of crystallisation in the
ratio [Ru(trpy)(L1)(H2O)][ClO4]2 :H2O = 1:1. The geometrical
constraints imposed on the meridional terpyridine ligand are
reflected in the trans angles, N(1)–Ru–N(3), 158.82(10)Њ for 1
and 157.9(2)Њ for 6. The Ru–N(2) distances (central pyridyl
group of terpyridine) in 1 and 6 are approximately 0.1 Å shorter
than the terminal Ru–N bonds [Ru–N(1) and Ru–N(3)]. To
optimise the chelation of terpyridine, the central Ru–N bond
shortens while the terminal one lengthens, which maintains a
typical trpy bite angle of ≈ 79Њ.3
The ligand L1 is bound to the ruthenium ion with the pyrid-
ine nitrogen (Np) and the azo nitrogen (Na) having a bite angle of
76.55(10)Њ in complex 1 and 76.2(2)Њ in 6. The shorter Ru–N(6)
(azo) distances [1.966(2) Å in 1 and 1.944(5) Å in 6] compared
to Ru–N(4)(pyridine) distances [2.055(2) and 2.051(5) Å] of
co-ordinated L1 are due to strong (dπ)RuII→π*(azo) back
bonding.8
The RuII–Cl distance, 2.4078(9) Å, in complex 1 is longer
than that observed in [Ru(trpy)(biq)Cl]PF6 (biq = 2,2Ј-biquino-
line), 2.378(2) Å,9 but close to those found in other RuII–trpy
complexes.10
The RuII–O(H2O) distance, 2.140(5) Å, found in complex 6 is
slightly longer than the other known RuII–O(H2O) distance,
2.122(16) Å.11 However, it is much longer than the RuII–O
(phenolato) distances 2.060(3), 2.064(4), 2.022(5) and 2.042(4) Å
found in [RuII(bpy)2(Schiff base)]ϩ,12 [RuII(bpy)2(pyridine
phenolate}]ϩ,13 [RuII(pap)2(catecholate)]14 [pap = 2-(phenyl-
azo)pyridine]
and
[RuII(bpy)2(salicylate)]15
complexes
respectively.
To the best of our knowledge, this work demonstrates the
first crystal structure of an aqua-derivative of ruthenium–
terpyridine as well as ruthenium–azopyridine complex systems.
The perchlorate (ClO4Ϫ) anion is tetrahedral with an average
Cl–O distance of 1.363(4) and 1.395(7) Å and an average O–Cl–
O angle of 109.416(4) and 109.458(6)Њ in complexes 1 and 6
respectively.
Spectral properties
There are four notable features in the IR spectra of the com-
plexes. (i) The νN᎐N stretching frequency of the co-ordinated L is
appreciably lowered (≈1300 cmϪ1) compared to that of free L
(≈1425 cmϪ1), evidently due to the (dπ)RuII→π*(L) back
bonding effect.16 (ii) The νRu–Cl stretching frequency of 1–5 has
been observed as a sharp singlet near 320 cmϪ1,5 which is sys-
tematically absent in the spectra of the aqua (6–10) and
hydroxo (11–15) complexes. (iii) ClO4Ϫ vibrations are observed
near 1100 and 630 cmϪ1. (iv) The νO–H vibration for the aqua
and hydroxo complexes appeared near 3400 cmϪ1 as expected.
The IR spectra of the isomers, A and B, of 4 are found to be
very similar to each other.
Fig. 3 1H NMR spectra of (a) [RuII(trpy)(L1)Cl]ClO4 1, (b) [RuII-
(trpy)(L4)Cl]ClO4 4 (isomer B) and (c) methyl peaks for isomers A
and B.
also recorded and exhibit one distinct methyl signal in each case
at the expected positions (δ 2.17 for isomer A, 2.15 for B). The
aromatic region of the spectra exhibit the calculated number of
protons corresponding to one particular isomer (Fig. 3b). The
methyl signal of complex 2 appears at δ 2.21 as a singlet. The
observed signals have been assigned to individual aromatic
hydrogens (Table 2) with the aid of 1H NMR correlation
spectroscopic (COSY) experiments.4,5,10,16
Electronic spectral data of complexes 1–15 in dichloro-
methane solvent are listed in Table 3. In order to study the effect
of solvents on the spectral features of the aqua derivatives
(6–10), data were also collected in acetonitrile, methanol and
water media (Table 3). Selected spectra are shown in Fig. 4. The
complexes display multiple transitions in the UV/visible region.
The lowest energy bands near 500 nm are assigned to
(dπ)RuII→π*(ligand) metal to ligand charge transfer (MLCT)
transitions.17 Bands in the UV region are believed to be ligand
based charge-transfer transitions. Multiple charge transfer
transitions may arise from lower symmetry splitting of the
metal level, the presence of different acceptor orbitals and from
mixing of singlet and triplet configurations in the excited state
through spin–orbit coupling.18 The MLCT band energy follows
the order: Cl < OH < H2O (Table 3, Fig. 4). This red shift in
λmax is due to the relative destabilisation of the (dπ)Ru electrons
while moving from H2O to OH to Cl complexes, making the
(dπ) → π* transition occur at relatively lower energy. Elec-
1H NMR spectra of the complexes 1–5 were recorded in
DMSO-d6 solvent and D2O was used for the aqua (6–10) and
hydroxo (11–15) complexes. All three sets of complexes (chloro,
aqua and hydroxo) display similar NMR spectra except for the
expected changes due to solvent variations. Therefore the data
are summarised only for the chloro complexes 1–5 (Table 2, Fig.
3). The complexes 1 and 2–5 exhibit fifteen and fourteen signals
respectively, nine and eight respectively from the non-
equivalent L1 and L2–L5 and six from the terpyridine (as the
plane of symmetry makes the two halves of terpyridine
equivalent).
In the case of complex 4 the aromatic region of the spectrum
is complicated due to the presence of two isomers. However, the
two well resolved upfield methyl signals at δ 2.17 and 2.15 and
direct comparisons of intensities of the individual methyl
groups with those of the respective aromatic protons have con-
firmed the presence of an approximately 1:3 ratio of A and B
isomers in the bulk solution (Fig. 3c). The NMR spectra of the
individual isomers A and B (separated from the crystals) were
J. Chem. Soc., Dalton Trans., 2000, 4209–4217
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Table 2 1H NMR spectral data for complexes 1–5 in DMSO-d6
δ(J/Hz)a
1
2
3
4
7
8
9
10
11
6.22
a
b
c
d
e
f
1
2
3
4
5
9.76
8.25
8.46
8.90
6.22
7.04
7.24
7.04
7.31
7.50
8.17
8.64
8.64
8.22
(5.9)b (6.9)c
(8.3)
(6.9)c
(7.6)
8.51)
(6.8)c
(7.4)
8.42
(6.9)b (6.8)b (6.8)c
(7.7)
(6.9)c
(6.9)
7.06
(6.8)c
(7.7)
6.91
(6.8)b (5.2)b (6.9)c
(6.9)
(6.9)c
(6.9)
8.12
(6.8)b (6.8)b (7.1)c
(8.2)
9.80
8.22
8.84
6.01d Me
6.58
7.34
7.53
8.72
8.59
8.20
(6.2)b (8.1)c
(7.9)
(7.9)b
(2.21) (7.3)b (7.27)c (7.9)b (6.0)b (6.3)c
(7.1)c
(7.0)
8.14
(6.7)b (6.3)b (6.0)c
(6.4)
(7.63)
7.10
(5.9)
7.76
9.72
8.24
8.91
6.66d Cl
7.61
6.24
7.27
8.70
8.62
8.24
(5.7)b (7.7)c
(7.6)
(8.2)c
(8.0)
8.45
(7.3)b
(6.7)b (8.2)c
(7.5)
(7.3)b (6.1)b (7.1)c
(6.9)
(7.0)c
(7.0)
8.16
(6.7)b (7.0)b (6.7)c
(6.7)
9.77
8.26
8.87
6.19
6.86
Me
6.86
6.19
7.29
7.49
8.66
8.66
8.22
(5.5)b (6.7)c
(8.5)
(8.5)c
(7.7)
8.45
(7.6)b (8.5)b (8.5)b (2.15) (8.5)b (8.5)b (5.7)b (6.7)c
(8.5)
(5.7)c
(5.7)
8.17
(8.5)b (8.5)b (5.7)c
(5.7)
9.76
8.29
8.93
6.31
7.15
Cl
7.15
6.31
7.29
7.50
8.68
8.65
8.27
(5.8)b (7.5)c
(7.9)
(6.5)c
(8.1)
(8.4)b (8.2)b (8.1)b
(8.1)b (8.2)b (6.4)b (5.8)c
(5.8)
(8.2)c
(8.2)
(5.0)b (6.1)b (8.2)c
(6.8)
a δ and J are given in ppm and Hz respectively. Tetramethylsilane is the internal standard. b Doublet. c Triplet. d Singlet.
Table 3 Electronic spectral data
UV/Vis, λmax/nm (ε/dm3 molϪ1 cmϪ1
)
Complex
CH2Cl2
CH3CN
MeOH
Water
1
2
3
4
5
6
513(5640), 315(13240),
273(11540), 236(13340)
515(11150), 315(26020),
273(24000), 235(27660)
515(7510), 314(19220),
273(16730), 232(20210)
514(10210), 314(23350),
273(21644), 237(27300)
517(5700), 316(14380),
273(12270), 232(16010)
494(5600), 332(8170),
311(9200), 270(15400)
228(13990)
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
493(9650), 327(23410),
282(24350)
490(12100), 332(27100),
281(28000), 270(29000),
226(35100)
487(9830), 330(21270),
308(22430), 270(24760)
7
8
495(11500), 365(6700),
330(19790), 315(18420),
285(21600), 232(19670)
500(10700), 331(24990),
314(25240), 282(25780),
271(25990), 229(27570)
495(6601), 384(7730),
331(15800), 284(18000),
272(16700), 232(17000)
498(5370), 371(6150),
330(13850), 316(11800),
285(14710), 231(14042)
504(6410), 315(17020),
273(23900), 236(22400)
505(10890), 312(29100),
272(34000), 230(37000)
511(10800), 316(29320),
273(27680), 230(32000)
503(11980), 370(16130),
317(33618), 273(36210),
235(44100)
496(11750), 314(27070),
328(29100), 283(30900)
499(10360), 363(7970),
329(24270), 314(25730),
283(25430), 272(26070)
497(7550), 315(19140),
272(19930)
490(7554), 328(16100),
282(19370), 270(19260)
499(7990), 327(20100),
313(18740), 283(20750)
489(8690), 330(18330),
316(17120), 281(20270),
270(20990)
490(6500), 379(6390),
329(12980), 281(17580),
271(17840)
491(9180), 328(18830),
315(17060), 282(21250),
271(21320)
9
490(7950), 375(9460),
329(20390), 282(21490)
495(14710), 377(17530),
330(37490), 272(39170)
10
497(4850), 365(4450),
327(13080), 314(11020),
284(13370), 270(11980)
—
498(3910), 365(4180),
329(10260), 315(10110),
283(9880), 272(9980)
—
11
12
13
14
—
—
—
—
—
—
—
—
—
—
15
512(6200), 316(15840),
273(16050), 230(20740)
—
—
—
tronic spectra of the aqua complexes (6–10) in co-ordinating
solvents, acetonitrile, methanol and water, show little variation
(Table 3). The spectra of isomers A and B of 4 are shown in
Fig. 4(c). It seems that they have very similar spectral patterns.
In all cases the shift in band maxima based on the ‘R’ groups
present in the framework of L is clear.
transition. The extent of this decrease essentially depends on
the third ligand present in the complex moiety, i.e. ClϪ, OHϪ or
H2O (Table 3). The overall lowering of the molecular symmetry
while moving from [Ru(trpy)2]2ϩ to the present set of complexes
along with the electronic effect of the σ-donor third ligand in
the complex moiety might be responsible for the observed
trend.20,21
Emission properties of complexes 1–5 have been studied in
optically dilute MeOH–EtOH (1:4 v/v) rigid glass at 77 K.
Excitations at the top of the lowest energy MLCT band (λmax
near 500 nm) exhibit moderately strong emissions near 600 nm
The lowest energy MLCT transition of [Ru(trpy)2]2ϩ appears
at 478 nm.19 Thus the replacement of one strong π-acidic tri-
dentate terpyridine (trpy) ligand by another well known
strong π-acidic bidentate azopyridine ligand (L) and one
monodentate σ-donor ligand decreases the energy of the same
4212
J. Chem. Soc., Dalton Trans., 2000, 4209–4217

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Table 4 Emission dataa
λmax/nm
Quantum
Compound
excitation
emission
yield (Φ)
1
2
3
4
5
506
508
509
510
510
571
582
589
567
595
0.72 × 10Ϫ2
0.55 × 10Ϫ1
0.42 × 10Ϫ2
0.98 × 10Ϫ2
0.31 × 10Ϫ2
a In ethanol–methanol (4:1 v/v) at 77 K.
Fig. 5 Cyclic voltammograms of ≈ 10Ϫ3 mol dmϪ3 solutions of the
complexes (a) [RuII(trpy)(L3)Cl]ClO4 3 in acetonitrile and (b) [RuII-
(trpy)(L3)(H2O)][ClO4]2ؒH2O 8 in water at pH 1.0.
13 < 15 for chloro and hydroxo complexes respectively (Table
5). The oxidation of the ruthenium() centre for the hydroxo
complexes 11–15 appears to be relatively easier, which is pos-
sibly due to better σ-donor character of the OH function com-
pared to ClϪ. The ruthenium()–ruthenium() couple of the
present set of chloro complexes 1–5 is more anodic than that of
previously reported analogous complexes, [RuII(trpy)(bpy)-
Cl]ϩ 27 and [RuII(trpy)(bpz)Cl]ϩ 28 (bpy = 2,2Ј-bipyridine;
bpz = 2,2Ј-bipyrazine). The better π-acidic nature of L com-
pared to bpy and bpz is believed to be the primary contributing
factor.29
The ruthenium()–ruthenium() couple of [Ru(trpy)]2ϩ
appears at 1.30 V.30 Therefore the potential of this couple
decreases while moving from [Ru(trpy)2]2ϩ to [Ru(trpy)(L)-
(X)]ϩ. The σ-donor nature of OHϪ/ClϪ provides electrostatic
stabilisation of the RuIII–L species which has originated from
reduction of the overall charge of ϩ2 in [Ru(trpy)2]2ϩ to ϩ1 in
the chloro (1–5) and hydroxy (11–15) complexes.
The complexes display three one-electron reductions at
negative potentials with respect to the SCE. The one-electron
nature of the responses has been established by differential
pulse voltammetry, which shows all the reduction waves to have
the same height as that of the oxidation wave. Since both the
co-ordinating ligands, trpy and L, are known to accept succes-
sively two electrons in their lowest unoccupied molecular
orbitals,31 the observed reductions are considered to be ligand
based processes. In the case of complexes 3 and 5 (where
R = Cl) all the expected four reductions are detected (Fig. 5a).
Although there is no direct experimental evidence in favour of
assigning the observed reductions corresponding to specific
ligands, trpy and L, the first two reductions are assumed to be
associated with L. The reductions at the higher potentials are
considered to be trpy based processes. The assumption is based
on the fact that the azopyridine ligand is known to be a better
π acceptor than polypyridyl ligands.32
Fig. 4 (a) Electronic spectra of [RuII(trpy)(L2)Cl]ClO4 2 (——), [RuII-
(trpy)(L2)(OH)]ClO4 12 (----) and [RuII(trpy)(L2)(H2O)][ClO4]2ؒH2O 7
(–ؒ–ؒ–) in dichloromethane. (b) Emission spectrum of 2 in EtOH–
MeOH 4:1 (v/v) at 77 K. (c) Electronic spectra of [RuII(trpy)(L4)Cl]-
ClO4 4, isomer A (----) and B (——), in dichloromethane.
(Table 4, Fig. 4b). The observed emissions are believed to
originate from the 3MLCT excited state.22
The quantum yields, Φem, of the emission processes were
measured in EtOH–MeOH (4:1 v/v) rigid glass at 77 K relative
to [Ru(bpy)3][PF6]2 for which Φem = 0.3523 by following the
reported procedure.24,25 The calculated quantum yields are
listed in Table 4 and show reasonable variations depending on
the nature of the ‘R’ functions.
Electron-transfer properties
Redox properties of the chloro (1–5) and hydroxo (11–15) com-
plexes have been studied in acetonitrile solvent using a platinum
working electrode at 298 K. Representative voltammograms
are shown in Fig. 5 and the data are presented in Table 5. All
potentials are referenced to the SCE. The complexes 1–5
and 11–15 display reversible ruthenium()–ruthenium()
couples in the ranges 1.10–1.24 and 0.79–0.92 V respectively.26
The one-electron nature of the responses is confirmed by
constant potential coulometry. Although the oxidised species
can be generated by constant-potential coulometry, the
oxidised solutions are found to be unstable at room
temperature. This prevented isolation of the oxidised species
for further characterisation. The formal potential of the
ruthenium()–ruthenium() couple varies depending on the
electronic nature and location of the ‘R’ groups present in L
and follows the order 4 < 2 < 1 < 3 < 5 and 14 < 12 < 11 <
The redox properties of the aqua complexes 6–10 have been
studied in aqueous media using a platinum working electrode.
They behave essentially in a similar manner. In the acidic pH
range (0.85–5.5) they show a single-step two-electron oxidation
process, Fig. 5(b). The oxidation potentials and the reversibility
of the peaks are found to be sensitive to the pH of the medium.
At pH > 5.5 the separation between Epa and Epc increases and
unfortunately the voltammograms become progressively ill
defined in the alkaline pH range, which has precluded electro-
chemical studies in this range. In the acidic region the potential
decreases linearly with increase in pH. The change in oxidation
potential as a function of pH is shown in Fig. 6 for a represen-
tative case. At pH р 5.5 a shift of peak potentials with pH at
the rate of about 60 mV per unit change of pH has been
observed (Fig. 6), implying the involvement of a reversible
single-step 2eϪ ϩ 2Hϩ electrode process, eqn. (3).28,33 Electro-
J. Chem. Soc., Dalton Trans., 2000, 4209–4217
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Table 5 Electrochemical data at 298 Ka
Ligand reductions EЊ298/V (∆Ep/mV)
RuIII–RuII couple
EЊ298/V (∆Ep/mV)
Compound
L-based
trpy-based
1
2
1.15(80)
1.13(80)
1.18(90)
1.10(100)
1.24(80)
0.84(80)
0.81(70)
0.88(90)
0.79(70)
0.92(80)
Ϫ0.79(60), Ϫ1.41(120)
Ϫ0.83(60), Ϫ1.50(110)
Ϫ0.72(70), Ϫ0.99(100)
Ϫ0.77(60), Ϫ1.42(90)
Ϫ0.66(60), Ϫ0.95(90)
Ϫ0.83(80), Ϫ1.49(100)
Ϫ1.01(90), Ϫ1.55(110)
Ϫ1.00(80), Ϫ1.45(120)
Ϫ0.95(80), Ϫ1.58(110)
Ϫ0.72(70), Ϫ1.25(100)
Ϫ1.62(100)
Ϫ1.81(100)
Ϫ1.38(90), Ϫ1.76(120)
Ϫ1.75(120)
Ϫ1.35(100), Ϫ1.65(110)
Ϫ1.75(120)
Ϫ1.83(130)
Ϫ1.70(120)
Ϫ1.81(120)
Ϫ1.64(110)
3
4
5
11
12
13
14
15
a Solvent, acetonitrile; supporting electrolyte, [NBu4][ClO4]; reference electrode, SCE; solute concentration, ≈10Ϫ3 M; working electrode, platinum
wire. Cyclic voltammetric data: scan rate, 50 mV sϪ1; EЊ298 = 0.5(Epa ϩ Epc) where Epa and Epc are the anodic and cathodic peak potentials,
respectively.
Table 6 pKa values of [Ru(trpy)(L)(H2O)]2ϩ complexes
Compound
pKa
6
7
8.3
8.5
8.1
8.5
7.9
8
9
10
Fig. 7 Change in absorbance of [RuII(trpy)(L4)(H2O)][ClO4]2ؒH2O 9
as a function of [CeIV]. Ratio 9: [CeIV] = (1) 1:0; (2) 1:0.5; (3) 1:1; (4)
1:2.5; (5) 1:5; (6) 1:10; (7) 1:15; (8) 1:18; (9) 1:25; (10) 1:28; (11) 1:30.
Inset shows the spectrum of [RuIV(trpy)(L4)(O)]2ϩ obtained separately
by using an excess of Ce4ϩ. The arrows indicate increase or decrease in
band intensities as the reaction proceeds.
lower than that of the bpy complex (pKa = 9.7), it can be
inferred that the acidity of the co-ordinated water molecule or
in other words the π-acidic property of the ancillary ligand (L)
in the monoterpyridine core is the primary controlling factor in
directing the electron-transfer properties of this class of com-
plexes. Thus the strong π acidity of L1–5 makes the Ru←OH2
σ bond correspondingly stronger, which on the other hand
facilitates the proton dissociation process.
Fig. 6 Variation of cyclic voltammetric peak potentials with pH for
[RuII(trpy)(L3)(H2O)][ClO4]2ؒH2O 8.
[RuIV(trpy)(L)(O)]2ϩ ϩ 2Hϩ ϩ 2eϪ
[RuII(trpy)(L)(H2O)]2ϩ (3)
Chemical oxidation of [RuII(trpy)(L)(H2O)]2؉ by aqueous Ce4؉ in
0.5 M H2SO4: role of [RuIV(trpy)(L)(O)]2؉ in catalytic water
oxidation
chemical oxidations of the complexes at a potential greater
than the corresponding Epa could not be performed due to
continuous coulomb count.16
The aqua complexes [RuII(trpy)(L)(H2O)]2ϩ 6–10 were treated
by aqueous cerium() ammonium sulfate at 0.5 M H2SO4. This
led to an immediate change from red to yellow. In the presence
of an excess of Ce4ϩ the MLCT band of [RuII(trpy)(L)(H2O)]2ϩ
near 500 nm disappears (Fig. 7). It, however, progressively
reappears as time passes and eventually attains the initial inten-
sity (Fig. 8). The yellow oxidised solution is found to be stable
only in the presence of an excess of Ce4ϩ ion, otherwise it slowly
turns back to the parent red aqua species. All our attempts to
isolate the oxidised [RuIV(trpy)(L)(O)]2ϩ complexes have failed
altogether as during the work up process they convert back into
the aqua derivatives, [RuII(trpy)(L)(H2O)]2ϩ. Spectrophoto-
metric monitoring of the oxidation of [RuII(trpy)(L)(H2O)]2ϩ by
using acidic Ce4ϩ solution (0.5 M H2SO4) shows the successive
development of a new band near 350 nm at the expense of the
The pKa values of complexes 6–10 have been determined by
spectrophotometric titrations using sodium hydroxide (Table
6). The values of similar ruthenium() monoterpyridine aqua
complexes incorporating bpy, bpz, phen and tmen ancillary
ligands (L) are found to be 9.7, 8.8, 9.6 and 10.2 respectively.28
It may be noted that at acidic pH the analogous bpy system,
[RuII(trpy)(bpy)(H2O]2ϩ, moves to the oxo species [RuIV-
(trpy)(bpy)(O)]2ϩ via an intermediate species [RuIII(trpy)-
(bpy)(OH)]2ϩ 27
, on the other hand the 2,2Ј-bipyrazine complex
[RuII(trpy)(bpz)(H2O)]2ϩ exhibits a one-step 2eϪ/2Hϩ trans-
formation to [RuIV(trpy)(bpz)(O)]2ϩ 28 like the present case.
Since the pKa values of the present set of complexes (7.9–8.5)
are closer to that of the bpz complex [pKa = 8.8) and reasonably
4214
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Table 7 Pseudo first order rate constants (k) for the conversion [RuIV
-
(trpy)(L)(O)]2ϩ → [Ru(trpy)(L)(H2O)]2ϩ in aqueous acidic media
at 298 K
Compound
103 k/sϪ1
[RuIV(trpy)(L1)(O)]2ϩ
[RuIV(trpy)(L2)(O)]2ϩ
[RuIV(trpy)(L3)(O)]2ϩ
[RuIV(trpy)(L4)(O)]2ϩ
[RuIV(trpy)(L5)(O)]2ϩ
7.6
6.8
8.2
7.1
8.8
Conclusion
The role of azo-imine based strong π-acidic ancillary ligands
L1–5 in the ruthenium monoterpyrdine [Ru(trpy)] core has been
scrutinised with particular reference to the spectroelectro-
chemical properties of this class of complexes. Thus series of
complexes [RuII/III(trpy)(L)Cl]ϩ/2ϩ 1–5, [RuII(trpy)(L)(H2O)]2ϩ
6–10, [RuII(trpy)(L)(OH)]ϩ 11–15 and [RuIV(trpy)(L)(O)]2ϩ
have been synthesized and studied. Their properties have been
compared with those of similar complexes incorporating L = bpy
and bpz. In the case of bpy the aqua-complex [Ru(trpy)(L)(H2-
O)]2ϩ undergoes stepwise 1eϪ/1Hϩ oxidations RuII(H2O) →
RuIII(OH) → RuIV(O); where L = 2,2Ј-bipyrazine, there is a
one step 2eϪ/2Hϩ transformation, RuII(H2O) → RuIV(O),
whereas in the present case (L = L1–5) the complexes 6–10 not
only display one step 2eϪ/2Hϩ oxidation, RuII(H2O) →
RuIV(O), but the resultant oxo-species also catalyse water
oxidation. The stronger acidity of the co-ordinated water
molecule (pKa ≈ 8.0) as well as the higher oxidation potential
(>1.1 V) of [Ru(trpy)(L1–5)(H2O)]2ϩ compared to those of
the complexes where L = bpy or bpz possibly make the oxo-
Fig. 8 Time evolution of the electronic spectra of a changing solution
of [RuIV(trpy)(L4)(O)]2ϩ → [RuII(trpy)(L4)(H2O)]2ϩ in water (0.5 M
H2SO4) at 303 K. Inset shows the spectrum of pure [RuII(trpy)-
(L4)(H2O)]2ϩ in water (0.5 M H2SO4). Arrows as in Fig. 7.
intense band near 500 nm, characteristic of the starting aqua
derivative (Fig. 7). Keeping the electrochemical observation of
a single step, 2eϪ ϩ 2Hϩ transfer in acidic media (Figs. 5 and 6)
in mind, it is logical to consider that in the presence of acidic
Ce4ϩ solution the aqua (6–10) complexes are converted into the
oxo-species, eqn. (4). However, the oxo-species further interacts
[RuII(trpy)(L)(H2O)]2ϩ ϩ 2Ce4ϩ →
[RuIV(trpy)(L)(O)]2ϩ ϩ 2Ce3ϩ ϩ 2Hϩ (4)
species of the present set of complexes, [Ru(trpy)(L1–5)(O)]2ϩ
,
with the solvent water and converts back into the parent aqua
derivative (eqn. 5).
effective in catalysing the oxidation of water to dioxygen.
Thus the present study implies a role for the electronic aspects of
the ancillary ligands L in the physico-chemical properties of the
monoterpyridine complexes. The complexes 1–5 are found to
exhibit moderately strong emissions at 77 K. Further reactivity
studies of the aqua- and oxo-complexes are in progress.
[RuIV(trpy)(L)(O)]2ϩ ϩ H2O →
[RuII(trpy)(L)(H2O)]2ϩ ϩ ¹O2 (5)
¯
²
The backward reduction process (eqn. 5) has been followed
spectrophotometrically with time (Fig. 8). The decrease in
intensity of the band near 350 nm, characteristic of the
oxo-species with concomitant growth of a new band near 500
nm, characteristic of the aqua complex (Fig. 8), reveals the
active role of [RuIV(trpy)(L)(O)]2ϩ species in oxidising the water
molecules (eqn. 5). However, sequential additions of acidic
Ce4ϩ to the reaction mixture essentially repeat the process, eqns.
(4) and (5), The overall process is therefore catalytic in nature,
Scheme 2. The progressive accumulation of dioxygen in such
Experimental
Materials
Commercial ruthenium trichloride (S.D. Fine Chemicals,
Bombay, India) was converted into RuCl3ؒ3H2O by repeated
evaporation to dryness with concentrated hydrochloric acid.
The complex [Ru(trpy)Cl3] and the ligands L1–5 were prepared
according to the reported procedures.3,35 2,2Ј:6,2Љ-Terpyridine
was obtained from Aldrich, USA. Other chemicals and solvents
were reagent grade and used as received. Silica gel (60–120
mesh) used for chromatography was of BDH quality. For
spectroscopic and electrochemical studies HPLC grade sol-
vents were used. Water of high purity was obtained by dis-
tillation of deionised water from KMnO4. Sodium perchlorate
for electrochemical work in aqueous media was recrystallised
from water.
Physical measurements
Scheme 2
UV-visible spectra were recorded by using a Shimadzu-2100
spectrophotometer, FT-IR spectra on a Nicolet spectro-
photometer with samples prepared as KBr pellets. Solution
electrical conductivity was checked using a Systronic 305 con-
ductivity bridge. Magnetic susceptibility was checked with a
PAR vibrating sample magnetometer. NMR spectra were
obtained with a 300 MHz Varian FT spectrometer. 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
solutions is established with the help of the observed reduction
Ϫ
peak of O2 ϩ eϪ → O2 at Ϫ0.3 V.34 However, whether the
dioxygen is formed directly or via the intermediacy of H2O2 is
still not clear.
The pseudo first order rate constant (k) of reaction (5) has
been determined spectrophotometrically for all the complexes
(Fig. 8, Table 7). The rate constant of the conversion process
[RuIV(trpy)(L)(O)]2ϩ → [RuII(trpy)(L)(H2O)]2ϩ is found to
vary systematically depending on the electronic nature of the
ancillary ligand moiety (L).
J. Chem. Soc., Dalton Trans., 2000, 4209–4217
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Table
8
Crystallographic data for [Ru(trpy)(L1)Cl]ClO4
1 and
electrode were used in a three electrode configuration. The sup-
porting electrolyte was NBu4ClO4 and the solute concentration
was ≈10Ϫ3 M. The half-wave potential EЊ298 was set equal to
0.5(Epa ϩ Epc), where Epa and Epc are anodic and cathodic cyclic
voltammetric peak potentials respectively. A platinum wire-
gauze working electrode was used in coulometric experiments.
All experiments were carried out under a dinitrogen atmos-
phere and were not corrected for junction potentials. The
elemental analyses were carried out with a Carlo Erba (Italy)
elemental analyser. Solution emission properties were checked
using a SPEX-fluorolog spectrofluorometer.
[Ru(trpy)(L1)(H2O)][ClO4]2ؒH2O 6
1
6
Formula
M
Crystal symmetry
Space group
a/Å
b/Å
c/Å
C26H20Cl2N6O4Ru
652.45
C26H24Cl2N6O10Ru
752.48
Monoclinic
P21/n
Monoclinic
P21/n
8.598(18)
15.974(3)
19.496(5)
99.388(18)
2642.1(0)
4
9.297(9)
16.402(9)
20.110(15)
103.076(7)
2987(4)
4
β/Њ
V/Å3
Kinetic measurements
Z
The
conversion
[RuIV(trpy)(L)(O)]2ϩ → [RuII(trpy)(L)-
µ/mmϪ1
0.841
0.030
0.770
0.052
(H2O)]2ϩ was monitored spectrophotometrically. For the
determination of k the increase in absorption (At) at 480 nm
corresponding to the aqua-species was recorded as a function
of time (t). A∞ was measured when the intensity changes levelled
off. Values of pseudo first order rate constants, k, were obtained
from the slopes of linear least-squares plots of Ϫln(A0 Ϫ At)
against t.36
R1
wR2
Measured/unique
reflections [Rint
0.079
4922/4640
[0.0108]
0.095
5392/5326
[0.0283]
]
ally at room temperature. The orange colour of the aqua
derivative gradually changed to pink. Stirring was continued for
1 h. The volume of the solution was reduced over a water bath
and a saturated aqueous solution of NaClO4 was added to the
cold solution. The dark coloured solid mass thus obtained was
filtered off, washed with ice-cold water and dried in vacuo over
P2O5. Yield: 67.5 mg, 80%.
Preparation of complexes
The complexes [Ru(trpy)(L1–5)Cl]ClO4 1–5 were synthesized by
following a general procedure. Yields vary in the range 70–75%.
Details are given for one representative complex 2.
[Ru(trpy)(L2)Cl]ClO4 2. A 100 mg (0.227 mmol) quantity
of [Ru(trpy)Cl3] was taken in 1:1 ethanol–water (25 ml) and
heated at reflux for 5 min. The ligand L2 (44.73 mg, 0.227
mmol) followed by LiCl (100 mg) and NEt3 (1.5 ml) were added
to the hot solution and the mixture was heated to reflux for 5 h.
The solvent was then removed under reduced pressure. The dry
mass was dissolved in the minimum volume of acetonitrile and
to it was added a saturated aqueous solution of NaClO4 (3 ml).
The mixture was kept in a refrigerator overnight. The crystal-
line mass thus obtained was filtered off, washed with a little ice-
cold water and dried in vacuo over P4O10. The dried product was
purified by using a silica gel column. With dichloromethane–
acetonitrile (5:1) a blue solution corresponding to ctc-
[Ru(L2)2Cl2] was separated initially; ttt-[Ru(L2)2Cl2] was eluted
next by dichloromethane–acetonitrile (4:1). Using dichloro-
methane–acetonitrile (2:1) as eluent a red band corresponding
to the desired product [Ru(trpy)(L2)Cl]ClO4 2 was separated
and collected. Evaporation of the solvent under reduced
pressure afforded a pure solid. Yield: 106 mg (70%).
Crystallography
Single crystals of the complexes were grown by slow diffusion
of an acetonitrile solution of 1 in benzene followed by slow
evaporation, and slow evaporation of an aqueous solution of 6
respectively. Significant crystal data and data collection param-
eters are listed in Table 8. Absorption correction was done by
performing psi-scan measurement.37 The data reduction was
done by using MAXUS and structure solution and refinement
using the programs SHELXS 97 and SHELXL 97 respect-
ively.38 The metal atom was located from the Patterson map and
the other non-hydrogen atoms emerged from successive Fourier
synthesis. The structures were refined by full-matrix least
squares on F2. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were included in calculated posi-
tions. The difference map revealed the presence of one molecule
of lattice water in 6.
CCDC reference number 186/2186.
See http://www.rsc.org/suppdata/dt/b0/b005789o/ for crystal-
lographic files in .cif format.
The aqua-complexes [Ru(trpy)(L)(H2O)][ClO4]2ؒH2O 6–10
were prepared from the corresponding chloro-derivatives 1–5
using aqueous AgNO3. Details are mentioned for one complex
7.
Acknowledgements
Financial support received from the Department of Science
and Technology, New Delhi and Council of Scientific and
Industrial Research, New Delhi, India, is gratefully acknow-
ledged. The X-ray structural studies were carried out at the
National Single Crystal Diffractomer Facility, Indian Institute
of Technology, Bombay. Special acknowledgement is made to
Regional Sophisticated Instrumental Center, RSIC, Indian
Institute of Technology, Bombay for providing the NMR
facility.
[Ru(trpy)(L2)(H2O)][ClO4]2ؒH2O 7. The chloro complex
[Ru(trpy)(L2)Cl]ClO4 2 (100 mg, 0.15 mmol) was taken in 20 ml
water and to it an excess of AgNO3 (102 mg, 0.6 mmol) was
added. The mixture was heated to reflux for 2 h, then kept in a
refrigerator for 1 h. The precipitated AgCl was separated by
filtration through a sintered glass crucible (G-4). The volume of
the filtrate was then reduced to 5 ml and saturated aqueous
NaClO4 solution added. The solution was kept in the refriger-
ator overnight. The precipitate of the aqua-species 7 thus
obtained was filtered off, washed with ice-cold water and dried
in vacuo over P4O10. Yield: 73 mg, 65%.
References
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2 F. Scandola, C. A. Bignozzi and M. T. Indelli, Photosensitization and
Photocatalysis Using Inorganic and Organometallic Compounds,
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1993, p. 161.
3 B. Mondal, S. Chakraborty, P. Munshi, M. G. Walawalkar and
G. K. Lahiri, J. Chem. Soc., Dalton Trans., 2000, 2327; K.
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The complexes [Ru(trpy)(L)(OH)]ClO4ؒH2O 11–15 were
prepared from the corresponding aqua-derivatives 6–10 in the
presence of NaOH. Details are given for one complex 12.
[Ru(trpy)(L2)(OH)]ClO4ؒH2O 12. The aqua-complex
[Ru(trpy)(L2)(H2O)][ClO4]2ؒH2O (100 mg, 0.13 mmol) was dis-
solved in 25 ml water and 10 ml of aqueous NaOH (20 mg, 0.5
mmol) were added dropwise. The mixture was stirred magnetic-
4216
J. Chem. Soc., Dalton Trans., 2000, 4209–4217

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