Broad HOMO-LUMO gap tuning through the coordination of a single phosphine, aminophosphine or phosphite onto a Ru(tpy)(bpy)2+ core

Article abstract of DOI:10.1039/b806325g

The synthesis of new ruthenium(ii) terpyridine bipyridine complexes bearing a phosphorus(iii) ligand is presented. The steric and electronic properties of the phosphorus ligand were varied using aminophosphines, alkyl and aryl phosphites and the bulky tri(isopropyl)phosphine. All complexes were characterized by multi-nuclear NMR spectroscopy, mass spectrometry and X-ray diffraction analysis. The electronic properties of the complexes were probed by cyclic voltammetry, absorption and luminescence spectroscopy. The complexes do not show luminescence at room temperature, whereas at 77 K in an alcoholic matrix, emission is observed in the range 600-650 nm with lifetimes of 3.5-5.5 μs, originating from ³MLCT states. The MLCT transition spans over 65 nm, which corresponds to a variation of 0.4 eV in the HOMO-LUMO gap. The oxidation potential of the ruthenium varies over a broad range of 290 mV, from +1.32 V vs. SCE with L = PⁱPr₃ to +1.61 V vs. SCE with L = P(OPh)₃. This range is unprecedented upon the variation of a single monodentate ligand coordinated by the same heteroatom in the same oxidation and charge states. This work underlines the specific capacity of phosphorus in bringing up a large variety of electronic properties by changing its substituents.

Full text of DOI:10.1039/b806325g

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Broad HOMO–LUMO gap tuning through the coordination of a single
phosphine, aminophosphine or phosphite onto a Ru(tpy)(bpy)2+ core†
Isabelle M. Dixon,a Emilie Lebon,a Gilles Loustau,a Pierre Sutra,a Laure Vendier,a Alain Igau*a and
Alberto Jurisb
Received 18th April 2008, Accepted 1st July 2008
First published as an Advance Article on the web 2nd September 2008
DOI: 10.1039/b806325g
The synthesis of new ruthenium(II) terpyridine bipyridine complexes bearing a phosphorus(III) ligand is
presented. The steric and electronic properties of the phosphorus ligand were varied using
aminophosphines, alkyl and aryl phosphites and the bulky tri(isopropyl)phosphine. All complexes were
characterized by multi-nuclear NMR spectroscopy, mass spectrometry and X-ray diffraction analysis.
The electronic properties of the complexes were probed by cyclic voltammetry, absorption and
luminescence spectroscopy. The complexes do not show luminescence at room temperature, whereas at
77 K in an alcoholic matrix, emission is observed in the range 600–650 nm with lifetimes of 3.5–5.5 ms,
originating from 3MLCT states. The MLCT transition spans over 65 nm, which corresponds to a
variation of 0.4 eV in the HOMO–LUMO gap. The oxidation potential of the ruthenium varies over a
broad range of 290 mV, from +1.32 V vs. SCE with L = PiPr3 to +1.61 V vs. SCE with L = P(OPh)3.
This range is unprecedented upon the variation of a single monodentate ligand coordinated by the same
heteroatom in the same oxidation and charge states. This work underlines the specific capacity of
phosphorus in bringing up a large variety of electronic properties by changing its substituents.
the literature. There is indeed only one example of a Ru(tpy)(bpy)2+
complex bearing a phosphorus ligand of the triphenylphosphine
Introduction
type.10 Phosphorus is nevertheless a widely used coordinating
heteroatom the electronic11 and steric12 properties of which can be
tremendously altered by changing its substituents and oxidation
state. Neutral phosphorus(III) ligands of various s and p abilities
were coordinated on Ru(tpy)(bpy)2+ to probe the limits of the
accessible redox range and to pave the way for future work. In
addition to phosphines, various types of P(III) ligands such as
aminophosphines and phosphites were selected in order to cover a
wider range of electronic properties, in particular the HOMO–
LUMO gap as reflected by the energy of the MLCT (metal-
to-ligand charge transfer) transition. The electronic properties
of the complexes have been studied by UV-visible absorption
spectroscopy, luminescence studies and electrochemistry.
Progress in solar energy conversion technology is a major chal-
lenge for sustainable development.1 This interdisciplinary field of
research is at the crossroads between material science, polymer
science, molecular chemistry and physics, as illustrated by the
variety of journals publishing reports in this field.2 Among all
types of solar energy conversion devices, dye-sensitized solar cells
(DSSC) seem to be the best compromise in terms of ease of
preparation, cost and efficiency.3 This research was initiated by
the pioneering work of Gra¨tzel and his group,4 which has greatly
contributed to the development of many families of ruthenium
dyes in the past 15 years.5 Much of the current research in this
field aims at improving the electrolyte6 and the semiconductor,7
but the increase of the performance of dye-sensitized solar
energy conversion devices also relies on the development of new
families of dye complexes.8 In the context of the development
of new complexes for DSSC, we started fundamental studies
on ruthenium terpyridine bipyridine complexes bearing one
monodentate phosphorus-based ligand,9 surprisingly neglected in
Results and discussion
Selection of the ligands
The donor and acceptor properties of P(III) compounds are
easily tuned by changing the nature of the substituents on the
phosphorus atom. Pyrrolyldiphenylphosphine 1 was chosen for
its p-acceptor properties,13 while pyrrolidinyldiphenylphosphine
2 has powerful s-donating properties.14 In addition, the p-
accepting properties of phosphites vary according to the sub-
stituents on the oxygen atoms of P(OR)3, and are gradually
increased on going from alkylphosphites15 3–5 to arylphosphites
6.16 Tri(isopropyl)phosphine 7 was coordinated to complement
our data on ruthenium phosphine complexes,9 and as a steric
probe for future developments of this work.
aLaboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 route
de Narbonne, 31077 Toulouse Cedex 4, France. E-mail: alain.igau@lcc-
toulouse.fr; Fax: +33 561 553 003; Tel: +33 561 333 159
bDipartimento di Chimica “G. Ciamician”, Universita` di Bologna, via Selmi
2, 40126 Bologna, Italy. E-mail: alberto.juris@unibo.it; Fax: +39 512 099
456; Tel: +39 512 099 481
† Electronic supplementary information (ESI) available: Cyclic voltam-
mograms of complexes [Ru]1–[Ru]9. Absorption and emission spectra
of complexes [Ru]1–[Ru]9. CCDC reference numbers 665729 (compound
1·BH3), 665730 (compound 2·Se) and 665731–665737 (complexes [Ru]1–
[Ru]7). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b806325g
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Synthesis of compounds 1, 1·BH3, 2 and 2·Se
Ligands 1 and 2 were synthesized according to the literature by re-
acting two equivalents of amine with chlorodiphenylphosphine.17
X-Ray quality crystals were obtained for 1·BH3, prepared by re-
acting 1 and BH3·THF in CH2Cl2,18 by evaporation of an Et2O so-
¯
lution of the compound. 1·BH3 crystallizes in the triclinic P1 space
˚
group (Fig. 1), with a P–B bond length of 1.910(5) A, in the range
of those reported for other aminophosphine boranes.19 The asym-
metric unit is composed of two molecules but only one set of bond
lengths and bond angles is reported since they are similar in both
molecules (Table 1). The selenide derivative of 2 was prepared by
refluxing it with elemental selenium in toluene, and single crystals
of 2·Se were obtained by the slow evaporation of a toluene solution
Fig. 1 ORTEP representation of compounds 1·BH3 and 2·Se with 30%
probability displacement ellipsoids. Hydrogen atoms are omitted for
clarity.
the precursor complex [Ru]NCCH3,21 under photolabilization
conditions.22 After irradiation of the solution in acetone by a
300 W halogen lamp in a closed Schlenk tube under autogenous
pressure, the reaction mixture is precipitated in 20 volumes of
diethylether to eliminate excess ligand and other neutral species in
the supernatant. The solid hereafter obtained is purified by column
chromatography on silica gel. In a final step, the nitrate counter
ions are replaced by hexafluorophosphates to ensure solubility in
organic solvents. Complexes [Ru]1–[Ru]9 are shown in Scheme 1.
˚
of the compound (Fig. 1). The P–Se bond length (2.1049(8) A) is in
the same range as those reported for aminophosphine selenides.20
Coordination on the Ru(tpy)(bpy)2+ core
The coordination of a monodentate phosphorus ligand 1–9 on
the Ru(tpy)(bpy)(PF6)2 fragment (abbreviated [Ru]) takes place
in the presence of a ten-fold excess of ligand with respect to
Table 1 Selected crystallographic data, collection parameters, bond
lengths and bond angles for compounds 1·BH3 and 2·Se
Compound
1·BH3
2·Se
Formula
FW
T
C16H17BNP
265.09
C16H18NPSe
334.24
160 K
150 K
¯
Space group
P1
P21/n
˚
a/A
9.503(2)
10.494(2)
15.347(3)
90.182(15)
99.474(16)
103.301(18)
1467.7(5)
4
1.200
3.08–25.68
0.172
10 776
5575
9.0551(15)
17.944(4)
9.1410(8)
90
97.767(10)
90
1471.7(4)
4
1.509
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
rcalcd
q range/◦
2.54–32.31
2.646
14 968
m/mm-1
Reflections collected
Unique reflections
Rint
Refinement method
Data/parameters
Goodness of fit
R1 (I>2s(I))
wR2 (I>2s(I))
4970
0.0914
0.0750
Refined on F2
5575/345
0.915
Refined on F2
4970/172
1.020
Scheme 1 The coordination of P(III) ligands 1–9 onto a Ru(tpy)(bpy)2+
core and NMR labelling scheme.
6.61%
14.47%
0.895/-0.387
4.84%
13.00%
1.047/-0.888
-3
˚
Drmin/Drmax/e A
Under the conditions used for the coordination of the ruthenium
fragment, tris(amino)phosphines P(NMe2)3 and P(NEt2)3 decom-
posed into a multitude of phosphorus compounds as judged by
31P NMR and hence, they could not be coordinated.
˚
Selected bond lengths/A
P1–C5
P1–C11
P1–N1
P1–B1
1.793(4)
P1–C5
1.807(3)
1.807(3)
1.642(3)
2.1049(8)
1.777(4)
1.729(4)
1.910(5)
P1–C11
P1–N1
P1–Se1
Characterizations
1
All complexes were fully characterized by H, 13C and 31P NMR
Selected bond angles/◦
spectroscopy, mass spectrometry and X-ray diffraction on mono
crystals. The complete assignment of 1H and 13C NMR signals was
enabled by 2D NMR experiments. Similarly to what was described
earlier for [Ru(tpy)(bpy)L]2+ type complexes,10,23 we found that
the Ha proton on the bpy moiety, pointing towards the sixth
C5–P1–C11
C5–P1–N1
C11–P1–N1
N1–P1–B1
106.63(18)
103.80(17)
105.32(18)
112.8(2)
C5–P1–C11
C5–P1–N1
C11–P1–N1
N1–P1–Se1
105.40(13)
104.12(13)
103.99(13)
116.43(9)
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Table 2 NMR chemical shifts of the complexes in d6-acetone
Complex
[Ru]1
[Ru]2
[Ru]3
9.93
5.4
126.3
[Ru]4
9.95
5.7
121.6
[Ru]5
9.91
5.4
118.3
[Ru]6
[Ru]7
[Ru]8a
[Ru]9a
1
d Ha/ppm
9.39
6.0
92.2
10.23
6.0
78.1
10.38
5.7
115.4
9.87
5.7
30.0
9.78
5.4
22.7
9.50
5.7
39.8
J (Ha–Hb)/Hz
d
31P/ppm
a From ref. 12.
monodentate ligand of the complex (Scheme 1), is sensitive to
the nature of this latter ligand. Thus, in most cases, its chemical
shift can be used as a probe to monitor the substitution of the
acetonitrile molecule of the precursor complex, whose Ha appears
as a doublet with d = 9.93 ppm (J = 5.6 Hz) in d6-acetone (Table 2).
The 31P NMR spectra of the complexes gave two signals, one
corresponding to the coordinated phosphorus ligand, and a hep-
tuplet, which corresponds to the presence of hexafluorophosphate
counter ions in the region of -144 ppm (708 < JPF < 712 Hz).
The seven crystallographic structures (Fig. 2 and Table 3) share
the same relatively disordered hexafluorophosphate counter ions,
and some of the structures having their diffracted intensities
recorded at 100 K. The Ru–P bond length (Table 4) varies
of angles 359.8◦) and compound 2·Se (sum of angles 344.3◦),
in which the nitrogen lone pair points trans to the phosphorus–
selenium bond. On the other hand, taking into account the esd’s,
the length of the Ru–N1 bond trans to the Ru–P bond varies
only slightly, which indicates that the electronic effects of the sixth
ligand mostly focus within the Ru–P bond. An intramolecular
stacking interaction in complex [Ru]2 (inter-plane distance d =
◦
˚
3.06 A, offset angle a = 34 ) could account for the different
orientation of the phenyl substituents on the phosphorus, although
the offset angle is relatively large.25 Besides, a close examination
of the structure of complex [Ru]6 shows a strong distortion of
the terpyridine ligand. The dihedral angles between the peripheral
pyridine rings and the central pyridine ring are 12.9 and 15.3◦.
Upon looking at the X-ray structure one can clearly not invoke
steric hindrance to account for such a distortion. This is consistent
with the fact that the cone angle of triphenylphosphite is 128◦,
and steric effects are expected to be relatively small.12 Crystal
packing forces, however, are evidenced by a series of p–p stacking
˚
significantly from 2.2268(11)–2.4376(9) A, but poorly correlates
with the Tolman cone angle of the ligand (not shown),12 showing
the interplay between the global electronic and steric effects of the
substituents on the phosphorus atom and their direct influence
upon the net donor power of the coordinating atom:24 p-Accepting
◦
˚
˚
phosphites result in the shortest Ru–P bond lengths (2.22–2.26 A),
interactions (inter-plane distance d = 3.65 A, offset angle a = 26 ,
◦
˚
intermediate aminophosphines give rise to Ru–P bonds of medium
and d = 3.75 A, a = 19 ), which are also found intramolecularly
◦ ◦
25
˚
˚
˚
˚
lengths (2.31–2.35 A), and phosphines yield the longest Ru–P bond
(d = 3.43 A, a = 7 and d = 3.36 A, a = 30 ). (Fig. 3).
9
lengths (2.35–2.44 A). In the case of the aminophosphine ligands,
the Ru–P bond is longer in [Ru]2 than in [Ru]1 as a result of both
electronic and steric effects. Indeed the N6 nitrogen atom is planar
in [Ru]1 (sum of angles at N6 = 359◦) while it is more pyramidal
and hence more bulky in [Ru]2 (sum of angles at N6 = 349 ◦). This
compares well with what was found in the adduct 1·BH3 (sum
Discussion
The main challenge in ruthenium polypyridine chemistry lies in the
ability to customize the net donating properties of the ancillary lig-
and(s) in order to modulate the RuIII/RuII potential and the energy
Fig. 2 ORTEP view of complexes [Ru]1–[Ru]7 with 30% probability displacement ellipsoids. Solvent molecules, hydrogen atoms and anions are omitted
for clarity.
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Fig. 3 Stacking interactions in the elementary motif of the crystallized
complex [Ru]6.
of the MLCT transition. In the case of the phosphorus ligands, 31
P
NMR spectroscopy of the corresponding selenide is a simple and
powerful tool to estimate the electronic properties of the parent
ligand, with the P–Se coupling constant illustrating the basicity
of the phosphorus atom,26 with little steric bias.27 Phosphorus(III)
compounds with strong s-donating properties give rise to a small
1JPSe, due to the limited s-character of the phosphorus lone pair.
The phosphine selenides and selenophosphates were prepared
according to the literature.17 As expected, the aminophosphines
1–2 show an intermediate basicity (JPSe ~ 800 Hz) between the
phosphines (JPSe ~ 700 Hz) and the phosphites (JPSe > 900 Hz).28–30
Triphenylphosphite is the weakest s-donating ligand and has a
coupling constant of 1039 Hz with selenium (Table 5).
All of the complexes [Ru]1–[Ru]9 are luminescent in an alcoholic
rigid matrix at 77 K, with an emission lifetime value in the 3.5–
5.5 ms range. The luminescence band maxima are listed in Table 6,
and the luminescence spectra are given in the ESI.† In all cases,
3
the luminescence emission clearly originates from MLCT states
involving the polypyridine ligands, as evidenced by the position
and shape of the emission band, and by the lifetime values,
which are in the expected range for Ru–polypyridine complexes.31
Additional support to this assignment comes from the good
5630 | Dalton Trans., 2008, 5627–5635
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˚
Table 4 Selected bond lengths (A) for complexes [Ru]1–[Ru]9
Complex
[Ru]1
[Ru]2
[Ru]3
[Ru]4
[Ru]5
[Ru]6
[Ru]7
[Ru]8a
[Ru]9a
Ru–P
Ru–N1
2.3104(15)
2.103(5)
2.347(3)
2.101(8)
2.2448(15)
2.130(5)
2.238(2)
2.130(6)
2.2644(16)
2.113(5)
2.2268(11)
2.099(3)
2.4284(19)
2.116(6)
2.4376(9)
2.112(3)
2.3437(16)
2.090(5)
a From ref. 9.
Table 5 Selected NMR data of the ligands and their selenides
P(III) ligand L
1
2
3
4
5
6
7
8
9
d
d
J
31P La/ppm
31P LSe/ppm
PSe/Hz
47.5
47.3
140.1
72.2b
945f
138.0
138.7
66.6b
924g
128.9
58.8c
1039e
19.8
69.2c
716e
10.0
57.8c
711e
-5.7
35.4c
745e
58.4b
71.3c
71.0b
812d
776e
940f
a In acetone-d6. b In CDCl3. c In toluene. d From ref. 28. e This work. f From ref. 29. g From ref. 30.
to 465 nm for PCy3.9 The MLCT transitions of the complexes
[Ru]1, [Ru]3, [Ru]4 and [Ru]5 are found at around 420 nm, while
those of [Ru]2 and [Ru]9 are red-shifted by 20 nm.
Table 6 Electrochemical and absorption data at RT in CH3CN, and
luminescence data at 77 K in EtOH : MeOH 1 : 4 (v/v). The potentials
correspond to |Epa + Epc|/2 (with Epa - Epc in parentheses)
The electronic properties of complexes [Ru]1–[Ru]9 were probed
by electrochemistry, in an acetonitrile solution. All the cyclic volt-
ammograms (CV) show a first reversible oxidation, except in the
case of [Ru]2, and the first two reductions are reversible or quasi-
reversible at 100 mV s-1 scan rate (see ESI†). For [Ru]2, the poor
but reproducible reversibility of the oxidation process could be ex-
plained by the presence of a very minor (4% on the basis of current
intensities) irreversible couple at Epa= 0.62 V/SCE and Epc= 0.49
V/SCE, corresponding to a species, which was not detected by 1H
NMR, and could not be identified by comparison with known re-
l
MLCT/nm
Complex
E
ox1/V/SCE
E
red1/V/SCE (e/mol-1 L cm-1)
lem/nm
[Ru]1
[Ru]2
[Ru]3
[Ru]4
[Ru]5
[Ru]6
[Ru]7
[Ru]8b
[Ru]9b
+1.53 (78)
+1.36a (73)
+1.46 (74)
+1.39 (74)
+1.40 (93)
+1.61 (98)
+1.35 (83)
+1.32 (82)
+1.41 (67)
-1.16 (68)
-1.23 (74)
-1.17 (67)
-1.24 (64)
-1.24 (64)
-1.15 (67)
-1.17 (68)
-1.18 (64)
-1.22 (64)
422 (6920)
440 (8140)
417 (9590)
421 (9020)
423 (9640)
401 (8400)
461 (8650)
465 (7000)
440 (8000)
602
620
608
602
606
602
645
648
615
dox couples. The extra wave seen in the reduction of [Ru]2 at Epa
=
a Irreversible since Ic/Ia = 0.5; the oxidation potential of [Ru]2 was also
measured by square-wave (SqW) voltammetry and equals +1.38 V/SCE;
for all other complexes, the potentials measured by SqW and CV differ by
less than 10 mV. b From ref. 9.
-0.24 V/SCE, is also reproducible and corresponds to the back ox-
idation of the component that is irreversibly reduced at Epc = -1.70
V/SCE (see ESI†). The elucidation of this latter phenomenon is
out of the scope of this work and does not interfere with the
processes under discussion (i.e. first oxidation and first reduction).
In comparison with the literature, and on the basis of DFT cal-
culations performed on [Ru(tpy)(bpy)(DMSO)]2+,34 [Ru(bpy)3]2+,35
and [Ru(tpy)2]2+,36 the oxidation is assigned to a metal-centered
process and the first two reductions involve mostly the polypyri-
dine ligands. The first reduction, centered on the most delocalized
terpyridine ligand, is relevant for comparison purposes with
the MLCT transition of lowest energy. Indeed, the transition
corresponding to the formal transfer of one electron from the
metal to the most reducible ligand (i.e. optical gap) should
correlation between the emission energy and the electrochemical
HOMO–LUMO gap (not shown).
As opposed to the low temperature experiments, no lumi-
nescence was detected from complexes [Ru]1–[Ru]9 in an ace-
tonitrile solution at room temperature, in agreement with the
general behaviour of ruthenium phosphine complexes containing
bipyridine32 or terpyridine33 ligands. It is known that phosphine
3
ligands have the effect of moving the emitting MLCT state to
higher energy, in close proximity to the d–d state, which plays
a crucial role in determining the luminescence properties of Ru–
polypyridine complexes. At room temperature the d–d state can be
accessed easily by thermal activation, thus opening a non-radiative
∑
correlate with the RuIII/RuII and tpy/tpy- redox potentials (i.e.
electrochemical gap) (Fig. 4).37
3
Regarding the ruthenium oxidation potential in the
pathway for the decay of the MLCT state. Conversely, at 77 K
[Ru(tpy)(bpy)L]2+ complexes, it appears in the order PCy3
ª
<
thermal energy is not available to reach the d–d state, and thus the
3MLCT state can display the usual luminescence properties.
The common features of the UV-vis absorption spectra (see
ESI†) are (i) intense ligand-centered p–p* transitions in the UV
region, and (ii) a broad band in the visible region corresponding
to the envelope of the Ru–bpy and Ru–tpy MLCT transitions.
The wavelengths reported in Table 6 correspond to the MLCT
transitions of lowest energy, i.e. the formal transfer of one electron
from the ruthenium to the terpyridine ligand. This transition is
bathochromically shifted upon increasing the overall donating
power of the phosphorus ligand, ranging from 401 nm for P(OPh)3
PiPr3 ª Ph2P-pyrrolidinyl < P(OEt)3 ª P(OiPr)3 ª PPh3
P(OMe)3 < Ph2P-pyrrolyl < P(OPh)3. Indeed an electron-rich
ligand such as PCy3 or PiPr3 should efficiently stabilize the
RuIII state and hence, should lower the oxidation potential. The
differences between the relative order of the JPSe values and the
relative order of the Eox values is due to the fact that the coupling
constant of the selenide derivative only gives an estimate of the
basicity, or s-donating capacity, of the phosphorus atom, whereas
the redox potentials are under the influence of a combination of
donating and accepting s and p effects.24
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cyclic voltammetry, a discussion around small potential variations
should be done with extreme caution. Within the aminophosphine
and phosphite families, the expected trend is found experimentally
and underlines the importance of the impact of the heteroatom (N,
O) coordinated a to the phosphorus upon its electron-donating
and accepting properties.
Conclusions
This is the first report of the coordination of aminophosphines and
phosphites onto a ruthenium bipyridine terpyridine complex. For
all the complexes described in this study, the X-ray diffraction
structures show a classical monodentate coordination on the
phosphorus atom. The electronic properties of the complexes were
probed by the means of electrochemistry, absorption spectroscopy
and luminescence studies. It appears that the range of properties
allowed by a single neutral two-electron phosphorus(III) ligand
is very broad, the MLCT transition wavelength spanning over
65 nm and the RuIII/RuII redox potential ranging over 290 mV
between the two extremes of this study, [Ru(tpy)(bpy)P(OPh)3]2+
and [Ru(tpy)(bpy)PCy3]2+. It is remarkable that such a broad
range is unprecedented for neutral L-type ligands coordinated
by the same heteroatom, in this case phosphorus(III). Further
work is underway to synthesize phosphorus ligands with more
elaborate heteroatomic substituents in order to fine-tune the
electronic properties of the corresponding ruthenium polypyridine
complexes.
Fig. 4 Correlation between optical (diamonds) and electrochemical
(triangles) gaps. The error bars are given for the uncertainties of 2 nm on
lMLCT and 15 mV for redox potentials.
The most striking feature of this electrochemical study lies in
the fact that the simple variation of the substituents on a neutral
phosphorus(III) ligand allows to cover a potential range of 290 mV,
a range, which is not accessible for ruthenium polypyridine
complexes bearing a sixth neutral nitrogen ligand (Chart 1).38 In
addition, it is noteworthy that phosphorus(III) ligands strongly
stabilize the RuII oxidation state as compared to nitrogen ligands.
Experimental
Ligands 3–7 are commercially available and were used without
further purification. Ru(tpy)(bpy)(NCCH3)(PF6)2 was synthesized
from [Ru(tpy)(bpy)Cl](PF6) according to the method described for
Ru(tpy)(phen)(NCCH3)(PF6)2.13 The abbreviations tpy and bpy
stand for 2,2¢:6¢,2¢¢-terpyridine and 2,2¢-bipyridine respectively,
and [Ru] stands for Ru(tpy)(bpy)(PF6)2. All solvents were dried
and distilled by standard methods. Column chromatography was
performed on silica gel from SDS (70–200 mm), using the ternary
mixture acetone–H2O–saturated aqueous KNO3 solution 90 : 5 :
0.5 as starting eluent, followed by a gradual increase of the H2O–
KNO3 amount. NMR spectra were performed on a Bruker AV300
spectrometer, in (CD3)2CO unless otherwise stated.
Monocrystals of all the complexes were obtained by the
slow liquid diffusion of diethylether into an acetone solution
of the complex. For X-ray analysis, data were collected at
low temperature on an Oxford Diffraction Xcalibur for 1·BH3,
2·Se, [Ru]1, [Ru]4, [Ru]5, [Ru]6 and [Ru]7, and on a Oxford
Diffraction Gemini for [Ru]3, using graphite monochromated Mo
Chart 1 Accessible RuIII/RuII potential range with neutral nitrogen and
phosphorus ligands in [Ru(tpy)(bpy)L]2+ complexes
The first reduction is expected to be predominantly terpyridine-
centered and varies over a narrow potential range of 90 mV
(Table 6). We found essentially two different values for this
reduction: -1.17 V/SCE for Ph2P-pyrrolyl, P(OMe)3, P(OPh)3,
PiPr3 and PCy3, and -1.23 V/SCE for Ph2P-pyrrolidinyl, P(OEt)3,
P(OiPr)3 and PPh3. Although the reduced state should intuitively
be stabilized by the most p-accepting ligands, this trend is not
found experimentally throughout the whole series. As discussed by
Giering et al. in their QALE approach,39 meaningful comparisons
can be made within a family of ligands but not always between
families, or ‘classes’. The trend in the phosphine family is
unexpected but the difference is small between Ered(PR3) and
˚
Ka radiation (l = 0.71073 A), and equipped with an Oxford
Instrument Cooler Device. For [Ru]2, data were collected on a
STOE-IPDS equipped with an Oxford Cryosystems Cryostream
Cooler Device, using graphite monochromated Mo Ka radiation
˚
(l = 0.71073 A). The final unit cell parameters have been obtained
by means of a least-squares refinement. The structures have been
solved by direct methods using SIR92,40 and refined by means
of least-squares procedures on F2 with the program SHELXL-
97,41 included in the software package WinGX version 1.63,42
except for the structure [Ru]2, which was refined on F using
Crystal.43 The atomic scattering factors were taken from the
E
red(PAr3), and given the inherent experimental uncertainty of
5632 | Dalton Trans., 2008, 5627–5635
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international tables for X-ray crystallography.44 All hydrogens
atoms were geometrically placed and refined by using a riding
model. All non-hydrogens atoms were anisotropically refined, and
in the last cycles of refinement a weighting scheme was used,
where weights are calculated from the following formula: w =
Hh + H4 + Hi), 7.85 (dd, 1H, Hb, 3J = J = 6.4 Hz), 7.55–7.46 (m,
4H, H5 + Hp), 7.37 (m, 1H, Hj), 7.34–7.24 (m, 4H, Ho), 6.84–6.74
(m, 6H, Hm + NCHCH), 6.33 (s, 2H, NCHCH). dP (ppm) = 92.2,
-144.2 (sept, JPF = 708 Hz). dC (ppm) = 157.81 (C2), 157.28 (d,
Ce, J = 2.2 Hz), 156.70 (C2¢), 156.61 (d, Ca, J = 3.8 Hz), 155.71
(Cf), 154.43 (C6), 148.55 (d, Cj, J = 1.4 Hz), 139.67 (Ch), 139.29
(C4), 139.08 (Cc), 137.81 (C4¢), 131.47 (d, Cp, J = 2.2 Hz), 130.64
(d, Cm, J = 10.9 Hz), 129.08 (d, Cipso, J = 46.5 Hz), 129.27 (d, Co,
J = 9.8 Hz), 128.62 (C5), 127.94 (Cb), 127.82 (d, Ci, J = 2.1 Hz),
126.02 (d, NCHCH, J = 4.4 Hz), 125.37 (Cd), 124.86 (C3), 124.70
(C3¢), 124.24 (d, Cg, J = 2.1 Hz), 113.00 (d, NCHCH, J= 5.3 Hz).
ES+-MS: m/z = 886.9 ([M–PF6]+), 371.1 ([M–2PF6]2+), mp 260 ◦C
(decomposition).
3
1/[s (Fo2) + (aP)2 + bP] where P = (Fo + 2Fc )/3. The disorder
models for [Ru]1, [Ru]4, [Ru]5, and [Ru]7 are the following: for
[Ru]1 the disorders on two aromatic cycles were treated using the
PART command in SHELXL-97. One PF6 is highly disordered,
such a disorder is not treated easily with the PART command in
SHELXL-97, since the F atoms are not disordered over only two
sites. This results in several ALERT-B in the checkcif. Concerning
[Ru]4, [Ru]5 and [Ru]7, the disorders of the carbon atoms of the
phosphorus moiety were treated using the PART command in
SHELXL-97. For complexes [Ru]4 and [Ru]7, it was not possible
to properly resolve the diffuse electron density residuals related
to an acetone crystallization solvent molecule. Treatment with
2
2
2
Complex [Ru]2
Ligand 2 (250 mg, 1 mmol) and the complex [Ru]NCCH3 (82 mg,
0.1 mmol) were placed in a closed Schlenk tube with freshly
distilled acetone (4 mL) and were irradiated for 2 h. After cooling,
the reaction mixture was poured into an excess of diethylether
to precipitate the complexes. The precipitate was recovered
by filtration, washed with diethylether, taken in acetone and
chromatographed on silica gel (acetone–H2O–saturated aqueous
KNO3 (v/v) from 100 : 5 : 0.5–100 : 6 : 0.6). In a final step, the
nitrate counter ions were replaced by hexafluorophosphate ions to
ensure solubility in organic solvents. Complex [Ru]2 was isolated
in 92% yield (95 mg) as a bright orange solid. dH (ppm) = 10.23
(d, 1H, Ha, 3J = 6.0 Hz), 8.96 (d, 1H, Hd, 3J = 6.0 Hz), 8.75 (d,
the SQUEEZE facility from PLATON45 with a localized void of
3
˚
about 2106.3 A and 378 recovered electrons for [Ru]4 and with
3
˚
a localized void of about 624.3 A and 109 recovered electrons
for [Ru]7 resulted in a smooth refinement. Since a few low-order
reflections are missing from the data set, the electron count is
underestimated. Consequently, the values given for rcalcd, F(000)
and the formula weight are only valid for the ordered part of the
structures. Figures of the molecules were drawn with the program
ORTEP3,46 with 30% probability displacement ellipsoids for non-
hydrogen atoms.
The electrochemical measurements were obtained using an
Autolab PGSTAT 100 potentiostat using tetrabutylammonium
hexafluorophosphate as the supporting electrolyte in freshly dis-
tilled acetonitrile and a platinum working electrode. Electrospray
mass spectrometry analyses were performed on a Perkin Elmer
Sciex API-365 spectrometer in positive mode. Melting points
were determined in capillaries using an Electrothermal melting
point apparatus. Luminescence experiments were conducted in
air-equilibrated acetonitrile solutions at room temperature, and
in an EtOH–MeOH 1 : 4 v/v matrix at 77 K. Uncorrected
emission spectra and emission lifetimes were obtained with an
Edinburgh FLS-920 spectrofluorimeter. Emission spectra at 77 K
were recorded using quartz tubes immersed in a quartz Dewar
filled with liquid nitrogen. Scheme 1 shows the numbering used
for the ligands and complexes.
1H, Hg, 3J = 9.0 Hz), 8.50 (ddd, 1H, Hc, 3J = J = 8.0 Hz, 4J =
3
3
3
1.3 Hz), 8.43 (d, 2H, H3¢, J = 8.1 Hz), 8.42 (d, 2H, H3, J =
3
7.8 Hz), 8.25–8.15 (m, 2H, Hb + H4¢), 8.10 (d, 2H, H6, J = 5.7
Hz), 8.08–8.00 (m, 3H, Hh + H4), 7.42–7.35 (m, 4H, Hp + H5),
3
7.30–7.20 (m, 6H, Hm + Hi + Hj), 7.04 (t, 4H, Ho, J = 8.7 Hz),
3.02 (m, 4H, NCH2CH2), 1.86 (m, 4H, NCH2CH2). dP (ppm) =
78.1, -144.2 (sept, 1JPF = 705 Hz). dC (ppm) = 157.83 (C2), 157.15
(d, Ce, J = 2.3 Hz), 156.71 (C2¢), 156.61 (d, Ca, J = 3.0 Hz), 155.72
(Cf), 154.38 (C6), 148.46 (d, Cj, J = 0.8 Hz), 139.14 (Ch), 138.60
(C4 + Cc), 136.76 (C4¢), 130.91 (d, Co, J = 9.8 Hz), 130.11 (d,
Cp, J = 2.3 Hz), 129.87 (d, Cipso, J= 46.8 Hz), 128.74 (d, Cm, J=
9.0 Hz), 128.28 (Cb), 128.11 (C5), 127.56 (d, Ci, J = 2.3 Hz), 125.12
(Cd), 124.31 (C3), 124.10 (C3¢), 124.00 (d, Cg, J = 1.5 Hz), 50.68
(NCH2CH2), 26.06 (d, NCH2CH2, J = 6.6 Hz). ES+-MS: m/z =
891.3 ([M–PF6]+), 373.0 ([M–2PF6]2+), mp 197 ◦C.
Complex [Ru]1
Complex [Ru]3
Ligand 1 (250 mg, 1 mmol) and the complex [Ru]NCCH3 (85 mg,
0.1 mmol) were placed in a closed Schlenk tube with freshly
distilled acetone (4 mL) and irradiated for 2 h. After cooling, the
reaction mixture was poured into an excess of diethylether to pre-
cipitate the complexes. The precipitate was recovered by filtration,
washed with diethylether, taken in acetone and chromatographed
on silica gel (acetone–H2O–saturated aqueous KNO3 (v/v) from
100 : 5 : 0.5–100 : 8 : 0.8). In a final step, the nitrate counter ions
were replaced by hexafluorophosphate ions to ensure solubility in
organic solvents. Complex [Ru]1 was isolated in 47% yield (50 mg)
as a bright orange solid. dH (ppm) = 9.39 (d, 1H, Ha, 3J = 6.0 Hz),
8.98 (d, 1H, Hd, 3J = 8.1 Hz), 8.80 (d, 1H, Hg, 3J = 8.1 Hz), 8.54
(d, 2H, H3¢, 3J = 8.1 Hz), 8.50–8.43 (m, 3H, H3 + Hc), 8.32 (t, 1H,
H4¢, 3J = 8.1 Hz), 8.29 (d, 2H, H6, 3J = 5.4 Hz), 8.15–8.08 (m, 4H,
Trimethylphosphite (150 mL, 1.2 mmol) and the complex [Ru]-
NCCH3 (100 mg, 0.12 mmol) were placed in a closed Schlenk
tube with freshly distilled acetone (4 mL) and were irradiated for
2 h. After cooling, the reaction mixture was poured into an excess
of diethylether to precipitate the complexes. The precipitate was
recovered by filtration, washed with diethylether, taken in acetone
and chromatographed on silica gel (acetone–H2O–saturated
aqueous KNO3 (v/v) from 90 : 5 : 0.5–90 : 6 : 0.6). In a final
step, the nitrate counter ions were replaced by hexafluoro-
phosphate ions to ensure solubility in organic solvents. Complex
[Ru]3 was isolated in 79% yield (88 mg) as a bright orange solid.
d
H (ppm) = 9.92 (d, 1H, Ha, 3J = 5.4 Hz), 8.91 (d, 1H, Hd, 3J = 8.4
Hz), 8.87 (d, 2H, H3¢, 3J = 8.1 Hz), 8.78–8.68 (m, 3H, Hg + H3),
Dalton Trans., 2008, 5627–5635 | 5633
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3
3
8.55 (t, 1H, H4¢, 3J = 8.1 Hz), 8.44 (dd, 1H, Hc, 3J = J = 7.7 Hz),
18H, J = 6.0 Hz, CHCH3). dP (ppm) = 118.3 (d, J = 5.8 Hz),
-144.2 (sept, 1JPF = 708 Hz). dC (ppm) = 158.10 (C2), 157.30 (C2¢),
156.82 (d, Ce, J = 2.8 Hz), 155.74 (d, Ca, J = 2.9 Hz), 155.33 (d, Cf,
J = 1.6 Hz), 153.84 (C6), 148.62 (d, Cj, J = 1.4 Hz), 139.30 (Ch),
138.92 (C4), 138.50 (Cc), 137.72 (C4¢), 128.42 (C5), 127.75 (Cb),
127.47 (d, Ci, J = 3.5 Hz), 124.96 (Cd), 124.41 (C3), 123.93 (C3¢),
123.81 (d, Cg, J = 2.9 Hz), 71.45 (d, CHCH3, J = 9.5 Hz), 23.24 (d,
CHCH3, J = 3.7 Hz). ES+-MS: m/z = 844.20 ([M–PF6]+), 349.74
([M–2PF6]2+), mp 242 ◦C.
8.20 (dd, 2H, H4, 3J = J = 7.5 Hz), 8.16–8.02 (m, 4H, Hb + Hh +
3
H6), 7.60–7.50 (m, 3H, Hj + H5), 7.39 (dd, 1H, Hi, 3J = J = 6.5
3
3
Hz), 3.46 (d, 9H, P(OCH3)3, JHP = 10.5 Hz). dP (ppm) = 126.3
(decaplet, 3JHP = 10 Hz), -144.3 (sept, 1JPF = 708 Hz). dC (ppm) =
157.91 (C2), 157.20 (C2¢), 156.69 (d, Ce, J = 2.9 Hz), 155.90 (d,
Ca, J = 2.5 Hz), 155.36 (d, Cf, J = 1.4 Hz), 153.70 (C6), 148.21
(d, Cj, J = 1.5 Hz), 139.39 (Ch), 139.11 (C4), 138.43 (Cc), 138.09
(C4¢), 128.33 (C5), 128.16 (d, Cb, J = 0.7 Hz), 127.49 (d, Ci, J =
3.5 Hz), 124.84 (Cd), 124.66 (C3), 123.98 (C3¢), 123.77 (d, Cg, J =
2.9 Hz), 53.00 (d, P(OCH3)3, J = 8.6 Hz). ES+-MS: m/z = 750.56
([M–PF6]+), 307.98 ([M–2PF6]2+), mp 244 ◦C (decomposition).
Complex [Ru]6
Triphenylphosphite (265 mL,
1 mmol) and the complex
[Ru]NCCH3 (83 mg, 0.1 mmol) were placed in a closed Schlenk
tube with freshly distilled acetone (4 mL) and were irradiated
for 2 h. After cooling, the reaction mixture was poured into an
excess of diethylether to precipitate the complexes. The precipitate
was recovered by filtration, washed with diethylether, taken
in acetone and chromatographed on silica gel (acetone–H2O–
saturated aqueous KNO3 (v/v) from 100 : 5 : 0.5–100 : 8 :
0.8). In a final step, the nitrate counter ions were replaced by
hexafluorophosphate ions to ensure solubility in organic solvents.
Complex [Ru]6 was isolated in 91% yield (100 mg) as a pale orange
solid. dH (ppm) = 10.38 (d, 1H, Ha, 3J = 5.7 Hz), 8.93 (d, 1H, Hd,
3J = 8.1 Hz), 8.72 (d, 1H, Hg, 3J = 8.1 Hz), 8.64 (d, 2H, H3¢, 3J =
Complex [Ru]4
Triethylphosphite (140 mL, 0.8 mmol) and the complex
[Ru]NCCH3 (67 mg, 0.08 mmol) were placed in a closed Schlenk
tube with freshly distilled acetone (4 mL) and were irradiated
for 2 h. After cooling, the reaction mixture was poured into an
excess of diethylether to precipitate the complexes. The precipitate
was recovered by filtration, washed with diethylether, taken
in acetone and chromatographed on silica gel (acetone–H2O–
saturated aqueous KNO3 (v/v) from 100 : 5 : 0.5–100 : 7 :
0.7). In a final step, the nitrate counter ions were replaced by
hexafluorophosphate ions to ensure solubility in organic solvents.
Complex [Ru]4 was isolated in 86% yield (66 mg) as a bright
orange solid. dH (ppm) = 9.95 (d, 1H, Ha, 3J = 5.7 Hz), 8.93–8.83
(m, 3H, Hd + H3¢), 8.77–8.68 (m, 3H, Hg + H3), 8.54 (t, 1H, H4¢,
3
8.1 Hz), 8.59 (d, 2H, H3, 3J = 8.1 Hz), 8.53 (dd, 1H, Hc, 3J = J =
3
8.1 Hz), 8.42 (t, 1H, H4¢, 3J = 8.1 Hz), 8.32 (dd, 1H, Hb, 3J = J =
6.6 Hz), 8.15–8.05 (m, 3H, Hh + H4), 7.89 (d, 2H, H6, 3J = 5.4 Hz),
3J = 8.1 Hz), 8.44 (dd, 1H, Hc, 3J = J = 7.8 Hz), 8.20 (dd, 2H,
7.38 (dd, 1H, Hi, 3J = J = 6.6 Hz), 7.30 (dd, 2H, H5, 3J = J =
3
3
3
3
3
3
3
H4, J = J = 7.8 Hz), 8.15–8.02 (m, 4H, Hb + Hh + H6), 7.61–
6.6 Hz), 7.25 (dd, 1H, Hj, J = J = 4.7 Hz), 7.19–7.05 (m, 9H,
Hm + Hp), 6.84 (d, 6H, Ho, 3J = 8.1 Hz). dP (ppm) = 115.4, -144.2
(sept, JPF = 708 Hz). dC (ppm) = 157.55 (C2), 156.83 (d, Ce, J =
2.9 Hz), 156.73 (C2¢), 155.80 (d, Ca, J = 2.6 Hz), 155.29 (d, Cf,
J = 1.4 Hz), 153.62 (C6), 151.01 (d, Cipso, J = 11.2 Hz), 147.41 (d,
Cj, J = 1.2 Hz), 139.93 (Ch), 139.41 (C4), 139.29 (C4¢), 139.19 (Cc),
130.34 (Cm), 128.72 (C5), 128.63 (Cb), 127.87 (d, Ci, J = 3.5 Hz),
125.33 (Cp), 125.27 (Cd), 125.18 (C3), 124.46 (C3¢), 124.05 (d, Cg,
J = 2.9 Hz), 119.37 (d, Co, J = 4.7 Hz). ES+-MS: m/z = 945.9
([M–PF6]+), 400.5 ([M–2PF6]2+), mp 248 ◦C (decomposition).
7.51 (m, 3H, Hj + H5), 7.40 (dd, 1H, Hi, 3J = J = 6.6 Hz), 3.82
3
(m, 6H, POCH2CH3), 0.97 (t, 9H, POCH2CH3, 3J = 6.9 Hz). dP
(ppm) = 121.6, -144.2 (sept, 1JPF = 708 Hz). dC (ppm) = 158.00
(C2), 157.23 (C2¢), 156.76 (large, Ce), 155.68 (d, Ca, J = 2.7 Hz),
155.34 (d, Cf, J = 1.5 Hz), 153.62 (C6), 148.44 (d, Cj, J = 1.2 Hz),
139.31 (Ch), 138.98 (C4), 138.40 (Cc), 137.84 (C4¢), 128.28 (C5),
128.01 (Cb), 127.45 (d, Ci, J = 3.5 Hz), 124.84 (Cd), 124.53 (C3),
123.90 (C3¢), 123.75 (d, Cg, J = 2.8 Hz), 62.39 (d, POCH2CH3, J=
8.9 Hz), 15.40 (POCH2CH3). ES+-MS: m/z = 802.2 ([M–PF6]+),
328.5 ([M–2PF6]2+), mp 255 ◦C.
Complex [Ru]7
Complex [Ru]5
Tri(isopropyl)phosphine (190 mL, 1 mmol) and the complex
[Ru]NCCH3 (82 mg, 0.1 mmol) were placed in a closed Schlenk
tube with freshly distilled acetone (4 mL) and were irradiated for
2 h. After cooling, the reaction mixture was poured into an excess
of diethylether to precipitate the complexes. The precipitate was
recovered by filtration, washed with diethylether, taken in acetone
and chromatographed on silica gel (acetone–H2O–saturated aque-
ous KNO3 (v/v) from 90 : 5 : 0.5–90 : 10 : 1). In a final step, the
nitrate counter ions were replaced by hexafluorophosphate ions to
ensure solubility in organic solvents. Complex [Ru]7 was isolated
in 63% yield (60 mg) as a bright orange solid. dH (ppm) = 9.87 (d,
Tri(isopropyl)phosphite (275 mL, 1.2 mmol) and the complex
[Ru]NCCH3 (99 mg, 0.12 mmol) were placed in a closed Schlenk
tube with freshly distilled acetone (4 mL) and were irradiated
for 2 h. After cooling, the reaction mixture was poured into
an excess of diethylether to precipitate the complexes. The
precipitate was recovered by filtration, washed with diethylether,
taken in acetone and chromatographed on silica gel (acetone–
H2O–saturated aqueous KNO3 (v/v) from 90 : 5 : 0.5–90 : 7 :
0.7). In a final step, the nitrate counter ions were replaced by
hexafluorophosphate ions to ensure solubility in organic solvents.
Complex [Ru]5 was isolated in 91% yield (109 mg) as a bright
orange solid. dH (ppm) = 9.91 (d, 1H, Ha, 3J = 5.4 Hz), 8.96–8.88
(m, 3H, Hd + H3¢), 8.77–8.70 (m, 3H, Hg + H3), 8.55 (t, 1H, H4¢,
3
1H, Ha, J = 5.7 Hz), 9.00–8.87 (m, 3H, Hd + H3¢), 8.72 (d, 2H,
H3, 3J = 7.8 Hz), 8.66 (d, 1H, Hg, 3J = 7.8 Hz), 8.55–8.42 (m, 2H,
Hc + H4¢), 8.24 (d, 2H, H6, 3J = 5.4 Hz), 8.20–8.10 (m, 3H, H4 +
3J = 8.4 Hz), 8.46 (dd, 1H, Hc, 3J = J = 7.8 Hz), 8.24–8.07 (m,
Hb), 7.98 (dd, 1H, Hh, 3J = J = 7.8 Hz), 7.52 (dd, 2H, H5, 3J =
3
3
3
6H, H4 + Hb + Hh + H6), 7.60–7.52 (m, 3H, H5 + Hj), 7.38 (dd,
3J = 6.4 Hz), 7.26 (dd, 1H, Hi, 3J = J = 6.6 Hz), 6.98 (d, 1H, Hj,
3
3
1H, Hi, J = J = 6.5 Hz), 4.62–4.44 (m, 3H, CHCH3), 0.98 (d,
3J = 5.4 Hz), 2.25 (m, 3H, CHCH3), 0.91 (m, 18H, CHCH3). dP
5634 | Dalton Trans., 2008, 5627–5635
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(ppm) = 29.4 (broad), -144.2 (sept, JPF = 708 Hz). dC (ppm) =
158.82 (C2¢), 158.73 (C2), 156.98 (d, Ca, J = 1.6 Hz), 156.90 (d,
Ce, J = 2.1 Hz), 156.14 (Cf), 154.71 (C6), 147.09 (Cj), 138.96 (C4),
138.81 (Ch), 138.75 (Cc), 137.39 (C4¢), 128.25 (C5), 128.00 (Cb),
127.63 (d, Ci, J = 1.7 Hz), 125.09 (Cd), 124.80 (C3), 124.53 (C3¢),
123.86 (d, Cg, J = 1.6 Hz), 24.44 (d, CHCH3, J = 19.4 Hz), 18.75
(CHCH3). ES+-MS: m/z = 796.0 ([M–PF6]+), 325.6 ([M–2PF6]2+),
mp 229 ◦C (decomposition).
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Acknowledgements
We thank the French Ministry of Education for a Ph.D. fellowship
to EL and Dr A. Sournia-Saquet for the electrochemical measure-
ments. Johnson Matthey is gratefully acknowledged for a loan of
RuCl3·xH2O. We would like to thank Oxford Diffraction, espe-
cially Dr Neil Brooks, for technical assistance and for recording
the diffracted intensities of complex [Ru]3 on their demonstration
Gemini R diffractometer. This work has been supported in Italy
by MIUR grant FIRB (LaTEMAR: Laboratorio di Tecnologie
Elettrobiochimiche Miniaturizzate per l¢Analisi e la Ricerca).
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