RuII multinuclear metallosupramolecular rack-type architectures of polytopic hydrazone-based ligands: Synthesis, structural features, absorption spectra, redox behavior, and near-infrared luminescence

Article abstract of DOI:10.1002/chem.200900632

A novel class of polytopic hydrazone-based ligands was synthesized. They gave heteroleptic Ru^II polynuclear rack-like complexes of formula [Ru_nterpy_n molecular strand)]²ⁿ⁺ (terpy = 2,2′:6′,2″-terpyridine). The new rack-like systems can be viewed as being made of two identical or roughly identical peripheral subunits separated by several similar metal-containing spacer subunits. The presence of pyrazine or pyrimidine units within the molecular multitopic strands introduces additional chemical diversity; whereas a pyrimidine unit leads to appended orthogonal subunits that are on the same side with regard to the main molecular strand, a pyrazine unit leads to orthogonal subunits that lie on different sides. Mixing pyrazine and pyrimidine units within the same (bridging) molecular strand also allows peculiar and topographically controlled geometries to be obtained. Redox studies provided evidence that each species undergoes reversible redox processes at mild potentials, which can be assigned to specific subunits of the multicomponent arrays. Non-negligible electronic coupling takes place among the various subunits, and some electron derealization ex-tending over the overall bridging molecular strand takes place. In particular, oxidation data suggest that the systems can behave as p-type "molecular wires" and reduction data indicate that n-type electron conduction can occur within the multimetallic framework. All the multinuclear racks exhibit ³MLCT emission, both at 77 K in rigid matrix and at 298 K in fluid solution, which takes place in the near-infrared region (emission maxima in the 1000-1100 nm region), and is quite structured. Rigidity of the molecular structures and derealization within the large bridging ligands are proposed to contribute to the occurrence of the rather uncommon MLCT infrared emission, which is potentially interesting for optical communication devices.

Full text of DOI:10.1002/chem.200900632

FULL PAPER
DOI: 10.1002/chem.200900632
II
Ru Multinuclear Metallosupramolecular Rack-Type Architectures of
Polytopic Hydrazone-Based Ligands: Synthesis, Structural Features,
Absorption Spectra, Redox Behavior, and Near-Infrared Luminescence
[
a, b]
[c]
[c]
Adrian-Mihail Stadler,
Fausto Puntoriero, Francesco Nastasi,
[c] [a]
Sebastiano Campagna,* and Jean-Marie Lehn*
This work is dedicated to Jean-Pierre Sauvage, on the occasion of his 65th birthday
Abstract: A novel class of polytopic
that lie on different sides. Mixing pyra-
zine and pyrimidine units within the
same (bridging) molecular strand also
allows peculiar and topographically
controlled geometries to be obtained.
Redox studies provided evidence that
each species undergoes reversible
redox processes at mild potentials,
which can be assigned to specific sub-
tending over the overall bridging mo-
lecular strand takes place. In particular,
oxidation data suggest that the systems
can behave as p-type “molecular
wires” and reduction data indicate that
n-type electron conduction can occur
within the multimetallic framework.
All the multinuclear racks exhibit
hydrazone-based ligands was synthe-
II
sized. They gave heteroleptic Ru poly-
nuclear rack-like complexes of formula
[
Ru terpy (bridging
molecular
n
n
n+
2
strand)]
(terpy=2,2’:6’,2’’-terpyri-
dine). The new rack-like systems can
be viewed as being made of two identi-
cal or roughly identical peripheral sub-
units separated by several similar
metal-containing spacer subunits. The
presence of pyrazine or pyrimidine
units within the molecular multitopic
strands introduces additional chemical
diversity: whereas a pyrimidine unit
leads to appended orthogonal subunits
that are on the same side with regard
to the main molecular strand, a pyra-
zine unit leads to orthogonal subunits
3
MLCT emission, both at 77 K in rigid
A
H
U
G
R
N
U
G
matrix and at 298 K in fluid solution,
which takes place in the near-infrared
region (emission maxima in the 1000–
1100 nm region), and is quite struc-
tured. Rigidity of the molecular struc-
tures and delocalization within the
large bridging ligands are proposed to
contribute to the occurrence of the
rather uncommon MLCT infrared
emission, which is potentially interest-
ing for optical communication devices.
Non-negligible electronic coupling
takes place among the various subunits,
and some electron delocalization ex-
Keywords: luminescence · molecu-
lar racks
·
molecular wires
·
N ligands · polynuclear complexes ·
AHCTUNGTRENNUNG
ruthenium · supramolecular chemis-
try
[
a] Dr. A.-M. Stadler, Prof. J.-M. Lehn
ISIS-Universitꢀ de Strasbourg, 8 Allee Gaspard Monge, BP 70028
7083 Strasbourg cedex (France)
Introduction
6
Fax : (+33)390-245-140
E-mail: lehn@isis.u-strasbg.fr
Molecular racks are structurally rigid and roughly linear
multicomponent species; they are made of multitopic, “pro-
grammed” molecular strands to which several suitable mo-
lecular subunits, capable to read out the coordination infor-
mation encoded in the strands, are appended/complexed.
The appended molecular subunits usually assume orthogo-
nal arrangements with respect to the main molecular strand
[
b] Dr. A.-M. Stadler
Karlsruhe Institute of Technology (KIT)
Forschungszentrum Karlsruhe (FZK)
Institute for Nanotechnology (INT)
Postfach 3640, 76021 Karlsruhe (Germany)
[
c] Dr. F. Puntoriero, F. Nastasi, Prof. S. Campagna
Dipartimento di Chimica Inorganica
Chimica Analitica e Chimica Fisica
Universitꢁ di Messina, Via Sperone 31, 98166 Messina (Italy)
Fax : (+39)090-393-756
[1]
dimension.
Architectures of this kind, which bring together a more or
less linear sequence of metal ions, are interesting for their
[2–4]
physical properties.
synthetic precursors for larger architectures.
metals are used in the works reported in the literature:
Some of them may also be used as
E-mail: campagna@unime.it
[5a,b]
Various
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900632.
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5645

II[2,3]
I [5a,b]
II [5c]
II [5d]
III
III
III [4]
Ru
Cu ,
Zn , Cu , Eu , La , or Y . The rigid
easier and milder and 2) superior chemical diversity can be
synthetically introduced, as various isomeric possibilities in-
volving the position of the hydrazonic C=N bond within the
molecular strands are possible. Recently we reported the
synthesis, characterization, as well as the study of the ab-
sorption spectra, luminescence properties, and redox behav-
structure of the molecular racks is not a necessary prerequi-
site of the molecular strand (which plays the role of a multi-
topic bridging ligand), but it can emerge upon racks forma-
tion following the coordination of the appended subunits.
This is the case of molecular racks made of hydrazone-
based molecular strands, such as the species studied here.
Hydrazone-based molecular racks have several unusual
properties compared to the formerly investigated molecular
racks based on ligand strands containing exclusively terpyri-
[
6]
II
ior of some dinuclear Ru molecular racks based on hydra-
[2a]
zone-based strands. Here we extend this study to larger
molecular racks, spanning nuclearities from two to six (com-
plexes 1–4,6; Figure 1).
[3]
dine-like coordination sites: 1) The synthetic routes are
[
2a]
Figure 1. Structural formulae of the metal complexes (charges omitted). Complexes 2a–c were studied previously and are reported for comparison.
Synthesis and characterization of 1 and 4b are reported, but the absorption, luminescence, and redox properties of these species were not studied for
technical reasons.
5646
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Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
FULL PAPER
The new species are based on photo- and redox-active
Ru ACHTUNGTRENNUNG( terpy) subunits (terpy=2,2’:6’,2’’-terpyridine) append-
Results and Discussion
2
+
ed to polytopic, molecular strands of increasing size, con-
taining hydrazone-based terpy-type coordination centers.
They can be viewed as being made of two identical (or
quasi-identical, in the case of 3a and 3b), peripheral sub-
Synthetic approach and characterization
Synthesis of ligands: Ligand 7 was synthesized (Scheme 1) as
[7]
previously described, by condensation of 2-pyridinecarbox-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
units separated by zero (the dinuclear species, 2d), one (the
aldehyde (25) with 2-(1-methylhydrazino)pyridine (24), ob-
[8]
trinuclear species, 3a and 3b), two (the tetranuclear species,
4
6
of pyrazine or pyrimidine units within the molecular strands
also introduces a source of additional chemical diversity: ac-
tually, whereas a pyrimidine unit leads to orthogonal sub-
tained from 2-chloropyridine (23) and methylhydrazine.
Reaction of aminopyrazine 20 with N-bromosuccinimide
a and 4b), and four (the hexanuclear compounds, 6a and
b) similar metal-containing spacer subunits. The presence
[9]
led to 2-amino-5-bromo-pyrazine (19), which, on treatment
[10]
with Br , HBr, and NaNO , yielded 2,5-dibromopyrazine
2 2
(18), the reaction of which with methylhydrazine led to 2,5-
bis(1-methylhydrazino)pyrazine (17). Reaction of 17 with
two equivalents of 2-pyridinecarboxaldehyde (25) led to
ligand 8 (Scheme 1).
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
units which are on the same side with regard to the main
molecular strand, a pyrazine unit leads to orthogonal sub-
units that lie on different sides (“alternate” racks). Mixing
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
The reaction of 2,5-dimethylpyrazine (16) with benzalde-
hyde, in the presence of benzoic anhydride, gave 2,5-distyr-
pyrazine and pyrimidine units within the same (bridging
ligand) molecular strand leads to quite unusual and topo-
graphically controlled geometries. An example is the com-
parison of the structures of the hexanuclear 6a and 6b mo-
lecular racks.
[
11]
ylpyrazine
(15), the ozonolysis (O , MeOH, ꢀ788C) of
3
which followed by reduction with an aqueous solution of
sodium metabisulfite (Na S O ) produced 2,5-pyrazinedicar-
boxaldehyde
2
2
5
[
12]
(14). Condensation of an excess of dialde-
hyde 14 with one equivalent of 2-(1-methylhydrazino)pyri-
dine (24) in EtOH, at room temperature, gave the monoal-
dehyde 13. Precursor 13 reacted with an excess of bishydra-
zine 17 or 21 to give precursors 5 or 11, respectively
(
Scheme 1).
Scheme 1. Synthesis of the heterocyclic building blocks 14, 15, 17–19, 21, and 24, of the precursors 5, 11, 13, and 26 and of the ligands 7 and 8.
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5647

S. Campagna, J.-M. Lehn et al.
Scheme 2. Synthesis of the ligands 9a,b and 12a,b.
Ligands 10a and 10b (Scheme 2) were synthesized by a
The complexes were prepared following a previously de-
[13]
II
[3a–d]
double chain-extension method consisting of the conden-
sation of one equivalent of 2,5-bis(1-methylhydrazino)pyra-
zine (17) or 4,6-bis(1-methylhydrazino)pyrimidine (21) with
two equivalents of precursor 13. Strand 10a has an inversion
scribed pathway for the synthesis of Ru racks.
[Ru-
[15]
AHCTUNGTREG(NNNU terpy)Cl ] was reacted with ligands 7, 8, 9a,b, 10a,b, and
3
12a,b in molar ratios described in the Experimental Section,
at reflux temperature, in mixtures of water and ethanol that
have reducing properties (donor of electrons for the process
center, whereas 10b has a C axis as symmetry element. Lig-
2
II
III
ꢀ
II
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ands 10a,b are insoluble in organic solvents, but their Ru
Ru +e !Ru ). The reactants were heated at reflux for 18–
21 h. The results showed that the hydrazone bonds were
stable under these conditions.
rack-like complexes are soluble in acetonitrile.
A double chain-extension reaction of two equivalents of 5
and 11 with 2,5-pyrazinedicarboxaldehyde (14) yielded
ligand strands 12a and 12b (Scheme 2). 2-Pyridinecarboxal-
dehyde (25) reacted with one equivalent of precursor 5 and
yielded the ligand strand 9a. Reaction of precursor 13 with
The complexes thus obtained had chlorides as counterions
and were water soluble. They were precipitated from water
by the addition of aqueous solutions of ammonium hexa-
fluorophosphate (NH PF ). Further purification was per-
4
6
[6]
precursor 26
[obtained from 2-pyridinecarboxaldehyde
formed by reprecipitation from CH CN with Et O or chloro-
3 2
(
(
25) and an excess of 4,6-bis(1-methylhydrazino)pyrimidine
21), see Scheme 1] gave ligand 9b.
form.
The complexes are colored solid materials (1 and 2d are
brown, the others are deep green), soluble in acetonitrile or
acetone, but insoluble in toluene, diethyl ether, diisopropyl
ether, benzene, or chloroform, properties that were used in
crystallization experiments to obtain single crystals.
In these ligands, the hydrazone group is the isomorphic
[14]
equivalent of a 2,6-disubstituted pyridine ring.
II
Synthesis of complexes: Ru rack-type complexes may be
formed by octahedral coordination of Ru with three nitro-
II
gen atoms of a terpy ligand and three of a hydrazone site
Crystallographic studies: Single crystals suitable for X-ray
(
py/pz/pym-hyz-pym/pz), so that, for example, a dihydrazone
diffraction were obtained by slow diffusion of Et O into a
2
ligand could generate a dinuclear complex.
solution of complex 1 or 2d in CH CN. Both crystals are
3
monoclinic. The expected structures (Figure 2) were con-
5648
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Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
FULL PAPER
Figure 2. Solid-state molecular structures of complexes 1 (left) and 2d
ꢀ
(
right). PF
6
anions are omitted for clarity.
firmed. All the measured distances are internuclear (cen-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
troid-to-centroid) distances. The RuꢀN coordination bond
lengths and the N-Ru-N coordination angles of the two
complexes 1 and 2d are very similar (Table 1).
Table 1. Length of the coordination bonds, RuꢀRu distance, and
N-Ru-N coordination angles in complexes 1 and 2d.
d [ꢃ]
1
2d
Angle [8]
1
2d
N
N
N
N
N
N
hyzꢀRu
1.947
2.052
–
2.061
2.003
2.066
–
1.978
2.086
2.064
–
1.988
2.074
6.896
N
N
N
N
ald-Ru-Nhyz
hyz-Ru-Nhy
hyz-Ru-Npz
py-e-Ru-Npy-i
79.66
77.93
–
79.31
78.09
78.82
–
78.95
78.74
78.92
aldꢀRu
pzꢀRu
hyꢀRu
py-iꢀRu
py-eꢀRu
1
1
3
Figure 3. H– H NOESY spectrum of ligand 9b (300 MHz, CDCl ).
RuꢀRu
NMR spectroscopy: Several ligands (9a, 10a,b, and 12a,b)
have low solubility in organic solvents, and therefore they
were characterized by mass spectrometry (when soluble
enough) and by means of their complexes.
2
In the free ligands, the sp N atoms separated by three
bonds, that is, those located in relative 1,4-positions (except
the two pyrazine N atoms with respect to one another), are
in transoid orientation. In the case of ligand 9b, this transoid
orientation is confirmed by the following NOE: (D,G),
1
Figure 4. H NMR spectra of compounds 5, 8, 9b, 11, and 13 (300 MHz,
CDCl ).
3
(
H,K), (H,P), and (K,P) (Figure 3).
1
H NMR spectra of the complexes present relatively high
complexity, due to close signals in the aromatic region, and
two-dimensional experiments (COSY, NOESY, ROESY)
were useful to assign the peaks. Some particularities should
be noted. Thus, for complexes containing pyrimidine units,
the zone between the two terpy ligands should be the most
shielded from the molecule, and this is confirmed by the ex-
tremely low chemical shift of the C2 pyrimidine proton
(
5
proton L of the pyrimidine; see Figures 4 and 5): d=
.04 ppm for 3b (Figure 5; but d=8.48 ppm in the corre-
sponding free ligand 9b; Figure 4), 5.08 ppm for 4b and
.07 ppm for 6b. On the other hand, around the pyrazine
5
unit, due to its inversion center, the two terpy units are on
opposite sides of the rack and this makes their shielding
effect on protons G, H, and L of the pyrazine weaker than
in the case of the pyrimidine.
In the complexes 1, 3a,b, 4a,b, and 6a,b, the terminal hy-
drazinopyridine protons (A–D) and the methyl group (pro-
1
Figure 5. H NMR spectra of complexes 1, 2d, 3a,b, 4a,b, and 6a,b
300 MHz, CDCl ). For clarity, only the letters corresponding to the
peaks of bridging ligands are shown.
(
3
Chem. Eur. J. 2010, 16, 5645 – 5660
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5649

S. Campagna, J.-M. Lehn et al.
1
tons E) display similar H NMR resonances. Racks contain
a determined number of types of terpy: one type (1, 2d),
two types (4a, 4b), or three types (3a, 3b, 6a, 6b). This is
1
13
confirmed by H NMR data and also by C NMR data, as
shown in Figure 6, in the case of carbon C5 of the terminal
pyridines of the terpy units.
1
3
Figure 6. C NMR spectra of the region containing the peaks of C5 (*)
of the monosubstituted pyridines in complexes 2d, 3b, 3a, 4b, and 6a
(
3
75 MHz, CD CN).
1
1
3
Figure 7. H– H NOESY spectrum of complex 4a (300 MHz, CD CN).
The coordination induces conformational changes. For ex-
3
3
3
ample, it causes C4pymꢀNsp , C6pymꢀNsp , C2pzꢀNsp ,
3
3
C5pzꢀNsp , and C2pyꢀNsp bonds to turn on their axes, and
3
2
3
the place of the Nsp ꢀNsp bond is taken by the Nsp ꢀCH
3
2
hyz
2
hyz
bond. It also causes the C2pzꢀCsp , C5pzꢀCsp , and
2
2
C2pyꢀCsp bonds to turn, and the place of the Csp
=
hyz
hyz
2
2
hyz
Nsp bond is taken by the Csp ꢀH bond.
hyz
The shapes of the coordinated strands as represented in
the structural formulae are confirmed by 2D NMR spectros-
copy, essentially by correlations due to the existence of
NOEs between several key protons closely involved in the
conformational change of the ligand before and after bind-
ing of the metal ion. The coordination induces the transfor-
mation of the all-transoid conformation into an all-cisoid
one. In the case of terpy, it produces the (T ,T ) NOE ob-
Figure 8. Dependence of the diffusion coefficients obtained by DOSY on
the nuclearity of the complexes.
4
5
served for all complexes. The shape of the coordinated
ligand is established by NOEs, which do not exist in the free
ligand, but appear after coordination, such as the following:
us to calculate an electronic coupling attenuation parameter
[2b]
(usually called b) for the inner metal-based modules. The
ꢀ
1
relatively low b parameter obtained (0.23 ꢃ ) indicates that
these molecular racks exhibit p-type “molecular wire” prop-
erties of potential interest for implementation in supra-
(
D,E), (F,G), (H,I), (J,K), (Kpym,M), (J,K), (L ,M), (P,U).
pz
Thus, the following NOEs were observed for 4a: (D,E),
[2b,17]
(
F,G), (H,I), (J,yK) (Figure 7).
molecular electronic devices.
Here we describe the com-
The DOSY spectrum (Figure 8) of an equimolar mixture
of complexes 1, 2d, 3b, 4b, and 6b resulted in the determi-
plete oxidation behavior of all the rack species studied.
The oxidation patterns of the Ru rack complexes show
II
nation of five diffusion coefficients with the following
values: 967, 686, 579, 503, 440 mm s . The diffusion coeffi-
cient exponentially decreases with increasing nuclearity. A
several processes (Table 2, Figure 9), most of them reversi-
ble, that can be classified as metal-centered, on the basis of
2
ꢀ1
[18]
literature data.
The dinuclear species 2d shows two
similar evolution was observed in the case of a series of
metal-centered oxidation processes, separated by 250 mV.
This potential splitting is connected to the electronic cou-
pling between the two metal-based orbitals, mediated by su-
perexchange through the ꢀNꢀpyrazineꢀNꢀ bridging-moiety
II
[
Ru
{methylphenylenebis(terpyridine)} ] (n=2–6) oligo-
n
ꢀ
1
n
[16a]
mers formed in a single-pot reaction,
of helicates of increasing nuclearity.
as well as a series
[16b]
orbitals, by taking advantage of both HOMO and LUMO
[2a]
Redox behavior
orbitals of the bridge. Electrostatic terms can also contrib-
ute to the interaction. The potential splitting in 2d is in line
with that found for the already investigated 2a–c racks, the
redox properties of which are also shown in Table 2 for
Oxidation: Recently, we partly reported the oxidation prop-
erties of 2c, 3a, 4a, and 6a, and the results obtained allowed
5650
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Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
Table 2. Spectroscopic and redox data.
FULL PAPER
The oxidation pattern of the trinuclear systems 3a and 3b
shows two mono-electronic processes. Whereas in the dinu-
clear systems the two metal centers were identical, in this
case the three metal centers of each species are all different
from one another, because they have different bridging
ligand moieties. The clear difference is that two metal cen-
ters are peripheral, and are therefore linked to a single
bridging moiety, and one is central, so linked to two bridging
residues. Because the backbonding ability of a peripheral
[
a]
[a,c]
Absorption
Luminescence
Redox data
[
a]
[b]
2
98 K
298 K
77 K
E
1/2 ox
E1/2 red
[V vs. SCE]
l
max [nm]
l
max [nm]
l
max [nm] [V vs. SCE]
ꢀ1
ꢀ1
(
e
A[ M cm ]) (t [ns])
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(t [ns])
[
d]
2
2
2
2
a
308 (70300) 758 (30)
741 (335) +1.28 (65)
+1.53 (70)
ꢀ0.98 (62)
ꢀ1.22 (65)
ꢀ1.35 (70)
ꢀ1.43 (80)
ꢀ0.45 (60)
ꢀ1.05 (65)
ꢀ1.46 (75)
ꢀ1.65 (82)
ꢀ0.50 (65)
ꢀ1.05 (65)
ꢀ1.49 (75)
ꢀ1.57 (82)
ꢀ0.88 (72)
ꢀ1.42 (75)
ꢀ1.59 (80)
3
4
4
30 (60300)
34 (22400)
70 (20700)
+1.78 (80)
[
d]
b
330 (42500)
+1.36 (68)
+1.66 (75)
430 (45700)
506 (12600)
630 (27000)
ꢀ
N=Cꢀpyridine residue is worse than that of a bridging
[2a]
moiety, the peripheral metal centers should have a larger
electron density than the central metal center, and they
would be oxidized at milder potentials. The peripheral metal
centers are also different from one another. For example, in
[
d]
c
272 (56700)
+1.27 (65)
+1.50 (70)
440 (47400)
614 (40600)
772 (2700)
3
a one metal is linked to an ꢀN=CꢀpyrazineꢀC=Nꢀ group,
d
273 (46000)
765 (60)
750 (400) +1.27 (72)
+1.52 (78)
and the other to an ꢀNꢀpyrazineꢀNꢀ bridging group (the
310 (65000)
406 (51000)
437 (44000)
other ligand moieties, identical for both metals, such as the
terpyridine ligands, are not considered). As models for the
oxidation potential of the two metal centers, we can consid-
er the first oxidation of 2c and 2d, anticipating that both of
them are slightly displaced to more positive potentials in the
trinuclear species because of the presence of the central
unit. In fact, the first oxidation process of 3a (+1.30 V) is
slightly more positive than those of 2c and 2d; however,
one cannot decide which peripheral metal center is involved
in the first oxidation process, because the oxidation poten-
tial of the models are too close (+1.28 and +1.27 V, see
Table 2). The second oxidation involves the not yet oxidized
peripheral metal center, and the potential splitting (130 mV)
is evidently reduced compared to that occurring in the dinu-
clear species, because of increased separation between the
redox-active centers. The identical arguments can be em-
ployed to discuss the oxidation behavior of the trinuclear
species 3b, with peripheral unit models 2a and 2c.
[
[
e]
e]
3
3
4
a
b
a
308 (79000)
1040
1022
1052
1038
1004
1048
+1.30 (90)
+1.43 (92)
ꢀ0.41 (65)
ꢀ0.86 (75)
ꢀ1.24 (78)
ꢀ0.38 (62)
ꢀ0.85 (70)
ꢀ1.09 (75)
ꢀ0.37 (65)
ꢀ0.47 (68)
ꢀ0.86 (75)
ꢀ1.10 (75)
ꢀ0.30 (57)
ꢀ0.39 (62)
4
6
47 (72000)
30 (47500)
308 (80000)
+1.34 (70)
+1.45 (72)
4
6
30 (53000)
31 (38000)
304 (118000)
+1.32 (65)
+1.37 (68)
+1.61 (75)
4
6
8
60 (86000)
50 (104000)
23 (11800)
6
6
a
305 (149000)
1078
1032
1044
1018
+1.34 (58)
+1.43 (62)
+1.64 (78)
4
6
8
58 (132000)
64 (152000)
45 (21500)
[f]
b
305 (155000)
+1.40 (72)
+1.54 (82)
ꢀ0.41 (65)
ꢀ0.56 (72)
4
6
40 (125000)
59 (139000)
[
7
a] In argon-purged acetonitrile. [b] In MeOH/EtOH (4:1, v/v) matrix at
7 K. [c] In parentheses, the peak-to-peak separations for the redox cou-
ples are given, in mV. [d] From reference [2a]. [e] Quasi-reversible pro-
cess. [f] Bi-electronic process.
In the tetranuclear species 4a, symmetry is restored: the
two peripheral metal centers are identical, and so are the
inner metal centers. The first two oxidation processes in-
volve successive one-electron oxidation of the peripheral
metal centers. The potential splitting, 50 mV, is in line with
the increased distance between the redox-active sites in-
volved, when compared to the lower nuclearity compounds.
A third, irreversible process, takes place at +1.61 V, and is
assigned to the oxidation of one of the two inner metal cen-
ters. Interestingly, the difference between second and third
oxidation potentials, 240 mV, related to the coupling be-
tween the peripheral and inner units, is almost identical to
the difference between two oxidation processes of 2c, which
contains the identical bridging moiety between the pertinent
metal center units (that is, the ꢀN=CꢀpyrazineꢀC=Nꢀ unit).
The hexanuclear species 6a and 6b exhibit symmetric
structures with respect to the central pyrazine ring. So, three
types of metal centers are present in both compounds. On
the basis of the discussion made for the trinuclear species,
the metal centers that are expected to be oxidized at milder
potentials are the two peripheral centers. Indeed, the first
oxidation process of both the hexanuclear species is bi-elec-
tronic, and is assigned to the simultaneous one-electron oxi-
Figure 9. Cyclic voltammogram of 3b in acetonitrile solution. The wave
at about 0.4 V is ferrocene (Fc), used as an internal standard.
comparison purposes, and this suggests that the metal–metal
interaction is roughly similar within the series of dinuclear
[2a,19–22]
species.
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5651

S. Campagna, J.-M. Lehn et al.
dation of the weakly interacting peripheral metal centers.
The absence of splitting of the peripheral oxidation in the
hexanuclear racks is not surprisingly: on considering the dis-
ꢀ1
tance between the sites and the value of 0.23 ꢃ calculated
for the b parameter in the 2c, 3a, 4a series, a splitting of
[2b]
1
0 mV is estimated for 6a, and this value is beyond detec-
tion by our differential pulse voltammetry apparatus. A fur-
ther, irreversible oxidation process is present in the oxida-
tion pattern of both 6b and 6a. Such a process is assigned to
an inner metal center, most likely one of the two metal cen-
ters not directly linked to the already oxidized peripheral
centers: the difference in potentials between the bi-electron-
ic process and the successive, mono-electronic process
(
110 mV in 6a and 140 mV in 6b) is indeed of the same
order of the difference between the two oxidation potentials
in the trinuclear species (130 and 110 mV in 3a and 3b, re-
spectively), supporting the assignment (in all cases, the dif-
ference in potential would be related to processes involving
metal centers separated by another interposed and not yet
oxidized metal center).
Figure 10. Schematic representation of the reduction potentials of the re-
ported complexes. In the first row, the various redox-active subunits ex-
pected to be reduced in a potential range (vertical borders) are shown.
Reduction: To discuss the reduction behavior of the rack
complexes, it is useful to identify the various redox-active
subunits and, on the basis of the results reported for the for-
merly investigated dinuclear species of the same family, to
make hypotheses on the probable potential range where the
reduction processes of such redox-active subunits can be ex-
pected.
On reduction, we can identify the following types of sub-
ACHTUNGTRENNUNGu nits, going from the subunits which are expected to be re-
duced at milder potentials to the ones which are expected to
be reduced at more negative potentials:
nal and inner locations for the various terpy ligands can
be identified, and such different locations are expected
to have a consequence on the electron density on the
terpy (terpy ligands linked to a terminal, electronically
richer Ru center should be more difficult to reduce, that
is, they should be reduced at more negative potentials
than terpy ligands linked to an inner, electronically
poorer Ru center). Second-order differences could also
be present as a consequence of the overall chemical envi-
ronment of the metal center to which the various terpy
ligands are linked (compare for example the two termi-
nal terpy ligands of the trinuclear species: one is linked
[2a]
II
1
) The ꢀN=CꢀpyrazineꢀC=Nꢀ units, that is, the bis-chelat-
ing bridging units containing a LUMO which extends
over a central pyrazine and two conjugate C=N bonds,
to a Ru center connected to a bridging delocalized
ꢀN=CꢀpyrazineꢀC=Nꢀ unit, whereas the other is linked
II
to a Ru center which is coordinated by a bridging pyra-
(
see genetic diagrams in Figure 10). From the informa-
zine/pyrimidine ring). However, also in this case it is
hard to predict a priori the electronic differences among
the various terpy units.
tion gained by the redox properties of dinuclear spe-
ACHTUNGTRENNUNGc ies, a subunit such as this should be mono-reduced at
[2a]
about ꢀ0.40 V versus SCE and can be doubly reduced in
the range ꢀ0.90 to ꢀ1.25 V. Note that in the tetranuclear
species 4a two identical ꢀN=CꢀpyrazineꢀC=Nꢀ bridging
In the light of the general guidelines derived from the
above-mentioned considerations, the reduction behavior of
the new complexes will be discussed (Table 2; the redox
properties of the dinuclear species 2a–c are also reported
for comparison). However, the analysis of the reduction be-
havior is complicated by the lack of reversibility of several
processes. For this reason, in many cases the assignment of
the reduction processes should be considered only as tenta-
tive.
The reduction pattern of 2d, which completes the dinu-
clear series, corresponds to those of the formerly studied
2a–c species: the three mono-electronic reduction processes
of 2d are straightforwardly assigned to reduction of the
bridging pyrazine subunit and to reduction of the two terpy
ligands. The two terpy-based reductions are split by 170 mV,
indicating a significant electronic interaction between the
(identical) terpy subunits.
units are present, each one connecting a peripheral Ru
center with an inner one, and in the larger hexanuclear
species two types of such units are present: two such
units are located at the periphery of the supramolecular
structure, connecting a terminal Ru unit with an inner
Ru center, whereas one such unit connects two inner Ru
centers. It is hard to predict a priori which can be the dif-
ference in redox potentials between these two types of
redox-active units.
2
) The ꢀNꢀpyrazineꢀNꢀ or ꢀNꢀpyrimidineꢀNꢀ bis-chelat-
ing units, in which the LUMO is essentially centered on
the pyrazine or pyrimidine ring (see Figure 10). This type
[2a]
of subunit is reduced in the range ꢀ0.80 to ꢀ0.95 V.
) The terpy ligands, which are mono-reduced at potentials
3
5
[18]
more negative than ꢀ1.30 V. Even in this case, termi-
652
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Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
FULL PAPER
For the trinuclear species 3a and 3b, the first reduction
process is assigned to the ꢀN=CꢀpyrazineꢀC=Nꢀ bridging
has to be due to the nature of the aromatic rings of the
strands separating the ꢀN=CꢀpyrazineꢀC=Nꢀ units: in 6a,
subunit and the second reduction process is assigned to the
second bridging ꢀNꢀpyrazineꢀNꢀ (in 3a) or to the ꢀNꢀpyr-
imidineꢀNꢀ (in 3b) unit (for the sake of simplicity, from
it is a pyrazine-based component, and in 6b, a pyrimidine-
based component. On comparing the first reduction poten-
tial of the homologue species 2d (reduction localized on
pyrazine-based unit, occurring at ꢀ0.88 V) and 2a (reduc-
herein these subunits will be named pyrazine-based and pyr-
imidine-based units, respectively). The practically coincident
potential of the second reduction potentials of the trinuclear
species (involving a pyrimidine/pyrazine-based unit) with
those occurring for the reduction of the equivalent bridging
units in the corresponding dinuclear species 2c and 2d is
most likely due to two contrasting effects. In fact, in the tri-
nuclear species, the LUMO centered in the bridging pyra-
zine/pyrimidine unit should be stabilized in comparison to
the dinuclear species (the inner Ru center of the trinuclear
compounds should backbond mainly towards the other
bridging ligand) and therefore its reduction potential should
move to less negative potentials; however, first reduction on
the other bridging ꢀN=CꢀpyrazineꢀC=Nꢀ unit would bal-
tion involving
a pyrimidine-based unit, occurring at
[2a]
ꢀ0.98 V ), it appears that pyrazine-based subunits of the
strands have lower-lying orbitals than pyrimidine-based sub-
units; this could contribute to stabilize the LUMOs involv-
ing ꢀN=CꢀpyrazineꢀC=Nꢀ units in 6a compared to 6b.
However, it is not the mere presence of the pyrazine- versus
pyrimidine-based separating unit that makes the difference,
because this is not reflected by the redox properties of 3a
[23]
and 3b. Most likely, it is the electronic coupling among
the three low-lying and redox-active ꢀN=CꢀpyrazineꢀC=Nꢀ
units that benefits from the presence of the para-disubstitut-
ed pyrazine-based bridging moiety. This increased coupling
leads to stabilization of all the ꢀN=CꢀpyrazineꢀC=Nꢀ units
ance this effect. The third reduction process occurs at poten-
tials similar to those of second reduction of theꢀN=Cꢀpyra-
zineꢀC=Nꢀ unit in the corresponding dinuclear species 2a
of 6a, the LUMO of which, although mostly involving the
central subunit, also receives significant contributions from
both the two peripheral ꢀN=CꢀpyrazineꢀC=Nꢀ units, as a
and 2b, and therefore it is assigned to second reduction of
this latter site also in the trinuclear species. At more nega-
tive potentials, terpy-based reductions take place. However,
the behavior of such successive reduction processes is irreg-
ular, making it impossible to discuss them in detail.
consequence of this inter-unit coupling. In other words, the
LUMO of 6a is partly delocalized over the entire molecular
strand, whereas this delocalization is much lower for the
LUMO of 6b, largely localized on the central ꢀN=Cꢀpyra-
zineꢀC=Nꢀ unit, because the meta-disubstituted pyrimidine-
For the tetranuclear species 4a, the first two mono-elec-
tronic reductions are assigned to the two ꢀN=Cꢀpyrazineꢀ
C=Nꢀ bridging units that are present. Their potential split-
based bridging units allow only reduced coupling between
the ꢀN=CꢀpyrazineꢀC=Nꢀ units they connect.
In the limiting case of complete delocalization of the
LUMO within the molecular frame of 6a, an alternative as-
signment of the first two reduction processes of this com-
pound could be proposed. In this limiting case, the two pro-
cesses could be both mono-electronic and ultimately lead to
partial localization of the added electrons on the two pe-
ripheral ꢀN=CꢀpyrazineꢀC=Nꢀ units, so minimizing repul-
ting (100 mV) is related to their electronic interaction
through the molecular strand. The third (irreversible) pro-
cess, occurring at about ꢀ0.86 V versus SCE, is tentatively
assigned to reduction of the inner bridging pyrazine-based
unit.
The reduction pattern of the hexanuclear species shows
two quasi-reversible processes, peaking, in the differential
pulse voltammetry experiments, at ꢀ0.30 and ꢀ0.39 V for
sion. A third electron could not be added in this case at
mild potentials, because the energy of the LUMO would in-
crease. With the data in our hands, we cannot verify the
extent of LUMO delocalization within the molecular frame,
so the hypothesis should be taken with care.
The case of complete delocalization, which can be valid
for 6a, is surely not true for 6b. However, an alternative as-
signment for the reduction pattern of this species has also to
be taken into account. If the reduction potentials of periph-
eral and central ꢀN=CꢀpyrazineꢀC=Nꢀ units are close
6
a and at ꢀ0.43 and ꢀ0.57 V for 6b. Successive ill-behaved
and irreversible processes take place at potentials more neg-
ative than ꢀ0.90 V. The irreversible nature of the reduction
processes makes it impossible to carefully determine the
number of electrons involved in each process. Tentatively,
because the hexanuclear species contain two different types
of bridging ꢀN=CꢀpyrazineꢀC=Nꢀ units, in a 1:2 ratio, and
assuming that the inner bridging ꢀN=CꢀpyrazineꢀC=Nꢀ
unit is easier to reduce than the peripheral ꢀN=Cꢀpyra-
zineꢀC=Nꢀ units, at a first sight it is reasonable to assign
enough, the first reduction process at ꢀ0.42 V could be a bi-
electronic one, involving two closely spaced one-electron re-
ductions involving the peripheral units. The second reduc-
tion at ꢀ0.56 V would be the reduction of the central
ꢀN=CꢀpyrazineꢀC=Nꢀ unit, displaced at more negative po-
the first process to reduction of the inner bridge and the
second reduction process to simultaneous one-electron re-
duction of the two peripheral bridges. Alternative assign-
ments are nonetheless possible, as will be discussed later in
this section.
tential by the presence of two added electrons on the pe-
A
H
U
G
R
N
U
G
ACHTUNGTRENNUeGN ry. This third electron could be introduced in 6b at
Independent of the detailed assignments of the two reduc-
tion processes, the LUMOs of 6a are significantly stabilized
compared to the LUMOs of 6b (compare the first reduction
potentials of the two complexes, Table 2). The difference
AHCTUNGTRENNUNG
Within such a hypothesis, the inter-unit interaction would be
responsible for the 140 mV shifting of the reduction of the
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5653

S. Campagna, J.-M. Lehn et al.
central ꢀN=CꢀpyrazineꢀC=Nꢀ redox-active site compared
to the reduction of the two peripheral almost identical sites.
This could appear in contradiction with the splitting
feature. A weak absorption tail is also present at wave-
lengths longer than 720 nm.
The bands in the l<360 nm region are assigned to spin-
allowed ligand-centered (LC) transitions, involving both the
terpy ligands and the main bridging strands. The bands be-
tween 360 and 520 nm are assigned to lower-lying spin-al-
lowed LC transitions involving the delocalized ꢀN=Cꢀpyra-
zineꢀC=Nꢀ units and (as far as the red side of the main ab-
(
100 mV) of the reduction potentials of the two ꢀN=Cꢀpyr-
azineꢀC=Nꢀ units in 4a (see above), because the pyrazine-
based “bridge” of 4a should guarantee a better electronic
interaction than the pyrimidine-based “bridges” of 6b, but
this apparent discrepancy is dispelled upon consideration
that in 4a the interaction involves only two redox-active
centers and not three as in 6b.
sorption band is concerned, that is, for the absorption fea-
ture between 410 and 520 nm) to spin-allowed metal-to-
ligand charge-transfer (MLCT) transitions, involving the ter-
pyridine ligands and the ꢀNꢀpyrazineꢀNꢀ or ꢀNꢀpyrimi-
dineꢀNꢀ bis-chelating moieties of the molecular strand. The
In summarizing the results obtained by the redox behav-
ior, it is interesting to note that whereas the weak b value
calculated from the oxidation potential data suggested that
the molecular racks studied here can be considered as p-
type molecular wires (see above and reference [17]), the de-
localization suggested by the reduction potential of the
band at wavelengths longer than 520 nm is assigned to spin-
allowed MLCT transitions involving the ꢀN=Cꢀpyrazineꢀ
C=Nꢀ subunits. Such assignments are based on the absorp-
[2a]
hexa
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
nuclear 6a indicates that these species can also behave
tion spectra of the free ligands of 2a–c, which allowed the
identification of LC bands involving the various subunits of
the strand(s) and the redox behavior (see above), clarifying
the reduction order of the various subunits.
as n-type molecular wires. A bifunctionality can therefore
be proposed for the studied racks, and could be useful for
their potential interest for implementation of supramolec-
ular electronic devices.
The absorption spectra of the racks are mainly additive,
with the molar absorptivity of each band increasing with the
number of subunits having transitions contributing to the
specific band present in the molecular frame. As evidenced
by the redox properties, there are slight differences within
this general view, with MLCT involving the same ligand unit
but different metal centers (e.g., inner or outer with respect
to the multinuclear rack structures) expected to occur at
slightly different energies. However, such differences appear
to be too small to be clearly evidenced in the absorption
properties.
Absorption spectra
II
The absorption spectra of the Ru rack complexes show in-
tense bands both in the UV and in the visible region
(
Table 2, Figure 11). A cursory look at the spectra shows
three main regions in which dominant absorption bands are
present: a) the wavelength region below 350 nm, with struc-
tured and intense bands; b) the 360<l<520 nm region,
containing a broad absorption, clearly comprising several
components; and c) the 520<l<720 nm region, dominated
by an intense, structureless, and relatively narrow absorption
Finally, the features at low energy (l>720 nm) are as-
signed to spin-forbidden MLCT transitions.
Luminescence properties
All the rack-type compounds investigated here show lumi-
nescence both at room temperature in acetonitrile fluid so-
lution and at 77 K in rigid matrix. Relevant data are collect-
ed in Table 2 and some spectra are shown in Figure 12. All
the emissions are assigned to triplet MLCT states.
The dinuclear compound 2d emits in the visible region,
similar, under both experimental conditions (77 and 298 K),
[2a]
to the emissions of compound 2a, formerly studied, and
containing very similar subunits. For all the other species,
emission occurs in the near-infrared region. For the already
studied dinuclear species 2b and 2c, which contain the
ꢀ
N=CꢀpyrazineꢀC=Nꢀ or ꢀN=Cꢀpyrimidine-C=Nꢀ unit, it
[2a]
was suggested
that an eventual emission could occur in
the near-infrared region, because the lowest-energy spin-al-
lowed MLCT absorption bands of these species lie at wave-
lengths longer than 600 nm and the energy difference be-
1
3
tween lowest-lying MLCT absorption maxima and MLCT
II
emission maxima in Ru polypyridine complexes is typically
ꢀ
1 [18,24]
5
000–6000 cm .
The high-nuclearity racks studied here
contain the ꢀN=CꢀpyrazineꢀC=Nꢀ unit, responsible for the
Figure 11. Absorption spectra of 3a (b), 6a (c), 4a (g), 3b
(
d), 6b (c).
low-lying MLCT absorption band, and therefore near-IR
5654
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
FULL PAPER
presence, in the vicinity of a ꢀN=CꢀpyrazineꢀC=Nꢀ unit, of
an ꢀNꢀpyrazineꢀNꢀ moiety or of an ꢀNꢀpyrimidineꢀNꢀ
moiety has different effects: apparently, the former moiety
contributes to the stabilization of the emitting MLCT level
to a larger extent than the latter one (e.g., compare the
emission of 6a with that of 6b, Table 2), most likely by in-
creasing interaction between the bridging ligand moieties
(
see reduction). Therefore, the overall structure of the racks
allows for fine tuning of the luminescence properties.
The net structure of the emission spectra of the com-
pounds yields other interesting information: highly struc-
tured MLCT emission spectra are connected with relatively
small Huang–Rhys coupling constants, that is, negligible dis-
placements of excited-state nuclear coordinates relative to
[30]
ground-state coordinates. This suggests that the emitting
MLCT states have geometries very similar to those of the
ground state in the studied complexes, most likely as a con-
sequence of the highly organized and rigid nature of the mo-
lecular racks. This is particularly shown in 4a and 6a, with a
Huang–Rhys factor S (0.50 and 0.43), which is smaller rela-
tive to that of the other species {e.g. S values of 3a and 3b
are 0.70 and 0.67, respectively; more details are given in the
Supporting Information; for comparison, the factor S for
Figure 12. Emission spectra of 3a (b), 6a (c), 4a (g), 3b (d),
6
b (c).
emissions could be inferred also for these species. However,
it should be considered that Ru-based MLCT emissions in
the near-infrared suffer from the effects of the energy-gap
2
+
[Ru ACHTUNGTERNN(UNG bpy) ] (bpy=2.2’-bipyridine) in the same experimen-
3
[32f]
tal conditions approaches 1.0 }, and could be a further in-
dication of significant electron delocalization in the MLCT
acceptor ligand of these larger racks due to the presence of
multiple ꢀN=CꢀpyrazineꢀC=Nꢀ units separated by pyra-
[
25]
law, which makes infrared MLCT emission quite rare, be-
cause of favorable Franck–Condon factors for radiationless
decay. The inverted dependence of the rate constant for the
[26]
radiative decay on emissive energy
further complicates
zine-based moieties. The similarity in ground- and excited-
state structures of the rack compounds, as suggested by the
emission spectra profiles, tends to reduce the rate constant
of radiationless decay, and would be at the origin of the de-
tectable emission in the near-infrared. Please also note that
the indications suggested by the emission spectral profiles,
as well as the energy difference in the emission spectra be-
tween the hexanuclear species mentioned above, strictly
agree with the interpretation of the reduction data, in partic-
ular with the delocalized nature assigned to the LUMO of
6a.
the possibility of detecting MLCT emission in the near-infra-
red. As a consequence, reported examples of Ru-based
MLCT emission spectra at wavelengths longer than 900 nm
[27]
are extremely limited.
In spite of the considerations mentioned above concern-
ing difficulties in obtaining MLCT infrared emissions, all the
II
Ru racks studied here show MLCT emissions peaking at l
longer than 1000 nm (Table 2, Figure 12). The spectra also
exhibit a net structural progression, even at room tempera-
ture, uncommonly found under these experimental condi-
tions. To the best of our knowledge, this is a first time that a
Unfortunately, information on excited-state lifetimes are
not yet available (the lifetime was shorter than 2 ns, the
time limit of our equipment), and quantum yield data are
not known yet, owing to difficulties in finding reliable quan-
tum yield standards in the infrared region, and therefore fur-
ther discussion of the excited-state properties cannot be
made. In particular, the lack of quantum yield data limits
the discussion of possible applicative developments. Never-
theless, the conclusions derived from the analysis of the
emission spectra profiles indirectly suggest that interesting
quantum yield values could be obtained. In this context, one
thinks of infrared emitters, the most often employed and
promising systems for optical communication, for example,
in short-range communication among computer peripherals
and personal digital assistants. Infrared light with a wave-
length between 1100 and 1700 nm, coupling least dispersion
with best transmission properties, is the best choice for stan-
II
net, structured emission for Ru complexes at wavelengths
longer than 1000 nm has been reported, even at room tem-
[28,29]
perature.
A closer look at the energy differences among the emis-
sion spectra of the various species provides interesting hints.
For example, the energy emission moves to lower energy in
the series 3b, 6b, 3a, 4a, and 6a. All the emissions are as-
signed to Ru-to-(ꢀN=CꢀpyrazineꢀC=Nꢀ unit) CT triplet
state(s). However, although the acceptor ligand of the
MLCT emitting level is roughly identical in all the species,
some slight differences exist, as shown by the reduction be-
havior. For example, the presence of multiple ꢀN=Cꢀpyra-
zineꢀC=Nꢀ units allows for some delocalization, decreasing
the energy of the MLCT state. This is reflected in the
energy emission of 3a (1040 nm), 4a (1052 nm), and 6a
(
1078 nm), in which the number of ꢀN=CꢀpyrazineꢀC=Nꢀ
[34]
acceptor units increases from one to three. Moreover, the
dard silica fibers.
Our results could therefore open new
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5655

S. Campagna, J.-M. Lehn et al.
and somewhat unexpected possibilities for MLCT emitters
in the optical communication field.
photomultiplier response by using a program purchased with the fluorim-
eter. The emission spectra in the near-IR region were recorded on an Ed-
inburgh FLS920 spectrofluorimeter, equipped with a liquid nitrogen
cooled hyperpure germanium crystal as a detector. Luminescence life-
times were determined by time-correlated single-photon-counting
(TCSPC) with an Edinburgh OB900 spectrometer (light pulse: Hama-
matsu PL2 laser diode, pulse width 59 ps at 408 nm; or nitrogen dis-
charge, pulse width at 337 nm: 2 ns). Electrochemical measurements
were carried out on samples in argon-purged acetonitrile at room tem-
perature with a PAR273 multipurpose equipment interfaced to a PC. The
Conclusion
A series of new metal-containing molecular racks, spanning
a nuclearity from two to six, was synthesized and the ab-
sorption spectra, redox behavior, and luminescence proper-
ties were investigated. The main results are summarized
here:
The chemical diversity introduced in the structures by the
isomeric possibilities offered by the hydrazone-based array
led to interesting differences in the properties of the rack-
type architectures. Further structural and functional diversi-
ties are offered by using pyrazine or pyrimidine subunits as
moieties of the bridging molecular strand polytopic ligands.
In spite of the complexity of the molecular arrays, spec-
troscopic as well as redox processes can be assigned to spe-
cific subunits.
2
working electrode was a glassy carbon (8 mm , Amel) electrode. The
counter electrode was a Pt wire, and the reference electrode was an SCE
separated through a fine glass frit. The concentration of the samples was
ꢀ4
about 5ꢅ10 m. Tetrabutylammonium hexafluorophosphate was used as
supporting electrolyte and its concentration was 0.05m. Cyclic voltammo-
ꢀ1
grams were obtained at scan rates of 20, 50, 200, and 500 mVs . For re-
versible processes, half-wave potentials (vs. SCE) were calculated as the
average of the cathodic and anodic peaks. The criteria for reversibility
were the separation of 60 mV between the cathodic and anodic peaks,
the close to unity ratio of the intensities of the cathodic and anodic cur-
rents, and the constancy of the peak potential on changing scan rate. The
number of exchanged electrons was measured with differential pulse vol-
ꢀ
1
tammetry experiments performed with a scan rate of 20 mVs , a pulse
height of 75 mV, and a duration of 40 ms, and by taking advantage of the
presence of ferrocene used as the internal reference.
Oxidation behavior shows that non-negligible electronic
interaction takes place between the peripheral metal cen-
ters, even in the larger species, thus indicating that the mul-
timetallic systems can behave as efficient p-type “molecular
wires”. Reduction processes show that the same species can
behave as n-type “molecular wires”, particularly in the spe-
cies containing pyrazine subunits in which some electron de-
localization can take place within the larger molecular
strands used as bridging ligand. The molecular racks can
therefore be seen as bifunctional molecular wires.
Experimental uncertainties are as follows: absorption maxima, 2 nm;
molar absorption coefficient, 10%; emission maxima, 5 nm; excited state
lifetimes, 10%; redox potentials, 10 mV.
The NMR assignments in the data below refer to formulae given in
Schemes 1 and 2 and Figure 13.
Synthesis
[Ru
A
H
U
G
R
N
U
G
6 2
ACHTUNGTNERNU[GN PF ] (1): An ethanol/water mixture (6 mL, 1:1 v/v) was
added to [Ru
A
H
U
G
R
N
U
G
3
] (23 mg, 0.052 mmol, 1.36 equiv) and free ligand
7
1
NH
4
6
Rather unusual room-temperature structured MLCT
emissions in the near-infrared region take place for most of
the complexes, a feature of potential interest for optical
communication devices.
3
1
3
1
5
.3, 8.2 Hz, 1H), 8.37 (dd, J=7.9, 8.4 Hz, 1H), 8.21 (ddd, J=0.8, 1.5,
.5 Hz, 1H), 7.9 (td, J=1.5, 6.2 Hz, 1H), 7.79–7.73 (m, 1H), 7.69–7.60
(
m, 2H), 7.37 (ddd, J=1.3, 5.5, 6.9 Hz, 1H), 7.14 (d, J=8.8 Hz, 1H),
6
1
.98–6.94 (m, 1H), 6.89 (ddd, J=1.5, 5.5, 7.3 Hz, 1H), 6.77 (ddd, J=0.7,
1
3
.6, 5.7 Hz, 1H), 6.59 ppm (ddd, J=1.1, 5.7, 6.8 Hz, 1H); C NMR
CN): d=161.07, 159.70, 158.60, 156.18, 153.69, 152.52,
50.83, 140.96, 139.24, 138.69, 137.16, 136.57, 128.57, 126.68, 126.59,
25.39, 124.53, 120.37, 110.34, 35.45 ppm; HRMS (ES): m/z: calcd for
Experimental Section
(75 MHz, CD
3
1
1
Materials and general methods: The following compounds were prepared
+
+
[
6]
[9]
[10]
[8]
[12]
C H N ·C H N ·Ru·PF : 692.0702 [MꢀPF ] ; found: 692.0692.
[{Ru
as previously described: 21 (from 22) and 26, 19, 18,
24, 14,
]. The following reagents were purchased from
commercial sources: RuCl (Aldrich, Avocado), terpy (Aldrich, Avoca-
do), 2,5-dimethylpyrazine (Aldrich), benzaldehyde (Aldrich), benzoic an-
hydride (Aldrich). The 400 MHz H and 100 MHz C NMR spectra were
recorded on a Bruker Ultrashield Avance 400 spectrometer and the
12 12
4
15 11
3
6
6
[
11]
[15]
1
5, and [Ru
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(terpy)Cl
3
A
H
U
G
R
N
U
G
2
(8)]
A
H
U
T
E
N
N
6 4
] ACHUTGNENRNUG
was added to [Ru
ligand 8 (7 mg, 0.02 mmol, 1 equiv). The mixture was heated under reflux
for 19 h, then cooled to room temperature and filtered. Excess aqueous
A
H
U
T
E
N
N
3
3
1
13
NH
4
PF
6
was added to the solution and the precipitate was collected. The
to afford
2d (20 mg, 62%). Brown solid; m.p. >3008C; H NMR (300 MHz,
CD CN): d=8.71 (d, J=8.1 Hz, 4H; T ), 8.53 (s, 2H; U), 8.51–8.41 (m,
6H; T +T ), 8.04–7.96 (m, 8H; T +T ), 7.67–7.60 (m, 4H; P+Q), 7.31
(ddd, J=1.3, 5.7, 7.1, 4H; T ), 6.88 (ddd, J=2.3, 5.7, 6.8 Hz, 2H; R),
6.78–6.74 (m, 2H; S), 6.38 (s, 2H; K), 3.61 ppm (s, 6H; J); C NMR
(75 MHz, CD CN): d=160.51, 158.66, 156.25, 153.90, 152.53, 152.01,
139.76, 139.35, 138.16, 137.76, 130.53, 128.50, 127.07, 126.93, 125.67,
125.03, 35.68 ppm; MS (ES): m/z: calcd for
·2Ru·3PF ] ; found: 1451.0815;
: 1451.0591 [MꢀPF
·2Ru·PF
387.0432 [Mꢀ3PF
1
13
3
3
00 MHz H and 75 MHz C NMR spectra were recorded on a Bruker
solid was purified by recrystallization from acetonitrile/CHCl
3
1
00 spectrometer. The solvent residual signal was used as an internal ref-
erence for both H (CD
C NMR (CD
spectra. The following notation is used for the H NMR spectral splitting
patterns: singlet (s), doublet (d), triplet (t), multiplet (m). The following
1
3
CN, d=1.94 ppm; CDCl
CN, d=118.26 ppm, CN group; CDCl
3
, d=7.26 ppm) and
, d=77.00 ppm)
3
5
1
3
3
3
4
6
1
3
1
2
1
3
2
D-NMR experiments were used: COSY, NOESY, and ROESY; they
3
were carried out on 300 MHz or 500 MHz Bruker spectrometers. FAB-
MS, EIMS, and ES-MS measurements were performed by the Service de
Spectromꢀtrie de Masse, Universitꢀ Louis Pasteur. Melting points were
recorded on a Bꢄchi B-540 melting point apparatus and are uncorrected.
+
+
C
18
H
18
N
8
·2C15
H
11
N
3
6
6
3
+
3+
m/z: calcd for
C
18
H
18
N
8
·2C15
H
11
N
3
6
:
6
] ;
found: 387.0543.
Absorption spectra were recorded on a JASCO V570 spectrophotometer.
For luminescence spectra at wavelengths shorter than 900 nm, a Jobin
Yvon-Spex Fluoromax 2 spectrofluorimeter was used, equipped with a
Hamamatsu R3896 photomultiplier, and the spectra were corrected for
[{Ru
was added to [Ru
ligand 9a (7.2 mg, 0.015 mmol, 1 equiv). The mixture was heated under
A
H
U
G
R
N
N
(terpy)}
3
A
H
U
G
E
N
N
(9a)]
A
H
N
T
E
U
G
6 6
] ACHUTGNENRNUG
A
H
U
G
R
N
G
3
5656
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
FULL PAPER
Figure 13. Structural formulae of new complexes 1, 2d, 3a,b, 4a,b, and 6a,b and identification of NMR sites.
reflux for 19 h, then cooled to room temperature and filtered. Excess
aqueous NH PF was added to the solution and the precipitate was col-
lected. The solid was purified by recrystallization from acetonitrile/
CHCl
to afford 3a (23 mg, ꢁ65%). Green solid; m.p. >3008C;
H NMR (300 MHz, CD CN): d=8.76 (d, J=8,1 Hz, 2H; 1T ), 8.67 (d,
J=8.3 Hz, 2H; 1T ), 8.66 (d, J=8.3 Hz, 2H; 1T ), 8.60–8.30 (m, 9H;
), 8.51 (s, 1H; U), 8.26 (s, 1H; F), 8.07 (s, 1H; I), 8.03–7.90 (m,
+1T ), 7.85 (dd, J=0.8, 5.5 Hz, 2H; 1T ), 7.72 (d, J=5.1 Hz,
), 7.68–7.61 (m, 3H; C+P+Q), 7.30–7.11 (m, 7H; D+3T ),
[{Ru
A
H
U
G
R
N
N
(terpy)}
3
A
T
N
T
N
U
G
A
H
U
G
R
N
U
G
6 6
] ACHUTGNENRNUG
4
6
was added to [Ru
ligand 9b (7.2 mg, 0.015 mmol, 1 equiv). The mixture was heated under
reflux for 19 h, then cooled to room temperature and filtered. Excess
aqueous NH
lected. The solid was purified by recrystallization from acetonitrile/
CHCl
(400 MHz, CD
7.8 Hz, 2H), 8.51–8.42 (m, 7H), 8.35–8.25 (m, 5H), 8.22 (s, 1H), 7.98–
7.79 (m, 9H), 7.69–7.63 (m, 2H), 7.60–7.56 (m, 2H), 7.25–7.07 (m, 11H),
6.93–6.89 (m, 2H), 6.79–6.76 (m, 1H), 6.67–6.62 (m, 1H), 6.47 (s, 1H),
5.04 (s, 1H), 4.13 (s, 3H), 4.00 (s, 3H), 3.95 ppm (s, 3H); C NMR
(75 MHz, CD CN): d=163.90, 163.03, 159.83, 159.21, 158.38, 158.19,
3
157.93, 157.70, 155.96, 155.77, 155.47, 155.44, 155.43, 154.80, 154.69,
153.91, 152.89, 150.76, 148.00, 146.60, 141.95, 141.85, 140.18, 139.95,
139.61, 139.54, 138.95, 138.85, 138.23, 137.58, 132.59, 128.66, 128.63,
128.57, 128.48, 127.84, 125.63, 125.29, 125.16, 125.08, 124.62, 124.07,
A
T
N
T
E
N
N
3
3
1
3
5
4 6
PF was added to the solution and the precipitate was col-
5
5
1
3
8
2
7
6
3
1
1
1
1
1
T
4
+3T
H; 3T
H; 1T
6
3
to afford 3b (24 mg, 68%). Green solid; m.p. >3008C; H NMR
CN): d=8.91 (s, 1H), 8.70 (d, J=8.2 Hz, 2H), 8.59 (d, J=
3
1
1
1
2
.03 (s, 1H; H), 6.90–6.84 (m, 2H; G+R), 6.78–6.73 (m, 2H; A+S),
.65–6.59 (m, 1H; B), 6.361 (s, 1H; K), 6.356 (s, 1H; L), 3.95 (s, 3H; E),
1
3
13
.60 (s, 3H; M), 3.48 ppm (s, 3H; J); C NMR (75 MHz, CD
3
CN): d=
58.18, 157.60, 156.44, 156.19, 155.76, 155.44, 154.84, 154.65, 153.81,
53.19, 152.04, 151.68, 150.76, 146.57, 145.76, 141.91, 140.18, 139.92,
39.79, 139.47, 139.42, 138.83, 138.22, 133.80, 132.91, 131.32, 130.27,
28.52, 128.46, 127.20, 127.07, 125.80, 125.71, 125.58, 125.45, 125.02,
21.49, 111.01, 35.86, 35.73 ppm.
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5657

S. Campagna, J.-M. Lehn et al.
1
C
21.70, 111.16, 88.79, 35.81, 35.62, 35.53 ppm; HRMS (ES): m/z: calcd for
156.77, 156.26, 155.71, 155.56, 155.40, 154.78, 154.73, 154.65, 152.42,
152.20, 150.74, 146.75, 146.02, 145.77, 141.93, 140.21, 139.92, 139.58,
3
+
ꢀ 3+
69
H
57
F
18
N
21
P
3
Ru
(10a)]
mL) was added to [Ru
3
: 639.706 [Mꢀ3PF
[PF (4a): An ethanol/water mixture (1:1, v/v;
(terpy)Cl ] (25 mg, 0.057 mmol, 4.8 equiv) and
6
] ; found: 639.706.
1
1
39.51, 138.85, 134.39, 132.91, 131.17, 131.10, 128.53, 128.47, 128.43,
25.83, 125.58, 125.42, 125.02, 121.52, 111.03, 35.98, 35.90, 35.75 ppm; MS
[
{Ru
A( terpy)}
C
H
T
U
N
G
T
R
E
N
N
U
N
G
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
6 8
] ACHUTGNENRNUG
7
A
H
U
G
R
N
N
3
3
+
(
ES): m/z: calcd for
C
42
H
42
N
24·6C15
H
11
N
3
·6Ru·9PF
found: 1398.037; elemental analysis calcd (%) for
O: C 32.01, H 2.93, N 11.88; found: C 31.75, H 2.93, N 11.95.
(terpy)} (12b)][PF (6b): An ethanol/water mixture (1:1, v/v;
5 mL) was added to [Ru(terpy)Cl ] (25 mg, 0.057 mmol, 7.1 equiv) and
6
:
1398.028
free ligand 10a (7.3 mg, 0.012 mmol, 1 equiv). The mixture was heated
under reflux for 19 h, then cooled to room temperature and filtered.
3
+
[
6
Mꢀ3PF
6
] ;
a·18H
2
Excess aqueous NH
was collected. The solid was purified by recrystallization from acetoni-
trile/CHCl
to afford 4a (21 mg, ꢁ57%). Green solid; m.p. >3008C;
H NMR (300 MHz, CD CN): d=8.72 (d, J=7.9 Hz, 4H; 1T ), 8.67 (d,
J=8.1 Hz, 4H; 1T ), 8.56–8.38 (m, 12H; 2T +2T ), 8.27 (s, 2H; F), 8.07
s, 2H; I), 8.00–7.89 (m, 8H; 2T ), 7.85 (dd, J=0.7, 5.7 Hz, 4H; 1T ),
), 7.64 (ddd, J=1.7, 7.4, 8.9 Hz, 2H; C),
.22 (ddd, J=1.1, 5.5, 7.5 Hz, 4H; 1T ), 7.20–7.10 (m, 6H; 1T +D), 7.05
4 6
PF was added to the solution and the precipitate
[{Ru
A
H
U
G
R
N
N
6
A
H
U
G
R
N
U
G
A
H
N
T
E
N
N
6 12
] ACHUTGNENRNUG
A
H
U
G
R
N
U
G
3
3
1
free ligand 12b (7 mg, 0.008 mmol, 1 equiv). The mixture was heated
under reflux for 19 h, then cooled to room temperature and filtered.
3
5
5
6
4
(
7
7
3
1
Excess aqueous NH
was collected. The solid was purified by recrystallization from acetoni-
trile/CHCl to afford 6b (12 mg, 38%). Green solid; m.p. >3008C;
H NMR (400 MHz, CD CN): d=8.69 (d, J=8.3 Hz, 4H), 8.54 (d, J=
4 6
PF was added to the solution and the precipitate
.68 (dd, J=0.8, 5.5 Hz, 4H; 1T
1
2
2
3
1
(
6
d, J=0.7 Hz, 2H; H), 6.87 (d, J=0.7 Hz, 2H; G), 6.77–6.73 (m, 2H; A),
.64–6.59 (m, 2H; B), 6.35 (s, 2H; K), 3.94 (s, 6H; E), 3.48 ppm (s, 6H;
3
7.84, 8H), 8.50–8.40 (m,12H), 8.38 (s, 2H), 8.30–8.25 (m, 8H), 8.22 (s,
2H), 7.93 (t, J=7.9 Hz, 4H), 7.98–7.78 (m, 12H), 7.64 (t, J=7 Hz, 2H),
7.54 (d, J=5.4 Hz, 4H), 7.47 (d, J=5 Hz, 4H), 7.24–7.10 (m, 10H), 7.08–
6.97 (m, 10H), 6.76 (d, J=5.8 Hz, 2H), 6.63 (t, J=6.2 Hz, 2H), 6.49 (s,
2H), 5.07 (s, 2H), 3.98 (s, 6H), 3.96 (s, 6H), 3.94 ppm (s, 6H).
1
3
J); C NMR (75 MHz, CD
1
1
1
3
CN): d=159.21, 158.30, 158.16, 157.76,
56.27, 155.71, 155.41, 154.74, 154.65, 152.33, 150.74, 146.71, 145.76,
41.91, 140.21, 139.92, 139.51, 138.85, 134.31, 132.92, 131.07, 128.53,
28.48, 125.84, 125.60, 125.43, 125.04, 121.51, 111.04, 35.95, 35.76 ppm;
3
+
HRMS (ES): m/z: calcd for C30
H
30
N
16·4C15H
11
N
3
·4Ru 5PF
6
: 892.7024
Pyridine-2-carboxaldehyde (pyrazine-2,5-diyl)dihydrazone (8): A solution
of 17 (10 mg, 0.059 mmol, 1 equiv) and 25 (19.1 mg, 3 equiv) in EtOH
(5 mL) was heated to reflux for 3 h. Then the mixture was cooled and fil-
3
+
[
Mꢀ3PF
{Ru(terpy)}
mL) was added to [Ru
6
]
; found: 892.7197.
(10b)][PF (4b): An ethanol/water mixture (1:1, v/v;
(terpy)Cl ] (23 mg, 0.052 mmol, 4.3 equiv) and
[
6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
6 8
] ACHUTGNENRNUG
A
H
U
G
R
N
N
3
tered. The precipitate was washed with EtOH and dried for 10 h under
1
high vacuum; this gave
400 MHz, CDCl
.01 (d, J=8.2 Hz, 2H), 7.74 (s, 2H), 7.70 (td, J=1.8, 7.9 Hz, 2H), 7.19
8
(16 mg, ꢁ78%). Yellow solid; H NMR
free ligand 10b (7.2 mg, 0.012 mmol, 1 equiv). The mixture was heated
under reflux for 19 h, then cooled to room temperature and filtered.
(
3
): d=8.69 (s, 2H), 8.56 (ddd, J=0.9, 1.4, 4.6 Hz, 2H),
8
Excess aqueous NH
was collected. The solid was purified by recrystallization from acetoni-
trile/CHCl
to afford 4b (24 mg, ꢁ 66%). Green solid; m.p. >3008C;
H NMR (400 MHz, CD CN): d=8.69 (d, J=8.3 Hz, 4H), 8.56 (d, J=
4 6
PF was added to the solution and the precipitate
1
3
(
ddd, J=1.2, 5, 7.3 Hz, 2H), 3.66 ppm (s, 6H); C NMR (100 MHz,
CDCl ): d=155.25, 149.22, 148.13, 136.31, 134.57, 128.97, 122.45, 119.18,
29.90 ppm; HRMS (ES): m/z: calcd for C H N +Li : 353.1809
3
3
+
1
18 18
8
3
+
[
M+Li] ; found: 353.1809.
Pyrazine-2,5-dicarboxaldehyde methyl{5-[1-methyl-2-(pyridin-2-ylmethyl-
ene)hydrazino]pyrazin-2-yl}hydrazone methyl(pyridin-2-yl)hydrazone
(9a): solution of (15 mg, 0.038 mmol, 1 equiv) and 25 (5 mg,
1.2 equiv) in CHCl (10 mL) was heated overnight at reflux. Then the so-
lution was concentrated to 2 mL, and filtered, and the precipitate was
washed with CHCl and EtOH and dried under vacuum for 10 h to
afford 9a (10 mg, 54%) as a yellow solid. Its low solubility did not allow
8
7
.3 Hz, 4H), 8.50–8.40 (m, 10H), 8.29 (d, J=7.9, 4H), 8.22 (s, 2H), 7.98–
.80 (m, 12H), 7.64 (t, J=7.9 Hz, 2H), 7.55 (d, J=5.4 Hz, 4H), 7.25–7.17
(
m, 6H), 7.17 (d, J=8.3 Hz, 2H), 7.10–7.04 (m, 6H), 6.76 (d, J=5.4 Hz,
H), 6.63 (t, J=6.6 Hz, 2H), 6.48 (s, 1H), 5.08 (s, 1H), 3.98 (s, 6H),
.94 ppm (s, 6H); C NMR (75 MHz, CD
ACHTUNGTRENNUNG
2
3
1
1
1
3
1
A
5
1
3
3
CN): d=163.41, 159.17,
3
58.16, 157.63, 155.62, 155.44, 155.40, 154.79, 154.68, 150.73, 148.13,
46.56, 141.93, 140.16, 139.93, 139.51, 138.99, 138.95, 138.01, 132.58,
28.64, 128.54, 125.61, 125.27, 125.07, 124.70, 121.70, 111.15, 89.35, 35.81,
3
2
+
+
5.66 ppm; MS (ES): m/z: calcd for C30
H
30
N
16·4C15H
11
N
3
·4Ru·6PF
6
:
the recording of NMR spectra. HRMS (ES): m/z: calcd for C24
487.2402 [M+Li] ; found: 487.2391.
H
24
N
12Li :
2
+
+
411.036 [Mꢀ2PF
6
]
; found: 1411.048.
Precursor 5: A solution of 17 (50 mg, 0.297 mmol, 2.23 equiv) in EtOH
30 mL) was added to a solution of 13 (32 mg, 0.133 mmol, 1 equiv) in
Pyrazine-2,5-dicarboxaldehyde methyl{6-[1-methyl-2-(pyridin-2-ylmethyl-
ACHTUNGTNEReNUNG ne)hydrazino]pyrimidin-4-yl}hydrazone methyl(pyridin-2-yl)hydrazone
(
EtOH (250 mL). The mixture was stirred overnight at room temperature.
Then the solution was concentrated to 20 mL, and filtered, and the pre-
(9b): A solution of 13 (10 mg, 0.041 mmol, 1 equiv) and 26 (11 mg,
1 equiv) in ethanol (30 mL) was heated overnight at reflux. Then the so-
lution was concentrated to 15 mL, and filtered, and the precipitate was
cipitate was washed with EtOH and dried under vacuum for 10 h to
1
afford 5 (27 mg, 52%). Yellow solid; H NMR (300 MHz, CDCl
3
): d=
washed with EtOH and dried under vacuum for 10 h to afford 9b (15 mg,
1
9
8
4
.14 (d, J=1.3 Hz, 1H), 9.12 (d, J=1.3 Hz, 1H), 8.63 ( d, J=1.7 Hz, 1H),
.26 (ddd, J=0.8, 1.9, 4.9 Hz, 1H), 8.17 (d, J=1.5 Hz, 1H), 7.78–7.62 (m,
H), 6.87 (ddd, J=1.1, 5.1, 7 Hz, 1H), 3.73 (d, J=0.8 Hz, 3H), 3.64 (d,
75%). Yellow solid; H NMR (300 MHz, CDCl
3
): d=9.33 (d, J=1.3 Hz,
1H), 9.20 (d, J=1.3 Hz, 1H), 8.62 (ddd, J=0.8, 1.7, 4.9 Hz, 1H), 8.48 (d,
J=0.9, 1H), 8.28 (ddd, J=0.8, 1.9, 4.9 Hz, 1H), 8.23 (dt, J=0.9, 8.1 Hz,
1H), 8.01–7.94 (m, 2H), 7.88 (s, 1H), 7.85 (s, 1H), 7.80–7.75 (m, 2H),
7.67 (ddd, J=1.9, 7.1, 8.9 Hz, 1H), 7.28 (ddd, J=1.1, 4.9, 7.5 Hz, 1H),
1
3
J=0.8, 3H), 3.27 ppm (s, 3H); C NMR (75 MHz, CDCl
1
1
3
): d=157.15,
53.36, 148.58, 148.14, 147.04, 146.06, 140.63, 140.53, 137.75, 132.22,
31.04, 129.76, 127.05, 116.63, 110.21, 41.98, 30.14, 20.70 ppm; HRMS
6.89 (ddd, J=1.1, 4.9, 7 Hz), 3.77 (d, J=0.8, 3H), 3.72 (d, J=0.6 Hz,
+
+
13
(
ES): m/z: calcd for C18
{Ru(terpy)} (12a)][PF
mL) was added to [Ru
H
21
N
11Na : 414.187 [M+Na] ; found: 414.191.
(6a): An ethanol/water mixture (1:1, v/v;
(terpy)Cl ] (25 mg, 0.057 mmol, 7.1 equiv) and
3H), 3.70 ppm (d, J=0.6 Hz, 3H); C NMR (75 MHz, CDCl
3
): d=
1
62.91, 162.53, 157.09, 156.88, 154.88, 149.33, 149.21, 147.96, 147.10,
[
5
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
6
]
12
ACHTUNGTRENNUNG
1
41.13, 140.75, 137.80, 137.20, 136.89, 134.64, 131.98, 123.22, 119.76,
A
H
U
G
R
N
U
G
3
1
16.83, 110.24, 89.10, 29.81, 29.64, 29.58 ppm; HRMS (ES): m/z: calcd for
free ligand 12a (7 mg, 0.008 mmol, 1 equiv). The mixture was heated
under reflux for 19 h, then cooled to room temperature and filtered.
+
+
C
24
H
24
N
12 +H : 481.2320 [M+H] ; found: 481.2314.
2,2’-bis[methyl(pyridin-2-yl)hydrazone]
,5’-[(pyrazine-2,5-diyl)bis(methylhydrazone)] (10a): A solution of 25
29 mg, 0.120 mmol, 2 equiv) and 17 (10 mg, 1 equiv) in CHCl (30 mL)
was heated overnight at reflux. Then the solution was concentrated to
5 mL, and filtered, and the precipitate was washed with CHCl and
Excess aqueous NH
was collected. The solid was purified by recrystallization from acetoni-
trile/CHCl
to afford 6a (16 mg, ꢁ51%). Green solid; m.p. >3008C;
H NMR (300 MHz, CD CN): d=8.71 (d, J=8.2 Hz, 4H; 1T ), 8.65 (d,
J=8.2 Hz, 8H; 2T ), 8.55–8.35 (m, 18H; 3T +3T ), 8.24 (s, 2H; F), 8.03
s, 2H; I), 8.01 (s, 2H; N), 7.97–7.88 (m, 12H; 3T ), 7.83 (dd, J=0.6,
), 7.21 (ddd, J=1.2, 5.6,
), 7.02 (s, 2H; H), 6.84 (s,
4
PF
6
was added to the solution and the precipitate
Pyrazine-2,5-dicarboxaldehyde
ACHTUNGTRENNUNG
5
(
3
3
1
3
5
1
3
5
6
4
dried under vacuum for 10 h to afford 10a (35 mg, ꢁ 96%) as a yellow
(
3
solid. Its low solubility did not allow the recording of NMR spectra.
5
7
2
2
.6 Hz, 4H; 1T
1 1
), 7.69–7.58 (m, 10H; C+2T
Hz, 4H; 1T ), 7.15–7.05 (m, 10H; D+2T
2
2
Pyrazine-2,5-dicarboxaldehyde
2,2’-bis AHCNUTGTERNNUN[G methyl(pyridin-2-yl)hydrazone]
H; G), 6.74 (dd, J=1.2, 5.6 Hz, 2H; A), 6.72 (s, 2H; O), 6.64–6.59 (m,
H; B), 6.335 and 6.328 (s, 2H+2H; K+L), 3.93 (s, 6H; E), 3.44 ppm
5,5’-[(pyrimidine-4,6-diyl)bis(methylhydrazone)] (10b): A solution of 25
(20 mg, 0.083 mmol, 2 equiv) and 21 (7 mg, 1 equiv) in ethanol (30 mL)
was heated overnight at reflux. Then the solution was concentrated to
1
3
(
3
s, 12H; J+M); C NMR (75 MHz, CD CN): d=158.20, 158.16, 157.79,
5658
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 5645 – 5660

Multinuclear Metallosupramolecular Ligands
FULL PAPER
1
5 mL, and filtered, and the precipitate was washed with EtOH and
Crystallographic data for 1: formula [Ru
; M =6.53; crystal system: monoclinic; space group: Cc
(No. 9), a=12.721(12), b=12.329(12), c=20.29(2) ꢃ; a=90, b=101.0(2),
12 4 11 3 6 2
ACHTUNGTRENNUNG( C12H N ) ACHTUNGTREN(GNU C15H N )] ACHTUNGTRENNUNG[ PF ] :
dried under vacuum for 10 h to afford 10b (20 mg, ꢁ78%). Its low solu-
C
27
H
23
N
7
RuF12
P
2
r
bility did not allow the recording of NMR spectra. Yellow solid; MS
+
+
3
ꢀ3
ꢀ1
(
ES): m/z: calcd for C30
Precursor 11: A solution of 21 (70 mg, 0.417 mmol, 2 equiv) in EtOH
30 mL) was added to a solution of 13 (50 mg, 0.207 mmol, 1 equiv) in
H
30
N
16 +H : 615.2912 [M+H] ; found: 615.2914.
g=908; V=3124(6) ꢃ ; Z=4; 1calcd =1.779 gcm ; m
(000)=1664; crystal size 0.04ꢅ0.05ꢅ0.06 mm; temperature 173 K;
MoKa radiation l=0.71073 ꢃ; 2.3ꢂqꢂ41.08; dataset: ꢀ19ꢂhꢂ19;
19ꢂkꢂ19; ꢀ30ꢂlꢂ31; total unique data, R(int)=22766, 12252, 0.069;
observed data [I>2.0s(I)]: 9238; ref =12252, par =431; R=0.0858,
ACHTUNGTNERNU(GN MoKa)=0.709 mm ;
FACHTUNGTRENNUNG
(
ꢀ
ACHTUNGTRENNUNG
EtOH (250 mL). The mixture was stirred overnight at room temperature.
Then the solution was concentrated to 20 mL, and filtered, and the pre-
cipitate was washed with EtOH and dried under vacuum for 10 h to
afford 11 (50 mg, ꢁ62%). Yellow solid; H NMR (300 MHz, CDCl
N
N
wR2=0.2333, S=1.09; flack x=0.02(5); minimum and maximum residual
density: ꢀ2.35, 1.65 eꢃꢀ
3
.
1
3
): d=
9
8
1
1
0
.20 (d, J=1.5 Hz, 1H), 9.17 (d, J=1.5 Hz, 1H), 8.35 (d, 0.9 Hz, 1H),
Crystallographic data of 2: formula [Ru
2
A
H
U
G
R
N
U
G
H
18
N
8
)
A
T
N
T
E
N
N
(C15
H
11
N
3
)
2
]-
.27 (ddd, J=0.9, 1.9, 4.9 Hz, 1H), 7.80 (s, 1H), 7.76 (dt, J=1, 8.7 Hz,
H), 7.72 (s, 1H), 7.66 (ddd, J=1.9, 7, 8.9 Hz, 1H), 7.07 (d, J=0.9 Hz,
H), 6.88 (ddd, J=1.1, 4.9, 7 Hz, 1H), 4.12 (s, br, 2H), 3.73 (d, J=
ACHUTNGRENNUG[ PF ] ·4CH CN: C H F N P Ru ; formula weight: 1759.18; crystal
6 4 3 56 52 24 18 4 2
system: monoclinic; space group: C2/c (No. 15), a=18.3090(4), b=
25.9760(6), c=16.7000(4) ꢃ; a=90, b=122.0311(13), g=908; V=
1
3
3
ꢀ3
ꢀ1
.6 Hz, 3H), 3.68 (d, J=0.6 Hz, 3H), 3.67 ppm (s, 3H); C NMR
75 MHz, CDCl ): d=164.95, 162.02, 157.05, 156.90, 148.93, 147.96,
47.08, 140.95, 140.83, 137.80, 134.18, 131.80, 116.84, 110.26, 84.96, 39.96,
6733.3(3) ꢃ ; Z=4; 1calcd =1.735 gcm ; m AHCTUNGTERNNUNG( MoKa)=0.664 mm ; F ACHTUNGTRENNUNG( 000)=
(
3
3512; crystal size 0.08ꢅ0.10ꢅ0.12 mm; temperature 173 K; MoKa radia-
tion l=0.71073 ꢃ; 2.6ꢂqꢂ30.08; dataset: ꢀ25ꢂhꢂ25; ꢀ36ꢂkꢂ33;
1
2
+
9.77, 29.74 ppm; HRMS (ES): m/z: calcd for C18
H
22
N
11 +H : 392.2054
ꢀ23ꢂlꢂ23; total unique data, R
ACHTUNGTRENUNNG( int)=17098, 9781, 0.045; observed data
+
[
M+H] ; found: 392.2043.
Ligand 12a: A solution of 5 (22 mg, 0.056 mmol, 1 equiv) and 14 (3.8 mg,
.023 mmol, 0.4 equiv) in CHCl (30 mL) was heated overnight at reflux.
Then the solution was concentrated to 15 mL, and filtered, and the pre-
cipitate was washed with CHCl and dried under vacuum for 10 h to
[I>2.0s(I)]: 7156; Nref =9781, Npar =470; R=0.0531, wR2=0.1472, S=
3
1
.02; minimum and maximum residual density: ꢀ0.78, 0.97 eꢃꢀ
.
0
3
CCDC-710685 (1) and CCDC-710684 (2) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
3
afford 12a (22 mg, ꢁ55%). Its low solubility did not allow the recording
+
of NMR spectra. Yellow solid; HRMS (ES): m/z: calcd for C42
H
42LiN24
:
+
8
89.418 [M+Li] ; found: 889.421.
Ligand 12b: A solution of 11 (30 mg, 0.077 mmol, 2 equiv) and 14 (5 mg,
.037 mmol, 1 equiv) in CHCl (30 mL) was heated overnight at reflux.
Then the solution was concentrated to 15 mL, and filtered, and the pre-
cipitate was washed with CHCl and dried under vacuum for 10 h to
0
3
Acknowledgements
3
We acknowledge Andrꢀ de Cian and Nathalie Kyritsakas for recording
the data and solving the structure of complex 2. We acknowledge the Eu-
ropean Synchrotron Radiation Facility for provision of synchrotron radia-
tion facilities and we thank Dr. Gavin Vaughan for recording the data of
complex 1 (ESRF experiment CH-2474, beamline ID11). We wish also to
thank Lionel Allouche for DOSY NMR, Patrick Wehrung, Dr. Ray-
monde Baltenweck-Guyot, and Romain Carriꢆre for mass spectrometry
analyses, Dr. Juan Ramirez for helpful discussions, Prof. Frꢀdꢀrique Loi-
seau for preliminary redox experiments, and Prof. Alberto Juris for assis-
tance in performing near-infrared luminescence experiments.
afford 12b (23 mg, ꢁ44%). Its low solubility did not allow the recording
+
of NMR spectra. Yellow solid; MS (ES): m/z: calcd for C42
H
42
N
24 +H :
+
+
8
[
83.4097 [M+H] ; found: 883.4056; calcd for C42H N24 +Na : 905.3916
42
+
M+Na] ; found: 905.3896.
Pyrazine-2,5-dicarboxaldehyde methyl(pyridin-2-yl)hydrazone (13): A so-
lution of 24 (130 mg, 1.057 mmol, 0.96 equiv) in EtOH (30 mL) was
added to a solution of 14 (150 mg, 1.103 mmol, 1 equiv) in EtOH
(
250 mL). The mixture was stirred overnight at room temperature. Then
the solution was concentrated to 30 mL and filtered. The solvent of the
filtrate was evaporated under vacuum and the solid thus obtained was
purified by flash chromatography (Al
(100 mg, 0.415 mmol, 43%). Yellow solid; H NMR (400 MHz,
CN): d=10.15 (s, 1H), 9.38 (d, J=1.8 Hz, 1H), 9.06 (d, J=1.2 Hz,
H), 8.31–8.28 (m, 1H), 7.78–7.75 (m, 1H), 7.74 (s, 1H), 7.70 (ddd, J=
2 3 3
O , CHCl /pentane 8:2) to afford
1
1
CD
1
1
3
[
[
[
1] a) J.-M. Lehn, Supramolecular Chemistry—Concepts and Perspec-
tives, VCH, Weinheim, 1995; b) M. Ruben, J. Rojo, F. J. Romero-
Salguero, L. H. Uppadine, J.-M. Lehn, Angew. Chem. 2004, 116,
3
1
3
.8, 6.7, 8.5 Hz, 1H), 6.97–6.92 m, 1H), 3.76 ppm (s, 3H); C NMR
): d=192.12, 156.67, 154.09, 147.21, 144.46, 143.01,
3
728; Angew. Chem. Int. Ed. 2004, 43, 3644, and references therein.
(
75 MHz, CDCl
3
2] a) A.-M. Stadler, F. Puntoriero, S. Campagna, N. Kyritsakas, R.
Welter, J.-M. Lehn, Chem. Eur. J. 2005, 11, 3997; b) F. Loiseau, F.
Nastasi, A.-M. Stadler, S. Campagna, J.-M. Lehn, Angew. Chem.
1
C
41.81, 137.98, 130.79, 117.70, 110.55, 30.12 ppm; MS (ES): m/z: calcd for
+
+
12
12 5
H N O+H : 242.1036 [M+H] ; found: 242.1048.
2
,5-Bis(1-methylhydrazino)pyrazine (17): Under magnetic stirring, 18
2
007, 119, 6256; Angew. Chem. Int. Ed. 2007, 46, 6144.
3] a) G. S. Hanan, C. R. Arana, J.-M. Lehn, D. Fenske, Angew. Chem.
995, 107, 1191; Angew. Chem. Int. Ed. 1995, 34, 1122; b) G. S.
Hanan, C. R. Arana, J.-M. Lehn, G. Baum, D. Fenske, Chem. Eur. J.
996, 2, 1292; c) B. Hasenknopf, J. Hall, J.-M. Lehn, V. Balzani, A.
(
(
300 mg) was slowly added in portions to ice-cooled methylhydrazine
3 mL). The mixture was heated to reflux for 4 days under argon. After
1
cooling of the mixture, methylhydrazine was evaporated, K
and CHCl (40 mL) were added to the solid residue, and the mixture was
stirred for 10 min. The liquid phase was filtered. The solid–liquid extrac-
tion procedure was repeated 3 times with CHCl (without adding more
CO ), and the combined liquid fraction was evaporated. Filtration on
alumina (CHCl ), followed by partial evaporation and precipitation with
diethyl ether afforded 17 (50 mg, 23%). Brown solid, air-unstable;
2 3
CO (0.3 g)
3
1
Credi, S. Campagna, New J. Chem. 1996, 20, 725; d) P. Ceroni, A.
Credi, V. Balzani, S. Campagna, G. S. Hanan, C. R. Arana, J.-M.
Lehn, Eur. J. Inorg. Chem. 1999, 1409; e) D. Brown, S. Muranjan, Y.
Jang, R. Thummel, Org. Lett. 2002, 4, 1253; f) D. Brown, R. Zong,
3
K
2
3
3
II
R. P. Thummel, Eur. J. Inorg. Chem. 2004, 3269; g) For related Ru
1
H NMR (400 MHz, CDCl
H); C NMR (100 MHz, CDCl ): d=142.84, 131.95, 40.75 ppm; MS
3
): d=8.09 (s, 2H), 3.88 (brs, 4H), 3.18 ppm (s,
complexes used for water oxidation, see: R. Zong, R. P. Thummel, J.
Am. Chem. Soc. 2005, 127, 12802.
1
3
6
(
3
+
+
ES): m/z: calcd for C
6
H
12
N
6
: 168.1118 [M] ; found: 168.1071.
[
[
4] M. M. Castano-Briones, A. P. Bassett, L. L. Meason, P. R. Ashton,
Z. Pikramenou, Chem. Commun. 2004, 2832.
2
,5-Dibromopyrazine (18): Prepared according to a literature proce-
[
10]
1
13
dure.
H NMR (400 MHz, CDCl
): d=147.45, 139.24 ppm.
-Amino-5-bromopyrazine (19): Prepared according to a literature proce-
3
): d=8.47 ppm (s, 2H); C NMR
5] a) M. Schmittel, V. Kalsani, J. W. Bats, Inorg. Chem. 2005, 44, 4115;
b) V. Kalsani, H. Bodenstedt, D. Fenske, M. Schmittel, Eur. J. Inorg.
Chem. 2005, 1841; c) M. Schmittel, V. Kalsani, R. S. K. Kishore, H.
Cçlfen, J. W. Bats, J. Am. Chem. Soc. 2005, 127, 11544; d) J. Ramꢇr-
ez, A.-M. Stadler, G. Rogez, M. Drillon, J.-M. Lehn, Inorg. Chem.
2009, 48, 2456.
(
100 MHz, CDCl
3
2
[
9]
1
dure.
H NMR (400 MHz, CDCl
3
): d=8.06 (s, 1H), 7.75 (s,1H),
): d=153.46, 144.12,
1
3
4
1
.76 ppm (s, br, 1H); C NMR (100 MHz, CDCl
31.73, 126.94 ppm.
3
Chem. Eur. J. 2010, 16, 5645 – 5660
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5659

S. Campagna, J.-M. Lehn et al.
[
[
[
[
6] J.-L. Schmitt, A.-M. Stadler, N. Kyritsakas, J.-M. Lehn, Helv. Chim.
Acta 2003, 86, 1598.
7] A.-M. Stadler, N. Kyritsakas, R. Graff, J.-M. Lehn, Chem. Eur. J.
way, G. F. Strouse, R. R. Ruminski, T. J. Meyer, Inorg. Chem. 2001,
40, 4508; c) S. M. Draper, D. J. Gregg, E. R. Schofield, W. R.
Browne, M. Duati, J. G. Vos, P. Passaniti, J. Am. Chem. Soc. 2004,
126, 8694; d) S. D. Bergman, D. Gut, M. Kol, C. Sabatini, A. Bar-
bieri, F. Barigelletti, Inorg. Chem. 2005, 44, 7943; e) M. I. J. Polson,
F. Loiseau, S. Campagna, G. S. Hanan, Chem. Commun. 2006, 1301.
[28] Near-infrared emission has been reported for species containing
2
006, 12, 4503.
8] M. A. Baldo, G. Chessa, G. Marangoni, B. Pitteri, Synthesis 1987,
20.
7
9] D. A. De Bie, A. Ostrowica, G. Geurtsen, H. C. Van Der Plas, Tetra-
hedron 1988, 44, 2977.
II
[29a]
[29b]
Ru subunits connected to Os
or Nd units,
but in those cases
[
[
10] R. C. Ellingson, R. L. Henry, J. Am. Chem. Soc. 1949, 71, 2798.
11] M. Hasegawa, Y. Asusuki, F. Susuki, H. Nakanishi, J. Polym. Sci.
Part A-1 1969, 7, 743–752.
the emission was Os-based or Nd-based and the Ru units played the
role of energy-transfer donors.
[29] a) A. Juris, V. Balzani, S. Campagna, G. Denti, S. Serroni, G. Frei,
H. U. Gꢄdel, Inorg. Chem. 1994, 33, 1491; b) C. Giansante, P.
Ceroni, V. Balzani, F. Vçgtle, Angew. Chem. 2008, 120, 5502;
Angew. Chem. Int. Ed. 2008, 47, 5422.
[
[
[
[
[
12] a) R. H. Wiley, US Patent 4260757, 1981; b) R. H. Wiley, J. Macro-
mol. Sci. Part A 1987, 24, 1183.
13] See, for example: a) C. S. Poss, S. L. Schreiber, Acc. Chem. Res.
1
994, 27, 9; b) S. R. Magnuson, Tetrahedron 1995, 51, 2167.
14] K. M. Gardinier, R. G. Khoury, J.-M. Lehn, Chem. Eur. J. 2000, 6,
124.
15] B. P. Sullivan, J. M. Calvert, T. J. Meyer, Inorg. Chem. 1980, 19,
404.
16] a) T. J. Cho, C. N. Moorefield, S.-H. Hwang, P. Wang, L. A. Godꢇnez,
E. Bustos, G. R. Newkome, Eur. J. Org. Chem. 2006, 4193; b) L. Al-
louche, A. Marquis, J.-M. Lehn, Chem. Eur. J. 2006, 12, 7520.
17] F. Puntoriero, S. Campagna, A.-M. Stadler, J.-M. Lehn, Coord.
Chem. Rev. 2008, 252, 2480.
[30] The structure of an emission spectrum is the result of the Franck–
Condon factors for radiative transition decay, which are connected
to the geometrical distortion (DQ) between the ground and excited
4
[
31,32]
states.
A single-mode Franck–Condon analysis of the emission
[
32]
1
spectra,
using Equation (1), allows parameters E
0
, ꢀh w, S
M
, and
Dn1/2 to be obtained.
ꢂꢀ
ꢁ ꢀ ꢁꢀ
ꢂ
ꢀ
ꢁ ꢃꢁꢃ
5
3
x
2
P
E ꢀ x ꢀh w
S
n ꢀ E þ x ꢀh w
0
0
IðnÞ ¼
exp ꢀ4 ln 2
(1)
(in
E
0
x!
Dn1=2
[
[
x¼0
In Equation (1), n˜ is the relative emission intensity at energy n˜
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
18] a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von
Zelewsky, Coord. Chem. Rev. 1988, 84, 85; b) J.-P. Sauvage, J. P.
Collin, J. C. Chambron, S. Guillerez, C. Coudret, V. Balzani, F. Bari-
gelletti, L. De Cola, L. Flamigni, Chem. Rev. 1994, 94, 993; c) V.
Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem. Rev.
ꢀ1
cm ), E
0
is the energy of the zero–zero transition (i.e., the energy
3
of the emitting MLCT state), ꢀh w is the average of the medium fre-
quency acceptor modes coupled to the radiative transition, x is the
quantum number of such an averaged medium frequency mode that
serves as the final vibronic states (x is usually limited to 5), Dn1/2 is
1
996, 96, 759.
the half-width of the individual vibronic bands, and S is the elec-
[
19] Redox splitting can be related to intercomponent electronic cou-
pling only for weakly coupled systems, that is when valence is vibra-
tionally trapped (Classes I and II of the Robin–Day mixed-valence
[32]
tron-vibrational coupling constant (Huang–Rhys factor).
The S
parameter is linked to DQ by Equation (2).
[
20]
systems). For strongly interacting species (Class III), valence de-
localization occurs and the potential separation between first and
second oxidation processes assumes different meanings. To discrimi-
nate between weakly and strongly coupled systems, intervalence
transfer bands of mono-oxidized species, investigated by spectro-
1 Mw
ꢀh
2
S ¼ 2
ðDQÞ
(2)
In Equation (2), M is the reduced mass of the oscillator, and w is
the frequency of the dominant vibrational mode. In general, a
highly structured emission spectrum indicates a low value of S, and
therefore of DQ. For transition-metal complexes, only ligand-cen-
tered (LC) or metal-to-ligand charge-transfer (MLCT) emissions
normally exhibit small values of DQ and can therefore show struc-
ACHTUNGTRENNUNGe lectrochemistry, are quite useful. We performed spectroelectro-
chemistry experiments for 2, and found (see the Supporting Infor-
mation to reference [2b]) that this species exhibits behavior belong-
ing to the transition regime between Class II and Class III, a border-
[
19,24,33]
[
21,22]
tured emission spectra.
line regime defined quite recently.
In other words, in 2, valence
[
[
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[
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[
[
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Received: March 10, 2009
Revised: January 7, 2010
Published online: April 8, 2010
5660
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Chem. Eur. J. 2010, 16, 5645 – 5660