Cation and anion fluorescent and electrochemical sensors derived from 4,4′-substituted biphenyl

Article abstract of DOI:10.1016/j.tet.2005.08.010

Five new fluorescent macrocyclic ligands derived from biphenyl are described. These new compounds can be used in cation and anion recognition and sensing. Their fluorescent and electrochemical properties are studied and the influence of the biphenyl conformation on these properties is considered. The X-ray single crystal structure of one of the described ligands has been determined.

Full text of DOI:10.1016/j.tet.2005.08.010

Tetrahedron 61 (2005) 10309–10320
Cation and anion fluorescent and electrochemical sensors derived
from 4,40-substituted biphenyl
a
a
b
c
Ana M. Costero,a, M. Jose Banuls, M. Jose Aurell, Luis E. Ochando and Antonio Domenech
*
´
´
´
˜
a
´ ´ `
Departamento de Quımica Organica, Universitat de Valencia, Dr. Moliner 50, 46100-Burjassot, Valencia, Spain
´ `
Departamento de Geologıa, Universitat de Valencia, Valencia, Dr. Moliner 50, 46100-Burjassot, Valencia, Spain
b
c
´ ´ `
Departamento de Quımica Analıtica, Universitat de Valencia, Valencia, Dr. Moliner 50, 46100-Burjassot, Valencia, Spain
Received 10 June 2005; revised 2 August 2005; accepted 2 August 2005
Abstract—Five new fluorescent macrocyclic ligands derived from biphenyl are described. These new compounds can be used in cation and
anion recognition and sensing. Their fluorescent and electrochemical properties are studied and the influence of the biphenyl conformation on
these properties is considered. The X-ray single crystal structure of one of the described ligands has been determined.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
used with different goals. Thus, some authors have reported
their use as anion and cation fluorescent sensors due to the
influence of the dihedral angle between both aromatic rings
in their fluorescent behaviour.3 On the other hand the
introduction of different substituents on the biphenyl system
gives rise to strong modifications not only in its fluorescent
but also its electrochemical properties. Recently our
research group has prepared different macrocyclic ligands
directly bound to the 2,20 positions of 4,40-disubstituted
biphenyls to be used as cationic or anionic sensors using
their fluorescent or electrochemical properties.4 These
studies demonstrated that both the size of the cavity and
the flexibility were two important factors not only for
complexation but also for sensing. For this reason we now
report the synthesis, complexation and electrochemical
studies of several new compounds containing different
functional groups and also different sizes in the binding
cavity.
Supramolecular chemistry has evolved to a coherent and
lively body of concepts and has been progressive growing
incorporating novel areas of investigation. Among all these
areas one of the most attractive fields of research is that of
programmed systems in which the recognition process is
further coupled to a specific action. Among several
examples of programmed supramolecular systems we will
focus our research on the field of chemosensing where the
recognition process is coupled to the specific action of
signalling. One of the best approximations towards the
development of chemosensors consists in coupling of two
subunits each one displaying differentiable functions: the
binding site and the signalling subunit. In the former resides
the role of coordination to a certain species, whereas the task
of the later consists in signalling the coordination event via
changes in a physical property. In the approach of ‘binding
site-signalling subunit’ these two subunits are covalently
linked. To use fluorescent signalling subunits gives rise to
fluorescent chemosensors whereas to use of electroactive
systems allow the design of electrochemical chemosensors.1
Most of the chemosensors prepared following this approach
work inducing modifications in the electronic properties of
the signalling unit but there are less examples of
chemosensors based on conformational changes induced
in the complexation process.2 Among these later systems
ligands derived from biphenyl systems have been widely
2. Results and discussion
The compounds studied in this research are reflected in
Chart 1. The synthesis of ligands 1, 2 and 3 is now described
whereas the synthesis of compounds 4, 5 and 6 has been
previously published.5 Preparation of 1 was accomplished
by the method outlined in Scheme 1.
Keywords: Chemosensors; Electrochemistry; Fluorescence; Biphenyl;
Cations; Anions.
Ligand 1 was prepared from 6 by reduction with borane–
tetrahydrofurane. Under these conditions the amide groups
were reduced providing 1 as a yellow solid (Scheme 1).
*
Corresponding author. Tel.: C34 963544410; fax: C34 963543152;
e-mail: ana.costero@uv.es
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.08.010

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A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
diester was reduced with lithium borohydride providing
2,6-bis[(2-hydroxy)ethoxymethyl]pyridine (9).8
2.1. X-ray
X-ray single-crystal studies were performed on compound 1
(Fig. 1). Suitable crystals were obtained by slow evaporation
of a solution of 1 in methanol–hexane. As can be seen in the
structure, ligand 1 presented a diminution of the rigidity in the
structure in comparison with other related compounds.4b,5
This fact leads to an increase of the size inside the cavity.
Chart 1.
Scheme 1.
Preparation of ligands 2 and 3 was carried out as shown in
Scheme 2 by reaction between 2,20-bis(chloromethyl)-4,40-
bis(dimethylamino)biphenyl, whose synthesis was pre-
viously described,6 and the corresponding chains. For the
obtention of these chains different pathways were used.
Thus, 2,6-pyridindicarbaldehyde, 7, was prepared from
2,6-pyridindimethanol employing selenium oxyde
following the procedure described in the literature.7 The
dialdehyde was then treated with N-methyl ethanolamine
and sodium borohydride to give 2,6-bis[N-methyl-N-(2-
hydroxyethyl)aminomethyl]pyridine (8) (Scheme 3).
Figure 1. Molecular structure with crystallographic numbering scheme for
compound 1. For clarity, only an average position of O(26) has been drawn.
This atom is disordered over two sites with occupancies at 50%.
The ester groups were almost coplanar to the biphenyl rings
(O(26A)–C(12)–C(2)–C(17)Z3.08, O(26B)–C(12)–C(2)–
C(17)Z22.08 O(15)–C(22)–C(9)–C(20)Z135.38). The
biphenyl unit shows a dihedral angle almost perpendicular
(Angle between the average planes of each phenyl ringZ
88.68). This fact seems to be related with the substitution in the
pyridine ring, thus, ligands containing amide groups instead
of amine groups present dihedral angles smaller than 908.
In the case of compound 2, 2,6-pyridinedimethanol was
reacted with methyl bromoacetate and then, the formed
On the other hand, methyl groups of amines lie in the same side
of the mean plane defined of the cavity (C(24)–N(5)–C(27)–
C(33)Z174.48, C(40)–N(23)–C(37)–C(35)Z167.58). This fact
is in agreement with the situation of maximum packing required
for the consecution of the single crystal. Stereoscopic view
of crystal packing of the structure is shown in Figure 2.
3. Complexation experiments
3.1. Cation complexation
Scheme 2.
The ability of ligands 1–4 in cation complexation was firstly
studied with Cd2C and Zn2C as their nitrate salts by using
1H NMR techniques. The four compounds were able to form
the corresponding complexes but the observed results
1
depended on the ligand used. Thus, the H NMR spectrum
of the complex formed between ligand 1 and Cd(NO3)2 and
Zn(NO3)2 in CDCl3 suggested that complexation was
Scheme 3.

A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
10311
Figure 2. Stereoscopic view of crystal packing of the structure of compound 1.
1
Table 1. H NMR shifts for the hydrogen in ligand 1 and its complexes with Cd(NO3)2 and Zn(NO3)2 in CDCl3
Ha
Hb
Hc
Hd
He
Hf
Hg
Hh
Hi
Hj
1
7.15
7.06
7.20
6.75
7.05
7.01
6.94
6.86
6.82
7.51
7.80
7.62
7.12
7.17
7.23
3.92
3.35
3.69
2.53, 2.27 3.69, 3.51 2.32
3.90, 2.53 4.83, 4.22 2.10
3.03, 2.73 4.15, 3.88 2.44
2.91
3.02
2.97
1$Cd(NO3)2
1$Zn(NO3)2
carried out with the nitrogen atoms. This proposal agreed
with the shift observed in the NMR signals of Hh (DdZ0.46,
0.37 ppm and 1.14, 0.71 for Zn2C and Cd2C, respectively),
Due to the fluorescent properties of the tetramethylbenzi-
dine moiety, studies of complexation using this technique
were carried out with ligands 1–4 (Fig. 4). These
experiments have allowed to determining not only the
complexation constants but also the 1:1 stoichiometry of the
Hg (DdZ0.50, 0.46 ppm and 1.37, 0.26 for Zn2C and Cd2C
,
respectively) and even Hd (DdZ0.11 ppm and 0.29 for
Zn2C and Cd2C, respectively) corresponding to the pyridine
moiety (Table 1).
When similar studies were carried out with ligands 2 and 3
all the signals in the NMR spectra of the complex were very
broad in CD3CN as well as in CDCl3. Probably the higher
flexibility of these ligands made the interchange process
faster than with ligand 1 but not faster enough in the NMR
time scale to observe sharp signals. Variable temperature
experiments were carried out from K40 to 55 8C but no
improvement was observed with both solvents.
By contrast, the alkaline cations LiC and NaC as their
picrate salts gave rise to better resolved signals in their
corresponding 1H NMR spectra in (CD3)2CO (Fig. 3). With
both ligands clear modifications were observed in the
benzylic hydrogen atoms of the biphenyl moiety what
agreed with the stronger interaction between the hard
alkaline cations with the ether groups than with the amine
nitrogen atoms. By using this technique the 1:1 stoichi-
ometry and the complexation constants for these salts were
determined. The results showed in Table 2 demonstrated
that the changes in the heteroatoms have not too much
influence in the complexation constant. What is more, these
values are close to that obtained with the related compound
containing five ether groups that showed a value of log KZ
2.0 for the complexation constant with sodium picrate in the
same solvent.9
Figure 3. 1H NMR spectra in (CD3)2CO of (a) free ligand 2; (b) ligand 2C
0.1 equiv LiPic; (c) ligand 2C0.3 equiv LiPic; (d) ligand 2C0.5 equiv
LiPic; (e) ligand 2C1.0 equiv LiPic.
Finally, the ability of ligand 4 for complexing Cd2C and
Zn2C salts was also studied using RMN techniques and the
modifications observed in the shifts of the signals are
showed in Table 3. It is clear that the presence of ester and
amide groups makes the ligand more rigid and precludes an
efficient binding of the cations.
Table 2. Log K for ligands 2 and 3 with alkaline cations in (CD3)2CO by
using NMR techniques
Ligand
Li Pic
Na Pic
2
3
1.9 (0.1)
2.4 (0.1)
2.1(0.1)
2.6 (0.1)

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A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
Table 3. 1H NMR shifts for the hydrogen in ligand 4 and its complexes with Cd(NO3)2 and Zn(NO3)2 in CDCl3
0
Hg,g ,h
NH
Ha
Hb
Hc
Hd
He
Hf
CH3
4
4$Zn(NO3)2
4$Cd(NO3)2 8.59
8.50
8.62
7.16
7.17
7.16
6.92
6.94
6.93
7.07
7.05
7.04
8.29
8.31
8.29
8.15
8.19
8.16
4.25–3.80
4.28–4.13
4.30–4.14
3.68–3.40
3.70–3.42
3.72–3.41
2.90
2.93
2.93
Figure 4. Fluorescence spectra of 3 after addition of (a) Zn2C and (b) Cd2C at 20 8C, lexcZ300 nm. Initial concentration of ligand was 10K5 M in CH3CN and
the salt was added until high excess.
formed complex (Fig. 5). All these experiments were carried
out in CH3CN with Zn2C and Cd2C as their triflate salts.
The values reflected in Table 4 demonstrated the influence
of the coronand amine groups on the complexation ability.
Thus, ligand 1 and 3 make stronger complex with both
cations than ligand 2. On the other hand, the similar results
obtained with ligands 1 and 3 demonstrated that the
substitution of an ester group for an ether group has a
small influence on complexation. Even though, the
experimental conditions were different the results
The four studied ligands gave rise to a quenching of the
fluorescence in the presence of different transition metal
cations. A similar quenching of the fluorescence has also
been observed in complexation experiments with other
ligands containing tetramethylbenzidine subunits.4a,10 In the
studied ligands, the coordination of cations inside the
coronand cavity modifies the dihedral angle between both
byphenyl aromatic rings and this variation gives rise to the
modification in the fluorescent properties.
Figure 6. (a) Fluorescence spectra of 3CHEPES after addition of
increasing of TBAHSO4 at 20 8C, lexcZ275 nm. Initial concentration of
ligand was 10K5 M in CH3CN; (b) stoichiometry determination.
Figure 5. Stoichiometry determination for ligand 3 with Cd2C in
acetonitrile using fluorescence meassurements.
Table 4. Log K values for the complexation of Cd2C and Zn2C as triflate salts with ligands 1–4 in CH3CN using fluorescence techniques
Ligand
1
2
3
4
5
6
Cd2C
Zn2C
5.0 (0.2)
5.1 (0.2)
3.6 (0.1)
—
6.2 (0.2)
5.8 (0.2)
2.8 (0.1)
2.9 (0.1)
1.6a
1.5a
1.9a
2.2a
a These values are determined for nitrate salts and by using 1H NMR techniques.5

A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
Table 5. Log K values for the complexation of FK, AcOK and HSO4K as
tetrabutylammonium salts with ligands 1 and 3 in CH3CN using
10313
fluorescence techniques
Ligand
FK
AcOK
HSOK4
1
3
4.9 (0.1)
5.7 (0.3)
5.0 (0.1)
5.3 (0.4)
4.7 (0.1)
5.0 (0.4)
previously published for ligand 5 and 6 demonstrated the
strong influence of the amide group reduction in the
complexing ability.
3.2. Anion complexation
Anion complexation was studied with ligands 1 and 3 due to
the presence of amino groups susceptible to be protonated
and became anion receptors. The studied anions were FK,
BrK, IK, AcOK and HSOK4 as their tetrabutylammonium
salts after protonation of the corresponding ligands by using
HEPES 0.01 M, pH 6, in dioxane/water 70:30. Fluorescence
studies showed that 1:1 complexes were produced with FK,
AcOK and HSOK4 (Fig. 6). The complexation constants
were determined by using Specfit11 and are shown in
Table 5.
BrK and IK showed a continuous quenching of the
fluorescence that can not be related to the complexation
event but to the heavy atom effect. On the other hand, the
obtained results showed that each ligand complexed the
three anions, FK, AcOK and HSOK4 with similar constants
values. However, ligand 3 showed higher constants than
ligand 1 probably due to its higher flexibility. The
presence of the ester groups in ligand 1 made it less suitable
for adopting the appropriate geometry for anion
complexation.
Figure 7. CVs of a 10K3 mM 4 solution in 0.10 M Bu4NPF6/MeCN.
Potential scan rate 100 mV/s.
cathodic counterparts at C0.76 (Ic),C0.87 (IIc), and
C1.06 V (IIIc). A similar response was obtained for all
receptors (peak potentials in Table 6).
The response of such bis(dimethylamino)biphenyl-contain-
ing receptors is consistent with that reported for the
oxidation of aromatic compounds.12 Thus, the parent neutral
ligand, L, is reversibly oxidized to the corresponding radical
cation L%C and a dication, L2C, in two successive one-
electron transfer steps. On a first examination, these can
correspond to the Ia/Ic and IIa/IIc couples. The presence of
an additional couple IIIa/IIIc can be rationalized taking into
account that the overall oxidation process is accompanied
by a significant stereochemical modification: there is a
transition from the dihedral neutral molecule, to the planar
dication. Accordingly, one can assume that the first
electron-transfer step yields a non-planar cation radical
(L%C) that undergoes to some extent a relatively slow
pre-organization process:13
4. Electrochemical studies
The electrochemical response of compounds 1–4 in MeCN
solution has been studied, as well as the electrochemistry of
such macrocyclic receptors in the presence of an excess of
different metal ions, namely, LiC, NaC, KC, Mg2C, Ca2C
,
Ba2C, Ni2C and Zn2C. The electrochemical response at
glassy carbon and platinum electrodes is dominated by two
successive one-electron transfer processes involving the
oxidation of the diaminobiphenyl moiety above C1.0 V
versus AgCl/Ag.
4.1. Electrochemistry of free ligand 1–4
L Z L$C CeK
(1)
(2)
As shown in Figure 7, corresponding to a 10K3 mM solution
of 4 in 0.10 M Bu4NPF6/MeCN, the CV response of that
receptor consists of three overlapped anodic peaks at C0.82
(Ia),C0.95 (IIa), and C1.10 V (IIIa) coupled with their
L$CðdihedralÞ Z ðL$CÞꢀðplanarÞ
followed by the second electron transfer of both radical
Table 6. Peak potential data in mV versus AgCl/Ag for ligands 1–4 in MeCN solution (0.10 M Bu4NPF6). From CVs at 100 mV/s at platinum electrode
Anodic peaks (Ep (mV))
IIa
Cathodic peaks (Ep (mV))
IIc
Ligand
Ia
IIIa
Ic
IIIc
1
2
3
4
C0.76
C0.74
C0.74
C0.82
C0.86
C0.92
C0.88
C0.95
C1.00
C1.09
C1.04
C1.10
C0.68
C0.72
C0.69
C0.76
C0.79
C0.84
C0.88
C0.87
C0.92
—
C1.0
C1.06

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A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
cations:
ðL$CÞꢀðplanarÞ Z ðL2CÞꢀðplanarÞ CeK
(3)
(4)
L$CðdihedralÞ Z ðL2CÞꢀðplanarÞ CeK
attributable, respectively, to the IIa/IIc and IIIa/IIIc couples
(Fig. 8).
Figure 8.
In both cases, however, the dication comproportionates with
the parent ligand within the second reduction process,
L
2C CL Z 2L$C
(5)
Figure 9. CVs of (a) a 2.0 mM 4 solution in 0.10 M Bu4NPF6/MeCN; (b)
4Cfour times excess of Zn2C. Potential scan rate 100 mV/s.
to yield the cation radical which can be further oxidized to
2C. Accordingly, the second voltammetric wave (II or III)
ꢀ
ꢀ
ꢀ
ꢁ
ꢀ
ꢁ
L$C
MCAK
L$C
MCAK
L
ðdihedralÞ Z
ꢀðplanarÞ
(7)
(8)
(9)
is enhanced with respect to the first (I) as described in
literature for different two-step redox processes.14 Depend-
ing on the extent of the comproportionation reaction (5), IIa/
IIc and IIIa/IIIc couples are more or less enhanced with
respect to the Ia/Ic couple, thus the voltammetric profile
consists on two separate peaks or two overlapping peaks.
ꢁ
ꢀ
ꢁꢀ
ꢀðplanarÞ Z
ðplanarÞ CeK
L$C
MCAꢁ
L2C
MCAꢁ
ꢁ
ꢀ
ꢁꢀ
L$C
L2C
ðdihedralÞ Z
ðplanarÞ CeK
MCAK
MCAK
4.2. Electrochemistry of L–MnC complexes
Peak potential values are summarized in Table 7.
The electrochemical response of L plus MnC solutions is
quite similar to that previously described. As shown in
Figure 9, in the presence of an excess (four times) of metal
salts, the voltammetric pattern is similar to that of the parent
ligands. However, some peak potential shifts and changes in
the voltammetric profile with respect the voltammetry of the
non-coordinated ligands were recorded. The voltammetry of
L plus MnC systems can be interpreted in terms of the
formation of ion-pair complexes that experience ligand-
centred electrochemical processes:
Accordingly, complexation may influence the potential and
relative height of the voltammetric peaks: (i) by modulating
the thermochemical stability of the complex species; (ii) by
modifying the kinetic of the comproportionation and
preorganization reactions. To rationalize these effects,
electrostatic, electronic, stereochemical and solvation
factors should be considered.15 First of all, it should be
noted that, assuming reversibility, the formal potentials of
the different couples are related with the equilibrium
constants of the complexes formed with the oxidized and
reduced forms. In view of the reversibility of the
electrochemical processes I, II and III, formal electrode
ꢀ
ꢁ
ꢀ
ꢁ
L
L$C
MCAK
Z
CeK
(6)
MCAK
0
0
potentials E , for such processes can be calculated from the
Table 7. Peak potential data in mV versus AgCl/Ag for ligands 1–4 in MeCN solution (0.10 M Bu4NPF6)
Anodic peaks
Cathodic peaks
IIc
Ligand
Ia
IIa
IIIa
Ic
IIIc
2
C0.74
C0.82
C0.92
C0.98
C1.09
C1.32
C0.99
C1.04
C0.98
—
C1.10
C1.1
—
C0.72
C0.77
C0.84
C0.94
—
2CLiC
2CZn2C
3
C1.18
C0.91
C1.0
C0.90
—
C1.06
C1.04
—
C0.85
C0.88
C0.83
C0.81
C0.95
C0.94
C0.93
C0.75
C0.88
C0.76
C0.72
C0.87
C0.89
C0.88
C0.74
C0.69
3CLiC
3CZn2C
4
C0.82
C0.83
C0.76
C0.73
4CLiC
4CZn2C
From CVs at 100 mV/s at platinum electrode.

A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
Table 8. Log Kcom and log Kcom for ligands 1–4 free and in the presence of cations
10315
0
Ligand
1
2
LiC-2
Zn2C-2
3
LiC-3
Zn2C-3
4
LiC-4
Zn2C-4
Log Kcom
Log Kcom
K1.78
K4.07
K2.54
—
K3.13
K7.71
—
K2.54
K2.79
K5.17
—
K2.45
—
—
K2.03
K4.19
—
K2.63
K2.15
—
0
0
0
half sum of the cathodic and anodic peak potentials (E Z
(EpcCEpa)/2). Thus, the shift in the formal potentials
recorded upon complexation satisfies (V at 298 K):
transfer represented by Eq. 8 predominates over that
represented by Eq. 9. In other cases, however, the
couple IIIa/IIIc disappears. This is attributable to a fast
acquirement of the planar geometry after the initial
one-electron transfer step.
ꢂ
ꢃ
0
KðLÞ
E0 ðIÞ Z 0:059 log
(10)
(11)
(12)
KðL$C
Þ
(d) In some cases, the voltammograms (LiC-4) show a
decrease in reversibility, as denoted by the appearance
of a significant decrease of cathodic peaks, as can be
seen on comparing Figure 11a and b. Reversibility of a
couple can be interpreted in terms of the ability of the
ligand or ligandCmetal ion systems to promote
ꢂ
ꢃ
KðL$C
Þ
E00ðIIÞ Z 0:059 log
KðL2C ꢀ
Þ
ꢂ
ꢃ
KðL$C
Þ
E00ðIIIÞ Z 0:059 log
KðL2C ꢀ
Þ
where K(L), K(L%C), K(L%C)*, and K(L2C)* are the
equilibrium constants for the complexation of a given
metal ion, MnC, with the neutral ligand, the radical cation
(non-planar and planar), and the dication, respectively.
Electrochemical data allow for a direct determination of the
equilibrium constant for the comproportionation reaction
described by Eq. 5 and its analogues for metal complexes:
ꢂ
ꢃ
ðE00ðIÞKE00ðIIÞ
0:059
log Kcom
Z
(13)
(14)
ꢂ
ꢃ
The estimated values are summarized in Table 8.
E00ðIÞKE00ðIIIÞ
0:059
log Kcom0
Z
The differences between ligand and ligandCmetal ion
voltammograms result from the superposition of thermo-
dynamic factors and kinetic ones and can be summarized in
the following way:
Figure 10. CVs of (a) a 2.0 mM 3 solution in 0.10 M Bu4NPF6/MeCN; (b)
3Cfour times excess of LiC. Potential scan rate 100 mV/s.
(a) In some cases (LiC-2) the peaks Ia/Ic are displaced
towards more positive potentials. This effect can be in
principle associated to an electrostatic cause: upon
complexation, the system acquires a certain additional
positive charge, thus decreasing its oxidation ability.
(b) In some cases (LiC-4) the peaks IIa/IIc are negatively
displaced even merging with the Ia/Ic couple that
remains essentially unchanged. This effect, consisting
in a relative destabilization of the L%C complexes
towards disproportionation, cannot be explained by
electrostatic considerations. Probably it should be
attributed to stereochemical factors: in these cases
metal coordination favours the acquirement of a nearly
coplanar geometry, thus favouring oxidation.
(c) In some cases (LiC-3) (Fig. 10), the couple IIIa/IIIc is
enhanced at the expense of the couple IIa/IIc. This
effect, observed for small cations, can be associated to
the adoption of a different coordination environment
that makes difficult the adoption of a coplanar
geometry. Then, Eq. 7 is fast, and the second electron
Figure 11. CVs of (a) a 2.0 mM 4 solution in 0.10 M Bu4NPF6/MeCN; (b)
4Cfour times excess of LiC. Potential scan rate 100 mV/s.

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A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
Figure 12.
structural rearrangements compatible with the struc-
In relation to anion recognition, it could be concluded that
tural requisites for stabilizing both oxidation states.16
Since in non-reversible cases the peak potential values
are similar to those in reversible systems, the loss of
reversibility can be associated to the lowering of the
rate of preorganization of the ligand. This result from
ligand topology and ligand flexibility.
anion geometry has not influence in the fluorescent response
what suggest the interaction with the anion is mostly related
to acid–base reactions.
On the other hand, electrochemical studies can be
summarized in two points: firstly, the existence of two
different pathways in the two-electron oxidation. One of
these routes goes through a dihedral radical-cation being the
intermediate in the second pathway a planar system.
Secondly, the obtained results in the presence of metal
cations suggest that transition metal cations place them-
selves close to the nitrogen atoms. By contrast, the alkaline
cations are preferably co-ordinated with the oxygen atoms
present in the ligand.
The most remarkable facts for compounds 3 and 4 were, that
in the case of complexation with alkaline and alkaline-earth
cations, peaks I and II overlapped, and peak III suffered an
increase in its intensity. These features suggest that in such
cases the dihedral/planar interconversion of the radical
cation is fast. By contrast, with zinc cation and ligand 4,
peak III disappeared totally and it was not observed
overlapping of peaks II and I. These facts were explained
based on the different character of lithium and zinc. Thus,
the hard character of lithium placed it close to the biphenyl
moiety, making difficult the oxidation through the planar
radical cation. In the case of zinc, because of its soft
character, it is located close to the pyridine zone, so the
oxidation through the planar geometry is possible (Fig. 12).
On the other hand, 2 with transition metals showed that the
preferred way for the oxidation was through the non-planar
radical cation. This behaviour can be related to the presence
of only one nitrogen atom in this compound that could allow
the complex to adopt a different geometry. Finally, the
addition of transition metal cations improved significantly
the reversibility of the cyclic voltammetry.
6. Experimental
6.1. General methods
All commercially available reagents were used without
further purification. Water sensitive reactions were per-
formed under argon. Column chromatography was carried
out on SDS activated neutral aluminium oxide (0.05–
0.2 mm; activity degree 1). IR spectra were recorded on a
Perkin-Elmer 1750 FT-IR and a Bruker Equinox 55 FT-IR.
NMR spectra were recorded with Bruker Avance 300/500
and Varian Unity-300/400 spectrometers. Chemical shifts
are reported in parts per million downfield from TMS.
Spectra were referenced to residual undeuterated solvent.
High-resolution mass spectra were taken with a Fisons VG-
AUTOSPEC and those using the eletrospray ionizing
technique were recorded on an HPLC-MS with ion trap
Bruker 3000-Esquire Plus. UV spectra were run at 20 8C
(thermostated) on a Shimadzu UV-2102 PC. Fluorescence
spectra were carried out in a Varian Cary Eclipse
Fluorimeter.
5. Conclusions
The results obtained in complexation experiments carried
out with ligands 1–4 allows to establish the influence of the
different functional group in the cavity on the binding
efficiency. Thus, substitution of amide groups by the
corresponding amine functions (compounds 1 and 6) has
stronger influence in complexation than the change of ester
by ether groups (compounds 1 and 3). On the other hand, the
rigidity of compound 4 (two ester and two amide groups)
makes it few suitable for complexing transition metal
cations, even though the size of the cavity is larger than in
the other described compounds. It is also remarkable the
small influence that the substitution of N by O atoms has in
the complexation of alkali metal cations (compounds 2
and 3).
6.1.1. Synthesis of N,N0-dimethyl-2,20-TMB-2,16-dicar-
boxy-6,9,12-triaza-8,10-pyridine-19-crown-2 (1). In a
two-neck round bottom flask, under argon, 6 (0.670 g,
1.2 mmol) was dissolved in dry THF (10 ml) and stirred at
room temperature. BH3–THF 1 M (24 ml) was added
dropwise over this solution. The mixture was stirred at
room temperature for 24 h. Then, the reaction was quenched
by adding aqueous HCl 1 M. The reaction was stirred for 2 h
and neutralizing with NaOH pellets until pH 7. THF was

A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
10317
removed under vacuum and the aqueous layer was
continuously extracted with hot chloroform for 24 h.
Organic layer was dried with anhydrous sodium sulphate
and filtered off. The solvent was evaporated under vacuum
to give, after column chromatography purification (neutral
alumina, AcOEt), compound 1 as a yellow solid (0.174 g,
32%). Mp: 168–169 8C. IR (KBr): 3140 (Ar–H), 2841 (CH3,
2.62 (4H, t, JZ6.2 Hz, CH2–N), 2.00 (6H, s, CH3–N). NMR
13C (CDCl3) d (ppm): 157.0, 149.5, 137.2, 136.2, 130.3, 127.4,
122.0, 111.8, 110.7, 70.2, 67.3, 62.4, 54.3, 43.0, 39.5. HRMS
(E.I.): MC calcd for C31H43N5O2 517.3417, found
517.3415.
6.1.4. Synthesis of 2,6-bis[N-methyl-N-(2-hydroxyethyl)
aminomethyl] pyridine (8). 2,6-Bis(hydroxymethyl)pyr-
idine (1.4 g, 10 mmol) and SeO2 (1.105 g, 10 mmol) were
refluxed for 4 h in dry 1,4-dioxane (25 ml), then filtered in
hot and the solvent evaporated under vacuum. 2,6-
Pyridinedicarbaldehyde was used in the following step
without further purification. 2,6-Pyridinedicarbaldehyde
(0.638 g, 4.7 mmol) and N-methyl ethanolamine (0.75 ml,
9.4 mmol) were refluxed in anhydrous ethanol (25 ml)
during 4 h. Then, the mixture was cooled at room
temperature, sodium borohydride was added (3.556 g,
94 mmol) and the reaction was stirred overnight. Finally,
the reaction was quenched by refluxing with water during
5 h. Solvent was removed by distillation under reduced
pressure and the aqueous layer was continuously extracted
in hot chloroform for 24 h. The organic layer was dried with
sodium sulphate and the solvent removed providing
compound gave product 8 as a pale yellow oil (0.975 g,
82%). IR (KBr): 3362 (OH), 2949, 1594 (C]C), 1455,
CH2), 1721 (C]O), 1607 (C]C), 1256 (C–O), 862 cmK1
.
1
NMR H (CDCl3) d (ppm): 7.51 (1H, t, JZ7.7 Hz, Py-H),
7.17 (2H, d, JZ2.6 Hz, Ar–H), 7.14 (2H, d, JZ7.7 Hz, Py-
H), 6.96 (2H, d, JZ8.5 Hz, Ar–H), 6.77 (2H, dd, J1Z
2.8 Hz, J2Z8.7 Hz, Ar–H), 4.05–3.91 (4H, m, CH2–OOC),
3.72 (2H, d, JZ13.2 Hz, CH2-py), 3.54 (2H, d, JZ13.3 Hz,
CH2-py), 2.91 (6H, s, (CH3)2N), 2.58 (2H, m, CH2–N), 2.32
(3H, s, CH3–N), 2.29 (2H, m, CH2–N). NMR 13C (CDCl3) d
(ppm): 166.9, 156.1, 148.0, 135.8, 130.5, 130.0, 129.4,
121.5, 114.2, 112.5, 61.9, 60.8, 52.6, 42.4, 39.6. HRMS
(E.I.): MC calcd for C31H39N5O4 545.2274, found
545.2276.
6.1.2. Synthesis of 2,20-TMB-9-aza-8,10-pyridine-19-
crown-4 (2). In a two-neck round bottom flask, under
argon, a stirred solution of 2,6-bis((2-hydroxy)ethoxy-
methyl) pyridine (9) (0.227 g, 1 mmol) and NaH 60%
mineral oil (0.120 g, 3 mmol) in THF0 (50 ml) was refluxed
for 2 h. 2,20-bis(chloromethyl)-4,4 -bis(dimethylamine)
biphenyl (0.335 g, 1 mmol) and sodium iodide (5 mg) in
THF (50 ml) was added dropwise over this solution. The
mixture was heated for 48 h and quenched with water. THF
was removed under vacuum, the aqueous layer was
extracted with ethyl acetate (3!25 ml) and organic layers
were washed with brine, dried with anhydrous sodium
sulphate and evaporate under reduce pressure. Column
chromatography (neutral alumina, AcOEt) gave ligand 2 as
pale brown oil (0.097 g, 20%). IR (KBr): 2858, 1607
(C]C), 1494, 1453, 1348 and 1084 cmK1. NMR 1H
(CDCl3) d (ppm): 7.60 (1H, t, JZ7.8 Hz, Py-H), 7.24
(2H, d, JZ7.8 Hz, Py-H), 6.92 (2H, d, JZ8.4 Hz, Ar–H),
6.87 (2H, d, JZ2.7 Hz, Ar–H), 6.65 (2H, dd, J1Z2.7 Hz,
J2Z8.4 Hz, Ar-H), 4.54 (2H, d, JZ12.3 Hz, CH2–OAr),
4.44 (2H, d, JZ12.3 Hz, CH2–O), 4.07 (4H, s, CH2–O),
3.60–3.55 (4H, m, CH2–O), 3.52–3.42 (4H, m, CH2–O),
3.31–3.24 (4H, m, CH2–O), 2.91 (12H, s, (CH3)2–N). NMR
13C (CDCl3) d (ppm): 158.0, 150.1, 137.9, 137.5, 134.6,
131.2, 122.3, 112.4, 111.6, 74.3, 71.5, 69.9, 69.6, 41.2.
HRMS (E.I.): MC calcd for C29H37N3O4 491.2784, found
491.2834.
1
1036 (C–O), 873, 787 cmK1. NMR H (CDCl3) d (ppm):
7.58 (1H, t, JZ7.6 Hz, Py-H), 7.06 (2H, d, JZ7.6 Hz,
Py-H), 4.32 (2H, br s, OH), 3.65 (4H, s, CH2–Ar), 3.60 (4H,
t, JZ5.2 Hz, CH2–O), 2.59 (4H, t, JZ5.2 Hz, CH2–N), 2.24
(6H, s, CH3–N).NMR 13C (CDCl3) d (ppm): 158.7, 137.6,
122.0, 63.1, 59.8, 59.2, 43.1. HRMS (E.I.): MC calcd for
C13H23N3O2 253.1790, found 253.1783.
6.1.5. Synthesis of 2,6-bis[(2-hydroxy)ethoxymethyl]
pyridine (9). Synthesis of dimethyl 2,6-bis-pyridinedi-
methoxyethanoate. The reaction was carried out under
argon atmosphere. A suspension of 2,6-pyridinedimethanol
(3 g, 21.5 mmol) and sodium hydride 60% mineral oil
(2.3 g, 57.2 mmol) in dry THF (100 ml) was stirred at room
temperature for 20 min. Then, methyl bromoacetate (6.2 ml,
64.5 mmol) in dry THF (40 ml) was added dropwise and the
mixture heated (60 8C) for 10 h. The reaction was stopped
by adding water; THF was evaporated under vacuum and
the aqueous layer was extracted with ethyl acetate (3!
25 ml), washed with brine and dried over anhydrous sodium
sulphate. Evaporation of the solvent and column chroma-
tography (silica, CH2Cl2/(CH3)2CO 95:5) provided the
desired compound as a yellow oil (5.492 g, 90%). NMR
1H (CDCl3) d (ppm): 7.34 (1H, t, JZ7.5 Hz, Py-H), 7.00
(2H, d, JZ7.5 Hz, Py-H), 4.29 (4H, s, CH2–Ar), 3.81 (4H, s,
CH2–O), 3.30 (6H, s, CH3–O). NMR 13C (CD3OD) d (ppm):
167.3, 154.5, 140.1, 120.4, 74.2, 66.8, 52.3.
6.1.3. Synthesis of N,N0-dimethyl-2,20-TMB-6,9,12-
triaza-8,10-pyridine-19-crown-2 (3). Following the same
procedure employed in the synthesis of 2, 2,6-bis(N-methyl,
N-2-hydroxyethyl)methylamine pyridine (8) (0.253 g,
1 mmol) and 2,20-bis(chloromethyl)-4,40-bis(dimethylami-
ne)biphenyl (0.335 g, 1 mmol) gave ligand 3 after column
chromatography (neutral alumina, AcOEt/MeOH, 98:2) as a
brown sirup (0.171 g, 33%). IR (KBr): 3376, 2850, 1607
Dimethyl 2,6-bis-pyridinedimethoxyethanoate (1.935 g,
6.83 mmol) in anhydrous ethanol (60 ml) was cooled at
0 8C and vigorously stirred. Lithium borohydride (0.458 g,
21 mmol) was carefully added in portions. The mixture
remained stirred at room temperature for 15 h, then
quenched with water and the ethanol was evaporated
under reduced pressure. The aqueous layer was continu-
ously extracted with hot chloroform for 24 h; the organic
layer was dried over anhydrous sodium sulphate and
concentrated to give 9 as pale yellow oil (1.240 g, 80%).
(C]C), 1494, 1453, 1348, 1089 and 806 (C–N) cmK1
.
1
NMR H (CDCl3) d (ppm): 7.67 (1H, t, JZ7.7 Hz, Py-H),
7.27 (2H, d, JZ7.7 Hz, Ar–H), 6.87 (2H, d, JZ7.7 Hz, Py-
H), 6.84 (2H, d, JZ3.0 Hz, Ar–H), 6.70 (2H, dd, J1Z
3.0 Hz, J2Z7.7 Hz, Ar–H), 4.27 (2H, d, JZ11.8 Hz, CH2–
OAr), 4.12 (2H, d, JZ11.8 Hz, CH2–OAr), 3.73 (4H, s,
CH2–N), 3.49–3.33 (4H, m, CH2–O), 2.97 (12H, s, (CH3)2–N),

10318
A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
1
NMR H (CDCl3) d (ppm): 7.63 (1H, t, JZ8.0 Hz, Py-H),
7.19 (2H, d, JZ8.0 Hz, Py-H), 5.00 (2H, br s, OH), 4.67
(4H, s, CH2–Ar), 3.77 (4H, br m, CH2–O), 3.73 (4H, br m,
CH2–O). NMR 13C (CD3OD) d (ppm): 158.2, 138.0, 120.9,
73.8, 73.3, 62.0.
coordinatesand torsion angles, excluding structure factors)
for this structure have been deposited with the Cambridge
Crystallographic Datra Centre as supplementary publication
number CCDC 273729. Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK.
6.2. X-ray structure analysis of ligand 1
6.3. NMR titrations with cations. General procedure
Intensity measurements were made on an Enraf-Nonius
CAD4 diffractometer using a colourless prismatic single
crystal of dimensions 0.41!0.20!0.18 mm. Graphite-
1
In a NMR tube, the NMR H spectrum of a well known
concentration of free ligand was recorded. Then, an aliquot
of a known solution of the metallic salt was added (0.1 equiv
aprox), the mixture was stirred for 30 s, the solvent was
evaporated to raise the initial volume (using argon flow) and
the spectrum was again recorded. This procedure was
repeated several times until the concentration of the salt
was larger than the ligand and no variation of the signals
was observed. The plot of the variation of the chemical
shifts versus the ratio between the salt and the ligand gave
the titration curve and so the stoichiometry of the complex.
The constant value was calculated making use of the
program Clinp 2.0.21
˚
monochromated Mo Ka radiation (lZ0.71073 A) and
u-scan technique was used. Data collection was carried
out at room temperature. Three reference reflections were
measured every 2 h as an intensity and orientation check and
no significant fluctuation was noticed during the collection
of the data. Lorentz-polarization correction was made. The
crystal structure was solved by direct methods using the
SHELX system16 and refined by full-matrix least-squares
techniques17 on F2. The non-hydrogen atoms were refined
anisotropically. All the hydrogen atoms were found by
Fourier synthesis in the refinement process. Molecular
graphics of the structure (Fig. 1) was made with ORTEP18.
Stereoscopic view of crystal packing of the structure (Fig. 2)
was performed with PLATON/PLUTON graphics system.19
Part of the publication material was made with WinGX
publication routines.20
Table 10. Atomic coordinates (!104) and equivalent isotropic displace-
2
ment parameters (A !10 ) for 1
3
˚
X
y
z
U(eq)
O(1)
C(3)
O(10)
N(8)
N(5)
C(4)
C(2)
C(7)
N(6)
6341(2)
4015(4)
4849(3)
1259(3)
9835(3)
3415(3)
4189(4)
5107(4)
5687(3)
8280(4)
5787(4)
4714(3)
3463(4)
1949(4)
3836(4)
5502(4)
1980(4)
9915(3)
1258(5)
4365(4)
7588(4)
6430(30)
6600(30)
5226(4)
10,555(5)
7927(4)
5049(4)
5424(6)
9938(6)
6618(7)
K300(5)
5852(7)
11,342(5)
7089(6)
10,611(4)
9980(5)
1988(7)
9209(5)
6898(7)
11,412(6)
10,719(6)
2898(2)
1870(3)
2364(2)
6329(3)
1328(3)
3108(3)
4041(3)
K528(3)
K1681(3)
1616(4)
4002(4)
1682(3)
5108(3)
3332(3)
816(3)
3803(2)
5282(3)
1452(3)
4385(3)
K350(3)
1831(3)
4750(20)
4930(20)
498(3)
3329(2)
3194(2)
703(2)
65(1)
41(1)
60(1)
70(1)
48(1)
38(1)
41(1)
45(1)
57(1)
52(1)
50(1)
40(1)
48(1)
50(1)
46(1)
85(1)
46(1)
61(1)
52(1)
47(1)
66(1)
108(9)
97(6)
46(1)
61(1)
59(1)
55(1)
68(1)
63(1)
78(1)
68(1)
71(1)
79(1)
72(1)
59(1)
66(1)
92(2)
77(1)
97(2)
94(2)
87(2)
Information concerning crystallographic data collection is
summarized in Table 9. Atomic coordinates and equivalent
isotropic displacement parameters are shown in Table 10.
Additional crystallographic data (bond lengths and angles,
3689(3)
2860(2)
3250(2)
3342(2)
3193(3)
3176(2)
2931(4)
3288(3)
2324(2)
3479(3)
3260(3)
4042(3)
1196(2)
3531(2)
609(2)
3390(3)
4050(3)
K457(2)
3390(20)
3080(20)
2329(3)
2083(3)
3196(4)
1369(3)
K311(3)
3787(3)
2266(4)
3808(4)
4067(4)
736(4)
anisotropic
displacement
parameters,
hydrogen
Table 9. Crystal data and structure refinement for 1
Empirical formula
Formula weight
Temperature
C31 H39 N5 O4
545.67
293(2) K
C(14)
C(12)
C(9)
˚
Wavelength
0.71073 A
Triclinic
P-1
C(17)
C(13)
C(18)
O(15)
C(16)
N(21)
C(19)
C(11)
N(23)
O(26A)
O(26B)
C(20)
C(27)
C(25)
C(22)
C(32)
C(24)
C(28)
C(34)
C(29)
C(30)
C(31)
C(33)
C(35)
C(36)
C(37)
C(40)
C(39)
C(38)
Crystal system
Space group
Unit cell dimensions
˚
aZ9.4339(2) A
aZ68.081(3)8
bZ71.502(3)8
gZ86.250(3)8
˚
˚
bZ11.3474(2) A
cZ15.6222(2) A
3
˚
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data
collection
1468.64(4) A
2
1.234 Mg/m3
0.083 mmK1
584
0.41!0.20!0.18 mm3
2.29–26.248
855(4)
2762(4)
2744(4)
3206(5)
419(5)
Index ranges
0%h%11, K14%k%14,
K18%l%19
6300
Reflections collected
Independent reflections
5923 [R(int)Z0.0418]
K1751(5)
6454(5)
K2613(4)
2996(5)
3126(4)
1821(4)
2281(4)
7164(6)
1813(6)
1108(7)
3809(6)
3444(5)
Completeness to thetaZ 99.9%
26.248
Refinement method
Full-matrix least-squares
on F2
5923/0/526
K695(3)
1093(3)
K289(3)
3946(6)
K814(4)
K843(5)
K182(5)
K701(4)
Data/restraints/
parameters
Goodness-of-fit on F2
Final R indices
[IO2sigma(I)]
0.908
R1Z0.0518,
wR2Z0.1246
R1Z0.2605,
wR2Z0.1775
R indices (all data)
K3
˚
0.198 and K0.246 e A
Largest diff. peak and
hole
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

A. M. Costero et al. / Tetrahedron 61 (2005) 10309–10320
10319
6.4. Fluorescence titrations with cations. General
procedure
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.08.
010
The spectrum of free ligand (3.0 ml, 10K5 M) in acetonitrile
was recorded. Then, an aliquot of a solution 10K3 M in the
metallic salt and 10K5 M in the ligand was added
(0.1 equiv), the mixture was stirred for 30 s and the
spectrum was recorded. This procedure was repeated
several times until the concentration of the salt was larger
than the ligand and no variation of the signals was
observed. The plot of the variation of the absorbance/
emission versus the ratio between the salt and the complex
gave the titration curve and so the stoichiometry of the
complex. The constant value was calculated making use of
the program Clinp 2.0.21
References and notes
1. (a) Fabbrizzi, L.; Poggi, A. Chem. Soc. Rev. 1995, 24,
197–202. (b) De Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsonn, T.; Huxley, A. J. M.; McCoy, C. P.;
Radamacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97,
1515–1566. (c) Full issue of Coord. Chem. Rev. 2000, 205
(d) Rurack, K.; Resch-Genger, U. Chem. Soc. Rev. 2002, 31,
6.5. Fluorescence titrations with anions. General
procedure
´ ´ ´
116–127. (e) Martınez-Man˜ez, R.; Sancenon, F. Chem. Rev.
The spectrum of free ligand (3.0 ml, 10K5 M) in dioxane/
water 70:30, HEPES 0.01 M, pH 6, was recorded. Then, an
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2. Weinig, H-G. ; Krauss, R.; Seydack, M.; Bendig, J.; Koert, U.
Chem. Eur. J. 2001, 7, 2075–2088.
aliquot of a solution 10K3 M in the metallic salt and 10K5
M
in the ligand was added (0.1 equiv), the mixture was stirred
for 30 s and the spectrum was recorded. This procedure was
repeated several times until the concentration of the salt was
larger than the ligand and no variation of the signals was
observed. The plot of the variation of the absorbance/
emission versus the ratio between the salt and the complex
gave the titration curve and so the stoichiometry of the
complex. The constant value was calculated making use of
the program Specfit.11
3. (a) McFarland, S. A.; Finney, N. S. J. Am. Chem. Soc. 2001,
123, 1260–1261. (b) Lee, D. H.; Im, J. H.; Lee, J-H. ; Hong,
J-I. Tetrahedron Lett. 2002, 43, 9637–9640. (c) Cody, J.;
Fahrni, C. J. Tetrahedron 2004, 60, 11099–11107.
´
4. (a) Costero, A. M.; Gil, S.; Sanchis, J.; Peransı, S.; Sanz, V.;
Williams, J. A. G. Tetrahedron 2004, 60, 6327–6334. (b)
Costero, A. M.; Aurell, M. J.; Ban˜uls, M. J.; Ward, M. D.;
Argent, S. Tetrahedron 2004, 60, 9471–9478.
5. Costero, A. M.; Aurell, M. J.; Ban˜uls, M. J.; Ochando, L. E.;
Tamarit, F. J.; Domenech, A. J. Inclus. Phen. And Macrocyclic
Chem. 2003, 45, 241–249.
6.6. Electrochemical experiments
´ ´
6. Costero, A. M.; Andreu, C.; Monrabal, E.; Martınez-Man˜ez,
R.; Soto, J.; Padrilla-Tosta, M. J.; Pardo, T.; Ochando, E. L.;
´
Amigo, J. M. J. Chem. Soc., Dalton Trans. 2000, 361–367.
Electrochemical measurements were performed at 298 K in
a conventional three-electrode cell under argon atmosphere.
Nominal ca. 2.0 mM concentrations of the ligand were used
in dry MeCN using tetrabuthylammonium hexafluoropho-
sphate (0.10 M) as a supporting electrolyte. A four times
excess of metallic salts was used in each case (triflate salts
for the transition metal cations and perchlorate for alkaline
cations). Experiments were performed using a BAS CV 50
W equipment using a BAS MF2012 glassy carbon
electrode (GCE) (geometrical area 0.071 cm2), and a BAS
MF2014 platinum electrode (geometrical area 0.018 cm2)
as a working electrode. A platinum wire auxiliary electrode
and a AgCl (3 M NaCl)/Ag reference electrode separated
from the test solution by a salt bridge only containing
supporting electrolyte completed the electrode
arrangement. The potential of such reference electrode
was K35 mV versus the saturated calomel reference
electrode (SCE).
7. Alcock, N. W.; Kingston, R. G.; Moore, P.; Pierpoint, C.
J. Chem. Soc., Dalton Trans. 1984, 1937–1943.
8. Neugebauer, F. A.; Funk, B.; Staab, H. A. Tetrahedron Lett.
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Acknowledgements
The present research has been financed by DGCYT
(PPQ2002-00986) and Generallitat Valenciana Grupos 03/
206. We acknowledge the Universitat de Valencia (S.C.I.E.)
for most of the equipment employed. One of us (M.J.B.)
acknowledges the Generalitat Valenciana for a PhD grant.

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