Redox-driven intramolecular anion translocation between a metal centre and a hydrogen-bond-donating compartment

Article abstract of DOI:10.1002/chem.200601865

Dicationic ligands incorporating two 2,2′-bipyridine units and two imidazolium moieties, [1]²⁺ and [2]²⁺, form stable chelate complexes with Cu^II and Cu^I in acetonitrile solution. Each Cu" complex binds two X

Full text of DOI:10.1002/chem.200601865

DOI: 10.1002/chem.200601865
Redox-Driven Intramolecular Anion Translocation between a Metal Centre
and a Hydrogen-Bond-Donating Compartment
Valeria Amendola, Benoît Colasson, Luigi Fabbrizzi,* and
Maria-Jesffls Rodriguez Douton[a]
Abstract: Dicationic ligands incorpo-
rating two 2,2’-bipyridine units and two
um compartment. CuI complexes are
able to host only one NO3 ion in the
bis-imidazolium cavity, while other
anions induce demetallation. Thus, in
the presence of one equivalent of
ꢀ
ꢀ
NO3 , the CuII/CuI redox change
imidazolium moieties, [1]2+ and [2]2+
,
makes the anion translocate quickly
and reversibly from one binding site to
the other within the [CuII,I(1)]4+/3+
system, as demonstrated by cyclic vol-
tammetry and controlled-potential
electrolysis experiments.
form stable chelate complexes with
CuII and CuI in acetonitrile solution.
Each CuII complex binds two Xꢀ ions
according to two stepwise equilibria,
the first involving the CuII centre and
the second involving the bis-imidazoli-
Keywords: anion recognition · cop-
per · hydrogen bonds · molecular
machines · redox chemistry
Introduction
location of transition-metal ions and are based on two dis-
tinct protocols: 1) the metal exists in two oxidation states of
comparable stability, and oxidation–reduction processes
cause its transfer between two compartments of definitely
different coordinating tendencies;[5–12] and 2) the coordinat-
ing properties of only one compartment towards a given
metal ion are altered through an acid–base process, so that
consecutive addition of H+ and OHꢀ ions induces a reversi-
ble intramolecular metal displacement.[13–18] On the other
hand, examples of anion translocation are rare and all refer
to the redox-driven transfer of an anion between two metal
centres within a hetero-dinuclear complex.[19] In particular,
the coordinating tendencies of a metal towards a given
anion are modified through a one-electron redox change
(e.g. NiII/NiIII), which induces the anion movement to/from a
proximate ancillary metal ion (e.g. a coordinatively unsatu-
rated CuII centre).[20,21]
The externally controlled motion of a movable part within a
supramolecular system provides a way to convert chemical
energy into mechanical work at a molecular level and forms
the basis of the design of molecular machines. Classical ex-
amples include 1) the sliding of the axle of a rotaxane (cou-
pled to either an auxiliary acid–base or redox process)[1] and
2) the half-turn of the ring of a [2]catenate (driven by the
CuI/CuII redox couple).[2] These have inspired the design of
a great number of molecular and supramolecular systems
consisting of one movable and one stationary part, whose
activity is operated by an input of a chemical, electrochemi-
cal or photonic nature.[3] A further type of controlled
motion concerns the translocation of an ionic particle be-
tween two non-equivalent compartments of a molecular
system.[4] Most examples involve the externally driven trans-
In recent years, great attention has been devoted to the
design of receptors capable of establishing electrostatic and/
or hydrogen-bonding interactions with anions, essentially in
order to develop optical sensors for anion determination in
the biomedical, environmental and food sciences.[22] In par-
ticular, guidelines for the synthesis of positively charged or
neutral receptors suitable for selective interaction with
anions of given size and geometrical features have been out-
lined.[23] On these bases and in the perspective of setting up
further experiments of anion translocation, we considered
the design of a ditopic receptor containing a purely organic
compartment, capable of establishing hydrogen-bonding in-
[a] Dr. V. Amendola, Dr. B. Colasson,+ Prof. L. Fabbrizzi,
Dr. M.-J. Rodriguez Douton
Dipartimento di Chimica Generale
Università di Pavia
via Taramelli 12, 27100 Pavia (Italy)
Fax : (+39)0382-528-544
E-mail: luigi.fabbrizzi@unipv.it
[+] Present address:
Laboratoire de Chimie et Biochimie Pharmacologiques et
Toxicologiques
UniversitØ Paris V
45 Rue des Saints P›res, 75270 Paris Cedex 06 (France)
4988
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Chem. Eur. J. 2007, 13, 4988 – 4997

FULL PAPER
teractions with anions. The other compartment had to con-
tain a redox-active metal centre in order to provide the
engine for the translocation process, which should be fuelled
by the energy released by an ancillary oxidation–reduction
process.
the imidazolium compartment). The hypothesised transloca-
tion process for [L]2+ =[1]2+ is depicted in Scheme 1.
In particular, we have synthesised the molecular dications
[1]2+ and [2]2+, in which two equivalent arms are appended
in the 1,4- and 1,3-positions of a phenyl platform, respective-
Scheme 1. The planned translocation of an anion Xꢀ (grey sphere) within
the [CuII,I(1)]4+/3+ system, driven by the CuII/CuI redox couple (CuI:
hashed sphere; CuII: black sphere). The redox change induces a change
of the coordination geometry at the metal centre, from tetrahedral (CuI)
to a five-coordinate arrangement (CuII, square pyramidal or trigonal bi-
pyramidal).
It should be noted that the positive electrical charge of
!
the two complexes [CuI(L)
ACHTREUNG
ꢀ
neutralised by two and three Yꢀ counterions (PF6 ,
ꢀ
ꢀ
ly. Each arm contains an imidazolium fragment and a 2,2’-
bipyridine subunit. The imidazolium moiety can behave as
an H-bond donor, through the polarised C–H fragment be-
tween the nitrogen atoms, and is widely used in the design
of receptors suitable for multipoint hydrogen-bonding inter-
actions with anions.[24] 2,2’-Bipyridine (bpy) is one of the
most classical and investigated ligands in metal coordination
chemistry, and forms stable complexes with most transition-
metal ions.[25]
CF3SO3 , ClO4 ), respectively, which are dispersed in the
acetonitrile (MeCN) solution and do not participate in the
ꢀ
ꢀ
translocation process. In particular, PF6 , CF3SO3 and
ꢀ
ClO4 have been chosen as counterions in view of their poor
or zero tendencies to coordinate metal centres and to estab-
lish hydrogen-bonding interactions.
The success of the translocation experiment illustrated
above requires the fulfilment of several conditions:
1) system [L]2+ should be able to form stable chelate com-
plexes with both CuI and CuII; the issue is not marginal, as
metal chelation could be contrasted with the electrostatic re-
pulsion by the facing imidazolium subunits; 2) in the
[CuII(L)(X)]3+ complex, the Xꢀ ion should exhibit a defi-
nitely higher affinity for the metal than for the imidazolium
compartment; and 3) the envisioned Xꢀ ion should not com-
pete with bpy for the metal, either CuI or CuII.
In this work, detailed equilibrium studies have been car-
ried out in MeCN solution on the interaction of [1]2+ and
[2]2+ with CuII and CuI, and on the association of the corre-
sponding complexes with a variety of inorganic anions, to
define the conditions for the occurrence of the anion trans-
location process. It is anticipated that a reversible, electro-
chemically driven translocation between the two compart-
ments of the [CuII,I(1)]4+/3+ system occurs with the nitrate
ion.
We considered that the chelate coordination by the bpy
subunits of [1]2+ or [2]2+ of a metal M should generate a
closed compartment capable of establishing hydrogen-bond-
ing interactions with a given anion. In particular, H-bonds
should be donated to the anion 1) by two C–H fragments
from the imidazolium subunits and 2) by the two C–H frag-
ments in the 3-positions of the bpy subunits, which have
been activated through the coordination of bpy to the
metal.[26] Then, we thought that the redox couple CuII/CuI
could work as an effective switch for anion translocation. In
fact, the model complex [CuII
anion Xꢀ to give the solution-stable five-coordinate ternary
species [CuII(bpy)2(X)]+, whose coordination polyhedron is
a trigonal bipyramid, slightly distorted towards the square
A
ACHTREUNG
pyramid.[27] Thus, in
a solution containing equimolar
amounts of [CuII(L)]4+ ([L]2+ =[1]2+ or [2]2+) and of a se-
lected Xꢀ ion, the anion should be bound to the metal
!
centre to give the [CuII(L)( X)]3+ complex (the arrow de-
notes the metal–anion coordinative bond). On one-electron
Results and Discussion
!
reduction of [CuII(L)( X)]3+, the CuI centre forms, which
exhibits a definite preference towards four-coordination by
the two bpy molecules, according to a flattened tetrahedral
arrangement.[28] Under these conditions, the Xꢀ anion will
be extruded from the coordination sphere of the metal and
will move to the close H-bond-donating compartment, to
The formation of [CuII(1)]4+ and [CuII(2)]4+ complexesand
their interactionswith anions : The complexation of CuII by
[1]2+ and [2]2+ was investigated by titrating a solution of [1]-
(PF6)2 in MeCN (3.70”10ꢀ4 m) with a standard solution of
ꢀ
ꢀ
CuII
(CF3SO3)2 in MeCN. PF6 and CF3SO3 were chosen as
U
give the translational isomer [CuI(L)
A
counter anions in view of their very poor or zero coordinat-
ing tendencies towards metal ions.
symbol indicating the hydrogen-bonding interaction within
Chem. Eur. J. 2007, 13, 4988 – 4997
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4989

L. Fabbrizzi et al.
Figure 1a shows the spectra taken over the course of the
titration of [1]2+. CuII binding is demonstrated by the dra-
matic modifications observed in the UV spectral region. In
profile over the 0–1 equivalent interval prevented any safe
determination of the complexation constants K1M, whose
value can only be estimated as >106. In any case, the fact
that logK1M >6 allows one to assume that, in the concentra-
tion scale used in the studies described below (ꢁ10ꢀ4 m) and
in a solution equimolar in both 1 and CuII
(CF3SO3)2, the
A
[CuII(1)]4+ complex is the highly dominant metal-containing
species present (>99%). On the other hand, the profile ob-
served after the addition of one equivalent (see Figure 1c)
shows a curvature suitable for a safe determination of the
pertinent equilibrium constant. In particular, on processing
the titration data by a non-linear least-squares procedure,[29]
a logK2M value of 4.4Æ0.1 was calculated. A similar spectral
pattern was obtained on titration with CuII of a solution of
[2]2+ in MeCN, and the formation of
a very stable
[CuII(2)]4+ complex could be ascertained (logK1M >6;
logK2M =3.3Æ0.1).
To study the interaction of the [CuII(1)]4+ receptor with
anions, a solution of [1]2+ and CuII
(CF3SO3)2 in MeCN
G
(both 10ꢀ4 m) was prepared and titrated with a standard solu-
tion of a [Bu4N]X salt in MeCN. Figure 2a shows the family
of spectra taken in the visible region over the course of the
titration of a solution of [CuII(1)]4+ (1.01”10ꢀ4 m) with
[Bu4N]Br.
Figure 1. a) UV spectra recorded over the course of the titration of a so-
lution of [1]2+ in MeCN (3.7”10ꢀ4 m) with a standard solution of CuII-
!
*
(CF3SO3)2 in MeCN; b) titration profiles at 284 ( ) and 304 nm ( ); and
A
c) titration profile at 304 nm, after addition of one equivalent of CuII.
particular, on CuII addition the intensity of the band centred
at 284 nm, which arises from p–p* transitions in the bpy
subunits, decreases, while a new band, split into two distinct
transitions centred at 304 and 316 nm, forms and develops.
These spectral changes may result from the alteration of the
p energy levels of the bpy moiety, following their interaction
with the dp orbitals of CuII. In any case, the course of the
metal complexation is unambiguously illustrated by titration
profiles at selected wavelengths. As an example, Figure 1b
shows the variation of the absorbance at 284 and 304 nm, in-
duced by CuII addition. The sharp decrease (284 nm)/in-
crease (304 nm) of the absorbances observed on addition of
the first equivalent of CuII is indicative of the formation of a
rather stable [CuII(1)]4+ complex, according to the equilibria
[Eqs. (1) and (2)]:
Figure 2. a) Spectra recorded in the visible region over the course of the
titration of a solution of [CuII(1)]4+ in MeCN (1.01”10ꢀ4 m) with a stan-
dard solution of [Bu4N]Br in MeCN. Optical path: 1 cm; e: molar absorb-
ance. b) Titration profile at 745 nm.
2þ
4þ
Cu2þ þ ½1Š Ð ½CuIIð1ފ
ð1Þ
ð2Þ
½CuIð1ފ4þ þ Cu2þ Ð ½Cu2 ð1ފ
II
6þ
In the [CuII(1)]4+ species, the CuII centre should be coor-
dinated by the two bpy subunits of [1]2+. An MeCN mole-
cule may complete the five-coordinate geometrical arrange-
ment preferred by the CuII centre. After the addition of one
equivalent of CuII, a neat slope change is observed, which
Attention is focussed on the region of d–d transitions. In
particular, before Brꢀ addition, a broad band centred at ap-
proximately 700 nm is observed. Such a band should be the
envelope of the d–d transitions occurring in the five-coordi-
nate [CuII
A
N
II
corresponds to the formation of the [Cu2 (1)]6+ dimetallic
each bpy fragment of 1. On Brꢀ addition, the band is red-
shifted, while its intensity progressively increases. Such a
spectral change has to be ascribed to the replacement of the
coordinated MeCN molecule by a bromide ion, to give the
species, according to the stepwise equilibrium (2). In this
species, the ligand [1]2+ has opened its arms, so that each
bpy subunit can coordinate one CuII ion. The steep titration
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Intramolecular Anion Translocation
FULL PAPER
[CuII
G
subunits can be explained by assuming that Brꢀ accepts H-
bonds not only from the C–Ha fragments of the imidazolium
subunits, but also from the C–Hb fragments of the close pyri-
dine rings, which have been polarised through the coordina-
tion of the bpy subunits to the CuII centre. Noticeably, the
establishing of a definite hydrogen-bonding interaction be-
tween the bromide ion and the C–Hb fragments in an FeII-
containing bpy–imidazolium cage has been recently demon-
Figure 2b shows an abrupt slope change on addition of one
equivalent of Brꢀ, indicating that the formation equilibrium
of the [CuII(1)(Br)]3+ species is characterised by a constant
K1 whose value is higher than 106. On further addition of
bromide, minor spectral changes are observed, as emphas-
ised by the very moderate slope of the titration profile over
the interval of 1–2 equivalents of Brꢀ. It is tentatively sug-
gested that such small changes can be ascribed to a rear-
rangement of the five-coordinate copper complex, induced
by the binding of a second Brꢀ ion by the bis-imidazolium
compartment. However, more direct evidence on the occur-
rence of this second equilibrium could be obtained from
spectra recorded in the UV region (see Figure 3a).
1
strated through crystallographic and H NMR spectroscopic
results.[26] Discontinuity in spectral effects induced by anion
addition is observed in any portion of the spectra. For in-
stance, the profile of the absorbance at 325 nm (circles in
Figure 3b) shows a definite change of slope after the addi-
tion of one equivalent of Brꢀ. Also in this case, the steep-
ness of the 0–1 equivalent segment prevents the determina-
tion of K1, whose value should be higher than 106. On the
other hand, the smooth profile observed after one equiva-
lent would allow the determination of a binding constant. In
particular, on multi-wavelength treatment of titration data
through a non-linear least-squares procedure,[29] a value of
logK2 =4.62Æ0.06 was calculated. Thus, the interaction of
the [CuII(1)]4+ receptor with the Brꢀ ion can be described
through the stepwise equilibria (3) and (4), which give the
!
receptor–anion complexes [CuII(1)
(
Br)]3+ and [CuII(1)-
!
Br) .
(···Br)]2+
(
N
3þ
½CuIIð1ފ4þ þ Brꢀ Ð ½CuIIð1Þð Brފ
ð3Þ
2þ
½CuIIð1Þð Brފ3þ þ Brꢀ Ð ½CuIIð1Þð BrÞðꢂ ꢂ ꢂBrފ
ð4Þ
The occurrence of two distinct consecutive equilibria was
ꢀ
also ascertained for the anions Clꢀ, NCSꢀ, and NO3
Figure 3. a) Spectra recorded in the UV region over the course of the ti-
tration of a solution of [CuII(1)]4+ in MeCN (1.01”10ꢀ4 m) with a stan-
dard solution of [Bu4N]Br in MeCN. Optical path: 0.1 cm. b) Titration
through analogous titration experiments: the corresponding
logK values are reported in Table 1.
!
*
profiles at 312 nm ( , left vertical axis) and at 325 nm ( , right vertical
axis).
Table 1. LogK values for anion complexation equilibria in MeCN solu-
tion at 258C. In parentheses: the standard deviation on the last figure.
ꢀ
Equilibrium
[CuII(1)]4+ +Xꢀ
Clꢀ
Brꢀ
NCSꢀ
NO3
In particular, subtle but significant spectral modifications
are observed over the course of the titration. For instance,
the band centred at 312 nm decreases steeply during the ad-
dition of the first equivalent of Brꢀ (see the titration profile,
triangles, in Figure 3b). However, after the addition of one
equivalent, the absorbance stops decreasing and increases
with a smooth curvature, which indicates the occurrence of
two distinct and consecutive processes. It is suggested that
in the 0–1 equivalent interval, the Brꢀ ion goes to bind the
logK1
logK2
logK
logK1
logK2
logK2
>6
>6
>6
5.35(4)
3.49(4)
3.29(1)
5.53(4)
3.69(4)
3.56(1)
!
A
5.20(8)
4.62(6)
3.62(3)
[CuI(1)]3+ +Xꢀ
–
–
–
[a]
[a]
[a]
[CuII(2)]4+ +Xꢀ
>6
>6
>6
!
[CuII(2) X]3+ +Xꢀ
5.03(9)
4.31(9)
3.41(9)
[CuI(2)]3+ +Xꢀ
–
–
–
[a]
[a]
[a]
[a] The anion demetallates the receptor.
metal and forms the [CuII(N^N)2(Br)]+ ternary complex.
G
The charge density transferred on the metal affects indirect-
ly, but significantly, the p–p* transition within the bpy sub-
Whereas for the strongly coordinating anions Clꢀ and
ꢀ
NCSꢀ logK1 values >6 were estimated, in the case of NO3
A
a definite value of the first binding constant could be deter-
mined (logK1 =5.35Æ0.04). On the other hand, logK2 values
were observed to decrease according to the sequence: Clꢀ >
abruptly decreases. It is then assumed that the second Brꢀ
ion goes to interact with the bis-imidazolium compartment:
this process still has an effect on the p–p* transition within
the bpy subunit, but it operates in the opposite direction, in-
ducing an increase of the absorbance. The interference of
the second Brꢀ ion with the electronic transitions of the bpy
ꢀ
Brꢀ >NCSꢀ ꢃNO3 , which reflects the decrease of the H-
bond acceptor tendencies of the anion. Titration with
ꢀ
ꢀ
[Bu4N]I resulted in oxidation to I3 . Addition of HSO4 and
H2PO4ꢀ induced precipitation.
Chem. Eur. J. 2007, 13, 4988 – 4997
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4991

L. Fabbrizzi et al.
The same behaviour was observed for the isomeric system
In particular, the best-fitting of titration data over the
200–600 nm spectral range was observed on assuming the
occurrence of the two equilibria (5) and (6), whose constants
are: logb1 =logK1M =4.26Æ0.01 and logb2 =6.58Æ0.05.
[2]2+, that is, a stable [CuII(2)]4+ complex formed, which be-
ꢀ
haved as an anion receptor for Clꢀ, Brꢀ, NCSꢀ and NO3 .
The logK1 and logK2 values, reported in Table 1, are quite
similar to those observed for the [CuII(1)]4+ receptor. Thus,
it is concluded that appending the two imidazolium–bpy
arms, whether in the 1,4- or 1,3-position of the phenyl plat-
form, neither significantly modifies the coordination geome-
try and the binding tendencies of the [CuII(L)2]2+ subunit
(K1) nor alters the spatial arrangement and the H-bond-do-
nating properties of the imidazolium compartment (K2).
2þ
3þ
Cuþ þ ½1Š Ð ½CuIð1ފ
ð5Þ
ð6Þ
ð7Þ
2þ
I
4þ
2 Cuþ þ ½1Š Ð ½Cu2 ð1ފ
I
4þ
½CuIð1ފ3þ þ Cuþþ Ð ½Cu2 ð1ފ
Notice that the constant referring to the stepwise equilib-
rium (7), logK2M, can be obtained from the difference (log-
b2ꢀlogb1)=2.32Æ0.06. The interaction of CuI with [1]2+
could also be followed by investigating the spectral changes
in the UV region (Figure 4c). In particular, on CuI binding,
the band centred at 280 nm, originating from p–p* transi-
tions within the bpy subunits, is split into two bands centred
at 265 and 300 nm.
The formation of [CuI(1)]3+ and [CuI(2)]3+ complexesand
their interactionswith anions : The complexation of CuI by
[1]2+ and [2]2+ in MeCN was first investigated by titrating a
solution of the ligand with a standard solution of [CuI-
A
subunits of [1]2+ and [2]2+ could be visually perceived, even
in a diluted solution, through the appearance of a brick-red
colour, which is ascribed to a rather intense absorption band
centred at 450 nm of metal-to-ligand charge-transfer
(MLCT) nature. The development of such a band in the
case of system [1]2+ is illustrated in Figure 4a, while the ti-
tration profile at 456 nm is shown in Figure 4b. The slope
change observed in the profile at one equivalent of added
CuI suggests the occurrence of two consecutive equilibria in-
volving: 1) the formation of the [CuI(1)]3+ complex, in
which the metal centre is coordinated by the two bpy sub-
Figure 5a shows the percentage concentration of the spe-
cies present at the equilibrium over the course of the titra-
tion experiment illustrated in Figure 4. It is observed that
the dimetallic species [Cu2 (1)]4+ is formed in an appreciable
I
amount only on addition of a substantial excess of CuI
(13% at 5 equiv). In any case, the formation of such a dime-
tallic species must be assumed to ensure a fully satisfactory
fitting of titration data over the entire titration range.
The interaction of the CuI ion with ligand [1]2+ was also
A
investigated by titrating a solution of [CuI
MeCN with a standard solution of [1](PF6)2 (i.e. in the oppo-
A
I
formation of the dimetallic complex [Cu2 (1)]4+, in which
ACHTREUNG
each bpy subunit of [1]2+ binds a copper(I) centre.
site way with respect to the titration experiment illustrated
in Figure 4).
Figure 6a shows the visible
portion of the spectra taken
over the course of the titration,
while Figure 5b displays the ti-
tration profile at 450 nm. Best-
fitting of titration data was
achieved by assuming the oc-
A
currence of equilibria (5) and
(6), to which the following con-
stants corresponded: logb1 =
4.23Æ0.01 and logb2 =6.7Æ0.1.
The values are in good agree-
ment with those obtained from
the titration experiments illus-
trated in Figure 4 (metal added
to the ligand). Figure 5b shows
the percentage concentration of
the species present at the equi-
librium over the course of the
titration experiments. It is ob-
served that the dimetallic spe-
I
cies [Cu2 (1)]4+ is formed in a
very small amount (the maxi-
mum concentration of about
1% being achieved on addition
Figure 4. Spectra recorded over the course of the spectrophotometric titration of a solution of [1](PF6)2 in
C
G
tions); c) UV portion (p–p* transitions within the bpy subunits); b,d) titration profiles at selected wave-
lengths.
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Chem. Eur. J. 2007, 13, 4988 – 4997

Intramolecular Anion Translocation
FULL PAPER
in most cases the decomposi-
tion of the [CuI(1)]3+ complex,
as inferred by a decrease of the
intensity of the MLCT band
and the disappearance of the
brick-red colour since the first
additions of the tetraalkylam-
monium salt.
As an example, Figure 7a
shows the spectra taken over
the course of the titration with
[Bu4N]NCS. The band pertinent
to the MLCT transition in the
CuI–polypyridine complex pro-
gressively decreases on anion
addition, as also indicated by
the absorbance profile in Fig-
ure 7b.
!
Figure 5. Percentage concentrations (c, left vertical axis) and molar absorbance at 450 nm ( , right vertical
axis) pertinent to titrations of a) a solution of [1](PF6)2 in MeCN with
standard solution of [CuI-
(MeCN)4]ClO4; and (b) a solution of [CuI
(MeCN)4]ClO4 in MeCN with a standard solution of [1](PF6)2 in
MeCN.
G
a
U
G
ACHTREUNG
Figure 6. Spectra recorded over the course of the spectrophotometric ti-
tration of a solution of [CuI(MeCN)4]ClO4 in MeCN (2.00”10ꢀ4 m) with a
standard solution of [1](PF6)2 in MeCN; a) visible portion (MLCT transi-
AHCTREUNG
ACHTREUNG
tions) and b) titration profile at 450 nm.
Figure 7. a) Spectra recorded over the course of the spectrophotometric
titration of a solution of [1]
U
G
of 0.8–1.0 equiv of [1]2+). In any case, it has to be noted that
tion profile at 450 nm.
the presence of the dimetallic complex [Cu2 (1)]4+ is not
I
strictly relevant for data fitting. Quite interestingly, for both
titration experiments the absorbances of the MLCT band
superimpose well on the concentration of the [CuI(1)]3+
complex (see triangles in Figure 5a and b).
Notice that a totally similar behaviour was also observed
on addition of NCSꢀ to an MeCN solution of the model
The higher stability of the [CuII(1)]4+ complex (logb1 >6)
with respect to the monovalent species [CuI(1)]3+ (logb1 =
4.2) is not surprising and reflects 1) the crystal-field stabili-
sation effects experienced by the CuII (d9) and not by CuI
(d10), 2) the higher coordination number n of the CuII com-
plex (n=5: 2N^N+1MeCN) with respect to CuI (n=4, no
solvent molecules in the coordination sphere; the compari-
son is fair, as both uncomplexed CuII and CuI are present in
solution as tetra-solvated cations).
complex [CuI(bpy)2]+. In particular, CuI–polypyridine com-
G
plexes in a MeCN solution are unstable with respect to the
a addition of Xꢀ anions, which form the more stable
[CuIX4]3ꢀ complexes. In this connection, one would expect
that [CuI(1)]3+ species should display extra stability due to
the chelate effect. However, it is suggested that such an ad-
ditional stability is cancelled by the electrostatic repulsions
exerted by the two facing imidazolium subunits within the
chelate complex.
ꢀ
Then, the capability of the [CuI(1)]3+ complex to act as an
anion receptor was investigated by titrating a solution of [1]-
Quite happily, on titration with NO3 , the MLCT band re-
mained intact and colour did not disappear even after addi-
tion of a large excess of anion. On the other hand, signifi-
cant spectral modifications were observed in the p–p*
region, as shown in Figure 8a.
A
N
with a solution of the chosen [Bu4N]X salt. Experiments
were frustrating with most anions, as anion addition induced
Chem. Eur. J. 2007, 13, 4988 – 4997
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4993

L. Fabbrizzi et al.
Electrochemically driven nitrate translocation: The occur-
ꢀ
rence of the redox-controlled NO3 translocation process
was first investigated through CV experiments. For compara-
tive purposes, the redox behaviour of the model system
[CuII,I(Mebpy)2]2+/+ was preliminarily investigated.
G
Figure 9a shows the CV profile obtained at a platinum
working electrode on a solution of [CuII
(Mebpy)2](ClO4)2 in
MeCN (2.00”10ꢀ3 m; dashed line). The process is poorly re-
A
ACHTREUNG
versible (Dp=150 mV at
a
potential scan rate of
500 mVsꢀ1), due to the high kinetic barrier associated with
the change of the coordination geometry of the redox-active
Figure 8. a) Spectra recorded in the UV region over the course of the ti-
tration of a solution of [CuI(1)]3+ in MeCN (3.01”10ꢀ4 m) with a standard
solution of [Bu4N]NO3 in MeCN. b) Titration profile at 296 nm.
Figure 8b shows the titration profile at a selected wave-
length. Best-fitting of spectral data was obtained on assum-
ing the formation of a 1:1 receptor/anion adduct, according
to Equation (8):
ꢀ
2þ
½CuIð1ފ3þ þ NO3 Ð ½CuIð1Þðꢂ ꢂ ꢂNO3ފ
ð8Þ
Figure 9. CV profiles obtained at the platinum working electrode (poten-
with an association constant logK=3.29Æ0.01. It is suggest-
ed that, in this complex, nitrate establishes hydrogen-bond-
ing interactions with the bis-imidazolium compartment. In
tial scan rate 500 mVsꢀ1) in a solution of [Bu4N]ClO4 in MeCN (0.1m).
a) (a) [ Cu
b) (a) [1]2+ and CuII
[Bu4N]NO3 (1.0 equiv).
II(Mebpy)2]2+ (2.00”10ꢀ3 m), (c)
+
excess [Bu4N]NO3;
N
+
ꢀ
particular, the NO3 ion should interact not only with imida-
zolium C–Ha fragments, but also with the C–Hb fragments
of metal-bound bpy subunits, which explains the effect on
the p–p* transitions centred on the bpy moieties. The occur-
rence of hydrogen-bonding interactions at the bis-imidazoli-
um compartment is indirectly demonstrated by the fact that,
metal centre: from distorted tetrahedral of CuI to distorted
trigonal-bipyramidal of CuII. On titration with nitrate, the
CV profile became more reversible (Dp=120 mV) and both
reduction and oxidation peaks were shifted towards more
negative potentials. The solid line in Figure 9a refers to the
CV profile taken in the presence of a large excess of
on titration of the [CuI(Mebpy)2]+ (Mebpy: 5-methyl-2,2’-bi-
A
pyridine) model system with nitrate, no spectral modifica-
tions were observed in either the UV or the visible regions.
The [2]2+ system exhibited a similar behaviour, as far as
the complexation equilibria (5) and (6) are concerned
(logb1 =4.55Æ0.02, logb2 =7.27Æ0.05, determined from the
metal-added-to-ligand procedure). Then, a logK=3.56Æ
0.01 was determined for the nitrate-association equilibri-
um (7).
[Bu4N]NO3; its E = value is 55 mV more negative than that
1
2
ꢀ
recorded in the absence of NO3 . The anion-induced in-
crease of electrochemical reversibility can be tentatively as-
cribed to a beneficial involvement of NO3 in the activated
complex that forms during the change of the coordination
geometry. On the other hand, the cathodic shift of the po-
tential simply reflects the thermodynamic stabilisation of
the CuII complex, which takes advantage from the coordina-
ꢀ
These studies suggest that translocation experiments
could be carried out by operating on a solution containing
ꢀ
ꢀ
equimolar amounts of NO3 and of the copper–L system.
tion of NO3 (which replaces a coordinated MeCN mole-
When copper is in the +2 oxidation state, the anion should
be bound to the metal centre. Electrochemical reduction to
CuI should make the anion translocate to the proximate bis-
cule). Needless to say, the coordinatively saturated CuI com-
plex cannot profit from such an effect.
Figure 9b displays the CV profile of a solution containing
ꢀ
imidazolium compartment. Then, the NO3 ion could be re-
equimolar amounts of [1]2+ and CuII
ACHTREUNG
located at will in one of the two sites by switching the poten-
tial of the working electrode between two definite values, in
electrochemical experiments that are both dynamic (cyclic
voltammetry, CV) and static (controlled-potential electroly-
sis).
-
A
rearrangement associated with the CuI-to-CuII change in-
volves a substantial change of the angle between the planes
containing the bpy subunits. It is possible that, in the course
4994
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 4988 – 4997

Intramolecular Anion Translocation
FULL PAPER
of the [CuII,I(1)]4+/3+ redox change, such a rearrangement is
made sterically difficult by the presence of the carbon chain
linking the two bpy moieties. Addition of one equivalent of
of the working electrode at 100 mV versus Fc+/Fc, the solu-
tion decolourises and the spectrum of the [CuII(1)-
!
NO3)]3+ complex is restored (dashed black line in Fig-
A
ꢀ
NO3 renders the CV profile more reversible (Dpꢃ300 mV)
ure 10a). Then, on exhaustive reduction at ꢀ700 mV, the
and, most interestingly, makes the cathodic peak (associated
with the CuII-to-CuI reduction) less negative, an opposite be-
spectrum of the [CuI(1)(···NO3)]2+ species forms again
ACHTREUNG
(dashed grey line in Figure 10a), without any substantial de-
composition. The switching nature of the process is illustrat-
ed in Figure 10b, which reports the absorbance at 295 nm
after the completion of every electrolytic cycle.
haviour with respect to what is seen with the [CuII,I
-
G
[CuI(1)]3+ form can be ascribed to the fact that, on CuII/CuI
ꢀ
reduction, the leaving NO3 ion does not move to the bulk
While the occurrence of a reversible translocation of a
ꢀ
solution but goes to the close bis-imidazolium compartment,
thus profiting from a favourable energy term not experi-
NO3 ion from the CuII metal centre to the bis-imidazolium
compartment of the [CuI(1)]3+ system has been demonstrat-
ed (in a process involving about 80% of the copper com-
plexes in solution, at the concentration level of the exhaus-
tive electrolysis experiment), one could ask if the anion-
transfer process is intra- or intermolecular, that is, whether
enced by the reference system [CuI
voltammetric cycles, NO3 moves back and forth, quickly
and reversibly, between the metal centre and H-bond-donat-
ing cavity.
A
ꢀ
ꢀ
The occurrence of the translocation process was charac-
terised under bulk conditions in a controlled-potential elec-
trolysis experiment. The electrolytic cell, equipped with a
working electrode (platinum gauze), a counter-electrode
(platinum coil) and a reference electrode (platinum wire, in-
ternally calibrated through the Fc+/Fc couple; Fc: ferro-
the NO3 ion moves directly from CuII to the H-bond-donat-
ꢀ
ing cavity or is released to the solution and another NO3
from the solution moves to the bis-imidazolium compart-
ment. The question is irrelevant for the bulk electrolysis ex-
periment, which is carried out over the course of minutes, a
ꢀ
period in which the labile NO3 ions continuously exchange
cene), contained a solution of [1]2+, CuII
C
from the binding sites to the solution. However, the intra-
molecular issue may have sense for a CV experiment carried
out at a relatively high-potential scan rate. According to the
!
NO3)]3+ species is
tions, it is calculated that the [CuII(1)
(
ꢀ
present at 81% (with respect to the total Cu). The spectrum
of this solution is shown in Figure 10a (black solid line).
Then, the potential of the platinum gauze was set at
ꢀ700 mV versus Fc+/Fc. The consumption of one equivalent
of electrons was ascertained through coulometry, while the
solution took on a brick-red colour. The spectrum of the
electrolysed solution, shown in Figure 10a (solid grey line),
is that previously observed in the formation of the [CuI(1)-
intramolecular hypothesis, the NO3 transfer should not
result from a jump from one site to the other, but, more re-
alistically, from the folding of the molecular framework of
[1]2+ that brings the anion from the metal to the bis-imid-
A
bis-imidazolium cavity and the metal centre, as evaluated
from molecular models, is about 7 Š. Thus, the concentra-
tion of the NO3ꢀ ion, useful for the intramolecular transloca-
tion, is given by one molecule over the volume of the
sphere whose centre is the centroid of the bis-imidazolium
compartment and whose radius is 7 Š (ꢃ1400 Š3). This cor-
responds to a 0.84m concentration, much higher than that of
A
age concentration of this complex, under the conditions of
the electrolysis experiment, is 77%. On setting the potential
ꢀ
NO3 in the bulk solution, under the typical conditions of
the electrochemical experiments (10ꢀ3–10ꢀ4 m) and, on statis-
tical bases, points towards the occurrence of a true intramo-
lecular process.
It is disappointing, but curious, that the redox-driven
ꢀ
translocation process takes place only with the NO3 ion.
This results from the unique combination of two properties
of nitrate: 1) its moderate coordinating tendency (which
does not afford the decomposition of the [CuI(1)]3+ system)
and 2) its definite, although not very pronounced, ability to
accept H-bonds. In this respect, it is interesting to consider
the behaviour of the single-arm system [3]+. On addition of
[CuI
(MeCN)4]ClO4 to an MeCN solution containing an
G
excess of the single-arm system [3]+, no colour develops and
metal coordination to the bpy fragment does not take place.
Figure 10. a) Spectra recorded after a cycle of exhaustive electrolysis ex-
periments on a solution of [Bu4N]ClO4 (0.1m), [1]2+ (4.00”10ꢀ3 m), CuII-
(CF3SO3)2 (4.00”10ꢀ3 m) and [Bu4N]NO3 (4.00”10ꢀ3 m) in MeCN. Optical
ꢀ
path: 0.01 cm. Spectral changes accompany the translocation of the NO3
!
anion, according to the redox equilibrium: [CuII(1)
A
NO3)]3+ +eꢀÐ
[CuI(1)(···NO3)]2+
U
; b) absorbance at 295 nm, after the completion of
every electrolytic cycle.
Chem. Eur. J. 2007, 13, 4988 – 4997
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4995

L. Fabbrizzi et al.
basis of correlated spectroscopy (COSY) experiments. Electrochemical
experiments (CV and controlled-potential coulometry, CPC) were per-
formed with a BAS 100B/W electrochemical workstation. In CV studies
(on a solution of [Bu4N]ClO4 in MeCN (0.1m)), the working electrode
was a platinum microsphere and the counter-electrode a platinum wire.
An Ag/AgCl electrode was used as pseudo-reference and was calibrated
versus ferrocene as an internal standard. The measured potentials were
referred to the Fc+/Fc couple (425 mV versus SCE, in CH3CN). CPC ex-
However, if [Bu4N]NO3 is added to this solution, the brick-
red colour of the [CuI(L)2]+ species develops, indicating that
metal complexation takes place and requires the additional
contribution from the hydrogen-bonding interaction be-
tween nitrate and imidazolium subunits. While this feature
provides a nice procedure for the colourimetric detection of
the elusive nitrate ion (other anions do not have any effect),
it also demonstrates the intrinsically poor binding tendencies
of the bpy–imidazolium system. Ligands [1]2+ and [2]2+ suc-
ceeded in forming relatively stable complexes with CuI be-
cause their bpy subunits belong to the same framework,
thus profiting from the chelate effect.
periments were performed on solutions of [1]2+, CuII
(CF3SO3)2 and
N
[Bu4N]NO3 (all 4.00”10ꢀ3 m), with a platinum gauze as working electrode
and a platinum coil as counter-electrode. The counter-electrode compart-
ment was separated from the working compartment by a glass frit. All
compartments were filled with
a solution of [Bu4N]ClO4 in MeCN
(0.1m). The reference electrode was an Ag/AgCl electrode calibrated
versus Fc+/Fc through CV experiments prior to the CPC.
Synthesis of [1](PF6)2: A mixture of imidazole (1.2 g, 19 mmol) and
A
sodium hydride (0.79 g, 34 mmol) in dry THF (100 mL) was stirred for
15 min at room temperature. 1,4-Di(bromomethyl)benzene (1.26 g,
Conclusion
The redox-driven transfer of an anion between two compart-
ments of different binding tendencies (metal–ligand and hy-
drogen-bonding interactions) has been described. The inves-
tigated systems ([1]2+ and [2]2+) allow the conversion of
electrochemical energy into a controlled and repeatable
movement and, in this sense, can be considered a further ex-
ample of a molecular machine.[30] The CuII/CuI couple plays
a primary role as it mediates the uptake of electrochemical
energy and determines, through the oxidation state of the
metal, the location of the anion in one or the other available
compartment of the ditopic receptor. This unique feature is
related to the fact that the one-electron uptake/release
makes the metal cross the border between two realms of
very different habits and lifestyles of transition (e.g. d9) and
of post-transition (e.g. d10) metals. It is not accidental that a
significant proportion of redox-driven supramolecular ma-
chines and motors are based on the CuII/CuI couple within a
polypyridine coordinative environment.[31] The system inves-
4.75 mmol) was added to the mixture, which was further stirred overnight
at room temperature. Water was then added and THF was rotary evapo-
rated. The water phase was extracted with CH2Cl2 (3”70 mL); the col-
lected organic phases were first washed with water (100 mL) and then
dried over Na2SO4. Filtration and evaporation of the solvent yielded a
white solid, which was immediately engaged in the next step. The crude
product (0.15 g, 0.63 mmol) was dissolved in dry MeCN (40 mL) and 5-
(bromomethyl)bipyridine (0.40 g, 1.60 mmol) was added. The mixture
was refluxed for three days, after which a white precipitate appeared.
The solution was cooled to room temperature. The white precipitate was
isolated by filtration, washed with MeCN and dissolved in the minimum
amount of methanol. A saturated aqueous solution of KPF6 was poured
onto the solution, and the resulting solid was collected and washed with
water. [1](PF6)2 was obtained as a white solid (0.28 g, 52% yield).
T
ꢀ
1H NMR (400 MHz, CD3CN): d=8.70 (m, 6H; Ha, H6, H6’), 8.52 (d, J=
8.0 Hz, 2H; H3), 8.46 (d, J=8.0 Hz, 2H; H3’), 7.95 (t, J=7.0, 8.0 Hz, 2H;
H4’), 7.90 (dd, J=2.0, 8.0 Hz, 2H; H4), 7.45 (m, 10H; Ha, Hb, Hb, Hc, H5’),
5.30 (s, 4H; Hd), 5.42 ppm (s, 4H; Hg); 13C NMR (400 MHz, CD3CN):
d=157.1, 155.4, 149.9, 149.8, 138.0, 137.8, 136.4, 135.1, 129.8, 124.9, 123.5,
123.3, 121.3, 117.8, 52.8, 50.8 ppm; ESI-MS for C36H32N8P2F12: m/z: 288
tigated here is able to translocate only the NO3 ion, while
more ligating anions, such as Clꢀ and Brꢀ, form stable
binary complexes with the CuI ion, inducing the decomposi-
tion of the polypyridine complex. This is an obligatory con-
sequence of the intrinsically low coordinating tendencies of
[1]2+ and [2]2+, which results from the electrostatic repul-
sions between the two imidazolium subunits.
[Mꢀ2PF6]2+
.
Synthesis of [2]
ACHTREUNG
scribed for [1]
ACHTREUNG
a
19 mmol) and sodium hydride (0.79 g, 34 mmol) in dry THF (100 mL).
After treatment with water, the mixture was extracted with CH2Cl2 (3”
70 mL). The organic phase was washed with water and dried over
Na2SO4. Evaporation of CH2Cl2 gave rise to a sticky white product,
which was employed in the next reaction without purification. The crude
product (0.15 g, 0.63 mmol) was dissolved in dry MeCN and treated with
5-(bromomethyl)bipyridine (0.40 g, 1.60 mmol). After the mixture was re-
fluxed for three days, the white precipitate was collected, dissolved in
methanol and the solution was treated with an aqueous solution of KPF6.
Experimental Section
General proceduresand materials : All reagents for syntheses were pur-
chased from Aldrich/Fluka and used without further purification. All re-
actions were performed under N2. 5-(Bromomethyl)bipyridine was pre-
pared according to the literature procedure.[32] UV/Vis spectra were re-
corded on a Varian CARY 100 spectrophotometer, with quartz cuvettes
of the appropriate path length (0.1 or 0.01 cm). In any case, the concen-
tration of the chromophore and the optical pathway were adjusted to
obtain spectra with AUꢄ1. In the titrations with anions, the UV/Vis
spectra of the samples were recorded after the addition of aliquots of an
alkylammonium salt solution of the envisaged anion ([Bu4N]+ for Brꢀ,
ꢀ
NO3 and NCSꢀ; [BnBu3N]+ for Clꢀ; Bn: benzyl). All spectrophotomet-
ric titration curves were fitted with the HYPERQUAD program.[29]
1H NMR spectra were obtained on a Bruker Avance 400 spectrometer
(400 MHz) operating at 9.37 T. Proton assignments were made on the
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Chem. Eur. J. 2007, 13, 4988 – 4997

Intramolecular Anion Translocation
FULL PAPER
[2]
A
[14] V. Amendola, L. Fabbrizzi, C. Mangano, P. Pallavicini, Acc. Chem.
Res. 2001, 34, 488–493.
vacuum (0.32 g, 58% yield).
1H NMR (400 MHz, CD3CN): d=8.82 (m, 6H; Ha, H6, H6’), 8.52 (d, J=
8.0 Hz, 2H; H3), 8.48 (d, J=8.0 Hz, 2H; H3’), 7.92 (m, 4H; H4, H4’), 7.40
(m, 10H; Ha, Ha’, Hb, Hg, Hb, Hc, H5’), 5.32 (s, 4H; He), 5.40 ppm (s, 4H;
Hd); 13C NMR (400 MHz, CD3CN): d=155.5, 154.2, 149.9, 148.8, 147.0,
139.2, 138.3, 136.4, 135.0, 130.6, 130.5, 129.9, 129.5, 125.4, 123.4, 121.8,
121.5, 120.8, 119.9, 117.8, 87.5, 83.1, 53.2, 50.7 ppm; ESI-MS for
[15] V. Amendola, C. Brusoni, L. Fabbrizzi, C. Mangano, H. Miller, P.
Pallavicini, A. Perotti, A. Taglietti, J. Chem. Soc. Dalton Trans.
2001, 3528–3533.
[16] V. Amendola, L. Fabbrizzi, C. Mangano, H. Miller, P. Pallavicini, A.
Perotti, A. Taglietti, Angew. Chem. 2002, 114, 2665–2668; Angew.
Chem. Int. Ed. 2002, 41, 2553–2556.
[17] L. Fabbrizzi, F. Foti, S. Patroni, P. Pallavicini, A. Taglietti, Angew.
Chem. 2004, 116, 5183–5187; Angew. Chem. Int. Ed. 2004, 43, 5073–
5077.
[18] A. Aurora, M. Boiocchi, G. Dacarro, F. Foti, C. Mangano, P. Pallavi-
cini, S. Patroni, A. Taglietti, R. Zanoni, Chem. Eur. J. 2006, 12,
5535–5546.
[19] L. Fabbrizzi, M. Licchelli, P. Pallavicini, Acc. Chem. Res. 1999, 32,
846–853.
[20] G. De Santis, L. Fabbrizzi, D. Iacopino, P. Pallavicini, A. Perotti, A.
Poggi, Inorg. Chem. 1997, 36, 827–832.
[21] L. Fabbrizzi, F. Gatti, P. Pallavicini, E. Zambarbieri, Chem. Eur. J.
1999, 5, 682–690.
[22] a) Supramolecular Chemistry of Anions (Eds.: A. Bianchi, K.
Bowman-James, E. García-EspaÇa), Wiley-VCH, New York, 1997;
b) F. P. Schmidtchen, M. Berger, Chem. Rev. 1997, 97, 1609–1646;
c) P. D. Beer, Acc. Chem. Res. 1998, 31, 71–80; d) P. D. Beer, P. A.
Gale, Angew. Chem. 2001, 113, 502–532; Angew. Chem. Int. Ed.
2001, 40, 486–516; e) P. A. Gale, Coord. Chem. Rev. 2003, 240, 1–
226, f) R. Martínez-Mꢁꢂez; F. Sancenón Chem. Rev. 2003, 103,
4419–4476; g) V. Amendola, D. Esteban-Gomez, L. Fabbrizzi, M.
Licchelli, Acc. Chem. Res. 2006, 39, 343–353; h) V. Amendola, M.
Bonizzoni, D. Esteban-Gómez, L. Fabbrizzi, M. Licchelli, F. Sance-
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Res. 2006, 39, 465–475.
C36H34N8P2F12: m/z: 288 [Mꢀ2PF6]2+
.
Synthesis of [3]PF6: A mixture of imidazole (0.65 g, 10.3 mmol) and
sodium hydride (0.43 g, 18.6 mmol) in dry THF (100 mL) was stirred for
15 min at room temperature. 5-(Bro-
momethyl)bipyridine
(1.28 g,
5.1 mmol) in THF (150 mL) was
added to the mixture, which was fur-
ther stirred overnight at room temper-
ature. Water was then added and THF
was rotary evaporated. The water
phase was extracted with CH2Cl2 (3”100 mL); the collected organic
phases were first washed with water (100 mL) and then dried over
Na2SO4. Filtration and evaporation of the solvent yielded an oily product,
which was immediately used in the next step. The crude product (0.29 g,
1.23 mmol) was dissolved in CHCl3 (40 mL) and benzyl bromide (175 mL,
1.47 mmol) was added. The mixture was refluxed for three days, then the
solvent was removed and the orange residue was dissolved in the mini-
mum amount of water. The obtained solution was treated with saturated
NH4PF6 until the complete precipitation of [3]PF6 as a light-orange solid
(0.23 g, 40% yield).
1H NMR (400 MHz, CD3CN): d=8.72 (broad s, 2H; H6, H6’), 8.65 (s,
1H; Ha), 8.52 (d, J=8.0 Hz, 1H; H3), 8.46 (d, J=8.0 Hz, 1H; H3’), 7.95
(t, J=7.0, 8.0 Hz, 1H; H4’), 7.90 (dd, J=2.0, 8.0 Hz, 1H; H4), 7.45 (m,
8H; HBz, Hb, Hc, H5’), 5.32 (s, 2H; Hb), 5.41 ppm (s, 2H; Ha); ESI-MS for
C21H19N4PF6: m/z: 327 [M]+.
[23] J. L. Sessler, P. A. Gale, W. S. Cho, Anion Receptor Chemistry,
(Monographs in Supramolecular Chemistry, Ed.: J. F. Stoddart),
Royal Society of Chemistry, Cambridge, 2006.
[24] J. Yoon, S. K. Kim, N. J. Singh, K. S. Kim, Chem. Soc. Rev. 2006, 35,
355–360.
Acknowledgements
[25] A. P. Smith, L. C. Fraser in Comprehensive Coordination Chemis-
try II, Vol. 1 (Eds.: J. A. McCleverty, T. J. Meyer), Elsevier, New
York, 2004, pp. 1–23.
[26] V. Amendola, M. Boiocchi, B. Colasson, L. Fabbrizzi, M.-J. Rodri-
guez Douton, F. Ugozzoli, Angew. Chem. 2006, 118, 7074–7078;
Angew. Chem. Int. Ed. 2006, 45, 6920–6924.
The financial support of the Italian Ministry of University and Research
(PRIN—Dispositivi Supramolecolari; FIRB—Project RBNE019H9K) is
gratefully acknowledged.
[1] R. A. Bissell, E. Córdova, A. E. Kaifer, J. F. Stoddart, Nature 1994,
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[27] Representative examples of X-ray-determined molecular structures
of [CuII(bpy)2(X)]+ complexes include: X=Clꢀ, Brꢀ, Iꢀ: a) C. OꢃSul-
A
[2] A. Livoreil, C. O. Dietrich-Buchecker, J.-P. Sauvage, J. Am. Chem.
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Received: December 22, 2006
Published online: March 15, 2007
Chem. Eur. J. 2007, 13, 4988 – 4997
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4997