Acid-base-driven interconversion between a mononuclear complex and supramolecular coordination polymers in a terpyridine-functionalized dioxocyclam ligand

Article abstract of DOI:10.1021/ic701584e

A polytopic cyclam-bis-terpyridine ligand has been used to accomplish an acid-base-triggered formation of either a mononuclear neutral complex or metallopolymers with Cu²⁺ ions. A controlled interconversion between these two forms was achieved through the reversible displacement of a Cu ²⁺ cation from the macrocycle to the terpyridine units.

Full text of DOI:10.1021/ic701584e

Inorg. Chem. 2008, 47, 1862-1864
Acid–Base-Driven Interconversion between a Mononuclear Complex and
Supramolecular Coordination Polymers in a Terpyridine-Functionalized
Dioxocyclam Ligand
Aurélien Gasnier,† Jean-Michel Barbe,‡ Christophe Bucher,† Franck Denat,‡ Jean-Claude Moutet,†
Eric Saint-Aman,† Pierre Terech,§ and Guy Royal*,†
Département de Chimie Moléculaire, UMR CNRS 5250, UniVersité Joseph Fourier, BP 53,
38041 Grenoble Cedex 9, France, Institut de Chimie Moléculaire de l’UniVersité de Bourgogne,
UMR CNRS 5260, 9 aVenue Alain SaVary, BP 47870, 21078 Dijon Cedex, France, and UMR
5819, CEA-Grenoble, INAC-SPrAM-LASSO, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
Received August 8, 2007
Scheme 1. Complexation Routes of Cu2+ by the L1 Ligand (A)
without and (B) with the Addition of a Base and (C) Acid–Base-Driven
Interconversion
A polytopic cyclam-bis-terpyridine ligand has been used to
accomplish an acid–base-triggered formation of either a mono-
nuclear neutral complex or metallopolymers with Cu2+ ions. A
controlled interconversion between these two forms was achieved
through the reversible displacement of a Cu2+ cation from the
macrocycle to the terpyridine units.
Supramolecular architectures containing labile and revers-
ible bonds between multifonctional ligands and transition-
metal ions open large possibilities for the conception of smart
molecular materials and devices.1 In particular, remarkable
coordination polymers2 or metal-containing molecular sys-
tems acting as dynamic molecular switches3 have been
recently reported. It thus appears highly challenging to
combine these two domains, i.e., to elaborate coordination
polymers displaying switching properties.1–4 Herein we
report a proton-triggered self-assembling-disassembling
process between mononuclear and polynuclear copper(II)
complexes based on the bis-terpyridine-dioxocyclam ligand
L1 (Scheme 1).
Chart 1. Structures of the Ligand L2 and the [L2–2HCuII]0 Complex
Used as Reference Compounds in This Study
* To whom correspondence should be addressed. E-mail: guy.royal@
ujf-grenoble.fr.
†
Université Joseph Fourier.
Institut de Chimie Moléculaire de l’Université de Bourgogne.
CEA-Institute for Nanoscience and Cryogenics.
‡
The 1,4,8,11-tetraazacyclotetradecane (cyclam) is a re-
markable molecular platform for the conception of elaborated
architectures because it can be N- and/or C-functionalized
and forms many stable complexes with a wide range of metal
ions.5,6 Here, the employed cyclam derivative L1 comprises
two types of coordination sites: (i) tridentate terpyridine
groups, well-known to form octahedral 2:1 (L/M) complexes
§
(1) Lehn, J.-M. Chem. Soc. ReV. 2007, 36, 151–160.
(2) (a) Oh, M.; Mirkin, C. A. Nature 2005, 438, 651–654. (b) Hoogen-
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Friese, V. A.; Kurth, D. G. Coord. Chem. ReV. 2008, 252, 199–211.
(d) Williams, K. A.; Boydston, A. J.; Bielawski, C. W. Chem. Soc.
ReV. 2007, 36, 729–744.
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2007, 46, 72–191. (b) Molecular Switches; Feringa, B. L., Ed.; Wiley-
VCH: Weinheim, Germany, 2001. (c) Balzani, V.; Venturi, M.; Credi,
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Thomas, F. Inorg. Chem. 2004, 43, 3777–377. (b) Padilla-Tosta, M. E.;
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1862 Inorganic Chemistry, Vol. 47, No. 6, 2008
10.1021/ic701584e CCC: $40.75
2008 American Chemical Society
Published on Web 02/19/2008

COMMUNICATION
Table 1. Characteristic Visible Absorption, FT-IR, and Electrochemical Data of the Ligands and Copper(II) Complexes Used in This Work
b c
,
p
FT-IR data ν (cm-1
)
electrochemical data E1/2 (∆E )
terpyridine-centered reduction
CuII/I
V
compound
L1
λ
max, nma (ꢀ, L mol-1
)
νCO
νCF SO -
CuIII/II
3
3
1645
1646
1645
-2.420 (0.070)
L2
2n+
[L1CuII]n
687 (69)
687 (73)
503 (156)
501 (115)
1259, 1033
1262, 1031
d
d
e
e
-0.719 (0.190)
-0.733 (0.080)
-1.475 (0.085)
-1.514 (0.090)
[(ttpy)CuII]2+
[L1–2HCuII]0
[L2–2HCuII]0
a
1544
1559
-2.465 (0.080)
+0.434 (0.085)
+0.429 (0.090)
b
In MeOH. Because of the low solubility of some compounds in MeOH in the presence of a supporting electrolyte, experiments were conducted in
c
d
c
e
a
c
a
DMF + 0.1 M TBAP; V vs Fc/Fc+ couple. E1/2 ) (Ep + Ep )/2 at 0.1 V s-1; ∆Ep ) Ep - Ep . Not observed. Not observed because of the prior
electrodeposition of Cu0.
with transition-metal cations and (ii) a dioxocyclam (1,4,8,11-
tetraazacyclotetradecane-5,7-dione) unit whose complexing
properties might be turned on upon deprotonation of the
amide functions.7 L1 was prepared by the reaction of 4′-[4-
(bromomethyl)phenyl]-2,2′:6′2′′-terpyridine with trans-di-
oxocyclam in refluxing CH3CN in the presence of a base.
The reaction of L1 in MeOH with Cu(CF3SO3)2 in a 1:1
molar ratio afforded a green solution, from which a light-
green powder was isolated upon precipitation by the addition
of diethyl ether.
This species presents an absorption band at λmax ) 687
nm in MeOH, identical with that obtained with the
Figure 1. (left) CV curves in DMF + TBAP 0.1 mol L-1 solutions and
(right) visible absorption spectra in MeOH of millimolar solutions of (A)
[(ttpy)2CuII]2+ reference compound (Chart 1 and Table 1).
This feature strongly suggests the formation of a copper(II)
bis-terpyridine based coordination complex in which the
macrocyclic dioxocyclam unit remains metal-free. This result
was further demonstrated by spectrophotometric titration of
L1 during the progressive addition of Cu2+ ions showing
the formation of a structure having a 1:1 (L1/Cu2+) ratio
(see the Supporting Information). FT-IR studies of the green
solid confirmed the presence of counteranions, in agreement
with the formation of a cationic copper(II) complex
([L1CuII]n2n+; Scheme 1).
[L1CuII]n2n+, (B) [(ttpy)2CuII]2+, (C) [L1–2HCuII]0, and (D) [L2–2HCuII]0.
too short (<7 Å)11a compared to the size of the bis-
(terpyridine)copper(II) unit (21 Å),11b preventing an intramo-
lecular 1 + 1 coordination mode (see the Supporting
Information). A mass spectrometry (MS) study (electrospray
ionization mode) of the isolated species gave a signal at m/z
) 1699.9 corresponding to the trinuclear {L13(CuII)3-
(CF3SO3)4}2+ dication. Unfortunately, certainly because of
the reversibility of the metal–ligand bonds, higher molecular
weight structures could not be observed.
The potential formation of coordination polymers was
then investigated by viscosimetry. Indeed, the reduced
viscosity ηr ) η/ηs (ηs is the viscosity of the solvent) can
be used to trace out the variation of the concentration of
particles with a given morphology in noninteracting
conditions and/or the variation of the morphology of these
particles in the dilute regime of concentrations. Various
theoretical models have been proposed to relate the
reduced viscosity to the form factor of the dispersed
particles.12 Figure 2 shows the variation of the reduced
The presence of metal-free dioxocyclam in [L1CuII]n
2n+
was corroborated by a νCO stretching band at 1645 cm-1,
identical in free L1.7,8 Cyclic voltammetry (CV) curves of
the complex displayed a reversible reduction wave at E1/2
)
-0.719 V, close to the wave obtained with [(ttpy)2CuII]2+
and attributed to the CuII/I system (Figure 1A,B and
Table 1).9
All of these experimental data proved the formation of
copper (II) bis-terpyridine units when L1 is mixed with 1
mol equiv of metal cations, but the exact nature of the species
remained unclear because linear chains (oligomers/polymers)
or [L1nMn]2n+ cyclic structures could theoretically be
formed.10 First, the formation of a mononuclear ring in which
both terpyridines of L1 would be bound to the same Cu2+
ion was excluded because the dioxocyclam spacer in L1 is
(9) Chen, S.-H. J. Electroanal. Chem. 1998, 457, 23–30.
(10) (a) Schmatloch, S.; Van Den Berg, A. M. J.; Alexeev, A. S.; Hofmeier,
H.; Schubert, U. S. Macromolecules 2003, 36, 9943–9949. (b)
Vermonden, T.; Van Der Gucht, J.; De Waard, P.; Marcelis, A. T. M.;
Besseling, N. A. M.; Sudhölter, E. J. R.; Fleer, G. J.; Stuart, M. A. C.
Macromolecules 2003, 36, 7035–7044. (c) Dobrawa, R.; Lysetska,
M.; Ballester, P.; Grüne, M.; Würthner, F. Macromolecules 2005, 38,
1315–1325. (d) Chow, H. S.; Constable, E. C.; Housecroft, C. E.;
Neuburger, M. Dalton Trans. 2003, 4568–4569.
(7) (a) Pallavicini, P.; Dacarro, G.; Mangano, C.; Patroni, S.; Taglietti,
A.; Zanoni, R. Eur. J. Inorg. Chem. 2006, 4649–465. (b) Fabbrizzi,
L.; Foti, F.; Patroni, S.; Pallavicini, P.; Taglietti, A. Angew. Chem.,
Int. Ed. 2004, 43, 5073–5077. (c) Aurora, A.; Boiocchi, M.; Dacarro,
G.; Foti, F.; Mangano, C.; Pallavicini, P.; Patroni, S.; Taglietti, A.;
Zanoni, R. Chem.sEur. J. 2006, 12, 5535–5546.
(11) (a) Gavrish, S. P.; Lampeka, Y. D.; Lightfoot, P. Inorg. Chim. Acta
2004, 357, 1023–1030. (b) Uma, V.; Vaidyanathan, V. G.; Nair, B. U.
Bull. Chem. Soc. Jpn. 2005, 78, 845–850.
(8) (a) Meyer, M.; Frémond, L.; Espinosa, E.; Guilard, R.; Ou, Z.; Kadish,
K. M. Inorg. Chem. 2004, 43, 5572–5587. (b) Hegedus, L. S.;
Sundermann, M. J.; Dorhout, P. K. Inorg. Chem. 2003, 42, 4346–
4354.
(12) (a) Einstein, A. Ann. Phys. (N.Y.) 1911, 34, 591–592. (b) Onsager, L.
Phys. ReV. 1932, 40, 1028. (c) Kuhn, W.; Kuhn, H. HelV. Chim. Acta
1945, 28, 97–127. (d) Jacobson, H.; Stockmayer, W. H. J. Chem. Phys.
1950, 18, 1600–1606.
Inorganic Chemistry, Vol. 47, No. 6, 2008 1863

COMMUNICATION
Although the formation of [L1nMn]2n+ rings cannot be fully
excluded, all of these results unambiguously confirmed the
major formation of metallopolymers when a 1:1 (M2+/L1)
ratio is reached (Scheme 1A). In contrast, metalation of L1
with 1 mol equiv of Cu(CF3SO3)2 in MeOH in the presence
of a mild base (2 equiv of K2CO3) gave a pink-red solution
displaying an absorption band at 503 nm, consistent with a
d f d transition in a square-planar Cu2+ complex.7,8 FT-IR
data of the isolated red powder showed no presence of anion,
indicating the neutral character of the complex, and the νCO
stretching band at 1544 cm-1 was strongly shifted compared
to the free ligand (1645 cm-1). These features are in
agreement with a metal-assisted deprotonation of the two
amide groups and their coordination to the metal ion, giving
rise to a neutral [L1–2HCuII]0 complex (Scheme 1B).7,8 This
formula was confirmed by MS data and elemental analysis.
For comparison, the L2 ligand and the corresponding red-
pink [L2–2HCuII]0 complex were studied as reference com-
pounds (Chart 1). The characteristics of this complex were
close to those of [L1–2HCuII]0. Because L2 has no terpyridine
units, all of these data clearly demonstrate that the terpyridine
groups in [L1–2HCuII]0 are not involved in the coordination
mode. As expected, CV curves of [L1–2HCuII]0 and
[L2–2HCuII]0 presented very similar features (Figure 1C,D
and Table 1), with the two waves around -1.50 and +0.43
V attributed to the metal-centered redox processes affording
the corresponding CuI and CuIII complexes, respectively.11
The presence of metal-free terpyridine in [L1–2HCuII]0 was
corroborated by the observation of a reduction wave at -2.46
V, close to the signal corresponding to the reduction of the
terpyridine units in free L1 (E1/2 ) -2.42 V). In addition,
the relative viscosity measured for a solution of [L1–2HCuII]0
was negligible (ηr ) 1; see Figure 2).
Figure 2. Viscosimetry experiments. Progressive addition of Cu(CF3SO3)2
(0.52 M in DMF) to a solution of L1 in DMF/CH2Cl2 (90:10): (b) [L1] )
26 mM; (O) [L1] ) 13 mM; (9) isolated [L1–2HCuII]0 (13 mM in DMF).
Figure 3. 1H DOSY NMR spectra of L1 + 1 mol equiv of Zn2+ in DMF-
d7 (90%)/CD2Cl2 (10%). (The signals at log D from about -9 to -8.5 are
attributed to the solvents and water molecules.)
viscosity of the media during stepwise addition of Cu2+
ions to a solution of L1.
The reduced viscosity is increasing up to an apex for a
Cu2+/L1 ratio of 1 that can be related to an increase of the
average chain length of 1D assemblies. Beyond this maxi-
mum, ηr is decreasing, which reveals the formation of extra
chain ends. Such evidence for the formation of shorter entities
is a good indication of the reversibility of the metal–ligand
bonds. Without going into the details of the functional
dependence of the viscosity of a solution of charged
supramolecular polymers on the volume fraction, morphol-
ogy, and balance between the hydrodynamic and Brownian
forces, the increase of the reduced viscosity can be related
to an increase of the molecular weight of the species. Indeed,
the concentrations involved in our experiments are not very
dilute, and the variation of the viscosity (between ca. 30 and
70%) is relatively important. These features, also observed
for other coordination chains,10a,b are not in favor of the
potential formation of rings but rather of linear species
following the description by Jacobson and Stockmayer12d
of the linear polycondensation of polymeric-like chains.
In order to confirm this result, NMR experiments were
carried out. Because of the paramagnetic character of the
copper(II) complexes, Zn2+ was used as the metal cation.
First, broad peaks and the absence of free L1 were logically
The possible interconversion (Scheme 1C) between
[L1–2HCuII]0 and [L1CuII]n
was then investigated. For
2n+
solubility reasons, these experiments could not be realized
in aqueous solvents, but upon the addition of K2CO3 to a
methanol solution of [L1CuII]n2n+, the green solution (λmax
) 687 nm) turned gradually red-pink (λmax ) 503 nm) in
accordance with the formation of the [L1–2HCuII]0 complex.
Conversely, upon the addition of triflic acid to a red solution
of [L1–2HCuII]0, the initial absorption at λmax ) 503 nm was
replaced by a band at 687 nm characteristic of the polymer
(see the Supporting Information). This process was also
readily confirmed by electrochemical experiments because
the CV curve representative of the [L1CuII]n2n+ complex was
obtained when acid was added to a solution of [L1–2HCuII]0.
In this work, we have designed a cyclam-bis-terpyridine
ligand allowing an acid–based-triggered formation of mono-
nuclear neutral complex or metallopolymers with Cu2+ ions.
We are now exploiting these promising systems for the
conception of heterometallic polymers.
1
observed by H NMR after the progressive addition of 1
equiv of Zn2+ to a solution of L1. The strongest chemical
shifts induced by the presence of metal ions were observed
in the terpyridine region, indicating that the metal insertion
occurs in these units.1H DOSY NMR experiments performed
on a solution containing a 1:1 (Zn2+/L1) molar ratio (Figure
3) showed the presence of species having a low diffusion
coefficient (D ) 6.3 × 10-11 to 2 × 10-10 m2 s-1). These
data corroborate the presence of high-molecular-weight
polymers.10b,c
Supporting Information Available: Experimental details, syn-
thesis, MS, UV–visible, and NMR results. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC701584E
1864 Inorganic Chemistry, Vol. 47, No. 6, 2008