Electron-Triggered Metamorphism in Palladium-Driven Self-Assembled Architectures

Article abstract of DOI:10.1021/acs.inorgchem.0c02365

A metal-induced self-assembly strategy is used to promote the π-dimerization of viologen-based radicals at room temperature and in standard concentration ranges. Discrete box-shaped 2:2 (M:L) macrocycles or coordination polymers are formed in solution by

Full text of DOI:10.1021/acs.inorgchem.0c02365

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Electron-Triggered Metamorphism in Palladium-Driven Self-
Assembled Architectures
̈
Christophe Kahlfuss, Shagor Chowdhury, Adérito Fins Carreira, Raymond Gruber, Elise Dumont,
Denis Frath, Floris Chevallier, Eric-Saint-Aman,* and Christophe Bucher*
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ABSTRACT: A metal-induced self-assembly strategy is used to
promote the π-dimerization of viologen-based radicals at room
temperature and in standard concentration ranges. Discrete box-
shaped 2:2 (M:L) macrocycles or coordination polymers are
formed in solution by self-assembly of a viologen-based ditopic
ligand with cis-[Pd(en)(NO3)2], trans-[Pd(CH3CN)2(Cl)2], or
[Pd(CH3CN)4(BF4)2]. Changing the redox state of the bipyridium
units involved in the tectons, from their dicationic state to their
radical cation state, results in a reversible “inflation/deflation” of
the discrete 2:2 (M:L) macrocyclic assemblies associated to a large modification in the size of their inner cavity. Viologen-centered
electron transfer is also used to trigger a dissociation of the coordination polymers formed with tetrakis(acetonitrile)Pd(II), the
driving force of the disassembling process being the formation of discrete box-shaped 2:2 (M:L) assemblies stabilized by π-
dimerization of both viologen cation radicals.
applications of stimuli-responsive metal−ligand assemblies
have indeed attracted the attention of many chemists over
the past decade, leading to the discovery of new “smart”
molecular materials capable of modifying one or more of their
properties in response to an external stimulus, such as pH,
light, or the addition of chemical species.38−45 Due to
conceptual, technical, or synthetic locks, much less progresses
have been achieved with systems capable to respond to an
electrical stimulation,46 the most common approach explored
so far relying on a change in the redox state of the metal
centers to trigger large-scale reorganizations affecting the
whole assembly.47−49
Applying our expertise in the field of molecular and
supramolecular metamorphism,50−56 we are now reporting
on the formation of discrete palladium-ligand assemblies and
coordination polymers whose structure/size/organization can
be tuned with an electrical input. The basic concept underlying
this novel approach is illustrated in Scheme 1 with simple
sketches. The idea was to introduce two carefully selected
metal coordinating units on both sides of a 4,4′-bipyridinium
unit so as to enable its self-association in the presence of
palladium to form either discrete macrocyclic architectures or
INTRODUCTION
■
Self-assembly is a unique spontaneous process allowing to
organize, without external constraints, a set of disordered
molecular units. It has proved extremely useful over the past
decades to give access to highly complex structures from very
simple building blocks.1−14 In addition, the labile/weak nature
of non-covalent connections endows the resulting dynamic
assemblies with numerous fascinating properties making them
perfect candidates for the development of responsive
materials.15−23 It includes the ability to undergo assembly/
disassembly processes under specific conditions, to adapt to
their environment with changes in shape and size or to
spontaneously repair structural damages (self-healing).24−32
One strategy that has been used to expand the application
scope of self-assembled systems relies on the use of metal ions
as building elements. The so-called “metal-driven self-assembly
approach” has indeed been explored since the early nineties
with most transition metals to give access to a wide range of
metal−ligand assemblies with interesting properties for
applications in various research fields ranging from catalysis
to material science.33−35 Fujita was a pioneer in this field in
showing that the well-defined and highly predictable square
planar coordination scheme of Pd2+, together with the lability
of the Pd−L bonds, are very suitable characteristics for the
construction of dynamic self-assembled supramolecular sys-
tems.36,37 These innovative contributions have since then led
to major developments motivated by the need to access ever
more complex assemblies and by the desire to control/exploit
their responsive character. The properties and potential
Received: August 7, 2020
Published: February 23, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c02365
© 2021 American Chemical Society
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molecule with metal-coordination abilities, has been obtained
in two steps with an overall 33% yield, starting with a
Sonogashira cross-coupling reaction between 2-iodo-1-methyl-
1H-imidazole 657 and 4-ethynylaniline. Intermediate 3 was
then submitted to a Zincke reaction requiring the use of 0.45
molar equiv of the bis-activated Zincke salt 22+.58
Scheme 1. Schematic Representation of the Concept
Developed in This Work
Complexation with Palladium. Complexation of 12+
with Pd(II) has been thoroughly investigated by NMR
spectroscopy, electrochemistry, and by spectroelectrochemistry
measurements using cis-[Pd(en)(NO3)2], trans-[Pd-
(CH3CN)2(Cl)2], or [Pd(CH3CN)4(BF4)2] as sources of
partially (cis- or trans-) or fully unprotected metal centers (en:
ethylenediamine). In the two first cases, only the cis and trans
positions occupied by weakly binding nitrate and acetonitrile
ligands are kinetically labile and thus prone to ligand exchange,
while the four positions of tetrakis(acetonitrile)palladium(II)
are potentially available for binding.
Selected 1H-NMR spectra collected with a millimolar
solution of 12+ in DMSO-d6 in the presence of increasing
amounts of cis-protected [Pd(en)(NO3)2] are shown in Figure
1. The kinetics of the ligand exchange processes occurring in
solution upon addition of metal cations are slow with respect
to the NMR time scale. As expected, coordination of the
imidazole rings to Pd(II) leads to a significant shift of the
signals attributed to Hb and Hc. The chemical shift of the
singlet attributed to the protons of the methyl substituent Ha
proved to be one of the most valuable and direct diagnostic
tool for analyzing the metal binding processes and for
identifying the most abundant species formed at given
metal:ligand ratios (M:L). Addition of substoichiometric
amounts of metal ions (0 < M:L < 1) was found to induce
the progressive disappearance of the initial signal resonating at
3.84 ppm at the expense of two very close singlets of equal
intensities centered at 3.87 and 3.88 ppm, which suggests the
formation of two species featuring similar structures. This
assumption was further supported by the observation of a
similar low field shift and splitting affecting the signal
attributed to the hydrogen of the imidazole ring Hc, while
that attributed to Hb appears in the form of a singlet. Another
important conclusion drawn from these data is that complex-
ation of palladium occurs simultaneously on both sides of the
ligands, as proved by the fact that no signal corresponding to a
“free” imidazole ring is observed in the signature of the two
symmetrical complexes developing over the course of the
titration experiment up to M:L = 1. The NMR spectra
recorded in the presence of one molar equiv of Pd2+ also reveal
coordination polymers. Based on the knowledge accumulated
in recent years in our group on palladium assemblies,55,56 we
considered that the type of assembly (discrete vs polymeric
assemblies) could be determined by a careful design of the
coordination unit (denticity, topicity, steric hindrance, etc.)
together with a rigorous choice of the palladium source and
more specifically the number and the relative position (cis or
trans) of the available binding sites on the palladium center (L
in Scheme 1). It was also anticipated that a dissociation of the
self-assembled polymers or a contraction of the discrete
coordination rings could be actuated by a suitable electrical
stimulation of the assemblies leading to a change in the redox
state of the bipyridiums, from their dicationic state to their
cation radical state, and to the coupled formation of
intramolecularly π-dimerized coordination rings.
In the following sections, we will report on the self-
assembling ability of the bis-imidazole-tethered viologen 12+
(Scheme 2) in the presence of cis, trans, or unprotected
palladium(II) centers yielding either macrocyclic coordination
rings or coordination polymers. Then, the electron-triggered
dissociation of the coordination polymers or the contraction of
the discrete self-assembled rings will be discussed based on
spectroscopic, (spectro)electrochemical, and computational
data.
RESULTS AND DISCUSSION
■
Synthesis. The targeted imidazole-viologen derivative 12+
(Scheme 2), featuring a rigid ethynylene spacer between the
imidazole and phenyl rings so as to provide the resulting
a
Scheme 2. Synthesis of Targeted Compound 1(PF6)2
a
(a) Pd(PPh3)2Cl2, CuI, Et3N, 60 °C, 18 h, 87%. (b) CH3CN, reflux, 24 h, 70%. (c) EtOH/CH3CN, 80 °C, 6 h/KPF6, 38%.
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Figure 1. Partial 1H-NMR spectra (400 MHz, DMSO-d6, 1.0 mM, 293 K) of 1(PF6)2 in the absence (1) and in the presence of (2) 0.5 molar equiv
of [Pd(en)(NO3)2], (3) 1.0 molar equiv of [Pd(en)(NO3)2], (4) 10 molar equiv of [Pd(en)(NO3)2], and (5) 200 molar equiv of [Pd(en)(NO3)2].
Figure 2. Partial 1H-NMR spectra (400 MHz, DMF-d7, 1.0 mM, 293 K) of 1(PF6)2 in the absence (1) and in the presence of (2) 0.5 molar equiv
of [Pd(CH3CN)2(Cl)2], (3) 1.0 molar equiv of [Pd(CH3CN)2(Cl)2], (4) 2.0 molar equiv of [Pd(CH3CN)2(Cl)2], and (5) 10 molar equiv of
[Pd(CH3CN)2(Cl)2].
that 19% of the ligand is uncomplexed. Further addition of
Pd(II) (1 < M:L) led to disappearance of these intermediate
signals at the expense of a new set of signals, reaching a
maximum intensity after addition of palladium in large excess,
a t t r i b u t e d t o t h e m e t a l - s a t u r a t e d c o m p l e x
[(Pd2+)2(en)2(S)2(12+)] (with S = solvent).
The array of spectra depicted in Figure 1 thus reveals that
the solution contains four different species present in
equilibrium over a wide range of concentration: the free
ligand 12+, two intermediate symmetric isomers and the final
metal-saturated complex [(Pd2+)2(en)2(S)2(12+)]. The eight
protons of the viologen units (Hf and Hg) are interestingly
found to resonate at similar frequencies in the free ligand and
in the final product obtained after addition of palladium in
excess. This however proved not true for the signals attributed
to the hydrogen atoms located on the phenyl (+0.1 and +0.2
ppm) and imidazole rings (+0.3 ppm) undergoing significant
downfield shifts upon formation of the 2:1 (M:L) complex
[(Pd2+)2(en)2(S)2(12+)]. Further investigations carried out in
DMF-d7 led to similar results (Figure S9).
The well-defined shape of all the signals observed
throughout the titration experiment is moreover a clear
experimental evidence supporting the formation of isolated
species rather than oligomers and polymers. This conclusion
was further confirmed by DOSY NMR measurements, showing
that the calculated diffusion coefficient remains unchanged
upon dilution of the sample from 1.0 to 0.5 mM. The 1H- and
DOSY-NMR data discussed above thus support the conclusion
that the box-shaped isomers noted cis-[(Pd2+)2(en)2(12+)2]
and trans-[(Pd2+)2(en)2(12+)2] in Figure 1 are formed in the
early stage of the titration (0 < M:L < 1) with a maximum
concentration reached at M:L = 1, each isomer being
1
distinguished on the H-NMR spectrum with specific signals
for protons Ha and Hc. This conclusion implies that the
electrostatic repulsion between two dicationic viologen units is
not large enough to prevent the formation of such a box-
shaped macrocyclic structure wherein two positively charged
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Figure 3. Partial 1H-NMR spectra (400 MHz, DMF-d7, 1.0 mM, 293 K) of 1(PF6)2 in the absence (1) and in the presence of (2) 0.5 molar equiv
of [Pd(CH3CN)4(BF4)2], (3) 1.0 molar equiv of [Pd(CH3CN)4(BF4)2], (4) 10 molar equiv of [Pd(CH3CN)4(BF4)2], and (5) 200 molar equiv of
[Pd(CH3CN)4(BF4)2].
viologens end up adopting a cofacial arrangement. Formation
of such discrete metallacyclic compounds is further supported
by a significant shielding of the viologen-based signals Hf and
Hg upon metalation. As previously established by Crowley et al.
with constrained bis-pyridinium derivatives,59,60 these shifts
can indeed be attributed to the spatial proximity and van der
Waals interactions occurring between both viologens in both
the cis- and trans-[(Pd2+)2(en)2(12+)2] isomers.
phenyl rings (Hd and He), and two singlets at 7.28 and 7.58
ppm attributed to the protons of the imidazole ring (Hb and
Hc), respectively. No significant modifications of the 1H-NMR
spectra could be observed upon further addition of trans-
[Pd(CH3CN)2(Cl)2].
Contrarily to what was observed with cis-[Pd(en)(NO3)2],
the spectra recorded in the presence of trans-[Pd-
(CH3CN)2(Cl)2] displays a series of non-identified weakly
intense signals suggesting the formation of side products most
probably resulting from kinetic issues in line with the known
trans-effect affecting the lability of metal−ligand bonds in
chlorinated square planar complexes.61 The data discussed
above nevertheless clearly indicate that the three main species
shown in Figure 2 are formed successively with the increasing
M:L ratio. A careful analysis of the signals assigned to the
protons of the imidazole (Ha, Hb, and Hc) and of the viologen
units (Hf and Hg) indeed reflects that the symmetric and
discrete complexes [(PdCl2)2(12+)2] and [(PdCl2)2(S)2(12+)]
are the main species in solution at M:L = 1 and M:L ≥ 2,
respectively (S = solvent). In mixtures involving substoichio-
metric amounts of trans-[Pd(CH3CN)2(Cl)2] (M:L < 1), the
box-shaped [(PdCl2)2(12+)2] complex and the free ligand 12+
are the main species in solution, while subsequent addition of
Pd(II), up to 2 molar equivalents, leads to the exclusive
formation of the 2:1 (M:L) complex [(PdCl2)2(S)2(12+)]
featuring two trans-coordinated chlorine ligands on each
palladium centers. Here again, the high field shift of the
viologen-based Hf signal (−0.05 ppm) observed upon
formation of the box-shaped [(PdCl2)2(12+)2] complex is
attributed to the spatial proximity between both viologens in
the macrocyclic structure. The trans- versus cis-coordination
modes imposed for 12+ by the chlorine and ethylene diamine
ligands lead however to major structural differences between
the box-shaped [(PdCl2)2(12+)2] and [(Pd2+)2(en)2(12+)2]
complexes, which include the distance and relative arrange-
ment between both viologen units. It should be stressed that
the formation of the macrocyclic 2:2 (M:L) compound
[(PdCl2)2(12+)2] has been further demonstrated by MS
analyses through the observation of signals attributed to
Subsequent addition of Pd(II) led to a dissociation of both
[2:2] metallacyclic isomers in favor of the metal-saturated
[2:1] complex [(Pd2+)2(en)2(S)2(12+)], which becomes
predominant only after addition of 10 molar equiv of
[Pd(en)(NO3)2] (10 < M:L). At M:L = 10, the ratio between
the macrocyclic and metal-saturated complexes
([(Pd2+)2(en)2(S)2(12+)]:[(Pd2+)2(en)2(12+)2]) could be esti-
mated to 1:4 from the integration of specific 1H-NMR signals.
A similar study was then conducted with trans-[Pd-
(CH3CN)2(Cl)2] used as a trans-protected metal source. As
can be seen in Figure 2, addition of this metal salt to a
millimolar solution of 12+ was found to result in the
disappearance of the initial singlets/multiplets at the expense
of new sets of signals. The changes observed throughout the
titration are consistent with the involvement of three main
species whose relative concentration evolves with the M:L
ratio. In particular, the singlet centered at 3.94 ppm attributed
to the N-methyl substituents of the free ligand 12+ (Ha) sees its
intensity gradually decrease at the expense of one main signal
developing at 3.97 ppm and reaching its maximum intensity at
M:L = 1 (Figure 2, spectrum 3). The initial signal of the
imidazole ring (Hb and Hc) undergoes similar changes with a
progressive disappearance at the expense of two novel
deshielded signals. Analogous changes are observed in the
aromatic domain with the development of two novel shielded
signals attributed to the viologen subunit (Hf and Hg) in a 2:2
(M:L) adduct. Further addition of metal salt then leads to the
emergence of a new sets of signals reaching maximum
intensities as M:L ≥ 10 (Figure 2, spectrum 5). The spectrum
of the final complex displays two doublets at 9.39 and 10.00
ppm attributed to Hg and Hf, one AB quartet at 8.27 and 8.33
ppm corresponding to the hydrogen atoms located on the
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Figure 4. Average diffusion coefficient recorded as function of (A) the number of molar equivalent of [Pd(CH3CN)4](BF4) added to a 1 mM
DMSO-d6 solution of 1(PF6)2 and of (B) the concentration of 1(PF6)2 and [Pd(CH3CN)4](BF4) in DMSO-d6 at a 1:1 (M:L) ratio (measured by
DOSY-NMR (400 MHz, 293 K).
Figure 5. Voltammograms recorded for 1(PF6)2 (0.4 mM, DMF + TBAP 0.1 M) in the presence of (A) cis-[Pd(NO3)2(en)]: 0 molar equiv (full
line), 1 molar equiv (bold line), and 10 molar equiv (dashed line) and (B) trans-[Pd(Cl)2(CH3CN)2]: 0 molar equiv (full line), 0.4 molar equiv
(dotted line), 1 molar equiv (bold line), and 3 molar equiv (dashed line) (VC, ⌀ = 3 mm, E vs Ag/Ag+ 10−2 M, ν = 0.1 V s−1).
−
−
[(PdCl2)2(12+)2(PF6 )]3+ and to [(PdCl2)2(12+)2(PF6 )2]2+
(Figure S3 and S4).
The formation of coordination polymers or oligomers in the
conditions described above has been further confirmed from
DOSY-NMR measurements carried out in DMSO-d6 at
different M:L ratio. As can be seen in Figure 4A, addition of
[Pd(CH3CN)4](BF4)2 to a 1 mM solution of 12+ was found to
result in a significant drop of the diffusion coefficient going
from 157 μm2.s−1 down to 85 μm2·s−1 after addition of 1 equiv
of Pd(II). Subsequent addition of Pd(II) then led to an
increase of the diffusion coefficient up 125 μm2·s−1 reached for
M:L = 5. Similarly, the diffusion coefficient calculated for a 1:1
(M:L) ratio was found to decrease with increasing concen-
tration, as expected for a concentration-dependent self-
assembly process (Figure 4B). The broadness of the DOSY
NMR signals also suggests a rather large polydispersity in the
investigated mixtures. All these data thus support the
conclusion that the largest oligomers are formed at M:L = 1
and that every deviation from that stoichiometry leads to the
displacement of the equilibria in favor of shorter assemblies.
Electrochemical Characterization. The free ligand 12+
has been studied by cyclic voltammetry (CV) in DMF.
Selected curves are shown in Figure 5 (full line) and in the
Supporting Information. The curve recorded at a glassy carbon
working electrode at 100 mV/s exhibits two reversible
All the data presented above support the conclusion that
discrete complexes rather than polymers are formed in solution
from mixtures of 12+ and cis-[Pd(en)(NO3)2] or trans-
[Pd(CH3CN)2(Cl)2]. Formation of coordination polymers
was conversely observed when using [Pd(CH3CN)4](BF4)2 as
the metal source, featuring four ″available″ binding positions
around the Pd center. Addition of this non protected metal salt
to a millimolar solution of 12+ indeed led to the progressive
disappearance of all the signals attributed to 12+ in favor of a
series of very broad and weakly intense signals observed until
addition of palladium in excess (Figure 3), which is compatible
with the existence of multiple equilibria involving various
coordination polymers/oligomers. Well-resolved NMR signals,
characteristic of smaller and well-defined metal−ligand
assemblies, could only be observed after addition of palladium
in excess (about 5 molar equiv). In the presence of a large
excess of [Pd(CH3CN)4](BF4)2, the signals attributed to the
protons of the pyridinium rings were shifted to 9.29 and 9.93
ppm, while those of the imidazole ring were seen to resonate at
7.43 and 7.76 ppm (Figure 3).
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reduction waves at [E1/2]1a = −545 mV ([ΔEp]1a = 63 mV, ν =
0.1 V s−1) and [E1/2]2a = −769 mV ([ΔEp]2a = 59 mV, ν = 0.1
V s−1) corresponding to the successive formation of the radical
cation 1+· and of the neutral quinonic species 10. One key
information extracted from these preliminary measurements,
and more specifically form the rather limited amplitude of the
recorded at M:L = 1 also suggests that the cis- and trans-
isomers exhibit similar electrochemical behavior, which is not
surprising given the great similarity between the environments
of the viologens in both macrocyclic compounds. Another
major finding is that the reduction of the free ligand is hardly
observed on the CV curve recorded at M:L = 1, a fact which
contrasts with the NMR data discussed above revealing that
19% of the ligand remains uncomplexed under these
experimental conditions. Failure to observe such wave
attributed to the reduction of the free ligand (12+/1+·) can
be explained by the dynamic nature of the electrochemical and
chemical equilibria involved in solution (see in Figure 6 the
K1‑6 and KDim constants involving a wide range of complex-
ation, reduction/oxidation, and π-dimerization processes). As
can be seen in Figure 5A, the first wave at approximately −0.4
V corresponds to the two-electron reduction of the metal-
lacyclic compound [(Pd2+)2(en)2(12+)2] ([E°]1b and [E°]2b in
Figure 6) yielding the doubly reduced complex
[Pd2+)2(en)2(1+·)2], which is readily transformed at the CV
time scale into the intramolecular π-dimer
[(Pd2+)2(en)2(1+)2]Dim (KDim in Figure 6). Our explanation
of the electrochemical signature recorded at M:L = 1 results
from this series of events triggered at the early stage of the
scanning experiment, the most important one being the
effective π-dimerization process [(Pd2+)2(en)2(1+·)2] →
[(Pd2+)2(en)2(1+)2]Dim leading, at the CV time scale, to a
displacement of the initial complexation equilibrium (K1 in
Figure 6) and thus to the consumption of the free ligand 12+.
Further addition of cis-[Pd(NO3)2(en)] led to the
observation of two different reduction waves centered at E1/2
= −0.532 V ([ΔEp]1c = 72 mV) and at E1/2 = −0.410 V (ΔEp =
27 mV). The CV curve recorded at M:L = 10 is shown in
Figure 5A. The first reduction at E1/2 = −0.410 V (could be
readily attributed to the EEC process involving the two-
electron reduction of [(Pd2+)2(en)2(12+)2] and its subsequent
π-dimerization to yield [(Pd2+)2(en)2(1+)2]Dim ([E°]1b, [E°]2b,
KDim in Figure 6), while the second and more intense wave at
E1/2 = −0.532 V (ΔEp = 72 mV) could be attributed to the
one-electron reduction of the bimetallic complex
[(Pd2+)2(en)2(S)2(12+)] ([E°]1c in Figure 6). Observation of
ΔE1/2 value ([E1/2 1a − [E1/2]2a = 224 mV) and from the
]
standard [ΔEp]1a value (|Epc − Epa|1a = 63 mV) measured on
the first reduction wave, is that the electrogenerated cation
radicals 1+· do not interact with each other in solution under
these experimental conditions.
The complexation of 12+ with palladium has been similarly
studied by electrochemical methods. Selected CV curves
recorded at different M:L ratios with cis-protected [Pd-
(NO3)2(en)] as the palladium source are shown in Figure
5A. These data reveal that the addition of 1 molar equiv of
metal (M:L = 1) leads to a major shift of the first viologen-
centered reduction wave toward a less negative potential
([E1/2]1b = −0.399 V, [ΔE1/2]1b/1a = [E1/2]1b − [E1/2]1a = +150
mV) and to a large decrease of the peak to peak potential shift
going from 59 (M:L = 0) to 38 mV (M:L = 1). It also led to a
less important shift of the second reduction potential toward
more negative values ([ΔE1/2]2b/2a = [E1/2]2b − [E1/2]2a = −50
mV, see Figure S12). As explained in previous reports,10,17,29,63
these changes support the conclusion that the presence of Pd2+
promotes an efficient π-dimerization of the electrogenerated
viologen-based cation radicals. This assumption is moreover
1
fully consistent with our analyses of the H-NMR titration
data, which have led to the identification of the metallacyclic
complexes (Figure 6) as the main species in solution at M:L =
1, the preorganization and spatial proximity between both
viologen sub-units held in a cofacial arrangement by two
Pd(II) hinges being perfectly suited to promote the electron-
triggered formation (+1e/viologen) of the intramolecular π-
dimer [(Pd2+)2(en)2(1+)2]Dim (Figure 6). The CV curve
1
both signals is here in agreement with the H-NMR data
depicted in Figure 1, allowing to estimate the concentration
ratio [(Pd2+)2(en)2(S)2(12+)]: [(Pd2+)2(en)2(12+)2] to 1:4 at
M:L = 10.
Similar measurements have been carried out using the trans-
protected palladium source [Pd(Cl)2(CH3CN)2] as a reactant.
Addition of 1 molar equiv of this metal salt to an electrolytic
solution of 12+ in DMF (0.4 mM) led to a large shift of the first
reduction wave toward less negative potential values (ΔE1/2
=
+ 135 mV, E1/2 = −0.411 V) coming along with a significant
decrease of the ΔEp value reaching 41 mV at M:L = 1 and ν =
100 mV/s (Figure 5B). As already discussed with the cis-
protected metal salt, these changes can be seen as
unambiguous experimental evidences demonstrating that the
complexation of 12+ with trans-[Pd(Cl)2(CH3CN)2] promotes
the intramolecular π-dimerization of the electrogenerated
viologen cation radicals within the intramolecular macrocyclic
complex [(Pd2+)2(Cl)2(1+)2]Dim
.
Figure 6. Schematic representation of the chemical and electro-
chemical reactions and equilibria involving 12+ and of cis-[Pd-
(NO3)2(en)]. Each electrochemical step (E) represented in this
mechanism are monoelectronic. In this schematic representation, the
organic fragment linking the viologens to the coordinating units is a
quite rigid acetylene spacer.
Further addition of trans-[Pd(Cl)2(CH3CN)2] (the curve
recorded at M:L = 3 is shown in Figure 5B) was conversely
found to result in the observation of ill-defined irreversible
signals at E < −0.4 V. From the NMR data discussed above, we
know that the main species in solution at M:L = 3 is the
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Figure 7. Schematic representation and DFT-minimized structures corresponding to the macrocyclic complexes formed at M:L = 1 with (a) cis-
[Pd(en)2(NO3)2] and (b) trans-[Pd(Cl)2(CH3CN)2] and to the corresponding π-dimers formed upon reduction of the viologen units (one
electron per viologen). Enlarged top views showing the relative position between both viologen cation radicals in the dimeric forms appear below in
boxes marked with one or two stars.
bimetallic complex [(Pd2+)2(Cl)2(12+)]. The reduction wave
observed in Figure 5B (dashed line) is thus potentially
attributed to the one-electron reduction of the single viologen
unit involved in this metal saturated complex. The unusual
shape of this signal is however not clearly understood, one
possible explanation supported by the observation of a deposit
onto the electrode surface being that the ″free″ metal salt,
present in solution at M:L > 2, gets reduced/physisorbed at
the electrode in this potential range (the CV curve of the free
salt is shown in the Supporting Information).
A closer look at the well-defined CV data collected at M:L =
1 with the cis- and trans-protected metal salts nevertheless
enables to compare the dimerization ability of both viologen
units incorporated in macrocyclic complexes cis-
[(Pd2+)2(en)2(12+)2] and trans-[(Pd2+)2(Cl)2(12+)2] (Figure
6). Both compounds are clearly prone to dimerization and the
similar drop in the ΔEP values (38 vs 41) and potential shifts
ΔE1/2 (+155 vs +135) measured in the presence of cis-
[Pd(en)(NO3)2] and trans-[Pd(CH3CN)2(Cl)2] suggest that
similar dimers (arrangement, orbital overlap, interplanar
distance) are formed with the cis or trans-protected metallic
hinges.
provided by computational analyses. The DFT-minimized
structures of the oxidized and two-electron reduced species
obtained at M:L = 1 are shown in Figure 7. These structures
reveal that (i) both viologens stand at the same distance from
each other (3.46 Å) in the cis- and trans-protected π-dimers cis-
[(Pd2+)2(en)2(1+)2]Dim and trans-[(Pd2+)2(Cl)2(1+)2]Dim and
that (ii) the trans-configuration of the PdCl2 hinge enforces a
twist of 36° between the viologen axis leading to a partial and
quite limited overlap between both viologen cation radicals
(see Figure 7 and Figures S16 and S18), while the cis-protected
palladium hinge allows both viologens to lie perfectly aligned,
which results in a much more extensive orbital overlap.
The calculated structures depicted in Figure 7, and most
notably the great difference observed between the structures of
the oxidized species cis-[(Pd2+)2(en)2(12+)2] and of the doubly
reduced π-dimerized compound cis-[(Pd2+)2(en)2(1+)2]Dim
,
bring to light the flexibility of the ligand and the existence of
a reversible “inflating/deflating” process associated to the
electron transfer centered on the viologen units. At the
oxidized state, the “inflated” oval-shaped form is imposed by
electrostatic repulsive forces occurring within the highly
positively charged (z = +8) complex cis-[(Pd2+)2(en)2(12+)2].
Deflation of this structure is then triggered by the one-electron
reduction of both viologens and by the coupled dimerization of
Further insights into the arrangements of the oxidized and
reduced forms of these macrocyclic complexes have been
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Figure 8. Cyclic voltammograms recorded for 1(PF6)2 (1 mM, DMF + TBAP 0.1 M) in the presence of Pd(ACN)4(BF4)2: 0 molar equiv (dotted
line), 1 molar equiv (full line) (VC, ⌀ = 3 mm, E vs Ag/Ag+ 10−2 M, ν = 0.1 V s−1).
Figure 9. Superimposition of UV−vis spectra recorded during the exhaustive reduction (1 electron per viologen subunit) of (A) 12+ alone (dashed
line) and in the presence of cis-[Pd(NO3)2(en)] (1 molar equiv; full line), (B) trans-[Pd(Cl)2(CH3CN)2] (1 molar equiv), (C) trans-
[Pd(Cl)2(CH3CN)2] (0.5 molar equiv), and (D) [Pd(CH3CN)4](BF4)2 (1 molar equiv) (DMF + 0,1 M TBAP, Eapp = −0.67 V, 0.4 mM, 15 mL, l
= 1 mm, t ≈ 30 min, Pt).
the resulting π radicals to yield a square-shaped intramolecular
dimer. From a quantitative perspective, the deflation associated
to electron transfer results in a large drop in the size of the
inner cavity from ∼11.3 down to 3.5 Å (Figure 7a). A similar
fully reversible electron-triggered “inflating/deflating” process
is involved with the trans-protected palladium hinge where the
inner dimension changes from ∼11.7 Å to 3.5 Å upon
stimulation (Figure 7b). We should stress out that the latter
distances are characterized as stationary points on the potential
energy surface and that the screening effect of the counterions
and the dynamics of the system in solution probably result in a
distribution of slightly more compact macrocycles in their
oxidized forms.
CV measurements carried out in the presence of the
palladium salt [Pd(CH3CN)4]2+ brought to light the ability of
the ″non-protected″ palladium center to promote the
dimerization of 1+·. This effect is mainly revealed on the
curve recorded at M:L = 1 through the large positive shift of
the first reduction wave associated to a small negative shift of
the second reduction wave (Figure 8). These changes were
also found to come along with a significant broadening of the
first reduction wave associated to a loss of its reversible
character. Based on our analyses of the NMR data collected at
this M:L ratio (see Figures 3 and 4), this unexpected shape of
the first viologen-centered reduction wave observed at
approximately −0.55 V was attributed to the presence of
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many different assemblies/oligomers in solution exhibiting
slightly different local environments around the viologens
units.
All the electrochemical data discussed above support the
conclusion that the intramolecular dimerization is highly
favored in the presence of one molar equiv of palladium(II),
which is consistent with the formation of macrocyclic square-
shaped 2:2 complexes initially proposed on the ground of
NMR data.
Spectroelectrochemical Characterizations. The con-
clusion reached from the CV data recorded with cis-
[Pd(en)2(NO3)2], trans-[Pd(Cl)2(ACN)2] and Pd(ACN)4]-
(BF4)2 have been further confirmed by spectroelectrochemical
measurements, which involved regularly recording absorption
spectra over time during the potentiostatic reduction of
selected complexes.
the dimerization equilibrium is strongly displaced toward the
π-dimerized species.
Carrying out similar spectroelectrochemistry (SEC) meas-
urements in the presence of 1 equiv of trans-[Pd-
(Cl)2(CH3CN)2] leads to the appearance of the same
absorption bands centered at 457 nm (45,300 L·mol−1·
cm−1), 611 nm (29,500 L·mol−1·cm−1), 648 nm (26,700 L·
mol−1·cm−1), and 1000 nm (5800 L·mol−1·cm−1) (Figure 9B).
At first sight, the great similarities observed between the SEC
curves recorded with the cis- and trans-protected metal ions
suggest that the bulk reduction of both mixtures leads to the
same π-dimers.
We also discovered that the exhaustive reduction of 12+
performed in the presence of only 0.5 equiv of trans-
[Pd(Cl)2(CH3CN)2] leads to the exact same set of signals
(Figure 9C). As can be seen in Figure 5B, the CV curves
recorded in substoichiometric conditions (M < L) exhibit
three different waves attributed to successive reduction of the
2:2 (M:L) macrocyclic compound trans-[(PdCl2)2(12+)2]
followed by that of an intermediate compound that we assume
to be a 1:2 (M:L) complex featuring two ligands bound to a
single metal ion, and of the free ligand 12+. It needs to be
mentioned that the formation of a 1:2 (M:L) intermediate
complex at the electrode interface stills remains speculative as
Initial measurements have been carried out in the absence of
metal, with the free ligand only. The exhaustive bulk reduction
of 12+ (10 mL at 0.4 mM in electrolytic DMF, Eapp = −0.67 V,
one electron per viologen subunit) led to a decrease in the
intensity of the absorption band centered at 305 nm (19,200 L·
mol−1·cm−1) at the expense of new signals developing at 468
nm (20,500 L·mol−1·cm−1), λmax = 668 nm (17,700 L·mol−1·
cm−1), and 728 nm (19,800 L·mol−1·cm−1) (molar extinction
coefficients have been calculated from the final spectrum,
shown in Figure 9A as a dashed curve, recorded after
completion of the electrolysis). The stability of 1+· at the
electrolysis time scale was checked with coulometric and
rotating disk electrode (RDE) measurements and upon
checking that the initial electrochemical and spectroscopic
signatures can be fully recovered by back-electrolysis
(reoxidation at 0 V). The signature recorded after completion
of the one-electron reduction is thus attributed to the
“isolated” cation radical 1+·, and the absence of signal in the
near-IR region confirms that no π-dimers are formed under
these experimental conditions (0.4 to 1.0 mM in DMF).
Similar measurements have then been carried out in the
presence of 1 molar equiv of cis-[Pd(NO3)2(en)]. As can be
seen in Figure 9A, the exhaustive one-electron reduction of a
1:1 (M:L) mixture at Eapp = −0.67 V led to the progressive
development of a new set of absorption bands centered at λmax
= 457 nm (46,400 L·mol−1·cm−1), 614 nm (29,100 L·mol−1·
cm−1), 651 nm (28,800 L·mol−1·cm−1), and 1000 nm (4500 L·
mol−1·cm−1). In agreement with the conclusion drawn above
from CV data, the spectrum recorded after completion of the
electrolysis exhibits typical features revealing the palladium-
assisted π-dimerization of 1+·, including the broad absorption
band observed in the NIR region and the blue shift of the
intense bands observed in the visible range.10,17,29,63 This new
set of signals observed in Figure 9A and the well-defined
isosbestic point at about 400 nm are thus fully consistent with
the conclusion that the intramolecular dimer cis-
[(Pd2+)2(en)2(1+)2]Dim is generated in solution by reduction
of the box-shaped metallocyclic 2:2 (M:L) complex cis-
[(Pd2+)2(en)2(12+)2]. As frequently seen with viologen-based
dimers, the equilibria between the dimerized and non-
dimerized forms (KDim in Figure 6) is revealed in Figure 9A
by the shoulder at about 730 nm attributed to the bis-radical
cis-[(Pd2+)2(en)2(1+·)2], incorporating two non-interacting
viologen-based cation radicals. Taken together, the weak
intensity of this shoulder, the large intensity of the near IR
band, and the CV data discussed above support the idea that
1
such species could not be observed on the H-NMR spectra
recorded in substoichiometric conditions. As a matter of fact,
the proposed attribution mostly relies on previous inves-
tigations showing that the electron-triggered intramolecular π-
dimerization of 1:2 (M:L) palladium complexes results in a
positive shift of the first viologen-centered reduction wave of
only +60 to 100 mV.56 Such limited stabilization effect is thus
in good agreement with the intermediate position of the wave
observed at approximately −0.5 V in Figure 5B, attributed to a
1:2 (M:L) complex, wherein the viologens are more easily
reduced than in the free ligand but harder to reduce than in the
macrocyclic 2:2 complex.
The UV/vis spectra recorded during the exhaustive
reduction of 12+ (0.4 mM in electrolytic DMF, Eapp = −0.67
V, one electron per viologen subunit) in the presence of
[Pd(CH3CN)4](BF4)2 are shown in Figure 9D. Here again, a
clean isosbestic point is observed at 395 nm and the signals
developing upon reduction of the viologen centers are the
same as those observed with cis-[Pd(NO3)2(en)] and trans-
[Pd(Cl)2(CH3CN)2]. It includes absorption bands at 460 nm
(47,200 L·mol−1·cm−1), 619 nm (30,800 L·mol−1·cm−1), and
656 nm (35,200 L·mol−1·cm−1), as well as a broad absorption
band centered at λmax = 1033 nm (4900 L·mol−1·cm−1). The
full reversibility of the phenomena involved in solution was
demonstrated by checking that the initial signature of the
sample can be recovered by reoxidation at Eapp = 0 V.
These results thus support the finding that the self-
assembled coordination polymer [(Pd2+)(S)2(12+)]n formed
in solution in the presence of [Pd(CH3CN)4](BF4)2 evolves
upon reduction (one electron/viologen) into a discreet
rectangle-shaped [2 + 2] macrocyclic complex whose
formation is driven by the π-dimerization of both viologen
cation radicals involved in the complex. This conclusion is
supported in the first instance by the unquestionable
similarities observed between the spectroscopic signatures of
the palladium-assisted π-dimerized species obtained after
reduction of equimolar mixtures of 12+ and cis-[Pd-
(NO3)2(en)], 12+, and trans-[Pd(Cl)2(CH3CN)2] or 12+ and
[Pd(CH3CN)4](BF4)2. It is also further supported by CV
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2+
2+
Figure 10. Optimized geometries for the cis − [(PdCl2)2(1+)2] and trans − [(PdCl2)2(1+)2] complexes at the DFT/M06-2X/6-31G(d) +
Dim
Dim
LANL2DZ(f) level of theory.
measurements carried out on the solution obtained at the end
of the spectroelectrochemical experiment displayed in Figure
9D, i.e., after completion of the exhaustive reduction of the
polymer (one electron/viologen) [(Pd2+)(S)2(12+)]n formed
in solution in the presence of [Pd(CH3CN)4](BF4)2. In
contrast to the broad, ill-defined, and irreversible wave
observed at Ep = −0.55 V on the voltammogram recorded
before electrolysis (Figure 8), which is attributed to the
reduction of viologen units involved in many different
oligomers and to the slow kinetics of the reorganization
steps coupled to electron transfer, the cyclic voltammogram
recorded at the end of the exhaustive reduction of [(Pd2+)-
(S)2(12+)]n (∼30 min at Eapp = −0.67 V) displays a single
since the spectral signature of π-dimers is known to mostly
depend on the relative distance/orbital overlap between the
two radicals and on their relative spatial arrangement. The
DFT-minimized structures of cis-[(Pd2+)2(en)(12+)2] and
trans-[(PdCl2)2(12+)2] and of the corresponding π-dimers cis-
[(Pd2+)2(en)(1+)2]Dim and trans-[(PdCl2)2(1+)2]Dim are shown
in Figure 7. These models clearly bring to light the great
structural differences imposed by the stereochemistry of the
palladium center. At the oxidized state, the cis or trans
coordination scheme of the inorganic spacer results in a
slightly different distance between both viologens (11.3 vs 11.7
Å, see Figure 7). The cis/trans effects are in fact found to be far
more important at the reduced state; i.e., between the π-
dimerized structures cis-[(Pd2+)2(en)(1+)2]Dim and trans-
[(PdCl2)2(1+)2]Dim, wherein the conformations of the
molecules are imposed by two different constraints: (i) the
stereochemistry of the metal center and (ii) the proximity (π
interplanar distance) required to achieve an orbital overlap
between both viologen cation radicals involved in the complex.
In the case of trans-[(PdCl2)2(1+)2]Dim, bringing the viologen
radicals at close distance can only be achieved through a
twisting of the macrocycle yielding a figure eight conforma-
tion62 with limited orbital overlaps between the π radicals. The
situation is very different for the cis-protected linkers as the
dimerization is greatly facilitated by the conformation of the
molecule enabling to bring both viologen cation radicals at a
suitably close distance while keeping a perfectly aligned co-
facial arrangement. The major differences (interplanar
distance, twist angle...) observed between the calculated π-
dimerized structures of cis-[(Pd2+)2(en)(1+)2]Dim and trans-
[(PdCl2)2(1+)2]Dim are thus not compatible with the nearly
identical absorption spectra shown in Figure 9A,B. This
assumption has been confirmed by TDDFT calculations
carried out on different cis and trans isomers (see Figure
S23), which suggest that the broad absorption band in the
near-IR range should only be observed for the cis isomers. This
inadequacy between structural feasibility and experimental data
led us to suspect the existence of a stabilizing trans → cis
isomerization coupled to the electron transfer centered on the
trans-protected compound trans-[(PdCl2)2(12+)2], the im-
proved stability of the cis-dimer being the driving force of
the reorganization or the trans-dimer (Figure 10).
reversible and well-defined oxidation wave centered at E1/2
=
−0.42 V (Figure S13). The shape and position of this new
wave, attributed to the oxidation of the only product of the
polymer dissociation, is here again similar to those obtained
with the rectangle shape [2 + 2] complexes formed in solution
in the presence of cis-[Pd(NO3)2(en)], 12+ and trans-
[Pd(Cl)2(CH3CN)2] (see bold lines in Figure 5A,B). All
these experimental data thus support the conclusion that a
viologen-centered reduction (12+ → 1+·) leads to a dissociation
of the polymer [(Pd2+)(S)2(12+)]n yielding the discreet
macrocyclic [2 + 2] π-dimer [(Pd2+)2(S)4(1+)2]Dim
.
1
Taken together, the H-NMR, CV, and SEC data discussed
above clearly demonstrate that the palladium ions promote the
π-dimerization of the electrochemically generated viologen-
based cation radicals through the formation of highly
preorganized metallacyclic architectures obtained by self-
assembly of two metal ions and two imidazole-tipped ditopic
ligands. The experimental data supporting the conclusion that
all the equilibria involved in solution are displaced in favor of
intramolecular macrocyclic dimers [(Pd2+)(X)n(1+)2]dim {(X)n
= (en)2, (Cl−)4, or (ACN)4} are (i) a major shift of the
reduction potential towards less negative values, which results
from a large stabilization of the electrogenerated radicals
through π-dimerization, (ii) a drop of the ΔEp value down to
almost 30 mV measured in the presence of palladium, (iii) the
inability of the free ligand to dimerize in palladium-free
solutions, and (iv) the observation of diagnostic spectroscopic
signatures revealing the near quantitative formation of π-
dimerized species featuring identical, if not very similar,
structures.
The influence of the geometry around the palladium center
during the π-dimerization process has been studied on the
One major result of our investigations is indeed the striking
similarities observed between the spectra recorded after
reduction of 12+ in the presence of cis-[Pd(NO3)2(en)] and
trans-[Pd(Cl)2(CH3CN)2]. Such results were quite unexpected
model species cis-[(PdCl2)2(1+)2]Dim and trans-
2+
[(PdCl2)2(1+)2]D2+im. Their geometry has been studied by the
density functional theory (DFT) at the M06-2X/6-31G(d) +
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LANL2DZ(f) level of theory (Figure 10). The optimized
structure of cis-[(PdCl2)2(1+)2]D2+im shows two parallel viologen
subunits at a distance of ∼3.18 Å, while the structure obtained
for trans-[(PdCl2)2(1+)2]D2+im shows a crossed conformation
that forces both viologens to be more distant from each other
(3.46 Å, twist angle 33.2°). The box-shaped structure thus
induces a deviation from the reference stacking distance (3.3
Å) resulting in either a compression for the cis isomer or in a
decompression for the trans isomer. The weak energy
difference estimated to be −3.2 kcal·mol−1 in favor of the cis
isomer thus corroborates the trans → cis isomerization
hypothesized on the ground of experimental data.
EXPERIMENTAL SECTION
The synthesis and characterizations of the ligands and complexes and
the instrumentation are provided in the Supporting Information.
■
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02365.
■
sı
Detailed synthetic procedures and characterization data
including additional CV curves, H-NMR spectra, and
computational methods (PDF)
1
AUTHOR INFORMATION
Corresponding Authors
■
CONCLUSIONS
■
As a conclusion, we have developed a metal-induced self-
assembly strategy promoting the π-dimerization of simple
viologen-based tectons at room temperature and in standard
concentration ranges. Our investigations revealed that discrete
box-shaped 2:2 (M:L) macrocycles are formed in solution in
the presence of cis-[Pd(en)(NO3)2] and trans-[Pd-
(CH3CN)2(Cl)2] and that the structure/geometry of the
self-assembled product is determined by the relative position of
the non-exchangeable ligands (cis-ethylenediamine vs trans-
dichloride) initially bound to the metal. Use of a “non-
protected” metal source, i.e., tetrakis(acetonitrile)Pd(II)
featuring four available/exchangeable binding sites, was
conversely found to enable the formation of coordination
polymers/oligomers reaching a maximum size at M:L = 1.
On the ground of detailed spectroscopic and electrochemical
data supported by computational calculations, we have
established that all the investigated palladium-based inorganic
hinges (cis, trans, or un-protected) promote an effective
intramolecular π-dimerization of the electrogenerated viologen
cation radicals involved in the macrocyclic or polymeric
assemblies. From a structural point of view, the reduction and
re-oxidation of both viologen units involved in the discrete 2:2
(M:L) assemblies result in a reversible “inflating/deflating” of
the macrocyclic structure associated to a large modification in
the size of the inner cavity going from ∼3.5 to 11.3 Å.
We have also established that the one-electron reduction of
the viologen units present at different metal:ligand ratios leads
to the formation of the same intramolecular π-dimer, regardless
of the initial environment around the metallic precursor (cis-,
trans-, or unprotected) and of the relative ratio between metal
and ligand initially introduced in solution. In line with this
finding, all the experimental and computational data discussed
above suggest that the electrical stimulation of the macrocyclic
assembly formed with trans-[Pd(CH3CN)2(Cl)2] triggers a
trans → cis isomerization of the coordinated ligands.
Eric-Saint-Aman − Univ. Grenoble Alpes, CNRS,
Département de Chimie Moléculaire, F38000 Grenoble,
France; Email: eric.saint-aman@univ-grenoble-alpes.fr
Christophe Bucher − Univ Lyon, Ens de Lyon, CNRS UMR
5182, Université Claude Bernard Lyon 1, Laboratoire de
Chimie, F69342 Lyon, France; orcid.org/0000-0003-
1803-6733; Email: christophe.bucher@ens-lyon.fr
Authors
Christophe Kahlfuss − Univ Lyon, Ens de Lyon, CNRS UMR
5182, Université Claude Bernard Lyon 1, Laboratoire de
Chimie, F69342 Lyon, France
Shagor Chowdhury − Univ Lyon, Ens de Lyon, CNRS UMR
5182, Université Claude Bernard Lyon 1, Laboratoire de
Chimie, F69342 Lyon, France
Adérito Fins Carreira − Univ Lyon, Ens de Lyon, CNRS
UMR 5182, Université Claude Bernard Lyon 1, Laboratoire
de Chimie, F69342 Lyon, France
Raymond Gruber − Univ Lyon, Ens de Lyon, CNRS UMR
̈
5182, Université Claude Bernard Lyon 1, Laboratoire de
Chimie, F69342 Lyon, France
Elise Dumont − Univ Lyon, Ens de Lyon, CNRS UMR 5182,
Université Claude Bernard Lyon 1, Laboratoire de Chimie,
F69342 Lyon, France; Institut Universitaire de France,
75005 Paris, France; orcid.org/0000-0002-2359-111X
Denis Frath − Univ Lyon, Ens de Lyon, CNRS UMR 5182,
Université Claude Bernard Lyon 1, Laboratoire de Chimie,
F69342 Lyon, France; orcid.org/0000-0001-6888-9377
Floris Chevallier − Univ Lyon, Ens de Lyon, CNRS UMR
5182, Université Claude Bernard Lyon 1, Laboratoire de
Chimie, F69342 Lyon, France; orcid.org/0000-0002-
7482-9351
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02365
Author Contributions
Dissociation of the coordination polymers/oligomers
formed in the presence of tetrakis(acetonitrile)Pd(II) was
also achieved upon reduction of the viologen units involved in
the organic tectons, the driving force of the disassembling
process being here again the formation of (2:2) (M:L) box-
shape π-dimers.
We believe that the metal-assisted metamorphic processes
reported therein with easy-to-make building blocks will be a
great source of inspiration for the future development of redox-
responsive dynamic supramolecular assemblies involving π-
dimerization as actuation forces. Ongoing efforts are directed
toward discovering new redox-responsive metal−organic
supramolecular assemblies with macroscopic responses.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
This work was supported by the “Agence National de la
Recherche” (ANR-12-BS07-0014-01) and by the region
̂
Auvergne-Rhone-Alpes. The authors also acknowledge finan-
cial support from the LABEX-IMUST (Gelly). Calculations
have been performed using the local HPC resources of PSMN
(ENS Lyon).
Notes
The authors declare no competing financial interest.
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