Sunlight-driven formation and dissociation of a dynamic mixed-valence Thallium(III)/Thallium(I) porphyrin complex

Article abstract of DOI:10.1002/anie.201411616

Inspired by a Newton's cradle device and interested in the development of redox-controllable bimetallic molecular switches, a mixed-valence thallium(III)/thallium(I) bis-strap porphyrin complex, with Tl^III bound out of the plane of the N core and Tl^I hung to a strap on the opposite side, was formed by the addition of TlOAc to the free base and exposure to indirect sunlight. In this process, oxygen photosensitization by the porphyrin allows the oxidation of Tl^I to Tl^III. The bimetallic complex is dynamic as the metals exchange their positions symmetrically to the porphyrin plane with Tl^III funneling through the macrocycle. Further exposure of the complex to direct sunlight leads to thallium dissociation and to total recovery of the free base. Hence, the porphyrin plays a key role at all stages of the cycle of the complex: It hosts two metal ions, and by absorbing light, it allows the formation and dissociation of Tl^III. These results constitute the basis for the further design of innovative light-driven bimetallic molecular devices. Newton's cradle: A mixed-valence thallium(III)/thallium(I) bis-strap porphyrin complex, with Tl^III bound to the N core and Tl^I attached to a strap, was generated by the addition of TlOAc to the free base and exposure to indirect sunlight. Exposure of the complex to direct sunlight leads to thallium dissociation and to the recovery of the free porphyrin.

Full text of DOI:10.1002/anie.201411616

Angewandte
Chemie
DOI: 10.1002/anie.201411616
Porphyrinoids
Sunlight-Driven Formation and Dissociation of a Dynamic Mixed-
Valence Thallium(III)/Thallium(I) Porphyrin Complex**
Victoria Ndoyom, Luca Fusaro, Vincent Dorcet, Bernard Boitrel,* and Stꢀphane Le Gac*
Abstract: Inspired by a Newtonꢁs cradle device and interested
in the development of redox-controllable bimetallic molecular
switches, a mixed-valence thallium(III)/thallium(I) bis-strap
porphyrin complex, with TlIII bound out of the plane of the
N core and TlI hung to a strap on the opposite side, was formed
by the addition of TlOAc to the free base and exposure to
indirect sunlight. In this process, oxygen photosensitization by
the porphyrin allows the oxidation of TlI to TlIII. The bimetallic
complex is dynamic as the metals exchange their positions
symmetrically to the porphyrin plane with TlIII funneling
through the macrocycle. Further exposure of the complex to
direct sunlight leads to thallium dissociation and to total
recovery of the free base. Hence, the porphyrin plays a key role
at all stages of the cycle of the complex: It hosts two metal ions,
and by absorbing light, it allows the formation and dissociation
of TlIII. These results constitute the basis for the further design
of innovative light-driven bimetallic molecular devices.
dynamic features associated with a specific class of bimetallic
porphyrin complexes, which incorporate a metal ion bound
out of the plane (OOP) of the N core and a second one hung
to a strap above the N core on the opposite side, in the so-
called hanging-atop coordination mode (HAT).[5] Thus far,
two types of double translocation processes have been
characterized, which correspond to an equilibrium between
two degenerate forms of the complex, as exemplified in
Figure 1 with lead(II)-based metalloporphyrins. Of particular
interest is the motion of the metal ions triggered by an
M
etal-ion-based molecular switches are particularly attrac-
tive for the design of molecular devices whose properties
depend on the oxidation state of the metal.[1] As observed in
catenane or rotaxane structures, motion of interlocked
subunits can be achieved under redox control owing to
a reorganization in the coordination sphere of the oxidized or
reduced metal center.[2] Other systems, albeit much less
developed, include a translocation of a metal ion between two
different coordination sites.[3] Considering bimetallic com-
plexes, there are very few molecular switches that integrate
a double translocation of two metal ions in their functioning,[4]
and a redox control of such a process is even rarer.[4b] In the
course of our research on the supramolecular coordination
chemistry of porphyrins, we have recently observed unique
Figure 1. Recent examples of double translocation processes with PbII
bimetallic complexes of a bis-strap porphyrin ligand bearing overhang-
ing carboxylic acid groups. a) Coupled and compartmentalized intra-
molecular migrations of PbII ions resembling the motion of spheres in
a Newton’s cradle device (upper strap to N core up coupled with
N core down to bottom strap).[5b] b) Intramolecular migration of HgII
or CdII ions through the N core coupled to the intermolecular
migration of PbII ions from one to the other strap (“defective”
Newton’s cradle motion, N core up to N core down coupled with
bottom strap to upper strap).[5e]
[*] V. Ndoyom, V. Dorcet, B. Boitrel, S. Le Gac
UMR CNRS 6226, Institut des Sciences Chimiques de Rennes
Universitꢀ de Rennes 1
263 avenue du Gꢀnꢀral Leclerc, 35042 Rennes cedex (France)
E-mail: bernard.boitrel@univ-rennes1.fr
stephane.legac@univ-rennes1.fr
L. Fusaro
allosteric effector (e.g., AcOꢀ) promoting the HAT coordi-
nation mode.[5b,d] Furthermore, switching between the two
types of motions as depicted in Figure 1 can be readily
achieved by a dynamic constitutional evolution of the system
triggered by a chemical effector.[5e] These recent findings
define a new class of molecular devices, whose working
principles resemble that of a Newtonꢀs cradle device. Aiming
at expanding our toolbox towards the redox control of such
double translocation processes, we turned our attention to the
complexation of thallium and describe herein an unprece-
Unitꢀ de Chimie des Nanomatꢀriaux (CNANO)
Universitꢀ de Namur
61 rue de Bruxelles, 5000 Namur (Belgium)
[**] We are grateful to ANR (research program ANR BLANC “BIBI-
CHEMAP”, ANR-12-B507-0006-01) for financial support. L.F. thanks
Prof. Michel Luhmer (Universitꢀ Libre de Bruxelles) for stimulating
discussions and access to the NMR facilities, and Philippe Thils
(Agilent Technologies) for his invaluable help with 205Tl heteronu-
clear NMR spectroscopy.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201411616.
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dented photoredox process for the formation and dissociation
of a dynamic mixed-valence TlIII/TlI porphyrin complex.
In light of previous work reporting very different chemical
and spectroscopic properties for thallium(III) versus thal-
lium(I) porphyrin complexes, a preliminary NMR spectro-
scopic study was conducted with tetraphenylporphyrin (TPP)
and the TlIII and TlI acetate salts. In a CDCl3/CD3OD solution
(9:1), the addition of five equivalents of Tl(OAc)3 or TlOAc
to TPP led to the mononuclear TlIII complex [(TPP)TlIIIOAc]
and to the dinuclear TlI complex [(TPP)TlI2], respectively,
which were both formed as minor products (Scheme 1; see
able to promote the photo-oxidation of TlI to TlIII, giving
evidence that TlI complexation to the macrocycle is not
a prerequisite for this process (see the Supporting Informa-
tion). Furthermore, we also observed that [(TPP)TlIIIOAc],
which is stable in the dark, could be efficiently demetalated
(> 90%) upon exposure to direct sunlight for approximately
16 hours, independent of the presence of oxygen, leading very
cleanly to the free base TPP as the only product (Scheme 1;
see also the Supporting Information). Although more
detailed studies are still needed, these two processes, photo-
induced formation and photo-dissociation of a TlIII porphyrin
complex, are very attractive as they only
require sunlight—and oxygen for the
former process—and can be controlled
by simply changing the light exposure of
the sample.
We next examined the complexation
properties of C2-symmetric bis-strap por-
phyrin 1 towards TlI and TlIII acetate
1
salts by H and 205Tl NMR spectroscopy
(Scheme 2). In a CDCl3/CD3OD (9:1)
solution, the monometallic TlIII complex
1TlIII was readily obtained by addition of
a slight excess of Tl(OAc)3 to 1, and no
other species were detected with
a higher amount of the metal salt (see
the Supporting Information).[10] The
4
b-pyrrolic protons exhibit a J coupling
constant of approximately 60 Hz with
the 205Tl isotope, giving rise to a dense set
of eight signals (dd) in the 8.5–9.5 ppm
region. 205Tl-decoupled 1H NMR spectra
were recorded to allow for the assign-
ment of these signals (see the Supporting
Scheme 1. Complexation properties of TPP towards TlOAc and Tl(OAc)3 and photoredox
behavior of the resulting complexes. Experimental conditions: 298 K, CDCl3/CD3OD (9:1),
DIPEA (10 equiv). DIPEA=N,N-diisopropylethylamine.
Information).
The
dissymmetric
1H NMR pattern indicates that TlIII is
also the Supporting Information). The latter bimetallic
complex was stable in the dark, but we observed the slow
formation of [(TPP)TlIIIOAc] upon leaving the NMR tube
under ambient light in the laboratory (Scheme 1, pathway B;
these conditions are defined as indirect sunlight). Typically,
approximately 90% conversion was achieved upon exposure
to indirect sunlight for about 36 hours. To the best of our
knowledge, such a process is unprecedented, the few photo-
chemical studies of bis-TlI porphyrins pointing to a photo-
induced degradation of the ligand.[6] Photo-oxidation of TlI
salts has been scarcely reported; it was always studied in
aqueous solution and without any photosensitizer, and its
mechanism is not fully understood.[7] Most likely, photo-
excitation of the TlI salts by UV irradiation is a primary step
that subsequently leads to oxygen activation, and reactive
oxygen species then oxidize the metal ion following the
general equation: TlI + 2H+ + O2!TlIII + 2H2O2.[7a] In the
present case of [(TPP)TlI2], the oxygen dependence of the
photo-oxidation process has been demonstrated (see the
Supporting Information), supporting a mechanism involving
singlet oxygen formed by porphyrin photosensitization.[8,9]
TlOAc was not photo-oxidized in the absence of porphyrin
under these experimental conditions, whereas [(TPP)Zn] was
located on one side of the macrocycle, which was confirmed
by an X-ray structure (Figure 2a). One of the carboxylic acids
behaves as an intramolecular counterion (Tl–O1: 2.328 ꢁ, Tl–
O2: 2.368 ꢁ), and pulls the metal 0.914 ꢁ away from the 24-
atom porphyrin mean plane (24MP, out-of-plane coordina-
tion). In solution, an intraligand exchange of TlIII between the
two sides of the macrocycle occurs by funneling through the
N core (Scheme 3a).[11] Indeed, a ROESY 2D NMR experi-
ment shows exchange correlations between protons of
opposite straps (see the Supporting Information). Similarly
to [(TPP)TlIIIOAc], 1TlIII was stable in the dark and tolerated
well the ambient light of the laboratory. However, exposure
to direct sunlight led to photo-demetalation and to the total
recovery of free base 1, which was not affected by this process
(Scheme 2, top; Figure 3a–c). Therefore, three metalation/
photo-demetalation cycles were achieved in a single NMR
tube (light-gray loop in Scheme 2; Figure 3a–g).
The further addition of TlOAc to a solution of 1TlIII
(Scheme 2, pathway A) led to a clear modification of the
1H NMR spectrum consistent with the complexation of TlI in
a fast exchange regime (Ka ꢁ 80mꢀ1; see the Supporting
Information). In the absence of base, the new species retains
a dissymmetric 1H NMR pattern, whereas a broad NMR
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Scheme 2. Complexation properties of bis-strap porphyrin 1 towards TlOAc and Tl(OAc)3 and photoredox behavior of the resulting complexes.
revealed the formation of a mixed-valence
TlIII/TlI complex, namely 1TlIII·TlI, owing to
the binding of TlI to the free COOꢀ group
of 1TlIII (Figure 2b). TlI was found at
2.973 ꢁ above the 24 MP with no interac-
tion with the N core (shortest TlI–N2 dis-
tance: 3.373 ꢁ) and with a TlIII–TlI distance
of 4.072 ꢁ. Its coordination sphere involves
five oxygen atoms: The O3 and O4 atoms
of the overhanging carboxylate ion, the Os
atom of a weakly bound methanol, which
also forms a hydrogen bond with an amide
group of the strap (dashed line), and the O3
atom of a second complex (intermolecular
coordination), which leads to a dimeric
assembly (see the Supporting Information).
This radiocrystallographic structure repre-
sents the first example of a hanging-atop
monovalent metal ion. Mixed-valence
bimetallic complexes with the two metal
ions located on opposite sides of a porphy-
rin system have not been reported thus far
and are of particular interest for the design
Figure 2. Crystal structures of 1TlIII (a) and 1TlIII·TlI (b). Ellipsoids set at 20% and 30%
probability, respectively; H atoms and solvents of crystallization were removed for clarity.
Selected distances [ꢁ] for 1TlIII : Tl–N1 2.226, Tl–N2 2.262, Tl–N3 2.190, Tl–N4 2.215, Tl–O1
2.328, Tl–O2 2.368, Tl–24MP 0.914; for 1TlIII·TlI: Tl1–N1 2.240, Tl1–N2 2.219, Tl1–N3 2.235,
Tl1–N4 2.204, Tl1–O1 2.267, Tl1–O2 2.588, Tl1–24MP 0.826, Tl1–Tl2 4.072, Tl2–24MP 2.973,
Tl2–O3 2.679, Tl2–O4 2.719, Tl2–Os 2.897, Tl2–O3 (inter) 2.762.
signature characteristic of a C2-symmetric compound with
4J(Hbpyr–TlIII) coupling constants of approximately 63 Hz was
observed in the presence of DIPEA (Figure 4 f, see also the
Supporting Information). An X-ray structure of this species
of bimetallic devices under redox control. A 2D NMR
ROESY experiment of 1TlIII·TlI also shows exchange correla-
tions between protons of the two straps (see the Supporting
Information), indicating that the metal ions exchange their
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an intramolecular motion for TlIII (funneling through the
N core as in 1TlIII) with an intermolecular pathway for TlI
exchanging between the two overhanging carboxylate groups,
similarly to that previously reported for PbII based hetero-
bimetallic complexes (see Figure 1b; “defective” Newtonꢀs
cradle motion).[5e] The photoredox behavior of 1TlIII·TlI is
similar to that of 1TlIII, that is, it is characterized by its stability
in the dark and a clean photodissociation of the TlIII ion upon
exposure to direct sunlight, leading to an average spectrum of
1 and 1·2TlI (see below) with the relative amount of
1 increasing with the time of light exposure (diagonal in
Scheme 2; see also the Supporting Information).
The complexation properties of the free base 1 towards
TlOAc were then investigated in the same CDCl3/CD3OD
solution. In the presence of DIPEA, a mixture of two species
was obtained with an excess of five equivalents of TlOAc
(Figure 4a; see also the Supporting Information). At 298 K,
the major species displayed an ill-defined spectrum whereas
a well resolved signature characteristic of a C2-symmetric
compound was observed for the minor one. A similar titration
experiment by UV/Vis spectroscopy indicates that the major
species presents a Soret band slightly red-shifted compared to
that of 1 (430 vs. 421 nm), whereas that of the C2-symmetric
species is observed at 488 nm (see the Supporting Informa-
tion). These data are in agreement with a mixture of two
bimetallic complexes, namely 1·2TlI and 1TlI2, for which the
two TlI ions are bound in a fast exchange regime to the strap
(HAT) and in a slow exchange regime to the N core (OOP),
presumably with the participation of the
Scheme 3. Dynamic behavior of 1TlIII (a) and 1TlIII·TlI (b).
carbonyl group of the overhanging carbox-
ylate groups, respectively (Scheme 2, bot-
tom).[6b] This mixture of bis-TlI complexes
did not show any evolution when kept in the
dark nor when exposed to indirect sunlight
in a deoxygenated solution. However, under
ambient atmosphere, both 1H and 205Tl
NMR monitoring revealed the gradual
appearance of the mixed-valence complex
1TlIII·TlI upon exposure of the NMR tube to
indirect sunlight (Figure 4b–e; Scheme 2,
pathway B), with approximately 60% con-
version after about 35 hours of light expo-
sure (Figure 5c,f). More detailed experi-
ments that are still in progress in our
laboratory tend to show that the binding of
TlI to the strap or to the N core has little
influence on the global process, that is, the
TlI to TlIII photo-oxidation and the insertion
of the latter ion into the N core. Although it
is difficult to estimate the proportion of TlIII
that binds to the porphyrin N core, its
Figure 3. 1H NMR study confirming the sunlight-driven formation and dissociation of 1TlIII
Conditions: CDCl3/CD3OD (9:1), 298 K, DIPEA (10 equiv). Three cycles were performed in
a single NMR tube (a–c, c–e, e–g). S=solvent, *=impurity.
.
insertion might be hindered by the HAT
TlI complexation, as it is supposed to pro-
ceed by a deconvolution mechanism involv-
ing the overhanging carboxylic acid.
As mentioned above, the exposure of
positions relative to the porphyrin plane (Scheme 3b). This
equilibrium between the two degenerate forms of the
complex corresponds to a double translocation combining
1TlIII·TlI to direct sunlight gives 1; therefore, two cycles were
performed in a single NMR tube (dark-gray triangle in
Scheme 2; Figure 5): The addition of TlOAc to 1 led to
4
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Figure 4. a) 1H (left) and 205Tl (right) NMR spectra of a mixture of 1·2TlI and 1TlI2, formed by the addition of five equivalents of TlOAc to 1.
Conditions: CDCl3/CD3OD (9:1), 298 K, DIPEA (10 equiv). b–e) Evolution of the NMR spectrum upon exposure of the NMR tube to indirect
sunlight for approximately 8 hours, showing the gradual apparition of 1TlIII·TlI (Scheme 2, pathway B). f) Reference spectra of a sample of 1TlIII·TlI
that was formed by the addition of Tl(OAc)3 (1.25 equiv) and TlOAc (5 equiv) to 1 (Scheme 2, pathway A). S=solvent, *=impurity.
molecular
chemistry of porphyrins. The
mixed-valence complex
coordination
1TlIII·TlI displays dynamic fea-
tures relying on metal ion
translocations, and both its
formation and dissociation
are driven by sunlight. In
this system, the porphyrin is
not only a ditopic host for
metal ions, but it also acts as
a
photosensitizer playing
a key role in the oxidation
or reduction of the metal
ions, hence triggering their
incorporation or release. It
thus operates at all stages of
the cycle of the device
(assembling/functioning/dis-
assembling). We are cur-
rently investigating the pho-
toredox behavior of related
Figure 5. 1H NMR study confirming the sunlight-driven formation and dissociation of 1TlIII·TlI. Conditions:
CDCl3/CD3OD (9:1), 298 K, DIPEA (10 equiv). Two cycles were carried out in a single NMR tube (a–d, d–g).
S=solvent, *=impurity.
heterobimetallic
thallium-
based complexes, aiming for
light-driven
molecular
switches[12] based on double
translocation processes.
a mixture of bis-TlI complexes, exposure to indirect sunlight
afforded 1TlIII·TlI, and subsequent exposure to direct sunlight
Received: December 2, 2014
Published online: && &&, &&&&
gave back the free base 1 (Figure 5a–d); this experiment was
repeated twice in the same NMR tube (Figure 5d–g), high-
lighting the remarkable properties of our system.
In conclusion, the site-selective binding of the redox-
active metal ions TlIII and TlI to ligand 1, which meets the
geometrical prerequisites of so-called Newtonꢀs cradle-like
bimetallic devices, opens new perspectives for the supra-
Keywords: molecular devices · photochemistry · porphyrinoids ·
redox chemistry · thallium
.
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Scheme 2.
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1
a H NMR spectrum of a 1:1 mixture of 1 and 1TlIII recorded at
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Communications
Porphyrinoids
Newton’s cradle: A mixed-valence
thallium(III)/thallium(I) bis-strap por-
phyrin complex, with TlIII bound to the
N core and TlI attached to a strap, was
generated by the addition of TlOAc to the
free base and exposure to indirect sun-
light. Exposure of the complex to direct
sunlight leads to thallium dissociation
and to the recovery of the free porphyrin.
V. Ndoyom, L. Fusaro, V. Dorcet,
B. Boitrel,* S. Le Gac*
&&& — &&&
Sunlight-Driven Formation and
Dissociation of a Dynamic Mixed-Valence
Thallium(III)/Thallium(I) Porphyrin
Complex
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These are not the final page numbers!