Reversible photoreduction of Cu(II)-coumarin metal-organic polyhedra

Article abstract of DOI:10.1039/c7cc04799a

We report a new approach for photoinduced reduction of Cu²⁺ that will enrich the structural diversity of coordination complexes and be a valuable contributor to the development of Cu⁺/Cu⁰-based catalysts. To realize controlled Cu²⁺ reduction, coumarin as a triplet quencher of excited benzophenone was tethered to Cu(ii)-metal-organic polyhedra (MOPs). The photoinduced catalytic activity of the coumarin-MOPs was also examined in a Cu(i)-catalyzed azide-alkyne cycloaddition (CuAAC).

Full text of DOI:10.1039/c7cc04799a

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DOI: 10.1039/C7CC04799A
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Reversible Photoreduction of Cu(II)-Coumarin Metal-Organic
Polyhedra
Jaeyeon Bae,a Kangkyun Baek,b Daqiang Yuan,c Wooram Kim,b,d Kimoon Kim,b,d Hong-Cai Zhou,e
Received 00th January 20xx,
Accepted 00th January 20xx
and Jinhee Parka*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a new approach for photoinduced reduction of Cu2+ that
will enrich the structural diversity of coordination complexes and
be a valuable contributor to the development of Cu+/Cu0-based
catalysts. To realize controlled Cu2+ reduction, coumarin as a triplet
quencher of excited benzophenone was tethered to Cu(II)-metal-
organic polyhedra (MOPs). The photoinduced catalytic activity of
the Coumarin-MOPs was also examined in a Cu(I)-catalyzed azide-
alkyne cycloaddition (CuAAC).
optically responsive solubility change of an azobenzene-
functionalized MOP through trans-cis isomerization.31
In this report, cuboctahedral MOPs bearing Cu-paddlewheel
clusters were employed to realize the controlled photoinduced
reduction of Cu2+. Since the organic ligands are located close to the
Cu-paddlewheel clusters in the MOPs, a reversible and controlled
Cu2+ reduction was accomplished through judicious ligand design. As
a triplet photosensitizer, benzophenone facilitates the reduction of
Cu2+ to Cu+ and then to Cu0. Coumarin ides an effective triplet
3
quencher of excited benzophenone due to the relatively low π,π*
Metal-organic polyhedra (MOPs) are discrete cage-like
supramolecules that have been studied extensively owing to their
designability, tailorability, solubility, and intrinsic porosity.1-8
Previously, various structural MOPs inspired by geometric entities
such as Platonic and Archimedean solids were extensively reported.3-
5, 8, 9 Recent research has focused on the development of functional
MOP materials based on direct self-assembly, post-synthetic
transition energy of coumarin compared with that of
benzophenone.32 Thus, coumarin-modified MOPs (Coumarin-MOPs)
can control Cu2+ reduction via energy transfer from benzophenone
to coumarin. Generally, a Cu+ oxidation state is difficult to maintain
due to its high reduction potential. However, Coumarin-MOPs
stabilize the Cu+ state by perturbing the direct electron transfer from
benzophenone to Cu2+ and Cu+. Notably, Cu+ states with long
lifetimes allow their catalytic activity to be tested. In general, such a
photoinduced reduction system is useful because experiments can
be designed with a stable Cu2+ pre-catalyst; light irradiation then
initiates the catalytic activity easily and on demand. The catalytic
activity of the reduced Cu+ state was examined in the Cu(I)-catalyzed
azide-alkyne cycloaddition (CuAAC).33, 34
As zero-dimensional materials, MOPs are easily dispersed in
solvents through the proper choice of organic linkers and solvents.
This differentiates MOPs from their 3-dimensional analogues, i.e.
metal-organic frameworks (MOFs) and porous coordination
polymers (PCPs).35 In this report, the coumarin groups in MOPs act
not only as the triplet quencher but also as a solubility enhancer. Cu2+
salts and MOPs in organic solvents such as N,N-dimethylformamide
(DMF) and N,N-dimethylacetamide (DMAc) becomes insoluble when
Cu2+ is reduced to Cu+. Once MOPs are aggregated, only the surface
exposed to the photosensitizer undergoes photoinduced reduction,
and only the surface exposed to air may be re-oxidized. Therefore,
maintaining high solubility is a prerequisite for the reversible redox
reaction of Cu2+ complexes. The coumarin moieties allow the MOPs
to be soluble during the redox processes, leading to reversible and
homogeneous transformations.
modification10-12
,
and assembly with other materials13-16 for
applications in guest molecule storage,17 separation,18, 19 catalysis,20
23-25
drug delivery,21,
sensing,14,
etc.26,
In particular, post-
22
27
synthetic modifications have been used to attach additional
functional groups to organic linkers that are not compatible with
typical MOP syntheses.28, 29 However, the study of MOPs as stimuli-
responsive materials remains in the early stages, and optically
responsive MOPs, in particular, have rarely been reported. For
example,
a diaryl ethane-based cage-like molecule showed
photoinduced guest molecule capture and release, which was caused
by changes in its rigidity during the electrocyclization and
cycloreversion reactions of the ligand.30 Park et al. demonstrated an
a.Department of Emerging Materials Science, Daegu Gyeongbuk Institute of Science
and Technology, Daegu 42988, Republic of Korea. E-mail: jinhee@dgist.ac.kr
b.Center for Self-assembly and Complexity, Institute for Basic Science, Pohang
37673, Republic of Korea
c. State Key Lab of Structure Chemistry, Fujian Institute of Research on the Structure
of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China
d.Department of Chemistry, Pohang University of Science and Technology, Pohang
37673, Republic of Korea,
e. Department of Chemistry, Texas A&M University, PO Box 30012, College Station,
TX 77842-3012, USA
Inspired by typical cuboctahedral MOP synthetic protocols, three
isophthalate-based ligands (L1, L2, and L3) were prepared in three
steps using dibromoalkane, 7-hydroxyl coumarin, and dimethyl 5-
Electronic Supplementary Information (ESI) available: Experimental details,
supporting figures and graphs, and crystallographic data tables. See
DOI: 10.1039/x0xx00000x
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hydroxyisophthalate (See the Supplementary Information for the reduction of Cu+ to Cu0. As a control experiment, theVrieewdAurctictileoOnnslionef
DOI: 10.1039/C7CC04799A
structures and synthetic procedure for L1, L2, and L3). Three Cu(CH3COO)2 and MOP-4 were irreversible and uncontrollable
different dibromoalkanes (i.e., dibromoethane (for L1), (Figure S24-25). The irreversible process is mainly caused by the
dibromopropane (for L2), and dibromohexane (for L3)) were used to formation of precipitates during the reduction from Cu2+ to Cu+/0
control the distance between coumarin and the Cu-paddlewheel
clusters (Figures 1c and S5) because we initially hypothesized that
this distance determines the reduction rate. For clarity, the MOPs
derived by L1, L2, and L3 were named as Coumarin-MOP-1,
Coumarin-MOP-2, and Coumarin-MOP-3, respectively. The optically
inert MOP-4 was also prepared with tert-butyl isophthalate and Cu2+
according to a previous report.11 Coumarin-MOP-1, which consists of
the least flexible ligand, crystallized into a blue, block-shaped crystal
via ether vaporization, and its structure was unambiguously
determined by single-crystal X-ray diffraction measurements.
Twenty-four coumarin groups dangled from the cuboctahedral cage
of Coumarin-MOP-1 (Figure 1a). The structures of Coumarin-MOP-2
and Coumarin-MOP-3 are analogues of Coumarin-MOP-1 in that the
coumarins were tethered to the cuboctahedral cages through
flexible propane and hexane moieties, respectively (Figures 1b and
S5).15, 36, 37 However, Coumarin-MOP-2 and -3 did not crystallize into
large crystals presumably due to the considerable flexibility of L2 and
L3. In addition, MOP crystals generally include large amounts of
solvent among the cages. Thus, single-crystal diffraction analyses of
Coumarin-MOP-2 and Coumarin-MOP-3 were not possible.
Nevertheless, atomic force microscopy (AFM) analysis of those
Coumarin-MOP-2 and -3 confirms the formation of cage-like particles
with 3–4 nm sizes (Figures 2e-f and S6-8).29 The simulated structures
of Coumarin-MOP-2 and Coumarin-MOP-3 were shown in Figure 1b
and S5. The Based on the crystallographic data and excluding the
alkyl coumarin groups, the cuboctahedral cage is ~2.1 nm. Because
all Coumarin-MOPs include the same cuboctahedral cage, the overall
sizes of the MOPs as measured from one coumarin to the other on
.
Figure 1. Crystal structure of Coumarin-MOP-1 (a) and simulated structure of
Coumarin-MOP-3 (b). (c) Ligand structures: n=1, 2, and 5 for L1, L2, and L3,
respectively. (d) Photographs of Coumarin-MOP-1, Coumarin-MOP-2, and
Coumarin-MOP-3 during the Cu2+ to Cu0 photoinduced reduction process. The
hours indicate the UV irradiation time. (e) Absorbance change at 720 nm for
Cu2+ to Cu+ reduction for Coumarin-MOP-1 and -3. (f) Absorbance change at
563 nm for Cu+ to Cu0 reduction for Coumarin-MOP-1 and -3.
the opposite side in the crystal and simulated structures can be
directly related with the distance between coumarin and the Cu-
paddlewheel clusters. In addition, not only the calculated sizes from
the crystalline and simulated structures but also the measured sizes
from the AFM and dynamic light scattering (DLS) data are critical as
direct evidence to prove the relationship between the distance and
the reduction rate (Figure S9). The MOP sizes from the above-
mentioned methods were added to Table S2. From the AFM images
and DLS data, Coumarin-MOP-3 is 0.5-0.6 nm larger overall than
Coumarin-MOP-1 and -2, and the sizes of Coumarin-MOP-1 and -2
are similar size.
A proposed mechanism for the sequential reduction of Cu2+ to
38, 39
Cu+ and then to Cu0 is given in Reactions [1]–[5].32,
Photoabsorption generates benzophenone in the n,π* triplet state.
Subsequently, the radical-like benzophenone in the n,π* triplet state
abstracts a hydrogen atom from methanol or ether, generating
reactive radical pairs [1]. The benzophenone radical then donates the
electron to Cu2+, and a proton is transferred to solvent or the acetate
remaining in solution [2]. When the Cu+ is exposed to oxygen in air,
one electron is transferred from Cu+ to O2 [3], followed by the
reduction of the oxidized solvent [4].
Controlled photoinduced reduction of Cu2+ was demonstrated in
the Coumarin-MOPs. First, the Coumarin-MOPs were dissolved in
DMF and methanol for their efficient hydrogen donating and
electron transfer properties. Adding diethyl ether to the solution
moderated the solubility change during the Cu2+ reduction process.
The ratio of DMF, methanol, and diethyl ether in the solution was
9:0.5:0.5, which was optimized for the reversible reduction and
oxidation process. Moles of benzophenone equivalent to those of
Cu2+ of the Coumarin-MOPs were added to the MOP solutions. All
MOP reduction experiments were conducted under inert conditions
with 365 nm UV irradiation. Additional experimental details are
described in the Supplementary Information. Reduction of all
Coumarin-MOPs was monitored not only by eye observation but also
by UV-Vis spectroscopy. The blue colour (absorption λmax= 720 nm)
of the MOP-solutions faded upon Cu2+ to Cu+ reduction, because of
the absence of d-d transitions in Cu+ (d10) (Figures 1d-e and S18-23).
Further, the new absorption band at 563 nm, which indicates Cu
nanoparticle formation (Figures 1f and S18-23), confirms the
[1] BP + Solvent + hv → BP-H· + Solvent·
[2] BP-H· + Cu2+ + CH3COO-/Solvent·→ [BP…Cu+] + CH3COOH/Solvent·+
[3] O2 + Cu+ → O2·- + Cu2+
[4] O2·- + Solvent·+ → O2 + Solvent
[5] BP + Coumarin + hv → BP* + Coumarin → BP + Coumarin*
For the two-electron reduction from Cu2+ to Cu0, [1] and [2] must
be repeated with Cu+ instead of Cu2+. This proposed mechanism
explains the two-electron reduction of Cu2+ with one equivalent of
2 | J. Name., 2012, 00, 1-3
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benzophenone. The reduction of Cu2+ to Cu+ in MOP-4 took less than
30 min under the same UV irradiation conditions, followed by the
reduction of Cu+ to Cu0 (Figure S24). This uncontrollable reduction
process is not suitable to isolate the Cu+ state for practical use.
Reaction [5] illustrates the energy transfer from benzophenone to
coumarin as an alternative step for excited-state benzophenone.
Excitation of coumarin with benzophenone is almost 10 times faster
than that of coumarin itself, as shown in the UV-Vis spectra (Figures
S15 and S16), indicating that coumarin can accept energy from
benzophenone. When coumarin was not tethered to the MOPs, the
reduction was not successful due to the rapid formation of
precipitates (Figure S26). In contrast, when coumarin was tethered
to the MOPs through the alkyl groups the reduction rates were
significantly attenuated, and even controlled by changing the
distance between coumarin and the Cu-paddlewheel clusters. 1.125
mM Coumarin-MOP-1, -2, and -3 were reduced from Cu2+ to Cu+
within 2.5, 2.0, and 1.5 h, respectively (Figures 1d-e and S18-23).
Notably, the Cu+ states in Coumarin-MOPs lasted for 1.5~2 h, which
is long enough to isolate the Cu+ state, even during continuous UV
irradiation. These results indicate that the energy transfer from
excited-state benzophenone to coumarin notably delays the Cu+
reduction process. We note that even the use of one-quarter
equivalent benzophenone relative to Cu2+ resulted in the similar
lifetime of the Cu+ state (Figure S27). Further, it is noteworthy that
even though Cu+ leaching from the MOP structures cannot be
prevented, the Cu+ state persisted for at least 1 month after the UV
irradiation was stopped (Figure S33).
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DOI: 10.1039/C7CC04799A
Figure 2. UV-vis spectra during the reversible reduction and oxidation
processes of Coumarin-MOP-1 (a) and Coumarin-MOP-2 (b). Monitoring of
colour change during the c) reversible reduction and oxidation processes for
Coumarin-MOP-3 and d) irreversible reduction and oxidation processes for
Coumarin-MOP-3. AFM images of Coumarin-MOP-3 before (e) and after the
Cu2+ to Cu+ reduction process (f).
The Cu+ to Cu0 reduction of Coumarin-MOP-1 and -2 did not
show any significant rate difference, indicating that coumarin in both
MOPs can equally deactivate the excited-state benzophenone
coordinated to Cu+ (Figure S18-21). However, the Cu+ to Cu0
reduction for Coumarin-MOP-1 and -3 took 8.5 and 3 h, respectively,
to reach an absorbance of 2 (Figure 1f). The absorbance at 563 nm of
Coumarin-MOP-1 after 13 h of UV exposure was even lower than that
of Coumarin-MOP-3 after 6.5 h of UV exposure (Figure 1f). This
substantial rate difference is indirectly indicative of the
benzophenone coordination to the Cu+ clusters following electron
transfer. The coumarin group is close to benzophenone in Coumarin-
MOP-1 and -2. Thus, it appears as though electron transfer from
excited-state benzophenone to Cu+ is perturbed more effectively by
the energy transfer from excited-state benzophenone to coumarin
(Reaction [5]). In fact, reduction of Cu+ to Cu0 generated Cu
nanoparticles, as evidenced by blue-shifted absorption bands upon
extended UV irradiation time (Figure S18–23). Notably, their
solubility was retained without any visible precipitates. Owing to the
rapid oxidation of the Cu+-MOPs and Cu nanoparticles to Cu2+ upon
air exposure, the further analysis such as X-ray photoelectron
spectroscopy (XPS) and scanning electron microscopy (SEM) was not
possible. Additionally, this Cu nanoparticle formation caused the
irreversible reduction and oxidation of the Cu-paddlewheels because
the bonds between Cu and the ligands broke (Figure 2d).
The reduction system developed herein has been utilized in the
CuAAC as a photoinduced homogeneous catalyst. The photoinduced
catalytic activities reported previously are mainly based on
photodissociation processes or electron transfer from the
coordinated benzophenone to Cu2+.38, 40 Previous reaction systems
did not consider strategic molecular design to control the reduction
kinetics or to retain the relatively unstable Cu+ states. As a model
reaction to probe the catalytic activity of the Cu+ states, the
cycloaddition reaction between phenylacetylene and phenyl azide
was selected. Based on the reduction time from Cu2+ to Cu+ with
Coumarin-MOP-1, a solution of phenylacetylene, phenyl azide,
Coumarin-MOP-1 (5 mol%), and benzophenone was irradiated by UV
light at 365 nm for 3 h (Scheme 1). The reaction was continued for
an additional 3 h without irradiation for its completion. 1,4-
Disubstituted 1,2,3-triazole was isolated as a crystalline precipitate
with 89% yield and analyzed by NMR spectroscopy. Under identical
reaction conditions, the reaction with CuI and CuCl as traditional
catalysts exhibited low yields of 47 % and 33%, respectively. We
assume that the low conversion rates resulted from the low solubility
of CuI and CuCl in the solution compared to Coumarin-MOP-1.41 The
detailed experimental procedures are described in Supporting
Information-Part S6. Overall, this result demonstrates the potential
application of a MOP as a photoinduced catalyst, which has been
seldom explored.
The Cu2+ and Cu+ states in the Coumarin-MOPs were reversible
upon alternating the UV irradiation and O2 exposure in three cycles
(Figures 2a-c and S30-32). AFM images of the Coumarin-MOPs after
three cycles (one cycle: Cu2+  Cu+  Cu2+) showed sphere-like
MOPs (Figures 2e-f and S6–8). There is no significant size difference
before or after the Cu2+ to Cu+ reduction process. The weak
coordination bond of the paddlewheel clusters with Cu+/0 leads to
limited reversibility, indicating the structural collapse of the Cu+/0
MOPs.
-
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14. C. Wang, J. Shang, Y. Lan, T. Tian, H. Wang, X. Chen, J.-Y. Gu,
View Article Online
J. Z. Liu, L.-J. Wan, W. Zhu and G. Li, ADdOv.I:F1u0n.1c0t3.9M/Ca7tCeCr.0,4270919A5,
25, 6009-6017.
15. Y.-H. Kang, X.-D. Liu, N. Yan, Y. Jiang, X.-Q. Liu, L.-B. Sun and
J.-R. Li, J. Am. Chem. Soc., 2016, 138, 6099-6102.
16. X. Qiu, W. Zhong, C. Bai and Y. Li, J. Am. Chem. Soc., 2016,
138, 1138-1141.
Scheme 1. Cu(I)-Catalyzed Azide-Alkyne Cycloaddition (CuAAC).
We have developed a photoinduced Cu2+ to Cu+/Cu0 reduction
system by employing Cu-cuboctahedral MOPs. The MOPs are ideal
compounds to study photoinduced Cu2+ reduction processes
because desired functional groups can be introduced relatively easily
into the system. The MOPs were functionalized by the triplet
quencher coumarin, yielding a relatively long lifetime of the Cu+
states, even during continuous UV irradiation. The distance between
the benzophenone coordinated to Cu+ and coumarin was
manipulated by changing the length of the alkyl groups on the
organic linkers. When the distance is less, Cu+ reduction is
significantly delayed due to an efficient transfer of energy from
benzophenone to coumarin. The reduction and oxidation between
the Cu2+ and Cu+ oxidation states in the Coumarin-MOPs were
reversible by alternating UV irradiation and air exposure, presumably
until the MOP structures became degraded. Finally, we successfully
applied the Coumarin-MOP photoinduced catalysts to the CuAAC.
Stabilizing the Cu+-MOPs and Cu0 nanoparticles generated by the
process developed in this report are currently being explored.
This work was partly supported by the National Research
Foundation of Korea (NRF) and grants funded by the Korean
government (Nos. NRF-2016R1C1B2009987 and NRF-
2016M2B2A9912217). J. Park thanks the support from the Faculty
Start-up Fund (2017010035) of the Daegu Gyeongbuk Institute of
Science and Technology (DGIST).
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DOI: 10.1039/C7CC04799A
Controlled reduction of Cu2+ to Cu+/Cu0 can be achieved by judicious ligand design in optically
responsive metal-organic polyhedra systems.