High conversion and selectivity of photodimerization under air conditions by supramolecular oxidation restraint within a metallocage-like nanoreactor

Article abstract of DOI:10.1039/d0ce00678e

The well-designed metal-organic cage Ce-BHP, with a size-suitable cavity, functional interaction sites and a flexible backbone, could encapsulate 9-anthraldehyde molecules and act as a nanoreactor via supramolecular behavior, avoiding the undesired oxidation forming 9,10-anthraquinone, and also as an efficient catalyst to successfully achieve high conversion of single photodimers under air conditions. This journal is

Full text of DOI:10.1039/d0ce00678e

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High conversion and selectivity of
photodimerization under air conditions by
supramolecular oxidation restraint within a
metallocage-like nanoreactor†
Cite this: DOI: 10.1039/d0ce00678e
Received 7th May 2020,
Accepted 23rd July 2020
Lu Yang,a Lan Qin,a Yong Dou,a Daopeng Zhang,a
DOI: 10.1039/d0ce00678e
b
Zhen Zhou
a and Suna Wang
*
*
rsc.li/crystengcomm
The well-designed metal–organic cage Ce-BHP, with a size-
suitable cavity, functional interaction sites and flexible
backbone, could encapsulate 9-anthraldehyde molecules and act
as nanoreactor via supramolecular behavior, avoiding the
seeking or developing appropriate systems participating in
photodimerization as catalysts or reaction media to improve
the conversion and selectivity appears to be particularly
significant.
a
a
undesired oxidation forming 9,10-anthraquinone, and also as an
efficient catalyst to successfully achieve high conversion of single
photodimers under air conditions.
On the other hand, photodimerization generally could
obtain several isomers within a single system. The accurate
control and improvement of its stereoselectivity and
regioselectivity have become a topical issue for the intensive
research of photodimerization.7–9 In particular, the
stereoselectivity and regioselectivity of photodimerization
become more complicated in solution, due to the freer and
more irregular motion among the molecules. Based on these
The remarkable photodimerization between two identical or
different molecules into photodimers has been one of the
most important categories in photochemical reactions.1–3
Depending on the different numbers of atoms in the formed
ring, the modes of photodimerization could be classified as [2
+ 2], [4 + 4], [4 + 2], [3 + 2], etc., which mainly occurs between
the organic molecules containing carbon–carbon double
bonds or aromatic molecules with good photoactivity.
Therefore, photodimerization could be regarded as a feasible
method for the direct formation of C–C bonds and is much
greener than other general organic reactions, owing to its
atom economy, less toxic/polluting agents involved and light-
driven behavior.4–6 Taking the reaction mechanism into
account, the substrates are firstly activated to the excited state
or radicals are generated under irradiation and then their
high energy and different electronic patterns are exploited for
the next combination. The excited state species and radicals
usually have short lifetimes and are difficult to regulate
during the photochemical reactions. From this point of view,
situations, the introduction of
a supramolecular self-
assembled structure into the photodimerization system as a
molecular reactor to restrict the substrates within well-
defined confined space proved to be an efficient approach for
the
improvement
of
The
the
excellent
stereoselectivity
and
regioselectivity.10–13
and
well-studied
supramolecular architectures in this field were mainly
focused on macrocyclic hosts formed through covalent
bonds, including cyclodextrins, calixarenes, covalent
capsules, and cucurbiturils, which have been widely
delineated by the pioneering work of many groups.14–19
Another class of interesting molecular hosts, coordination-
driven metal–organic cages (MOCs), which are self-assembled
by metal ions and organic ligands as linkers containing
internal cavities with well-defined shapes and sizes, have
achieved increasing prominence.20–24 These materials, as the
subunits of artificial supramolecular host platforms, could be
easily functionalized with multiple active sites and accurately
control the size and shape of the structures via the
coordination of specific metal ions with well-designed
ligands. The promising functionalities and coordination
abilities of MOCs provide them with advantages to mimic
protein receptors or enzymes for the effective binding ability
of molecules within their microenvironments to stabilize
reactive intermediates or catalyze chemical transformations
a School of Chemistry and Chemical Engineering, Shandong University of
Technology, Zibo, 255000, P. R. China. E-mail: zhouzhen@sdut.edu.cn
b Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry and Chemical Engineering, Liaocheng University,
Liaocheng 252059, P. R. China
† Electronic supplementary information (ESI) available: Supplementary
structural figures, characterization of 1, including PXRD patterns, TG curve, IR
spectra and luminescence, catalysis details of hydrolytic cleavage and crystal
data. CCDC 1983149 and 2011937. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d0ce00678e
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avoiding the undesired reaction pathways.25–28 Therefore,
compared with the traditional macrocyclic hosts, the
advantages of coordination-driven MOCs mainly are: (1) the
coordinated ligands could be easily modified with functional
groups as required; (2) the introduction of metal ions into
the system could improve the potential activity; (3) the sizes
and shapes of structures and cavities could be modulated via
the assembly of the extensive choice of metal ions and well-
designed ligands.
Recently, we reported
a flexible ligand-based metal–
organic cage by the incorporation of Ce(III) ions synthesized
via a vapor diffusion method.29 The 4f metal atoms in the
metal-based complex with alterable valence could perform
multiple redox states that facilitate the electron or proton
transfer and thus it is regarded as potential
photocatalyst.30–32 The functional amide and secondary
amino groups of ligands as interaction sites play an
important role in the host–guest supramolecular behaviour
for the selective recognition of RNA-based nucleoside
derivatives with fluorescence response. The self-adaptive
behaviour of the flexible ligand backbone presented in this
work inspired us to further study the field of
photodimerization of 9-anthraldehyde. From the structural
point of view, this metal–organic cage, Ce-BHP, contains
open windows of 8.52 × 9.56 Å2 and a well-defined cavity,
which are suitable for the accommodation of 9-anthraldehyde
molecules with a width of 5.8 Å and a length of 9.2 Å.
Additionally, the functional amide and secondary amino
groups of ligands in the backbone and the aldehyde group in
9-anthraldehyde exhibit potential hydrogen-bonding, together
with π⋯π stacking interactions, to recognize and construct
the supramolecular system. Besides, the flexible backbone
with self-adaptivity of Ce-BHP facilitates the encapsulation
and maintains the stability of the complexation in solution
(Fig. 1). Based on these considerations, we envision that Ce-
BHP is an ideal candidate as a molecular reactor and a
catalyst for the catalysis of photodimerization with feasible
selectivity.
Fig. 2 (a) Main peaks in the 1H NMR spectra of the cage Ce-BHP upon
the addition of 2 mole ratio of 9-anthraldehyde; (b) free 9-anthraldehyde
in the mixture solution of CDCl3 :DMSO-d6 (v:v = 1:1).
8.95 ppm, respectively (Fig. 2), while other peaks have
remained unchanged. These protons could be attributed to
the aldehyde group and aromatic ring of 9-anthraldehyde
and the shifts provided an indicator for the strong
interaction between 9-anthraldehyde and the cage Ce-BHP via
supramolecular behavior.33
Fluorescence spectroscopy of 9-anthraldehyde in DMF
solution upon the addition of Ce-BHP was also performed
with excitation at 340 nm. Fluorescence titration revealed
that the addition of Ce-BHP in a solution of 9-anthraldehyde
(10.0 μM) caused significant emission quenching (Fig. 3).
The emission intensity exhibited almost complete quenching
when 0.5-fold Ce-BHP (5.0 μM) was added to the system. The
quenching process could probably be attributed to the
interaction between the host Ce-BHP and the guest
9-anthraldehyde molecules.34 The titration profiles upon the
addition of Ce-BHP are consistent with the Hill plot. The best
fitting of the titration profiles suggests a 1 : 2 host–guest
behavior, with the association constant (Kass.–G) being
calculated to be 4.33 × 109 M−1.
ESI-MS suggested that metal–organic cage Ce-BHP could
be regarded as a stable homogeneous catalyst in solution and
9-anthraldehyde was chosen as the model system to study its
photodimerization in the solution. Under typical conditions,
the photodimerization was conducted in a test tube by
The supramolecular interaction between the host Ce-BHP
and the guest 9-anthraldehyde molecules was investigated by
NMR spectroscopy. The 1H NMR titration spectrum of
9-anthraldehyde (10 mM) in a mixture solution of CDCl3 and
DMSO-d6 (v : v = 1 : 1) upon the addition of 0.5 mole ratio of
Ce-BHP (5 mM) to the above solution exhibited downfield
shifts of the protons from 11.50 to 11.62 ppm and 8.85 to
Fig.
3 (a) The family of normalized fluorescence spectra of
9-anthraldehyde (10.0 μM) in DMF solution upon the addition of 1.0,
2.0, 3.0, 4.0 and 5.0 μM Ce-BHP, excited at 340 nm; (b) quenching
fluorescence with different ratios of Ce-BHP as
a host and
Fig. 1 Structural features of the metal–organic cage Ce-BHP.
9-anthraldehyde as a guest molecule.
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adding 0.05 mmol of 9-anthraldehyde and Ce-BHP (1 mol%)
conditions in the dark (entry 5). According to previous
reports, the formation of the side product 9,10-
as
tertbutylammonium bromide (TBABr, 5 mol%) in a mixed
solution of CDCl3/DMSO-d6 (1mL/0.2 mL) in mild
a
homogeneous catalyst with the addition of
anthracenedione
from
9-anthraldehyde
needs
the
a
participation of oxygen or an oxidizing agent.37,38 The high
selectivity to the target photodimer in the presence of Ce-
BHP, together with the results of the control experiments,
indicated that the cage structure could be regarded as a
molecular reactor to effectively block the oxidation of
9-anthraldehyde during the photocatalysis. In order to
environment and under Xe lamp irradiation. After
1
irradiation, the reaction system was directly monitored by H
NMR analysis to determine the conversion and selectivity.
confirm
this
speculation,
we
carried
out
the
photodimerization under a saturated argon and oxygen
atmosphere with other fixed conditions. Almost complete
conversion (>99%) to the desired photodimer adduct
(selectivity >99%) was observed after 8 h irradiation in an
argon atmosphere (entry 6), and only 65% conversion without
adding Ce-BHP, suggesting that the presence of Ce-BHP
indeed has a positive effect on the increase of the conversion
(entry 7). However, 9-anthraldehyde could all convert into the
side product 9,10-anthracenedione in an oxygen atmosphere
in the absence of Ce-BHP, giving 54% selectivity to afford the
photodimer with the addition of a catalytic amount of Ce-
BHP (entries 8 and 9). These results exhibited full evidence
that the photodimerization occurred within the cavity of the
Surprisingly, unlike the normal photodimerization, the
obtained photoproducts were not postulated to the ideal syn-
and anti-dimers via the analysis of the 1H NMR spectrum.
Separated by silica-gel column chromatography and further
identified by GC-MS and X-ray single crystal analysis, the
final photoproducts were confirmed as anti-dimers and 9,10-
anthraquinone, which was synthesized from the oxidation of
9-anthraldehyde. The anti-dimer adducts of 9-anthraldehyde
were observed exclusively without the formation of
syn-dimers upon photodimerization35,36 and the reaction
route is illustrated in Table 1.
Irradiation of 9-anthraldehyde with Ce-BHP and TBABr
was carried out under the optimal conditions for 8 hours and
afforded 92% conversion and 90% selectivity for the main
product anti-dimer (entry 1). The control experiment shows
only 47% conversion and 83% selectivity in the absence of
the co-catalyst TBABr; however, without adding the cage Ce-
BHP into the system, relatively low reaction conversion and
selectivity are afforded (entries 3 and 4). These results
indicate that the presence of both Ce-BHP and TBABr plays a
significant role in the catalysis of photodimerization and we
deemed that Ce-BHP and TBABr act synergistically as
catalysts in the catalytic process. Additionally, the
photodimerization could not occur without irradiation, and
almost no conversion is observed under the typical
Ce-BHP cage, and Ce-BHP is regarded not only as
a
homogeneous catalyst for this reaction, but also as a shelter
to avoid the formation of the side product 9,10-
anthracenedione in the catalytic system. The free ligand BHP
(4 mol%) resulted in 82% conversion and 64% selectivity,
both lower than those of cage Ce-BHP. At this point, the
combination of Ce(III) centers and ligands into the metal–
organic cage system is a promising approach to achieve high
reaction efficiency and selectivity in photodimerization
catalysis.
The catalytic efficiency and selectivity depend on the
amount of Ce-BHP in a certain range and can be reduced by
adding 0.5% mol of Ce-BHP (entry 12). Fortunately, the single
crystals of photoproducts isolated from the catalytic system
after the reaction were obtained and the quality of the
crystals was sufficient for X-ray structure analysis. From the
Table 1 Ce-BHP-catalyzed and control experiments for the photodimerization of 9-anthraldehydea
Entry
Catalyst
Condition
Adduct
Conversionb (%)
Selectivityb (%)
1
2
3
4
5
6
7
8
Ce-BHP
Ce-BHP
—
Air
Air
Air
Air
TBABr
—
TBABr
—
TBABr
TBABr
TBABr
TBABr
TBABr
TBABr
—
92
47
72
22
Trace
>99
65
>99
>99
82
90
83
45
39
Trace
99
90
54
0
—
Ce-BHP
Ce-BHP
—
Ce-BHP
—
Ligand BHP
Ligand BHP
Ce-BHP
Air, in the dark
Ar
Ar
O2
O2
Air
Air
Air
9
10c
11c
12d
64
46
78
38
84
TBABr
a
Conditions: 9-anthraldehyde (0.05 mmol), TBABr (5 mol%), Ce-BHP (1 mol%), CDCl3/DMSO-d6 (1mL/0.2 mL), 25 °C, 8 h Xe lamp irradiation.
b
c
Conversion and selectivity are determined by 1H NMR analysis. With 4 mol% free ligand BHP. d With 0.5 mol% catalyst.
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structure, the asymmetric unit contains three independent
anti-dimer photoproducts and one 9,10-anthracenedione
molecule (Fig. 4). This crystal structure with a 3 : 1 ratio was
approximately in accordance with the calculation of the
catalytic selectivity (78%) and also provided full evidence for
the formation of 9,10-anthracenedione via the oxidation in
the catalytic system. Additionally, the single crystal of the
pure anti-dimer product could also be obtained and isolated
from the catalytic system after the reaction (Fig. S6†).
Catalytic traces of photodimerization have been further
obtained with cage Ce-BHP and the coexistence of TBABr
under the optimal conditions at different reaction times. As
shown in Fig. 5a, the conversion has increased with the
reaction time and remained unchanged after 8 hours. It was
worth noticing that there was almost no difference in the
selectivity with the increase of reaction time in the presence
of cage Ce-BHP. Additionally, the photoreaction process has
also been monitored by UV-vis spectroscopy. The substrate
9-anthraldehyde and the side product 9,10-anthracenedione
showed main absorption at 398 nm and 322 nm, respectively,
while no obvious absorption was observed in the UV-vis
range for the pure photodimer product (Fig. 5b). Depending
on the difference in UV-vis absorption of these molecules, it
was convenient for us to monitor the photoreaction process
via the consumption of 9-anthraldehyde and the formation of
9,10-anthracenedione. As shown in Fig. 5c and d, the UV-vis
spectral traces of photodimerization both exhibited the
decrease of 9-anthraldehyde and the increase of 9,10-
anthracenedione at 1 h, 4 h and 8 h reaction times with or
without addition of cage Ce-BHP, respectively. However, the
peak of 9,10-anthracenedione was found to grow slowly in
the presence of Ce-BHP, giving a stark contrast to the growth
of the peak of 9,10-anthracenedione without Ce-BHP under
the same conditions. These results were consistent with the
conclusion that the cage structure Ce-BHP could be regarded
as an effective molecular reactor to block oxidation of
Fig. 5 (a) Catalytic traces of photodimerization with cage Ce-BHP and
coexistence of TBABr under the optimal conditions at different
reaction times; (b) UV-vis spectra of the substrate 9-anthraldehyde
(black), pure photodimer product (blue) and side product 9,10-
anthracenedione (red); (c) UV-vis spectral traces of photodimerization
without cage Ce-BHP under 1 h (black), 4 h (red) and 8 h (blue)
irradiation; (d) UV-vis spectral traces of photodimerization with cage
Ce-BHP under 1 h (black), 4 h (red) and 8 h (blue) irradiation.
photodimerization and oxidation of 9,10-anthracenedione.
From a mechanism point of view, the avoidance of 9,10-
anthracenedione formation is not attributed to the cage's
ability to exclude dioxygen, but rather to the restricted
generation of a large intermediate product that does not fit
within
the
cavity
of
Ce-BHP.
Therefore,
the
photodimerization of 9-anthraldehyde exhibited not only
high conversion with a catalytic amount of Ce-BHP, but also
excellent selectivity based on the remarkable well-confined
effect of the Ce-BHP cage under air conditions.
Conclusions
In conclusion, the photodimerization of 9-anthraldehyde in
the presence of metal–organic cage Ce-BHP under air
conditions has been studied in this work. The size-suitable
open windows and cavity of Ce-BHP were able to
accommodate 9-anthraldehyde molecules, which was
9-anthraldehyde during the photocatalysis.
A proposed
mechanism for the photodimerization of 9-anthraldehyde
has been shown to illustrate the role of cage Ce-BHP (Fig.
S7†). First, 9-anthraldehyde molecules were accommodated
in cage Ce-BHP to form the 1 : 2 inclusion complex. Then, Br−
as the nucleophilic reagent could attack the excited
substrates, resulting in a corresponding intermediate, which
is expected to both participate in the next steps of
1
monitored by H NMR and fluorescence titration. GC-MS and
X-ray single crystal analyses indicated that the final
photoproducts of 9-anthraldehyde were anti-dimers and 9,10-
anthraquinone synthesized from the oxidation of
9-anthraldehyde. Ce-BHP could be regarded as an ideal
molecular reactor to avoid the oxidation of 9-anthraldehyde
and an effective catalyst for the formation of anti-dimers with
high conversion and selectivity under air conditions via the
supramolecular confined effect. Such a strategy could be
applied to the construction of more modified molecular
reactors to functionalize the catalysis of other photochemical
reactions.
Conflicts of interest
Fig. 4 The crystal structure and the packing mode of photoproducts
isolated from the catalytic system.
There are no conflicts to declare.
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