Heteroternary cucurbit[8]uril complexes as supramolecular scaffolds for self-assembled bifunctional photoredoxcatalysts

Article abstract of DOI:10.1039/d0cc08025j

The self-assembly of bifunctional photoredoxcatalysts is reported. A series of photosensitizers and water-reducing catalysts were functionalized with viologen- and naphthol-units, respectively. Subsequent formation of the heteroternary cucurbit[8]uril-viologen-naphthol complexes was used for the constitution of bifunctional photoredoxcatalysts for hydrogen generation.

Full text of DOI:10.1039/d0cc08025j

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Heteroternary cucurbit[8]uril complexes as
supramolecular scaffolds for self-assembled
bifunctional photoredoxcatalysts†‡
Cite this: Chem. Commun., 2021,
57, 2887
Received 10th December 2020,
Accepted 4th February 2021
b
Fabian Lutz,a Nerea Lorenzo-Parodi,b Torsten C. Schmidt
and
a
Jochen Niemeyer
*
DOI: 10.1039/d0cc08025j
rsc.li/chemcomm
The self-assembly of bifunctional photoredoxcatalysts is reported. integrating both metal centers into a single molecule, enabling
A series of photosensitizers and water-reducing catalysts were intramolecular electron transfer.7 Unfortunately, the exact
functionalized with viologen- and naphthol-units, respectively. influence of the ligands on electron transfer is hard to predict8
Subsequent formation of the heteroternary cucurbit[8]uril-viologen– and many bimetallic complexes are sparingly soluble in pure
naphthol complexes was used for the constitution of bifunctional water.4b,9
photoredoxcatalysts for hydrogen generation.
Supramolecular self-assembly is a powerful tool to induce
spatial proximity in a bifunctional catalyst,10 which may also
Since the industrial revolution, mankind has steadily increased facilitate the electron transfer between both subunits of a
its power demands. Especially in the last hundred years, power photocatalyst. Based on the use of hydrophobic interactions,
consumption has risen drastically due to population growth supramolecular self-assembly can also be applied in aqueous
and rising living standards.1 As of today, most energy sources solvents.11 In this context, Konig and coworkers employed
¨
are based on limited fossil fuels like oil, gas and coal and functionalized vesicles together with alkylated PSs and WRCs
represent a main factor in global warming.2 One possible to achieve
a
2D-confinement of the photocatalyst
alternative is solar energy, harnessed by photovoltaics, by components.12 The alkyl-chains are anchored onto the vesicle
thermal collectors or via photocatalysis. Especially the photo- surface, leading to a strongly increased local concentration.
catalytic production of dihydrogen is attractive, since H2 can be Although dynamic in nature, this approach enhances photo-
stored and subsequently be transformed into electrical energy catalytic efficiency.
in fuel cells in an emission-free fashion.3
We envisaged that supramolecular chemistry can also be
Photocatalytic hydrogen production can be effected by used to achieve a more defined spatial preorganisation of two
bifunctional photocatalysts consisting of a photosensitizer subcomponents. Especially the use of macrocyclic hosts, such
(PS) and a water reducing catalyst (WRC). Starting with research as cyclodextrins, calixarenes or cucurbiturils, seems promising
by Lehn and Sauvage in 1977, the development of such bifunctional in this regard, since these host-molecules can bind hydrophobic
(often bimetallic) catalysts has been met with high interest.4
guests in their inner cavity with high association constants.13 In
Initially, heterobimetallic photocatalysts were generated by this work, we employ cucurbit[8]uril (CB8), which possesses a
dissolving two suitable metal complexes in aqueous or organic nonpolar cavity surrounded on both sides by carbonyl groups
solutions.5 However, the efficiency of the intermolecular electron (Fig. 1B). CB8 itself is almost completely insoluble in water, but
transfer between PS and WRC is distant-dependent, so that can be solubilized by formation of the corresponding host–guest
simple dissolution of both components renders such systems complexes. CB8 can bind up to two guests within its cavity at
inefficient at low concentrations.6 This can be circumvented by once, leading to binary 2 : 1 or heteroternary 1 : 1 : 1 complexes.
For the formation of 1 : 1 : 1 complexes, dialkylviologen and
a Faculty of Chemistry, Organic Chemistry and Center for Nanointegration
2-naphthol are suitable guest molecules. Although 2-naphthol
itself does not bind within CB8, it does bind to the preformed
CB8–viologen complex, leading exclusively to the 1 : 1 : 1 hetero-
ternary complex with association constants of 1.1 ꢀ 105 Mꢁ1 for
the first and 5.9 ꢀ 105 Mꢁ1 for the second step (Fig. 1A).14 This is
due to the formation of a charge-transfer complex between the
electron poor viologen and the electron rich naphthol unit.15
¨
Duisburg-Essen (CENIDE), University of Duisburg-Essen, Universitatsstrasse. 7,
Essen 45141, Germany. E-mail: jochen.niemeyer@uni-due.de
b Instrumental Analytical Chemistry and Centre for Water and Environmental
¨
Research (ZWU), Universitatsstrasse 5, Essen 45141, Germany
† In memory of Prof. Carsten Schmuck.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc08025j
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5.84 ppm for free CB8) and the signals for the viologen-protons
were severely broadened, which is characteristic for formation of
the charge-transfer complex (Fig. S19, ESI‡).16 Yet, the CB8-unit
did not show a reduced symmetry upon complex formation,
which we attribute to a fast guest exchange on the NMR time
scale, as found by other groups.17 Furthermore, the samples
showed a change in color from yellow to purple due to formation
of the charge-transfer complex.16 In the UV/Vis spectra, the
charge transfer band could not be observed directly due to
overlap with the absorption bands of the coloured PSs.18
However, the UV/Vis absorption and the fluorescence of the
heteroternary complexes are decreased in comparison to the
mixture of PSs and WRCs alone. For example, the 3 + 5 + CB8
mixture shows a 12% decrease for the UV/Vis absorption at
320 nm and even a 36% decrease in the fluorescence intensity of
the iridium complex at 575 nm (Fig. S35 and S36, ESI‡). The
decrease in fluorescence indicates a non-emissive deactivation of
the excited state of the PS, e.g. by PET to the WRC.7b,8a This hints
at a spatial proximity of the PS and WRC, which in conjunction
with the data from NMR, optical spectroscopy and solubility
indicate the formation of the desired CB8-based host–guest
complexes.
Fig. 1 (A) Concept for the self-assembly of photocatalysts for hydrogen-
generation based on heteroternary 1 : 1 : 1 host–guest complexes of CB8.
(B) Molecular structures of the functionalized photosensitizers, functionalized
water reducing catalysts and cucurbit[8]uril.
Next, the application of the bifunctional supramolecular
catalysts in hydrogen evolution was tested. Irradiation experiments
With this in mind, we aimed for the functionalization of were conducted at a minimum concentration of 0.25 mM for each
2-naphthol and viologen with photosensitizers and water- component, which should allow sufficient complex formation
reducing-catalysts, respectively. In presence of CB8, a hetero- based on the reported association constants (4105 Mꢁ1). To ensure
ternary PS–WRC–CB8 complex with controlled stoichiometry full homogeneity, the mixture was sonicated in an ultrasonic bath
should be formed, leading to a spatial proximity of the catalyst for 15 minutes before being placed in a lab shaker for 10 hours in
subcomponents and thus enhancing photocatalytic efficiency. the dark at 60 1C. The temperature was chosen to accommodate for
Different combinations of PS and WRC were investigated with the low solubility of the water reducing catalysts. After flushing
regard to their ability for hydrogen generation, leading to the carefully with argon, irradiation was carried out at 60 1C for
identification of an Ir/Pt pair as the most efficient photocatalyst. 11 hours with LEDs of suitable wavelength for each PS (528/470/
First, we synthesized the functionalized PSs and WRCs. As 365 nm for 1/2/3). The amount of generated hydrogen was
PSs, trisbipyridine-ruthenium(II) and bipyridine-bis(2-phenyl- determined by headspace gas chromatography (based on external
pyridine)-iridium(III) complexes were used as well as a metal-free calibration, Table S1, ESI‡) and is reported as turn-over-number
eosine Y derivative. WRCs are based on bipyridine-palladium and (TON), normalized to a concentration of the least concentrated
-platinum complexes. As viologen is highly water soluble it was component (always 0.25 mM).
coupled to the uncharged Pd- and Pt-complexes, whereas the
First, all six possible combinations of PSs 1/2/3 and WRCs 4/
nonpolar naphthol-unit was conjugated with the water soluble 5 with CB8 were tested in a 1 : 1 : 1 stoichiometry (0.25 mM
Ru- and Ir-complexes (also via the bipyridine ligand) as well as each). For palladium complex 4, we did not observe any
the eosin Y derivative (via the carboxylic acid functionality). This relevant generation of hydrogen in combination with either of
resulted in three functionalized PSs 1/2/3 and two WRCs 4/5 the photosensitizers (Tables S2, S3 and S5, ESI‡). In contrast,
(Fig. 1B). All compounds were fully characterized by standard platinum complex 5 was suitable as the WRC in this approach:
techniques (see ESI‡).
in combination with eosin-based photosensitizer 1, we
Subsequently, we generated the heteroternary complexes in observed 1.31 ꢂ 0.02 TONs (Table S2, ESI‡). However, an
water containing 15% (v/v) triethanolamine (TEOA), which acts identical amount of hydrogen was produced in the absence of
as a sacrificial electron donor (for the effect of different TEOA- CB8. We attribute this to the low solubility of 1: even in the
concentrations on the H2-production see Table S8, ESI‡). CB8, presence of CB8, which allowed complete dissolution of all
which is insoluble in this solvent mixture, could be dissolved in components for 2/3 in combination with 5 (vide infra), a red
the presence of both guest molecules (presence of PS or WRC precipitate was observed in case of 1.
alone was not sufficient), indirectly proving the successful
To our delight, the combination of PS 2 and WRC 5 showed
formation of the heteroternary 1 : 1 : 1 complexes. The complex a positive effect of CB8 in photocatalytic hydrogen generation
1
formation could also be followed by H-NMR spectroscopy: in (Table S3, ESI‡). For the 1 : 1 : 1 stoichiometry, we observed only
the 3 + 10 + CB8 mixture (10: viologen-bipyridine ligand, Fig. S2, 0.50 ꢂ 0.06 TONs in the absence of CB8, while addition of
ESI‡), the signals for CB8 were shifted to 6.08/5.78 ppm (cf. 6.18/ CB8 gave increased TONs, albeit with low reproducibility
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Fig. 2 Amount of hydrogen generated by combination of PS 3 and WRC 5
at different stoichiometries at concentrations of 0.25 mmol lꢁ1 or 1.0 mmol lꢁ1
for each component in a mixture of water/triethanolamine (85/15). Turnover
numbers are normalized for a concentration of 0.25 mM.
Fig. 3 Amount of hydrogen generated by combinations of PS 3 and WRC
5
and different mechanistic control reactions at concentrations of
0.25 mmol lꢁ1 for each component in a mixture of 15 : 85 triethanolamine :
water. Turnover numbers are normalized for a concentration of 0.25 mM.
(1.19 ꢂ 1.26 TONs). Further experiments were executed in
which one compound was added in excess (4 equivalents, i.e.
1 mM concentration), showing a drastic increase in photocata-
lytic activity in presence of four equivalents of WRC 5, although
the effect of the CB8 is rather low (9.38 ꢂ 0.75/11.8 ꢂ 0.45 TONs
in absence/presence of CB8, i.e. +26%, Table S4, ESI‡).
binding of the metal to the anchoring unit is necessary by using
the mixtures of 3 + EV + CB8 with Pt(dmb)Cl2 or Naph + 5 + CB8
with Ir(ppy)2(dmb)Cl, respectively (ppy: 2-phenyl-pyridyl, dmb:
4,40-dimethyl-2,20-bipyridine). Surprisingly, this led to a catalytic
activity (3.77 ꢂ 0.06/3.19 ꢂ 0.49 TONs) comparable to the system
consisting of 3 + 5 + CB8, (3.11 ꢂ 0.01 TONs). Although other
explanations might be possible, we believe that this might
indicate an anti-orientation of the metal centers in the 3 + 5 +
CB8 system, spatially separating photosensitizer and water
reducing catalyst, thus obliviating the effect of the covalent
linking to the anchoring unit.
Thus, we envisaged the application of the bis-Pt viologen
derivative 6 (Fig. 4, for the synthesis of 6 see the ESI‡), which
would ensure that one Pt-center is always in close proximity to the
Ir-center in the 3 + 6 + CB8 complex.20 Force-field calculations
show that the bis-Pt complex is able to thread the CB8 macrocycle
despite the two bipyridine-PtCl2 termini (Fig. S37 and S38, ESI‡).
The formation of the heteroternary complexes is also supported
by decreases in absorbance (37% decrease at 320 nm) and
fluorescence (50% decrease at 575 nm) upon addition of CB8 to
the 3 + 6 mixture (Fig. S35 and S36, ESI‡).
Finally, we investigated the use of the Ir-based PS 3 in
combination with WRC 5 (Fig. 2 and Tables S5, S6, ESI‡). In
comparison to the 2 + 5 combination, significantly higher TONs
were observed when used in equimolar ratio (1.26 ꢂ 0.51/3.11 ꢂ
0.01 TONs for 3 + 5 in the absence/presence of CB8, cf. 0.50 ꢂ
0.06/1.19 ꢂ 1.26 for 2 + 5). More importantly, the 1 : 1 : 1 ternary
mixture now gives reproducible results and the presence of CB8
leads to strong increase in hydrogen evolution (+147%). Once
again, we checked the influence of adding each component in
excess (4 equiv.). Here, the addition of excess WRC did not lead
to an increased activity (1.65 ꢂ 0.87 TONs for 4 equiv. of 5, cf.
3.11 ꢂ 0.01 TONs for the 1 : 1 : 1 system). In contrast, increasing
the concentration of PS 3 led to increased TONs, indicating that
the PS deteriorates more quickly than the WRC in this case.
Without CB8, the mixture of 3 (1 mM) with 5 (0.25 mM) gives
rise to 6.42 ꢂ 1.17 TONs, while complexation by CB8 leads to an
increased activity of 11.1 ꢂ 0.11 TONs (+73%).
Next, a series of control experiments was performed in order
to check the viability of our proposed mechanism, namely the
improved photoinduced electron transfer by spatial proximity
of the reaction partners in the 3 + 5 + CB8 host guest complex
(Fig. 3 and Table S7, ESI‡). To demonstrate the photocatalytic
character of the hydrogen generation, the reaction was carried
out without irradiation and indeed no hydrogen generation was
observed. Then, we checked the possible formation of catalytically
active colloidal Pt-nanoparticles, by addition of elemental
mercury, which however did not change the catalytic activity
(3.11 ꢂ 0.01/3.72 ꢂ 0.50 TONs in absence/presence of Hg).19
Next, the importance of each component in the heteroternary
complex was tested by substituting PS 3, WRC 5 or both by
unfunctionalized 2-naphthol (naph) or ethylviologen (EV) moieties.
The metal-free combination (naph + EV + CB8) showed hardly
any hydrogen generation and the heteroternary complexes 3 +
EV + CB8 and naph + 5 + CB8 only gave a reduced catalytic
activity (0.72 ꢂ 0.60/1.16 ꢂ 0.34 TONs, cf. 3.11 ꢂ 0.01 TONs for
the 3 + 5 + CB8 mixture). Also, we investigated if the covalent
In the catalytic application, we used the optimized stoichiometry
(4 equiv. PS, 1 equiv. WRC, 1 equiv. CB8), to test the
Fig. 4 (A) Structure of bisplatinated complex 6. (B) Comparison between
the system of 3, 5, and CB8 with an excess of PS 3, and the use of bisplatinated
complex 6 at concentrations of 0.25 mmol lꢁ1 or 1.0 mmol lꢁ1 for each
component in a mixture of water/triethanolamine (85/15).
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Table 1 Summary of the photocatalytic activity of different combinations
of PS and WRC, with and without CB8 (as shown in Fig. 2–4)
¨¨
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TON
Without CB8
With CB8
Experiment
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3 + 5
1.26 ꢂ 0.51
3.11 ꢂ 0.01
3.34 ꢂ 1.88
1.65 ꢂ 0.87
11.1 ꢂ 0.11
0
3.72 ꢂ 0.50
0.72 ꢂ 0.05
1.16 ꢂ 0.34
3.77 ꢂ 0.06
3.19 ꢂ 0.49
29.1 ꢂ 0.53
3 + 5 (4 equiv. CB8)
3 + 4 equiv. 5
4 equiv. 3 + 5
3 + 5 (dark)
3 + 5 + Hg
3 + EV
Naph + 5
3 + EV + Ptdmb
Naph + 5 + Irdmb
4 equiv. 3 + 6
—
1.66 ꢂ 0.07
6.43 ꢂ 1.17
—
—
—
—
—
¨
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—
4.23 ꢂ 2.09
¨
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observed (4.23 ꢂ 2.09 TONs for 3 + 6, cf. 6.43 ꢂ 1.17 TONs for
3 + 5), which also indicates that the effective increase in total
Pt-concentration has no effect (as expected based on the
stoichiometry experiments for the 3 + 5 mixture, see Table S6,
ESI‡). However, in the presence of CB8, a significant increase
in hydrogen-production was observed when using the bis-Pt
system 6 (29.1 ꢂ 0.53 TONs for 3 + 6 + CB8, cf. 11.1 ꢂ 0.11 TONs
for 3 + 5 + CB8). One possible explanation for this substantial
increase in hydrogen production (+263%) is that the spatial
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In summary, we have reported a supramolecular concept
for the photocatalytic generation of hydrogen from aqueous
solutions. Formation of heteroternary 1 : 1 : 1 complexes of CB8
with naphthol-functionalized PSs and viologen-functionalized
WRCs allowed for an efficient screening of different PS/WRC
combinations, leading to the identification of the Ir/Pt pair as
the best candidate. The initial hit (3 + 5 + CB, 3.11 ꢂ 0.01 TONs)
was systematically investigated and optimized by variation of
stoichiometry and a series of control experiments, finally lead-
ing to the 3 + 6 + CB8 system with a largely increased activity
(29.1 ꢂ 0.53 TONs with 4 equiv. 3, i.e. +935% vs. 3 + 5 + CB8).
Thus, CB8-based supramolecular self-assembly can be an
efficient tool for the formation of bifunctional catalysts in
aqueous solution and we are currently expanding this concept
towards combined transition-metal and organocatalysts.
Funding from MERCUR (AN-2016-007) and the Fonds der
Chemischen Industrie (Liebig-Fellowship) is gratefully
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acknowledged. We thank Prof. Burkhard Konig and Dr.
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Conflicts of interest
There are no conflicts to declare.
Notes and references
1 British Petroleum Company, BP Statistical Review of World Energy,
British Petroleum Co, London, 2019.
2890 | Chem. Commun., 2021, 57, 2887ꢁ2890
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