Redox-active copper triangles as an enzymatic molecular flask for light-driven hydrogen production

Article abstract of DOI:10.1039/c7ra09285g

A positively charged redox-active metal-organic triangle containing three redox-active copper centres was developed to encapsulate anionic organic dyes (fluorescein) through a weak host-guest interaction for photocatalytic hydrogen production. The unique

Full text of DOI:10.1039/c7ra09285g

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Redox-active copper triangles as an enzymatic
molecular flask for light-driven hydrogen
production†
Cite this: RSC Adv., 2017, 7, 48989
Liang Zhao,
and Chunying Duan
Jianwei Wei, Feili Zhang, Cheng He,
Sijia Zheng
*
A positively charged redox-active metal–organic triangle containing three redox-active copper centres was
developed to encapsulate anionic organic dyes (fluorescein) through a weak host–guest interaction for
Received 22nd August 2017
Accepted 13th October 2017
photocatalytic hydrogen production. The unique geometry enforces
a distorted square planar
coordination suitable for proton reduction. Control experiments with a mononuclear copper complex as
a reference photocatalyst and inactive ATP as an inhibitor were performed to confirm this enzymatic
photocatalytic behaviour.
DOI: 10.1039/c7ra09285g
rsc.li/rsc-advances
The development of catalytic synthetic methods inspired by application in light-driven hydrogen production and has indi-
natural enzyme prototypes that react under an ambient atmo- cated that these supramolecular systems are superior to other
sphere and use benign solvents and clean energy is a major relevant systems. Accordingly, the construction of host–guest
endeavour in synthetic chemistry.1–4 To match the efficiency and photosynthetic systems should be a promising approach to
selectivity of enzymatic systems, chemists use small molecules increase hydrogen production efficiency.24,25
with dened hydrophobic cavities that emulate enzyme active
Copper is an earth-abundant and low-cost material that also
site properties to catalyse specic chemical transformations.5–8 participates in many biological metabolic processes, for
An exciting area in this research includes the incorporation of example, photosynthesis and respiration, with various func-
transition-metal moieties as redox-active centres that mimic tions.26–29 A CuII complexes with well-dened coordination
highly evolved9 and nely tuned natural photocatalytic systems chemistry and diverse redox chemistry have attracted extensive
by catching organic dyes in their pockets.10,11 When positively attention and have been used as catalysts for various trans-
charged, these pockets have important properties in anion formations.30–32 To our surprise, only a few copper complexes
encapsulation.12–14 As the majority of enzyme substrates are have been used for the electrocatalytic reduction33 and oxida-
anionic, any replication of their catalysed reactivity under mild tion of water.34,35 Herein, we report a copper triangular metallo-
biological conditions using synthetic hosts represents a useful macrocycle Cu–OBP (where H2OBP ¼ 2,20-(([1,10-biphenyl]-4,40-
advance.
diylbis(azanylylidene))bis(methanylyl idene))diphenol) encap-
Photocatalytic hydrogen production from water represents sulating an organic dye molecule for homogeneous light-driven
an important process in future sustainable solar energy hydrogen production. To the best of our knowledge, this is the
conversion. Inspired by photosynthetic complexes in nature, rst example of a homogeneous copper metal–organic macro-
this process has been realized using a homogeneous reaction cycle for photo-catalytic hydrogen generation from water
environment towards the optimal availability of active catalytic (Scheme 1).
sites for solar hydrogen production.15–18 New reaction pathways
have emerged for such substrate molecules inside these
containers19–21 by enhancing the proximity between the
substrate and the catalytic centre and by increasing the effective
concentration of the reaction within the conned space.22,23
This novel, well-elucidated reaction strategy has developed for
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116024, P. R. China. E-mail: cyduan@dlut.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
characterization data, as well as additional tables and gures. CCDC 1483351.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7ra09285g
Scheme 1 Procedure for the synthesis of the molecular Cu–OBP
triangle and construction of the artificial supramolecular system for
photocatalytic proton reduction.
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The ligand H2OBP and complex Cu–OBP were obtained
according to previous literature procedures.36 Single-crystal
X-ray structural analysis indicated that two of three ligands
were linked by one metal centre during self-assembly, while the
third ligand position on the same side as the other ligands
constructed a non-C3 symmetry triangular geometry.37–40 Due to
the comfortable metal/metal distances of approximately
˚
12.2 A, the three ligands overlap to form a double-layer structure
˚
with a height of approximately 9.3 A. The structural features of
the molecular triangle most likely allow the encapsulation of
one planar uorescein (Fl) molecule in the pocket,41 producing
an articial supramolecular system for the light-driven gener-
ation of hydrogen from water.42
The solution-phase stability of Cu–OBP was characterized by
electrospray ionisation-mass spectrometry (ESI-MS), revealing an
intense peak at m/z ¼ 1362.20 (Fig. 1). A simple comparison with
the simulation results based on natural isotopic abundances
suggests that this peak is properly assigned to the singly charged
[HCu3(OBP)3]+ species, indicating the formation and stability of
the M3L3 species in solution. When an equimolar amount of Fl
Fig. 2 (a) ITC experiments of Cu–OBP upon addition of Fl or ATP in
EtOH solution, showing the formation of host–guest complex species.
(b) Cyclic voltammograms of 0.1 mM Cu–OBP (blue line) and 0.3 mM
Cu–OMP (red line), which is
a mononuclear copper complex
possessing the same coordination geometry as Cu–OBP, in CHCl3
was added into the solution of Cu–OBP, a new intense peak at containing 0.1 M TBAPF6. Scan rate: 100 mV sꢀ1. (c) Set of emission
spectra for Fl (10 mM) upon the addition of Cu–OBP. The inset shows
m/z ¼ 1694.28 was observed, which could clearly attributed to
the normalized luminescence vs. [Cu–OBP]. (d) Light-driven hydrogen
a [HCu3(OBP)3(Fl)]+ species. A comparison of the experimentally
evolution of systems containing Fl (4.0 mM), NEt3 (15% v/v), and
obtained peak with that obtained via simulation based on natural
Cu–OBP in an EtOH/H2O solution (1 : 1, pH
concentration of Cu–OBP fixed at 5.0 mM (black), 10.0 mM (red line),
¼
12.5) with the
isotopic abundances conrms the formation of a 1 : 1 stoichio-
metric host–guest complex species Fl 3 Cu–OBP in the solution. and 20.0 mM (blue line).
Isothermal titration microcalorimetry (ITC) was used to
ensure the quantitative accuracy of the titration data and
provide insight into the thermodynamics for host–guest (Fig. 2b).33,46 The CuI/Cu0 potential falls well within the range
complexation.43–45 A typical titration curve is shown in Fig. 2a; of that of proton reduction in aqueous media, indicating that
the enthalpy change (DH) and entropy change (TDS) for Cu–OBP the reduced Cu–OBP complex is capable of directly reducing
I Fl are ꢀ1.5 and 30.0 kJ molꢀ1, respectively, with a good “n” protons.46–48 The molecular Cu–OBP triangle is also demon-
value of 1.0 in the curve tting by computer simulation using an strated to be an efficient quencher of the Fl excited state. The
“independent” model.
addition of an equivalent molar ratio of Cu–OBP to an EtOH/
Cyclic voltammetry of the molecular Cu–OBP triangle H2O solution (1 : 1 in volume, pH ¼ 12.5, ensuring the same pH
(1.0 mM) recorded in CHCl3 solution shows the coupled CuI/Cu0 condition as that of the reaction mixture mentioned below for
reduction process at approximately ꢀ0.75 V (vs. Ag/AgCl) photocatalytic hydrogen evolution) containing Fl (10 mM)
quenched approximately 50% of the Fl emission intensity
(Fig. 2c). The quenching behaviour is attributed to the photo-
induced electron transfer (PET) process from the excited state
Fl* to the redox catalyst Cu–OBP.49
Irradiation of a solution containing Fl (4.0 mM), Cu–OBP
(10.0 mM), and triethylamine (Et3N, 15% in volume) in an EtOH/
H2O (1 : 1 in volume) solution at 25 ꢁC resulted in direct
hydrogen generation.25,50 A higher hydrogen production effi-
ciency was achieved at pH ¼ 12.0–13.0. Control experiments
revealed that the absence of any of these individual components
led to failure to produce hydrogen, thus demonstrating that all
three species are essential for hydrogen generation. Of course,
the articial system could not function well in the absence of
light. When the concentrations of Fl (4.0 mM) and Et3N (15% in
volume) were xed, the produced hydrogen volume exhibited
a linear relationship with the Cu–OBP catalyst concentration in
the range of 5.0 mM to 20.0 mM (Fig. 2d). The calculated turnover
number (TON) was approximately 1200 moles of hydrogen per
Fig. 1 ESI-MS spectra of Cu–OBP in EtOH solution (top) and of Fl in
the aforementioned solution (bottom). The insets show the measured
mole of catalyst (Fig. 3a). The modied supramolecular system
and simulated isotopic patterns at m/z ¼ 1362.20 (top) and 1694.28
(bottom).
exhibits the highest TON among related homogenous copper/FI
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suggested that hydrogen production occurred within the cavity
of Cu–OBP.51,52
To further investigate the potential factors that inuence the
photocatalytic hydrogen evolution process, a mononuclear
copper complex Cu–OMP (where H2OMP ¼ 2-((phenylimino)
methyl)phenol) resembling a corner of the molecular triangular
metal–organic macrocycle Cu–OBP was designed and prepared.
The ligand HOMP was obtained by a simple Schiff-base reaction
of salicylaldehyde with aniline in a CH3OH solution. The
mononuclear copper complex Cu–OMP was synthesized in
approximately 77% yield by reacting Cu(BF4)2 with HOMP in the
presence of NaOH in CH3OH. ESI-MS spectrum of the solution-
phase Cu–OMP complex exhibited [NaCu(OMP)2]+ and {Na
[Cu(OMP)2]2}+ peaks at m/z ¼ 478.11 and 935.22, suggesting the
formation of the mononuclear species in solution. Cyclic vol-
tammetry of Cu–OMP exhibited a broad peak at approximately
ꢀ0.75 V (vs. Ag/AgCl, Fig. 2b), corresponding to the CuI/Cu0
couple. This potential is consistent with the Cu–OBP triangle
and permits the exploration of redox-induced reactions near the
H2/H+ couple. Single-crystal structure analysis revealed that the
mononuclear Cu–OMP copper centre was affixed to two
nitrogen and two oxygen atoms. These donors belonged to two
bidentate six-membered ring chelators, and the copper centre
possessed the same coordination environment as those in Cu–
OBP (Scheme 2). Therefore, Cu–OMP was considered an ideal
reference compound for Cu–OBP in the investigation of pho-
tocatalytic hydrogen evolution from water within a supramo-
lecular system.
Fig. 3 (a) Produced hydrogen volume by systems containing Fl (4.0
mM), Et3N (15% in volume), and redox catalysts (red bars) in EtOH/H2O
(1 : 1 in volume, pH ¼ 12.5): Cu–OBP (10.0 mM), Cu–WBP (10.0 mM, in
EtOH/H2O ¼ 2 : 8, in volume, pH ¼ 12.5), and Cu–OMP (30.0 mM). The
cyan bars show the aforementioned systems in the presence of ATP
(4.0 mM). (b) Light-driven hydrogen evolution of systems containing Fl
(4.0 mM), NEt3 (15% v/v) and Cu–WBP in an EtOH/H2O solution
(1 : 1, pH ¼ 12.5) with the Cu–WBP concentration fixed at 5.0 mM
(black), 10.0 mM (red line) and 20.0 mM (blue line).
Photolysis of a solution containing Fl (4.0 mM) and Cu–OMP
(30.0 mM) in a solvent mixture containing Et3N (15% in volume)
and EtOH/H2O (1 : 1 in volume) resulted in hydrogen genera-
tion under the photocatalytic conditions optimized at pH ¼
12.5. The TON was approximately 100 moles of hydrogen per
mole of redox catalyst aer irradiating for 12 h, and the addition
systems. This is the rst example of a homogeneous copper of ATP (4.0 mM) did not obviously change the produced
metal–organic macrocycle for photocatalytic hydrogen genera- hydrogen volume, demonstrating the normal homogeneous
tion from water,33,46 providing an opportunity for the develop- behaviour dynamics of the system (Fig. 3a). The generated
ment of highly efficient copper-based catalysts for
photocatalytic proton reduction.
To further determine whether photocatalytic hydrogen
evolution occurred inside the pocket of Cu–OBP through
a typical enzymatic fashion or outside of the pocket in a normal
homogeneous manner, an important biomolecule without any
suitable redox potential for hydrogen production, adenosine
triphosphate (ATP), was chosen as an inhibitor for our enzy-
matic system.9 The inhibition of the photocatalytic reaction was
displayed through the addition of this non-reactive species.24
Microcalorimetric titration curve generated for Cu–OBP upon
addition of ATP revealed the formation of a host–guest system.
The higher DH (ꢀ11.0 kJ molꢀ1) and TDS (22.9 kJ molꢀ1
)
compared to those of the Cu–OBP/Fl system suggested that ATP
could replace Fl and become encapsulated in the pocket of the
macrocycle (Fig. 2a). As expected, in the presence of 4.0 mM
ATP, photocatalytic hydrogen production by the Fl (4.0 mM)/
Cu–OBP (10.0 mM)/Et3N (15%) system dropped to 20% of the
original value under the same experimental conditions. Thus,
Scheme 2 Square planar coordination geometries of the copper ions
in Cu–OMP and Cu–OBP. The copper, oxygen, nitrogen, and carbon
atoms are drawn in cyan, red, blue, and grey, respectively. aThe
the competitive inhibition behaviour was enzymatic-like and numbers in parentheses represent the deviation for the front number.
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hydrogen volume was approximately 30% of that produced by Cu– the formation of a 1 : 1 stoichiometric complex species Fl 3
OBP under the same reaction conditions (10 mM Cu–OBP, ensuring Cu–WBP in solution. In addition, loading an identical amount
the same concentration of copper ions), and with an equal copper of the water-soluble Cu–WBP analogue with the Cu–OBP
ion concentration, Cu–OMP exhibited a considerably lower cata- provided the same catalytic efficiency for light-driven hydrogen
lytic activity. The superiority of the Cu–OBP/Fl/Et3N system over the production. The results of the catalytic experiments suggest that
Cu–OMP/Fl/Et3N system is attributed to the new reaction pathways the introduction of water-soluble functional groups does not
of the Cu–OBP/Fl/Et3N system and to the increased concentration inuence the photo-catalytic properties of the AP system. Even
of the reaction within the conned space.53
upon increasing the EtOH/H2O volume ratio to 2 : 8, the
The application of supramolecular articial photosynthetic hydrogen evolution reaction nearly maintained the original
(AP) systems is oen limited by aqueous solubility, especially in reaction rate (Fig. 3), providing a strategy for the construction of
photocatalytic proton and CO2 reduction. Solubility improve- water-soluble systems as efficient homogeneous catalysts for
ments are thus crucial, a problem that can be overcome by photocatalytic proton reduction.
a tailor-made supramolecular ask possessing high solubility in
aqueous solution. Therefore, by the introduction of six
hydroxymethyl groups, a triangular analogue Cu–WBP (where
Conclusions
H2WBP ¼ 2,20-(((2,20-bis(hydroxy methyl)-[1,10-biphenyl]-4,40- In summary, we applied our strategy for the construction of Cu-
diyl)bis(azanylylidene))bis(methan ylylidene))diphenol) with based AP systems for hydrogen generation from water by
similar structural features but with a 2,20-bis(hydroxymethyl) encapsulating an organic photosensitizer in the pocket of
benzidine moiety to substitute the benzidine moiety was a metal–organic triangle. For the rst time, copper ions were
designed and prepared. The six hydroxymethyl groups on the introduced into a metal–organic macrocycle as a highly effective
periphery of Cu–WBP provided the assembly with water solu- visible-light photocatalyst for homogeneous splitting water to
bility.54,55 The hydroxymethyl substituents not only make the hydrogen. The copper triangle exhibited a suitable redox
pocket water soluble but also form a hydrophobic environment potential for hydrogen production. The results provide an
from the outward direction of the hydroxymethyl groups, opportunity for the development of highly efficient copper-
thereby allowing easier encapsulation of Fl for light-driven based supramolecular catalysts for photocatalytic proton
hydrogen production.
reduction. The introduction of hydroxymethyl groups provides
The triangular Cu–WBP exhibited a suitable reduction aqueous solubility for the supramolecular catalyst without
potential at ꢀ0.76 V. The solution-phase Cu–WBP structure was inuencing the original photocatalytic properties. The superior
further characterized by ESI-MS analysis. The intense peaks at activity and stability suggest that this novel approach for the
m/z
¼
771.65, 782.64 and 793.62 were assigned to construction of water-soluble supramolecular AP systems as
[H2Cu3(WBP)3]2+, [HNaCu3(WBP)3]2+ and [Na2Cu3(WBP)3]2+, efficient homogeneous catalysts is promising and could be
respectively, by a simple comparison with the simulation results extended to other aqueous chemical transformations.
based on natural isotopic abundances, suggesting the forma-
tion and stability of the M3L3 species in solution. The ESI-MS
spectrum of the Cu–WBP solution containing an equimolar
Conflicts of interest
amount of Fl also exhibited a new peak at m/z ¼ 937.67, which The authors declare no conict of interest.
was assigned to a [H2Cu3(WBP)3(Fl)]2+ species (Fig. 4), implying
Acknowledgements
We are grateful for the National Natural Science Foundation of
China (21501019, 21531001, and 21421005).
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