Negatively charged metal-organic hosts with cobalt dithiolene species: Improving PET processes for light-driven proton reduction through host-guest electrostatic interactions

Article abstract of DOI:10.1039/c9cc03871j

By incorporating 1,2-benzenedithiol as a chelator to construct cobalt dithiolene species, two negatively charged redox-active metal-organic hosts were obtained. By taking advantage of electrostatic interactions, cationic Ru-based photosensitizers were con

Full text of DOI:10.1039/c9cc03871j

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Negatively charged metal-organic hosts with cobalt dithiolene
species: Improving PET processes for light-driven proton reduction
through host-guest electrostatic interactions
Received 00th January 20xx,
Accepted 00th January 20xx
Junkai Cai, Liang Zhao,* Jianwei Wei, Cheng He, Saran Long and Chunying Duan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
By incorporating 1,2-benzenedithiol as a chelator to construct
The synergistic interaction between redox catalysts and
cobalt dithiolene species, two negatively charged redox-active photosensitizers plays an important role in the efficient charge
metal-organic hosts were obtained. By taking advantage of and energy transfer processes of photosynthetic systems.10
electrostatic interactions, cationic Ru-based photosensitizers were Because the metal centres of hosts carry inherent positive
constrained to improve photoinduced electron transfer processes charges,5 they usually can hardly form host-guest species with
for efficient photocatalytic proton reduction.
metal-based photosensitizers due to Coulombic repulsion. To
the best of our knowledge, few negatively charged cages have
been reported to date,14-16 and only one metal-organic cage
constructed by Raymond and co-workers can be applied to
photocatalytic research via the PET processes.16 In terms of
availability, utilizing electrostatic interactions to construct
host-guest species to improve PET processes is a promising
approach that has not been reported in the field of light-driven
H2 evolution. In this work, we successfully synthesized two
novel negatively charged metal-organic hosts containing CoS4
cores, which are the first example of the formation of a host-
Supramolecular approaches have attracted considerable
attention with regard to controlling molecular behaviour.1,2 In
nature, a plethora of mechanisms are harnessed to optimize
chemical processes via the cumulative influence of many
noncovalent bonds, such as hydrophobic interactions,
hydrogen bonds and the Coulombic force.3,4 Metal-organic
architectures constructed by the coordination of metal ions
and organic linkers have been considered powerful
supramolecular hosts,5 in which geometric and electronic
characteristics could be embedded within the individual
components for recognition, stabilization of reactive
intermediates and catalysis with promising supramolecular
active sites reminiscent of natural enzymes.6-8
2+
guest system with the star molecule Ru(bpy)3 (bpy = 2,2’-
bipyridine) by electrostatic interactions for application in light-
driven H2 production. Cobalt dithiolene compounds containing
CoS4 cores are types of artificial hydrogenase mimics that have
particularly low overpotentials and display outstanding redox
properties.17 Importantly, the strong coordinating ability of the
benzenedithiolate chelator was expected to enhance the
stability of hosts and maintain the intrinsic redox potential of
cobalt dithiolenes.
On the other hand, converting solar energy into other
forms of energy has initiated interest in fundamental
photophysics research for effective chemical transformations.9
The efficiency of light energy conversion is always determined
by the photoinduced electron transfer (PET) processes and the
form of a long-lived charge separation state.10 The host-guest
supramolecular strategy was considered a highly promising
way to force close proximity between electron acceptors and
the donors to improve the PET processes.11,12 With abundant
weakly interacting sites on the metal-organic hosts, guests
could be caught within the supramolecular hosts in a
favourable conformation to form host-guest systems13 and
avoid unwanted energy transfer processes.12
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian,
116024, P. R. China. E-mail: zhaol@dlut.edu.cn; cyduan@dlut.edu.cn
†Electronic Supplementary Information (ESI) available: Experimental details, crystal
structure and data. CCDC 1916150-1916152. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/x0xx00000x
Scheme 1 Construction of an Artificial Supramolecular System for Photocatalytic
Proton Reduction.
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H4NAS was synthesized by reducing the reaction product of in the crystal structure (Fig. 1b)
Journal Name
The nearest C−H…O, C−H…S
.
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1,5-diaminonaphthalene and 2,3-bis(isopropylthio)benzoyl and C−H … π distances were only 2.5DÅO,I: 210.8.10Å39/aCn9dCC30.3087Å1J,
chloride according to the reported literature.15 The reaction of respectively, which are typical of hydrogen bonds. These
H4NAS and Co(BF4)2·6H2O in a DMF solution containing NaOH multiple interactions might play an auxiliary role in
and NEt4Cl yielded 68% of the compound Co−NAS. ESI-MS thermodynamically promoting the binding and maintaining the
spectrum of Co−NAS exhibited an intense peak at m/z = stability of the host-guest species in peripheral binding manner,
548.9222 with isotopic distribution patterns separated by 0.5 which is a special host-guest interaction mode seldom
2+
Da (Fig. S4.1†) and was clearly assignable to the [Co2(NAS)2]2− observed.18 Although Ru(bpy)3 is a spherical molecule with a
species, indicating the formation of M2L2 species with high diameter of ca. 11.5 Å, which is larger than the inner window
stability in solution. Single-crystal structure analysis of Co−NAS of Co−NAS, electrostatic interactions and hydrogen bonding
2+
revealed that two deprotonated H4NAS units and two cobalt prompted Co−NAS to capture Ru(bpy)3 ions on both sides of
ions formed a negatively charged quadrangular molecule its quadrangular window similar to a trap. This peripheral
2+
Co−NAS with an ideal C2 symmetry via coplanar coordination binding mode maintained a distance between Ru(bpy)3 and
(Fig. 1a). The average Co−S bond distances of approximately the redox centres of approximately 9.0 Å, and the shortened
2.2 Å were in good agreement with the reported for CoS4, distance between the electron donors and electron acceptors
implying that the CoS4 core retained its original redox activity was beneficial to the subsequent PET processes.
after modification on the supramolecular hosts. The cobalt
ions were located on two parallel edges of the quadrangle with
an average distance of 13.2 Å, and the distance between
naphthalene groups was approximately 7.7 Å, giving a window
of approximately 100 Å2 and providing potential space to
interact with guest molecules.
2+
Fig. 2 (a) ITC experiments of Ru(bpy)3 upon addition of Co−NAS or Co−TAS
showing the formation of host-guest complex in DMF. (b) Cyclic voltammogram
of catalysts (0.1 mM) in DMF containing 0.10 M TBAPF6. Scan rate: 100 mV/s. (c)
2+
H2 evolution by system containing Ru(bpy)3 (0.5 mM) and AA (0.1 M) with
various concentration of Co−NAS. (d) H2 evolution by system containing Co−NAS
2+
(20 μM), AA (0.1 M) with various concentration of Ru(bpy)3
.
2+
The modes of interaction between Co−NAS and Ru(bpy)3
2+
Fig. 1 Crystal structure of Co−NAS (a) and Co−NAS/Ru(bpy)3 (b) with the
was further studied by ITC assays and the combined number
indicated the presence of Ru(bpy)32+ triggered the formation of
Co−NAS/Ru(bpy)32+ 1:2 complexes in solution (Fig. 2a).19 Curve
fitting by computer simulation using an “independent” model
cumulative noncovalent bonds shown. (c) The filling pattern of the host and
guests. Co cyan, Ru purple, S yellow, N blue, O red, C grey and H white.
+
As expected, two NEt4 counter cations were located on
showed a large Gibbs free energy (ΔG) of –30.46 kJ·mol−1 for
both sides of the Co−NAS quadrangular plane through host-
2+
the Co−NAS/Ru(bpy)3
complex, suggesting strong
guest electrostatic interactions (Fig. S3.1†). When the
2+
combination between the host Co−NAS and the guest
photosensitizer ion Ru(bpy)3 with two positive charges was
2+
1
+
Ru(bpy)3 (Fig. S5.9†). H NMR titration experiments between
added instead of NEt4 , we speculated that Co−NAS could bond
2+
2+
Co−NAS and Ru(bpy)3 were also executed (Fig. S5.1†). The
addition of Co−NAS to Ru(bpy)3 showed significant upfield
shifts in all signals of Ru(bpy)32+, indicating that Ru(bpy)3
with Ru(bpy)3 in a similar manner to obtain a host-guest
2+
complex. Fortunately, the single crystal structure of the
2+
2+
Co−NAS/Ru(bpy)3 complex was obtained, as shown in Fig. 1b
combined with Co−NAS to give host-guest species. These
results were consistent with the obtained crystal structure.
and 1c. The negatively charged quadrangular structure was
2+
maintained, and the two Ru(bpy)3 molecules occupied the
positions of NEt4+ through host-guest electrostatic interactions.
At the same time, close contacts between the H-atoms of the
pyridine ring on the bpy ligand and the oxygen atoms, sulfur
atoms and naphthalene nucleus of Co−NAS were also observed
Cyclic voltammograms displayed a pair of reversible redox
peaks of the coupled CoIII/CoII reduction process at
approximately −0.55 V (vs. Ag/AgCl) (Fig. 2b), which was in
good agreement with reported cobalt dithiolene species.17 The
2 | J. Name., 2012, 00, 1-3
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result indicated that the CoS4 groups embedded within 3d). Obviously, a comparison of the H2 evolution activity
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2+
2+
DOI: 10.1039/C9CC03871J
Co−NAS still maintained redox activity. The PET processes between Co−NAS/Ru(bpy)3 and Co−BAS/Ru(bpy)3 systems
between Ru(bpy)3
2+
and Co−NAS were investigated by revealed that the formation of host-guest species through
fluorescence titration (Fig. S5.3†). The addition of Co−NAS to a electrostatic interactions is a promising method for improving
2+
DMF solution containing Ru(bpy)3 caused obvious emission the PET processes.
quenching following linear Stern-Volmer behavior with a
constant of 2.7 × 104 M−1, which was assignable to the
2+
photoinduced electron transfer from excited Ru(bpy)3 to the
CoS4 cores in Co−NAS, indicating that Co−NAS was capable of
being activated directly for proton reduction. In fact, such
intense quenching between host and guest enabled effective
photoinduced electron transfer via a pseudo-intramolecular
pathway comparable to that of reported host-guest system.12
Hydrogen evolution was carried out in a typical light-driven
reduction system containing the catalyst Co−NAS (0.04 mM),
2+
the photosensitizer Ru(bpy)3 (0.5 mM) and the electron
donor ascorbic acid (AA, 0.1 M). As shown in Fig. 2c, under the
optimized conditions, the volume of H2 evolution was up to
400 μL in 9 h with an initial turnover frequency (TOF) of
approximately 40 h−1. The efficiency is comparable to that of
previous relative host-guest systems for the photocatalytic
2+
Fig. 3 (a) Emission spectra of Ru(bpy)3 (10 μM) upon the addition of catalysts
hydrogen production.20,21 Under saturated reaction conditions,
2+
(40 μM) and AA (0.1 M). (b) Emission spectra of Ru(bpy)3 (10 μM) upon
2+
when the concentrations of Ru(bpy)3 and AA were fixed, the
addition of 40 mM Et4NCl (black line), 40 μM Co−TAS (red line) and both 40 μM
initial rate constant of H2 production exhibited a linear
Co−TAS and 40 mM Et4NCl (blue line). The spectra show the recovery of emission
relationship with the concentration of the Co−NAS. When the
2+
in the presence of Et4NCl. (c) H2 evolution by system containing Ru(bpy)3 (0.5
concentrations of Co−NAS and AA fixed, TOF maintained
mM), AA (0.1 M) with various concentration of Co−TAS. (d) H2 evolution by
unchanged with the variation of the concentration of
Ru(bpy)3 (Fig. 2d). We inferred that the increase of catalyst
2+
system containing Ru(bpy)3 (0.5 mM), AA (0.1 M) and catalysts (Co−NAS: 0.04
2+
mM, Co−TAS: 0.04 mM, Co−BAS: 0.08 mM); Cyan and pink bars show the
facilitated the formation of host-guest species and the rate of
absence and presence of Et4NCl (2 M), respectively.
H2 evolution depend on the concentration of the host-guest
2+
2+
complexes Co−NAS/Ru(bpy)3 rather than that of Ru(bpy)3
.
At the same time, a triangular prism with three CoS4 cores
was prepared to sufficiently elucidate the role of peripheral
binding by electrostatic interactions in PET processes. Single-
crystal Co−TAS was obtained by vapour diffusion of diethyl
ether into a DMF solution of H6TAS (4,4',4”-tri(2,3-dimercapto
benzamido)triphenylamine) and Co(BF4)2·6H2O containing
NaOH and NEt4Cl with a yield of 48%. The ESI-MS spectrum of
Co−TAS in DMF exhibited an intense peak at m/z = 584.9067
with the exact isotopic distribution fingerprint of the species
[Co3(TAS)2]3− (Fig. S4.2). Single-crystal analysis of Co−TAS
2+
Control experiments with either free Ru(bpy)3 or Co−NAS
produced trace H2, demonstrating that both species
contributed to the PET processes for H2 production. The
fluorescence quenching efficiency of Co−NAS to Ru(bpy)3 is
significantly higher than that of AA under the reaction
conditions (Fig. 3a). From a mechanistic point of view, the tight
2+
2+
combination of Co−NAS and Ru(bpy)3 enhanced the electron
transfer from excited Ru(bpy)3 to Co−NAS by oxidation
2+
quenching processes in a pseudo-intramolecular pathway
similar to internal host-guest systems.12 The reduced Co−NAS
could further transfer electrons to protons to complete the H2
production. To verify PET processes, the transients absorption
spectrum of Ru(bpy)32+ with Co−NAS in the presence of AA was
investigated. The appearance of a new absorption peak at
approximately 420 nm at 0.8 μs assigned to the absorption
maximum of RuIII ion confirmed the occurrence of efficient
photoinduced electron transfer processes from the excited-
state *Ru(bpy)32+ to the Co−NAS (Fig. S5.8†).22
revealed
a pseudo-C3 symmetric M2L3 triangular prism
constructed by two ligands on the top and bottom faces of the
triangular prism defined by three cobalt ions located on its
three edges (Fig. 4). The three separate dithiolene chelators of
each ligand were coordinated to three different metal centres,
while each metal cenre coordinated to two dithiolene
chelators from two deprotonated ligands via the same
coplanar mode as Co−NAS. The separation of Co−Co on the
edges was approximately 7.4 Å. Two triangular faces kept a
height of approximately 8.5 Å, producing a potential space
between each two edges for the recognition of guest
molecules. As shown in Fig. 4a, three Et4N+ counter cations
Et4N+ were evenly distributed at the opening windows of
Co−TAS in exactly the same way as Co−NAS, which indicated
To further investigate the potential factors that influence
the photocatalytic H2 evolution, a mononuclear compound,
Co–BAS (where H2BAS = 2,3-dimercapto-N-phenyl benzamide),
resembling a corner of the Co−NAS compound was designed
and prepared. However, only a trace amount of H2 was
detected after 9 h under the same conditions (0.08 mM Co–
BAS while ensuring the same concentration of cobalt ions) (Fig.
2+
that Ru(bpy)3 could replace Et4N+ to form a Co−NAS/
Ru(bpy)32+ complex for light-driven proton reduction.
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Although we did not obtain direct evidence from the single- to ensure the formation of negatively charged metal-organic
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crystal structure of Co−TAS together with Ru(bpy)32+, the mode hosts and enhance their stability. The host-guest species with
DOI: 10.1039/C9CC03871J
2+
of interaction between them was confirmed by ITC assays and the photosensitizer Ru(bpy)3 and the supermolecular redox
provided insight into the thermodynamics of host-guest hosts assembled by peripheral binding way were first
species. Microcalorimetric titration of host-guest complexation characterized by single-crystal diffraction. The experiments
verified the inclusion number of 3 with a ΔG of −33.26 kJ·mol−1 indicated that the host-guest system obtained through
2+
for the Co−TAS/Ru(bpy)3 complex (Fig. 1a). The quenching electrostatic interaction was
a promising platform for
constant of 1.8 × 104 M−1 was slightly weaker than that of the improving the PET processes from an excited-state
Co−NAS/Ru(bpy)3 complex (Fig. S5.4†). The redox potential photosensitizer to the redox sites and provided a new
2+
of Co−TAS was −0.55 V, similar to that of Co−NAS (Fig. 2a). reference for other fields, such as biosensing and conduction.
Irradiation of a solution containing Co−TAS (0.04 mM),
This work was supported by the National Natural Science
Foundation of China (No: 21531001 and 21820102001) and
the Fundamental Research Funds for the Central Universities
(DUT18LK53 and DUT19ZD102).
Ru(bpy)32+ (0.5 mM) and AA (0.1 M) gave 350 μL of H2 (Fig. 3c).
The results suggested that Co−TAS was able to combine
2+
Ru(bpy)3 with the assistance of electrostatic interactions for
light-driven proton reduction in a pseudo-intramolecular
pathway.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1 C. J. Brown, F. D. Toste, R. G. Bergman and K. N. Raymond,
Chem. Rev., 2015, 115, 3012.
2
X. Jing, C. He, L. Zhao and C. Duan, Acc. Chem. Res., 2019, 52,
100.
3 K. Drauz and H. Waldmann, Enzyme Catalysis in Organic
Synthesis, Wiley-VCH, Weinheim, 2002.
4 D. Voet and J. G. Voet, Biochemistry, Wiley, New York, 2004.
5 T. R. Cook and P. J. Stang, Chem. Rev., 2015, 115, 7001.
6 Y. Ueda, H. Ito, D. Fujita and M. Fujita, J. Am. Chem. Soc.,
2017, 139, 6090.
7 P. Mal, B. Breiner, K. Rissanen and J. R. Nitschke, Science,
2009, 324, 1697.
8 W. Cullen, A. J. Metherell, A. B. Wragg, C. G. P. Taylor, N. H.
Williams and M. D. Ward, J. Am. Chem. Soc., 2018, 140, 2821.
9 S. Berardi, S. Drouet and A. Llobet, Chem. Soc. Rev., 2014, 43,
7501.
10 P. D. Frischmann, K. Mahata and F. Würthner, Chem. Soc. Rev.,
2013, 42, 1847.
11 A. M. Kluwera, M. Lutzb, P. W. N. M. van Leeuwena and J. N.
H. Reek., Proc. Natl. Acad. Sci. U.S.A., 2009, 106, 10460.
12 X. Jing, C. He, Y. Yang and C. Duan, J. Am. Chem. Soc., 2015,
137, 3967.
13 L. J. Jongkind, X. Caumes, A. P. T. Hartendorp and J. N. H.
Reek, Acc. Chem. Res., 2018, 51, 2115.
Fig. 4 (a) Crystal structure of Co−TAS with three Et4N+ inlaying on the window. (b)
Crystal structure of Co−TAS. (c) The filling pattern of the host and guests. Co cyan,
Ru purple, S yellow, N blue, O red, C grey and H white.
To validate whether the PET processes in photoinduced H2
production either occurred by
a pseudo-intramolecular
pathway or just proceeded through an intermolecular pathway,
an inhibition experiment was performed. The addition of non-
light-absorbing species Et4NCl efficiently recovered the
2+
emission of both the Co−NAS/Ru(bpy)3
and Co−TAS/
2+
Ru(bpy)3 systems (Fig 3b and S5.6†), indicated that Et4NCl
could act as an inhibitor to extrude Ru(bpy)3 out of the
2+
window of hosts. As shown in Fig. 3d, when Et4NCl was added
to aforementioned two reaction systems, the volume of the H2
produced was only 40% and 25% of the original value under
the same experimental conditions for Co−NAS and Co−TAS,
respectively. The exhibited inhibition behaviours in
photocatalytic proton reduction suggested that the
combination of host and guest through electrostatic
interactions played an important role in photoinduced
electron transfer processes for efficient H2 evolution.
14 I. M. Müller, D. Möller and C. A. Schalley, Angew. Chem. Int.
Ed., 2005, 44, 480.
15 B. Birkmann, R. Fröhlich and F. E. Hahn, Chem. Eur. J., 2009,
15, 9325.
16 D. M. Dalton, F. D. Toste, R. G. Bergman and K. N. Raymond, J.
Am. Chem. Soc., 2015, 137, 10128.
17 W. R. McNamara, Z. Han, P. L. Holland and R. Eisenberg, J. Am.
Chem. Soc., 2011, 133, 15368.
18 V. McKee, J. Nelsonbc and R. M. Town, Chem. Soc. Rev., 2003,
32, 309.
19 J. Demers and A. Mittermaieri, J. Am. Chem. Soc., 2009, 131,
4355.
20 C. He, J. Wang, L. Zhao, T. Liu, J. Zhang and C. Duan, Chem.
Common., 2013, 49, 627.
21 L. Yang, X. Jing, C. He, Z. Chang and C. Duan, Chem. Eur. J.,
2016, 22, 18107.
In summary, we reported two new negatively charged
metal-organic hosts containing redox active cobalt dithiolenes
and both of them were devoted to developing a new strategy
for the construction of host-guest system by electrostatic
interactions to accelerate the light driven H2 generation.
Cobalt dithiolene species have not only suitable potential for
the H2 evolution reaction but also special coordination modes
22 D. Thiruppathi, P. Karuppasamy, M. Ganesan, V. K.
Sivasubramanian, T. Rajendran and S. Rajahopal, J.
Photochem. Photobiol. A, 2014, 295, 70.
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