Synthetic Approaches to Metallo-Supramolecular CoIIPolygons and Potential Use for H2O Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.0c02182

Metal-directed self-assembly has been applied to prepare supramolecular coordination polygons which adopt tetrahedral (1) or trigonal disklike topologies (2). In the solid state, 2 assembles into a stable halide-metal-organic material (Hal-MOM-2), which catalyzes H2O oxidation under photo- and electrocatalytic conditions, operating with a maximum TON = 78 and TOF = 1.26 s-1. DFT calculations attribute the activity to a CoIII-oxyl species. This study provides the first account of how CoII imine based supramolecules can be employed as H2O oxidation catalysts.

Full text of DOI:10.1021/acs.inorgchem.0c02182

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Article
Synthetic Approaches to Metallo-Supramolecular CoII Polygons and
Potential Use for H2O Oxidation
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Ayuk M. Ako, Amal Cherian Kathalikkattil, Rory Elliott, Joaquín Soriano-Lopez, Ian M. McKeogh,
Muhammad Zubair, Nianyong Zhu, Max García-Melchor, Paul E. Kruger, and Wolfgang Schmitt*
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ABSTRACT: Metal-directed self-assembly has been applied to prepare
supramolecular coordination polygons which adopt tetrahedral (1) or
trigonal disklike topologies (2). In the solid state, 2 assembles into a stable
halide−metal−organic material (Hal-MOM-2), which catalyzes H2O
oxidation under photo- and electrocatalytic conditions, operating with a
maximum TON = 78 and TOF = 1.26 s−1. DFT calculations attribute the
activity to a CoIII-oxyl species. This study provides the first account of how
CoII imine based supramolecules can be employed as H2O oxidation
catalysts.
of solar energy in energy-dense chemical bonds. However,
breakthroughs are hampered by the lack of efficient and cost-
effective catalysts for the highly endergonic H2O oxidation
half-reaction.27,28 To date, mainly noble-metal oxides (or
related molecular species) provide stable catalysts with
satisfactory O2 conversion rates.29,30 In recent years, new
catalysts containing earth-abundant metal ions have been
emerging.31−37 Molecular species are noteworthy, containing
CoII,31,32 CuII,31,33 and FeII or NiII ions,31,34−37 whose
coordination environments are commensurable to those of
imine-based complexes.31
INTRODUCTION
■
Coordination cages and polygons represent metallo-supra-
molecular species in which metal ions, or small polynuclear
complexes, are linked through organic ligands to produce
molecular entities with well-defined geometries and cavities.1−3
Over the last decades, research has focused on the preparation
of topologies whose structural, constitutional, and electronic
attributes affect catalysis,4,5 drug delivery,6 sensing,7 and other
areas of interest.8−13 Directed syntheses often employ
adaptions of “reticular” synthesis concepts using carboxy-
late-,14,15 pyridine-, catecholate-, or imine-based li-
gands.1−3,16−26 Tetrahedral M4L6 and M4L4 cages are
remarkable,16−19,24−26 some of which exemplify enzymatic
reaction characteristics.4,5 Cages containing pseudo-C3-sym-
metric tris-bidentate-imine moieties include face-capped M4L4
tetrahedral cages that often incorporate ZnII and FeII metal
ions.16−19,24−26 Although it is well understood that the nature
of the metal ion strongly influences the physicochemical
characteristics of imine-based cage systems,16−19,24−26 surpris-
ingly, the corresponding CoII-based assemblies remain
relatively unexplored, whereby structural data on tetrahedral
Herein a new class of heterogeneous, molecule-based H2O
oxidation catalysts is reported. The synthetic methodology
involves CoII-directed self-assembly using N,N′,N″-tris(1-
methyl-1H-imidazol-2-ylmethylene)-1,3,5-triphenylbenzene
(L1) and N,N′,N″-tris(1H-imidazol-4-ylmethylene)-1,3,5-tri-
phenyl benzene (L2) to form tetrahedral [CoII (L1)4]8+ (1)
4
and disklike [CoII (L2)2(H2O)6]6+ (2) supramolecules. In the
3
solid state, 2 assembles through Cl−−CH interactions into a
stable, water-insoluble halide−metal−organic material (Hal-
MOM-2) with large solvent-accessible voids. Hal-MOM-2
promotes light-driven and electrocatalytic H2O oxidation;
face-capped CoII L4 species have yet to be reported.
4
To establish new supramolecular CoII catalysts, our efforts
were directed toward the synthesis of {CoII/imine} species
with partially hydrated coordination environments that
facilitate substrate binding and organo-catalytic oxidations4
or H2O oxidation. The latter is motivated by the necessity to
develop sustainable, carbon-neutral energy concepts.27 Owing
to its abundance, H2O represents an obvious source of
reducing equivalents to produce H2thus enabling the storage
Received: July 27, 2020
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Figure 1. (a) Crystal structure of [CoII (L1)4]8+ (1). (b) Structure of [CoII (L2)2(H2O)6]6+ (2) (H atoms omitted for clarity). (c, d) Views of the
4
3
3D structure of Hal-MOM-2 in the directions of the crystallographic a and c axes, respectively. Color code: CoII, red polyhedra; C, gray; N, blue;
Cl, green; H, white. Yellow spheres highlight the void space.
a
Scheme 1. Representation of the Formation of the Tetra- and Trinuclear Species in Solution
a
R = Me (L1).
DFT studies elucidate the mechanism and attribute activity to
a CoIII-oxyl species.
rise to a T-symmetric, face-capped tetrahedral cage whose
symmetry is characterized by four 3-fold and three 2-fold axes.
The CoII−CoII distances are ca. 14.58 Å, while the Co−N
bond lengths are 2.0564(5) and 2.1841(5) Å. Thus, 1 is related
to reported ZnII and FeII cages.16−19,24−26
RESULTS AND DISCUSSION
■
Initial UV−vis titrations in DMF using 1,3,5-tri(4-
aminophenyl)benzene (A), 1-methyl-2-imidazolecarboxalde-
hyde (B), and Co(NO3)2 ·6H2O confirm the in situ synthesis
of L1 and the formation of CoII complexes in solution (Figure
1 and Scheme 1; also see the Supporting Information).
Depending on the relative reactant concentrations, {Co4L14}
and {Co3L12} species are identifiable using ESI-MS (Support-
ing Information). Consequent optimization of the reaction
conditions and the use of either L1 or L2 resulted in the
crystallization of [CoII L14]8+ (1) and [CoII L22(H2O)6]6+
Hal-MOM-2 crystallizes in the hexagonal space group P6122.
It contains the trinuclear, trigonal-disklike complex
[Co3(L2)2(H2O)6]6+ in which the CoII centers are coordinated
by two L2 ligands whose aromatic ring systems align parallel to
each other and are stabilized by π−π interactions (Figure 1b).
The CoII centers in 2 display distorted-octahedral coordination
geometries. The imine-imidazole moieties act as cis-coordinat-
ing bidentate functionalities, and their N-donor atoms are
located in the equatorial plane. H2O ligands occupy the
remaining apical CoII coordination sites. The packing of
[Co3(L2)2(H2O)6]6+ molecules generates a hexagonal assem-
bly via H bonds, involving coordinated H2O and constitutional
solvent molecules. The complexes are further connected
through interactions between Cl− counterions and H atoms
of L2 to form the extended halide−metal−organic material
Hal-MOM-2. The Cl−···Himidazole interactions occur over
4
3
(2), respectively. 1 is a tetranuclear complex in which the
four CoII ions are located on the vertices of a tetrahedral
topology (Figure 1a). Each CoII ion is chelated by three imine-
imidazole moieties that are derived from three different L1
ligands, resulting in an octahedral {N6} coordination geometry.
Altogether, the structure is stabilized by four L1 ligands, giving
B
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distances of ca. 2.19−2.29 Å. On consideration of interatomic
N···Cl− distances varying between 3.07 and 3.17 Å, these
interactions can be classified as moderately strong.38,39 The
formation of a highly interconnected network involving various
such supramolecular interactions is expected to contribute
significantly to the stability of the 3D structure.
Within the crystallographic ab plane, the CoII complexes
adopt honeycomb-type layers which assemble in the c direction
to form helical channels with 63 screw symmetry (Figure 1c,d
and Figures S2 and S3). The resulting extended supra-
molecular network is characterized by solvent-accessible voids
corresponding to ca. 58% of the unit cell volume (see the
Supporting Information). Hal-MOM-2 is insoluble in H2O
and alcohols; upon reflux in DMF only small quantities
dissolve, whereby mass spectra confirm the integrity of the
dissolved [Co3(L2)2(H2O)6]6+ species.
The open supramolecular network structure containing
hydrated CoII centers prompted us to assess Hal-MOM-2 as a
light-driven heterogeneous H2O oxidation catalyst in a three-
component system using a Clark electrode (Figure 2a).
Optimal catalytic performance was observed using Hal-
MOM-2 loadings of between 0.04 and 0.08 mg and 2.0 mg
of [Ru(bpy)2(deeb)](PF6)2 photosensitizer in phosphate-
buffered aqueous solutions (5 mL, 0.01 M, initial pH 7)
containing Na2S2O8 as a sacrificial two-electron acceptor.
Control experiments in which one of each of these three
components were removed resulted in negligible O2 evolution
(see the Supporting Information). Upon light irradiation (λ =
470 nm), the dissolved O2 concentration continuously
increases, reaching a plateau at up to 640 μmol/L before
leveling off and slowly decreasing after ca. 200 s due to
equilibration with the headspace of the reactor.
Under these conditions Hal-MOM-2 functions as an
effective H2O oxidation catalyst operating with a maximum
TON = 78 and reaching an O2 yield of 12.9% at a catalyst
loading of 0.08 mg. The maximum TOF = 1.26 s−1 was
achieved using a catalytic loading of 0.06 mg. Postcatalytic
characterization experiments could not detect in situ cobalt
oxide (CoOx) or cobalt phosphate formation or the leaching of
CoII ions from Hal-MOM-2 under the working conditions
(Supporting Information).
Recycling tests, involving the addition of photosensitizer and
oxidant, demonstrate that Hal-MOM-2 retains its activity after
O2 evolution; however, the evolved O2 quantity is reduced in
consecutive runs (Figure S20). This reduction can be
attributed to a reduction in pH value.
Dissolving the residual material in DMF after three
photocatalytic tests and analyzing using ESI-MS revealed that
the signal for {Co3L22(H2O)5Cl6}+ at m/z 1669.18 had
essentially disappeared, while residual signals corresponding to
the amine and aldehyde hydrolysis products remained present
at m/z 96.03 and 352.18 (Figures S21 and S22), confirming
the hydrolytic disassembly of the supramolecular structure
during catalysis.
To further confirm and characterize the photocatalytic O2
evolution reaction (OER), cyclic voltammetry (CV) experi-
ments were carried out in phosphate buffer at pH 7 using
modified carbon-paste electrodes containing 5 wt % catalyst
loadings (Hal-MOM-2/CP). Figure 2b compares their
response during 150 cycles with a control experiment in the
absence of Hal-MOM-2. The Hal-MOM-2/CP electrode
clearly exhibits catalytic OER behavior at an onset potential
of 1.28 V vs NHE, which corresponds to an onset
Figure 2. (a) Light-induced H2O oxidation catalysis at various Hal-
MOM-2 loadings: gray, 0 mg; blue, 0.04 mg (TON = 44; TOF = 1.02
s−1; yield (O2) = 3.6%); yellow, 0.06 mg (TON = 66; TOF = 1.26
s−1; yield (O2) = 8.2%); red, 0.08 mg (TON = 78; TOF = 1.09 s−1;
yield (O2) = 12.9%). Conditions: LED (λ = 470, 10 mW cm−2), 2.0
mg of [Ru(bpy)2(deeb)](PF6)2, 11.9 mg of Na2S2O8 in 5 mL of 0.01
M phosphate buffer (initial pH 7, T = 25 °C). (b) Repetitive CV: red,
first scan; blue, last scan after 150 cycles (scan rate = 100 mV/s);
black, blank experiment without catalyst.
overpotential of 465 mV. The shoulder at ca. 1.45 V can be
attributed to the one-electron CoII/CoIII oxidation, which
shifts ca. 120 mV to cathodic potentials when the experiment is
performed at pH 9 (Figure S23), indicating that the CoII/CoIII
oxidation proceeds via a proton-coupled electron transfer
(PCET) step. Similar to the recycling tests under light-driven
conditions, the OER activity decreases within the first 10
cycles; however, it then remains relatively stable for the rest of
the experiment. The maintained activity can be attributed to a
stabilizing effect of the carbon blend.32 The electrochemical
response confirms that the OER activity under the working
conditions is not caused by oxides (CoOx), as these would
result in higher OER activity at lower potentials upon
cycling.40 Such an activity enhancement is indeed observed
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under highly basic conditions, under which in situ CoOx
formation can occur (Figure S23).
Density functional theory (DFT) calculations at the B3LYP
level were applied to elucidate the catalytic pathway (Figure 3a,
[CoIV(L2′)2(OH)2]2+. These PCET events are calculated to
require 1.64 and 1.67 V (vs NHE, pH 7), respectively, in good
agreement with the recorded experimental CV. Nucleophilic
H2O binding followed by PCET generally occurs when a
catalyst comprises an oxo group with an unoccupied molecular
orbital localized at the O atom, which acts as an electrophile.41
Thus, for [CoIV(L2′)2(OH)2]2+, additional deprotonation
events are required, as the LUMO shows no contribution of
the hydroxyl O atoms (Figure 3b, top). For imines, proton
transfer (PT) preferentially occurs intramolecularly, whereby
the most basic N atom acts as an H+ acceptor, yielding
[(NH)CoIII(L2’)2(O•)(OH)]2+, which formally represents a
CoIII-oxyl radical. The prevalence of CoIII-oxyl over CoIV-oxo
species has previously been reported for other CoII-based OER
catalysts.42 This intramolecular PT step entails the breaking of
the Co−N bond and requires a Gibbs energy of only 0.20 eV
(see the Supporting Information for details), whereby the
LUMO of the species is mainly localized at the oxyl radical O
atom (Figure 3b, bottom). The alternative PT from
[CoIV(L2′)2(OH)2]2+ to the bulk solution demands a Gibbs
energy of 1.69 eV, rendering this process highly unlikely.
Hence, the reaction is calculated to proceed via a [(NH)-
CoIII(L2′)2(O•)(OH)]2+ intermediate, which undergoes nu-
cleophilic attack by a H2O molecule to form the O−O bond.
Subsequent PCET events require 1.29 and 0.56 eV to yield
[(NH)CoIII(L2′)2(OOH)(OH)]2+ and the superoxide species
[(NH)CoIII(L2′)2(OO•)(OH)]2+, respectively, prior to the
release of O2.
Finally, the initial CoII catalyst is recycled upon H2O
coordination, intramolecular PT, and re-formation of the Co−
N bond. This final chemical step is exergonic by −2.16 eV (see
the Supporting Information). The DFT-computed mechanism
defines the second PCET event as the potential-determining
step (PDS). The calculations predict a theoretical over-
potential of 853 mV associated with this step, which agrees
well with the experimental value of 846 mV to reach 1 mA/
cm2.
The proposed mechanism is in line with the cyclo-
voltammetry data, which support that the H2O oxidation is
initiated by two consecutive proton-coupled-electron-transfer
events. In addition, the observed deactivation pathway under
acidic conditions and the mass spectra identifying the ligand
hydrolysis products are supportive of the DFT calculations.
The single-crystal structure identifies the H2O binding sites
and supports that the O−O bond formation cannot proceed
through the interaction with two metal oxo species (I2M) and
most likely occurs via water nucleophilic attack.
It is informative to relate the OER activity of Hal-MOM-2
to those of other reported OER catalysts containing Schiff base
ligands. However, direct quantitative comparisons are difficult
to establish (see Table S7 in the Supporting Information),
considering various working conditions and the heterogeneous
nature of our catalyst. In comparison, the selected Ru- and Ir-
based catalysts yield very high TONs due to their intrinsic
catalytic activities and their higher stability under harsh
conditions. However, Hal-MOM-2 performs favorable kinetics.
Examples of OER catalysts containing Schiff base ligands and
earth-abundant metal ions are relatively rare.31 A dimanganese
tetrakis-Schiff base macrocycle was reported as an OER
catalyst, achieving a TON of 11.2 and a maximum O2
evolution rate of ca. 15 nmol of O2 per minute.43 The
heterogeneous electrocatalytic OER activity of two Co-
containing, O-phenylenediamine-derived imine complexes,
Figure 3. (a) Proposed OER mechanism for [Co(L2′)2(H2O)2]2+ on
the basis of DFT-B3LYP calculations. Inset: ball and stick
representation of [Co(L2′)2(H2O)2]2+. (b) LUMOs of the α and β
electrons of [CoIV(L2′)2(OH)2]2+ (top) and [(NH)CoIII(L2′)2(O•)-
(OH)]2+ (bottom) species. The LUMO of the β electrons of
[(NH)CoIII(L2′)2(O•)(OH)]2+ is mainly localized on the oxyl radical
(right bottom), indicating that this is the active species for accepting
the WNA (N∩N = L2′ = (1H-imidazol-4-ylmethylenenimine)-
phenyl).
see also the Supporting Information). As the locations of the
CoII centers in Hal-MOM-2 prohibit the direct coupling of
two oxo moieties, O−O bond formation was assumed to occur
via a water nucleophilic attack (WNA) pathway.41 Thus, the
single-site model [Co(L2′)2(H2O)2]2+ was employed, in which
L2 was replaced by (1H-imidazol-4-ylmethylenenimine)phenyl
(L2′). DFT calculations show that the reaction proceeds via
two consecutive PCET steps, in which the two axial H2O
molecules are deprotonated followed by the concomitant two-
electron oxidation of the CoII center to form
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denoted MRC and RSP, was studied, achieving an onset
overpotential of 450 mV for RSP in slightly basic, borate-
buffered solution.44 However, the activity of both catalysts
decreases upon lowering the pH of the reaction media to
neutral values. Hal-MOM-2 reveals an OER activity com-
parable to that of RSP. The differences in electrocatalytic
activity may partially stem from a lower number of exposed
active sites of Hal-MOM-2 within the CP blend. Moreover,
higher pH values increase the OER due to a higher
concentration of OH− ions. The provided comparison,
although qualitative, illustrates the good capabilities of Hal-
MOM-2 toward H2O oxidation and places this catalyst among
the best of its kind.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02182.
■
sı
Experimental section and methods, single-crystal X-ray
diffraction, additional and enlarged images of 1 and 2,
UV−vis titration analysis, mass spectrometry of the
metallo-supramolecular compounds, infrared spectra of
the coordination complexes, X-ray powder diffraction,
thermogravimetric analysis, light-driven water oxidation
measurements, electrochemical analysis of [2]Cl6·DMF·
9H2O, DFT calculations, and OER activity comparison
with literature examples (PDF)
Accession Codes
CONCLUSIONS
■
CCDC 1867727−1867728 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
In conclusion, we report novel {CoII/imine} polygons with
tetrahedral and trigonal-disk topologies and demonstrate how
supramolecules with hydrated CoII centers can catalyze the
highly endergonic H2O oxidation reaction. The supramolecular
3D network structure of Hal-MOM-2 promotes both electro-
and photocatalytic OER, leading to an onset overpotential of
ca. 465 mV and a relatively high TOFmax = 1.26 s−1. DFT
calculations support the experimental data, attributing the
catalytic activity to a CoIII-oxyl species, and propose an
intermediate Co−N bond cleavage. This possible hydrolytic
decomposition pathway is in line with the detection of ligand
reactants in the postcatalytic reaction media and might explain
the experimentally observed decrease in catalytic activity.
Considering that these and other reported supramolecules can
form through subcomponent synthesis,1−3 reversible disassem-
bly/assembly cycles may in the future allow the preparation of
catalysts with prolonged activity, hence applying supra-
molecular concepts to H2O oxidation catalysts.45
AUTHOR INFORMATION
Corresponding Author
■
Wolfgang Schmitt − School of Chemistry & AMBER Center,
Trinity College, University of Dublin, Dublin D02 PN40,
Ireland; orcid.org/0000-0002-0058-9404;
Email: schmittw@tcd.ie
Authors
Ayuk M. Ako − School of Chemistry & AMBER Center, Trinity
College, University of Dublin, Dublin D02 PN40, Ireland
Amal Cherian Kathalikkattil − School of Chemistry &
AMBER Center, Trinity College, University of Dublin, Dublin
D02 PN40, Ireland; orcid.org/0000-0002-7351-0357
Rory Elliott − School of Chemistry & AMBER Center, Trinity
College, University of Dublin, Dublin D02 PN40, Ireland
EXPERIMENTAL SECTION
■
Synthesis of [Co4(L1)4]8+ (1). A slurry of 1,3,5-tris(4-
aminophenyl)benzene (120 mg, 0.34 mmol) and 1-methyl-2-
imidazolecarboxaldehyde (110 mg, 1.00 mmol) in methanol (20
mL) was stirred for 30 min and then heated at reflux for 2 h, after
which time Co(NO3)2·6H2O (151 mg, 0.52 mmol) in methanol (10
mL) was added dropwise. The resulting slurry was heated at reflux for
a further 4 h, and the precipitate was collected and the filtrate set
aside. The solid residue was then washed with ether, dried, and
dissolved in DMF. Slow diffusion of THF into this solution afforded
single crystals of ([1](NO3)8·(solv) after 5 days. Yield: 39 mg. Anal.
Found: C, 55.5; H, 4.2; N, 18.3. Calcd for Co4C156H144N44O30
([1](NO3)8·6H2O): C, 55.9; H, 4.3; N, 18.4. FT-IR (cm−1): 3246
(br), 3082 (w), 2932 (w), 2253 (w), 2595 (w), 1714 (s), 1598 (s),
1514 (m), 1381 (s), 1244 (m), 1139 (m), 1012 (m), 839 (s), 786
(m), 713 (m).
́
Joaquín Soriano-Lopez − School of Chemistry & AMBER
Center, Trinity College, University of Dublin, Dublin D02
PN40, Ireland; orcid.org/0000-0002-9203-3752
Ian M. McKeogh − School of Chemistry & AMBER Center,
Trinity College, University of Dublin, Dublin D02 PN40,
Ireland
Muhammad Zubair − School of Chemistry & AMBER Center,
Trinity College, University of Dublin, Dublin D02 PN40,
Ireland
Nianyong Zhu − School of Chemistry & AMBER Center,
Trinity College, University of Dublin, Dublin D02 PN40,
Ireland
Max García-Melchor − School of Chemistry & AMBER Center,
Trinity College, University of Dublin, Dublin D02 PN40,
Ireland; orcid.org/0000-0003-1348-4692
Paul E. Kruger − MacDiarmid Institute for Advanced Materials
and Nanotechnology, School of Physical and Chemical Sciences,
University of Canterbury, Christchurch 8041, New Zealand;
orcid.org/0000-0003-4847-6780
Synthesis of [Co3(L2)2(H2O)6]6+ (2). Ligand L2 (100 mg, 0.17
mmol) in DMF (7 mL) was treated with CoCl2·6H2O (60 mg, 0.25
mmol) and then heated at 75 °C for 2 h to give a green solution. This
solution was filtered, and slow diffusion of THF into this solution
afforded orange crystals of [2]Cl6·(solv) after 24 h. Yield: 50 mg. FT-
IR (cm−1); 3365 (m), 1611 (s), 1589 (s), 1497 (s), 1440 (s), 1386
(s), 1324 (m), 1292 (w), 1253 (w), 1173 (w), 1096 (m), 1015 (w),
978 (w), 906 (w), 835 (m), 781 (w), 660 (s). Anal. Found: C, 48.9;
H, 5.5; N, 13.0. Calcd for Co3C75H91N19O16Cl6 ([2]Cl6·DMF·
9H2O): C, 48.9; H, 5.2; N, 13.4.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02182
Crystal data and details of data collection and refinement of the
compounds are summarized in Table S1 in the Supporting
Information. Crystallographic data, CCDC 1867727 and 1867728,
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Author Contributions
W.S. conceived the project and performed data analyses.
A.M.A., I.M.M., and M.Z. synthesized and characterized the
compounds. A.C.K. and R.E. performed the light-induced
E
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⊥A.M.A., A.C.K., R.E., and J.S.-L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(18) Xu, W.-Q.; Fan, Y.-Z.; Wang, H.-P.; Teng, J.; Li, Y.-H.; Chen,
C.-X.; Fenske, D.; Jiang, J.-J.; Su, C.-Y. Investigation of Binding
Behavior between Drug Molecule 5-Fluoracil and M4L4-Type
Tetrahedral Cages: Selectivity, Capture, and Release. Chem. - Eur. J.
2017, 23 (15), 3542−3547.
Funding from the European Research Council (CoG 2014−
647719) and Science Foundation Ireland (13/IA/1896) is
acknowledged. J.S-L. acknowledges funding from the European
Union’s Horizon 2020 research and innovation programme
under the Marie Skłodowska-Curie grant agreement No
713567. ICHEC and the TCHPC unit are acknowledged for
the provision of computational resources.
(19) Zhang, D.; Ronson, T. K.; Mosquera, J.; Martinez, A.; Nitschke,
J. R. Selective Anion Extraction and Recovery Using a FeII L4 Cage.
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