Inorganic Chemistry
Article
3
5,36
while MOC-2 only contains Co−O (O from H O).
For
by using circulating water. After that, the photoinduced oxygen
evolution started by turning on a 470 nm LED lamp and the
generated oxygen was detected by a gas chromatography (GC9790II)
with a TCD detector.
2
MOC-3 and MOC-4, all the cobalt ions were coordinated by
anions (NO3 and Br for MOC-3 and MOC-4, respectively)
or nitrogen atoms from the ligands. As a result, no such active
sites exist. Indeed, as expected, MOC-1 showed the highest
activity in those MOCs, while MOCs 2−4 showed lower or no
activity. Isotope experiments proved that molecular oxygen was
produced from water oxidation. Photophysical and electro-
chemical studies reveal that photoinduced water oxidation in
MOC-1 initiates via electron transfer from the excited
−
−
3
4
Isotope-Labeling Experiment. The isotope-labeling experiment
was also carried out on WP-TEC-1020H with a quartz tube. In detail,
a mixture of 2.0 mg of photocatalyst, 3.7 mg [Ru(bpy) ]Cl (5 μmol,
3
2
working as photosensitizer), 9.5 mg of Na S O (40 μmol, working as
2
2
8
1
8
the sacrificial agent), 0.1 mL of 97% H2 O, and 0.9 mL of borate
solution (0.2 M, pH = 9.0) was added into a quartz transparent tube.
The above mixture was then stirred and bubbled with Argon for 30
min under room temperature by using circulating water. After that,
the photoinduced oxygen evolution started by turning on a 470 nm
LED lamp and the generated oxygen was detected by gas
chromatography−mass spectrometry (Agilent 5977B).
2
+
3+
[
Ru(bpy) ] * to Na S O , and the oxidized [Ru(bpy) ]
3 2 2 8 3
further accepts electrons from the bis(μ-oxo)dicobalt sites and
Co−O sites of MOC-1 to drive water oxidation.
Electrochemical Measurements. The cyclic voltammetry (CV),
linear sweep voltammetry (LSV), and chronoamperometric i-t
experiments were conducted under the heterogeneous condition by
dropcasting the MOC materials onto the carbon paper electrode. In
detail, 4 mg of MOC material (MOC-1, 2, 3, or 4) was grinded first
and dispersed into 1 mL of ethanol solution containing 25 μL of
Nafion D-520 dispersion (5% w/w solution) under sonification to
form a homogeneous mixture. Then 50 μL of the mixture was dripped
on a carbon paper electrode with a geometrical surface area of 0.72
EXPERIMENTAL SECTION
■
Synthesis of MOC-1. MOC-1 was synthesized based on our
35
previously published paper. A mixture of Co(BF ) ·6H O (17.0 mg,
4
2
2
0
.05 mmol), 2-methyl-1H-imidazole-4-carbaldehyde (6.6 mg, 0.06
mmol), m-xylylenediamine (4.1 mg, 0.03 mmol), tetraethylammo-
nium perchlorate (45.9 mg, 0.2 mmol), N,N-dimethylformamide
DMF, 2.0 mL), and methanol (1.0 mL) was sealed in a Pyrex glass
(
tube and heated in an oven at 100 °C for 3 days. Then, the mixture
2
−
1
cm to prepare the working electrode. After that, the CV, LSV, and
was cooled down to room temperature at a rate of 5 °C h . Deep red
chronoamperometric i-t curves were recorded on the CHI760E
electrochemical workstation with a standard three-electrode system, in
which MOC-modified carbon paper electrode worked as the working
electrode, platinum wire as the counter electrode, and the saturated
calomel electrode as a reference electrode. A 0.2 M borate solution
(pH = 9.0) was used as the electrolyte.
polyhedron-like crystals were obtained, and the isolated yield was
6
[
3.4%, based on Co(BF ) ·6H O. MOC-1 was formulated as
4 2 2
Co L1 (OH) (H O) ]·(ClO ) (H L1 = 1,3-bis[(2-methyl-1H-
20 12 12 2 4 4 8 2
imidazole-4-yl)methyleneaminomethyl]benzene).
Synthesis of MOC-2. MOC-2 was synthesized based on our
36
previously published paper. A mixture of Co(BF ) ·6H O (13.6 mg,
4
2
2
0
.04 mmol), 5-methyl-1H-imidazole-4-carbaldehyde (6.6 mg, 0.06
mmol), m-xylylenediamine (4.1 mg, 0.03 mmol), N,N-dimethylace-
tamide (DMA) (2.0 mL), and ethanol (1.0 mL) was sealed in a Pyrex
glass tube and heated in an oven at 100 °C for 3 days. Then, the
RESULTS AND DISCUSSION
■
Catalyst Synthesis and Characterization. MOC-1 was
synthesized according to our previously reported subcompo-
n e n t s e l f - a s s e m b l y s t r a t e g y , f o r m u l a t e d a s
−
1
mixture was cooled down to room temperature at a rate of 5 °C h .
Dark red polyhedron-like crystals were obtained, and the isolated
yield was 30.5%, based on Co(BF ) ·6H O. MOC-2 was formulated
[
Co L1 (OH) (H O) ]·(ClO ) (H L1 = 1,3-bis[(2-meth-
20 12 12 2 4 4 8 2
35
4
2
2
as [Co L2 (H O) ]·(BF ) , (H L2 = 1,3-bis[(5-methyl-1H-imida-
yl-1H-imidazole-4-yl)methyleneaminomethyl]benzene). It
features a 20-nucleus Co-based tetartoidal (tetragonal
pentagonal dodecahedral) structure and contains two types
of geometrically independent mononuclear cobalt centers
8
6
2
6
4
6
2
zole-4-yl)methyleneaminomethyl]benzene).
Synthesis of MOC-3. MOC-3 was synthesized based on our
35
previously published paper. A mixture of Co(NO ) ·6H O (11.6
3
2
2
mg, 0.04 mmol), 1H-imidazole-4-carbaldehyde (5.8 mg, 0.06 mmol),
m-xylylenediamine (4.1 mg, 0.03 mmol), DMA (2.0 mL), and
methanol (1.0 mL) was sealed in a Pyrex glass tube and heated in an
oven at 100 °C for 3 days. Then, the mixture was cooled down to
1
6
by six nitrogen atoms from three L1 ligands with Co−N bond
the Co ion in Co1 (Figure S1). Co2 (CoN O) is tetrahedrally
−
1
room temperature at a rate of 5 °C h . Deep red block crystals were
obtained, and the isolated yield was 32.4%, based on Co(NO ) ·
3
3
2
6
H O. MOC-3 was formulated as [Co L3 (NO ) ], (H L3 = 1,3-
coordinated by three nitrogen atoms from three imidazolyl
groups and one oxygen atom from the aqua ligand (Figure S1).
from four L1 ligands (Figure S1). Compared with Co1, the
Co−N bond lengths in Co2 and bis(μ-oxo)dicobalt ranging
from 2.001 to 2.185 Å are longer, indicating +2 valence of Co
ions. The Co−Co distance of bis(μ-oxo)dicobalt is only
2
8
6
3
6
2
bis[(1H-imidazole-4-yl)methyleneaminomethyl]benzene).
Synthesis of MOC-4. MOC-4 was synthesized based on our
34
previously published paper. A mixture of Co(NO ) ·6H O (0.08
3
2
2
mmol, 23.3 mg), sodium bromide (0.20 mmol, 20.6 mg), 5-methyl-
imidazole-4-carboxaldehyde (0.12 mmol, 13.2 mg), 4-methylbenzyl-
amine (0.12 mmol, 14.5 mg), N,N-diethylformamide (DEF, 0.8 mL),
and EtOH (1.6 mL) was sealed in a Pyrex glass tube and heated in an
oven at 120 °C for 3 days. Then, the mixture was cooled down to
3
.0511(26) Å, which benefits the synergistic catalysis of the
−
1
water oxidation. Twelve L1, four Co1, four Co2, and 6 bis(μ-
oxo)dicobalt construct this unusual 20-nucleus Co-based
MOC. The structure and high purity of the synthesized
MOC-1 were confirmed by the powder X-ray diffraction
PXRD) measurements (Figure 2a) and scanning electron
microscopy (SEM) images (Figure S5). Furthermore, the FT-
room temperature at a rate of 5 °C h . Black green block-like crystals
were obtained, and the isolated yield was 54.7%, based on Co(NO ) ·
3
2
III
II
6
(
H O. MOC-4 was formulated as [(Co Co L4 Br )]·(Br) ·
2 4 4 12 4 3
NO ), (H L4 = N-((5-methyl-1H-imidazole-4-yl)methylene) (4-
3 2
methylphenyl)-methanamine).
Photoinduced Oxygen Evolution Measurement. The photo-
induced oxygen evolution experiments were carried out on WP-TEC-
(
IR spectrum with obvious CN specific absorbance bands at
1
020H with a quartz tube. In detail, a mixture of 2.0 mg of
−
1
photocatalyst, 37.4 mg of [Ru(bpy) ]Cl (50 μmol, working as
3
2
photosensitizer), 95.2 mg of Na S O (400 μmol, working as
2
2
8
sacrificial agent), and 10 mL of borate solution (0.2 M, pH = 9.0)
was added into a quartz transparent tube. The above mixture was then
stirred and bubbled with Argon for 30 min under room temperature
Photocatalysis. The photoinduced water oxidation of
MOC-1 was conducted with [Ru(bpy) ]Cl as the photo-
3
2
1
0381
Inorg. Chem. 2021, 60, 10380−10386
Chen, Zi-Ye
