Multifaceted Bicubane Co4Clusters: Magnetism, Photocatalytic Oxygen Evolution, and Electrical Conductivity

Article abstract of DOI:10.1002/ejic.201600510

The use of 1-(hydroxymethyl)-3,5-dimethylpyrazole (HL), a functionalized pyrazole ligand, to assemble with CoX₂(X = Cl or Br) in the presence of triethylamine under low-temperature solvothermal conditions gave rise to two tetranuclear cobalt(II) clusters, [Co₄L₆X₂] [X = Cl (1), Br (2)]. Both Co^II₄clusters are isostructural and protected by four μ₂-N¹:O²and two μ₃-N¹:O³L^–as well as terminal X anions to form a face-shared open bicubane structural motif. Magnetic susceptibility measurements indicated that there is an intramolecular antiferromagnetic interaction between four Co^IIatoms in 1 and 2. Although the core motif of 1 and 2 is not classic Co₄O₄monocubane, both are active catalysts for water oxidation, and their relative O₂-evolution rates are dependent on the halogen terminal ligands, which is also supported by spin-polarized density functional theory (DFT) calculations. Both clusters exhibit semiconductor behavior with σ values on the 10^–9S cm^–1scale at room temperature; however, mechanical iodine doping results in up to an astonishing 10⁵-fold maximum enhancement of solid-state conductivity relative to the undoped samples. This work therefore presents a new core type of cobalt cluster that possesses photocatalytic oxygen-evolution capabilities, provides new insight into the catalysis-related mechanism based on the relative oxygen-evolution efficiency, and applies the iodine-doping strategy to boost the conductivity of cluster compounds.

Full text of DOI:10.1002/ejic.201600510

DOI: 10.1002/ejic.201600510
Full Paper
Water Oxidation Catalysts
Multifaceted Bicubane Co Clusters: Magnetism, Photocatalytic
4
Oxygen Evolution, and Electrical Conductivity
[
a,b][‡]
[a][‡]
[a][‡]
[c]
[c]
Wan-Feng Xie,
Ling-Yu Guo,
Jia-Heng Xu,
Marko Jagodič, Zvonko Jagličić,
[a]
[d]
[a]
[a]
[a]
Wen-Guang Wang, Gui-Lin Zhuang, Zhi Wang, Chen-Ho Tung, and Di Sun*
Abstract: The use of 1-(hydroxymethyl)-3,5-dimethylpyrazole gands, which is also supported by spin-polarized density func-
(
(
HL), a functionalized pyrazole ligand, to assemble with CoX2 tional theory (DFT) calculations. Both clusters exhibit semicon-
X = Cl or Br) in the presence of triethylamine under low-tem- ductor behavior with σ values on the 10– S cm scale at room
9
–1
perature solvothermal conditions gave rise to two tetranuclear temperature; however, mechanical iodine doping results in up
II
5
cobalt(II) clusters, [Co L X ] [X = Cl (1), Br (2)]. Both Co clusters to an astonishing 10 -fold maximum enhancement of solid-
4
6
2
4
1
2
are isostructural and protected by four μ -N :O and two μ - state conductivity relative to the undoped samples. This work
2
3
1
3 –
N :O L as well as terminal X anions to form a face-shared open therefore presents a new core type of cobalt cluster that pos-
bicubane structural motif. Magnetic susceptibility measure- sesses photocatalytic oxygen-evolution capabilities, provides
ments indicated that there is an intramolecular antiferromag- new insight into the catalysis-related mechanism based on the
II
netic interaction between four Co atoms in 1 and 2. Although relative oxygen-evolution efficiency, and applies the iodine-
the core motif of 1 and 2 is not classic Co O monocubane, doping strategy to boost the conductivity of cluster com-
4
4
both are active catalysts for water oxidation, and their relative pounds.
O -evolution rates are dependent on the halogen terminal li-
2
[
6]
Introduction
dence in functional materials as solar water-splitting catalysts.
[
7]
Several X-ray structural studies confirmed that an Mn CaO5
4
Molecular clusters with multiple-spin metal centers have re-
ceived considerable attention because of their molecular aes-
thetics as well as some promising applications in quantum com-
cluster is the key site for the natural photosystem II (PSII) re-
sponsible for biocatalytic water oxidation. Thus, the mimics of
this structural motif – that is, twisted cubane with three Mn
atoms and one Ca atom occupying four corners, and a fourth
Mn atom outside the cubane bridged by two oxo bridges – is
an ideal pursued by synthetic chemists. The most important
breakthrough has been the successful synthesis of
Mn CaO (tBuCO ) (tBuCO H) (py)] (py = pyridine), which has
thus far found its best structural match in the full site of the
Mn CaO cluster in PSII. Although structural mimics of oxygen-
[
1]
[2]
[3]
putation, molecular spintronics, magnetic cooling, and
[
4]
nanoscale magnets. Undoubtedly, this field is witnessing its
greatest period of success along with some record-breaking nu-
clearity of transition-metal clusters such as Mn , Fe , Co ,
[8]
8
5]
4
168
36
[
Ni , Cu , Er , La Ni , Cu Mn , and Fe Ln . In the face of
[9]
3
4
44
60
76 60
17
28
16
4
[
4 4 2 8 2 2
such rich achievements, the direction in which transition-metal
clusters should go to retain their vitality is a huge challenge.
Thanks to recent research on mimics of photosynthetic sys-
tems, polynuclear transition-metal clusters have restored confi-
4
5
evolving complexes (OECs) have undergone huge advances, the
exploration of efficient and well-defined molecular catalysts for
water oxidation is still appealing. Recently cobalt-based molec-
[
a] Key Lab for Colloid and Interface Chemistry of Education Ministry,
School of Chemistry and Chemical Engineering,
Shandong University,
Jinan 250100, People's Republic of China
E-mail: dsun@sdu.edu.cn
http://www.researcherid.com/rid/G-3352-2010
b] School of Physics, Shandong University,
ular catalysts, a strong competitive alternative to noble-metal-
[10]
based catalysts,
have proven to be promising catalysts for
[11]
water oxidation. Among these molecular materials, the char-
acteristic Co O cubane core was considered to be the crucial
4
4
[
12]
active center for efficient catalysis of water oxidation as seen
in inorganic polyoxometalate (POM) cobalt complexes. Recently,
Dismuke's group also compared the rates of water oxidation by
six cobalt clusters with four different Co O cores and claimed
[
[
Jinan 250100, People's Republic of China
c] Faculty of Civil and Geodetic Engineering & Institute of Mathematics,
Physics and Mechanics, University of Ljubljana,
Jamova 2, 1000 Ljubljana, Slovenia
x
y
[
13]
that Co O was the best.
But is the Co O motif really the
4 4
4
4
[
[
d] College of Chemical Engineering and Materials Science,
Zhejiang University of Technology,
Hangzhou 310032, People's Republic of China
‡] These authors contributed equally to this work.
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600510.
sole criterion for assessing catalytic activity toward water oxid-
ation for the cobalt cluster family? To determine the answer, it
is worth exploring new cobalt clusters with different Co O
cores, then comparing their catalytic performance in water
oxidation, which would not only highlight the water oxidation
x
y
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4 6 2
Scheme 1. Schematic representation of ligand synthesis, assembly of [Co L X ] [X = Cl (1), Br (2)] and ligand bonding mode.
chemistry of Co-OECs but might also be helpful in providing 1 and 2 were pure phases for the subsequent characterizations.
insight into the mechanism of OECs in PSII. Their TGA curves (Figure S2 in the Supporting Information)
The hydroxymethyl-pyrazole ligand contains both pyrazole N show three similar weight-loss steps; the first weight loss for 1
and methoxy O coordination atoms. For this reason, it should and 2 occurs at 169 and 198 °C, respectively, which indicates
possess excellent potential as a bridging ligand in the construc- that complex 2 has higher thermal stability than 1.
tion of polynuclear metal clusters. However, its usage in polynu-
II
II
II
[14]
clear metal clusters has been rare (e.g., Co , Ni , Cu clusters ),
which might be due to the instability that is related to the
decomposition reverse reaction, especially in the presence of a
II
[15]
Cu atom.
For this purpose, we used 1-(hydroxymethyl)-3,5-
dimethylpyrazole (HL) to construct two isostructural tetra-
Cluster Structures of [Co L X ] [X = Cl (1), Br (2)]
4
6 2
nuclear cobalt(II) clusters, [Co L X ] [X = Cl (1), Br (2)], which
4
6 2
have the face-shared open bicubane Co O structural motif but
4
6
with different halogen anion ligands (Scheme 1). Magnetic sus-
ceptibility measurements indicated an intramolecular antiferro-
magnetic interaction between four Co atoms in 1 and 2. Al-
As indicated by X-ray single-crystal diffractions, clusters 1 and
both crystallize in the monoclinic P2 /n space group; each
2
1
II
asymmetric unit contains half of a complete molecule that sits
on the crystallographic inversion center (Figure 1). They have
though the core motif of 1 and 2 is not classic Co O mono-
4
4
cubane, both of them are intrinsically active catalysts for water
II
very similar defect double-cubane structures. Two Co atoms
oxidation, and their relative O -evolution rates are dependent
i
2
(Co1 and Co1 ) adopt an octahedral coordination environment
on the terminal halogen ligands. Interestingly, the iodine-
doping strategy effectively enhanced the electrical conductivity
of these tetranuclear Co clusters.
–
completed by two pyrazolyl N atoms from two L and four
–
II
methoxy O atoms from four L , whereas the remaining two Co
i
atoms (Co2 and Co2 ) are coordinated by three methoxy O
atoms from three L , one pyrazolyl N atom, and one Cl or Br
–
anion to give Co2 a trigonal-bipyramidal geometry (τ = 0.95
5
Results and Discussion
[18]
for 1 and 0.94 for 2).
The Co–O and Co–N bond lengths are
in and
.954(3)–2.266(3) and 2.052(4)–2.164(4) Å in 2. The larger radius
1
1
.9631(19)–2.2918(17) and 2.052(2)–2.164(2)
Å
1
Synthetic Aspects and General Characterization
–
–
As we know, most cobalt clusters are synthesized by means of Br with respect to that of Cl results in longer Co–Br bonds
[
16]
of room-temperature solution reactions or high-temperature [2.4750(9) Å] with respect to the Co–Cl in 1 [2.3198(9) Å]. Both
[
17]
II
II
1
2
1
3 –
solvothermal reactions (>100 °C)
between Co salts and li- Co clusters are protected by four μ -N :O and two μ -N :O L
4 2 3
gands. However, these two cobalt clusters were synthesized by as well as terminal X anions to form a Co O core, which could
4
4
means of low-temperature solvothermal reactions, which is un- be described as two defect Co O cubane subunits fused to-
3
4
common in the synthesis of discrete molecular clusters. We gether by sharing a Co O face. The Co···Co separations be-
2
2
adopted this eclectic tactic mainly to avoid decomposition of tween neighboring atoms are in the ranges of 3.2069(5)–
the HL ligand at high temperature and to facilitate its crystalli- 3.3144(8) Å and 3.1911(10)–3.3200(15) Å for 1 and 2, respec-
zation at the same time. Comparative PXRD patterns (Figure S1 tively. The structures of 1 and 2 are not common in the Co
in the Supporting Information) indicated that both compound cluster family and share similarities with only three complexes,
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[
19]
[
[
[
Co (OC H OEt) Cl ] (HOC H OEt
=
2-ethoxyethanol),
4
2
4
6
2
2 4
[
20]
Co (hqdH) Cl ] (hqd
=
8-hydroxyquinaldine),
and
4
6
2
[
21]
Co (L ) Cl ] (HL = pyridine-2-ylmethanol).
4
1 6
2
1
Figure 2. (a) Susceptibility (inset) and product ꢀ·T as a function of tempera-
ture in H = 1000 Oe. (b) Magnetization curves at 1.8 K.
Below 100 K the product ꢀT gradually decreases and finally
reaches a value of 0.31 emu K mol– at 1.8 K for both 1 and 2,
which indicates an antiferromagnetic interaction between the
1
II
Co ions. The existence of an antiferromagnetic interaction can
also be deduced from the M(H) curves (Figure 2, b), in which
the magnetization M versus the magnetic field H is only slightly
S-shaped for magnetic fields above 20 kOe even at the lowest
temperature of 1.8 K. This is not compatible with paramagnetic
behavior and a Brillouin function for localized magnetic mo-
ment that corresponds to spin 3/2, as shown by a dashed curve
in Figure 2 (b).
Figure 1. Labeled ORTEP (50 % probability) representation of clusters (a) 1
and (b) 2. Hydrogen atoms are omitted for clarity. Symmetry code i: (i) –x,
–y + 1, –z.
A detailed investigation of the susceptibility below 20 K (in-
set in Figure 2, a) shows a maximum of ꢀ(T) at Tmax = 2.6 K for
Magnetic Properties
2, whereas in 1 the maximum was not reached down to the
Magnetic susceptibility ꢀ = M/H was investigated between 1.8 lowest experimentally obtained temperature of 1.8 K. The maxi-
and 300 K under a constant magnetic field H = 1000 Oe; iso- mum in susceptibility is a clear demonstration of the antiferro-
II
thermal magnetization M was measured up to 70 kOe at several magnetic interaction between the magnetic moments of Co
temperatures below 8 K by using a Quantum Design MPMS-XL- ions. However, a relatively round maximum of ꢀ(T) without a
7
SQUID magnetometer. The data were corrected for sample sharp phase transition, no difference between zero-field and
holder contribution and a temperature-independent magnetic field-cooled susceptibilities (see inset in Figure 2, b), and the
susceptibility of inner-shell electrons (Larmor diamagnetism) as low temperature of Tmax signify that the antiferromagnetic in-
[
22]
obtained from Pascal's tables.
The product of ꢀT (Figure 2, a) is almost the same for both nearest Co ions in a molecular unit.
and 2 and practically constant between 300 and 100 K. Its In addition to the antiferromagnetic interaction, the single-
teraction must be weak and effective only between the four
II
1
–1
II
[20]
value at room temperature is 2.72 emu K mol Co, and its effec- ion effects of the Co ions – mainly spin–orbit coupling
–
tive magnetic moment per Co atom is μeff = √8ꢀT = 4.7 μ . This should be taken into account when trying to describe the devi-
B
value is larger than the high-spin-only S = 3/2 system value ation of susceptibility from perfect paramagnetic behavior [i.e.,
(
3.9 μ ) but is almost equal to 4.8 μ , which is the value that is a constant ꢀT product and Brillouin-shaped M(H)]. Because both
B
B
II
7
typically measured for a S = 3/2 Co ion in the 3d electron contributions are present simultaneously, modeling the meas-
configuration with non-zero orbital contribution to the mag- ured susceptibility becomes very difficult. A similar temperature
[
23]
netic moment.
dependence of the ꢀT product can be attributed either to the
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antiferromagnetic interaction parameter J or to the spin–orbit
coupling constant λ.
To further investigate the interactions in the Br sample, for
which a maximum was observed in the susceptibility at low
[
24]
temperatures, simulations using PHI software
were per-
formed. The Co ions in the double-cubane structure are cou-
pled through superexchange interactions. To determine the dis-
tances between Co ions and Co–O–Co angles, a simplified spin-
coupling diagram was constructed (Figure 3) on the basis of a
simple interaction Hamiltonian:
Hint = –J ·(S S + S S + S S + S S ) – J ·S S
2 3 4
1
1
2
1
4
2
3
2
3
Figure 5. Temperature dependence of the in-phase component of the ac
susceptibility ꢀ′ of 2 measured under 2 Oe at various frequencies. The inset
shows the out-of-phase component of ac susceptibility ꢀ′′.
Preliminary Visible-Light-Driven Water Oxidation by 1 and 2
Figure 3. Simplified spin-coupling diagram for the Br sample showing the Co
ions at two different sites and the double oxygen bridges.
In typical photocatalytic water oxidation experiments for 1 and
2
9
, irradiation of 1 and 2 in aqueous borate buffer that contained
The temperature dependence of the susceptibility data up
to 20 K measured under 1000 Oe and the magnetization curve
mM
K S O as sacrificial oxidant and 1 m [Ru(bpy) ]Cl as
M
2
2
8
3
2
photosensitizer were performed with an LED lamp at λ =
50 nm. In control experiments performed in the absence of
one of the factors such as light, K S O , [Ru(bpy) ]Cl , or Co
at 3 K were fitted simultaneously using four S = 3/2 spins cou-
i
4
pled as shown in Figure 3. Although a simplified model was
used, neglecting the spin–orbit coupling, the main feature of
the susceptibility curve (namely, the maximum at 2.6 K), was
reproduced (Figure 4). Furthermore, an excellent agreement of
the simulated magnetization curve and experimental measure-
2
2
8
3
2
clusters, we found that no O production could be detected,
2
which suggests that all components are prerequisites for light-
driven O production in our system (Figure S4 in the Supporting
2
–
1
Information). Three different pH environments were evaluated
for oxygen-evolution performance. Owing to the insolubility of
ments was achieved. Coupling constants of J = –0.77 cm and
1
–
1
J2 = –0.83 cm were obtained, which is in agreement with a
weak antiferromagnetic interaction between Co ions. The dis-
II
1
and 2 in such a reaction system, 2 mg of each solid sample
was taken for tests every time. Photocatalytic oxygen genera-
tion was monitored through the detection of dissolved O using
crepancy between the measured data and the fitted curve that
increases at temperatures above 8 K might be attributed to the
spin–orbit coupling we ignored in the fit.
2
a Clark-type electrode. As shown in Figure 6, the O evolution
2
with 1 and 2 increases steadily with irradiation time, then
gradually reaches the maximum at around 1 min. The initial
rate of O evolution was obtained on the basis of the slope of
2
the initial linear portion of the plots in Figure 6 (a and c). For
1
9
, the initial rates of O evolution are 8.30, 11.36, and
2
.30 mmol s–
1
g
–1
at pH = 7.4, 8.0, and 9.0, respectively (Fig-
ure 6, b), whereas these values under the same conditions are
only 1.82, 2.18, and 0.91 mmol s–
1
g
–1
for 2 (Figure 6, d). Thus,
the best oxygen-generation pH value of borate aqueous solu-
tion is 8 for both 1 and 2. As the driving force for water oxid-
ation increased at higher pH but was accompanied by the easy
[
25]
decomposition of the photosensitizer,
similar results can be seen in a 3d–4f {Co Ln(OR) } cubane-
catalyzed water oxidation system.
this might be why
II
3
4
[
26]
The overall photocatalytic
Figure 4. Temperature dependence of the susceptibility of 2 measured under
efficiency of 1 is clearly superior to that of 2, which is probably
caused by manifold competing factors involved in oxygen evo-
lution but is most likely related to the coordination differences
between Cl and Br. The mechanism of water oxidation catalyzed
1
000 Oe (circles) and the simulated curve (red line). The inset shows the
magnetization curve of 2 (circles) and the curve obtained by simulation (red
line) at 3 K.
Additionally, ac measurements were performed on 2. Maxi- by 1 and 2 is the focus of ongoing investigation but is difficult
mums in the in-phase ac susceptibility at temperatures inde- to determine on the basis of the data presented above. At any
pendent (or very weakly dependent) of frequency and almost rate, a preliminary mechanistic framework could be proposed
constant out-of-phase components of the ac susceptibility sug- as shown in Scheme 2. In particular, the equilibria of halogen
gest that no single-molecule magnetism is present in this sys- anion coordination and the nucleophilic attack by water could
tem (Figure 5).
generate metal–water intermediates, which are then oxidized
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II
to high-valent metal-oxo species by means of multielectron indeed difficult to completely exclude the free Co ions or CoOx
transfer. Hydrolysis of the high-valent intermediate species or Co(OH) nanoparticles from the water oxidation catalysis ac-
x
along with proton transfers would liberate O and then close tivity.
2
[
27]
the catalytic cycle.
Thus, the free water molecule should be
To identify the mechanism behind the photocatalytic differ-
involved in the formation of the O–O bond, which suggests that ences, spin-polarized DFT calculations were conducted using
[
29]
the most likely mechanism should follow the water nucleophilic the DMol3 module. The initial structures, which were derived
[
28]
attack (WNA). Such similar clusters with clearly different cata- from X-ray diffraction analysis, were first optimized without any
–
–
lytic abilities provides evidence that water oxidation catalysis is constraints. Essentially, the Cl or Br ions were removed from
mainly controlled by molecular species 1 and 2. However, it is the Co Cl or Co Br core to expose the catalytically active sites.
4
2
4
2
3
Figure 6. Photocatalytic oxygen evolution as a function of time from a 2 mL of solution containing 2 mg of (a) 1 and (c) 2 with [Ru(bpy) M) and
]2+ (1 m
K
2
S
2
O
8
(9 m
M) in borate buffer upon light irradiation (λ = 450 nm, 48 W). The kinetics of O
2
evolution rates of (b) 1 and (d) 2 based on linear fitting of the
linear portion of the plots in (a) and (c), respectively.
Scheme 2. Proposed catalytic cycle for water oxidation catalyzed by 1 or 2.
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+
+
[30]
In this way, the stable configurations of Co Cl and Co Br were found in some MOF materials.
However, the “through-space
4
4
[
31]
identified. The gap between the highest occupied molecular approach” renews the possibility that the cluster compounds
orbital (HOMO) and the lowest unoccupied molecular orbital are semiconductive or even conductive. The electrical conduc-
+
+
(
LUMO) were 0.73 eV for Co Cl and 0.62 eV for Co Br , which tivity properties of 1 and 2 were assessed using a two-probe
4
4
+
+
hints at the greater stability of Co Cl over Co Br ions. Inspec- dc measurement on compressed pellets of polycrystalline sam-
4
4
+
+
tion of the frontier molecular orbitals of Co Cl and Co Br ions ples. The I–V curves were plotted in the range of –20 to 20 V
4
4
reveals that both HOMOs consist of d orbitals of the middle by using an Agilent B1500A semiconductor parameter analyzer
z2
2
+
two Co ions, and 2p orbitals of O and N atoms of the ligands, at room temperature. As shown in Figure 8, the I–V curves fol-
z
whereas both of the LUMOs concentrate on dz orbitals of one low Ohm's law as indicated by their good linearity; the currents
2
II
terminal pentacoordinate Co ion, a few 2p orbitals of O and were able to reach the nanoampere order in the measured volt-
z
N atoms, and π* antibonding orbitals of one pyrazole group, as age range, which suggests semiconductive behavior with a con-
9
–1
–9
–1
shown in Figure 7. Essentially, dz
Lewis acid active sites. Upon exposure to light, electrons can 2. These values are lower than that for many chalcogen-con-
2
orbitals in the LUMO act like ductivity of 1.78 × 10– S cm for 1 and 2.91 × 10 S cm for
[
32]
transfer from the pyrazole groups to the dz
2
orbitals and further taining coordination compounds.
However, their intrinsic
donate to H O molecules. To examine the adsorption of H O, it conductivities could be modulated and enhanced by guest
2
2
is usually helpful to understand the difference in photocatalytic molecule doping, which has been responsible for the huge suc-
[
33]
properties. Therefore we further identified the adsorption en- cess of conductive polymers.
The outstanding breakthrough
in MOFs that was achieved by such a strategy is 7,7,8,8-tetra-
is much larger than that of Co Br (0.17 eV). It can be observed cyanoquinodimethane (TCNQ)-doped HKUST-1, which dis-
+
ergy of H O on the catalysts. The E value of 0.19 eV for Co Cl
2
ad
4
+
4
+
–8
–1 [34]
that H O molecules prefer to adsorb onto Co Cl rather than played tunable conductivity from 10 to 0.07 S cm . In spite
Co Br and thereby enhance the possibility of activating H O in of such advances through this doping strategy in the field of
Co Cl . By comparing the catalyst stability and the adsorption coordination compounds, its application is not very extensive
energy of H O of the two catalysts, we can satisfactorily posit and the related mechanism is still unclear.
2
4
+
4
2
+
4
[
35]
The conductivity
2
+
that the photocatalytic properties of Co Cl are better than that performances of 1 and 2 can be further enhanced through a
4
+
of Co Br .
simple doping process in which the compounds and iodine are
4
mechanically mixed by grinding in a mortar. For 1, the conduc-
–
7
–4
tivity values gradually increase from 5.8 × 10 to 4.7 × 10 and
to a maximum of 5.4 × 10– S cm as the I mass fraction in-
4
–1
2
creases from 5 to 10 and to 15 %. A further increase in
doped I to 20 wt.-% conversely decreases the conductivity to
2
4
–1
1
.4 × 10– S cm (inset plot in Figure 8, a). A similar conductiv-
+
Figure 7. Frontier molecular orbitals: (a) HOMO and (b) LUMO of Co
4
Cl . (c)
+
HOMO and (d) LUMO of Co
4
Br .
Electrical Conductivities of 1 and 2
In contrast to the widely studied properties of the coordination
clusters, semiconductivity or conductivity is functionality that
has long been sought after but is hard to access in cluster com-
pounds owing to their intrinsic discrete characteristics, includ-
ing dense packing in the solid state, and the lack of a continu-
ous 1D, 2D, or 3D electron-transport pathway such as that
Figure 8. I–V curves of (a) 1 and (b) 2 with different amounts of I
2
doping.
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ity evolution upon I doping is also observed for 2. The maxi- Photocatalytic Water Oxidation: The amount of evolved O
dis-
2
2
solved in solution was measured in situ by means of a standard
Clark-type oxygen electrode (Hansatech Instruments). The electrode
was calibrated following standard procedures before measure-
ments. In a typical experiment, each component was mixed in
buffer solution, then degassed with argon for 30 min. The degassed
solution was then irradiated using a 450 nm LED lamp. The amount
mum conductivity increments between undoped and doped
15 %) 1 and 2 are 5 and 3 orders of magnitude, respectively.
It is worth noting that conductivity boosts are an emergent
functionality that are observed neither in tetranuclear Co clus-
(
ters nor in the iodine molecules. The I doping that induced
2
better conductive performance should follow an oxidative dop-
of O evolved in solution was measured in situ.
2
ing mechanism that could produce a highly efficient n→σ*
Electrical Conductivity Measurement: The dc current–voltage (I–
V) measurements were performed at room temperature using a
two-probe method with an Agilent B1500A semiconductor parame-
ter analyzer. Compressed circular pellet samples were made as fol-
lows: Single crystals of 1 and 2 were ground and pressed into pel-
lets 0.4 cm in diameter and with thicknesses of 0.04–0.07 cm. The
measurements were performed on these circular pellet samples
with silver paint coated on both sides as electrodes. We repeated
the experiments on samples with different thicknesses. The ob-
served results are consistent within experimental errors.
[
36]
charge transfer, as evidenced by the gradually enhanced ab-
sorption band above 700 nm relative to the undoped sample
(
Figure S6 in the Supporting Information).
Conclusion
We have isolated and structurally characterized two new tetra-
nuclear cobalt(II) clusters with a face-shared open bicubane
structural motif. An intramolecular antiferromagnetic interac-
Synthesis of Clusters
1
and 2:
A mixture of CoCl ·6H O
2 2
(
0.25 mmol, 72 mg) or CoBr ·4H O (0.25 mmol, 83 mg) and HL
2
2
II
tion between four Co atoms is confirmed by magnetic suscep-
(0.5 mmol, 61 mg) were dissolved in acetonitrile (5 mL), then trieth-
tibility measurements. More interestingly, although the core
ylamine (60 μL) was added with a pipette. Afterwards, the resulting
mixtures were sealed in a 25 mL Teflon-lined stainless steel auto-
motif of 1 and 2 is not classic Co O4 monocubane, both of
4
them are intrinsically active catalysts for water oxidation, and clave and heated at 60 °C for 3500 min, after which the mixtures
their relative O -evolution rates are dependent on the halogen were cooled over 500 min to room temperature. The products were
2
terminal ligands. The electrical conductivity measurements obtained as deep blue-purple rodlike crystals in yields of 70 and
gave the intrinsic σ value of approximately 10–9 S cm–1 at room
85 % (based on cobalt) for 1 and 2, respectively. For 1: Selected IR
peaks: ν˜ = 3058 (w), 2936 (w), 2869 (w), 1672 (m), 1485 (m), 1435
temperature, thus indicating their semiconductive behavior. In-
terestingly, mechanical iodine doping was able to enhance the
conductivity to approximately 10–4 S cm relative to the un-
doped samples. The present work presents a new model of
a molecular cobalt cluster-based photocatalytic water oxygen
catalyst, provides new insight into the catalysis-related mecha-
(
m), 1383 (m), 1366 (m), 1265 (m), 1150 (m), 1103 (m), 1020 (m),
1
4
010 (m), 826 (m), 741 (s), 694 (s), 632 (s), 548 (m), 508 (s), 479 (m),
53 (m) cm . Elemental analysis calcd. (%): C 40.89, H 5.15, N 15.89;
–1
–
1
found C 40.77, H 5.50, N 15.75. For 2: Selected IR peaks: ν˜ = 2988
w), 2827 (w), 2741 (w), 1545 (m), 1490 (m), 1455 (m), 1406 (m),
364 (m), 1239 (m), 1148 (m), 1119 (s), 1037 (m), 982 (m), 829 (m),
(
1
nism by comparing oxygen-evolution efficiency, and applies the 801 (m), 777 (m), 685 (s), 663 (m), 627 (m), 594 (m), 554 (m), 486
iodine-doping strategy to boost the conductivity of cluster (m) cm . Elemental analysis calcd. (%): C 37.72, H 4.75, N 14.66;
–
1
compounds.
found C 37.50, H 4.88, N 14.76. Crystal data for 1 and 2 are provided
in Table 1. Selected bond lengths and angles are shown in Table S1
of the Supporting Information.
Experimental Section
Table 1. Crystal data for 1 and 2.
Materials and Methods: Commercially available solvents and
1
2
metal salts were used without further purification. The ligand 1-
Formula
M
r
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C
36
H
54Cl
2
Co
4
N
12
O
6
36 2 4 12 6
C H54Br Co N O
1146.45
298(2)
(hydroxymethyl)-3,5-dimethylpyrazole was synthesized by the reac-
1057.55
298(2)
monoclinic
P2 /n
10.4457(8)
18.4395(15)
11.6468(10)
96.384(6)
2229.4(3)
2
1.5753
1.635
1091.7
23253
3925 [Rint = 0.0445]
3925/276
1.049
tion of 3,5-dimethylpyrazole and paraformaldehyde as described by
Driessen[ and used after crystallization from CH CN. IR spectra
37]
3
monoclinic
P2 /n
were recorded using a PerkinElmer Spectrum Two in the frequency
1
1
–
1
range of 4000–400 cm . Elemental analyses for C, N, and H were
performed using a PerkinElmer 2400 CHN elemental analyzer. Pow-
der X-ray diffraction (PXRD) data were collected using a Philips X′
Pert Pro MPD X-ray diffractometer with Mo-Kα radiation equipped
with an X′Celerator detector. Thermogravimetric analyses (TGA)
were performed using a SHIMADZU DTG-60A thermal analyzer from
room temperature to 800 °C under a nitrogen atmosphere at a heat-
10.565(3)
18.662(4)
11.445(3)
95.689(3)
2245.4(9)
2
1.6955
3.278
1162.1
9347
ꢀ
[°]
3
V [Å ]
Z
–3
ρ
calcd. [g cm ]
–1
μ [mm ]
–1
ing rate of 10 °C min . The diffuse-reflectance spectrum was deter-
mined using a UV/Vis spectrophotometer (Evolution 220, ISA-220
accessory, Thermo Scientific) with a built-in 10 mm silicon photodi-
ode with a 60 mm Spectralon sphere. Variable-temperature mag-
netic susceptibilities were collected using a magnetic property
measurement system (MPMS), SQUID-VSM (superconducting quan-
tum interference device-vibrating sample magnetometer) (Quan-
tum Design, USA).
F(000)
Reflections collected
Independent reflections
Data/parameters
GoF on F
Final R indexes [I ≥ 2σ(I)]
Final R indexes [all data]
4408 [Rint = 0.0533]
4408/275
1.002
2
R
1
R
1
= 0.0323, wR
= 0.0530, wR
2
= 0.0680
= 0.0763
R
R
1
1
= 0.0470, wR
= 0.1062, wR
1.07/–0.82
2
= 0.0821
= 0.1006
2
2
–3
Largest diff. peak/hole [e Å ] 0.47/–0.41
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CCDC 1450527 (for 1), and 1450528 (for 2) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre.
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This work was supported by the National Natural Science Foun-
dation of China (NSFC) (grant number 21571115), the Research
Award Fund for Outstanding Middle-aged Young Scientist of
Shandong Province (grant number BS2013CL010), the Young
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Keywords: Cluster compounds · Cobalt · Water splitting ·
Magnetic properties · Semiconductivity · Doping
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Full Paper
Water Oxidation Catalysts
This work presents a new core type of
cobalt cluster that possesses photocat-
alytic oxygen-evolution capabilities,
provides new insight into the catalysis-
related mechanism based on the rela-
tive oxygen evolution efficiency, and
applies the iodine-doping strategy to
boost the conductivity of cluster com-
pounds.
W.-F. Xie, L.-Y. Guo, J.-H. Xu, M. Jagodič,
Z. Jagličić, W.-G. Wang,
G.-L. Zhuang, Z. Wang, C.-H. Tung,
D. Sun* ............................................... 1–10
Multifaceted Bicubane Co Clusters:
4
Magnetism, Photocatalytic Oxygen
Evolution, and Electrical Conductiv-
ity
DOI: 10.1002/ejic.201600510
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