Synthesis, characterization, magnetic anisotropy and catalytic behaviors of a cobalt complex of S,S′-bis(2-pyridylmethyl)-1,2-thiobenzene

Article abstract of DOI:10.1016/j.ica.2019.119400

S,S′-bis(2-pyridylmethyl)-1,2-thiobenzene (bptb) reacts with CoBr₂ to form a cobalt complex, [(bptb)CoBr₂]. Its structure has been determined by X-ray crystallography. Studies show that [(bptb)CoBr₂] can act as both a single-molecule magnet and a molecular catalyst. As a single-molecule magnet, magnetic anisotropy for [(bptb)CoBr₂] is revealed via the analyses of the direct-current magnetic data with a zero-field splitting (ZFS) parameter (D) of 69.88 cm^?1. Catalytic activities for hydrogen evolution from water are found. As an electrocatalyst, [(bptb)CoBr₂] can provide 870 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalyst/h) at an overpotential (OP) of 837.6 mV. As a photocatalyst, mixing with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, [(bptb)CoBr₂] can afford 19290 mol H₂ per mole of catalyst during 80 h irradiation. The highest apparent quantum yield (AQY) is ~24.30%.

Full text of DOI:10.1016/j.ica.2019.119400

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Research paper
Synthesis, characterization, magnetic anisotropy and catalytic behaviors of a
cobalt complex of S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene
Wei-Xia Liu, Zhen-Lang Xie, Jun-Zheng Zhan, Shu-Zhong Zhan, Song-Ping
Wu
PII:
S0020-1693(19)31752-9
https://doi.org/10.1016/j.ica.2019.119400
ICA 119400
DOI:
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Inorganica Chimica Acta
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11 November 2019
22 December 2019
22 December 2019
Please cite this article as: W-X. Liu, Z-L. Xie, J-Z. Zhan, S-Z. Zhan, S-P. Wu, Synthesis, characterization, magnetic
anisotropy and catalytic behaviors of a cobalt complex of S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene, Inorganica
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Synthesis, characterization, magnetic anisotropy and catalytic
behaviors of cobalt complex of
S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene
a
Wei-Xia Liu, Zhen-Lang Xie*, Jun-Zheng Zhan, Shu-Zhong Zhan*, Song-Ping Wu
College of Chemistry ＆ Chemical Engineering, South China University of
Technology, Guangzhou,510640, China
_
____________________________________________________________________
Abstract
S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene (bptb) reacts with CoBr to form a
2
cobalt complex, [(bptb)CoBr ]. Its structure has been determined by X-ray
2
crystallography. Studies show that [(bptb)CoBr ] can act as both a single-molecule
2
magnet and a molecular catalyst. As a single-molecule magnet, magnetic anisotropy
for [(bptb)CoBr ] is revealed via the analyses of the direct-current magnetic data with
2
-1
a zero-field splitting (ZFS) parameter (D) of 69.88 cm . Catalytic activities for
hydrogen evolution from water are found. As an electrocatalyst, [(bptb)CoBr ] can
2
provide 870 moles of hydrogen per mole of catalyst per hour (mol H /mol catalyst/h)
2
at an overpotential (OP) of 837.6 mV. As a photocatalyst, mixing with CdS nanorods
(
CdS NRs) as a photosensitizer, and ascorbic acid (H A) as a sacrificial electron
2
donor, [(bptb)CoBr ] can afford 19290 mol H per mole of catalyst during 80 h
2
2
irradiation. The highest apparent quantum yield (AQY) is 24.30%.
Keywords: Cobalt complex; crystal structure; magnetic anisotropy; electro and
photocatalytic activities for hydrogen evolution.
_
__________________
1

*
To whom correspondence should be addressed.
E-mail address: shzhzhan@scut.edu.cn (S.-Z. Zhan)
1
. Introduction
Considering that hydrogenase enzymes based on nickel or iron complexes can
efficiently catalyze both the production and the oxidation of dihydrogen [1],
researchers still focus their interests on the designs and studies of metal complexes,
which have catalytic activities for hydrogen generation [2-5]. To a typical
FeFe-hydrogenases supported by dithiolate ligands [2,6,7], the amine [8,9] cofactor is
suggested to facilitate the cleavage/formation of the hydrogen-hydrogen bond and the
transfer of protons to and from the distal metal center [9]. To mimic this natural
performance, an approach is the design and preparation of unique ligands that impart
novel chemistry when incorporated into the metal coordination sphere. These
considerations have led to the development of catalysts based on complexes with
varying ligands, and several complexes [10-13] have been developed as
electrocatalysts for hydrogen production. Encouraged by that transition metal
complexes supported by S atom ligands can serve as catalysts for photochemical
driven hydrogen evolution [14,15], a new cobalt-based catalyst, [(bptb)CoBr ] has
2
been designed and synthesized in our lab. This cobalt complex acts as an efficient
catalyst for H production both in an electrocatalytic system and a photocatalytic
2
system combining CdS NRs as a photosensitizer, and ascorbic acid (H A) as a
2
sacrificial electron donor. Additionally, this mononuclear cobalt complex displays
magnetic anisotropies. As new results, we present the synthesis, characterization,
magnetic properties and catalytic behavior of the cobalt complex, [(bptb)CoBr2].
2

2
. Experimental Section
Physical measurements, crystal structure determination, and equations for the
calculations of turnover frequency (TOF) and turnover number (TON) were showed
in “Supplementary Materials”. CdS nanorods (CdS NRs) were prepared by using the
reported method [16].
2
.1 Preparation of S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene (bptb)
Based on the reported procedure [17], bptb was prepared, with the
1
characterization via NMR measurements and analysis shown in Figs. S1 and S2. H
NMR (600 MHz, Chloroform-d)：δ 8.51 (d, 2H), 7.54 (td, 2H), 7.27 (d, 2H), 7.23 –
1
3
7
.20 (m, 2H), 7.11 (dd, 2H), 7.03 (dd, 2H), 4.25 (s, 4H). C NMR (151 MHz,
Chloroform-d) δ 157.28, 149.20, 136.62, 136.49, 129.99, 126.84, 123.16, 122.09,
3
9.76.
2
.2 Synthesis of the cobalt complex, [(bptb)CoBr2]
Addition of a 10 mL methanol solution of CoBr ·6H O (0.16 g, 0.5 mmol ) to a
2
2
solution of S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene (bptb) (0.16 g, 0.5 mmol ) in
0 mL ethanol, a red solution was formed. Under room temperature, slow evaporation
1
afforded pink needle-shaped crystals (0.21 g, 77%). Elemental measurements and
analysis, calcd for C H Br CoN S : C: 46.67; H: 3.38; N: 6.05. Found: C: 46.51, H:
1
8
16
2
2 2
3
.40, N: 6.09.
3

3
. Results and discussion
3
.1 General characterizations for CdS NRs and [(bptb)CoBr2]
As shown in Scheme 1, the cobalt complex, [(bptb)CoBr ] was obtained by the
2
reaction of CoBr and bptb. The result is consistent with the following structural and
2
ESI-MS analysis. According to the reported method [16], CdS nanorods (CdS NRs)
was prepared, with the characterizations by Transmission Electron Microscopy
(TEM), Scanning Electron Microscopy (SEM) shown in Fig. S3.
Scheme 1 Synthesis of the ligand, bptb and [(bptb)CoBr2].
To determine the structure of [(bptb)CoBr ], X-ray crystallography was
2
employed, giving the results listed in Tables S1-S2 and Fig. 1. In solid, the cobalt
atom is coordinated by two nitrogen atoms, two sulfur atoms from bptb, and two Br
-
atoms. Two Br ions are positioned in cis type. The bond lengths of Co–N are 2.150(7)
and 2.151(8) Å, and the Co-S distances are 2.494(2) and 2.501(2) Å, respectively. The
4

distances between Co and Br are 2.5101(12) and 2.5438(12) Å, respectively.
Moreover, using powder X-ray diffraction to check the purity of [(bptb)CoBr2]
sample, with the results shown in Fig. S4. The peak positions of the simulated
patterns were consistent with the experimental ones, indicating the [(bptb)CoBr2]
sample is in good purity.
To characterize the structure of [(bptb)CoBr ] in solution, ESI-MS spectrum of
2
[
(bptb)CoBr ] was measured in CH CN, with the result shown in Fig. S5. ESI-MS
2 3
spectrum of [(bptb)CoBr ] showed one peak at a mass-to-charge ratio (m/z) of
2
+
4
63.9224 in CH CN, which is assigned to the species of [(bptb)CoBr] . The
3
-
dissociation of a Br ion leaves an empty position on the cobalt center, amenable to
+
binding to H or H O and hydrogen production. Such results prompted possible usage
2
of this cobalt complex as a catalyst for hydrogen generation.
3
.2 Magnetic behavior of the cobalt complex, [(bptb)CoBr2]
Magnetic behaviors of [(bptb)CoBr ] were investigated in the temperature range
2
3
-1
2
-300 K, with the results shown in Fig. 2. The χ T value was 2.91 cm Kmol at 300
M
3
-1
K, much higher than the spin-only value of 1.875 cm Kmol for a mononuclear
II
high-spin Co (S = 3/2, g = 2.0). This value is in the reported range of 2.1–3.4
3
-1
II
cm Kmol for the anisotropic high-spin Co center [18], attributed to the orbital
contribution [19]. Moreover, the χ T values slowed down with decreasing
M
temperature from 300 K to 130 K, then decreased quickly below 130 K, and the
3
-1
minimum 1.82 cm Kmol was reached at 2 K. The downturn of the χ T value at low
M
5

II
temperatures can be attributed to the inherent magnetic anisotropy of the Co ion.
To further understand the magnetic anisotropy, the measurements for
variable-field magnetization of [(bptb)CoBr ] were carried out in the range from 0 to
2
7
T at temperatures of 2.0, 3.0 and 5.0, respectively. As shown in Fig. 2-inset, the
values of magnetization of [(bptb)CoBr ] at 7 T were lower than the theoretical
2
saturation value (3 NB), suggesting the presence of the magnetic anisotropy.
To further quantify the anisotropy of [(bptb)CoBr ], the Hamiltonian (1) and PHI
2
program [20] were employed to fit the experimental data, with the results plotted in
Fig. 2.
Where D, E, S, H and μ represent the axial zero field splitting (ZFS) parameter, the
B
rhombic or transverse ZFS parameter, the spin projection, magnetic field vector, and
the Bohr magneton, respectively.
-1
According to the data listed in Fig. 2, D= 69.88 cm , E= 8.19, g =g = 2.62, g =
x
y
z
2
.39. The similar magnetic properties were also observed in the reported cobalt
complexes [21,22].
3
.3 Electrocatalytic behavior of [(bptb)CoBr2]
Encouraged by that cobalt complexes can serve as electrocatalysts for hydrogen
generation via an unstable hydride intermediate [5,23], we first investigated
electrochemical behaviors of [(bptb)CoBr ]. With 0.10 M [(n-Bu) N]ClO as a
2
4
4
supporting electrolyte, cyclic voltammograms (CVs) of CoBr , the ligand (bptb) and
2
6

the cobalt complex, [(bptb)CoBr ] were measured in CH CN, respectively, giving the
2
3
results shown in Fig. S6 and Fig. 3. As shown in Fig. 3a, [(bptb)CoBr ] showed one
2
II/I
reduction peak at -1.26 versus Ag/AgNO , which can be assigned to the Co couple.
3
As shown in Fig. 3b, voltammetric currents emerging at 1.26 V versus
Ag/AgNO exhibited a systematic increase with addition of varied content of acetic
3
acid from 0.0 to 4.69 mM. This rise in current can be attributed to the catalytic
evolution of dihydrogen from acetic acid [5,23]. Additionally, upon addition of 3.35
mM acetic acid, a new reduction wave was observed near -1.61 V versus Ag/AgNO3.
Further addition of acetic acid led to an increase in the current, consistent with a
catalytic process [24].
Together literature precedents [25-27], with above observations and analysis, a
catalytic cycle for H evolution from acid mediated by [(bptb)CoBr ] was depicted in
2
2
I
Scheme 2. First, a cobalt(I) species, [(bptb)Co -Br] was formed via an one-electron
I
+
+
III
reduction of [(bptb)Co -Br] . Upon addition of hydrogen proton (H ) led a Co -H
III
+
III
species, [H-Co (bptb)-Br] to be formed. Next, one-electron reduction of the Co -H
II
II
species resulted in the formation of a Co -H species, [H-Co (bptb)-Br]. Further
II
addition of hydrogen proton to [H-Co (bptb)-Br] afforded a H molecule.
2
7

Scheme 2 Possible electrocatalytic mechanism for proton reduction to hydrogen by
(bptb)CoBr2].
[
Next, the electrocatalytic activity of [(bptb)CoBr ] was investigated via bulk
2
electrolysis, with the results listed in Fig. S7. Under -1.45 V versus Ag/AgNO , the
3
addition of [(bptb)CoBr ] led to the presence of 139 mC charge used 2 min of
2
electrolysis, with accompanying generation of H , which was confirmed by gas
2
chromatography. However, only a charge of 8 mC was afforded without
[
(bptb)CoBr ] (Fig. S7b), indicating that [(bptb)CoBr ] can serve as an electrocatalyst
2
2
for hydrogen generation. The electrocatalytic efficiency (turnover frequency (TOF))
8

of [(bptb)CoBr ] also can be estimated by equation 2 [28], and equation 3 [29] was
2
used to calculate overpotential (OP), with the results listed in Fig. S8. For instance, at
an OP of 941.6 mV, [(bptb)CoBr ] can afford 103 moles of hydrogen per mole of
2
catalyst per hour (mol H /mol catalyst/h).
2
TOF = C / (F*n *n *t)
(2)
1
2
Overpotential = Applied potential -E⊙
HA
=
Applied potential – (E⊙H+-(2.303RT/F)pKaHA) (3)
Next, electrocatalytic behavior of [(bptb)CoBr ] was characterized in aqueous
2
media, with the results shown in Fig. S9. The strength of the reduction of
[
(bptb)CoBr ] wave increased with decreasing pH values from 7.0 to 4.5 and the onset
2
of the catalytic wave were shifted to higher potentials, which are attributed to a
catalytic process [30].
To further understand the electrocatalytic activity of [(bptb)CoBr ] in aqueous
2
media, bulk electrolysis was carried out in buffer, with the results plotted in Fig. S10.
Under 1.45 V versus Ag/AgCl, only 312 mC of charge was provided from a neutral
buffer used 2 min of electrolysis (Fig. S10a). Surprisingly, the introduction of
[
(bptb)CoBr ] led to the formation of 1454 mC of charge under the same conditions
2
(
Fig. S10b). According to Fig. S11a, 48 mL of H was afforded during 1 h electrolysis,
2
and the Faradaic efficiency was 98% (Fig. S11b). Based on equations (2) and (4) [31],
TOFs and OPs were estimated, with the results listed in Fig. S12. For example, at an
OP of 837.6 mV, [(bptb)CoBr ] can electro-catalyze hydrogen evolution with a TOF
2
9

of 870 mol H /mol catalyst/h.
2
Overpotential = Applied potential- E(pH) = Applied potential- (-0.059pH) (4)
The result that [(bptb)CoBr ] is responsible indeed for the electrocatalytic
2
hydrogen evolution was also supported by other pieces of evidence: 1) CVs of the free
ligand (bptb), acetic acid and CoBr were each measured under identical conditions.
2
From Figs. 3, S6, S13, the catalytic competency achieved with the cobalt complex
was not matched by just bptb, acetic acid and CoBr . 2) Controlled-potential
2
electrolysis (CPE) of an aqueous buffer solution, bptb, CoBr and [(bptb)CoBr ] were
2
2
each measured under the same conditions. According to Fig. S14, a neutral buffer (pH
.0) only afforded 313 of charge during 2 min of electrolysis under -1.45 V versus
7
Ag/AgCl. The use of solely bptb or CoBr as a catalyst afforded hydrogen with 336
2
mC and 386 mC of charge, respectively. However, when [(bptb)CoBr ] was used as a
2
catalyst, bulk electrolysis provided H with 1347 mC of charge. Thus, the formation
2
of [(bptb)CoBr ] is essential for catalytic activity in above catalytic system. This can
2
be attributed to the ligand, bptb, that coordinates to the cobalt center and can stabilize
+
the low oxidation state of cobalt (such as Co ) well.
Under the same conditions, the catalytic activity of [(bptb)CoBr ] (870 mol
2
H /mol catalyst/h at an OP of 837.6 mV) is higher than some reported molecular
2
catalysts based on cobalt complexes for electrocatalytic H production from a neutral
2
water.
For
example,
a
cobalt
complex
with
N,N'-bis(2'-pyridinecarboxamide)-1,2-benzene (bpbH ), Co(bpbH )Cl (810 mol
2
2
2
1
0

H /mol catalyst/h at an OP of 837.6 mV) [23] (Table 1) and a cobalt complex,
2
[
CoA (H O) ] (A: picolinic acid ion) that can afford 722 mol of hydrogen per mole of
2 2 2
catalyst per hour at an OP of 837.6 mV [32]. Under 1.45 V versus Ag/AgCl, the
electrocatalytic activity of [(bptb)CoBr ] is lower than that of [(phen) Co(CN) ]·ClO
4
2
2
2
(
phen: 1,10-phenanthroline; TOF: 1113 mol H /mol catalyst/h) [5] (Table 1). It is not
2
easy to make a comparison for the catalytic activity among different electrocatalytic
systems in term of TOF values, because of possible differences in the experimental
conditions conducted and calculated methods. The catalytic efficiencies of some
reported catalysts based on cobalt complexes [33-35] were listed in Table 1.
To investigate the electrocatalytic performance of [(bptb)CoBr ] in
2
heterogeneous medium, electrochemical measurements were carried out by using a
modified electrode with [(bptb)CoBr ] as the working electrode. As shown in Fig.
2
S15, in 0.5 M Na SO aqueous solution (pH 7.0), [(bptb)CoBr ] could electrocatalyze
2
4
2
hydrogen evolution with a current density of -3.10 at an overpotential of 877 mV.
Similar electrocatalytic behaviors were found in the reported samples [36,37]. A
modified
gold
electrode
with
[Co-P]
II
(
cobalt -5,10,15,20-tetrakis[3-(p-acetylthiopropoxy)phenyl]porphyrin) could afford
hydrogen with a current density of -1.6 at an overpotential of 700 mV [36]. To a
modified glassy carbon with a cobalt-tetraimine complex, at pH 2.0, onset of catalysis
occurred at 0.24 V overpotential, and bulk electrolysis at 0.54 V overpotential
2
operated at ∼1 mA/cm with 80 10% Faradaic efficiency [37].
1
1

3
.4 Heterogeneous photocatalytic system with [(bptb)CoBr ] for H generation
2 2
Inspired by that transition metal complexes supported by S atom ligands often
serve as catalysts for photochemical driven hydrogen evolution [14,15], a
photocatalytic system containing ascorbic acid (H A) as an electron donor,
2
[
(bptb)CoBr ] as a catalyst and CdS NRs as a photosensitizer was designed. From
2
Figs. S3a, S16 and S17, CdS NRs were dispersed on the surface of [(bptb)CoBr2]
particle.
To get an optimal photocatalytic system for hydrogen production, a series of
measurements were carried out. First, impact of pH of media on the photocatalytic
activity of H production was investigated, with the results listed in Fig. S18. At pH
2
4
3
.5, the highest catalytic activity for hydrogen evolution was found with a TON of
−
1
190 mol of H (mol of cat) used 2 h of irradiation.
2
To investigate the impact of the amount of photosensitizer on the photocatalytic
H generation, photocatalytic systems containing 0.10 M ascorbic acid, [(bptb)CoBr ]
2
2
(0.010 mM) and a varying content of CdS NRs were designed. As shown in Fig. S19,
upon increasing the amount of CdS NRs, the TON increased until a highest value of
−
1
-1
3
720 mol of H (mol of cat) was obtained at 0.14 mg.mL used 2 h of irradiation.
2
Then, when the amount of CdS NRs was more, the TON decreased (Fig. S19). Next,
-1
photocatalytic systems containing CdS NRs (0.14 mgmL ), [(bptb)CoBr ] (0.010
2
mM) and varying contents of ascorbic acid were assembled to investigate the effect of
an amount of ascorbic acid on the catalytic activity, with the results listed in Fig. S20.
The TON increased with increasing the amount of ascorbic acid until a highest value
1
2

−
1
of 6000 mol of H (mol of cat) was got at 0.28 M used 2 h of irradiation. Then, the
2
TON decreased when the amount of ascorbic acid was more.
Based on the above observation and analysis, an optimal three-component
-1
system containing 0.14 mgmL CdS NRs, 0.28 M ascorbic acid and [(bptb)CoBr ]
2
(0.010 mM) was obtained. As shown in Fig. 4, upon light irradiation, the
photocatalytic system afforded H and could work for about 100 h. More details, H
2
2
evolution started immediately and the rate increased sharply for about 40 h. Then the
build-up of H slightly increased until H formation ceased after about 100 h. For
2
2
instance, during 60 h irradiation, the photocatalytic system with [(bptb)CoBr ] can
2
−
1
afford 18350 mol of H (mol of cat) .
2
Remarkably, [(bptb)CoBr ] exhibits higher efficiency for hydrogen evolution
2
than [(bpte)CoCl ] (bpte: S,S’-bis(2-pyridylmethyl)-1,2-thioethane; TON: 6740 mol
2
−
1
of H (mol of cat) ) [38], indicating that bptb can constitute the better active catalyst
2
for hydrogen generation than bpte. This result can be attributed to the electronic
properties of the ligands (bptb and bpte). The introduction of phenyl makes the
electron distribution more uniform in the cobalt complex, letting it easier to form the
III
Co -H species, further promoting the formation of hydrogen [39-41].
To investigate the apparent quantum yields (AQYs), a photocatalytic system was
irradiated under monochromic light with a band-pass filter (= 469 nm), giving the
results listed in Fig. 5. Based on equation 5 [42], the AQYs of the photocatalytic
system for H generation were calculated, with the results shown in Fig. 5. The AQY
2
was 19.90% used the first 2 h irradiation, and then it increased until 24.30% at 6 h
1
3

irradiation.
AQY (%) = (2· ·N ·h·c)/(t ·λ·I·A)·100
(5)
H2
A
irr
To find factors responsible for the photocatalytic H evolution in the above
2
photocatalytic system, bptb, CoBr or [(bptb)CoBr ] was employed as a catalyst to
2
2
assembly three component systems with ascorbic acid and CdS NRs, respectively. As
shown in Fig. S21, as a catalyst, bptb or CoBr only gave 5.9 and 19 mol H ,
2
2
respectively. However, when [(bptb)CoBr ] was used as a catalyst, the photocatalytic
2
system could afford 321 mol H . Thus, the formation of [(bptb)CoBr ] is essential
2
2
for catalytic activity in above photocatalytic system.
3
.5 Study for photocatalytic mechanism
In order to find the photocatalytic procedure, several measurements and analysis
were carried out. First, we measured electronic spectra of [(bptb)CoBr ] and the
2
related components, with the results shown in Fig. S22. Based on CdS NRs, the
introduction of [(bptb)CoBr ] into CdS NRs led to red shift of the absorption onset
2
and reduction of Eg of CdS NRs, indicating that the range and ability of visible light
absorption of CdS NRs are improved.
Next, measurements and analysis of photoluminescence (PL) spectra of
[
(bptb)CoBr ] and the related components were carried out, giving the results shown
2
in Fig. S23. With the excitation of 450 nm, CdS NRs exhibited two emission peaks at
59 and 681 nm, respectively. However, with the immobilization of [(bptb)CoBr2],
the peak intensity of CdS NRs at 681 nm decreased, showing a lower possibility of
5
1
4

electron–hole pair recombination due to the fast electron transfer from CdS NRs to
(bptb)CoBr ] [43]. Therefore, as an electron acceptor loaded on the CdS NRs
[
2
surfaces, [(bptb)CoBr ] can trap efficiently photo-generated electrons and promoted
2
+
their combination with H to form H . Based on the above investigations and analysis,
2
it is reasonable to speculate that a cobalt(I) species is generated via the reduction of
[
(bptb)CoBr ], which was found in the reported samples [44]. The results are also
2
consistent with the following orbital calculation.
To carry out orbital calculation of [(bptb)CoBr ], “Gaussian B3LYP/6-31G”
2
was employed. According to Fig. 6, the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of [(bptb)CoBr ] were -4.492 and
2
-
1.662 eV, respectively. Considering that ELUMO (-1.662 eV) of [(bptb)CoBr ] is
2
higher than the conducting band of CdS (-3.8 eV) [45], the photogenerated electrons
transfer from CdS to [(bptb)CoBr ] is energetically favorable.
2
Next, electrochemical impedance spectra of [(bptb)CoBr ] and the related
2
components were measured, with the results shown in Fig. 7. Compared with CdS
NRs, the mixture of CdS NRs and [(bptb)CoBr ] showed much smaller arc radius,
2
indicating that the introduction of [(bptb)CoBr ] makes interfacial charge transfer
2
faster, and separation efficiency of photogenerated charge carriers much higher
[8,46].
Combined these observation, analysis and literature precedents [47-49], a
possible mechanism for the photocatalytic H production was schemed. As outlined in
2
Scheme 3, the conduction band (CB) of CdS NRs provided electrons via irradiation;
1
5

then the introduction of the photoexcited electron into [(bptb)CoBr ] led to the
2
formation of a reduced cobalt(I) species, a high reactive intermediate. The
accumulation of the Co(I) species will undergo oxidative addition of a proton to
afford the CoIII hydride species. Further introduction of hydrogen proton to the
III
Co -H species provided H and regenerated the starting cobalt complex.
2
Scheme 3 Possible mechanism for H production by the photocatalytic system with
2
[(bptb)CoBr2].
4
. Conclusions
This paper presents the synthesis and characterization of a new cobalt(II)
complex, [(bptb)CoBr ], coming from the reaction of bptb and cobalt salt. As a
2
single-molecule magnet, this cobalt complex exhibits magnetic anisotropy with a
-1
zero-field splitting (ZFS) parameter (D) of 69.88 cm . As a molecular catalyst,
[
(bptb)CoBr ] shows catalytic activity for both electrochemical and photochemical
2
driven hydrogen evolution from water. Mixing with CdS NRs and ascorbic acid
H A), [(bptb)CoBr ] can photocatalyze hydrogen evolution under blue light and can
(
2
2
work for about 100 h.
Acknowledgements
This work was supported by the National Science Foundation of China (No.
1
6

2
1271073 and 21875074), and the Student Research Program (SRP) of South China
University of Technology.
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2
0

Hanan, Inorg. Chem. 58 (2019) 9127-9134.
TOF
TON
Catalysts
Electrolyte solution
Applied potential
mol H
2
/mol
mol H
2
/mol
Ref
This work
[5]
catalyst/h
catalyst
[
(bptb)CoBr
2
]
0.25
M
phosphate buffer
-1.45 V vs Ag/AgCl
-1.45 V vs Ag/AgCl
-1.45 V vs Ag/AgCl
-1.30 V vs SHE
870
1113
810
/
solution, pH 7.0
[
(phen)
2
Co(CN)
2
]·ClO
4
0.25 M phosphate buffer solution,
pH 7.0
/
[
Co(bpbH
2
)Cl
2
]
0.25 M phosphate buffer solution,
pH 7.0
/
[23]
[
(PY5Me
2
)Co(OH
2
)](CF
3
SO
3
)
2
917
5.5×104
[33]
2
.0 M phosphate buffer solution,
pH 7.0
A solution of 160 mM
cobalt(II) porphyrins
with -SO
-1.6 V vs NHE
-1.6 V vs NHE
-1.6 V vs NHE
-1.6 V vs NHE
-1.6 V vs NHE
-1.6 V vs NHE
−1.20 V vs SHE
9.12
0.23
4.54
0.56
4.04
0.31
/
104
1.9
51.8
6.4
46
[34]
[34]
[34]
[34]
[34]
[34]
[35]
3
H
AcOH in DMSO
A solution of 160 mM
AcOH in DMSO
cobalt(II) porphyrins
with -COOMe
cobalt(II) porphyrins
with -COOH
A solution of 160 mM
AcOH in DMSO
cobalt(II) porphyrins
with -H
A solution of 160 mM
AcOH in DMSO
cobalt(II) porphyrins
A solution of 160 mM
AcOH in DMSO
with -COONH
2
cobalt(II) porphyrins
with -OH
A solution of 160 mM
AcOH in DMSO
1.5
40
[
Co(bpyPY2Me)(CH
3
CN)(CF
3
0.3 M ascorbate, pH 7.0
SO )](CF SO )
3
3
3
Table 1. Electrocatalytic activities of some cobalt-based catalysts for H generation
2
PY5Me : 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine; bpyPY2Me: polypyridyl ligand.
2
Statements
I am Shu-Zhong Zhan, a corresponding author, from South China University of
Technology. My work focuses on the design and studies of new catalysts for
hydrogen generation. Now I’d like to report some new results to readers via Inorg.
2
1

Chim. Acta.
Shu-Zhong Zhan: Writing- reviewing and editing of the manuscript.
Wei-Xia Liu: Synthesis and characterization of the cobalt complex, [(bptb)CoBr2].
Zhen-Lang Xie: Investigation for catalytic activities of [(bptb)CoBr ]; revision of the
2
manuscript.
Jun-Zheng Zhan: Synthesis and characterization of the ligand, bptb.
Song-Ping Wu: Design of this subject.
Statements
I am Shu-Zhong Zhan, a corresponding author, from South China University of
Technology. My work focuses on the design and studies of new catalysts for
hydrogen generation.
No conflict of interest.
2
2

Fig. 1. ORTEP drawing of the [(bptb)CoBr ] with thermal ellipsoids on the 50%
2
probability level (hydrogen atoms are not shown) (up). Two-dimensional layer of
compound [(bptb)CoBr ] along the ab plane (down).
2
2
3

3.0
2.5
2.0
1.5
1.0
0.5
0.0
2
2
1
1
.5
.0
.5
.0
2
3 K
5
K
K
0.5
0
.0
0
1
2
3
4
5
6
7
H/T
0
50
100
150
200
250
300
T/K
Fig. 2. Temperature dependence of the  T product for [(bptb)CoBr ] under
M
2
temperature range of 2-300 K in the field of 1 kOe. The inset shows the M vs. B/T
plots in the temperature range 2.0-5.0 K. The solid lines are the best-fit curves using
the PHI program.
2
4

(a)
1
5
0
5
0
5
1
-
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V s Ag/AgNO3)
cacetic acid
(b)
0.00 mM
0.67 mM
1.34 mM
2.01 mM
2.68 mM
3.35 mM
4.69 mM
2
0
0
0
1
-
10
20
-
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V s Ag/AgNO3)
Fig. 3. (a) Cyclic voltammogram (CV) of [(bptb)CoBr ] (1.59 mM) in 0.10 M of
2
[
n-Bu N]ClO CH CN solution at a glassy carbon electrode and a scan rate of 100
4 4 3
-1
mVs . (b) CVs of a solution of [(bptb)CoBr ] (1.59 mM) with varying amounts of
2
acetic acid. Conditions: 0.10 M [n-Bu N]ClO as supporting electrolyte, scan rate:
4
4
-1
1
00 mVs , glassy carbon working electrode (1 mm diameter), Pt counter electrode,
Ag/AgNO reference electrode.
3
2
5

1000
20000
800
600
400
200
0
15000
10000
5000
0
0
20
40
Time (h)
60
80
100
Fig. 4. Hydrogen evolution kinetics obtained upon continuous visible irradiation (λ=
-1
4
69 nm) of a pH 4.5 buffer solution containing 0.14 mgmL CdS NRs, 0.28 M
ascorbic acid, and [(bptb)CoBr ] (0.01 mM).
2
2
6

600
500
400
300
200
100
H2
AQY
80
60
40
20
0
2
4
6
8
10
Time (h)
Fig. 5. Photocatalytic H production of [(bptb)CoBr ] under visible light (λ = 469 nm)
2
2
and an apparent quantum yield (AQY) of [(bptb)CoBr ] under monochromatic light (λ
2
=
469 nm). The reaction system contained 0.14 mg mL−1 CdS NRs, 0.010 mM
[
(bptb)CoBr ], 0.28 M ascorbic acid (pH 4.5).
2
1
9.9, 19.9, 24.3, 23.0, 22.7
0
-1
-2
-3
-4
-5
-6
LUMO
HOMO
Fig. 6. Calculated molecular orbital energy levels of [(bptb)CoBr ] with the molecular
2
orbital maps of the HOMO and LUMO.
2
7

1
1
5
0
5
0
CdS NRs
CdS NRs/[(bptb)CoBr2]
4
5
50
55
60
65
70
Z' / (ohm)
Fig. 7. Electrochemical impedance spectroscopy Nyquist plots of CdS NRs and CdS
NRs/[(bptb)CoBr ] in 0.010 M K Fe(CN) /K Fe(CN) electrolyte in the dark.
2
3
6
4
6
[
(bptb)CoBr ], a catalyst for electrochemical and photochemical driven hydrogen
2
evolution with high efficiency.
2
8

>
The cobalt complex, [(bptb)CoBr ] is formed by the reaction of CoBr and
2 2
S,S’-bis(2-pyridylmethyl)-1,2-thiobenzene (bptb).
[(bptb)CoBr ] exhibits magnetic anisotropy with a zero-field splitting (ZFS)
>
2
-1
parameter (D) of 69.88 cm .
Together with CdS NRs and ascorbic acid, [(bptb)CoBr ] can provide 19290 moles
>
2
H per mole of catalyst during 80 h irradiation.
2
2
9