Enhanced photocatalytic hydrogen production by introducing the carboxylic acid group into cobaloxime catalysts

Article abstract of DOI:10.1039/c5dt02645h

A series of cobaloxime complexes, [Co(iii)(dmgH)₂(py-m-X)Cl] (dmgH = dimethylglyoxime, py-m-X = meta-substituted pyridine, X = COOH (2), COOCH₃ (3), CH₂CH₂COOH (6), and CH₂CH₂COOCH₃ (7)), and [Co(iii)(dmgH)₂(py-p-X)Cl], (py-p-X = para-substituted pyridine, X = COOH (4) and COOCH₃ (5)), were synthesized and their photocatalytic H₂ production activities were compared in an artificial photosynthesis system containing a xanthene dye Eosin Y as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial reductant (SR) in CH₃CN/H₂O (1:1, pH = 7.5). Irrespective of substitution by an electron-donating or electron withdrawing group, the photocatalytic H₂ production activities of 2-7 are all higher than that of [Co(iii)(dmgH)₂(py)Cl] (1). Importantly, meta-substitution is more efficient than para-substitution, and COOH is more efficient than COOCH₃, in enhancing the photocatalytic activities. 6 showed the highest activity among the examined complexes. The -CH₂CH₂- chain linking COOH and pyridine might play a role in the promising performance of 6, which makes the proton relay via interaction between COOH and dmgH possible. This work may open new avenues for developing more efficient cobaloxime-based H₂ evolution catalysts (HERs).

Full text of DOI:10.1039/c5dt02645h

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PAPER
Enhanced Photocatalytic Hydrogen Production by Introducing
Carboxylic Acid Group into Cobaloxime Catalysts
Junfei Wang,a,b Chao Li,a Qianxiong Zhou,a Weibo Wang,a Yuanjun Hou,*,a Baowen Zhang,a
Xuesong Wang*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of cobaloxime complexes, [Co(III)(dmgH)2(py-m-X)Cl] (dmgH = dimethylglyoxime, py-m-X = meta-substituted
pyridine, X = COOH (2), COOCH3 (3), CH2CH2COOH (6), and CH2CH2COOCH3 (7)), and [Co(III)(dmgH)2(py-p-X)Cl], (py-p-X
=para-substituted pyridine, X = COOH (4) and COOCH3 (5)), were synthesized and their photocatalytic H2 production
activities were compared in an artificial photosynthesis system containing a xanthene dye Eosin Y as photosensitizer (PS)
and triethanolamine (TEOA) as sacrificial reductant (SR) in CH3CN/H2O (1:1, pH = 7.5). Irrespective of substitution by an
electron-donating or electron withdrawing group, the photocatalytic H2 production activities of 2-7 are all higher than that
of [Co(III)(dmgH)2(py)Cl] (1). Importantly, meta-substitution is more efficient than para-substitution, and COOH is more
efficient than COOCH3, in enhancing the photocatalytic activities. 6 showed the highest activity among the examined
complexes. The -CH2CH2- chain linking COOH and pyridine might play a role in the promising performance of 6, which
makes proton relay via interaction between COOH and dmgH possible. This work may open new avenues for developing
more efficient cobaloxime-based H2 evolution catalysts (HERs).
www.rsc.org/
realized.10 Irrespective of photocatalysis or electrocatalysis, it
is generally recognized that cobaloxime complexes are
reduced to Co(I) or Co(0) species at first, and then protonated
Introduction
As a promising solution for energy crisis and climate changes,
H2 generation via water splitting has been intensively and
extensively studied for decades.1 Both photocatalytic and
electrocatalytic H2 evolution catalysts (HERs) of high efficiency
and low cost are highly pursued, making the first row
transition metal complexes the focus of related researches.
Among them, cobaloxime catalysts are particularly attractive
by virtue of their high activity and easy preparation.2 Lehn and
coworkers reported the first H2-production artificial
to form the key intermediate of Co(III)H or Co(II)H, by which H2
generation may be achieved via either protonolysis or
bimetallic reaction.11 As a result, the catalytic activity is highly
dependent on the reduction potentials and pKa values of these
oxidation states of the cobaloxime complexes,12 and tuning of
them via ligand modification has been explored to optimize
the H2 generation activity.10 It was found that introducing an
electron-donating substituent in the axial pyridine ligand of a
cobaloxime complex may enhance its electrocatalytic H2
production activity remarkably.10 In contrast, Eisenberg and
photosynthesis (AP) system using Co(dmgH)2(dmgH
=
dimethylglyoxime) as HER, Ru(bpy)32+ (bpy = 2,2’-bipyridine) as
photosensitizer (PS), and triethanolamine (TEOA) or
triisopropanolamine (TIPOA) as sacrificial reductant (SR).3
Since then, lots of cobaloxime-based AP systems were
scrutinized, and many viable PSs, including Pt,4 Re,5 Ru,6 Ir,7
and Au-based8 complexes and metal free organic dyes,9 have
been developed. Meanwhile, efficient electrocatalytic H2
production based on cobaloxime complexes were also
coworkers improved the photocatalytic activity of
a
cobaloxime complex by tethering an electron-withdrawing
group in the axial pyridine ligand.4b
Engineering on proton transportation may also boost
activities of HERs significantly. In [Fe-Fe]-hydrogenase enzymes,
the pendant amine in the dithiolate ligand serves as a proton
relay to facilitate heterolytic formation (or cleavage) of
hydrogen.12 Inspired by the unique structure and ultrahigh
activity of [Fe-Fe]-hydrogenase enzymes, the team of D. L.
DuBois and M. R. DuBois constructed a series of novel nickel
complexes using cyclic diphosphine ligands, abbreviated
a.Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
100190, P. R. China. E-mail: xswang@mail.ipc.ac.cn, yuanjunhou@yahoo.com.
b.Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. China.
asPR NR’2(1,5-R’-3,7-R derivative of 1,5-diaza-3,7-diphosphacyc-
2
looctane (R, R’ = aryl or alkyl)).13 When the pendant amines in
the second coordination sphere are positioned close to the Ni
ion, they facilitate the cleavage or formation of the H–H bond
and promote proton transfer between the metal center and
solution, leading to highly efficient electrocatalytic H2
Electronic Supplementary Information (ESI) available: Synthetic routes for 1-7
,
high-resolution ESI MS and 1H NMR spectra of 1–7, UV-visible absorption spectrum
of Eosin Y, The effect of solution composition and the concentrations of TEOA, 6,
and Eosin Y for the photocatalytic activities, Stern-Volmer plot of the fluorescence
quenching of Eosin Y by 1-7 , Stern-Volmer plot of the fluorescence quenching of
Eosin Y by TEOA, The reduction currents in the presence of 15 equiv acetic acid See
DOI: 10.1039/x0xx00000x
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generation.13 Though the proton relay strategy has been
successfully applied in electrocatalytic HERs,14 its exploitation
in photocatalytic HERs is still in its infancy. Up to now, three
examples were reported only.15 In the first case, a HER of
DuBois-type was used to construct a homogeneous and a
heterogeneous AP systems using a phosphonated Ru(II)
complex as PS.15a In the second case, a cationic analogue of
DuBois-type catalyst by functionalizing its outer ligand sphere
with primary amines realized photocatalytic H production in
self-assembling hydrogel scaffolds.15b In the third case, Wu and
coworkers synthesized a branched poly ethylenimine grafted
[FeFe]-H2ase mimic and realized photocatalytic H2 generation
in water in combination with quantum dots-based PS and
ascorbic acid-based SR.15c In light of the appealing
R
H
1
4
COOH
COOCH3
5
Eosin Y
performance of [Co(III)(dmgH)2(py)Cl] (
1 in Scheme 1) in
catalytic H2 generation and the fact that its chemical Scheme 1. Chemical structures of the examined cobaloximes
modification was focused on substituent’s electronic effect and Eosin Y.
and was performed mainly at para-position of the axial
pyridine ligand,4b, 10 we herein designed and synthesized a pyridine ligand is not considered, since steric hindrance will
series of new cobaloxime complexes, [Co(III)(dmgH)2(py-m- hamper its coordination to the Co center.
X)Cl], (py-m-X = meta-substituted pyridine, X = COOH (
2),
COOCH3 (3), CH2CH2COOH (6), and CH2CH2COOCH3 (7), Scheme Optimization of photocatalytic H2 production conditions
1) in the hope that the COOH group may act as a proton relay
to enhance the photocatalytic H2 generation activities of the
corresponding complexes. For comparison, [Co(III)(dmgH)2(py-
We at first used the complex
6 to optimize the photocatalytic
H2 production conditions. Eosin Y was selected as PS due to its
noble metal-free character and its successful combination with
the complex
1
in photocatalytic H2 production.9a Figure S1
p-X)Cl], (py-p-X = para-substituted pyridine, X = COOH (
COOCH3 ), Scheme 1) were also studied. Among these
cobaloxime complexes, the complex showed the best
4) and
(
5
shows the UV-visible absorption spectrum of Eosin Y in
CH3CN/H2O (1:1). In our AP systems, TEOA was chosen as SR.
6
photocatalytic behavior and proton relay in the outer
coordination sphere may play a role.
As shown in Table 1, 6, Eosin Y and TEOA are all necessary for
photocatalytic H2 production (run 1-3).
We then fixed the concentrations of the three components
at 0.2 mM for 6, 0.05 mM for Eosin Y and 5% volume ratio for
Results and discussion
TEOA, and examined the influence of solution pH on the H2
generation activities of our AP systems. As shown in Figure 1b,
with the gradual increase of the solution pH from 4.0 to 10.0,
the turnover number (TON, vs. HER) of the system after 5 h
irradiation increased at first and then decreased.
Design of HER
To improve activities of HERs, pendant amine groups have
been utilized widely to serve as proton relays to mimic the
structure and function of [Fe-Fe]-hydrogenase enzymes.13-15
Similar to the amine group, the carboxylic acid/carboxylate
group is also a proton relay candidate. Though electrocatalytic
HERs using pendant carboxylic acid/carboxylate group as
proton relay have been examined,11c, 14c, 16 to the best of our
knowledge, such a strategy has not been explored yet in
developing new photocatalytic HERs, which becomes the
impetus of this work.
Table 1. Photocatalytic H2 production of the selected AP
systems.
run HER Eosin HER
TEOA
Solvent
TON
(5h)
Y
(mM) (%,v/v) (1:1,v/v)
As a proton relay, the carboxylic acid/carboxylate group
should be in close proximity to the reaction center of the
(mM)
0
1
2
3
4
5
6
7
6
6
6
6
6
1
1
0.2
0
5
5
0
5
4
5
5
CH3CN/water
CH3CN/water
CH3CN/water
0
0
0
cobaloxime catalysts. Taking this in mind, the complexes
2 and
0.05
0.05
0.05
6
were synthesized, in which the carboxylic acid group is either
0.2
0.2
attached directly to the axial pyridine ligand at the meta-
position or through a short but flexible CH2CH2 chain. To better
understand the role of the carboxylic acid group in
CH3CN/water 60
CH3CN/water 111
CH3CN/water 55
CH3CN/water 72.6a
0.025 0.1
0.05
0.05
0.2
photocatalytic H2 production, the complexes
prepared as controls. Additionally, the complexes
were also studied for comparison. While and
studied before as HERs,4b, 9a
and
3
and
7
were
and
had been
7 are all new
0.248
1,
4
5
a, a TON of 360 vs. PS was obtained, corresponding to a TON of
72.6 vs. HER 9a
1
5
2
,
3,
4,
6
compounds. Modification at the ortho-position of the axial
2 | J. Name., 2012, 00, 1-3
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a
ARTICLE
b
120
100
80
60
40
20
0
1
2
3
4
5
6
7
50
50
Complex 6
pH4
pH5
pH6
pH7
40
30
20
10
0
40
pH7.5
pH8
pH9
30
pH10
20
10
0
0
1
2
3
4
5
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
Time (h)
Time (h)
pH
Figure 2. Photocatalytic H2 production profiles of the AP
systems containing 0.025 mM Eosin Y, 0.1 mM HER and 4%
TEOA in CH3CN/H2O (1:1, pH = 7.5).
Figure 1
．
(a) Photocatalytic H2 production profiles of the AP
systems containing 0.05 mM Eosin Y, 0.2 mM
6, and 5%TEOA
in CH3CN/H2O (1:1) of varied pH. (b) H2 production amounts of
the AP systems after 5 h irradiation.
In our experimental conditions (pH = 7.5), the COOH group
should be deprotonated and becomes a weak electron-
donating functionality, since the pKa values of isonicotinic acid,
nicotinic acid, and acetic acid are 1.94, 3.13, and 4.75,
The maximum TON was obtained at pH 7.5. The same trend
was also observed in other irradiation time windows, e.g. 1 h
or 3 h (Figure 1a). Below pH 5, no H2 generation was observed,
and above pH 10, the H2 amounts were less than 20% of that
obtained at pH 7.5. At pH lower than 5.0, the protonation of
TEOA eliminates its electron-donating feature, whereas, at pH
higher than 9.0, the thermodynamic driving force for
protonation of either Co(I), Co(0), or Co(III)H and Co(II)H
becomes unfavorable.9a Similar pH effects have been observed
in many AP systems.3,4, 19
respectively.20 Interestingly, compared with
5,
the
photocatalytic activity of 4 has a further increase albeit not
very large (Figure 2). In sharp contrast, Eisenberg and
coworkers observed a decrease in H2 production activity when
1
is substituted by a strong electron-donating group of N(CH3)2
at the para-position of pyridine.4b
It has been well recognized that introducing an electron-
withdrawing substituent in the axial pyridine ligand will shift
Similar to solution pH, other factors, including solution
composition (Figure S2) and the concentrations of TEOA
the reduction potentials of the oxidation states of
a
(Figure S3),
6 (Figure S4), and Eosin Y (Figure S5), also affected
cobaloxime anodically, facilitating their electron harvesting
from an excited PS and thus H2 production.21 On the other
hand, the electron-withdrawing substituent will decrease the
pKa values of the oxidation states of a cobaloxime, hindering
their protonation and thus H2 production.21 Such opposite
effects will also appear in the case of electron-donating
substituent. Theoretically, both electron-donating and
electron-withdrawing groups may improve the photocatalytic
activity of a HER, provided the modulation on reduction
potentials and pKa values are properly balanced. Bearing these
considerations in mind, it is not surprising anymore that both
COOCH3 (electron-withdrawing) and COOH (electron-donating
in our conditions) can lead to activity improvement of the AP
systems.
the photocatalytic activities of the systems remarkably. After
careful optimization, the best condition was set up in which
the concentrations of 6, Eosin Y, and TEOA are, respectively,
0.1 mM, 0.025 mM, and 4% in volume ratio, while the solution
pH is 7.5 and the solution composition is 1:1 of CH3CN/H2O. In
this optimal condition, a TON (vs. HER) of 111 (5 h irradiation)
and a TOF (vs. HER) of 44 h-1 (during the initial 2.5 h) were
obtained for 6 (Figure 2).
Photocatalytic activity comparison of HERs
Under the optimized condition, the HER activities of 1-7 were
compared in detail (Figure 2 and Table 2). For 1, a TON (vs.
HER) of 55 was obtained after 5 h irradiation. Under a similar
condition (different light source), Eisenberg and coworkers
Once the meta-substitution is compared with the para-
substitution, interesting phenomena may be observed
immediately, i.e. the photocatalytic activities were augmented
reported a TON (vs. HER) of 72.6 for an AP system containing
1,
Eosin Y and TEOA.9a When the electron-withdrawing COOCH3
group is anchored at the para-position of pyridine, a small but
further and followed the order of
6 > 7 > 2 > 4 > 3 > 5 > 1
reproducible enhancement in H2 production was observed for
(Figure 2). Such activity order is highly reproducible in five
independent measurements, though the TON differences are
not significant between the adjacent catalysts in this order. It
is well known that both electronic induction effect and
electronic conjugation effect are much lower at meta- than at
para-position. Though the observed activity enhancement in
the cases of meta-substitution might be attributed to a better
balance between the modulation of reduction potentials and
pKa values, a through-space effect cannot be rule out,
5
(Figure 2). In a 5-based AP system using a Pt complex as PS,
Eisenberg and coworkers also found a slight improvement in
photocatalytic H2 production with respect to 1-based system.4b
Table 2. The TON for the AP systems containing 0.025 mM Eosin Y,
0.1 mM HER and 4% TEOA in CH3CN/H2O (1:1, pH = 7.5) .
complex
TON(5h)
1
2
3
4
5
6
7
particularly, in the case of
6. For 6, its carboxylic acid or
79
102
88
98
83
111
102
carboxylate group may interact with dmgH via H-bonding, and
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thus playing a role of proton relay. Unlike HERs of DuBois-type,
in which the pendant amine may direct a proton directly to the
hydride atom coordinated on the Co center, the carboxylic
H
BF2
BF2
N
N
N
N
O
H
O
O
N
N
O
N
N
O
O
acid/carboxylate group in
6 cannot approach the hydride atom
Co
N
Co
N
O
O
which is on the opposite side of the [Co(dmgH)2] plane, and
have to find a relay to realize proton transfer, as shown in
Scheme 2. Clearly, the flexible -CH2CH2- alkyl chain makes such
interaction possible. Recently, Norton and coworkers observed
the UV-Vis absorption spectra of Co(dmgBF2)2L2 in CH3CN
under H2 pressure of up to 70 atm.22 In this work, a Co(I)
compound with an exchangeable proton is proposed to have
an equilibrium with Co(III)H species, as shown in eq1,22a
suggesting proton relay in the manner shown in Scheme 2 is
feasible. We performed theoretical calculation and obtained
an optimized structure as shown in Scheme 3, in which the
distance (1.738 Å) between the H atom of the COOH group
and the O atom of one dmgH moiety falls in the region of O-
H…O bonding,23 supporting our proposed mechanism. Kahnt,
F2B
L
F2B
L = CH3CN
------ (1)
Guldi and coworkers studied the kinetics of the reduction of
1
in detail and proposed a dimer structure as the key
intermediate, where the dmgH ligand and the solvent water
molecules make
interaction.22b
a
contribution through H-bonding
Scheme 3. Optimized ground-state structure of 6.
In addition, by combination of theoretical calculations and
electrochemical experiments, Artero, Field, and coworkers24 as
well as Hammes-Schiffer and coworkers25 proposed an
electrocatalytic mechanism for cobalt diimine-dioxime
complexes, in which the first proton-coupled electron transfer
process leads to a Co(I) compound containing a protonated
oxime bridge, Co(I)LH. Then, the second proton-coupled
electron transfer process gives rise to a Co(II) hydride species
bearing a protonated ligand, Co(II)HLH. Finally, H2 is produced
either through external protonation or through an
intramolecular mechanism.26 The photocatalytic mechanism
shown in Scheme 2 is very similar to this mechanism in the
ligand protonation. The difference lies in that proton relay by
way of the COOH group might play a role in the ligand
Singlet excited state quenching of PS
Generally, an AP system may realize photocatalytic H2
production through either reductive quenching mechanism or
oxidative quenching mechanism, depending on which process,
between quenching of the excited PS by SR or HER, is
preferential. To understand the mechanism involved in our AP
systems, the fluorescence quenching of Eosin Y by the
examined cobaloxime complexes and TEOA were investigated.
As shown in Figure S6-S13,
fluorescence of Eosin Y effectively. Based on Stern-Volmer
equation (eq 2), where I0 and are the integrated emission
intensity of Eosin Y in the absence or presence of a quencher,
and
0 (4.73 ns27) is the singlet excited state lifetime of Eosin Y
1-7 and TEOA may quench the
I
protonation in the complex
6.
τ
without the presence of any quencher, the bimolecular
quenching rate constants of the complexes and TEOA were
calculated, as shown in Table 3. The apparent quenching rate
constants of the complexes are higher than the diffusion
H
H
H
H
N
N
N
N
O
O
N
N
N
N
H
O
O
O
O
Co0
CoII
N
O
O
CoI
O
O
O
N
N
O
N
H
× ℃
1010 M-1s-1 for CH3CN at 25 ),28
e
H
O
H
O
H
O
constant of solvent (2.2
N
N
O
O
N
O
implying the presence of static interaction between Eosin Y
and the complexes. Though the quenching rate constants of
the cobaloxime complexes are two orders of magnitude larger
than that of TEOA, the singlet excited state of Eosin Y should
be mainly quenched by TEOA in our AP systems, since the
concentration of TEOA (4% in volume ratio) is three thousand
times higher than those of the complexes.
H
H
e
H
N
H
O
H2
H
H
N
N
O
N
N
O
O
O
CoII
CoII
O
O
N
O
N
N
H
O
H
O
N
O
N
O
I0/I = 1 + kqτ0[Q]
------ (2)
Scheme 2. A possible proton relay mechanism in
6.
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Table 3. Fluorescence quenching rate constants of Eosin Y by
TEOA and the cobaloxime complexes in CH3CN/H2O (1:1, pH =
7.5)
b
Quencher
1
2
3
4
5
6
7
TEOA
kq
2.7 27.0 5.4 18.8 6.0 10.5 3.9 0.028
(1011M-1s-1)
From Table 3, one may find that the quenching rate
constants of the complexes substituted by COOH group are
3~5-fold higher than that by COOCH3 group, e.g.
2 vs. 3, 4 vs. 5
and 6 vs. 7, in line with their photocatalytic H2 production
activities. Due to ionization in pH 7.5 solution, the COOH group
will take a negative charge and presents an electron-donating
feature, both disfavor the electron harvesting from anionic
Eosin Y. A possible explanation may be that the COOH group
may interact with Eosin Y through H-bonding, facilitating the
photoinduced electron transfer process between them.
However, the overall trend of fluorescence quenching
efficiencies of these cobaloxime catalysts is not in line with
their photocatalytic H2 production activities. For example, both
c
2
and
efficiently than
activities are lower than that of
4
can quench the fluorescence of Eosin Y more
, yet their photocatalytic H2 production
. These results suggest that
6
6
10-5M)
c) Eosin
the photocatalytic activities should not be related to the
photoinduced electron transfer from Eosin Y to the catalysts. A
proton relay mechanism might make somewhat contribution
Figure 3. Transient absorption spectra of (
alone, ( ) Eosin Y (1
10-5M) and TEOA (4
Y(1 (1
10-5M) and 10-4M) in CH3CN/H2O (1:1, pH = 7.5)
a
) Eosin Y (1
×
b
×
％
,v/v) and (
×
6
×
to the unique performance of 6.
the decay of the transient spectra at 560 nm, the triplet
excited state lifetimes of Eosin Y were measured to be 100μs
and 6.3μs, respectively, in the absence and presence of TEOA,
suggesting an efficient reductive quenching. In sharp contrast,
Triplet excited state quenching of PS
Due to the high intersystem crossing efficiency of Eosin Y (Φisc
= 0.8
±
0.08),26 its triplet excited state should take part in the
the transient absorption spectra of Eosin Y in the presence of
6
photocatalytic H2 production also. Figure 3a shows the
transient absorption spectra of Eosin Y obtained upon
irradiation of 532 nm-laser in CH3CN: H2O (1:1, pH = 7.5).
Besides the strong ground state bleaching centered at 520 nm,
a weak absorption band at around 560 nm was also observed,
which may be attributed to the triplet excited state absorption
of Eosin Y.28 When 4% TEOA was present, the transient
absorption spectra underwent significant changes (Figure 3b).
One peak centered at 410 nm appeared, assignable to the
radical anion of Eosin Y29 and indicating the electron transfer
from TEOA to the triplet excited state of Eosin Y. By monitoring
(0.1mM) are nearly the same as that of Eosin Y alone in both
spectrum profile and decay kinetics(Figure 3c), demonstrating
that the oxidative quenching cannot occur in the triplet excited
state of Eosin Y.
Electrochemical properties
To get more insight into the photocatalytic H2 production
behavior of 6, its electrochemical properties were also studied.
As shown in Figure 4a, upon cathodic scan, an irreversible
wave at -0.66 V (vs. SCE, peak potential) was followed by a
reversible redox process at -0.92 V (half wave potential),
assignable to Co(III) to Co(II) and Co(II) to Co(I), respectively.
The irreversible character of the first reduction process
suggests that the axial Cl ligand may dissociate from the Co(II)
center. Further scan to more negative potential led to a new
wave at -1.64 V, which may be attributed to the reduction of
Co(I) to Co(0). Many cobaloxime complexes have similar redox
processes at similar potentials.24a,30
a
Upon addition of acetic acid, large catalytic reduction
current was observed beyond -1.5 V (As shown in Figure 4b). In
the absence of 6, reduction currents in this electrochemical
window were much lower even in the presence of 15 equiv
acetic acid (As shown in Figure S14).The large catalytic current
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ARTICLE
a
Journal Name
Materials
b
Cobaltous chloride hexahydrate, methyl isonicotinate, and
methyl nicotinate were purchased from Adamas.
Dimethylglyoxime (dmgH2), pyridine, niacin acid, isonicotinic
acid, 3-pyridinepropionic acid, methyl 3-(3-pyridyl) propionate,
triethanolamine (TEOA), tetrabutylammoniumhexafluorophos-
15 equiv acetic acid
12 equiv acetic acid
9 equiv acetic acid
6 equiv acetic acid
3 equiv acetic acid
0 equiv acetic acid
6
0.4
0.3
0.2
0.1
0.0
0.02
0.01
0.00
-0.01
phate
，and Eosin Y were obtained from Alfa. All solvents were
-2.0 -1.5 -1.0 -0.5 0.0
E vs. SCE / V
-2.0 -1.5 -1.0 -0.5
E vs. SCE / V
0.0
of analytical purity and used without further treatment.
Distilled water was used in all experiments.
Figure 4. Electrochemical properties of
6
in CH3CN in the
[Co(dmgH)2pyCl](
[Co(dmgH)2(py-m-COOCH3)Cl](
), [Co(dmgH)2(py-p-COOCH3)Cl](
CH2CH2COOH)Cl]( ), [Co(dmgH)2(py-m-CH2CH2COOCH3)Cl](7)
1
),
[Co(dmgH)2(py-m-COOH)Cl](
), [Co(dmgH)2(py-p-COOH)Cl]
), [Co(dmgH)2(py-m-
2),
absence (
acid.
a) or presence (b) of varied concentrations of acetic
3
(
4
5
6
were prepared following the reported methods,4b,17 and their
1H NMR and high resolution MS data are included in
supporting information (ESI).
may be attributed to the electrocatalytic H2 production
process of the Co(0) species. The redox process of Co(II)/Co(I)
remained reversible, demonstrating that the pKa of Co(I) is not
high enough to be protonated by acetic acid. Thus, the
Instruments
electrocatalytic H2 production of
3 –eq 8).31
6 may proceed as follows (eq
1H NMR spectra were obtained on a Bruker DMX-400 MHz
spectrophotometer. HR-MS were taken on a Brucker APEX IV
(7.0T) FT_MS. UV-Vis absorption spectra were recorded on a
Shimadzu UV-1601 spectrophotometer. Fluorescence emission
Co(III) + e-
Co(II) + e-
→
Co(II)
(3)
(4)
(5)
(6)
(7)
(8)
spectra were run on
spectrophotometer.
H2 evolution studies
a Hitachi F-4600 fluorescence
→
Co(I)
Co(I) + e-
→
Co(0)
A 10 mL of CH3CN/H2O (1:1, v/v) solution containing 0.025 mM
Eosin Y, 0.1mM cobaloxime complex, and TEOA (4%, v/v) were
put into a 40 mL glass vial equipped with a rubber-septum-
sealed outlet. After bubbling with argon for 25 min, 3 mL of
gas in the vial was removed by a syringe and 3 mL of methane
was then injected into the vial to serve as the internal standard
for H2 quantification. The vial was irradiated with visible light,
which was obtained from a 1000 W solar simulator (Oriel
91192) using a 400-nm-long pass glass filter and a distilled
water pool to cut off the UV and IR light, respectively. The
production of H2 was monitored and quantified by gas
chromatography on a Shimadzu GC-2014 (thermal conductivity
detector, 5 Å molecular sieve, 30 m×0.53 mm column, N2 gas
carrier).
Co(0) + H+
→ Co(II)H
Co(II)H + H+
→
Co(II) + H2
2Co(II)H
→
2Co(I) + H2
Conclusions
In this work, the photocatalytic H2 production activities of the
cobaloxime complexes were compared in
1-7
a
multicomponent system, using Eosin Y as PS and TEOA as SR.
No matter the substituent in the axial pyridine ligand is
electron-withdrawing or electron-donating in nature, the
corresponding complexes showed enhanced photocatalytic
Electrochemistry
The redox potentials were measured on an EG&G model 283
potentiostat/galvanostat in a three-electrode cell with a
platinum-wire working electrode, a platinum-plate counter
electrode, and a SCE (saturated calomel electrode) reference
electrode. Cyclic voltammetry was conducted at a scan rate of
50mV s−1 in an argon-saturated anhydrous CH3CN containing
0.1 M n-Bu4NPF6 as the supporting electrolyte.
activities with respect to their parent complex
1. More
interestingly, meta-substitution is somewhat more efficient
than para-substitution and COOH is slightly more efficient than
COOCH3, in improving the photocatalytic H2 generation activity.
In particular,
6 showed the highest activity among the
examined complexes. Its unique structure, i.e. the short but
flexible methylene chain, makes the direct interaction
between COOH and dmgH possible, hinting at a role of proton
Quenching experiments
All quenching experiments were done by adding calibrated
amounts of quencher into the Eosin Y solution at room
temperature. The Eosin Y concentration was fixed at 1.0×10-6
M in CH3CN/H2O (1:1, v/v). The solutions were degassed with
argon for 25 min. Steady-state luminescence spectra were
then collected for each sample. The relative emission intensity
of the chromophore in the absence and presence of a
relay in 6. These interesting findings encourage us to develop
new proton relay based cobaloxime catalysts for more efficient
photocatalytic H2 production.
Experimental
6 | J. Name., 2012, 00, 1-3
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4952-4962; (c) X. H. Wang, S. Goeb, Z. Q. Ji, N. A.
quencher was used to calculate the quenching rate constant
by the Stern-Volmer equation.
Pogulaichenko and F. N. Castellano, Inorg. Chem., 2011, 50
,
Transient Absorption
705-707.
Nanosecond transient absorption measurements were
performed on a LP-920 laser flash photolysis setup (Edinburgh).
Excitation at 532 nm with a power of 2.0 mJ pulse−1 from a
computer-controlled Nd: YAG laser/OPO system from
Opotek(Vibrant 355 II) operating at 10 Hz was directed to the
sample with anoptical absorbance of 0.96 at the excitation
wavelength. The laser and analyzing light beam passed
perpendicularly through a 1 cm quartz cell. The complete time-
resolved spectra were obtained using a gated CCD camera
(AndoriSTAR); the kinetic traces were detected by a Tektronix
TDS 3012B oscilloscope and a R928P photomultiplier and
analyzed by Edinburgh analytical software (LP920). All samples
used in the flash photolysis experiments were deaerated for 30
min with argon before measurements.
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All calculations were performed with the Gaussian 03 (G03)
program package18a employing the DFT method with Becke’s
three-parameter hybrid functional18b and Lee-Yang-Parr’s
gradient corrected correlation functional (B3LYP).18c The
LanL2DZ basis set and effective core potential were used for
the Co atom, and the 6-31 G* basis set was applied for H, C, N,
O and Cl. The ground-state geometries of the complex were
optimized in CH3CN using the conductive polarizable
continuum model (CPCM).
2115-2120.
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Acknowledgements
This work was financially supported by the Ministry of Science
and Technology (2013CB933801) and NSFC (21390400,
21172228, 21273259, 21301182).
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[Co(III)(dmgH)2(py-m-CH2CH2COOH)Cl] showed much improved photocatalytic H2 production
activity than Co(III)(dmgH)2(py)Cl], and the COOH group may serve as a proton relay to account
for its promising performance.