Ligand assisted electrocatalytic water oxidation by a copper(ii) complex in neutral phosphate buffer

Article abstract of DOI:10.1039/c9cc00971j

The electrocatalytic water oxidation activity of a copper(ii) complex, 1, [Cu(L₁H)(L₁)(OH₂)](ClO₄), with a redox active aryl oxime ligand, L₁H [L₁H = 1-(pyridin-2-yl) ethanone oxime] has been investigated. Complex 1 shows a remarkably high turnover frequency of ～100 s^-1 in neutral phosphate buffer at about 675 mV overpotential with ～94% faradaic efficiency. Electrochemical analysis suggests the involvement of a ligand moiety in a proton-coupled-electron-transfer (PCET) step during the catalytic cycle of complex 1, which in turn provides a route for accumulation of high oxidizing equivalents at the reaction center.

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Ligand Assisted Electrocatalytic Water Oxidation by a Copper (II)
Complex in Neutral Phosphate Buffer
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
a
a
Hemrupa Kuilya, Noohul Alam, Debajit Sarma, Diganta Choudhury,* and Apurba Kalita*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Electrocatalytic water oxidation activity of a copper (II) complex,
, [Cu(L H)(L )(OH )](ClO ), with a redox active aryl oxime ligand, [Cu (L
H [L H = 1-(pyridin-2-yl) ethanone oxime] has been investigated. nitrogen donor atoms from two ligands and one coordinated
Complex 1 shows a remarkably high turnover frequency of ~100 s
in neutral phosphate buffer at about 675 mV overpotential with Cu-O bond length of 2.284 Å was found between the axial
94% Faradic efficiency. Electrochemical analysis suggest the aqua ligand and the central copper atom. The close proximity
The single-crystal X-ray structure of complex 1,
H)(L )(OH )] , revealed that Cu (II) is surrounded by four
1 1 2
II
+
1
1
1
2
4
L
1
1
-1
water molecule in a square pyramidal geometry (Figure 1). A
~
involvement of ligand moiety in a proton-coupled-electron- of the hydroxyl group gives rise to interesting hydrogen bond
transfer (PCET) step during the catalytic cycle of complex 1, which interactions of the O–H•••O type. The important O–H•••O
in turn provides a way for accumulation of high oxidizing interactions with O•••O contact distances are in the range of
equivalent at the reaction center.
2.415(9) – 3.14(2) and the O–H•••O bond angles are in the
range of 150 – 164 (Table S2). This suggests that the hydrogen
Electrocatalytic oxidation of water by transition metal bond interactions are relatively strong. Complex 1 in neutral
-1
-1
complexes is a highly active field of research at present time phosphate buffer exhibit broad d-d bands at λmax (ε/ M cm )
due to its implications in the conversion of electrical energy to = 580 nm (138), along with relatively strong intraligand
chemical fuels.1 A large number of metal complexes have absorptions in the UV region (Figure S6). Complex 1 show
been reported with the ability of electro catalytic oxidation of magnetic moment corresponding to one unpaired electron
water in neutral, acidic and basic medium. In view of cost (μobs = 1.7 μB). Molar conductivity of complex 1 was measured
-6
2
-1
effectiveness as well as robustness, copper based complexes to be 117 Scm mol in methanol medium which indicates that
have emerged as the potential water oxidation catalyst. Since it behaves as an 1:1 electrolyte.
the report of the first copper based electrocatalyst by Mayer To investigate the electrochemistry of complex 1, cyclic
et al.,1 a large number of copper complexes have been voltammograms (CVs) were recorded in 0.1 M neutral sodium
7
-11
reported with successive improvement in catalytic activity.
Copper complexes containing redox active ligand framework electrode in a single compartment electrochemical cell.
phosphate buffer using glassy carbon (GC) as working
7
-8
are reported to be efficient water oxidation catalyst. It is Potentials were measured versus an Ag/AgCl reference
noteworthy to mention that in Photosystem II, a redox active electrode and are reported versus the normal hydrogen
tyrosine residue (tyrosine Z or YZ) facilitate the oxidation of electrode (NHE) through the addition of 0.197 V to the
water by serving as a mediator in the electron transfer process measured potential. A typical CV of complex 1 is shown in
between catalytic center (CaMn
4
cluster) and the oxidant Figure 2(a).
In the present work we have
reported a copper (II) complex, 1, with a redox active oxime
ligand L H [L H = 1-(pyridin-2-yl) ethanone oxime] which
1
2-14
H
H
(photoexcited chlorophyll).
O
N
N
(ClO4)
Cu
1
1
N
O
N
O
efficiently oxidize water in neutral phosphate buffer with high
turnover frequency (TOF). The oxime ligands are chosen as
they are known to stabilize the metal centre at high oxidation
state due to the high concentration of negative charge on
H
(L1H)(L1)CuII(OH2)](ClO4)
[
Figure 1. Left: The structure of complex 1. Right: ORTEP
diagram (50 % thermal ellipsoid plot) of complex 1 (counter-
anion and solvent molecules are removed for clarity).
1
5-16
donor atoms and strong metal-ligand σ-bond formation.
a. Department of Chemistry, B. Borooah College, Guwahati 781007, Assam (India)
E-mail: apurbakalitabbc@gmail.com
Department of Chemistry, IIT Patna, Patna 801103, Bihar (India)
The differential pulse voltammogram of 0.5 mM solution of
complex 1 in neutral phosphate buffer from -0.3 to +1.6 V
exhibited five anodic peaks around -0.09, +0.64, +0.88, +0.93
and +1.49 V. The first anodic peak at E° = -0.09 V vs. NHE is
b.
†
Electronic Supplementary Information (ESI) available: Experimental Section, FT-
IR, UV-visible, NMR, EPR spectra, Crystallographic data. (CCDC 1879870). See
DOI: 10.1039/x0xx00000x
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II
I
associated with the Cu /Cu couple (Scheme 1). This Scheme 2. Oxidation of Copper (II) to Copper (III) at +0.64 V.
View Article Online
DOI: 10.1039/C9CC00971J
III
II
electrochemically quasi reversible redox couple exhibits a pH- The Cu /Cu couple is found to be quasi reversible when we record
dependence of 0.059 V per pH unit [Figure 2(b)] indicating the the cyclic voltammograms in the range 0 to 1.4 V vs NHE (Figure
involvement of one electron and one proton in this step.
S11) and the peak current varies linearly with scan rate (Figure S12).
-7
2
A diffusion coefficient (DCu) of 6.71 × 10 cm /s was determined for
complex 1 from the dependence of the anodic peak currents (ip,a
)
III
II
1/2
for the Cu /Cu couple on square root of scan rates (ʋ ) by
1
7
application of the Randles-Sevcik relation in equation 1.
ip = 0.446nFA[C]{(nFvDCu )/RT }1/2
(1)
-
In equation 1, n(= 1e ) is the number of electrons transferred, F is
the Faraday constant, A is the area of the working electrode (0.07
II
I
Scheme 1. Redox reaction of quasi reversible Cu /Cu couple.
2
cm ), R is the ideal gas constant, [C] is the concentration of the
catalyst, and T is the temperature.
The second anodic peak at +0.64 V vs. NHE decreases by 61 mV per
pH unit from pH 7.0 to 8.0 [Figure 2(b)] which is very close to the
value of 59 mV per pH unit predicted by the Nernst equation for 1e-
The third anodic peak near +0.88 V vs. NHE is also found to be pH
dependent (decreases by 0.06 V per pH unit) and is assigned for a
ligand based 1e / 1H redox process (Scheme 3) by comparing with
the cyclic voltammogram of the analogous Zn complex (Figure
S13) in neutral phosphate buffer. The fourth anodic peak at around
0.93 V is found to be pH independent. This peak is also assigned
for a ligand based redox process (Scheme 3). It is important to
mention here that aromatic Ketoxime ligands are known to be
redox active and generate iminoxy radical by monoelectronic
transfer and deprotonation step either chemically or
electrochemically. Through this Ligand based oxidation steps a
highly reactive species [(L )(L )Cu -OH] is believed to be formed
1 1
+
8
/
1H redox couple. This peak is assigned for the formation of a d
-
+
III
+
Cu (III) species, [(L
1
H)(L
1
)Cu -OH] ,
(Scheme 2).
A
similar
2
+
III
II
assignment of reduction potential for Cu /Cu couple was observed
in a recent report by Antoni Llobet et.al. UV-visible (Figure S9) and
7
+
EPR spectroscopic studies (Figure S10) of the solution obtained
from the control potential electrolysis (CPE) experiment at +0.64 V
vs. NHE also indicate the formation of Cu(III) species. Phosphate
buffer solution of complex 1 was found to display a characteristic
four line EPR spectrum at room temperature before electrolysis and
was observed to be EPR inactive after electrolysis at +0.64 V. This is
attributed to the formation of diamagnetic Cu(III) species from the
oxidation of Cu(II) complex.
1
8
●
●
III
+
with the concurrent accumulation of high oxidizing equivalent at
the reaction center.
Scheme 3. Ligand based oxidation of complex 1 at +0.88 and +0.93
V vs. NHE leading to formation of a reactive catalytic species.
●)(L
●
)Cu -OH] , once formed immediately
III
+
The reactive species, [(L
react with water to generate a Copper (III)- hydroperoxo species
(L
1 1
III
+
1 1
[ H)(L )Cu -OOH] , which again oxidise at +1.49 V to liberate
molecular oxygen and regenerate the Cu (II) species (Scheme 4).
1 1
Scheme 4. Reaction of reactive species, [(L )Cu -OH] with
●)(L
●
III
+
water to generate molecular oxygen.
Cyclic voltammetry measurements of complex 1 revealed that
catalytic current at 1.49 vs. NHE vary linearly with the
Figure 2. (a) Base line corrected cyclic voltammogram of complex 1
in 0.1 M pH 7 phosphate buffer with a glassy carbon (GC) working
V
-1
concentrations of the catalyst at a scan rate of 100 mVs (Figure
S14 and S15). Catalytic current increases sharply with the increase
in catalyst concentration upto a concentration of around 0.3 mM.
This first order kinetics suggests a single site mechanism for water
oxidation catalysis. The dependence of the catalytic current (icat) on
electrode,
a Ag/AgCl reference electrode and a Pt counter
-1
electrode, scan rate 100 mV s . (b) Differential pulse voltam-
mogram (DPV) of complex 1 in 0.1 M phosphate buffer at pH 7.0
(blue), 7.6 (green), 7.8 (red) and 8.0 (black).
1
7
the catalyst concentration can be described by equation 2.
icat = ncat FA[cat] ( kcat Dcu )1/2
(2)
In equation 2, ncat (= 4) is the number of electrons involved in water
oxidation and kcat is the rate constant for water oxidation.
2
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A comparison of scan-rate-normalized CVs (icat/ʋ1/2) at scan rates reaction conditions of some recently reported coppeVriebwaAsretidclewOantliener
-1
from 50 to 100 mV s show a decrease in icat with increasing ʋ oxidation catalysts.
DOI: 10.1039/C9CC00971J
(
Figure 3), which is consistent with a contribution to the current
1
7
from a rate-limiting water oxidation catalysis.
Figure 3. Cyclic voltammograms of complex 1 with currents
-1
normalized to the square root of the scan rates (50-100 mVs )
measured in 0.1 M neutral phsphate buffer.
CV carried out in D
S16), suggesting that the transfer of proton(s) from H
2
O shows a much smaller catalytic current (Figure
O is a
2
significant component of the rate-determining step. A solvent
kinetic isotope effect (KIE) of 2.46 was determined according to
1
9
equation 3.
KIE = k
/ k
= ( i
/ i
)2
cat, D O
2
(3)
cat, H O
cat, D O
cat, H O
2
2
2
To obtain kinetic information about the catalytic process, a foot-of-
the-wave analysis was carried out following the methodology
described by Saveant et al.20 Under catalytic conditions equation 4
is operative for an oxidation involving four electrons.
Figure 4. a) Background-corrected CVs of complex 1 in 0.1 M
neutral phosphate buffer at 10 (black), 15 (red), 20 (blue), 30
icat / i = [ 2.24 n (RT kobs / F n)1/2] / [1 + exp{(E° - E) F/RT}] (4) (green) and 50 (magenta) mVs scan rates. b) Foot-of-the-wave
-1
d
cat
analysis plotting icat/i
same color code).
vs. 1/[1+exp{(F/RT)(Eocat-E)}] at each scan rate
d
where E°cat is the standard potential for the catalysis-initiating
redox couple (1.49 V vs NHE), icat is the current intensity in the
(
presence of substrate, i
substrate (here we approximate this current to the current
associated with the Cu /Cu couple), n (= 4) is the number of
electrons involved in the catalytic cycle, F is the faraday constant, ν
d
is the current intensity in the absence of
The oxygen evolution was investigated through controlled potential
electrolysis (CPE) at 1.55 V vs. NHE with a large surface-area ITO
working electrode (4 cm ) in a gas-tight cell. The oxygen formed in
III
II
2
the solution was measured using a calibrated Ocean Optics FOXY
probe and a faradic efficiency close to 94 % was estimated in a long
period of electrolysis for 4 hour (Figure 5). CV and UV-visible
spectra of complex 1 recorded before and after bulk electrolysis
experiment (Figure S19 and S20) along with FE-SEM and EDX data of
fresh and used ITO working electrode shows decomposition of
negligible amount of complex 1 during long term electrolysis
experiment.
is the scan rate, kobs is defined as “kcat*C°
A
” where C°
A
is the
-
concentration of substrate (55.56 M for water), and R is 8.314 Jmol
1
-1
K .
Background-corrected CVs of complex 1 at different scan rates (10–
-1
5
0 mVs ) are shown in Figure 4(a). Now, kobs can be extracted from
the plot of icat/i versus 1/{1+exp[(F/RT)(E°cat -E)]} as shown in Figure
b and in Figure S17 in the supporting information. The largest
slope at the very beginning of the catalytic process gives an
d
4
-1
impressive value of kobs~100 s , which is equal to the turnover
frequency (TOF) of single-site water oxidation catalyst. To the best
Table 1. Kinetic data of complex 1 and some related copper based
water oxidation catalyst
of our knowledge, this is the highest TOF reported for
mononuclear copper complex in neutral phosphate buffer. The high
TOF recorded for complex 1 indicate the involvement of highly
a
Catalyst
TOF
pH
Overpotential
Ref.
-
1
(
s )
2
1
oxidized species in the catalytic cycle of the complex. Recently Li-
(bpy)Cu(OH)
2
100
33
13.1 ~ 750
1
2
2
Zhu Wu et. al, has reported the water oxidation activity of two
4-
]2-
[
[
[
[
(TGG )Cu-OH
Cu(TMC)(H
Cu
2
11
7
~ 520
810
2
-1
Cu
4 4 4
O cubane complexes with high TOF of 267 s for [(LGly- Cu) ] at
O)](NO
3
)
2
30
10
11
22
22
This
-1
2
1
.70 V and 105 s for [(LGlu-Cu) ] at 1.56 V in basic pH.
4
(BPMAN)(µ-OH)]3+ 0.6
7
~ 800
620
2
The onset potential for water oxidation is located near 1.491 V vs
L
Gly-Cu)
4
]
267
105
100
12
12
7
NHE and an over potential for 1 mM solution of complex 1 to attain
2
a current density of 1mA/cm was determined to be ~675 mV
[LGlu-Cu)4]
Cu(L H)(L
760
(Figure S18). In Table 1, we summarize the TOF values as well as the
[
1
1
)(OH
2
)]ClO
4
675
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pH calculated from the FOWA for complex 1 indicatesVitehweArpticrloebOanblinlee
involvement of highly oxidized species in thDeOcI:a1t0a.l1y0t3ic9/cCy9cCleC.0In09t7h1eJ
The total charge passed after 1 hr. of electrolysis under the same
condition varied linearly with the initial concentration of complex 1
with no evidence for an induction period at an early stage of
electrolysis, providing evidence for homogeneous single site water
oxidation catalysis (Figure 6). CVs of the ITO working electrode
before and after each electrolysis experiment in the absence of
catalyst were essentially identical with no evidence for deposition
of electroactive species to the working electrode surface during
1
electrocatalytic cycle of complex 1, the pyridyl oxime ligand (L H)
assisted in the accumulation of high oxidizing power at the reaction
center via involving in the proton-coupled-electron-transfer (PCET)
process and mimics the functions of tyrosine
photosynthesis.
Z of natural
This work was supported by DST (SERB) of India (award no.
YSS/2015/000878). The authors are also grateful to CIF IIT Guwahati
and SAIF IIT Patna for the EPR and single crystal XRD data.
electrolysis (Figure S21). The lack of evidence for
a new
electroactive species which is either adsorbed on the working
electrode or present in the electrolyte mixture is collectively Notes and references
consistent with homogeneous catalysis.
1
2
3
4
5
.
.
.
.
.
S. M.Barnett, K. I. Goldberg and J. M. Mayer, Nat. Chem., 2012,
, 498-502.
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Meyer, J. Am. Chem. Soc., 2014, 136, 5531-5534.
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Hayami, V. K. K. Praneeth, M. Yoshida, K. Yoneda, S. Kawata
and S. Masaoka, Nature, 2016, 530, 465-468.
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M. Blasco-Ahicart, J. Soriano-López, J. J. Carbó, J. M. Poblet and
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37, 6758-6761.
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1. X. J. Su, M. Gao, L. Jiao, R. Z. Liao, P. E. M. Siegbahn, J. P. Cheng
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Figure 5. (a) Bulk electrolysis with (blue line) and without (red line)
complex 1 in 0.1 M neutral phosphate buffer at 1.55 V versus NHE.
Working electrode, ITO; reference electrode, Ag/AgCl; counter
electrode, Pt. (b) The oxygen evolution during the bulk electrolysis
at an ITO electrode at 1.55 V versus NHE for 4 hr with (blue line)
and without copper complex (red line). Black line indicates the
theoretical amount of oxygen as assumed by charge passed with
2
013, 46, 1588-1596.
3. A. Klauss, M. Haumann and H. Dau, Proc. Natl. Acad. Sci., 2012,
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4
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972, 11, 2064-2069.
1
00% faradic efficiency.
1
1
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fundamentals and applications, Wiley, New York, 2001.
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2
2
Figure 6. (a) Plot of bulk electrolysis with different concentration of
complex 1. (b) Plot of total charge vs. concentration of complex 1
after 1hr of electrolysis.
1
F. L. Formal, E. Pastor, S. D. Tilley, C. A. Mesa, S. R. Pendlebury,
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In conclusion, complex 1 behaves as highly efficient, robust,
2
2
X. Jiang, J. Li, B. Yang, X. -Z. Wei, B. -W. Dong, Y. Kao, M. -Y.
Huang, C. -H. Tung and L. -Z. Wu, Angew. Chem. Int. Ed., 2018,
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homogeneous electrocatalyst for water oxidation in neutral
-1
phosphate buffer. The high turnover frequency (~100 s ) in neutral
4
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