Homogeneous photochemical water oxidation with metal salophen complexes in neutral media

Article abstract of DOI:10.1039/c9pp00254e

The development of water oxidation catalysts based on Earth-abundant metals that can function at neutral pH remains a basic chemical challenge. Here, we report that salophen complexes with Ni(ii), Cu(ii), and Mn(ii) can catalyse photochemical water oxidat

Full text of DOI:10.1039/c9pp00254e

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Homogeneous photochemical water oxidation
with metal salophen complexes in neutral media†
Cite this: Photochem. Photobiol. Sci.,
2019, 18, 2782
Md. Ali Asraf,a,b Chizoba I. Ezugwu,a C. M. Zakariab and Francis Verpoort
*
a,c,d
The development of water oxidation catalysts based on Earth-abundant metals that can function
at neutral pH remains a basic chemical challenge. Here, we report that salophen complexes with Ni(II),
Cu(II), and Mn(II) can catalyse photochemical water oxidation to molecular oxygen in the presence of
[Ru(bpy)3]2+ as a photosensitizer and Na2S2O8 as an oxidant in phosphate buffer of pH 7.0. Experimental
results including CV, SEM, EDS, ESI-MS, and DLS measurements on the metal salophen complex-
catalysed water oxidation to oxygen suggest that the catalytic activity of the catalysts is molecular in origin.
Received 2nd June 2019,
Accepted 30th September 2019
DOI: 10.1039/c9pp00254e
rsc.li/pps
capable of stabilizing metals in different oxidation states in
four coordination geometries and controlling the activity of
Introduction
Solar energy is employed with excellent efficiency to provide metals in a large diversity of productive catalytic transform-
high-energy chemicals such as sugar by photosynthesis, ations.64 Schiff base metal complexes are of enormous impor-
during which nature’s cuboidal CaMn4O5 oxygen-evolving tance for catalysis.65 More recently, it is reported that cobalt
complex (OEC) of photosystem II (PSII) oxidizes H2O to extract salophen and salen complexes can act as homogeneous or
protons and electrons.1–6 In order to mimic the function and heterogeneous catalysts for the oxidation of water, depending
structure of the OEC CaMn4O5 in PSII, scientists have made on the reaction conditions.29,38,66 Inspired by these reports, we
considerable efforts towards the development of the model decided to check the water oxidation activity of other metal
complexes of PSII for water oxidation.7–12 For instance, numer- salophen complexes such as copper salophen, nickel salophen,
ous manganese complexes have been examined for the oxi- and manganese salophen. From the literature, it is found that
dation of water.7,13–16 Additionally, a series of complexes and most of the water oxidation catalysts operate under acidic or
oxides of Ru and Ir have been studied as WOCs, since the late basic conditions, whereas only few operate at neutral pH.67–72
1970s.17–23 Nevertheless, because of the low abundance and The development of WOCs made of Earth-abundant, first-row
high price of these noble metals, limitations might arise for metals that operate under neutral conditions remains a basic
the application of these catalysts in establishing an artificial challenge. Herein, we report that metal [Cu(II), Ni(II) and
photosynthetic system.1 As a consequence, water oxidation cat- Mn(II)] salophen complexes act as stable catalysts in photo-
2+
alysts with low overpotential and high efficiency, made of chemical water oxidation in the presence of Ru(bpy)3 as the
cheap and abundant metals, are extremely desirable.24–28 In photosensitizer and S2O8 ions as sacrificial electron accep-
2−
recent years, catalysts (homogeneous and heterogeneous) tors at neutral pH.
based on first-row transition metals have attracted a lot of
attention for the oxidation of water.29–62 Salophen and Salen
Schiff bases can be synthesized by the condensation of amines
Experimental section
Materials
and aldehydes under different reaction conditions.63 They are
All chemical reagents, including o-phenylenediamine (99.0%),
salicylaldehyde (98.0%), copper acetate monohydrate
aLaboratory of Organometallics, Catalysis and Ordered Materials, State Key
Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan
University of Technology, Wuhan 430070, China.
(Cu(OAc)2·H2O;
99.5%),
nickel
acetate
tetrahydrate
(Ni(OAc)2·4H2O; 98.0%), manganese acetate tetrahydrate
(Mn(OAc)2·4H2O; 98.0%), tris(2,2′-bipyridyl)dichlororuthenium(II)
hexahydrate ([Ru(bpy)3]Cl2·6H2O; 99.95%), sodium persulfate
(Na2S2O8; 99.99%), sodium dihydrogen phosphate anhydrous
(NaH2PO4; 99.0%), sodium phosphate dibasic (Na2HPO4; 99.0%),
sodium hydroxide (NaOH; 96.0%), ethanol (EtOH; 99.7%), and
methanol (MeOH; 99.5%), were purchased from Aldrich or
E-mail: Francis.Verpoort@ghent.ac.kr; Fax: +862787879468; Tel: +8615071172245
bDepartment of Chemistry, Rajshahi University, Rajshahi-6205, Bangladesh
cNational Research Tomsk Polytechnic University, Lenin Avenue 30, 634050 Tomsk,
Russian Federation
dGhent University, Global Campus Songdo, 119 Songdomunhwa-Ro, Yeonsu-Gu,
Incheon, Korea
†Electronic supplementary information (ESI) available: Characterization studies
(1H and 13C NMR). See DOI: 10.1039/c9pp00254e
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Across. All chemicals and solvents were used as received. Purified phate buffer of pH 7.0 using a glassy carbon electrode. The
water obtained from Milli-Q system (resistivity: ∼18 MQ cm) was electrolysis was carried out for 2 h under rapid (400 rpm) stir-
used for the preparation of aqueous solutions.
ring at 1.2 V. After 2 h of bulk electrolysis, the glassy carbon
electrode was taken out and gently rinsed with Millipore water,
air dried, and then dried under vacuum. Field-Emission
Spectroscopic measurements
1H and 13C NMR spectra were recorded with a Bruker 500 MHz Scanning Electron Microscopy (FE-SEM) images and SEM-EDX
NMR spectrometer using TMS as the internal standard and data were obtained using a ZEISS ULTRA PLUS-43-13 on con-
[d6] DMSO as the solvent. ESI-MS spectra were performed nection to an Energy-Dispersive Spectrometer (OXFORD X-Max
using an Agilent Technologies MSD SL Trap mass spectrometer 50). The acceleration voltage was 15 kV.
with an ESI source coupled with an 1100 Series HPLC system.
Synthesis of the salophen ligand (Slp)
FT-IR spectra were recorded using a Nicolet 5700 FT-IR instru-
ment. DLS (dynamic light scattering) measurements were per-
formed using a Zetasizer Nano ZS; Malvern Instruments (par-
ticle size distribution from 0.6 to 6000 nm and detection limit
of 0.1 ppm) for all the reaction solutions.
The ligand, salophen was prepared according to the reported
procedure.73 1 mM o-phenylenediamine solution in ethanol
was added dropwise to the 2 mM solution of salicylaldehyde in
ethanol and then the mixture of the solutions was refluxed for
2 h at 85 °C. The precipitated Schiff-base ligand was filtered,
washed with cold ethanol and finally recrystallized from hot
ethanol giving the desired product, Slp.
Elemental analysis
Elemental analyses for CHNO were performed using a Vario
EL cube [Elements (Elemental) analysis system, Germany].
Salophen ligand (Slp)
Electrochemical measurements
Orange crystals; yield 90%; 1H NMR ([d6]DMSO): 12.94 (s, 2H),
8.93 (s, 2H), 7.66 (dd, 2H), 6.93–7.50 (m, 10H). 13C NMR ([d6]
DMSO): 164.53 (C–OH), 160.96 (CvN), 142.85, 133.92, 132.90,
128.31, 120.23, 119.97, 119.56, 117.09 (all are Ar.C). FT-IR
(KBr, cm−1): 3203–2400 (m, OH), 3051 (w, aromatic C–H), 1610
(s, CvN), 1190 (phenolic C–O). Elemental Analysis:
Calculated: C, 75.93; H, 5.10; N, 8.86; O, 10.11%. Found: C,
76.04; H, 5.24; N, 8.90; O, 9.82%.
Cyclic voltammetry experiments were carried out using a
CHI660E Electrochemical Analyzer Series/Workstation. A stan-
dard three-electrode electrochemical cell consisting of a glassy
carbon electrode (working electrode), Ag/AgCl (reference elec-
trode) and a Pt wire (auxiliary electrode) was used. All the
cyclic voltammetry experiments were carried out with a scan-
ning rate of 100 mV s−1. Prior to electrochemical measure-
ments, the glassy carbon electrode was polished with 0.3, 0.1,
and 0.05 μm of alumina slurry for 5 min each to get a mirror
surface, followed by ultrasonication in ethanol and Millipore
water, respectively, at least two times (3–5 min).
Preparation of the salophen Ni(II) complex, 1
The salophen Ni(II) complex was prepared as follows: 2 mM
solution of Ni(OAc)2·4H2O and 2 mM solution of the salophen
ligand in 50 mL of ethanol were refluxed with vigorous stirring
for 2 h. The reaction progress was monitored by TLC. The reac-
tion mixture was then cooled to room temperature and the
obtained solid product was filtered, washed with cold EtOH
and Et2O, and dried.74,75
Photochemical water oxidation and oxygen evolution
quantified by GC
Photochemical water oxidation experiments were performed as
follows: A salophen complex (1–50 μM) was added to a phos-
phate buffer (0.1 M, pH 7.0, 5.0 mL) containing [Ru(bpy)3]
Cl2·6H2O (1.0 mM) and Na2S2O8 (5.0 mM) deaerated by
purging with N2 gas for 30 min in a 34 mL flask sealed with a
rubber septum. The water oxidation reaction was carried out
by irradiating the solution with a 500 W Xe-lamp through a
transmitting glass filter (λ ≥ 420 nm) at room temperature.
After each sampling time, 100 µL of N2 was injected into the
flask and then 100 µL of the gas sample was withdrawn using
a SGE gas-tight syringe and analysed by a gas chromatographer
(GC) equipped with a thermal conductive detector and 5 Å
molecular sieve column (2 mm × 3 mm) and with He as a
carrier gas. The total amount of evolved O2 was calculated
based on the concentration of O2 in the headspace gas. The
pH of the reaction solution was monitored after the water oxi-
dation reaction using a Sartorius PB-10 pH meter.
Salophen Ni(II) complex, 1
Yield 83%; 1H NMR ([d6]DMSO): 8.91 (s, 2H), 8.16 (q, 2H), 7.62
(dd, 2H), 7.37 (m, 2H), 7.32 (m, 2H), 6.90 (d, 2H), 6.68 (t, 2H).
13C NMR ([d6]DMSO): 165.70 (CvN), 157.08 (C–O), 142.97,
135.66, 134.87, 128.08, 120.77, 116.59, 115.80, 100.17 (all are
Ar.C). FT-IR (KBr, cm−1): 3063 (w, aromatic C–H), 1601 (s,
CvN), 1194 (phenolic C–O). ESI-MS (H2O, m/z): 372.1656
[{Ni(C20H14N2O2)}H]+. Elemental Analysis: Calculated: C,
64.40; H, 3.78; N, 7.51; O, 8.58%. Found: C, 64.62; H, 3.82; N,
7.53; O, 8.64%.
Synthesis of the salophen Cu(II) complex, 2
0.5 mM solution of the salophen ligand in ethanol (10 mL)
and 0.5 mM solution of Cu(OAc)2·H2O in water (1 mL) were
mixed and refluxed with vigorous stirring for 3 h. The solution
Electron microscopy measurements
The sample for SEM measurements was prepared from the mixture was then cooled to room temperature and filtered.
bulk electrolysis of a solution of 0.5 mM catalyst in 0.1 M phos- After filtration, the obtained solid product was washed
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thoroughly with water, ethanol and diethyl ether, then dried 13C NMR spectroscopy, FT-IR spectroscopy, and ESI-MS
in vacuo to afford the desired product.76
spectroscopy.
1H and 13C NMR spectra (Fig. S1 and S2†) of the prepared
ligand confirm the formation of the ligand. 1H NMR spectrum
of the ligand, Slp, shows the OH proton and imine proton fea-
tures at 12.94 and 8.93 ppm, respectively. 13C NMR spectra
also support the proposed structure of the ligand, showing the
signals at 164.53 ppm (C–OH) and at 160.96 ppm (CvN). The
Salophen Cu(II) complex, 2
Yield 74%; FT-IR (KBr, cm−1): 3065 (w, aromatic C–H), 1600
(s, CvN), 1195 (phenolic C–O). ESI-MS (H2O, m/z): 377.0716
[{Cu(C20H14N2O2)}H]+. Elemental Analysis: Calculated: C,
63.57; H, 3.73; N, 7.41; O, 8.47%. Found: C, 63.65; H, 3.78; N,
7.44; O, 8.52%.
1
OH proton peak in the H NMR spectra of the free ligand dis-
appears in the case of complex 1, confirming that the OH
group of the ligand has been deprotonated and coordinated to
the metal, nickel (Fig. S4†). The imine proton signal
Synthesis of the salophen Mn(II) complex, 3
Owing to its air sensitivity, the salophen Mn(II) complex was
synthesized under argon with strict exclusion of air. To a
4 mM solution of the salophen ligand in 40 mL of deaerated
absolute EtOH was added an 8 mM solution of KOH dissolved
in 10 mL of deaerated absolute EtOH through a cannula
needle. To this resulting solution was added dropwise a 4 mM
solution of Mn(OAc)2·4H2O in 10 mL of deaerated absolute
EtOH using a Teflon cannula. The solution mixture was stirred
vigorously for 1.5 h at room temperature and refluxed for 5 h
at 90 °C. The solution mixture was then cooled to room temp-
erature and the obtained yellow product was filtered, washed
with deaerated EtOH and dried in vacuo.77
1
(8.93 ppm) in the H NMR spectra of the free ligand has been
shifted to higher field (8.88 ppm). Moreover, the 13C NMR
spectra of complex 1 also support the proposed structure
(Fig. S5†). Furthermore, it is important to mention that in the
tetracoordinated Ni(II) complex, the electronic magnetic
field shortens the nuclear longitudinal relaxation times
of the nuclei less effectively, thus sharpening the broad
peaks typically associated with paramagnetic substances.
Tetracoordinated Ni(II) with 3-fold orbitally degenerate ground
states often has short electron–spin lifetimes that permit well-
resolved NMR signals.78–80 In this case, hyperfine shifts are
concerned, the larger magnetic anisotropy induces large
dipolar contributions; negligible in the case of octahedral
Ni(II).81–83 The FT-IR spectra of the ligand and complexes
Salophen Mn(II) complex, 3
Yield 68%; FT-IR (KBr, cm−1): 3057 (w, aromatic C–H), 1606 (Fig. S3 and S6–S8†) confirm the formation of the ligand and
(s, CvN), 1197 (phenolic C–O). ESI-MS (H2O, m/z): 369.0103 complexes with the proposed structures (Scheme 2). By com-
[{Mn(C20H14N2O2)}H]+. Elemental Analysis: Calculated: C, 65.05; plexation, the broad O–H absorption bands of the salophen
H, 3.82; N, 7.59; O, 8.67%. Found: C, 65.11; H, 3.88; N, 7.62; ligand in the region from 3203 to 2240 cm−1 are absent in the
O, 8.73%.
FT-IR spectra of the complexes 1–3, confirming that the salo-
phen ligand is deprotonated in the complexation and the
oxygens are coordinated to the metal atom. In addition, the IR
spectra of 1–3, compared with the ligand, indicate that the
ν(CvN) stretching band at 1610 cm−1 is shifted to lower
energy by 4–10 cm−1 for salophen complexes, confirming that
Results and discussion
Structural characterization studies of the ligand and catalysts
The ligand, Slp (Scheme 1), and the complexes 1–3 (Scheme 2) the ligand Slp is coordinated to the metal ions through nitro-
were prepared and characterized by elemental analysis, 1H and gen atoms of the azomethine groups. The electrospray mass
spectrum of the salophen complexes (Fig. 1) also supports the
proposed structure of the complexes, showing ions at mass-to-
charge ratio (m/z) of 372.1656, 377.0716, and 369.0103 match-
ing the calculated values for the ions [{Ni(C20H14N2O2)}H]+,
[{Cu(C20H14N2O2)}H]+ and [{Mn(C20H14N2O2)}H]+, respectively.
Electrochemical studies
Cyclic voltammograms of three different water-soluble com-
Scheme 1 Preparation of the Schiff base ligand, salophen (Slp).
plexes, 1–3, in 0.1 M NaPi buffer of pH 7.0 were obtained on a
Scheme 2 Structure of the complexes studied in this work.
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Scheme 3 Catalytic cycle of visible light-driven water oxidation with
the [Ru(bpy)3]2+/S2O82− system catalysed by the salophen complex.
Scheme 3. Photoinduced electron transfer from [Ru(bpy)3]2+*
(where
* denotes the excited state) to Na2S2O8 affords
[Ru(bpy)3]3+, which can oxidize H2O to O2 in the presence of
water oxidation catalysts.
Fig. 1 ESI-MS spectra of a 10−5 M solution of (a) 1, (b) 2, and (c) 3 in
H2O.
Visible light-driven water oxidation experiments were
carried out as follows: A salophen complex (1–50 μM) was
added to a phosphate buffer (0.1 M, pH 7.0, 5.0 mL) contain-
ing [Ru(bpy)3]Cl2·6H2O (1.0 mM) and Na2S2O8 (5.0 mM) deaer-
ated by purging with N2 gas for 30 min in a 34 mL flask sealed
with a rubber septum. The water oxidation reaction was
carried out by irradiating the solution with a 500 W Xe-lamp
through a transmitting glass filter (λ ≥ 420 nm) at room temp-
erature. After each sampling time, 100 µL of N2 was injected
into the flask and then 100 µL of the gas sample was with-
drawn using a SGE gas-tight syringe and analysed by a gas
chromatographer (GC) equipped with a thermal conductive
detector and 5 Å molecular sieve column (2 mm × 3 mm) and
with He as a carrier gas. The total amount of evolved O2 was
calculated based on the concentration of O2 in the headspace
gas. The pH of the reaction solution was monitored after the
water oxidation reaction using a Sartorius PB-10 pH meter.
The maximum turnover numbers and the total quantity of
O2 formed in the photocatalytic system depend on the concen-
tration of the catalyst (Fig. 3). Both increased with an increase
in catalyst concentration, indicating single-site water oxidation
catalysis.
The water oxidation reaction catalyzed by the complex 2
under different concentrations of the photosensitizer,
[Ru(bpy)3]2+, and the sacrificial electron acceptor, Na2S2O8, was
performed (Fig. 4 and 5). The O2 yield increased when the con-
centration of the photosensitizer increased from 0.4 to
1.0 mM. Nonetheless, the O2 yield started to decrease with
increasing the concentration of the photosensitizer above
1.0 mM (for example, 1.5 mM) (Fig. 4). We also examined the
concentration effect of the oxidant Na2S2O8 on the quantity of
O2 formed in the water oxidation reaction catalyzed by the
Fig. 2 Cyclic voltammograms of 0.5 mM 1 (black), 2 (red), and 3 (blue)
recorded in 0.1 M phosphate buffer of pH 7.0 using a glassy carbon as
the working electrode, Ag/AgCl as the reference electrode and a Pt wire
as the auxiliary electrode at a scan rate of 100 mV s−1 at 1.5 V.
glassy carbon electrode (Fig. 2). Potentials were measured vs.
an Ag/AgCl reference electrode. The CVs of all three complexes
exhibited large, irreversible oxidation waves on scanning at
100 mV s−1 from 0 to +1.5 V that correspond to the catalytic
oxidation of H2O to O2. The onset potential for the catalytic
oxidation of H2O for all the catalysts ranges from 0.98 to 1.16 V
(vs. Ag/AgCl). The peak currents were 116 μA, 157 μA, and
67 μA at 100 mV s−1 using a 0.07 cm2 glassy carbon electrode
for the complexes 1, 2, and 3, respectively.
Visible light-driven water oxidation
The catalytic activity of the complexes 1–3 for photochemical complex 2 (Fig. 5). The amount of produced O2 increased with
water oxidation was investigated using [Ru(bpy)3]Cl2·6H2O as an increasing concentration of the oxidant up to 5 mM, but
2−
the photosensitizer and S2O8
as the sacrificial electron decreased when the concentration of the oxidant was more
acceptor, at pH 7.0 in 0.1 M phosphate buffer. The catalytic than 5 mM. The observed results at 1.5 mM concentration of
cycle of the photochemical water oxidation is shown in [Ru(bpy)3]2+ and 7.0 mM Na2S2O8 can be attributed to non-O2
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productive decomposition reactions between Na2S2O8 and
[Ru(bpy)3]2+.32,84,85 Thus, the optimal concentrations of the
photosensitizer and the sacrificial electron acceptor for the water
oxidation reaction were 1 mM and 5 mM, respectively. There was
no O2 formation in the absence of the photosensitizer,
[Ru(bpy)3]2+, and the sacrificial electron acceptor, Na2S2O8.61
Maximum O2 formation amounts of 3.25, 2.50, and
1.0 µmol were obtained from 50 µM 1, 2, and 3, respectively, in
0.1 M NaPi buffer of pH 7.0 after 2.5 h of photocatalysis
(Fig. 6). The low oxygen yields at a lower catalyst concentration
are probably because the diffusion-controlled interactions
between the catalyst and photosensitizer were not sufficient.86
The O2 evolution kinetics of the photoactivated system cata-
lyzed by complexes 1 and 2 showed a short induction period
and is possibly due to a slow solution-to-gas transfer of the O2
or a slow mass transfer of O2 into a head-space.87,88 The initial
sluggish kinetics of the O2 evolution reaction may also be
observed if initially not enough amount of reactive metal-oxo or
-hydroperoxidic intermediates34,89–94 was generated during the
reaction between Ru(III) and metal–salophen complexes. The O2
evolution during the photochemical water oxidation reaction
using metal–salophen complexes showed saturation at
∼100 minutes and the pH of the resulting solution dropped from
7.0 to 6.74. The saturation observed for the O2 evolution and a
decrease in the pH of the solution were probably due to proton
accumulation in the reaction mixture, which is attributed to
most proton-coupled electron transfer (PCET) processes.71,95
PCET pathways produce reactive MvO intermediates (M =
metal), which are primed to form an O–O bond with either a
water molecule or another catalyst molecule.96 The complex 2
showed a high activity with a turnover number of 13 for O2 evol-
ution, whereas the complexes 1 and 3 exhibited water oxidation
activity with turnover numbers of 10 and 4, respectively.
Fig. 3 O2 evolution kinetics in the photoactivated system, [2]
=
1–50 μM, [Ru(bpy)32+] = 1.0 mM, [S2O82−] = 5.0 mM in 0.1 M NaPi buffer,
pH = 7.0; illumination using a 500 W Xe-lamp (λ ≥ 420 nm).
Fig. 4 Kinetics of O2 formation under photoirradiation [500 W Xe-lamp
(λ ≥ 420 nm)] of a phosphate buffer solution (0.1 M, pH 7.0, 5.0 mL) con-
taining 5.0 mM [S2O82−] and 50 μM complex 2 using different concen-
trations of the photosensitizer, [Ru(bpy)3]2+ (1.0 mM, black; 0.7 mM,
blue; 0.4 mM, red; 1.5 mM, olive).
The identity of the true catalyst
To examine the nature of true catalytic materials in the water
oxidation reaction by 1, 2, and 3 under neutral conditions, we
Fig. 5 Kinetics of O2 formation under photoirradiation [500 W Xe-lamp
(λ ≥ 420 nm)] of a phosphate buffer solution (0.1 M, pH 7.0, 5.0 mL) con-
taining 1.0 mM [Ru(bpy)3]2+ and 50 μM complex 2 using different con-
centrations of Na2S2O8 (5.0 mM, black; 7.0 mM, olive; 4.0 mM, blue;
2.0 mM, red).
Fig. 6 Time courses of oxygen evolution under photoirradiation [500
W Xe-lamp (λ ≥ 420 nm)] of a phosphate buffer solution (0.1 M, pH 7.0,
5.0 mL) containing 1.0 mM [Ru(bpy)32+], 5.0 mM [S2O82−], and 50 μM
catalyst.
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performed 20 CV scans of 0.5 mM complexes 1, 2, and 3 in 0.1
M NaPi buffer of pH 7.0 at a glassy carbon electrode. The
working electrode was thoroughly rinsed with water after the
above CV scans, but not polished, and then the electrode was
cycled in a fresh catalyst free 0.1 M NaPi buffer of pH 7.0. No
considerable catalytic current was produced relative to a
freshly polished electrode (Fig. 7), indicating that the electro-
catalysis of 1, 2, and 3 did not lead to any catalytic film depo-
Fig. 8 Cyclic voltammograms of 0.1 M phosphate buffer of pH 7.0
without any catalyst (black solid line), with 1 mM Cu2+ in 0.1 M phos-
phate buffer (green solid line), with 0.5 mM complex 2 in 0.1 M phos-
phate buffer (red solid line), and with 2,2’-bipyridine (bpy) (after several
CV scans with the complex 2, excess bpy is added to the solution, blue
short dash line) at a GC electrode. Potential: 0–1.5 V vs. Ag/AgCl and
scan rate: 100 mV s−1
.
sition which means that the complexes work as homogeneous
catalysts for water oxidation under the neutral conditions.97
The catalytic current increases with the increasing concen-
tration of the complex 2 (Fig. 8), demonstrating single-site
copper catalysis. Furthermore, the addition of 1 mM Cu(NO3)2
to 0.1 M phosphate buffer resulted in an immediate precipi-
tation, and the resulting suspension/solution did not show any
catalytic water oxidation activities (Fig. 8), indicating that the
water oxidation was due to the complex rather than free Cu(II)
ions in the solution. In addition, an inhibition experiment was
performed to check the formation of Cu(II) ions from the
decomposition of the catalyst in the solution. The ligand 2,2′-
bipyridine (bpy) was used as the copper metal ion inhibitor
during water oxidation catalysis. The addition of an excess
amount of bpy to the solution using the complex 2 for catalysis
did not result in any significant changes of catalytic activities
(Fig. 8), confirming that no Cu2+ ions were produced from the
decomposition of the complex during the water oxidation
reaction.
We performed bulk electrolysis experiments of a 0.5 mM
solution of 1, 2, and 3 in 0.1 M phosphate buffer of pH 7.0 at
1.2 V for 2 h, then scanning electron microscopy (SEM) and
energy-dispersive X-ray spectroscopy (EDS) were performed to
check the morphology of the surface of the working electrode.
SEM measurements after controlled potential electrolysis on a
glassy carbon electrode confirmed that no heterogeneous film
was deposited on the surface of the working electrode. The
surface of the washed working electrode after controlled poten-
tial electrolysis at 1.2 V showed very similar features compared
to the bare electrode (Fig. 9). EDS measurements also indi-
cated no elemental metal and P except C on the surface of the
working electrode. The element carbon in the EDS spectra was
originated from the glassy carbon electrode (Fig. 10).
Fig. 7 Cyclic voltammograms of 0.5 mM 1 (top), 2 (middle), and 3
(bottom). Black: CV using catalysts. Red: CV of the fresh buffer solution
using an unpolished glassy carbon electrode. Blue: CV of the fresh
buffer solution using a freshly polished working electrode. Phosphate
buffer concentration and pH: 0.1 M and 7.0, potential: 0–1.5 V vs. Ag/
To check the stability of the catalysts during photochemical
water oxidation and further follow up the formation of nano-
AgCl and scan rate: 100 mV s−1
.
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Fig. 9 SEM images of the surface of the glassy carbon electrode; (a)
before bulk electrolysis and after 2 hours of bulk electrolysis at 1.2 V (vs.
Ag/AgCl) in 0.1 M phosphate buffer of pH 7.0 containing 0.5 mM (b) 1,
(c) 2, and (d) 3.
Fig. 11 ESI-MS spectra of a solution containing 0.2 mM complex,
2.4 mM [Ru(bpy)3]2+ and 5.0 mM Na2S2O8 in a 0.1 M NaPi buffer of pH
7.0. Before photolysis: (a), (c), and (e) are for catalysts 1, 2, and 3. After
photolysis: (b), (d), and (f) are for catalysts 1, 2, and 3. Irradiation with
light (λ ≥ 420 nm) for 60 min at 23 °C.
Fig. 10 EDS histogram of the glassy carbon electrode after 2 h of bulk
electrolysis at 1.2 V (vs. Ag/AgCl) in 0.1 M phosphate buffer of pH 7 con-
taining 0.5 mM (a) 1, (b) 2, and (c) 3.
particles, we performed the following experiment. A solution
containing 0.2 mM catalyst, 2.4 mM [Ru(bpy)3]2+ and 5.0 mM
2−
S2O8 in a NaPi buffer (0.1 M) at pH 7.0 was irradiated under
visible light for 60 min at room temperature. The solution
mixture was analysed by ESI-MS and dynamic light scattering
(DLS) measurements before and after 60 min of photolysis.
The species observed in the electrospray ionization mass spec-
trum of the reaction mixture of 1 before photolysis were
[{Ni(Slp)}H]+ (m/z: 372.2023), [Ru(bpy)3]2+ (m/z: 285.141) and
Fig. 12 DLS measurements before and after 60 min of light irradiation
of a solution containing 2.4 mM [Ru(bpy)3]2+, 5.0 mM S2O82−, and
0.2 mM complex in 0.1 M NaPi buffer of pH 7.0. (a): Complex 1, (b):
{[Ru(bpy)3]Cl}+ (m/z: 605.0018) (Fig. 11(a)). The ESI-MS spectrum complex 2 and (c): complex 3.
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(Fig. 11(c)) of the solution mixture of 2 showed the presence of
[{Cu(Slp)}H]+ (m/z: 377.0345), [Ru(bpy)3]2+ (m/z: 285.1302) and
{[Ru(bpy)3]Cl}+ (m/z: 605.0487) before photolysis. The electro-
spray mass spectrum of the solution mixture of 3 before photo-
lysis (Fig. 11(e)) exhibited ions at mass-to-charge ratio (m/z) of
369.0714, 285.2053, and 605.0018 matching the calculated
values for the ions [{Mn(Slp)}H]+, [Ru(bpy)3]2+ and {[Ru(bpy)3]
Cl}+, respectively. Consequently, the ESI-MS spectra of the
solution mixture after photolysis (Fig. 11(b), (d), and (f)) were
nearly identical to those of the solution mixture before photo-
lysis; no other Co(II) species or the free salophen ligand was
observed at m/z between 50 and 1000 in the ESI-MS, indicating
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Conclusion
In summary, we have reported Earth-abundant metal-based
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2+
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to the “State Key Lab of Advanced 21 N. D. McDaniel, F. J. Coughlin, L. L. Tinker and
Technology for Materials Synthesis and Processing” for finan- S. Bernhard, J. Am. Chem. Soc., 2008, 130, 210–217.
cial support. F. V. appreciates the financial support from 22 M. Graetzel, Acc. Chem. Res., 1981, 14, 376–384.
Tomsk Polytechnic University Competitiveness Enhancement 23 W. Su, H. A. Younus, K. Zhou, Z. A. Khattak,
Program grant (VIU-2019). M. A. A. and C. I. E. thank the
Chinese Scholarship Council (CSC) for the PhD grants
2013GXZ983 and 2013GXZ987.
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