An efficient oxygen evolving catalyst based on a μ-O diiron coordination complex

Article abstract of DOI:10.1039/c4cc04118f

A family of oxygen evolving catalysts was investigated, which was based on the most desired first-row transition metal iron. Among them, the highest turnover number of 2380 was obtained in acetate buffer at pH 4.5 with [(TPA)₂Fe₂(μ-O)(μ-OAc)]³⁺. This journal is

Full text of DOI:10.1039/c4cc04118f

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An efficient oxygen evolving catalyst based
on a l-O diiron coordination complex†
Cite this:DOI: 10.1039/c4cc04118f
Yongdong Liu,a Rui Xiang,a Xiaoqiang Du,a Yong Ding*ab and Baochun Ma*a
Received 29th May 2014,
Accepted 29th August 2014
DOI: 10.1039/c4cc04118f
www.rsc.org/chemcomm
A family of oxygen evolving catalysts was investigated, which was range of 40–360 and 1050 using CeIV and IO4À, respectively.
based on the most desired first-row transition metal iron. Among Other similar samples were studied as well.11–13 Compared with
them, the highest turnover number of 2380 was obtained in acetate the homogenous earth-scarce metal based WOCs, the activity of
buffer at pH 4.5 with [(TPA)2Fe2(l-O)(l-OAc)]3+
.
these iron-centered organometallic WOCs is relatively low.
Therefore, much more efforts are needed for the developments
Conversion of solar energy into clean and efficient chemical of efficient and stable oxygen evolving complexes (OECs) with
energy sources through artificial photosynthesis has become of iron cores.
increasing interest, since the human community is confronted with
Up to now, various water oxidation systems have appeared in
fierce energy and environmental challenges.1,2 One promising the literature, and different sacrificial oxidants were involved
3+
approach for H2 formation is the reduction of H+ by the splitting (e.g. CAN, Na2S2O8, Ru(bpy)3 etc.). Among them, potassium
of water artificially. However, a complicated process of water peroxymonosulfate (Oxone) was chosen as an oxidant for the
oxidation reaction is involved, 2H2O - O2 + 4eÀ + 4H+, which is evaluation of the activity of WOCs. The reduction potential of
supposed to be the major obstacle for water splitting.3 Water Oxone is 1.82 V vs. NHE, which provides a moderate thermal
oxidation catalysts (WOCs) are expected to solve this problem potential for water oxidation.14 Moreover, Oxone can be especially
and developments are realized with regard to the discovery of useful for characterizing first-row transition metal based WOCs,
numerous heterogeneous WOCs and homogeneous catalysts. because it is a two-electron oxidant.15
During the past three decades, after the first example of a
Here, we investigated a series of iron-based organometallic
dinuclear Ru complex ‘‘blue dimer’’,4 extensive studies have complexes for oxygen evolution in the presence of Oxone in acetate
been carried out on valuable transition metal based molecular buffer solution. Compound [(TPA)2Fe2(m-O)Cl2]2+ (2) showed the
catalysts.5,6 Earth-scarce ruthenium and iridium metals combined best activity with a TON of 2380 and a TOF (TON5min/300 s) of
with organic ligand frameworks were employed as significantly 2.2 sÀ1, the TON of which represents the highest value for any
active WOCs under homogeneous conditions.7,8
first-row transition metal–organic complex based homogeneous
Aimed at large scale utilization, structural models of organo- WOCs reported to date. The 18O-labeling experiments showed
metallic complexes with earth-abundant cores, especially the that the O atoms in O2 were derived from both water and oxone
most abundant transition metals, as oxygen evolving centers (Fig. S1 and S2; Table S1, ESI†).
are highly desired. In 2010, Collins et al. reported Fe-TAML for
Initially, an iron-based metal–organic complex 1 (Fig. 1) bearing
the oxidative conversion of water to dioxygen (TON = 16, TOF 4 a neutral tripodal tetraamine ligand was synthesized, which was
1.3 sÀ1) with ceric ammonium nitrate (CAN) as an oxidant.9 recently reported to catalyze the formation of dioxygen from water
Then, in 2011, Costas10 reported a family of iron complexes using a one-electron oxidant CAN.10 Importantly, the reaction of
bearing tetradentate nitrogen ligands and featuring cis labile FeTPA with 1 equiv. of peracetic acid results in the formation of an
sites that catalyzed the evolution of oxygen with TONs in the intermediate [FeIV(O)(TPA)]2+ quantitatively, which is an effective
oxygen-atom transfer agent.16 A previous report showed that the
a State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, China.
high-valent oxo–iron intermediates were responsible for the O–O
formation event.10 These active properties inspired us to explore
the possibility of FeTPA catalyzing oxygen evolution with a solution
of Oxone as the terminal electron acceptor.
As shown in Fig. S3 (ESI†), the catalytic activity of FeTPA
for oxygen-evolving reaction was investigated in acetate buffer
E-mail: dingyong1@lzu.edu.cn, mabaochun@lzu.edu.cn
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
† Electronic supplementary information (ESI) available: Experimental details and
the spectral characterization of catalysis reaction. See DOI: 10.1039/c4cc04118f
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Fig. 1 Molecular structures of iron complexes studied in this communication.
solution with Oxone as an oxidant. A very high catalytic activity
(mol O2 produced per mol iron used = 1190) was obtained
under the catalytic conditions (i.e. FeTPA, 1.5 mM; Oxone,
95 mM), which, to our knowledge, represents the highest turn-
over number per metal among the reported values for any
homogeneous system based on first-row transition metals.
Fig. 2 Oxygen evolution plots for 1 (15 mM, black), 2 (7.5 mM, red), 3
(15 mM, blue), 4 (15 mM, dark cyan), 5 (15 mM, wine), 6 (15 mM, pink), FeCl3
(15 mM, magenta), nano-Fe2O3 (2.5 mg, dark yellow) and blank (navy) in
20 mL acetate buffer (pH 4.5, 0.23 M) with Oxone (10 mM).
The measured value of 41.1 sÀ1 (mol O2 mol ironÀ1
s
À1) was
comparable to the best value reported before (4 1.3 sÀ1
,
complex Fe-TAML functioned as the WOC, but its high
performance vanished within a few seconds9). Relevant data
are summarized in Table S2 (ESI†). Prompted by the results
mentioned above, the catalytic behavior of FeTPA was further
investigated in detail.
As stated in the literature, metal coordination complexes
tend to decompose during turnovers and the metal centers are
liberated and transferred to other species (i.e. Fe2O3,17 CoOx
18
and Mn-oxide19) which function as the true water oxidation
catalysts. In our case, to eliminate the possibility that the iron
metal center was liberated from the tetradentate ligand and
converted into the iron ion, Fe2O3 or other insoluble species
(e.g. Fe(OH)3) which are the actual catalysts, detailed stability
studies were carried out. Firstly, an acetate buffer solution of
FeTPA (pH 4.5) remains homogeneous within months without
hydrolysis-precipitation. Secondly, dynamic light scattering
(DLS) was used to rule out the possibility that the oxygen
evolution activity was a result of the in situ formation of
nanoparticles (Fig. S4, ESI†). As is revealed by DLS analysis,
Fig. 3 Absorbance spectra of 1 (0.2 mM) (dashed lines) and 2 (0.1 mM)
(solid lines) in 230 mM acetate buffer solution (black), 40 mM Britton–
Robinson buffer solution (red), and water (blue). Compounds 1 and 2
converge to the same spectrum in the buffer solution.
no nanoparticles were found, indicating that the observed at 700 nm (Fig. 3). The carboxylate anion serving as a connector
catalysis truly originates from the water-soluble iron molecular for the di-nuclear site along with bridging oxygen chelates two
complex. Thirdly, injection of 1 mL of acetate buffer (with no mononuclear iron metal complexes containing the N-donor
catalyst) into an Oxone solution led to minimum O2 generation ligands to form a (m-oxo)diiron core. When 1 was dissolved in
monitored by gas chromatography, as well as injection of 1 mL pure water or a Britton–Robinson buffer solution with a pH 4.5,
of preformed nano-Fe2O3, commercial Fe2O3 and FeCl3 only one weak wave in the range of 400 to 800 nm (Fig. 3)
solution into an Oxone solution respectively (Fig. 2). The appeared in UV-visible spectrophotometry without carboxylate.
implication of all these control experiments and DLS analysis When 2 was dissolved in acetate buffer, the UV-vis spectrum
results is that the resultant aqueous species of FeTPA in acetate was consistent with that of the (m-oxo)(m-carboxylato)diiron
buffer act as the true catalyst.
unit. While diiron 2 was dissolved in either Britton–Robinson
An interesting phenomenon was observed through UV-visible buffer or water, a colourless solution was produced, which is
spectroscopic measurements when 1 was dissolved in acetate consistent with the performance of mononuclear 1. When 1
buffer solution, a brown iron dimer of Fe–O–Fe was formed and 2 were dissolved in acetate buffer, the same species were
in situ. The absorbance spectra exhibiting similar features of found to be formed with a dibridged (m-O)(m-OAc)diiron struc-
the bridging components have a peak at 330 nm, three features ture by ESI-MS and Raman (Fig. S6 and S7, ESI†). The presence
from 400 to 500 nm, a shoulder near 525 nm, and a broad band of the OAc bridges constrains the two trans free coordination
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sites of diiron 2 and gives the dibridged complex with two cis
free coordination sites which could represent a key structural
aspect of the process of oxygen evolution. However, the replace-
ment of the carboxylate with other anionic conjugate bases and
the protonation of the oxo bridge engender the expected changes
in the (m-oxo)(m-carboxylato)diiron mode (Fig. S8, ESI†).
The catalytic activity of FeTPA in Britton–Robinson buffer
solution was evaluated, and a monomer species existed as stated
above in the system. As is listed in Table S2 (ESI†), the activity
of monomeric FeTPA in Britton–Robinson buffer solution was
an order of magnitude lower than that of the dimeric species
in acetate buffer. All these results mentioned above clearly
indicate that the (m-oxo)(m-acetato)diiron formed and func-
tioned as the active species.
A number of parameters must be considered when an
oxygen-evolving reaction is conducted, and among them, pH is
always a key factor that influences the performance of an OEC. As
is depicted in Fig. S9 (ESI†), titration of FeTPA was conducted in
acetate buffer between pH 3.5 and 5.5 with 4 M solution of NaOH.
The absorbance of the characteristic peaks of the Fe–O–Fe dimer
in acetate buffer increased as more NaOH solution was added
from pH 3.5 to 4.5. This demonstrates that more iron dimers
existed in pH 4.5 acetate buffer solution since the monomeric
species shows no obvious response to visible light (Fig. 3). We
speculated that this was because the oxo bridges of the as-formed
Fe–O–Fe dimer were susceptible to protons and more H+ resulted
in more monomeric species in the solution.
Based on the discussion mentioned above, the catalytic
activity of FeTPA should be dependent on pH values. In order
to confirm this speculation, oxygen evolution reactions were
carried out in acetate buffer solutions with different pH values
(pH = 3.5; 4.5; 5.5) in parallel. As depicted in Fig. S10 (ESI†),
reaction at pH 4.5 gave the highest O2 yield and TON. Combining
the results of titration measurements with the results of oxygen
evolution experiments gives the conclusion that the optimum
pH value is 4.5 and further supports the above statement that
the Fe–O–Fe dimer is the actual catalyst for oxygen evolution.
Under other pH conditions, the oxygen evolution is poorer.
Therefore, subsequent experiments involving catalytic O2
evolving processes were carried out in acetate buffered (0.23 M,
pH = 4.5) aqueous solution.
The catalytic activity of the title complex was also compared
with catalysts 3, 4, 5 and 6 (Fig. 2). As is revealed in Fig. 2, the
fastest O2 liberation was observed with complex 3 and oxygen
evolution reached a plateau for only a few minutes; however,
the turnover number was quite low compared with catalyst 2.
The relatively low TON obtained over catalyst 3 might be due to
its inferior stability. The trigonal bipyramidal geometry ligand
with a particularity of phenolate oxygen in compound 3 is
labile and prone to oxygenolysis in its high oxidation states
(Fig. S11 and S12, ESI†). A small amount of oxygen was released
when catalyzed by complex 5, which dissociates under the test
conditions (Fig. S13, ESI†). No oxygen evolution was observed
when catalysts 4 and 6 were tested. These results show that
the m-oxo-bridged diiron units are necessary and capable of
homogenous catalytic oxygen-evolution.
Fig. 4 UV-vis spectral evolution of 2 (0.5 mM) before (black) and after
(color) addition of 20 equiv. of Oxone in acetate buffer at room
temperature.
To investigate what species are involved in the oxygen evolution
processes, further experiments were carried out. Fig. 4 depicts the
UV-vis spectral evolution of diiron in acetate buffer when 20 equiv.
of Oxone was added. An isosbestic point is observed and the peak
at 700 nm red shifted gradually to 726 nm with the addition of
Oxone, indicating the formation of a new species with a higher
molar absorptivity. Analogous spectra of [Fe(O)(TPA)(ClO4)]+ are
reported through the reaction of [Fe(TPA)(NCCH3)2]2+ with 1 equiv.
CH3CO3H.16 Therefore, a highly oxidizing Fe(IV)QO intermediate
was supposed to be formed on the basis of the experimental
evidence discussed above.
In summary, a series of coordination complexes based on earth-
abundant, environmentally benign iron metal were designed and
examined for oxygen evolution. Several lines of evidence suggested
that complexes 1 and 2 convert to (m-O)(m-OAc)diiron(III) in
acetate buffer, which is responsible for the O–O bond formation.
A quite high TON (2380) and TOF (2.2 sÀ1) were obtained over
[(TPA)2Fe2(m-O)(m-OAc)]3+ under the optimum conditions. To the
best of our knowledge, this TON represents the highest value for
any first-row transition metal–organic complex based homo-
geneous WOCs reported to date. The key m-oxo structure is
useful for the conceptual design of iron-based homogeneous
oxygen evolving complexes. A high-valent iron oxo species
observed in UV-vis spectra maybe an intermediate in the catalytic
cycle of O2 evolution with Oxone. Compared with the heteroge-
neous Fe based WOCs, taking the Fe2O3–NA/RGO/BiV1ÀxMoxO4
heterojunction20 for example, the diiron unit exhibits a quite high
efficiency. However, its stability is inferior to that of the hetero-
geneous ones due to dimer splitting.
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 21173105 and 21172098)
and the Fundamental Research Funds for the Central Universities
(lzujbky-2014-67).
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