hydrogenase, hydrogen evolution catalysts based on other first-row transition metals,
including macrocyclic cobalt-based [46–52] and nickel-based [53–58] compounds,
have been studied for decades [59].
1.3 Water Oxidation
Up to now, water oxidation has become a bottleneck restricting the development of
artificial photosynthesis, which is manifested that the catalytic performance of water
oxidation catalyst cannot match that of reduction catalyst. The main challenges in
water oxidation research are as follows:
1. Involving multi-proton and electron transfer – for a redox process, generally only
one or two electrons are involved, while the process of water oxidation to give
oxygen requires four consecutive four electrons oxidation and four protons
release;
2. Involving multiple bonds breakage and formation – the breakage of OÀH bonds
and the formation of OÀO bond;
3. Ambiguous mechanism of OÀO bond formation – as a recognized key step in
catalysis, the formation of OÀO bond is still unclear.
In the photosynthetic system (PSII), the site of water oxidation is located in the
Mn 4 Ca cluster (Fig. 4). It consists of four manganese atoms and one calcium atom,
and five metal atoms are grasped together by surrounding μ-oxo and μ-hydroxo
ligands to form a cubic-like structure, in which three manganese atoms in the
manganese center form a cubane structure with calcium atom, and the fourth
manganese atom, like a pendant, is connected to the cube by two oxo groups
[60–70]. The mechanism of water oxidation catalyzed by the oxygen evolving
complex has also been extensively studied [71–73]. The general view of the OEC
cycle is that five oxidation states are involved in the catalytic process, designated as
S 0 -S 4 (Kok cycle) [73]. S 0 represents the most reduced state, reaching the most
oxidized state of S 4 through four consecutive photo-induced proton-coupled electron
transfer (PCET) processes, and then the high-energy intermediate releases oxygen
back to the initial state of S 0 (Scheme 1). In many biological and chemical processes,
proton-coupled electron transfer (PCET) is an effective way to generate highly
oxidized species without changing the total charge of molecules. It is also an
important feature of the four electrons oxidation of water molecule in artificial
photosynthesis.
Since 1982, a large number of novel molecular catalysts for water oxidation have
been developed. In order to evaluate the catalytic activity of these artificial catalysts,
it is often necessary to utilize oxidants with high oxidation potential to drive water
oxidation catalyst (WOC) to react with water to produce oxygen. One of the
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H.-T. Zhang and M.-T. Zhang
including macrocyclic cobalt-based [46–52] and nickel-based [53–58] compounds,
have been studied for decades [59].
1.3 Water Oxidation
Up to now, water oxidation has become a bottleneck restricting the development of
artificial photosynthesis, which is manifested that the catalytic performance of water
oxidation catalyst cannot match that of reduction catalyst. The main challenges in
water oxidation research are as follows:
1. Involving multi-proton and electron transfer – for a redox process, generally only
one or two electrons are involved, while the process of water oxidation to give
oxygen requires four consecutive four electrons oxidation and four protons
release;
2. Involving multiple bonds breakage and formation – the breakage of OÀH bonds
and the formation of OÀO bond;
3. Ambiguous mechanism of OÀO bond formation – as a recognized key step in
catalysis, the formation of OÀO bond is still unclear.
In the photosynthetic system (PSII), the site of water oxidation is located in the
Mn 4 Ca cluster (Fig. 4). It consists of four manganese atoms and one calcium atom,
and five metal atoms are grasped together by surrounding μ-oxo and μ-hydroxo
ligands to form a cubic-like structure, in which three manganese atoms in the
manganese center form a cubane structure with calcium atom, and the fourth
manganese atom, like a pendant, is connected to the cube by two oxo groups
[60–70]. The mechanism of water oxidation catalyzed by the oxygen evolving
complex has also been extensively studied [71–73]. The general view of the OEC
cycle is that five oxidation states are involved in the catalytic process, designated as
S 0 -S 4 (Kok cycle) [73]. S 0 represents the most reduced state, reaching the most
oxidized state of S 4 through four consecutive photo-induced proton-coupled electron
transfer (PCET) processes, and then the high-energy intermediate releases oxygen
back to the initial state of S 0 (Scheme 1). In many biological and chemical processes,
proton-coupled electron transfer (PCET) is an effective way to generate highly
oxidized species without changing the total charge of molecules. It is also an
important feature of the four electrons oxidation of water molecule in artificial
photosynthesis.
Since 1982, a large number of novel molecular catalysts for water oxidation have
been developed. In order to evaluate the catalytic activity of these artificial catalysts,
it is often necessary to utilize oxidants with high oxidation potential to drive water
oxidation catalyst (WOC) to react with water to produce oxygen. One of the
384
H.-T. Zhang and M.-T. Zhang