metal peroxide species (M-OO-H), which can be further oxidized to release oxygen.
In I2M mechanism, two metal-oxo species which hold significant radical character
generate a (M-O-O-M) species by radical coupling, and then M-O-O-M species is
further oxidized to release oxygen. Even though these two mechanisms have been
generally recognized, it is still necessary to study in depth which mechanism is
preferred in different catalysts and catalytic systems.
1.4 Types of Pincer Ligand Used in HER/OER
Pincer ligands are a kind of tridentate ligand, which can form a planar coordination
mode around the metal center, resulting in the unique stability and reactivity of
pincer-metal complexes [87]. The design and synthesis of pincer ligands become
easily in a few steps, so that their electronic effect and steric nature can be easily
adjusted. Based on the special coordination mode and function of pincer ligands,
transition metal complexes containing pincer ligands exhibit excellent performance
in the fields of synthesis, bond activation, and catalysis [88–90].
Pincer ligands have been applied for proton reduction catalysts and water oxidation catalysts because of catalyst structure and activity could be easily tuned based
on pincer platform. For hydrogen evolution, the catalysts are required to increase the
density of electron cloud in the metal center with the enhancement of reduction
ability. Based on this consideration, some phosphine-based, sulfur-based, and
anionic pincer ligands with strong electron-donating ability will have priority in
the synthesis of proton reduction catalysts. For water oxidation, the metal centers of
catalysts often need to reach a high oxidation state to drive the conversion of water to
oxygen, which requires that the ligands have strong antioxidant capacity and stability. And thus tpy and its derivatives are extensively used in water oxidation catalysis.
In the HER/OER, various pincer-type ligands have been employed in designing
catalysts and regulating the catalytic activity. According to the difference of skeletal
structure, the ligands will be divided into three classes – tpy and its derivatives
L1–34 (Fig. 5), non-completely rigid tridentate pincer ligands L35–71 (Fig. 6), and
unconventional pincer ligands L72–74 (Fig. 7). As mentioned earlier, tpy-type
ligands are widely used for both synthetic HECs (hydrogen evolution catalysts)
and WOCs (water oxidation catalysts). Besides that, other NNN-, NNP-, PNP-,
PCP-, ONO- ligands based on pincer platform have also been employed for their
strong plasticity in redox, electron-donating ability and degree of deformation.
Interestingly, some multi-dentate ligands (Hthiop, dpktsc, and Cl-TMPA) can also
be regarded as pincer-like ligands when they are used as tridentate ligand.
The Application of Pincer Ligand in Catalytic Water Splitting
387
In I2M mechanism, two metal-oxo species which hold significant radical character
generate a (M-O-O-M) species by radical coupling, and then M-O-O-M species is
further oxidized to release oxygen. Even though these two mechanisms have been
generally recognized, it is still necessary to study in depth which mechanism is
preferred in different catalysts and catalytic systems.
1.4 Types of Pincer Ligand Used in HER/OER
Pincer ligands are a kind of tridentate ligand, which can form a planar coordination
mode around the metal center, resulting in the unique stability and reactivity of
pincer-metal complexes [87]. The design and synthesis of pincer ligands become
easily in a few steps, so that their electronic effect and steric nature can be easily
adjusted. Based on the special coordination mode and function of pincer ligands,
transition metal complexes containing pincer ligands exhibit excellent performance
in the fields of synthesis, bond activation, and catalysis [88–90].
Pincer ligands have been applied for proton reduction catalysts and water oxidation catalysts because of catalyst structure and activity could be easily tuned based
on pincer platform. For hydrogen evolution, the catalysts are required to increase the
density of electron cloud in the metal center with the enhancement of reduction
ability. Based on this consideration, some phosphine-based, sulfur-based, and
anionic pincer ligands with strong electron-donating ability will have priority in
the synthesis of proton reduction catalysts. For water oxidation, the metal centers of
catalysts often need to reach a high oxidation state to drive the conversion of water to
oxygen, which requires that the ligands have strong antioxidant capacity and stability. And thus tpy and its derivatives are extensively used in water oxidation catalysis.
In the HER/OER, various pincer-type ligands have been employed in designing
catalysts and regulating the catalytic activity. According to the difference of skeletal
structure, the ligands will be divided into three classes – tpy and its derivatives
L1–34 (Fig. 5), non-completely rigid tridentate pincer ligands L35–71 (Fig. 6), and
unconventional pincer ligands L72–74 (Fig. 7). As mentioned earlier, tpy-type
ligands are widely used for both synthetic HECs (hydrogen evolution catalysts)
and WOCs (water oxidation catalysts). Besides that, other NNN-, NNP-, PNP-,
PCP-, ONO- ligands based on pincer platform have also been employed for their
strong plasticity in redox, electron-donating ability and degree of deformation.
Interestingly, some multi-dentate ligands (Hthiop, dpktsc, and Cl-TMPA) can also
be regarded as pincer-like ligands when they are used as tridentate ligand.
The Application of Pincer Ligand in Catalytic Water Splitting
387