withdrawing groups in bpy, the rate of Cl
À replacement will slow down. Through the
electrochemical study of this group of compounds, it was found that the influence of
electronic effect on the redox potential of ruthenium centers by substituted tpy
ligands was no more important than the modification of bpy. More interestingly,
when the bpy ligand was introduced into the electron-withdrawing group, the
catalytic rate of water oxidation decreased, but when tpy ligand was introduced
into the electron-withdrawing group, the catalytic rate increased. In view of this
phenomenon, it is difficult for researchers to give a regular summary
[195, 196]. Next, through extensive mechanism studies of catalysts O32, O34, and
O40, it was found that modification of bpy ligands would lead to greater catalytic
difference of water oxidation, which was due to the axial para-position of RuÀO
bond occupied by a nitrogen atom of bpy. The electronic density and reactivity of
RuÀO bond will be directly affected by the change of electrical properties on bpy
ligand. The mechanism of OÀO bond formation by high-valent [Ru
V
¼O] species
was also proposed for this group of catalysts [197, 198]. Isotope labeling experiments showed that only one O atom was labeled by
18 O in generated O 2 molecule
and unlabeled O atom came from NO 3 - while WO catalysis was studied in H 2
18 O
and [Ce(NO 3 ) 6 ]
2+ as oxidant. So, this OAT (oxygen atom transfer) pathway of Ce
IV
has also been suggested by many researchers [199, 200], thus this method provides
us with an effective way to prove the catalytic mechanism.
In 2010, Meyer and coworkers synthesized a series of mononuclear Ru-OH 2
compounds O42–52 (Fig. 35) with pincer ligands as catalysts for water oxidation,
and explored how the diversity of ligand environments affected the reaction rate and
thermodynamics in water oxidation. Based on a large number of experimental data, a
platform for the design of catalysts with controllable reactivity was provided
[201]. In the subsequent paper, the electrocatalytic water oxidation by catalyst
O42 with carbene ligand was studied in detail [202]. The experimental results of
catalyst O42 supported the previously proposed WNA catalytic mechanism. It was
believed that the multi-step PCET process resulted in the formation of highly active
[Ru
V
¼O] intermediate, which were attacked by H 2 O to form OÀO bond. Next, the
[Ru
III -OOH] species was oxidized and oxygen was released. By comparing catalyst
O45, it was found that carbene ligands with strong electron donation ability can
effectively increase the catalytic rate of water oxidation. The electrochemical performance also showed that the rate-limiting step of the whole catalytic reaction
should be before the electrons are transferred to the electrodes. In other words, it may
be a chemical process or diffusion process that becomes the limiting factor of the
catalytic rate. In order to eliminate the influence of diffusion rate of catalyst molecule
on the catalytic rate, a series of assemblies have been carried out to attach catalyst to
semiconductor surface through covalent bond [203–211]. In 2017, Chen group
employed a tridentate pincer ligand with carbene structure to synthesize two
ruthenium-based catalysts O53–54 (Fig. 35) for water oxidation [212]. The
tridentate ligand contains two strong electron donors, carbene and carboxyl, which
greatly reduces the overpotential of catalytic water oxidation by O53–54. Therefore,
The Application of Pincer Ligand in Catalytic Water Splitting
419
À replacement will slow down. Through the
electrochemical study of this group of compounds, it was found that the influence of
electronic effect on the redox potential of ruthenium centers by substituted tpy
ligands was no more important than the modification of bpy. More interestingly,
when the bpy ligand was introduced into the electron-withdrawing group, the
catalytic rate of water oxidation decreased, but when tpy ligand was introduced
into the electron-withdrawing group, the catalytic rate increased. In view of this
phenomenon, it is difficult for researchers to give a regular summary
[195, 196]. Next, through extensive mechanism studies of catalysts O32, O34, and
O40, it was found that modification of bpy ligands would lead to greater catalytic
difference of water oxidation, which was due to the axial para-position of RuÀO
bond occupied by a nitrogen atom of bpy. The electronic density and reactivity of
RuÀO bond will be directly affected by the change of electrical properties on bpy
ligand. The mechanism of OÀO bond formation by high-valent [Ru
V
¼O] species
was also proposed for this group of catalysts [197, 198]. Isotope labeling experiments showed that only one O atom was labeled by
18 O in generated O 2 molecule
and unlabeled O atom came from NO 3 - while WO catalysis was studied in H 2
18 O
and [Ce(NO 3 ) 6 ]
2+ as oxidant. So, this OAT (oxygen atom transfer) pathway of Ce
IV
has also been suggested by many researchers [199, 200], thus this method provides
us with an effective way to prove the catalytic mechanism.
In 2010, Meyer and coworkers synthesized a series of mononuclear Ru-OH 2
compounds O42–52 (Fig. 35) with pincer ligands as catalysts for water oxidation,
and explored how the diversity of ligand environments affected the reaction rate and
thermodynamics in water oxidation. Based on a large number of experimental data, a
platform for the design of catalysts with controllable reactivity was provided
[201]. In the subsequent paper, the electrocatalytic water oxidation by catalyst
O42 with carbene ligand was studied in detail [202]. The experimental results of
catalyst O42 supported the previously proposed WNA catalytic mechanism. It was
believed that the multi-step PCET process resulted in the formation of highly active
[Ru
V
¼O] intermediate, which were attacked by H 2 O to form OÀO bond. Next, the
[Ru
III -OOH] species was oxidized and oxygen was released. By comparing catalyst
O45, it was found that carbene ligands with strong electron donation ability can
effectively increase the catalytic rate of water oxidation. The electrochemical performance also showed that the rate-limiting step of the whole catalytic reaction
should be before the electrons are transferred to the electrodes. In other words, it may
be a chemical process or diffusion process that becomes the limiting factor of the
catalytic rate. In order to eliminate the influence of diffusion rate of catalyst molecule
on the catalytic rate, a series of assemblies have been carried out to attach catalyst to
semiconductor surface through covalent bond [203–211]. In 2017, Chen group
employed a tridentate pincer ligand with carbene structure to synthesize two
ruthenium-based catalysts O53–54 (Fig. 35) for water oxidation [212]. The
tridentate ligand contains two strong electron donors, carbene and carboxyl, which
greatly reduces the overpotential of catalytic water oxidation by O53–54. Therefore,
The Application of Pincer Ligand in Catalytic Water Splitting
419