in the three-component photo-induced water oxidation system, O54 has comparable
catalytic activity with state-of-the-art catalysts, and TON can reach 273 [212].
In 2011, Llobet group synthesized single-site ruthenium compounds O55–58
(Fig. 36) with a structure similar to that of binuclear Ru-Hbpp catalyst O8
[213]. All four Ru-aqua compounds were equipped with a tpy ligand and a Hbpp
ligand or its analogues. In organic solution, the electrochemical measurements of
Ru-Cl species which were the precursors of catalysts O55–58 showed that only one
Ru
III/II redox couple existed, while Ru
III/II and Ru
IV/III couples appeared in active
complexes O55–58, which indicated that the existence of protons could make the
metal centers reach higher oxidation state through PCET pathway. More importantly, the investigation indicated that the out isomer had lower redox potential than
the corresponding in isomer. In the process of catalytic oxygen evolution, not only
the formation of O 2 was detected, but also the CO 2 was also discovered in chemicaldriven water oxidation process. It was found that CO 2 may originate from the
oxidative decomposition of phenyl in catalysts O55 and O56. Because no CO 2
was detected in the process of complexes O57 and O58 catalyzed water oxidation
under the same conditions, this may be due to the increase of oxidation potential of
pyridine rings after protonation under acidic conditions, which makes pyridine rings
not easy to be oxidized and decomposed. In 2016, Llobet group first developed
tri-heteronuclear Ru 2 M (M ¼ Mn, Co) precursors O59–O62 (Fig. 36) containing tpy
ligands for water oxidation catalysis based on corresponding Ru-Cl species of
catalyst O58 [214]. They can be converted into Ru 2 M-(H 2 O) 4 (M ¼ Mn, Co)
O63–O64 (Fig. 36) as real catalytic active substances in the catalytic environment.
Compared with catalyst O58, the catalytic activity of Ru 2 M-(H 2 O) 4 species O63–
O64 in the process of catalytic oxygen evolution was greatly improved whether in
photocatalysis or chemical oxidation. The results showed that the active center was
still on metal Ru, and the O-O bond was formed by WNA mechanism.
In 2011, Yagi and Fujita studied the catalytic differences of two geometric
isomers O65–66 (Fig. 37) as catalysts for water oxidation. Isomer O66 was obtained
by photoisomerization of O65 [113, 215]. Compared with Ru(tpy)(bpy) catalyst
O32, compounds O65 and O66 containing pynap ligand have lower Ru
III/II redox
potential. However, the two geometric isomers O65 and O66 with such similar
structures differ greatly in their electrochemical properties. It was found that the
uncoordinated pyridinium nitrogen atom in isomer O66 was very close to the
coordinating H 2 O molecule and formed intramolecular hydrogen bond, which
increased the difficulty of producing high-valent Ru ¼ O and eventually led to its
use as a poor catalyst for water oxidation. But it is the presence of intramolecular
hydrogen bond that makes O66 a more active proton reduction catalyst than
O65 [113].
In 2012, Thummel group synthesized a large number of [Ru(NNN)(NN)(X)] and
[Ru(NNN)(pic) 2 (X)]-type ruthenium compounds O67–90 (Fig. 38) to study the
relationship between catalyst structure and activity [216, 217]. When Ce
IV was
employed as oxidant, most of the catalysts in this group exhibited corresponding
catalytic activity for water oxidation, except compounds O75, O77, and O80.
According to the data analysis, the catalysts containing picoline ligands had higher
The Application of Pincer Ligand in Catalytic Water Splitting
421
catalytic activity with state-of-the-art catalysts, and TON can reach 273 [212].
In 2011, Llobet group synthesized single-site ruthenium compounds O55–58
(Fig. 36) with a structure similar to that of binuclear Ru-Hbpp catalyst O8
[213]. All four Ru-aqua compounds were equipped with a tpy ligand and a Hbpp
ligand or its analogues. In organic solution, the electrochemical measurements of
Ru-Cl species which were the precursors of catalysts O55–58 showed that only one
Ru
III/II redox couple existed, while Ru
III/II and Ru
IV/III couples appeared in active
complexes O55–58, which indicated that the existence of protons could make the
metal centers reach higher oxidation state through PCET pathway. More importantly, the investigation indicated that the out isomer had lower redox potential than
the corresponding in isomer. In the process of catalytic oxygen evolution, not only
the formation of O 2 was detected, but also the CO 2 was also discovered in chemicaldriven water oxidation process. It was found that CO 2 may originate from the
oxidative decomposition of phenyl in catalysts O55 and O56. Because no CO 2
was detected in the process of complexes O57 and O58 catalyzed water oxidation
under the same conditions, this may be due to the increase of oxidation potential of
pyridine rings after protonation under acidic conditions, which makes pyridine rings
not easy to be oxidized and decomposed. In 2016, Llobet group first developed
tri-heteronuclear Ru 2 M (M ¼ Mn, Co) precursors O59–O62 (Fig. 36) containing tpy
ligands for water oxidation catalysis based on corresponding Ru-Cl species of
catalyst O58 [214]. They can be converted into Ru 2 M-(H 2 O) 4 (M ¼ Mn, Co)
O63–O64 (Fig. 36) as real catalytic active substances in the catalytic environment.
Compared with catalyst O58, the catalytic activity of Ru 2 M-(H 2 O) 4 species O63–
O64 in the process of catalytic oxygen evolution was greatly improved whether in
photocatalysis or chemical oxidation. The results showed that the active center was
still on metal Ru, and the O-O bond was formed by WNA mechanism.
In 2011, Yagi and Fujita studied the catalytic differences of two geometric
isomers O65–66 (Fig. 37) as catalysts for water oxidation. Isomer O66 was obtained
by photoisomerization of O65 [113, 215]. Compared with Ru(tpy)(bpy) catalyst
O32, compounds O65 and O66 containing pynap ligand have lower Ru
III/II redox
potential. However, the two geometric isomers O65 and O66 with such similar
structures differ greatly in their electrochemical properties. It was found that the
uncoordinated pyridinium nitrogen atom in isomer O66 was very close to the
coordinating H 2 O molecule and formed intramolecular hydrogen bond, which
increased the difficulty of producing high-valent Ru ¼ O and eventually led to its
use as a poor catalyst for water oxidation. But it is the presence of intramolecular
hydrogen bond that makes O66 a more active proton reduction catalyst than
O65 [113].
In 2012, Thummel group synthesized a large number of [Ru(NNN)(NN)(X)] and
[Ru(NNN)(pic) 2 (X)]-type ruthenium compounds O67–90 (Fig. 38) to study the
relationship between catalyst structure and activity [216, 217]. When Ce
IV was
employed as oxidant, most of the catalysts in this group exhibited corresponding
catalytic activity for water oxidation, except compounds O75, O77, and O80.
According to the data analysis, the catalysts containing picoline ligands had higher
The Application of Pincer Ligand in Catalytic Water Splitting
421