of solar to chemical energy in the future. Therefore, the development of earthabundant first-row transition metal catalysts has become an interesting research
field for many researchers. In the field of water oxidation catalysts, after decades
of development, a large number of MnÀ, FeÀ, CoÀ, NiÀ, CuÀbased water
oxidation catalysts have been developed [61, 230–234], then their catalytic activity,
catalytic mechanism, and other scientific issues have been extensively explored.
Although Ru-based catalysts for water oxidation based on pincer ligands such as
tpy and pdc have been widely developed, excellent catalytic performance has been
demonstrated. However, pincer ligand-based catalysts using the first-row transition
metals for water oxidation have not been widely developed and explored. In 1999,
Crabtree and his coworkers reported a Mn-based catalyst O111 containing
tpy-ligand for water oxidation (Scheme 15) [235]. The discovery of this catalyst
means a major breakthrough, due to the first Mn-based catalyst with the ability to
catalyze water oxidation by chemical oxidation. It is found that the catalytic center of
catalyst O111 is bis(μ-O)Mn
III Mn
IV composed of two different valent Mn atoms. In
fact, the two Mn atoms are identical due to the average valence. In addition, each Mn
center contains a water molecule coordination, which is the basic structure of the
catalyst for water oxidation. In this work, when unconventional NaCIO was used as
oxidant, catalyst O111 could effectively catalyze water oxidation to oxygen, accompanied by general TOF of 12 h
À1 and TON of 4 in 6 h. According to the analysis of
UVÀvis spectra, the ligands on catalyst O111 were oxidized and decomposed to
release manganese ions in the process of catalytic water oxidation driven by chemical oxidation, and manganese ions were further oxidized to form permanganate,
which deactivates the catalyst, thus resulting in modest catalytic capacity. Permanganate and sodium hypochlorite are both oxygen-producing reagents, so the oxygen
source of O 2 evolution by catalytic water oxidation needs to be determined. The
isotope labeling experiments with H 2
18 O confirmed that 75% of the oxygen released
came from water, while the remaining 25% came from hypochlorite. Based on the
experimental results, a possible catalytic mechanism was proposed: Firstly, compound O111 was oxidized by one electron to form Mn
IV Mn
IV species O112; Then it
underwent ligand exchange reaction of H 2
18
O-Cl
16 O
À to form the intermediate
O113 containing Cl
16 O
À coordination; Then chlorine atom coupled two electrons
to leave in the form of Cl
À ion, and species O113 containing
16 O ¼ Mn
V Mn
V was
obtained. Species O114 could convert itself into species O114 containing
Mn
V Mn
V
¼
18 O through equilibrium reaction; Whether O114 or O115, the final
reaction of H 2 O molecules with electron-deficient Mn
V
¼O produced Mn
III Mn
III
species O116 and released oxygen. Under this catalytic mechanism, it can be
calculated by statistics that 75% of the oxygen atoms in released O 2 are from
water molecules and 25% are from ClO
À
, which is consistent with the experimental
results of isotope labeling experiment.
Catalyst O111 has been further extensively studied with different oxidants. When
Oxone (KSO 5 ) replaces ClO
À as oxidant, the catalytic activity of catalyst O111 for
oxygen evolution becomes enhanced [236–238]. The improvement of catalytic
performance is attributed to the higher reactivity between Oxone and catalyst
O111, and production of bis(μ-O)Mn
IV
Mn
V
¼O active species becomes easy,
430
H.-T. Zhang and M.-T. Zhang
field for many researchers. In the field of water oxidation catalysts, after decades
of development, a large number of MnÀ, FeÀ, CoÀ, NiÀ, CuÀbased water
oxidation catalysts have been developed [61, 230–234], then their catalytic activity,
catalytic mechanism, and other scientific issues have been extensively explored.
Although Ru-based catalysts for water oxidation based on pincer ligands such as
tpy and pdc have been widely developed, excellent catalytic performance has been
demonstrated. However, pincer ligand-based catalysts using the first-row transition
metals for water oxidation have not been widely developed and explored. In 1999,
Crabtree and his coworkers reported a Mn-based catalyst O111 containing
tpy-ligand for water oxidation (Scheme 15) [235]. The discovery of this catalyst
means a major breakthrough, due to the first Mn-based catalyst with the ability to
catalyze water oxidation by chemical oxidation. It is found that the catalytic center of
catalyst O111 is bis(μ-O)Mn
III Mn
IV composed of two different valent Mn atoms. In
fact, the two Mn atoms are identical due to the average valence. In addition, each Mn
center contains a water molecule coordination, which is the basic structure of the
catalyst for water oxidation. In this work, when unconventional NaCIO was used as
oxidant, catalyst O111 could effectively catalyze water oxidation to oxygen, accompanied by general TOF of 12 h
À1 and TON of 4 in 6 h. According to the analysis of
UVÀvis spectra, the ligands on catalyst O111 were oxidized and decomposed to
release manganese ions in the process of catalytic water oxidation driven by chemical oxidation, and manganese ions were further oxidized to form permanganate,
which deactivates the catalyst, thus resulting in modest catalytic capacity. Permanganate and sodium hypochlorite are both oxygen-producing reagents, so the oxygen
source of O 2 evolution by catalytic water oxidation needs to be determined. The
isotope labeling experiments with H 2
18 O confirmed that 75% of the oxygen released
came from water, while the remaining 25% came from hypochlorite. Based on the
experimental results, a possible catalytic mechanism was proposed: Firstly, compound O111 was oxidized by one electron to form Mn
IV Mn
IV species O112; Then it
underwent ligand exchange reaction of H 2
18
O-Cl
16 O
À to form the intermediate
O113 containing Cl
16 O
À coordination; Then chlorine atom coupled two electrons
to leave in the form of Cl
À ion, and species O113 containing
16 O ¼ Mn
V Mn
V was
obtained. Species O114 could convert itself into species O114 containing
Mn
V Mn
V
¼
18 O through equilibrium reaction; Whether O114 or O115, the final
reaction of H 2 O molecules with electron-deficient Mn
V
¼O produced Mn
III Mn
III
species O116 and released oxygen. Under this catalytic mechanism, it can be
calculated by statistics that 75% of the oxygen atoms in released O 2 are from
water molecules and 25% are from ClO
À
, which is consistent with the experimental
results of isotope labeling experiment.
Catalyst O111 has been further extensively studied with different oxidants. When
Oxone (KSO 5 ) replaces ClO
À as oxidant, the catalytic activity of catalyst O111 for
oxygen evolution becomes enhanced [236–238]. The improvement of catalytic
performance is attributed to the higher reactivity between Oxone and catalyst
O111, and production of bis(μ-O)Mn
IV
Mn
V
¼O active species becomes easy,
430
H.-T. Zhang and M.-T. Zhang