which further reacts with Oxone to release oxygen. However, the oxygen evolution
capacity in this system is correlated with the ratio between catalyst and oxidant.
When the oxidant content is 500 times higher than that of the catalyst, most of the
catalysts are oxidized to permanganate, and only 10% of the oxygen released comes
from water; When the content of oxidant is less than 100 times, it is no longer easy
for the catalyst to be deactivated by oxidation, and 50% of the oxygen atoms in the
released oxygen comes from water. From these experimental results, it can be seen
that the oxygen transfer reagent must be used to drive the oxidation of water that
becomes the main limiting factor of catalyst O111. The utilization of Ce
IV as an
oxidant to drive O111 catalytic water oxidation at the molecular catalysis has been
reported, but different views have been maintained among researchers [239–
242]. Subsequent studies have found that catalyst O111 and its analogues can
dimerize into tetranuclear manganese compounds, known as “dimer of dimers”
[243–245]. It was found that the dimers also had the ability of electrocatalytic
water oxidation with poor TON of 2.8. However, DFT calculations showed that
the dimers should first be oxidized and decomposed into binuclear Mn species, and
then react with water molecules to produce oxygen [245].
In 2005, Crabtree and his coworkers synthesized a series of catalysts O117–128
(Fig. 45) with Mn(μ-O) 2 Mn core employed various planar tridentate ligands, and
extensively studied the structure-activity relationship of catalysts [246]. In 2012,
Yagi group studied the substituent effect of catalysts O111, O117, O122, O129–133
(Fig. 45) containing 4
0 -substituted terpy (R-terpy) ligands as O111 derivatives.
Those derivatives were attached to the surface of mica for catalytic water oxidation,
and obvious substituent effect was observed [247].
In 2016, mononuclear manganese compounds O134–135 (Fig. 46) containing
bipy-alkH ligand were reported by Crabtree group [248]. Oxone was used as oxidant
to investigate the oxygen evolution of complexes O134–135. When 25 μm compound O134 was directly added into the solution containing low concentration
Oxone, the initial oxygen release rate maintained 2.15 h
À1 ; When catalyst O134
was pre-oxidized and then added into the reaction system, the initial oxygen
evolution rate increased to 5.27 h
À1 . However, when the concentration of O134
was increased to 125 μm, untreated and pre-oxidized compounds maintained basically the same rate of oxygen evolution. This indicated that mononuclear Mn
compound O134 only existed as a catalytic precursor, and the real active intermediate was formed by interacting with oxygen transfer reagent. Complex O135 with
saturated coordination has no empty coordination site to accommodate a water
molecule, which indicates that it needs to dissociate the ligand before it can obtain
a truly catalytic intermediate. These hypotheses were confirmed in 2019 [249]. Compound O134 was treated with oxidant Oxone, and the binuclear catalyst O136
containing Mn 2 O 2 core was obtained by separation and characterization to catalyze
water oxidation. With Oxone as sacrificial oxidant, catalyst O136 exhibited an initial
TOF of 0.0055 s
À1 , which is only half of that of compound O134 with the same Mn
concentration in catalytic system (TOF ¼ 0.0099 s
À1 ). This indicated that compound
O134, as a catalytic precursor, undergoes a rather intricate process to form truly
active species. Based on isotope labeling experiments, a possible mechanism of
catalytic oxidation of water by catalyst O136 was proposed. Firstly, catalyst O136
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H.-T. Zhang and M.-T. Zhang
capacity in this system is correlated with the ratio between catalyst and oxidant.
When the oxidant content is 500 times higher than that of the catalyst, most of the
catalysts are oxidized to permanganate, and only 10% of the oxygen released comes
from water; When the content of oxidant is less than 100 times, it is no longer easy
for the catalyst to be deactivated by oxidation, and 50% of the oxygen atoms in the
released oxygen comes from water. From these experimental results, it can be seen
that the oxygen transfer reagent must be used to drive the oxidation of water that
becomes the main limiting factor of catalyst O111. The utilization of Ce
IV as an
oxidant to drive O111 catalytic water oxidation at the molecular catalysis has been
reported, but different views have been maintained among researchers [239–
242]. Subsequent studies have found that catalyst O111 and its analogues can
dimerize into tetranuclear manganese compounds, known as “dimer of dimers”
[243–245]. It was found that the dimers also had the ability of electrocatalytic
water oxidation with poor TON of 2.8. However, DFT calculations showed that
the dimers should first be oxidized and decomposed into binuclear Mn species, and
then react with water molecules to produce oxygen [245].
In 2005, Crabtree and his coworkers synthesized a series of catalysts O117–128
(Fig. 45) with Mn(μ-O) 2 Mn core employed various planar tridentate ligands, and
extensively studied the structure-activity relationship of catalysts [246]. In 2012,
Yagi group studied the substituent effect of catalysts O111, O117, O122, O129–133
(Fig. 45) containing 4
0 -substituted terpy (R-terpy) ligands as O111 derivatives.
Those derivatives were attached to the surface of mica for catalytic water oxidation,
and obvious substituent effect was observed [247].
In 2016, mononuclear manganese compounds O134–135 (Fig. 46) containing
bipy-alkH ligand were reported by Crabtree group [248]. Oxone was used as oxidant
to investigate the oxygen evolution of complexes O134–135. When 25 μm compound O134 was directly added into the solution containing low concentration
Oxone, the initial oxygen release rate maintained 2.15 h
À1 ; When catalyst O134
was pre-oxidized and then added into the reaction system, the initial oxygen
evolution rate increased to 5.27 h
À1 . However, when the concentration of O134
was increased to 125 μm, untreated and pre-oxidized compounds maintained basically the same rate of oxygen evolution. This indicated that mononuclear Mn
compound O134 only existed as a catalytic precursor, and the real active intermediate was formed by interacting with oxygen transfer reagent. Complex O135 with
saturated coordination has no empty coordination site to accommodate a water
molecule, which indicates that it needs to dissociate the ligand before it can obtain
a truly catalytic intermediate. These hypotheses were confirmed in 2019 [249]. Compound O134 was treated with oxidant Oxone, and the binuclear catalyst O136
containing Mn 2 O 2 core was obtained by separation and characterization to catalyze
water oxidation. With Oxone as sacrificial oxidant, catalyst O136 exhibited an initial
TOF of 0.0055 s
À1 , which is only half of that of compound O134 with the same Mn
concentration in catalytic system (TOF ¼ 0.0099 s
À1 ). This indicated that compound
O134, as a catalytic precursor, undergoes a rather intricate process to form truly
active species. Based on isotope labeling experiments, a possible mechanism of
catalytic oxidation of water by catalyst O136 was proposed. Firstly, catalyst O136
432
H.-T. Zhang and M.-T. Zhang