28
2 Selective Production of Methanol …
(Fig. 2.3) have been characterized using various kinetic and spectroscopic methods
[19–24], from which the following catalytic cycle has been developed.
Step 1: Reduction of MMOH
The oxidized form of MMOH (MMOH ox ), in which the oxidation state of the
di-iron center is Fe
II Fe
II , accepts two electrons from NADH via MMOR to give
MMOH red , which has a high-spin weakly ferromagnetically coupled Fe
II Fe
II center.
Only MMOH red can react with molecular oxygen to initiate the catalytic cycle.
Step 2: Binding of molecular oxygen to MMOH
In the presence of two equivalents of MMOB, MMOH red reacts with molecular
oxygen to form intermediate O. In this intermediate, molecular oxygen binds to
MMOH, but not directly to the di-nuclear iron center.
Step 3: Formation of the peroxo intermediates
Intermediate O transforms into intermediate P*, which has a peroxo di-iron center.
The binding mode of O 2 to the di-nuclear iron center is unknown, and the oxidation
state of the di-iron center is still controversial. Fe
III Fe
III species that differ in ligand
arrangement from the subsequent intermediate P have been found. However, when
the rate of conversion of the intermediate P* into P was slowed, an Fe
II Fe
II species
was observed. This intermediate is proposed to form via replacement of the bridging
H 2 O ligand by the oxygen molecule without electron transfer from the di-iron center.
Intermediate P* then transforms into a peroxo di-iron species referred to as intermediate P, in which the oxidation state of the two irons is Fe
III . Based on spectroscopic observations and density functional theory (DFT) calculations, a μ-1,2-peroxo
bridge between the two irons has been suggested. The formation of this intermediate
depends on the pH, and shows a kinetic solvent isotope effect in D 2 O, indicating that
proton transfer is involved in the formation of intermediate P.
Intermediate P can also be generated by the reaction of MMOH ox with H 2 O 2 via
a mechanism known as the peroxide shunt. The apparent K m value of the peroxide
shunt is about 66 mM, while that of the reaction of molecular oxygen with MMOH ox
is about 3 μM. Therefore, a high concentration of H 2 O 2 is required to drive the
peroxide shunt.
Step 4: O–O bond cleavage
Intermediate P is then converted into intermediate Q. This transformation depends
on the pH and shows a normal kinetic solvent isotope effect in D 2 O, indicating that
the formation of intermediate Q requires proton transfer. Intermediate Q exhibits a
yellow color due to its absorption at 430 and 330 nm (E 430 , 330 = 7500 M
−1 cm
−1 )
and has a lifetime of several seconds, making it possible to investigate its structure
and properties.
2 Selective Production of Methanol …
(Fig. 2.3) have been characterized using various kinetic and spectroscopic methods
[19–24], from which the following catalytic cycle has been developed.
Step 1: Reduction of MMOH
The oxidized form of MMOH (MMOH ox ), in which the oxidation state of the
di-iron center is Fe
II Fe
II , accepts two electrons from NADH via MMOR to give
MMOH red , which has a high-spin weakly ferromagnetically coupled Fe
II Fe
II center.
Only MMOH red can react with molecular oxygen to initiate the catalytic cycle.
Step 2: Binding of molecular oxygen to MMOH
In the presence of two equivalents of MMOB, MMOH red reacts with molecular
oxygen to form intermediate O. In this intermediate, molecular oxygen binds to
MMOH, but not directly to the di-nuclear iron center.
Step 3: Formation of the peroxo intermediates
Intermediate O transforms into intermediate P*, which has a peroxo di-iron center.
The binding mode of O 2 to the di-nuclear iron center is unknown, and the oxidation
state of the di-iron center is still controversial. Fe
III Fe
III species that differ in ligand
arrangement from the subsequent intermediate P have been found. However, when
the rate of conversion of the intermediate P* into P was slowed, an Fe
II Fe
II species
was observed. This intermediate is proposed to form via replacement of the bridging
H 2 O ligand by the oxygen molecule without electron transfer from the di-iron center.
Intermediate P* then transforms into a peroxo di-iron species referred to as intermediate P, in which the oxidation state of the two irons is Fe
III . Based on spectroscopic observations and density functional theory (DFT) calculations, a μ-1,2-peroxo
bridge between the two irons has been suggested. The formation of this intermediate
depends on the pH, and shows a kinetic solvent isotope effect in D 2 O, indicating that
proton transfer is involved in the formation of intermediate P.
Intermediate P can also be generated by the reaction of MMOH ox with H 2 O 2 via
a mechanism known as the peroxide shunt. The apparent K m value of the peroxide
shunt is about 66 mM, while that of the reaction of molecular oxygen with MMOH ox
is about 3 μM. Therefore, a high concentration of H 2 O 2 is required to drive the
peroxide shunt.
Step 4: O–O bond cleavage
Intermediate P is then converted into intermediate Q. This transformation depends
on the pH and shows a normal kinetic solvent isotope effect in D 2 O, indicating that
the formation of intermediate Q requires proton transfer. Intermediate Q exhibits a
yellow color due to its absorption at 430 and 330 nm (E 430 , 330 = 7500 M
−1 cm
−1 )
and has a lifetime of several seconds, making it possible to investigate its structure
and properties.