Orbital Concept for Methane Activation
17
is trapped during the reaction. On the other hand, as shown at the center (right),
Siegbahn [36, 36] and Morokuma and coworkers [37] proposed that a C–H bond of
methane is cleaved in a homolytic manner by six-coordinate diiron model complexes
in the high-spin states. After the dissociation of a C–H bond of methane, the resultant
methyl radical is shifted to the formed OH group. These authors proposed that Qmediated hydroxylation should proceed along the radical rebound mechanism of
cytochrome P450.
In contrast to the radical mechanism, Friesner, Lippard, and coworkers [38]
proposed a non-synchronous concerted mechanism, as indicated at the right of
Fig. 12. Since the C–H activation and the rebound process depend on the coordination sphere and spin state of diiron models adopted, it is important to use an
appropriate diiron model of Q. Detailed discussion about the mechanism of sMMO
is developed in Chapter “Enzymatic Methane Hydroxylation: sMMO and pMMO”.
3.3 Methane Hydroxylation Mechanisms by pMMO
The structure of pMMO determined to 2.8 Å resolution shows a trimeric arrangement and overall folds of three subunits [39]. There are three metal centers per
protomer in the crystal structure. Two of these, which were modeled as mononuclear
and dinuclear copper species, are located within the soluble regions of the pmoB
subunit, the two copper sites being 21 Å apart from each other. In a previous DFT
study [35], we proposed a mechanism for the C–H cleavage and the recombination between CH 3 and OH ligands using a simple mixed-valent dinuclear Cu
II Cu
III
cluster with ammonia and hydroxo ligands. This (μ-O) 2 Cu
II Cu
III species has good
power for methane hydroxylation compared with the μ-η
2 :η
2 -peroxoCu
II Cu
II and
(μ-O) 2 Cu
III Cu
III species, which show no reactivity for the activation of methane.
We reported the mechanisms of methane hydroxylation at the mononuclear and
dinuclear copper sites on the basis of the crystal structure of pMMO and considered
how reactive copper species are formed in the protein environment [40, 41]. By
looking at the coordination environments, we set up three kinds of model complexes
for DFT calculations to search the reaction pathway for the conversion of methane to
methanol. One is a mononuclear Cu
III –O (or Cu
II –O
· ) model with two imidazole and
one acetate, and others are mixed-valent (μ-O) 2 Cu
II Cu
III and (μ-O) (μ-OH)Cu
II Cu
III
models with three imidazole and one acetate.
Figure 13 shows a mechanism for methane hydroxylation by the monocopper-oxo
species. The copper-oxo species optimized by using a small model in the gas phase
is in good agreement with the one optimized in the protein environment with respect
to the coordination bonds around the central copper atom. In the initial stages of the
reaction, methane is weakly bound to the monocopper active center, and after that,
one of the C–H bonds of methane is cleaved by the oxo species via the first transition
state (TS1).
The resultant radical species leads to a non-radical intermediate, which is
extremely stable in energy. Since the formal charge of the copper ion is changed from
17
is trapped during the reaction. On the other hand, as shown at the center (right),
Siegbahn [36, 36] and Morokuma and coworkers [37] proposed that a C–H bond of
methane is cleaved in a homolytic manner by six-coordinate diiron model complexes
in the high-spin states. After the dissociation of a C–H bond of methane, the resultant
methyl radical is shifted to the formed OH group. These authors proposed that Qmediated hydroxylation should proceed along the radical rebound mechanism of
cytochrome P450.
In contrast to the radical mechanism, Friesner, Lippard, and coworkers [38]
proposed a non-synchronous concerted mechanism, as indicated at the right of
Fig. 12. Since the C–H activation and the rebound process depend on the coordination sphere and spin state of diiron models adopted, it is important to use an
appropriate diiron model of Q. Detailed discussion about the mechanism of sMMO
is developed in Chapter “Enzymatic Methane Hydroxylation: sMMO and pMMO”.
3.3 Methane Hydroxylation Mechanisms by pMMO
The structure of pMMO determined to 2.8 Å resolution shows a trimeric arrangement and overall folds of three subunits [39]. There are three metal centers per
protomer in the crystal structure. Two of these, which were modeled as mononuclear
and dinuclear copper species, are located within the soluble regions of the pmoB
subunit, the two copper sites being 21 Å apart from each other. In a previous DFT
study [35], we proposed a mechanism for the C–H cleavage and the recombination between CH 3 and OH ligands using a simple mixed-valent dinuclear Cu
II Cu
III
cluster with ammonia and hydroxo ligands. This (μ-O) 2 Cu
II Cu
III species has good
power for methane hydroxylation compared with the μ-η
2 :η
2 -peroxoCu
II Cu
II and
(μ-O) 2 Cu
III Cu
III species, which show no reactivity for the activation of methane.
We reported the mechanisms of methane hydroxylation at the mononuclear and
dinuclear copper sites on the basis of the crystal structure of pMMO and considered
how reactive copper species are formed in the protein environment [40, 41]. By
looking at the coordination environments, we set up three kinds of model complexes
for DFT calculations to search the reaction pathway for the conversion of methane to
methanol. One is a mononuclear Cu
III –O (or Cu
II –O
· ) model with two imidazole and
one acetate, and others are mixed-valent (μ-O) 2 Cu
II Cu
III and (μ-O) (μ-OH)Cu
II Cu
III
models with three imidazole and one acetate.
Figure 13 shows a mechanism for methane hydroxylation by the monocopper-oxo
species. The copper-oxo species optimized by using a small model in the gas phase
is in good agreement with the one optimized in the protein environment with respect
to the coordination bonds around the central copper atom. In the initial stages of the
reaction, methane is weakly bound to the monocopper active center, and after that,
one of the C–H bonds of methane is cleaved by the oxo species via the first transition
state (TS1).
The resultant radical species leads to a non-radical intermediate, which is
extremely stable in energy. Since the formal charge of the copper ion is changed from