60
T. Yumura et al.
After the formation of the hydoxo intermediate, the hydrogen atom of the OH
group rotates out of the (μ-O) 2 Fe 2 plane, and the methyl group migrates toward
the OH group via a three-centered transition state in the second step (TS2). In this
transition state, the Fe–C bond is cleaved, and instead the C–O bond is newly formed.
The barrier height of TS2 is 15.7 kcal/mol relative to the dissociation limit toward
methane and the intermediate Q. After TS2, a methanol complex is formed by the
binding of the methyl and hydroxo ligands. As a result, the oxygen atom of the formed
methanol binds into two iron atoms in this complex. The final step in the reaction is
returning hydrogen-bonded water molecule to the five-coordinate iron atom to lead
to a coordinatively saturated methanol complex.
Figure 8 shows seven local minima and two transition states connecting two local
minima on the potential energy surface of this reaction starting from MMOH red ,
methane and dioxygen [52]. From Fig. 8, we found that all reaction intermediates
and transition states are energetically stable relative to the initial state (MMOH red +
CH 4 + O 2 ), and therefore the entire reaction is downhill. More importantly, Fig. 8
shows the most energy-consuming process is the H-abstraction from methane by
MMOH Q via the four-centered transition state. This process can be overcome by the
excess energy released in the dioxygen activation step, although this energy can be
partially absorbed by the protein pocket.
5.2 Radical Rebound Mechanism
In the radical rebound mechanism, the active site of MMOH Q consists of a coordinatively saturated diiron core, whose bridging oxygen atom acts as a reactive center
toward methane. Morokuma group used
9 A and
11 A spin states for the active site
of MMOH Q , and found that the bridging oxygen atoms have radical characters [66,
68]. According to their DFT calculations, a methane C–H bond is dissociated in a
homolytic manner by a radical bridging oxygen atom in the active site of MMOH Q .
This homolytic C–H bond dissociation proceeds via a transition state with a linear C–
H–O alignment, whose potential energy surface in the
9 A spin state is energetically
favorable relative to that in the
11 A spin state. When the model (n) in Fig. 4 was used,
the activation energy for this process in the
9 A spin state depends on the location of
a bridging oxygen atom on H 2 O or NH 2 side: 23.2 kcal/mol for the H 2 O-side and
19.5 kcal/mol for the NH 2 -side [66, 68]. Their small model was improved by using
imidazole and carboxylate ligands (model (o) in Fig. 4), and obtained similar value
(21.8 kcal/mol) for the activation energy [74]. The homolytic C–H bond dissociation
is rate-determining, because its activation energy is higher than that in the second
step (less than ten kcal/mol), where the OH group is rotated, and then methyl radical
is rebound to the OH group in MMOH Q . After the second step, a methanol complex
is formed, whose ground spin state is
11 A (Fig. 9).
Similar radical rebound mechanism was also investigated by Siegbahn et al. [57–
65] But a new intermediate Q (MMOH Q’ ) with a mixed valence Fe(III)Fe(IV) state
was considered in the
9 A spin state (model (m) in Fig. 4) [63]. MMOH Q’ , that also has
T. Yumura et al.
After the formation of the hydoxo intermediate, the hydrogen atom of the OH
group rotates out of the (μ-O) 2 Fe 2 plane, and the methyl group migrates toward
the OH group via a three-centered transition state in the second step (TS2). In this
transition state, the Fe–C bond is cleaved, and instead the C–O bond is newly formed.
The barrier height of TS2 is 15.7 kcal/mol relative to the dissociation limit toward
methane and the intermediate Q. After TS2, a methanol complex is formed by the
binding of the methyl and hydroxo ligands. As a result, the oxygen atom of the formed
methanol binds into two iron atoms in this complex. The final step in the reaction is
returning hydrogen-bonded water molecule to the five-coordinate iron atom to lead
to a coordinatively saturated methanol complex.
Figure 8 shows seven local minima and two transition states connecting two local
minima on the potential energy surface of this reaction starting from MMOH red ,
methane and dioxygen [52]. From Fig. 8, we found that all reaction intermediates
and transition states are energetically stable relative to the initial state (MMOH red +
CH 4 + O 2 ), and therefore the entire reaction is downhill. More importantly, Fig. 8
shows the most energy-consuming process is the H-abstraction from methane by
MMOH Q via the four-centered transition state. This process can be overcome by the
excess energy released in the dioxygen activation step, although this energy can be
partially absorbed by the protein pocket.
5.2 Radical Rebound Mechanism
In the radical rebound mechanism, the active site of MMOH Q consists of a coordinatively saturated diiron core, whose bridging oxygen atom acts as a reactive center
toward methane. Morokuma group used
9 A and
11 A spin states for the active site
of MMOH Q , and found that the bridging oxygen atoms have radical characters [66,
68]. According to their DFT calculations, a methane C–H bond is dissociated in a
homolytic manner by a radical bridging oxygen atom in the active site of MMOH Q .
This homolytic C–H bond dissociation proceeds via a transition state with a linear C–
H–O alignment, whose potential energy surface in the
9 A spin state is energetically
favorable relative to that in the
11 A spin state. When the model (n) in Fig. 4 was used,
the activation energy for this process in the
9 A spin state depends on the location of
a bridging oxygen atom on H 2 O or NH 2 side: 23.2 kcal/mol for the H 2 O-side and
19.5 kcal/mol for the NH 2 -side [66, 68]. Their small model was improved by using
imidazole and carboxylate ligands (model (o) in Fig. 4), and obtained similar value
(21.8 kcal/mol) for the activation energy [74]. The homolytic C–H bond dissociation
is rate-determining, because its activation energy is higher than that in the second
step (less than ten kcal/mol), where the OH group is rotated, and then methyl radical
is rebound to the OH group in MMOH Q . After the second step, a methanol complex
is formed, whose ground spin state is
11 A (Fig. 9).
Similar radical rebound mechanism was also investigated by Siegbahn et al. [57–
65] But a new intermediate Q (MMOH Q’ ) with a mixed valence Fe(III)Fe(IV) state
was considered in the
9 A spin state (model (m) in Fig. 4) [63]. MMOH Q’ , that also has