58
T. Yumura et al.
5 Details in Mechanisms for the Methane Hydroxylation
by MMOH Q
In this section, let us focus on details on the energetics in the methane hydroxylation
by MMOH Q via the radical rebound, nonsynchronous concerted, and nonradical
mechanisms.
5.1 Nonradical Mechanism
In order to proceed the methane hydroxylation by MMOH Q via the nonradical mechanism [44, 46, 52], the presence of a coordinaitively unsaturated diiron(IV) active
site in its catalytic reaction is indispensable. The formation of such active site can
be rationalized by DFT calculations where the catalytic cycle from MMOH red to
MMOH Q was investigated in terms of the energetics [52]. In this catalytic cycle,
dioxygen is incorporated into the diiron active site of MMOH red , which results in
the formation of peroxo intermediates (MMOH peroxo ) and MMOH Q . According to
the observed X-ray structure of MMOH red , Ref. [52] assumed that its active site
consists of five-coordinate iron cations, before the reaction with dioxygen, as shown
in Fig. 8. In the optimized model of MMOH red , a formate ligand forms a bidentate
chelating interaction with the left-hand iron atom and a single bond with the righthand iron atom. Each iron atom of MMOH red has a five-coordinate environment, and
thus the active site directly interacts with dioxygen to form peroxo intermediates with
different dioxygen binding modes (MMOH peroxo ) After that, the peroxo intermediates
can convert into MMOH Q with a five-coordinate iron atom. In fact, the coordinatively unsaturated diiron active site of MMOH Q is 56.3 kcal/mol stable relative to
the dissociation limit toward MMOH red and dioxygen. This result strongly suggested
the formation of a coordinatively unsaturated diiron(IV) active site of MMOH Q in
the catalytic cycle is feasible from a viewpoint of the energetics [52].
After the formation of the coordinatively unsaturated diiron active site, the fivecoordinate iron center can utilize its nonbonding d orbital to directly interact with
methane to form a methane complex In the methane complex, attractive interactions
can operate between methane and the coordinatively unsaturated diiron active site,
which is in contrast to the other radical-related mechanisms, as will be described.
These interactions through direct Fe–C couplings facilitate to abstract an H atom
from methane via a four-centered transition state (TS1). This process requires an
activation energy of 30.4 kcal/mol relative to the dissociation limit toward methane
and MMOH Q [52]. The intramolecular H atom migration leads to the formation
of an important intermediate where newly formed OH and CH 3 groups directly
coordinate to an iron atom as ligands. This intermediate, called hydoxo intermediate,
corresponds to the important insertion species (HO–Fe
+ –CH 3 ) formed in the methane
to methanol conversion by FeO
+ in the gas phase [118–121].
T. Yumura et al.
5 Details in Mechanisms for the Methane Hydroxylation
by MMOH Q
In this section, let us focus on details on the energetics in the methane hydroxylation
by MMOH Q via the radical rebound, nonsynchronous concerted, and nonradical
mechanisms.
5.1 Nonradical Mechanism
In order to proceed the methane hydroxylation by MMOH Q via the nonradical mechanism [44, 46, 52], the presence of a coordinaitively unsaturated diiron(IV) active
site in its catalytic reaction is indispensable. The formation of such active site can
be rationalized by DFT calculations where the catalytic cycle from MMOH red to
MMOH Q was investigated in terms of the energetics [52]. In this catalytic cycle,
dioxygen is incorporated into the diiron active site of MMOH red , which results in
the formation of peroxo intermediates (MMOH peroxo ) and MMOH Q . According to
the observed X-ray structure of MMOH red , Ref. [52] assumed that its active site
consists of five-coordinate iron cations, before the reaction with dioxygen, as shown
in Fig. 8. In the optimized model of MMOH red , a formate ligand forms a bidentate
chelating interaction with the left-hand iron atom and a single bond with the righthand iron atom. Each iron atom of MMOH red has a five-coordinate environment, and
thus the active site directly interacts with dioxygen to form peroxo intermediates with
different dioxygen binding modes (MMOH peroxo ) After that, the peroxo intermediates
can convert into MMOH Q with a five-coordinate iron atom. In fact, the coordinatively unsaturated diiron active site of MMOH Q is 56.3 kcal/mol stable relative to
the dissociation limit toward MMOH red and dioxygen. This result strongly suggested
the formation of a coordinatively unsaturated diiron(IV) active site of MMOH Q in
the catalytic cycle is feasible from a viewpoint of the energetics [52].
After the formation of the coordinatively unsaturated diiron active site, the fivecoordinate iron center can utilize its nonbonding d orbital to directly interact with
methane to form a methane complex In the methane complex, attractive interactions
can operate between methane and the coordinatively unsaturated diiron active site,
which is in contrast to the other radical-related mechanisms, as will be described.
These interactions through direct Fe–C couplings facilitate to abstract an H atom
from methane via a four-centered transition state (TS1). This process requires an
activation energy of 30.4 kcal/mol relative to the dissociation limit toward methane
and MMOH Q [52]. The intramolecular H atom migration leads to the formation
of an important intermediate where newly formed OH and CH 3 groups directly
coordinate to an iron atom as ligands. This intermediate, called hydoxo intermediate,
corresponds to the important insertion species (HO–Fe
+ –CH 3 ) formed in the methane
to methanol conversion by FeO
+ in the gas phase [118–121].