24
Y. Shiota and K. Yoshizawa
first-row MO
+ complexes and CH 4 [28, 30], demonstrating that late transition-metal
MO
+ complexes can activate CH 4 . The reaction efficiency and the CH 3 OH branching
ratio are strongly dependent on the metal. For example, FeO
+ efficiently reacts with
CH 4 under ion cyclotron resonance (ICR) conditions, forming CH 3 OH in 41% yield
[21]. Although MnO
+ reacts with CH 4 very efficiently, the CH 3 OH branching ratio is
less than 1% [24]. CoO
+ exhibits low reactivity toward CH 4 ; however, the branching
ratio to CH 3 OH is 100% [27]. Both the reactivity and the CH 3 OH branching ratio are
high in the case of NiO
+ [28]. By contrast, the early transition-metal MO
+ complexes
(ScO
+ , TiO
+ , and VO
+ ) exhibit no reactivity toward alkanes and alkenes because
of their strong metal–oxo bonds. Interestingly, Sc
+ reacts with CH 3 OH to yield
ScO
+ and CH 4 in the gas phase [29], which is precisely the reverse reaction of CH 4
hydroxylation.
From density functional theory (DFT) computations, we determined the reaction
pathway and its detailed energetics for the direct CH 4 –CH 3 OH conversion by FeO
+
[31–37]. Two transition states (TS1 and TS2) were confirmed from intrinsic reaction coordinate (IRC) and femtosecond dynamics calculations to correctly connect
the reaction pathway. Of course, this reaction mechanism is not limited to the gasphase CH 4 hydroxylation by bare MO
+ complexes. We have extended the reaction
mechanism to the CH 4 hydroxylation catalyzed by soluble methane monooxygenase (sMMO) [37–42], particulate methane monooxygenase (pMMO) [43–49], and
metal-exchanged zeolites [50–56].
2 Electronic Structures of MO + Ions
The gas-phase reaction between bare MO
+ cations and CH 4 is particularly interesting because it can be viewed as the simplest system for various oxidation reactions
by enzymatic and zeolitic systems. Carter and Goddard [57] predicted the general
bonding characters of the MO
+ complexes on the basis of all-electron ab initio generalized valence bond calculations. Moreover, Schwarz et al. used DFT computations
to investigate the electronic structures of MO
+ complexes in an attempt to deduce
the reactivity manifold of FeO
+ , CoO
+ , NiO
+ , and CuO
+ with CH 4 [30, 58]. The
reactivity of MO
+ can, in general, be understood by considering how the d-orbital
occupation of the metal dictates the type of metal–oxygen bond formed, with early
transition metals forming strong, unreactive triple bonds and late transition metals
forming weak, reactive biradical double bonds. Figure 1 shows the orbital occupancies in the molecular orbitals of ScO
+ (d
0 ), FeO
+ (d
5 ), and CuO
+ (d
8 ), which can
be partitioned into bonding (2σ and 1π ), nonbonding (1σ and 1δ), and antibonding
(2π * and 3σ *) orbitals. A metal–oxo bond and its catalytic function are strongly
dependent on how the d orbitals are occupied.
In the
1
+ ground state of ScO
+ , all bonding orbitals are doubly occupied,
suggesting a triple bond between the Sc and the O atoms. Therefore, the ground state
of ScO
+ is analogous to that of dinitrogen, the dissociation energy of ScO
+ being
156.1 kcal/mol at the B3LYP level of theory. TiO
+ has a similar ScO
+ bond strength
Y. Shiota and K. Yoshizawa
first-row MO
+ complexes and CH 4 [28, 30], demonstrating that late transition-metal
MO
+ complexes can activate CH 4 . The reaction efficiency and the CH 3 OH branching
ratio are strongly dependent on the metal. For example, FeO
+ efficiently reacts with
CH 4 under ion cyclotron resonance (ICR) conditions, forming CH 3 OH in 41% yield
[21]. Although MnO
+ reacts with CH 4 very efficiently, the CH 3 OH branching ratio is
less than 1% [24]. CoO
+ exhibits low reactivity toward CH 4 ; however, the branching
ratio to CH 3 OH is 100% [27]. Both the reactivity and the CH 3 OH branching ratio are
high in the case of NiO
+ [28]. By contrast, the early transition-metal MO
+ complexes
(ScO
+ , TiO
+ , and VO
+ ) exhibit no reactivity toward alkanes and alkenes because
of their strong metal–oxo bonds. Interestingly, Sc
+ reacts with CH 3 OH to yield
ScO
+ and CH 4 in the gas phase [29], which is precisely the reverse reaction of CH 4
hydroxylation.
From density functional theory (DFT) computations, we determined the reaction
pathway and its detailed energetics for the direct CH 4 –CH 3 OH conversion by FeO
+
[31–37]. Two transition states (TS1 and TS2) were confirmed from intrinsic reaction coordinate (IRC) and femtosecond dynamics calculations to correctly connect
the reaction pathway. Of course, this reaction mechanism is not limited to the gasphase CH 4 hydroxylation by bare MO
+ complexes. We have extended the reaction
mechanism to the CH 4 hydroxylation catalyzed by soluble methane monooxygenase (sMMO) [37–42], particulate methane monooxygenase (pMMO) [43–49], and
metal-exchanged zeolites [50–56].
2 Electronic Structures of MO + Ions
The gas-phase reaction between bare MO
+ cations and CH 4 is particularly interesting because it can be viewed as the simplest system for various oxidation reactions
by enzymatic and zeolitic systems. Carter and Goddard [57] predicted the general
bonding characters of the MO
+ complexes on the basis of all-electron ab initio generalized valence bond calculations. Moreover, Schwarz et al. used DFT computations
to investigate the electronic structures of MO
+ complexes in an attempt to deduce
the reactivity manifold of FeO
+ , CoO
+ , NiO
+ , and CuO
+ with CH 4 [30, 58]. The
reactivity of MO
+ can, in general, be understood by considering how the d-orbital
occupation of the metal dictates the type of metal–oxygen bond formed, with early
transition metals forming strong, unreactive triple bonds and late transition metals
forming weak, reactive biradical double bonds. Figure 1 shows the orbital occupancies in the molecular orbitals of ScO
+ (d
0 ), FeO
+ (d
5 ), and CuO
+ (d
8 ), which can
be partitioned into bonding (2σ and 1π ), nonbonding (1σ and 1δ), and antibonding
(2π * and 3σ *) orbitals. A metal–oxo bond and its catalytic function are strongly
dependent on how the d orbitals are occupied.
In the
1
+ ground state of ScO
+ , all bonding orbitals are doubly occupied,
suggesting a triple bond between the Sc and the O atoms. Therefore, the ground state
of ScO
+ is analogous to that of dinitrogen, the dissociation energy of ScO
+ being
156.1 kcal/mol at the B3LYP level of theory. TiO
+ has a similar ScO
+ bond strength