42
Y. Shiota and K. Yoshizawa
In the early transition-metal oxides, the crossing point exists prior to TS2, the threecentered transition state for methyl migration. The crossing point of the NiO
+ exists
prior to TS1 (the four-centered transition state for the cleavage of a C–H bond of
CH 4 ), and that of CuO
+ exist after TS1. For the NiO
+ potential-energy surface, there
is another crossing point in exit channel; this crossing point does not play a substantial
role.
Reasonably detailed potential-energy surfaces for this conversion can explain the
experimental observations that the early transition-metal oxides hardly react with
CH 4 , whereas the late transition-metal oxides react with CH 4 . In conclusion, the
early transition-metal oxides cannot proceed to the product complex via TS2, which
is very high in energy, as the rate-determining step. After H-atom abstraction, the
late transition-metal oxides proceed to the product complex.
In the FeO
+ /CH 4 system, crossing seams along the quartet and sextet IRC paths
and the SOC effects for the conversion of CH 4 were calculated. As a result of the
IRC path, there are three crossing seams between the sextet and quartet potentialenergy surfaces, and spin inversion should occur twice: FeO
+ (sextet) + CH 4 →
[FeO
+ (CH 4 )] (sextet) → spin inversion → TS1 (quartet) → [HO–Fe
+ –CH 3 ] (quartet)
→ TS2 (quartet) → [Fe
+ (CH 3 OH)] (quartet) → spin inversion → Fe
+ (sextet) +
CH 3 OH. The first crossing seam is the most important aspect of this reaction pathway
because a spin inversion from the sextet to the quartet state decreases the activation
barriers for the C–H bond dissociation of CH 4 . From CASSCF wavefunctions, SOC
was computed along the reaction pathway to estimate the probability of spin inversion. The SOC value decreases along the reaction pathway, approaching zero in the
product complex. The SOC analysis indicated that the spin inversion from the sextet
state to the quartet state should occur in the first crossing seam, whereas the spin
state should remain unchanged in both the second and third crossing seams.
The reaction in the condensed phase must be considered to predict the activity
of catalysts in experiments. The two-step concerted mechanism presented here is
expected to not be limited to the hydroxylation of gas-phase CH 4 by MO
+ , and these
results can be used to understand the catalytic and enzymatic processes concerning
alkane hydroxylation. We believe that the results of the gas-phase experiments and
calculations provide valuable insight into the elementary step of the oxidation process
and that the combination of experimental and theoretical studies enables us to realize
the design of effective catalysts for the hydroxylation of alkanes.
Acknowledgements This project was partially supported by JST-CREST “Innovative Catalysts”
JPMJCR15P5.
References and Notes
1. The Activation of Saturated Hydrocarbons by Transitions Metal Complexes, Shilov AE (1984)
Riedel Publishing: Dordrecht, The Netherlands
2. Shilov AE, Shul’pin GB (1997) Chem Rev 97:2879–2932
Y. Shiota and K. Yoshizawa
In the early transition-metal oxides, the crossing point exists prior to TS2, the threecentered transition state for methyl migration. The crossing point of the NiO
+ exists
prior to TS1 (the four-centered transition state for the cleavage of a C–H bond of
CH 4 ), and that of CuO
+ exist after TS1. For the NiO
+ potential-energy surface, there
is another crossing point in exit channel; this crossing point does not play a substantial
role.
Reasonably detailed potential-energy surfaces for this conversion can explain the
experimental observations that the early transition-metal oxides hardly react with
CH 4 , whereas the late transition-metal oxides react with CH 4 . In conclusion, the
early transition-metal oxides cannot proceed to the product complex via TS2, which
is very high in energy, as the rate-determining step. After H-atom abstraction, the
late transition-metal oxides proceed to the product complex.
In the FeO
+ /CH 4 system, crossing seams along the quartet and sextet IRC paths
and the SOC effects for the conversion of CH 4 were calculated. As a result of the
IRC path, there are three crossing seams between the sextet and quartet potentialenergy surfaces, and spin inversion should occur twice: FeO
+ (sextet) + CH 4 →
[FeO
+ (CH 4 )] (sextet) → spin inversion → TS1 (quartet) → [HO–Fe
+ –CH 3 ] (quartet)
→ TS2 (quartet) → [Fe
+ (CH 3 OH)] (quartet) → spin inversion → Fe
+ (sextet) +
CH 3 OH. The first crossing seam is the most important aspect of this reaction pathway
because a spin inversion from the sextet to the quartet state decreases the activation
barriers for the C–H bond dissociation of CH 4 . From CASSCF wavefunctions, SOC
was computed along the reaction pathway to estimate the probability of spin inversion. The SOC value decreases along the reaction pathway, approaching zero in the
product complex. The SOC analysis indicated that the spin inversion from the sextet
state to the quartet state should occur in the first crossing seam, whereas the spin
state should remain unchanged in both the second and third crossing seams.
The reaction in the condensed phase must be considered to predict the activity
of catalysts in experiments. The two-step concerted mechanism presented here is
expected to not be limited to the hydroxylation of gas-phase CH 4 by MO
+ , and these
results can be used to understand the catalytic and enzymatic processes concerning
alkane hydroxylation. We believe that the results of the gas-phase experiments and
calculations provide valuable insight into the elementary step of the oxidation process
and that the combination of experimental and theoretical studies enables us to realize
the design of effective catalysts for the hydroxylation of alkanes.
Acknowledgements This project was partially supported by JST-CREST “Innovative Catalysts”
JPMJCR15P5.
References and Notes
1. The Activation of Saturated Hydrocarbons by Transitions Metal Complexes, Shilov AE (1984)
Riedel Publishing: Dordrecht, The Netherlands
2. Shilov AE, Shul’pin GB (1997) Chem Rev 97:2879–2932