26
Y. Shiota and K. Yoshizawa
perturbation, which induces mixing between the low-lying quartet and the doublet
electronic excited states of NiO
+ .
The high-oxidation-state Cu(III) species in CuO
+ is difficult to understand because
the +3 formal charge of Cu and the −2 formal charge of O are unrealistic in general.
However, the high-oxidation-state Cu(III) species has been increasingly recognized
as a chemically useful species from unknown species in theoretical studies. Bera
et al. discussed high-oxidation-state copper organometallics using the MP2 level of
theory [62]. According to their computed properties of the complex of CuO
+ and
ethylene, the Cu(III) complexes do not have a d
8 electronic configuration. Actually,
the electronic configurations of formally Cu(III) species in the metal were calculated
to be similar to those expected for Cu(I) species as a d
10 closed-shell system. We
agree with the assignment by Bera et al. of the Cu(I) and the O fragments for CuO
+
because the Mulliken charges were also calculated to be 1.26 for Cu and −0.26 for
O at the B3LYP level of theory. The computed Cu–O bond energy in the
3
ground
state was 37.6 kcal/mol, in good agreement with the reported experimental value of
37 kcal/mol [60].
Bridging these extremes in behavior are CrO
+ and MnO
+ . The ground state
of CrO
+ can oxidize saturated hydrocarbons larger than CH 4 . The Cr–O bond
strength for the ground
4
− state of CrO
+ is weak compared with that of other
early transition-metal MO
+ . The computed and experimental bond energies for Cr–
O are 81.3 kcal/mol and 85.3 kcal/mol, respectively [60]. Carter and Goddard [57]
suggested quintet ground states (
5
+ or
5
). In the case of the bond dissociation
energy and the energetics of MnO
+ , the difference between the
5
+ or
5
states is
negligible; thus, the exact energy separation remains unclear. According to computational results for MnO
+ , the
5
+ ground state of MnO
+ is lower in energy than the
5
state. The computed and experimental bond energies for Mn–O are 56.4 kcal/mol
and 68 kcal/mol, respectively [60].
3 Potential-Energy Diagrams
for the Methane-To-Methanol Conversion
As shown in Scheme 1, CH 4 hydroxylation by bare MO
+ ions is predicted to occur
via a nonradical mechanism through heterolytic C–H bond cleavage [31]. In the first
half of the reaction, a H-atom of the adsorbed CH 4 molecule in a reactant complex
(RC) migrates to the O atom of the active site via a four-centered transition state
(TS1), leading to the formation of a hydroxo intermediate (HI), where methyl and
hydroxo moieties are formed on the active site. In the second half of the reaction,
HO–CH 3 recombination occurs via a three-centered transition state (TS2) to form a
product complex (PC).
Y. Shiota and K. Yoshizawa
perturbation, which induces mixing between the low-lying quartet and the doublet
electronic excited states of NiO
+ .
The high-oxidation-state Cu(III) species in CuO
+ is difficult to understand because
the +3 formal charge of Cu and the −2 formal charge of O are unrealistic in general.
However, the high-oxidation-state Cu(III) species has been increasingly recognized
as a chemically useful species from unknown species in theoretical studies. Bera
et al. discussed high-oxidation-state copper organometallics using the MP2 level of
theory [62]. According to their computed properties of the complex of CuO
+ and
ethylene, the Cu(III) complexes do not have a d
8 electronic configuration. Actually,
the electronic configurations of formally Cu(III) species in the metal were calculated
to be similar to those expected for Cu(I) species as a d
10 closed-shell system. We
agree with the assignment by Bera et al. of the Cu(I) and the O fragments for CuO
+
because the Mulliken charges were also calculated to be 1.26 for Cu and −0.26 for
O at the B3LYP level of theory. The computed Cu–O bond energy in the
3
ground
state was 37.6 kcal/mol, in good agreement with the reported experimental value of
37 kcal/mol [60].
Bridging these extremes in behavior are CrO
+ and MnO
+ . The ground state
of CrO
+ can oxidize saturated hydrocarbons larger than CH 4 . The Cr–O bond
strength for the ground
4
− state of CrO
+ is weak compared with that of other
early transition-metal MO
+ . The computed and experimental bond energies for Cr–
O are 81.3 kcal/mol and 85.3 kcal/mol, respectively [60]. Carter and Goddard [57]
suggested quintet ground states (
5
+ or
5
). In the case of the bond dissociation
energy and the energetics of MnO
+ , the difference between the
5
+ or
5
states is
negligible; thus, the exact energy separation remains unclear. According to computational results for MnO
+ , the
5
+ ground state of MnO
+ is lower in energy than the
5
state. The computed and experimental bond energies for Mn–O are 56.4 kcal/mol
and 68 kcal/mol, respectively [60].
3 Potential-Energy Diagrams
for the Methane-To-Methanol Conversion
As shown in Scheme 1, CH 4 hydroxylation by bare MO
+ ions is predicted to occur
via a nonradical mechanism through heterolytic C–H bond cleavage [31]. In the first
half of the reaction, a H-atom of the adsorbed CH 4 molecule in a reactant complex
(RC) migrates to the O atom of the active site via a four-centered transition state
(TS1), leading to the formation of a hydroxo intermediate (HI), where methyl and
hydroxo moieties are formed on the active site. In the second half of the reaction,
HO–CH 3 recombination occurs via a three-centered transition state (TS2) to form a
product complex (PC).