96
M. H. Mahyuddin
neighboring [Cu 2 (μ-O)]
2+ active sites via a transition structure of [Cu
III –(μO) 2 –
Cu
III ]
2+ is energetically more favorable with an O–O bond activation barrier of only
10.5 kcal/mol [32].
Figure 7 (black lines) shows detailed energy diagrams of O 2 activation on 2[Cu
I
2 ]MOR, where two [Cu 2 (μ-O)]
2+ active sites are simultaneously formed. In this mechanism, there are two Cu
I ··· Cu
I pairs hosted on four Al tetrahedra within the 8-MR
side-pocket windows of MOR. These windows are neighboring to each other and
separated by about 4 Å. The initial complex (IC) is formed in the triplet ground
state since the Cu
I centers have a CSS ground state and O 2 in the gas phase (O=O =
1.207 Å) has a triplet ground state. Subsequently, the formation of μ-η
2 :η
2 -peroxoCu
II
2 precursor (O–O = 1.465 Å) in the OSS ground state is extremely exothermic
with a binding energy of −53.1 kcal/mol. Here, one of the Cu
I ··· Cu
I pairs is oxidized
to Cu
II ··· Cu
II while the other one remains unchanged. The peroxo bond is then
cleaved via a first transition state (TS1) having a structure of bis(μ-O)Cu
III
2 (O ···
O = 2.201 Å) in the CSS state. The activation energy required for this process is
10.5 kcal/mol, which is expectedly higher than that for the N 2 –O cleavage discussed
in Sect. 3.1. At this step, a four-electron process of O=O bond cleavage is already
complete. However, to form two [Cu 2 (μ-O)]
2+ active sites, one of the separated O
atoms should migrate to the neighboring 8-MR side-pocket window through the
formation of an intermediate species (Int). This intermediate structure is energetically very stable in the triplet ground state with all Cu centers having an oxidation
state of +2. Finally, the formation of two [Cu
II
2 (μ-O)]
2+ active sites in the triplet
state then takes place via a second transition state (TS2), requiring an activation
energy of 12.0 kcal/mol.
In the formation of tricopper [Cu 3 (μ-O) 3 ]
2+ active site from O 2 activation on CuMOR, Yoshizawa and co-workers recently suggested a [Cu
I
2 Cu
II O]
2+ species in the
doublet ground state as a theoretically possible precursor [32]. This species is formed
as a result of two consecutive oxidations of methane over the tricopper active site
[33, 34]. In this case, since the precursor can provide four electrons required to break
the O=O bond of O 2 molecule forming a [Cu
III
2 Cu
II (μ-O) 3 ]
2+ [32] or [Cu
II
2 Cu
III (μO) 2 (μ-O
· )]
2+ [35] active species, no spectator Cu cations are needed. As shown in
Fig. 8, IC has a quartet ground state (precursor in the doublet state + O 2 in the
triplet state) and the reaction begins with the adsorption of an O 2 molecule on one
Cu center of the precursor with a slightly elongated O–O bond length from 1.207
to 1.260 Å, suggesting that the molecule is already activated even at this stage of
reaction. An intermediate structure μ-η
2 :η
2 -peroxo-Cu
II
3 O (Int) is then formed via
TS1, where the activated O 2 now forms a Cu
II O 2
· superoxo structure with a further
elongated O–O bond length of 1.398 Å. The formation of Int requires an activation
energy of 24.2 kcal/mol, and a spin inversion from the quartet state to the open-shell
doublet state (i.e. two Cu centers have α-spin electron while the other Cu center
has a β-spin electron) is expected to occur after TS1. Subsequently, the peroxo
bond of Int is cleaved via TS2 with a cleaved O ··· O distance of 1.810 Å to form
the [Cu 3 (μ-O) 3 ]
2+ active site in the quartet ground state. Given another activation
barrier of 10.4 kcal/mol for this cleavage, we obtain an effective activation barrier of
31.6 kcal/mol.
M. H. Mahyuddin
neighboring [Cu 2 (μ-O)]
2+ active sites via a transition structure of [Cu
III –(μO) 2 –
Cu
III ]
2+ is energetically more favorable with an O–O bond activation barrier of only
10.5 kcal/mol [32].
Figure 7 (black lines) shows detailed energy diagrams of O 2 activation on 2[Cu
I
2 ]MOR, where two [Cu 2 (μ-O)]
2+ active sites are simultaneously formed. In this mechanism, there are two Cu
I ··· Cu
I pairs hosted on four Al tetrahedra within the 8-MR
side-pocket windows of MOR. These windows are neighboring to each other and
separated by about 4 Å. The initial complex (IC) is formed in the triplet ground
state since the Cu
I centers have a CSS ground state and O 2 in the gas phase (O=O =
1.207 Å) has a triplet ground state. Subsequently, the formation of μ-η
2 :η
2 -peroxoCu
II
2 precursor (O–O = 1.465 Å) in the OSS ground state is extremely exothermic
with a binding energy of −53.1 kcal/mol. Here, one of the Cu
I ··· Cu
I pairs is oxidized
to Cu
II ··· Cu
II while the other one remains unchanged. The peroxo bond is then
cleaved via a first transition state (TS1) having a structure of bis(μ-O)Cu
III
2 (O ···
O = 2.201 Å) in the CSS state. The activation energy required for this process is
10.5 kcal/mol, which is expectedly higher than that for the N 2 –O cleavage discussed
in Sect. 3.1. At this step, a four-electron process of O=O bond cleavage is already
complete. However, to form two [Cu 2 (μ-O)]
2+ active sites, one of the separated O
atoms should migrate to the neighboring 8-MR side-pocket window through the
formation of an intermediate species (Int). This intermediate structure is energetically very stable in the triplet ground state with all Cu centers having an oxidation
state of +2. Finally, the formation of two [Cu
II
2 (μ-O)]
2+ active sites in the triplet
state then takes place via a second transition state (TS2), requiring an activation
energy of 12.0 kcal/mol.
In the formation of tricopper [Cu 3 (μ-O) 3 ]
2+ active site from O 2 activation on CuMOR, Yoshizawa and co-workers recently suggested a [Cu
I
2 Cu
II O]
2+ species in the
doublet ground state as a theoretically possible precursor [32]. This species is formed
as a result of two consecutive oxidations of methane over the tricopper active site
[33, 34]. In this case, since the precursor can provide four electrons required to break
the O=O bond of O 2 molecule forming a [Cu
III
2 Cu
II (μ-O) 3 ]
2+ [32] or [Cu
II
2 Cu
III (μO) 2 (μ-O
· )]
2+ [35] active species, no spectator Cu cations are needed. As shown in
Fig. 8, IC has a quartet ground state (precursor in the doublet state + O 2 in the
triplet state) and the reaction begins with the adsorption of an O 2 molecule on one
Cu center of the precursor with a slightly elongated O–O bond length from 1.207
to 1.260 Å, suggesting that the molecule is already activated even at this stage of
reaction. An intermediate structure μ-η
2 :η
2 -peroxo-Cu
II
3 O (Int) is then formed via
TS1, where the activated O 2 now forms a Cu
II O 2
· superoxo structure with a further
elongated O–O bond length of 1.398 Å. The formation of Int requires an activation
energy of 24.2 kcal/mol, and a spin inversion from the quartet state to the open-shell
doublet state (i.e. two Cu centers have α-spin electron while the other Cu center
has a β-spin electron) is expected to occur after TS1. Subsequently, the peroxo
bond of Int is cleaved via TS2 with a cleaved O ··· O distance of 1.810 Å to form
the [Cu 3 (μ-O) 3 ]
2+ active site in the quartet ground state. Given another activation
barrier of 10.4 kcal/mol for this cleavage, we obtain an effective activation barrier of
31.6 kcal/mol.