excited triplet state
3 A complex would generate a neat spin of 0, i.e., the dioxygen
approximates in an antiferromagnetic manner to the complex. The lower reaction
path starts with the coupling of one unpaired electron on the dioxygen with one
unpaired electron of the metal center, forming intermediate B on the open-shell
singlet (OSS) surface. In B, one electron is localized in the terminal oxygen, and the
other one is arranged antiparallel in the σ* Pt—Me orbital. This intermediate is found at
23.9 kcal/mol above the initial reactants (A +
3 O 2 ). This step is barrierless on the
Gibbs energy surface, but we can estimate an entropic barrier of ca. 8 kcal/mol.
Finally, the system evolves to intermediate C on the closed shell singlet as a result of
the quenching between the two antiparallel electrons. The side-on peroxide-kind
coordination of the dioxygen molecule moves the methyl group of the complex to
the apical position. Intermediate C has an energy of 17.6 kcal/mol above the
reactants. Free singlet oxygen has an energy of 22.7 kcal/mol; therefore, this
pathway is more favorable than the generation of singlet oxygen.
An alternative mechanism involves a single electron transfer step (SET) from the
metal to the dioxygen molecule, which implies the homolytic dissociation of the Pt–
Me bond. We were able to find this transition state TS
3 A-D with an activation
barrier of 12.9 kcal/mol, from intermedia
3 A. As a result of this event, two fragments
are generated: a tricoordinate Pt(II) complex with a vacant site on the fourth
coordination position and a methylperoxo radical. Nevertheless, this alternative
pathway is not competitive for the parent system because the direct coupling
between the dioxygen and the complex is favored by ca. 5 kcal/mol.
To sum up, we have presented the mechanism for the insertion of dioxygen into
the Pt(II)–methyl bond of the [Pt(terpyridine)Me]
+ complex, upon irradiation of the
complex to its excited triplet state. To determine the nature of the photoexcitation,
TD-DFT methods have shown a good agreement between the computed and the
experimental absorption bands. In the early steps of the mechanism, the
3 O 2 molecule is trapped by the excited complex on the open-shell singlet (OSS) surface with a
low barrier. The Pt(IV)-peroxo intermediate generated after the coupling between
the excited complex and the dioxygen is lower in energy than the free singlet
oxygen, confirming that there is not an energy transfer event to form free singlet
dioxygen.
4 Conclusions
Photoactivated processes are gaining importance because of their versatility and the
access they provide to new products. They pose a challenge for computational
chemistry because of their intrinsic electronic complexity and the size of the systems
involved. But recent years have witnessed the increased application of DFT calculations to the field through a combination of TD-DFT treatments for photoexcitation,
conventional DFT calculations for selected regions of the Gibbs energy profile, and
reasonable assumptions for the parts (intersystem crossing between same spin
spates) that cannot be properly computed with this treatment. In this chapter, we
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A. de Aguirre et al.
3 A complex would generate a neat spin of 0, i.e., the dioxygen
approximates in an antiferromagnetic manner to the complex. The lower reaction
path starts with the coupling of one unpaired electron on the dioxygen with one
unpaired electron of the metal center, forming intermediate B on the open-shell
singlet (OSS) surface. In B, one electron is localized in the terminal oxygen, and the
other one is arranged antiparallel in the σ* Pt—Me orbital. This intermediate is found at
23.9 kcal/mol above the initial reactants (A +
3 O 2 ). This step is barrierless on the
Gibbs energy surface, but we can estimate an entropic barrier of ca. 8 kcal/mol.
Finally, the system evolves to intermediate C on the closed shell singlet as a result of
the quenching between the two antiparallel electrons. The side-on peroxide-kind
coordination of the dioxygen molecule moves the methyl group of the complex to
the apical position. Intermediate C has an energy of 17.6 kcal/mol above the
reactants. Free singlet oxygen has an energy of 22.7 kcal/mol; therefore, this
pathway is more favorable than the generation of singlet oxygen.
An alternative mechanism involves a single electron transfer step (SET) from the
metal to the dioxygen molecule, which implies the homolytic dissociation of the Pt–
Me bond. We were able to find this transition state TS
3 A-D with an activation
barrier of 12.9 kcal/mol, from intermedia
3 A. As a result of this event, two fragments
are generated: a tricoordinate Pt(II) complex with a vacant site on the fourth
coordination position and a methylperoxo radical. Nevertheless, this alternative
pathway is not competitive for the parent system because the direct coupling
between the dioxygen and the complex is favored by ca. 5 kcal/mol.
To sum up, we have presented the mechanism for the insertion of dioxygen into
the Pt(II)–methyl bond of the [Pt(terpyridine)Me]
+ complex, upon irradiation of the
complex to its excited triplet state. To determine the nature of the photoexcitation,
TD-DFT methods have shown a good agreement between the computed and the
experimental absorption bands. In the early steps of the mechanism, the
3 O 2 molecule is trapped by the excited complex on the open-shell singlet (OSS) surface with a
low barrier. The Pt(IV)-peroxo intermediate generated after the coupling between
the excited complex and the dioxygen is lower in energy than the free singlet
oxygen, confirming that there is not an energy transfer event to form free singlet
dioxygen.
4 Conclusions
Photoactivated processes are gaining importance because of their versatility and the
access they provide to new products. They pose a challenge for computational
chemistry because of their intrinsic electronic complexity and the size of the systems
involved. But recent years have witnessed the increased application of DFT calculations to the field through a combination of TD-DFT treatments for photoexcitation,
conventional DFT calculations for selected regions of the Gibbs energy profile, and
reasonable assumptions for the parts (intersystem crossing between same spin
spates) that cannot be properly computed with this treatment. In this chapter, we
150
A. de Aguirre et al.