Complex 1 is a photoactive species with a MLCT band at 444 nm (ƒ ¼ 0.0596).
After the photoactivation process, the excited form of the complex would evolve to a
triplet state with an energy of 45.2 kcal/mol. This complex would fade back to the
ground state as it would not be able to activate BrCCl 3 because the estimated barrier
for the SET process is prohibitively high (40.1 kcal/mol). We found a similar
behavior for the coordinatively saturated complex 2. The complex can absorb light
promoting an MLCT with an associated band at 373 nm (ƒ ¼ 0.1704), but the barrier
for SET is too high.
Finally, complex 3 has an MLCT band at 387 nm with an oscillator force of
0.1482. Most importantly, its excited triplet form A can perform the outer-sphere
SET event with a low barrier of 5.1 kcal/mol for the activation of the BrCCl 3
substrate, confirming that this is the active photosensitizer species [64].
In summary, the full mechanism for the trichloromethylation of 2-acylpyridines
was calculated with DFT tools. We were able to reproduce all the experimental
observations using standard DFT, TD-DFT, and Marcus theory calculations.
3 Photoactivated Reactions
3.1 Photoinduced Oxidative Decarboxylation of a Nonheme
Iron(III) Complex
Photoactive iron(III) complexes bearing carboxylato ligands are well characterized
in coordination and bioinorganic chemistry [65, 66]. Iron(III) complexes of simple
organic acids like malonate or citrate can be activated by UV and blue light
[67, 68]. The process results ultimately in CO 2 extrusion and reduction of the
metal center to iron(II) [69, 70]. This process is used by plants and microorganisms
for iron processing in cases of scarcity of the element, but its mechanism is poorly
understood.
In this section, we describe our computational study on the light-driven irreversible oxidation of the ligand of an iron(III) complex, [Fe(tpena)]
2+ (tpena ¼ N,N,
N
0 -tris(2-pyridylmethyl)ethylenediamine-N
0 -acetate). This work was carried out
with the experimental group of McKenzie [71]. The reaction sequence starts with
the photoactivation of the complex followed by CO 2 extrusion. Then, the system can
trap a triplet dioxygen molecule to produce a putative alkylperoxide complex. The
latter can easily evolve to a Fe(IV)-oxo species. This species undergoes a proton
transfer from methylene to oxo, which results in the release of a formaldehyde
molecule. Finally, the iron(III) hydroxide complex is reduced. The presentation
here will focus on the computational work on the steps involving light. All the
energies reported in this section are Gibbs energies obtained with B3LYP-D3
functional, including zero-point energies, entropic corrections, solvation, and
dispersion.
144
A. de Aguirre et al.
After the photoactivation process, the excited form of the complex would evolve to a
triplet state with an energy of 45.2 kcal/mol. This complex would fade back to the
ground state as it would not be able to activate BrCCl 3 because the estimated barrier
for the SET process is prohibitively high (40.1 kcal/mol). We found a similar
behavior for the coordinatively saturated complex 2. The complex can absorb light
promoting an MLCT with an associated band at 373 nm (ƒ ¼ 0.1704), but the barrier
for SET is too high.
Finally, complex 3 has an MLCT band at 387 nm with an oscillator force of
0.1482. Most importantly, its excited triplet form A can perform the outer-sphere
SET event with a low barrier of 5.1 kcal/mol for the activation of the BrCCl 3
substrate, confirming that this is the active photosensitizer species [64].
In summary, the full mechanism for the trichloromethylation of 2-acylpyridines
was calculated with DFT tools. We were able to reproduce all the experimental
observations using standard DFT, TD-DFT, and Marcus theory calculations.
3 Photoactivated Reactions
3.1 Photoinduced Oxidative Decarboxylation of a Nonheme
Iron(III) Complex
Photoactive iron(III) complexes bearing carboxylato ligands are well characterized
in coordination and bioinorganic chemistry [65, 66]. Iron(III) complexes of simple
organic acids like malonate or citrate can be activated by UV and blue light
[67, 68]. The process results ultimately in CO 2 extrusion and reduction of the
metal center to iron(II) [69, 70]. This process is used by plants and microorganisms
for iron processing in cases of scarcity of the element, but its mechanism is poorly
understood.
In this section, we describe our computational study on the light-driven irreversible oxidation of the ligand of an iron(III) complex, [Fe(tpena)]
2+ (tpena ¼ N,N,
N
0 -tris(2-pyridylmethyl)ethylenediamine-N
0 -acetate). This work was carried out
with the experimental group of McKenzie [71]. The reaction sequence starts with
the photoactivation of the complex followed by CO 2 extrusion. Then, the system can
trap a triplet dioxygen molecule to produce a putative alkylperoxide complex. The
latter can easily evolve to a Fe(IV)-oxo species. This species undergoes a proton
transfer from methylene to oxo, which results in the release of a formaldehyde
molecule. Finally, the iron(III) hydroxide complex is reduced. The presentation
here will focus on the computational work on the steps involving light. All the
energies reported in this section are Gibbs energies obtained with B3LYP-D3
functional, including zero-point energies, entropic corrections, solvation, and
dispersion.
144
A. de Aguirre et al.