On the Accuracy of Calculated Reduction Potentials
of Selected Group 8 (Fe, Ru, and Os) Octahedral Complexes
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Abstract
The
theoretical calculations of reduction potentials for the [M(H<sub>2</sub>O)<sub>6</sub>]<sup>2+/3+</sup>, [M(NH<sub>3</sub>)<sub>6</sub>]<sup>2+/3+</sup>, [M(<i>en</i>)<sub>3</sub>]<sup>2+/3+</sup>, [M(<i>bipy</i>)<sub>3</sub>]<sup>2+/3+</sup>, [M(CN)<sub>6</sub>]<sup>4–/3–</sup>, and [MCl<sub>6</sub>]<sup>4–/3–</sup> systems (M = Fe, Os, Ru) were carried out.
The DFT(PBE)/def2-TZVP//DFT(PBE)/def2-SVP quantum chemical method
was employed to obtain presumably accurate ionization energies, whereas
the conductor-like screening model for real solvents (COSMO-RS) was
selected as the most suitable method for calculations of solvation
energies of the oxidized and reduced forms of the studied species.
It has been shown that COSMO-RS may overcome problems related to directionality
of hydrogen bonds in the second solvation sphere that previously led
to errors of ∼1 V for the [Ru(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> complex employing PCM-like models. Thus, most of the values
for (2+) → (3+) oxidations are now within 0.1–0.2 V
from the experimental data, once the anticipated spin–orbit
coupling effects in Os complexes (downshifting the calculated reduction
potentials by ∼0.3 V) are taken into account. The robustness
of the DFT(PBE)/COSMO-RS computational protocol is further verified
by showing that reduction potentials obtained for selected 2+/3+ redox
pairs with and without the inclusion of explicit second-sphere water
molecules are almost identical. At the same time, it must be admitted
that the calculated values of reduction potentials for systems involving
quadruple charged species, exemplified here by [M(CN)<sub>6</sub>]<sup>4–/3–</sup> and [MCl<sub>6</sub>]<sup>4–/3–</sup> redox pairs, might still not be within the grasp of contemporary
solvation models, possibly due to the large values of solvation energies
of their reduced (4−) forms that are in the range of 700–750
kcal mol<sup>–1</sup> (30–33 eV) and possibly larger
errors inherent in their calculations. Finally, a comparison is made
with M06-L//SMD computational protocol, which is also shown to correct
some of the deficiencies of previous PCM models