2 research outputs found
Redox Reactions of Nickel, Copper, and Cobalt Complexes with “Noninnocent” Dithiolate Ligands: Combined in Situ Spectroelectrochemical and Theoretical Study
The
redox properties of copper, nickel, and cobalt complexes (MePh<sub>3</sub>P)[M(bdt)<sub>2</sub>] with the ligand benzene-1,2-dithiolate
(bdt) and synthesized complexes (MePh<sub>3</sub>P)[M(bdtCl<sub>2</sub>)<sub>2</sub>] with the ligand 3,6-dichlorobenzene-1,2-dithiolate
(bdtCl<sub>2</sub>) have been studied by cyclic voltammetry and in
situ EPR–UV/vis/NIR spectroelectrochemistry. The addition of
chlorine substituents to the 3- and 6-positions of the benzene ring
not only facilitates the reduction of [M(bdtCl<sub>2</sub>)<sub>2</sub>]<sup>−</sup> complexes but also leads to the remarkable stabilization
of [M(bdtCl<sub>2</sub>)<sub>2</sub>]<sup>2–</sup> dianions
in solution. In contrast to the EPR-silent copper complexes, the solutions
of nickel samples exhibit a broad singlet EPR signal at room temperature
which becomes anisotropic at 100 K with a characteristic rhombic pattern.
Cathodic reduction of copper and cobalt complexes leads to paramagnetic
species having an EPR signal with splitting from <sup>63,65</sup>Cu
for copper and from <sup>59</sup>Co for cobalt samples, confirming
a strong contribution of the central atom with substantial delocalization
of the unpaired spin onto the central atom. B3LYP/6-311g*/pcm calculations
of the monoanions as well as of their oxidized and reduced forms were
performed. The spin density of all open-shell ground states calculated
for the investigated complexes in different redox states corresponds
well to the experimental spectroelectrochemical data
Stable Radical Trianions from Reversibly Formed Sigma-Dimers of Selenadiazoloquinolones Studied by In Situ EPR/UV–vis Spectroelectrochemistry and Quantum Chemical Calculations
The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-<i>h</i>]quinolines and 8-substituted 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-<i>f</i>]quinolines with R<sub>7</sub>, R<sub>8</sub> = H, COOC<sub>2</sub>H<sub>5</sub>, COOCH<sub>3</sub>, COOH, COCH<sub>3</sub>,
and CN has been studied by in situ EPR and EPR/UV–vis spectroelectrochemistry
in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron
reduction process to form the corresponding radical anions. Their
stability strongly depends on substitution at the nitrogen atom of
the 4-pyridone ring. The primary generated radical anions from <i>N</i>-ethyl-substituted quinolones are stable, whereas for the
quinolones with imino hydrogen, the initial radical anions rapidly
dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions.
These are reversibly oxidized to the initial compounds at potentials
considerably less negative than the original reduction process in
the back voltammetric scan. The dimer dianion can be further reduced
to the stable paramagnetic dimer radical trianion in the region of
the second reversible reduction step. The proposed complex reaction
mechanism was confirmed by in situ EPR/UV–vis cyclovoltammetric
experiments. The site of the dimerization in the σ-dimer and
the mapping of the unpaired spin density both for radical anions and
σ-dimer radical trianions with unusual unpaired spin distribution
have been assigned by means of density functional theory calculations