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 (MePh₃P)­[M­(bdt)₂] with the ligand benzene-1,2-dithiolate (bdt) and synthesized complexes (MePh₃P)­[M­(bdtCl₂)₂] with the ligand 3,6-dichlorobenzene-1,2-dithiolate (bdtCl₂) 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(bdtCl₂)₂]⁻ complexes but also leads to the remarkable stabilization of [M­(bdtCl₂)₂]^2– 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 ^63,65Cu for copper and from ⁵⁹Co 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 R₇, R₈ = H, COOC₂H₅, COOCH₃, COOH, COCH₃, 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