Tailoring Valence Tautomerism by Using Redox Potentials: Studies on Ferrocene-Based Triarylmethylium Dyes with Electron-Poor Fluorenylium and Thioxanthylium Acceptors

Dimerization of 9-Phenyl-ferroceno[2,3]indenylmethyl Radicals: Electrochemical and Spectroelectrochemical Studies

We report on three new 9-phenyl-substituted ferroceno[2,3]indenylmethylium dyes 1 + -3 + with electron-donating (OMe, Me) or electron-withdrawing (CF3) substituents. Complexes 1 + -3 + exist as racemic mixtures of Rp and Sp enantiomers. Pyramidalization at the methyl C atom in the precursor carbinol species 1-OH-3-OH or the corresponding one-electron reduced radicals induces a second stereocenter, as the 9-phenyl substituent may reside in an endo or an exo position. Indeed, alcohol 2-OH crystallizes as a racemate of Rp,S and Sp,R isomers. Cationic complexes 1 + -3 + are of deep green color and show intense electronic absorption in the visible. The oxidation and reduction processes are thoroughly investigated by means of cyclic voltammetry and UV/vis/NIR spectroelectrochemistry, the latter showing their electrochromic behavior. T-dependent EPR spectroscopy, EPR spin counting at 20 °C, as well as the UV/vis/NIR spectra of the reduced samples suggest that the one-electron reduced, neutral radicals dimerize nearly quantitatively (≥99.98%). Chemical reduction of 2 + furnished an isomeric mixture of dimeric 2-2. As was shown by cyclic voltammetry and UV/vis/NIR spectroelectrochemistry, the latter dimer redissociates to monomers 2 + upon oxidation, thereby closing a reversible cycle of redox-induced C-C bond making and breaking.

Gradual solid-state redox-isomerism in the lanthanide series

Oxidation of [(ArBIG-bian)2-Yb2+(dme)] (1) (ArBIG-bian = 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene; dme = 1,2-dimethoxyethane) by 0.5 equivalent of Me2NC(S)S-S(S)CNMe2 in dme at ambient temperature affords a mixture of two products, [(ArBIG-bian)2-Yb3+{SC(S)NMe2}1-(dme)] and [(ArBIG-bian)1-Yb2+{SC(S)NMe2}1-(dme)], which represent two redox-isomers (2a and 2b, respectively). Their ratio in solution depends on the solvent as well as on the temperature. In the solid state, a decrease of temperature (350 → 100 K) caused an electron transfer from the Yb2+ ion to the ArBIG-bian radical-anion in isomer 2b to afford isomer 2a. Accordingly, the ratio of isomers 2a and 2b changes from 1 : 1 (350 K) to 3 : 1 (100 K). In contrast, in the dimer [(dme)(dpp-bian)1-Yb2+(μ-Cl)2Yb3+(dpp-bian)2-(dme)] (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene), which is the sole example of a lanthanide complex that reveals solid-state redox-isomerism (valence tautomerism) reported so far, the electron transfer from the Yb2+ ion to the dpp-bian radical-anion takes place at around 150 K and is completed within a temperature interval of ca. 7 K.

A dicyanomethyl radical stabilized by ferrocene: a new building block for radical-based dynamic covalent chemistry with redox activity

A novel dicyanomethyl radical conjugated with ferrocene as an electron donating unit through a benzene linker is firstly prepared. This radical is in equilibrium with the σ-dimer in solution, and...

Five shades of green: substituent influence on the (spectro-) electrochemical properties of diferrocenyl(phenyl)methylium dyes

Five new, intensely green diferrocenylphenylmethylium complexes 1+-5+ with electron donating (EDG: 4-MeO, 4-Me, 4-Br) or withdrawing (EWG: 3,5-CF₃, 4-ⁿC₆F₁₃) substituents were synthesized and fully characterized. The substituent influence on their electrochemical and spectroscopic properties was investigated by cyclic voltammetry, UV/Vis/NIR and T-dependent EPR spectroscopy of the cationic as well as the oxidized (12+-52+) and reduced (1˙-5˙) species. The reduced forms equilibrate with their corresponding dimers (65-83%) with a clear substituent influence as expressed by their Hammett parameters in an ordering 4+ > 5+ > 3+ > 2+ > 1+. The structures of all five precursor carbinols 1-OH-5-OH and those of three of the diferrocenylphenylmethylium cations (1+, 4+-5+) were established by X-ray crystallography.

Tailoring Valence Tautomerism by Using Redox Potentials: Studies on Ferrocene-Based Triarylmethylium Dyes with Electron-Poor Fluorenylium and Thioxanthylium Acceptors

Three new electrochromic ferrocenyl triarylmethylium dyes with fluorenylium (1 a⁺ , 1 b⁺ ) or thioxanthylium (1 c⁺ ) residues were selected in order to keep the intrinsic differences of redox potentials for ferrocene oxidation and triarylmethylium reduction small and to trigger valence tautomerism (VT). UV/Vis/NIR and quantitative EPR spectroscopy identified paramagnetic diradical isomers 1 a^..+ -1 c^..+ alongside diamagnetic forms 1 a⁺ -1 c⁺ , which renders these complexes magnetochemical switches. The diradical forms 1 a^..+ -1 c^..+ as well as the one-electron-reduced triarylmethyl forms of the complexes were found to dimerize in solution. For radical 1 a^. , dimerization occurs on the timescale of cyclic voltammetry; this allowed us to determine the kinetics and equilibrium constant for this process by digital simulation. Mößbauer spectroscopy indicated that 1 a⁺ and 1 b⁺ retain VT even in the solid state. UV/Vis/NIR spectro-electrochemistry revealed the poly-electrochromic behaviour of these complexes by establishing the distinctly different electronic absorption profiles of the corresponding oxidized and reduced forms.