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.