Electrochemical Oxidation and EPR Spectroscopy of Radical Cations of N-Substituted 2,3,4,5-Tetramethylpyrroles.
作者:Robert Klusák、Pavel Kubáček、Domenico C. Cupertino、Lage Pettersson、Louisa Barré、Ole Hammerich、Inger Søtofte、Bengt Långström
DOI:10.3891/acta.chem.scand.52-0399
日期:——
Electrochemical oxidation of 19 N-substituted 2,3,4,5-tetramethylpyrroles has been studied in acetonitrile and dichloromethane by means of slow cyclic voltammetry and coulometry. The first oxidation consumes one electron and occurs within the potential range 0.60-0.94 V in acetonitrile and 0.78-1.17 V in dichloromethane (vs. SCE). Twelve in situ generated primary radical cations were sufficiently stable al lowered temperature in dichloromethane for EPR measurement and showed well resolved HFS. The g-values (approximate to 2.0026) and the coupling constants of 2,5-methyls (a(H) approximate to 1.5 mT), 3,4-methyls (a(H) approximate to 0.35 mT), and of the pyrrole nitrogen (a(N) approximate to 0.42 mT) are very proximate for all 12 radical cations. It can be concluded, with support from quantum chemical calculations, that the odd electron is localised entirely on the pyrrole ring in the a(2) HOMO of the parent molecule. Despite the odd electron distribution, the stability of the radical cations depends on the particular substituent attached to the pyrrole nitrogen.