Radical cations of some low-potential viologen compounds. Reduction potentials and electron-transfer reactions

Abstract

The one-electron reduction potentials (E¹) of certain pyrazinediium, diazepinediium and diazocinediium viologen compounds substituted with methyl groups, V²⁺, have been determined from the position of the one-electron transfer equilibria with reference compounds using pulse radiolysis. E¹ ranges from –491 ± 6 mV (vs NHE) for 6,7-dihydro-2,11-dimethyldipyrido-[1,2-a : 2′,1′-c]pyrazinediium dibromide (V21²⁺) to –832 ± 11 mV for 6,7,8,9-tetrahydro-2,3,12,13-tetramethyldipyrido[1,2-a : 2′,1′-c][1,4]diazocinediium dibromide (V42²⁺).

The rates of reduction by e^–_aq were found to be independent of E¹ whereas the rates of electron transfer from CO^˙–₂ and (CH₃)₂ĊOH species do show a dependence on E¹ for V²⁺ compounds of lowest E¹. Marcus-type treatment of the rate-constant data yields a rate constant of electron exchange between propan-2-oxyl radicals and acetone in the range (2–6)× 10⁶ dm³ mol^–1 s^–1 and between the radical cation of 6,7-dihydro-2,3,10,11-tetramethyldipyrido[1,2-a : 2′,1′-c]-pyrazinediium dibromide (V22^˙+) and its unreduced form (V22²⁺) of 5 × 10⁷ dm³ mol^–1 s^–1.