Study of bipyridyl radical cations. Part 4. Reaction of diquat radical cation with oxygen and copper(II)

Abstract

The radical cation (I) of diquat (6,7-dihydrodipyrido[1,2-a :1′,2′-c]pyrazinium dibromide), differs from that of paraquat and morphamquat in that it does not dimerise to diamagnetic dimer molecules on lowering the temperature. The reaction of the radical cation of diquat with oxygen was studied by the stopped-flow technique over the temperature range +40 to –80 °C in methanol and +40 to –100 °C in ethanol. The reaction was second order in radical cation in both solvents at +40 °C, first order in radical cation in ethanol at –100 °C, and first order in oxygen under all conditions. At +40 °C the third-order rate constants are (1.5 ± 0.2)× 10⁹ and (8.8 ± 0.7)× 10⁷ I² mol^–2 s^–1 in methanol and ethanol respectively. The reaction of diquat radical cation with Cu^II in methanol at –80 °C is first order in radical cation and in Cu^II. The reaction of the radical cation of morphamquat with Cu^II in methanol at –80 °C is first order in radical cation dimer and second order in Cu^II.