Free Radical Reactions of Some Transition and Organotransition Metal Complexes

摘要

A method for producing superoxide ion in aqueous solution has been developed, based upon photochemical generation of a reducing radical species and its subsequent reaction with dioxygen to produce O sub 2- . Optimal working conditions were 5.0 M 2-propanol, 6.3 mu M Ph sub 2 CO, dioxygen saturated at pH > 11 with irradiation performed using a Hg arc lamp photoreactor, producing nearly 400 mu M O sub 2- in 30 s. The reactions of O sub 2- with several cobalt(III) complexes and ferricinium ion proceed through an outer sphere electron transfer pathway. Second-order rate constants at 25.0 exp 0 C are: 31.3, 23.8, and 16.4 M exp -1 s exp -1 for Co(NH sub 3 ) sub 6+3 , Co(en) sub 3+3 , and Co(trans-1,2-cyclohexanediamine) sub 3+3 , respectively, and 8.6 x 10 exp 6 M exp -1 s exp -1 for Cp sub 2 Fe exp + . For the Co(III) reactions, several physical (temperature, pH) and chemical (isotopic labeling of complex and solvent, choice of counter anion, presence of dioxygen) effects were investigated. Some observations are made on the O sub 2 (aq)/O sub 2- electron self-exchange rate constant. The stoichiometry and kinetics of the reactions of bis(dimethylglyoximato)cobalt(II) complexes with polyhalomethanes are also reported. The first and rate limiting step is halogen atom abstraction by the cobalt complex and formation of a free radical. (ERA citation 09:017019)