Kinetics of dissociation of molecular oxygen from a superoxorhodium(III) complex and reactivity of a macrocyclic rhodium(II) ion

The kinetics of disappearance of the superoxorhodium complex L-2(H2O)RhOO2+ (L-2 = meso-hexamethylcyclam) were determined in the presence of several oxidants (H2O2, (NH3)(5)CoBr2+, and IrCl62-) in both air-free and air-saturated aqueous solutions. Under air-free conditions, the reaction obeyed first-order kinetics. After the correction for the appropriate stoichiometric factors, the value of the rate constant k(h) was the same irrespective of the oxidant, kh = 2.18 (+/- 0.37) x 10(-4) s(-1) at 25.0 degrees C in acidic solutions. The disappearance of L-2(H2O)RhOO2+ was slower in the presence of O-2. All the data suggest a sequence of reactions beginning with homolytic dissociation of O-2 from L-2(H2O)RhOO2+, followed by capture of the newly generated L-2(H2O)Rh2+ by added oxidants in competition with O-2. The equilibrium constant for O-2 binding by L-2(H2O)Rh2+ is 10(9)-fold greater than that for the cobalt analogue. This difference is attributed to the lower reduction potential of the rhodium complex.