Acid-catalyzed oxidation of iodide ions by superoxo complexes of rhodium and chromium

Superoxometal complexes L(H2O) MOO2+ (L = (H2O)(4), (NH3)(4), or N-4-macrocycle; M = Cr-III, Rh-III) react with iodide ions according to the stoichiometry L(H2O)MOO2+ + 3l(-) + 3H(+) - L(H2O)MOH2+ + 1.5l(2) + H2O. The rate law is -d[L(H2O)MOO2+]/dt = k [L(H2O) MOO2+][l(-)][H+], where k = 93.7 M-2 S-1 for CraqOO2+, 402 for ([14]aneN(4))(H2O)CrOO2+, and 888 for (NH3)(4)(H2O)RhOO2+ in acidic aqueous solutions at 25degreesC and 0.50 M ionic strength. The CraqOO2+/l(-) reaction exhibits an inverse solvent kinetic isotope effect, k(H2O)/kD(2)O = 0.5. In the proposed mechanism, the protonation of the superoxo complex precedes the reaction with iodide. The related CraqOOH2+/l(-) reaction has k(H2O)/k(D2O) = 0.6. The oxidation of (NH3)(5)Rupy(2+) by CraqOO2+ exhibits an [H+]-dependent pathway, rate = (7.0 x 10(4) + 1.78 x 10(5)[H+])[Ru(NH3)(5)py(2+)][CraqOO(2+)]. Diiodine radical anions, l(2)(.-), reduce CraqOO2+ with a rate constant k 1.7 x 10(9) M-1 s(-1).