Oxygen activation by a macrocyclic chromium complex.Mechanism of hydroperoxo-chromium(III) to oxo-chromium(V) transformation

摘要

The transformation of the hydroperoxo complex L~1 (H_2O)CrOOH~(2+) (L~1=1,4 8,11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined,1.7 < pH < 9.2.The pH dependence of the rate constant (kappa_1) yielded an apparent pK_a of 5.6 for L~1(H_2O)CrOOH~(2+).In the acidic range,(pH <4),the value of kappa_1 is 0.191 s~(-1).At the other extreme,pH >7.5,kappa_1=0.025 s~(-1).No [H~+]-dependence is observed within the two limiting regimes,clearly ruling out a simple attack by H~+ at the hydroperoxo group.The temperature dependence of kappa_1 in 0.020 M HC1O_4 yielded the activation parameters DELTAH=53.7 kJ mol~(-1) and DELTAS=-80.5 Jmol~(-1) K~(-1).