Ligand effect on the kinetics of hydroperoxochromium(iii)–oxochromium(v) transformation and the lifetime of chromium(v)†
Abstract
A macrocyclic superoxochromium complex L2(H2O)CrOO2+ (L2 = meso-Me6-[14]aneN4) is generated from L2Cr(H2O)22+ and O2 with kon = (2.80 ± 0.07) × 107 M−1 s−1. One-electron reduction of L2(H2O)CrOO2+ produces a transient hydroperoxo complex that readily undergoes intramolecular conversion to L2Cr(V), k1 = 1.00 ± 0.01 s−1 in acidic aqueous solutions, and 0.273 ± 0.010 s−1 at pH >7, with an apparent pKa of 5.9. The decay of L2Cr(V) in the pH range 1.3–6.2 obeys the rate law, −d[L2Cr(V)]/dt = (0.0080 (± 0.0049) + 8.19 (± 0.13) [H+])[L2Cr(V)]. Both the kinetics of formation and lifetime of L2Cr(V) are significantly different from those for the closely related [14]aneN4 complex. The X-ray structure of the parent Cr(III) complex, [L2Cr(H2O)2](ClO4)3·4H2O, shows that the macrocyclic ligand adopts the most stable, “two up-two down” configuration around the nitrogens.