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 k_on = (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 pK_a 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.