Competing Coordination and Oxygen Atom Transfer in a Reaction Between Triarylphosphines and a Chromium Hydroperoxo Complex

Abstract

A macrocyclic hydroperoxo chromium(III) complex, L 1 (H 2 O)CrOOH 2+ (L 1 = cyclam), reacts with triphenylphosphine and para-substituted derivatives in an acidcatalyzed O-atom transfer reaction in competition with intramolecular conversion to a Cr(V) complex. Oxygen atom transfer is complicated by another process, believed to be a rapid ligand exchange reaction that generates equilibrium amounts of less reactive L 1 (P(p-C 6 H 4 X) 3 )CrOOH 2+ . The fit of the data for O-atom transfer to the Hammett equation yields a reaction constant ρ = -0.24, consistent with a nucleophilic attack by P(p-C 6 H 4 X) 3 at one of the peroxo oxygens of the protonated hydroperoxo complex. The upper limit of <10 6 s -1 was estimated for the rate constant for hydrolysis of triphenylphosphine radical cation, PPh 3 •+ , in acidic 2:1 acetonitrile/H 2 O.