Kinetics of the Reaction of Mer-Tris(Picolinato) Iron(III) with Hydrogen Peroxide in Pyridine: Role of Hydroxyl Radicals in Subsequent Catalytic Oxygenation of Cyclohexane to the Ketone.

Abstract

Reaction of pale-green high spin mer-tris(picolinato)iron(III) with t-butylhydroperoxide in pyridine gives rise to an EPR signal for the t-butylperoxyl radical via a short lived purple intermediate which itself decays eventually to a yellow-brown high spin iron(III) product. The kinetics of the corresponding reaction with hydrogen peroxide have been studied in regard to the dependence of the hydrogen peroxide and picolinic acid concentration on the rate of the initial stages. The results support two rate-determining initial steps (minutes) involving the formation of a short lived purple high spin hydroperoxoor tbutylperoxoiron(III) intermediate (λmax = 530 nm, ε ~ 1,000 dm3 mol-1 cm-1 for the hydroperoxo species) via pre-equilibrium loss of one picolinic acid which then undergoes homolytic Fe-O bond cleavage to give iron(II) and hydroperoxyl (t-butylperoxyl) radical resulting in eventual formation of deep yellow-brown solution which undergoes further complex UV-visible changes over a period of several hours. During this latter timescale these solutions are able to carry out Gif-type catalytic oxygenation of cyclohexane to cyclohexanone in the presence of H2O2 (or O2/pyH/Zn powder) a process which however is completely inhibited in the presence of small amounts of dimethylsulfoxide, an efficient scavenger of the hydroxyl radical. Bis(picolinato)copper(II) was found to be a poor oxygenation catalyst, a finding consistent with its inability to generate the hydroxyl radical via Cu(I) under the same conditions. These results confirm that generation of hydroxyl radicals (via reaction of H2O2 with iron(II) or any other suitably reactive lower valent state) is central to the oxygenation chemistry carried out by these solutions.