The reaction of [Fe(pic)3] with hydrogen peroxide: a UV-visible and EPR spectroscopic study (Hpic = picolinic acid)

Abstract

The Gif family of catalysts, based on an iron salt and O₂ or H₂O₂ in pyridine, allows the oxygenation of cyclic saturated hydrocarbons to ketones and alcohols under mild conditions. The reaction between [Fe(pic)₃] and hydrogen peroxide in pyridine under GoAgg^III (Fe(III)/Hpic catalyst) conditions was investigated by UV-visible spectrophotometry. Reactions were monitored at 430 and 520 nm over periods ranging from a few minutes to several hours at 20 °C. A number of kinetically stable intermediates were detected, and their relevance to the processes involved in the assembly of the active GoAgg^III catalyst was determined by measuring the kinetics in the presence and absence of cyclohexane. EPR measurements at 110 K using hydrogen peroxide and t-BuOOH as oxidants were used to further probe these intermediates. Our results indicate that in wet pyridine [Fe(pic)₃] undergoes reversible dissociation of one picolinate ligand, establishing an equilibrium with [Fe(pic)₂(py)(OH)]. Addition of aqueous hydrogen peroxide rapidly generates the high-spin complex [Fe(pic)₂(py)(η¹-OOH)] from the labilised hydroxy species. Subsequently the hydroperoxy species undergoes homolysis of the Fe–O bond, generating HOO˙ and [Fe(pic)₂(py)₂], the active oxygenation catalyst.