Near-Stoichiometric Conversion of H2O2 to FeIV=O at a Nonheme Iron(II) Center. Insights into the O−O Bond Cleavage Step

Abstract

Near-quantitative formation of an oxoiron(IV) intermediate [Fe^IV(O)(TMC)(CH₃CN)]²⁺ (2) from stoichiometric H₂O₂ was achieved with [Fe^II(TMC)]²⁺ (1) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane). This important outcome is best rationalized by invoking a direct reaction between 1 and H₂O₂ followed by a heterolytic O−O bond cleavage facilitated by an acid−base catalyst (2,6-lutidine in our case). A sizable H/D KIE of 3.7 was observed for the formation of 2, emphasizing the importance of proton transfer in the cleavage step. Pyridines with different pK_a values were also investigated, and less basic pyridines were found to function less effectively than 2,6-lutidine. This study demonstrates that the reaction of Fe(II) with H₂O₂ to form Fe(IV)═O can be quite facile. Two factors promote the near-stoichiometric conversion of H₂O₂ to Fe(IV)═O in this case: (a) the low reactivity between 1 and 2 and (b) the poor H-atom abstracting ability of 2, which inhibits subsequent reaction with residual H₂O₂. Both factors inhibit formation of the Fe(III) byproduct commonly found in reactions of Fe(II) complexes with H₂O₂. These results may shed light into the nature of the O−O bond cleaving step in the activation of dioxygen by nonheme iron enzymes and in the first step of the Fenton reaction.