Near-stoichiometric conversion of H(2)O(2) to Fe(IV)=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)(CH(3)CN)](2+) (2) from stoichiometric H(2)O(2) 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 H(2)O(2) 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 H(2)O(2) to form Fe(IV)= O can be quite facile. Two factors promote the near-stoichiometric conversion of H(2)O(2) 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 H(2)O(2). Both factors inhibit formation of the Fe(III) byproduct commonly found in reactions of Fe(II) complexes with H(2)O(2). 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.

Figure 1

Yield of 2 vs. equiv. H₂O₂ added. Experimental conditions: 2.0 mM 1, 2.0 mM 2,6-lutidine, -40 °C in CH₃CN. The yield of 2 was determined from its absorption at 820 nm (ε = 400 M^-1cm^-1). Inset: Spectral changes observed during the formation of 2 (b = 1 cm).

Figure 2

Left panel: Dependence of the pseudo-first order rate constant for formation of 2 (k_obs) and yield of 2 on [2,6-lutidine] (conditions: 1.0 mM 1 in CH₃CN and 20 equiv. H₂O₂ at -40 °C). Right panel: [H₂O₂/D₂O₂] dependence of k_obs (conditions: 2.0 mM 1, 2.0 mM 2,6-lutidine in CH₃CN at -40 °C). See SI for further experimental details.

Scheme 1

Left: Ligands used in this study. Right: Proposed mechanism for the base-catalyzed formation of 2 from 1 and H₂O₂.