The Reaction of [Fe^II(tpa)] with H₂O₂ in Acetonitrile and Acetone—Distinct Intermediates and Yet Similar Catalysis

Abstract

The reaction of [FeII(tpa)(OTf)2] (tpa=tris(2-pyridylmethyl)amine) and its related 5-Me3-tpa complex with hydrogen peroxide affords spectroscopically distinct iron(III)-peroxo intermediates in CH3CN and acetone. The reaction in acetonitrile at -40 degrees C results in the formation of the previously reported Fe(III)-OOH intermediate, the end-on hydroperoxo coordination mode of which is established in this paper by detailed resonance Raman isotope-labeling experiments. On the other hand, the reaction in acetone below -40 degrees C leads to the observation of a different peroxo intermediate identified by resonance Raman spectroscopy to be an FeIII-OOC (CH3)2OH species; this represents the first example of an intermediate derived from the adduct of H2O2 and acetone. The peroxoacetone intermediate decays more rapidly than the corresponding FeIII-OOH species and converts to an FeIV=O species by O-O bond homolysis. This decay process is analogous to that observed for [FeIII(tpa)(OOtBu)]2+ and in fact exhibits a comparable enthalpy of activation of 54(3) kJ mol(-1). Thus, with respect to their physical properties at low temperature, the peroxoacetone intermediate resembles [FeIII(tpa)(OOtBu)]2+ more than the corresponding FeIII-OOH species. At room temperature, however, the behavior of the Fe(tpa)/H2O2 combination in acetone in catalytic hydrocarbon oxidations differs significantly from that of the Fe(tpa)/tBuOOH combination and more closely matches that of the Fe(tpa)/H2O2 combination in CH3CN. Like the latter, the Fe(tpa)/H2O2 combination in acetone catalyzes the hydroxylation of cis-1,2-dimethylcyclohexane to its tertiary alcohol with high stereoselectivity and carries out the epoxidation and cis-dihydroxylation of olefins. These results demonstrate the subtle complexity of the Fe(tpa)/H2O2 reaction surface.

Identification of a low-spin acylperoxoiron(III) intermediate in bio-inspired non-heme iron-catalysed oxidations

“…This reaction afforded a species that exhibits an absorbance maximum at 460 nm ( Supplementary Fig. 1), a brown chromophore that is distinct from the purple chromophore obtained in the absence of AcOH and associated with the Fe III -OOH intermediate 2 (l max 540 nm) 25 . The brown intermediate, designated as 4, has a lifetime of only 5 min at À 40°C and decays to the corresponding (TPA)Fe IV (O) species 3 (l max ¼ 720 nm) in about 70% yield, while 2 persists for up to 3 h under similar conditions 15,16 , indicating that 4 is significantly more reactive than the Fe III -OOH complex 2.…”

Iron‐Promoted ortho‐ and/or ipso‐Hydroxylation of Benzoic Acids with H2O2

“…Such species have been observed for both 1 [47,48] and 2. [50,51] Al- Figure 3. Resonance Raman spectra of the green complexes generated in acetonitrile at À40 8C by mixing 1, 2-methoxybenzoic acid and hydrogen peroxide (A) and 1, 2,6-dimethylbenzoic acid and hydrogen peroxide (B [48] its optical signature remains to be identified.…”

“…However, only limited characterization is available for the rather short-lived intermediates derived from 1, [47,48] while the intermediates associated with 2 are longer lived and better characterized spectroscopically. These include Fe III A C H T U N G T R E N N U N G (OOH), [49][50][51] Fe III A C H T U N G T R E N N U N G (OOR), [29,52,53] and Fe IV = O species. [54] In the present work, the reactivities of non-heme iron complexes 1 and 2 are compared in the carboxylate-directed regioselective hydroxylation of various substituted benzoic acids.…”

Formation of an Aqueous Oxoiron(IV) Complex at pH 2–6 from a Nonheme Iron(II) Complex and H2O2

“…[19][20][21] These differences suggest a divergence in the mechanisms of oxoiron(IV) formation. Since the latter complexes are very likely generated by O À O bond homolysis of their peroxoiron(III) precursors, we postulate that 2 forms directly from the reaction of 1 with H 2 O 2 by iron(II)-promoted OÀO bond heterolysis.…”