Nonheme Iron Mediated Oxidation of Light Alkanes with Oxone: Characterization of Reactive Oxoiron(IV) Ligand Cation Radical Intermediates by Spectroscopic Studies and DFT Calculations

Abstract

The oxidation of light alkanes that is catalyzed by heme and nonheme iron enzymes is widely proposed to involve highly reactive {Fe V = O} species or {Fe IV = O} ligand cation radicals. The identification of these high-valent iron species and the development of an iron-catalyzed oxidation of light alkanes under mild conditions are of vital importance. Herein, a combination of tridentate and bidentate ligands was used for the generation of highly reactive nonheme {Fe=O} species. A method that employs [Fe III (Me 3 tacn)(Cl-acac)Cl] + as a catalyst in the presence of oxone was developed for the oxidation of hydrocarbons, including cyclohexane, propane, and ethane (Me 3 tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane; Cl-acac = 3-chloro-acetylacetonate). The complex [Fe III (Tp) 2 ] + and oxone enabled stoichiometric oxidation of propane and ethane. ESI-MS, EPR and UV/Vis spectroscopy, 18 O labeling experiments, and DFT studies point to [Fe IV (Me 3 tacn)({Cl-acac}C + )(O)] 2+ as the catalytically active species.

Bioinspired Nonheme Iron Catalysts for C–H and C═C Bond Oxidation: Insights into the Nature of the Metal-Based Oxidants

“…35 Interestingly, the EPR properties of the new signals resemble those of the three bona fide oxoiron(V) complexes characterized thus far, 50 (Table 2). 56,57 The authors propose these new S = 1/2 species as having either (L)Fe V (O) or (L • )Fe IV (O) centers, but the low yields of 2−3% make it very difficult to establish the iron oxidation states of these novel species. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.5b00053.…”

“…35 Interestingly, the EPR properties of the signals resemble those of the three bona fide oxoiron(V) complexes characterized thus far, 50−52 namely [(TAML)Fe V (O)] − and [(TMC)Fe V (O)(NR)] + , and of species either proposed or shown to be oxoiron(IV)−ligand radical complexes (Table 2). 56,57 The authors propose these new S = 1/2 species as having either (L)Fe V (O) or (L • )Fe IV (O) centers, but the low yields of 2−3% make it very difficult to establish the iron oxidation states of these novel species.…”

n -Alkanes to n -alcohols: Formal primary C─H bond hydroxymethylation via quadruple relay catalysis

“…1B) (12). Likewise, n-alkane hydroxylations catalyzed by high valent porphyrin metal oxides and relevant complexes mainly produce branched alcohols (i-alcohols) (13,14). CYP153A enzyme catalyzes terminal-selective hydroxylations of n-alkanes, but the reactions are plagued by overoxidation, particularly for those higher than n-heptane (C7) (15).…”

Origins of the Selective C−S Bond Formation by the Non-Heme Iron EgtB

“…As shown in Figure 2, the following O-O cleavage via TS2 is quite facile, involving a small barrier of 3.3 kcal/mol (IC1→TS2), and leading to the Fe(III)=O species and sulfoxide radical in IC2 (Figure 2a). Unlike the normal Fe(IV)=O species, 61,[84][85][86][87][88][89][90][91][92][93] , the Fe-O distance of the Fe(III)=O species is significantly elongated in IC2 (1.72 Å in Figure 2b), which augments the basicity of the iron-oxo species. Starting from IC2, iron(III)-oxo mediates a facile proton abstraction from Y94, which is coupled to the proton transfer from Y93 to Y94.…”