Stereoelectronic Tuning of Bioinspired Nonheme Iron(IV)-Oxo Species by Amide Groups in Primary and Secondary Coordination Spheres for Selective Oxygenation Reactions

Abstract

Two mononuclear iron(II) complexes, [(6-amide₂-BPMEN)Fe^II](OTf)₂ (1) and [(6-amide-Me-BPMEN)Fe^II(OTf)](OTf) (2), supported by two BPMEN-derived (BPMEN = N¹,N²-dimethyl-N¹,N²-bis(pyridine-2-yl-methyl)ethane-1,2-diamine) ligands bearing one or two amide functionalities have been isolated to study their reactivity in the oxygenation of C–H and C═C bonds using isopropyl 2-iodoxybenzoate (iPr-IBX ester) as the oxidant. Both 1 and 2 contain six-coordinate high-spin iron(II) centers in the solid state and in solution. The 6-amide₂-BPMEN ligand stabilizes an S = 1 iron(IV)-oxo intermediate, [(6-amide₂-BPMEN)Fe^IV(O)]²⁺ (1A). The oxidant (1A) oxygenates the C–H and C═C bonds with a high selectivity. Oxidant 1A, upon treatment with 2,6-lutidine, is transformed into another oxidant [{(6-amide₂-BPMEN)-(H)}Fe^IV(O)]⁺ (1B) through deprotonation of an amide group, resulting in a stronger equatorial ligand field and subsequent stabilization of the triplet ground state. In contrast, no iron-oxo species could be observed from complex 2 and [(6-Me₂-BPMEN)Fe^II(OTf)₂] (3) under similar experimental conditions. The iron(IV)-oxo oxidant 1A shows the highest A/K selectivity in cyclohexane oxidation and 3°/2° selectivity in adamantane oxidation reported for any synthetic nonheme iron(IV)-oxo complexes. Theoretical investigation reveals that the hydrogen bonding interaction between the −NH group of the noncoordinating amide group and Fe═O core smears out the equatorial charge density, reducing the triplet–quintet splitting, and thus helping complex 1A to achieve better reactivity.