Nonheme Fe^IVO Complexes That Can Oxidize the C−H Bonds of Cyclohexane at Room Temperature

Abstract

Nonheme oxoiron(IV) complexes of two pentadentate ligands, N4Py (N,N-bis(2-pyridylmethyl)-bis(2-pyridyl)methylamine) and Bn-tpen (N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), have been generated and found to have spectroscopic properties similar to the closely related tetradentate TPA (tris(2-pyridylmethyl)amine) complex reported earlier. However, unlike the TPA complex, the pentadentate complexes have a considerable lifetime at room temperature. This greater thermal stability has allowed the hydroxylation of alkanes with C-H bonds as strong as 99.3 kcal/mol to be observed at room temperature. Furthermore, a large deuterium KIE value is found in the oxidation of ethylbenzene. These observations lend strong credence to postulated mechanisms of mononuclear nonheme iron enzymes that invoke the intermediacy of oxoiron(IV) species.

Axial ligand tuning of a nonheme iron(IV)–oxo unit for hydrogen atom abstraction

“…However, the electrochemical behavior of nonheme iron(IV)-oxo complexes exhibits some complexity that is not straightforward to interpret. For example, a reduction wave with an E p,c value of Ϫ0.44 V vs. (41), which has been shown to be reactive enough to oxidize cyclohexane at room temperature (38). However, subsequent spectropotentiometric experiments established its actual redox potential to be significantly more positive, 0.9 V vs. Fc ϩ/o (42).…”

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…It would appear then that there may be a larger kinetic barrier for oxidation by an [Fe IV 2 ( -O) 2 ] center than for an Fe IV AO center. Indeed, related, more thermally stable mononuclear oxoiron(IV) complexes have been shown to attack COH bonds as strong as 99 kcal⅐mol Ϫ1 (39). MMO-Q oxidizes methane and is thus a superior oxidant than any of these synthetic low-spin iron(IV) complexes.…”

Non‐Heme Fe(IV)—Oxo Intermediates

“…1,2 Significant insight into the geometric and electronic structures of high-valent non-heme Feoxo complexes and their reactivity was obtained in parallel from elegant studies of inorganic complexes [11][12][13][14][15] (see 1 for a recent review), but these studies will not be reviewed here due to the brevity of this article.…”