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Steric control of dioxygen activation pathways for MnII complexes supported by pentadentate, amide-containing ligands.

ABSTRACT: Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O2 as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear MnII complexes supported by pentacoordinate amide-containing ligands, [MnII(dpaq)](OTf) and the sterically modified [MnII(dpaq2Me)](OTf). Dioxygen titration experiments demonstrate that [MnII(dpaq)](OTf) reacts with O2 to form [MnIII(OH)(dpaq)](OTf) according to a 4?:?1 Mn?:?O2 stoichiometry. This stoichiometry is consistent with a pathway involving comproportionation between a MnIV-oxo species and residual MnII complex to form a (?-oxo)dimanganese(iii,iii) species that is hydrolyzed by water to give the MnIII-hydroxo product. In contrast, the sterically modified [MnII(dpaq2Me)](OTf) complex was found to react with O2 according to a 2?:?1 Mn?:?O2 stoichiometry. This stoichiometry is indicative of a pathway in which a MnIV-oxo intermediate abstracts a hydrogen atom from solvent instead of undergoing comproportionation with the MnII starting complex. Isotopic labeling experiments, in which the oxygenation of the MnII complexes was carried out in deuterated solvent, supported this change in pathway. The oxygenation of [MnII(dpaq)](OTf) did not result in any deuterium incorporation in the MnIII-hydroxo product, while the oxygenation of [MnII(dpaq2Me)](OTf) in d3-MeCN showed [MnIII(OD)(dpaq2Me)]+ formation. Taken together, these observations highlight the use of steric effects as a means to select which intermediates form along dioxygen activation pathways.

SUBMITTER: Parham JD

PROVIDER: S-EPMC6733413 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Steric control of dioxygen activation pathways for MnII complexes supported by pentadentate, amide-containing ligands.

Parham Joshua D JD Wijeratne Gayan B GB Mayfield Jaycee R JR Jackson Timothy A TA

Dalton transactions (Cambridge, England : 2003) 20190813 34

Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O2 as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear MnII complexes supported by pentacoordinate amide-containing ligands, [MnII(dpaq)](OTf) and the sterically modified [MnII(dpaq2Me)](OTf). Dioxygen titration experiments demonstrate that [MnII< ...[more]

PMID: 31406966

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Project description:Oxoiron(IV) species have been found to act as the oxidants in the catalytic cycles of several mononuclear nonheme iron enzymes that activate dioxygen. To gain insight into the factors that govern the oxidative reactivity of such complexes, a series of five synthetic S = 1 [Fe(IV)(O)(L(N5))](2+) complexes has been characterized with respect to their spectroscopic and electrochemical properties as well as their relative abilities to carry out oxo transfer and hydrogen atom abstraction. The Fe=O units in these five complexes are supported by neutral pentadentate ligands having a combination of pyridine and tertiary amine donors but with different ligand frameworks. Characterization of the five complexes by X-ray absorption spectroscopy reveals Fe=O bonds of ca. 1.65 Å in length that give rise to the intense 1s?3d pre-edge features indicative of iron centers with substantial deviation from centrosymmetry. Resonance Raman studies show that the five complexes exhibit ?(Fe=O) modes at 825-841 cm(-1). Spectropotentiometric experiments in acetonitrile with 0.1 M water reveal that the supporting pentadentate ligands modulate the E(1/2)(IV/III) redox potentials with values ranging from 0.83 to 1.23 V vs. Fc, providing the first electrochemical determination of the E(1/2)(IV/III) redox potentials for a series of oxoiron(IV) complexes. The 0.4-V difference in potential may arise from differences in the relative number of pyridine and tertiary amine donors on the L(N5) ligand and in the orientations of the pyridine donors relative to the Fe=O bond that are enforced by the ligand architecture. The rates of oxo-atom transfer (OAT) to thioanisole correlate linearly with the increase in the redox potentials, reflecting the relative electrophilicities of the oxoiron(IV) units. However this linear relationship does not extend to the rates of hydrogen-atom transfer (HAT) from 1,3-cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA), and benzyl alcohol, suggesting that the HAT reactions are not governed by thermodynamics alone. This study represents the first investigation to compare the electrochemical and oxidative properties of a series of S = 1 Fe(IV)=O complexes with different ligand frameworks and sheds some light on the complexities of the reactivity of the oxoiron(IV) unit.

Project description:This study describes the O2 reactivity of a series of high-spin mononuclear Fe(II) complexes each containing the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine ((Ph2)TIP) ligand and one of the following bidentate, redox-active ligands: 4-tert-butylcatecholate ((tBu)CatH(-)), 4,6-di-tert-butyl-2-aminophenolate ((tBu2)APH(-)), or 4-tert-butyl-1,2-phenylenediamine ((tBu)PDA). The preparation and X-ray structural characterization of [Fe(2+)((Ph2)TIP)((tBu)CatH)]OTf, [3]OTf and [Fe(2+)((Ph2)TIP)((tBu)PDA)](OTf)2, [4](OTf)2 are described here, whereas [Fe(2+)((Ph2)TIP)((tBu2)APH)]OTf, [2]OTf was reported in our previous paper [Bittner et al., Chem.-Eur. J. 2013, 19, 9686-9698]. These complexes mimic the substrate-bound active sites of nonheme iron dioxygenases, which catalyze the oxidative ring-cleavage of aromatic substrates like catechols and aminophenols. Each complex is oxidized in the presence of O2, and the geometric and electronic structures of the resulting complexes were examined with spectroscopic (absorption, EPR, Mössbauer, resonance Raman) and density functional theory (DFT) methods. Complex [3]OTf reacts rapidly with O2 to yield the ferric-catecholate species [Fe(3+)((Ph2)TIP)((tBu)Cat)](+) (3(ox)), which undergoes further oxidation to generate an extradiol cleavage product. In contrast, complex [4](2+) experiences a two-electron (2e(-)), ligand-based oxidation to give [Fe(2+)((Ph2)TIP)((tBu)DIBQ)](2+) (4(ox)), where DIBQ is o-diiminobenzoquinone. The reaction of [2](+) with O2 is also a 2e(-) process, yet in this case both the Fe center and (tBu2)AP ligand are oxidized; the resulting complex (2(ox)) is best described as [Fe(3+)((Ph2)TIP)((tBu2)ISQ)](+), where ISQ is o-iminobenzosemiquinone. Thus, the oxidized complexes display a remarkable continuum of electronic structures ranging from [Fe(3+)(L(2-))](+) (3(ox)) to [Fe(3+)(L(•-))](2+) (2(ox)) to [Fe(2+)(L(0))](2+) (4(ox)). Notably, the O2 reaction rates vary by a factor of 10(5) across the series, following the order [3](+) > [2](+) > [4](2+), even though the complexes have similar structures and Fe(3+/2+) redox potentials. To account for the kinetic data, we examined the relative abilities of the title complexes to bind O2 and participate in H-atom transfer reactions. We conclude that the trend in O2 reactivity can be rationalized by accounting for the role of proton transfer(s) in the overall reaction.

Dataset Information

Steric control of dioxygen activation pathways for MnII complexes supported by pentadentate, amide-containing ligands.

Publications

Steric control of dioxygen activation pathways for Mn<sup>II</sup> complexes supported by pentadentate, amide-containing ligands.

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