Activation of Dioxygen by Iron and Manganese Complexes: A Heme and Nonheme Perspective

Abstract

The rational design of well-defined, first-row transition metal complexes that can activate dioxygen has been a challenging goal for the synthetic inorganic chemist. The activation of O2 is important in part because of its central role in the functioning of metalloenzymes, which utilize O2 to perform a number of challenging reactions including the highly selective oxidation of various substrates. There is also great interest in utilizing O2, an abundant and environmentally benign oxidant, in synthetic catalytic oxidation systems. This Perspective brings together recent examples of biomimetic Fe and Mn complexes that can activate O2 in heme or nonheme-type ligand environments. The use of oxidants such as hypervalent iodine (e.g., ArIO), peracids (e.g., m-CPBA), peroxides (e.g., H2O2) or even superoxide is a popular choice for accessing well-characterized metal-superoxo, metal-peroxo, or metal-oxo species, but the instances of biomimetic Fe/Mn complexes that react with dioxygen to yield such observable metal-oxygen species are surprisingly few. This Perspective focuses on mononuclear Fe and Mn complexes that exhibit reactivity with O2 and lead to spectroscopically observable metal-oxygen species, and/or oxidize biologically relevant substrates. Analysis of these examples reveals that solvent, spin state, redox potential, external co-reductants, and ligand architecture can all play important roles in the O2 activation process.

Figure 1

Transient absorption spectral changes of the [(TBP₈Cz)Mn^III]* (⁵T₁) (530 nm) and [(TBP₈Cz)Mn^III]* (⁷T₁) (774 nm) states, generated from photoexcitation of (TBP₈Cz)Mn^III in benzonitrile. Reprinted with permission from ref . Copyright 2013 American Chemical Society.

Figure 2

Nonheme iron enzymes and the various transformations that they catalyze. Adapted with permission from ref . Copyright 2008 Macmillan Publishers Ltd.

Figure 3

(a) Tetradentate N4 ligands and dioxygenase reactivity for the corresponding mono- and doubly deprotonated catechol complexes. (b) Dioxygenase reactivity for Fe^III–catecholate complexes with tridentate N3 ligands. (c) Reaction of iron(II)–aminophenolate complex with dioxygen. Adapted with permissions from refs , , and . Copyright 2008 American Chemical Society, Copyright 2013 American Chemical Society, and Copyright 2014 American Chemical Society.

Scheme 1

Dioxygen-Mediated Autoxidation Mechanism for Ferrous–Porphyrin Complexes

Scheme 2

Formation of an Fe^III(O₂⁻) Species Generated from the Reaction of Ferrous–Porphyrin with O₂, and the One-Electron Reduction of the Fe^III(O₂⁻) Complex^{a a}Adapted with permission from refs and . Copyright 2009 John Wiley & Sons, Inc., and Copyright 2010 American Chemical Society.

Scheme 3

Photoinitiated Dioxygen Activation by a Mn^III–Corrolazine Complex and the Mechanism of Formation of a Mn^V(O) Species^{a a}Adapted with permission from refs and . Copyright 2012 American Chemical Society, and Copyright 2013 American Chemical Society.

Scheme 4

Proposed Mechanism for the Acid-Assisted Catalytic Oxidation of Hexamethyl Benzene^{a a}Reprinted with permission from ref . Copyright 2016 American Chemical Society.

Scheme 5

Proposed Dioxygen Activation Pathways for Synthetic Nonheme Iron Complexes

Scheme 6

Formation and Reactivity of the [Fe^III(O₂⁻)(BDDP)] (Top) and [Fe^III(O₂⁻)(Tp^Me2)(L^Ph)] (Bottom) Complexes^{a a}Bottom panel adapted with permission from ref . Copyright 2015 John Wiley & Sons, Inc.

Scheme 7

Formation of Nonheme Iron(III)–Peroxo and Iron(IV)–Oxo Complexes Derived from the Reaction of Fe^II Complexes and O₂^{a a}Dotted arrows indicate formation of a proposed but nondetected intermediate, and solid arrows indicate that the species was detected spectroscopically characterized.

Scheme 8

O₂ Activation by [Ni^IIFe^II] Complexes (Crystal Structure, Bottom Left) To Form an Fe^IV(O₂²⁻) Species (Crystal Structure, Bottom Right) and Subsequent 2e⁻ Reduction To Generate H₂O^{a a}Adapted with permission from ref . Copyright 2016 John Wiley & Sons, Inc.

Scheme 9

Reactions Catalyzed by Intra- and Extradiol Cleaving and 2-Aminophenol Dioxygenases^{a a}Reprinted with permission from refs and . Copyright 2007 American Chemical Society, and Copyright 2014 American Chemical Society.

Scheme 10

Ligands containg N,N,O Donor Atoms and Dioxygen Reaction Products for the Fe^II–Catecholate Complexes^{a a}Adapted with permission from ref . Copyright 2007 American Chemical Society.

Scheme 11

Interception of a Putative Fe^IV(O) Intermediate, Generated from the Reaction of [Fe^II(Tp^Ph2)(BF)] and O₂^{a a}Reprinted with permission from ref . Copyright 2009 John Wiley & Sons, Inc.

Scheme 12

Reaction of [Fe^II(Tp^Ph2)(benzilate)] with O₂ and the Interception of Various Active Oxidant Species^{a a}Adapted from refs and . Copyright 2016 John Wiley & Sons, Inc., and Copyright 2015 John Wiley & Sons, Inc.

Scheme 13

Examples of S-Oxygenation Reactions with Iron(II)–Thiolate Complexes and O₂^{a a}Adapted with permission from refs and . Copyright 2010 American Chemical Society, and Copyright 2011 American Chemical Society.

Scheme 14

Double Oxygenation of Sulfur, Derived from the Reaction of an Fe^II–Thiolate Complex and O₂^{a a}Adapted with permission from refs and . Copyright 2012 American Chemical Society, and Copyright 2012 John Wiley & Sons, Inc.

Scheme 15

Catalytic Reduction of O₂ to H₂O via the Intermediacy of Mn^III–Peroxo and Mn^III–O(H) Complexes^{a a}Adapted with permission from refs and . Copyright 2008 American Chemical Society, and Copyright 2011 American Chemical Society.

Scheme 16

Low-Temperature Formation of a Peroxo-Bridged Dimanganese(III) Species from the Reaction of a Mn^II–Thiolate Complex + O₂, and Its Subsequent Conversion to a µ-Oxo-Bridged Dimeric Complex^{a a}Adapted with permission from ref . Copyright 2015 American Chemical Society.

Scheme 17

Stepwise Oxidation of Benzylic C–H Bonds Using O₂ by a Mn^II Complex^{a a}Adapted with permission from ref . Copyright 2016 John Wiley & Sons, Inc.