Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic Epoxidation of Olefins by Hydrogen Peroxide.

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)FeIV (O)(CH3 CN)]2+ (2; cyclam=1,4,8,11-tetraazacyclotetradecane), is reported in the reactions of [(cyclam)FeII ]2+ with aqueous hydrogen peroxide (H2 O2 ) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo- and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)FeII ]2+ and H2 O2 . In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high-yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.

Observation of Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Iron Complexes.

To probe the possibility that carbodicarbenes (CDCs) are redox active ligands, all four members of the redox series [Fe(1)2 ]n+ (n=2-5) were synthesized, where 1 is a neutral tridentate CDC. Through a combination of spectroscopy and DFT calculations, the electronic structure of the pentacation is shown to be [FeIII (1.+ )2 ]5+ (S= 1 / 2 ). That of [Fe(1)2 ]4+ is more ambiguous, but it has significant contributions from the open-shell singlet [FeIII (1)(1.+ )]4+ (S=0). The observed spin states derive from antiferromagnetic coupling of their constituent low-spin iron(III) centres and cation radical ligands. This marks the first time redox activity has been observed for carbones and expands the diverse chemical behaviour known for these ligands.

A New Domain of Reactivity for High-Valent Dinuclear [M(µ-O)2 M'] Complexes in Oxidation Reactions.

The strikingly different reactivity of a series of homo- and heterodinuclear [(MIII )(µ-O)2 (MIII )']2+ (M=Ni; M'=Fe, Co, Ni and M=M'=Co) complexes with ß-diketiminate ligands in electrophilic and nucleophilic oxidation reactions is reported, and can be correlated to the spectroscopic features of the [(MIII )(µ-O)2 (MIII )']2+ core. In particular, the unprecedented nucleophilic reactivity of the symmetric [NiIII (µ-O)2 NiIII ]2+ complex and the decay of the asymmetric [NiIII (µ-O)2 CoIII ]2+ core through aromatic hydroxylation reactions represent a new domain for high-valent bis(µ-oxido)dimetal reactivity.

A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3 O Macrocyclic Ligand.

The sluggish oxidants [FeIV (O)(TMC)(CH3 CN)]2+ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [FeIV (O)(TMCN-d12 )(OTf)]+ (TMCN-d12 =1,4,7,11-tetra(methyl-d3 )-1,4,7,11-tetraazacyclotetradecane) are transformed into the highly reactive oxidant [FeIV (O)(TMCO)(OTf)]+ (1; TMCO=4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane) upon replacement of an NMe donor in the TMC and TMCN ligands by an O atom. A rate enhancement of five to six orders of magnitude in both H atom and O atom transfer reactions was observed upon oxygen incorporation into the macrocyclic ligand. This finding was explained in terms of the higher electrophilicity of the iron center and the higher availability of the more reactive S=2 state in 1. This rationalizes nature's preference for using O-rich ligand environments for the hydroxylation of strong C-H bonds in enzymatic reactions.

Oxidation Reactions with Bioinspired Mononuclear Non-Heme Metal-Oxo Complexes.

The selective functionalization of strong C-H bonds and the oxidation of water by cheap and nontoxic metals are some of the key targets of chemical research today. It has been proposed that high-valent iron-, manganese-, and copper-oxo cores are involved as reactive intermediates in important oxidation reactions performed by biological systems, thus making them attractive targets for biomimetic synthetic studies. The generation and characterization of metal-oxo model complexes of iron, manganese, and copper together with detailed reactivity studies can help in understanding how the steric and electronic properties of the metal centers modulate the reactivity of the metalloenzymes. This Review provides a focused overview of the advances in the chemistry of biomimetic high-valent metal-oxo complexes from the last 5-10 years that can be related to our understanding of biological systems.

C-H activation and nucleophilic substitution in a photochemically generated high valent iron complex.

The photochemical oxidation of a (TAML)FeIII complex 1 using visible light generated Ru(bpy)33+ produces valence tautomers (TAML)FeIV (1+ ) and (TAML˙+)FeIII (1-TAML˙+ ), depending on the exogenous anions. The presence of labile Cl- or Br- results in a ligand-based oxidation and stabilisation of a radical-cationic (TAML˙+)FeIII complex, which subsequently leads to unprecedented C-H activation followed by nucleophilic substitution on the TAML aryl ring. In contrast, exogenous cyanide culminates in metal-based oxidation, yielding the first example of a crystallographically characterised S = 1 [(TAML)FeIV(CN)2]2- species. This is a rare report of an anion-dependent valence tautomerisation in photochemically accessed high valent (TAML)Fe systems with potential applications in the oxidation of pollutants, hydrocarbons, and water. Furthermore, the nucleophilic aromatic halogenation reaction mediated by (TAML˙+)FeIII represents a novel domain for high-valent metal reactivity and highlights the possible intramolecular ligand or substrate modification pathways under highly oxidising conditions. Our findings therefore shine light on high-valent metal oxidants based on TAMLs and other potential non-innocent ligands and open new avenues for oxidation catalyst design.

A cobalt(ii) iminoiodane complex and its scandium adduct: mechanistic promiscuity in hydrogen atom abstraction reactions.

In addition to oxometal [M(n+)[double bond, length as m-dash]O] and imidometal [M(n+)[double bond, length as m-dash]NR] units, transient metal-iodosylarene [M((n-2)+)-O[double bond, length as m-dash]IPh] and metal-iminoiodane [M((n-2)+)-N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible "second oxidant" responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.