On the Reactivity of Mononuclear Iron(V)oxo Complexes

Abstract

Ferric tetraamido macrocyclic ligand (TAML)-based catalysts [Fe{C(6)H(4)-1,2-(NCOCMe(2)NCO)(2)CR(2)}(OH(2))]PPh(4) [1; R = Me (a), Et (b)] are oxidized by m-chloroperoxybenzoic acid at -40 °C in acetonitrile containing trace water in two steps to form Fe(V)oxo complexes (2a,b). These uniquely authenticated Fe(V)(O) species comproportionate with the Fe(III) starting materials 1a,b to give μ-oxo-(Fe(IV))(2) dimers. The comproportionation of 1a-2a is faster and that of 1b-2b is slower than the oxidation by 2a,b of sulfides (p-XC(6)H(4)SMe) to sulfoxides, highlighting a remarkable steric control of the dynamics. Sulfide oxidation follows saturation kinetics in [p-XC(6)H(4)SMe] with electron-rich substrates (X = Me, H), but changes to linear kinetics with electron-poor substrates (X = Cl, CN) as the sulfide affinity for iron decreases. As the sulfide becomes less basic, the Fe(IV)/Fe(III) ratio at the end of reaction for 2b suggests a decreasing contribution of concerted oxygen-atom transfer (Fe(V) → Fe(III)) concomitant with increasing electron transfer oxidation (Fe(V) → Fe(IV)). Fe(V) is more reactive toward PhSMe than Fe(IV) by 4 orders of magnitude, a gap even larger than that known for peroxidase Compounds I and II. The findings reinforce prior work typecasting TAML activators as faithful peroxidase mimics.

Ferryl and Ferrate Species: Mössbauer Spectroscopy Investigation

“…The electron-transfer steps of the oxidation of Met by Fe(VI) may be given as Equations (6)- (8). This reaction scheme is consistent with the observation of Mössbauer spectrum (i.e.…”

“…Interestingly, the reaction schemes, given by Equations (4), (5) and Equations (6)- (8), are in agreement with the suggested initial electron transfer steps from the correlation of the rate constants with reduction potentials for the oxidation of Cys and Met by Fe(VI), respectively. [32] …”

“…[1][2][3][4] For example, the activation of dioxygen to oxidize unactivated C-H bond of the substrates involved oxoiron(IV) (Fe IV =O) and oxoiron(V) (Fe V =O), generally called ferryl species. [5][6][7][8][9] Figure 1 represents characterization of the three kinds of high-valent iron-oxo active sites in haem and non-haem enzymes. In the first kind, haem-comprising enzymes contain mononuclear iron-protophrin IX active sites.…”

The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations

“…Corma and co-workers found that a chiral Au(III) complex (LAuCl 3 , L: 2,6-bis[(4R)-phenyl-2-oxazolin-2-yl]pyridine) can enantioselectively epoxidize olefins with dioxygen. Isotopic 18 O 2 labelling experiments reveal that 18 O has been incorporated into the epoxide product, indicating that the gold complex can directly activate dioxygen and transfer oxygen to olefin by a non-radical pathway. 19 Upon adding NaOCl to the gold complex, a new band at 460 nm in the UV-vis spectrum and one at 262 cm À1 in the Raman spectrum were assigned to a gold-oxygen vibration by the authors, indicating that a gold oxo or peroxo species has been formed under oxidation conditions.…”

Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs toward organic sulfides in water: resolving a fast catalysis versus slower single-turnover reactivity dilemma

“…Later, the kinetics and mechanism of this transformation were explored in detail in collaboration with Professor Rudi van Eldik, to whom this issue is devoted [14]. Our investigations of the reactivity of Fe V and Fe IV complexes derived from TAMLs, which were performed at -40 °C in MeCN, revealed that both forms oxidize thioanisoles, though Fe V is by several orders of magnitude more reactive than Fe IV [15]. More recently we explored the reactivity of iron(IV) species in aqueous solution towards hexacyanoferrate(II) as an electron donor [16].…”