Electron‐Transfer Properties of an Efficient Nonheme Iron Oxidation Catalyst with a Tetradentate Bispidine Ligand

Abstract

The interest in nonheme iron systems has lead to an increasingly detailed knowledge of the coordination geometries, electronic structures, and reaction mechanisms of oxygenases and halogenases (for example TauD and SyrB2), and this is due to numerous studies involving the biological processes, small molecule model systems, and quantumchemical model studies. [1][2][3][4][5][6][7] The thorough analysis of a variety of model systems has yielded a detailed understanding of the nature of the catalytically active high-valent iron oxo intermediates. [2,[4][5][6][7][8] Current focal points in experimental and theoretical modeling of the enzyme reactions are ambiguities in the degree of protonation, the oxidation and spin states of the catalytically active high-valent iron complex, and their relation to the reactivities (Fe IV vs. Fe V ; S = 1 versus S = 2; O versus OH versus OH 2 ).Important parameters for the characterization of the highvalent iron oxo complex and for the interpretation of its reactivity are 1) the reduction potential E red of the ferryl complex, [9][10][11] 2) the kinetic barrier for the electron-transfer process, 3) the basicity of the oxidized and reduced iron oxo species, [12] and 4) the energy gap between the various spin states, because computational studies indicate that the potential energy for oxidation reactions on the high-spin surface has lower barriers, whereas most low-molecularweight biomimetic Fe IV = O systems, in contrast to the enzymes, have an intermediate spin (S = 1) ground state. [6,[13][14][15] Herein, we report the electron-transfer properties of an Fe IV =O complex with the tetradentate bispidine ligand L (Scheme 1).[16] The coordination chemistry of ligand L has been thoroughly studied by computational (molecular mechanics, DFT) and experimental methods, and it was shown to be enforced by the rigid adamantane-type ligand backbone. A variety of X-ray crystal structures have been used to show that tetra-and pentadentate derivatives of L with different donor sets have an elastic coordination sphere as a consequence of the flat potential energy surface with various close-to-degenerate minima. The rigid and relatively large cavity of the bispidines is one reason for their high metal ion selectivity, and these ligands exhibit uncommon dependencies of the stability constants. [17,18] Their iron chemistry has been developed in the fields of alkane and alkene oxidation and biomimetic nonheme-halogenase reactivity. [19][20][21][22][23] Computational studies related to the bispidine-Fe IV = O species reveal a very small energy gap between the intermediate-spin (S = 1) and high-spin (S = 2) electronic configurations. [21][22][23] Furthermore, the reorganization energy of the electron transfer between the oxo Fe IV and the oxo Fe III species is expected to be small owing to the rigid ligand backbone, which is a possible reason for the exceptionally high reactivity. Therefore, we report herein the fundamental electron-transfer properties of [Fe IV =O(L)(NCMe)] 2+ (1). The high-valent comple...

Enhanced Electron-Transfer Reactivity of Nonheme Manganese(IV)–Oxo Complexes by Binding Scandium Ions

“…It should be noted that [(N4Py)Mn IV (O)] 2+ – (Sc 3+ ) 2 has the most positive E red value as compared with those of iron(IV)–oxo complexes. 23 Thus, [(N4Py)Mn IV (O)] 2+ – (Sc 3+ ) 2 is regarded as the strongest oxidant among high-valent metal–oxo complexes reported so far.…”

“…23b This indicates that the one-electron reduction of high-valent metal–oxo complexes generally requires a large reorganization energy probably due to the significant elongation of metal–oxo bond upon one-electron reduction. The smaller λ value of [Mn IV (O)] 2+ –(Sc 3+ ) 2 , as compared with [Mn IV (O)] 2+ , may be explained by the elongation of the Mn IV –O bond prior to electron transfer because of binding of two Sc 3+ ions, which results in smaller change in the Mn–O bond distance upon the electron transfer.…”

Deformylation Reaction by a Nonheme Manganese(III)–Peroxo Complex via Initial Hydrogen‐Atom Abstraction

“…Recent work of metal-bispidine ligand systems showed these complexes to react efficiently with substrates through hydrogen atom abstraction, due to their rigid ligand framework, and reaction rates could be monitored effectively over a broad temperature and concentration range. [10] ; with a half-life ~60 minutes, Figure 1a The nucleophilic and electrophilic character of 1 was then investigated in a reaction with 2-PPA as a substrate. Previous work showed that manganese(III)-peroxo complexes react with aldehydes to give the corresponding deformylated products by attacking the carbonyl group in a nucleophilic reaction.…”

Iron-catalysed oxidation and halogenation of organic matter in nature

“…5). [84,101] The very high Fe IV/III redox potentials [123,124] suggest that the bispidine-based ferryl complexes should even be able to oxidise the strong C-H bonds of alkanes. [89,124] With bispidines and several other ligand systems, [125][126][127] various possible high-valent iron species were proposed to be responsible for substrate activation, i.e.…”

“…[151,152] In a recent series of studies, biomimetic non-heme iron complexes were used to model the coordination of iron cations to natural organic matter in soils. Iron bispidine complexes were chosen for these experiments because they provide a well-defined coordination sphere, enforced by the rigid adamantane-derived backbone (see Figs 3, 5 and 7 above), [123,153,154] and the Fe II -bispidine-H 2 O 2 system has been studied extensively and thoroughly with trapped and well-characterised metastable intermediates. [57,155,156] Environmentally relevant organic reaction products Although oxidation processes under Fenton-or Fenton-like conditions were intensively studied for hazardous waste treatment, [157,158] their relevance for natural abiotic chemistry was not appreciated until the abiotic formation of halomethanes was studied in detail (Fig.…”