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We report two new FeIII complexes [L1FeIII(H2O)](OTf)2 and [L2FeIII(OTf)], obtained by replacing pyridines by phenolates in a known non‐heme aminopyridine iron complex. While the original, starting aminopyridine [(L52)FeII(MeCN)](PF6) complex is stable in air, the potentials of the new FeIII/II couples decrease to the point that [L2FeII] spontaneously reduces O2 to superoxide. We used it as an O2 activator in an electrochemical setup, as its presence allows to generate superoxide at a much more accessible potential (>500 mV gain). Our aim was to achieve substrate oxidation via the reductive activation of O2. While L2FeIII(OTf) proved to be a good O2 activator but a poor oxidation system, its association with another complex (TPEN)FeII(PF6)2 generates a complementary tandem couple for electro‐assisted oxidation of substrates, working at a very accessible potential: upon reduction, L2FeIII(OTf) activates O2 to superoxide and transfers it to (TPEN)FeII(PF6)2 leading in fine to the oxidation of thioanisole. [ABSTRACT FROM AUTHOR]

We report on the synthesis and characterization of three iron(III) phosphasalen complexes, [FeIII(Psalen)(X)] differing in the nature of the counter‐anion/exogenous ligand (X−=Cl−, NO3−, OTf−), as well as the neutral iron(II) analogue, [FeII(Psalen)]. Phosphasalen (Psalen) differs from salen by the presence of iminophosphorane (P=N) functions in place of the imines. All the complexes were characterized by single‐crystal X‐ray diffraction, UV/Vis, EPR, and cyclic voltammetry. The [FeII(Psalen)] complex was shown to remain tetracoordinated even in coordinating solvent but surprisingly exhibits a magnetic moment in line with a FeII high‐spin ground state. For the FeIII complexes, the higher lability of triflate anion compared to nitrate was demonstrated. As they exhibit lower reduction potentials compared to their salen analogues, these complexes were tested for the coupling of 2‐naphthol using O2 from air as oxidant. In order to shed light on this reaction, the interaction between 2‐naphthol and the FeIII(Psalen) complexes was studied by cyclic voltammetry as well as UV/Vis spectroscopy. [ABSTRACT FROM AUTHOR]

Inspired by the sulfur‐rich environment found in active hydrogenase enzymes, a Ni‐based proton reduction catalyst with pentadentate N2S3 ligand was synthesised. When coupled with [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) as photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared with a homologous counterpart bearing a tetradentate N2S2 ligand. The mechanistic pathway of photoinduced hydrogen evolution was comprehensively analysed through optical transient absorption and time‐resolved X‐ray absorption spectroscopy, which revealed important electronic and structural changes in the catalytic system during photoirradiation. The NiII catalyst undergoes a photoinduced metal‐centred reduction to form a NiI intermediate with distorted square‐bipyramidal geometry. Further kinetic analyses revealed differences in charge‐separation dynamics between the pentadentate and tetradentate forms. [ABSTRACT FROM AUTHOR]

Rieske dioxygenases are metalloenzymes capable of achieving cis‐dihydroxylation of aromatics under mild conditions using O2 and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis‐FeV(O)(OH) moiety. Molecular models allow the generation of a FeIII(OOH) species with H2O2, to yield a FeV(O)(OH) species with tetradentate ligands, or {FeIV(O); OH.} pairs with pentadentate ones. We have designed a new pentadentate ligand, mtL42, bearing a labile triazole, to generate an "in‐between" situation. Two iron complexes, [(mtL42)FeCl](PF6) and [(mtL42)Fe(OTf)2]), were obtained and their reactivity towards aromatic substrates was studied in the presence of H2O2. Spectroscopic and kinetic studies reflect that triazole is bound at the FeII state, but decoordinates in the FeIII(OOH). The resulting [(mtL42)FeIII(OOH)(MeCN)]2+ then lies on a bifurcated decay pathway (end‐on homolytic vs. side‐on heterolytic) depending on the addition of aromatic substrate: in the absence of substrate, it is proposed to follow a side‐on pathway leading to a putative (N4)FeV(O)(OH), while in the presence of aromatics it switches to an end‐on homolytic pathway yielding a {(N5)FeIV(O); OH.} reactive species, through recoordination of triazole. This switch significantly impacts the reaction regioselectivity. [ABSTRACT FROM AUTHOR]

The reactivity and selectivity of non‐heme FeII complexes as oxidation catalysts can be substantially modified by alteration of the ligand backbone or introduction of various substituents. In comparison with the hexadentate ligand N,N,N′,N′‐tetrakis(pyridin‐2‐ylmethyl)ethane‐1,2‐diamine (TPEN), N,N′‐bis[1‐(pyridin‐2‐yl)ethyl]‐N,N′‐bis(pyridin‐2‐ylmethyl)ethane‐1,2‐diamine (2MeL62) has a methyl group on two of the four picolyl positions. FeII complexation by 2MeL62 yields two diastereomeric complexes with very similar structures, which only differ in the axial/equatorial positions occupied by the methylated pyridyl groups. In solution, these two isomers exhibit different magnetic behaviors. Whereas one isomer exhibits temperature‐dependent spin‐state conversion between the S=0 and S=2 states, the other is more reluctant towards this spin‐state equilibrium and is essentially diamagnetic at room temperature. Their catalytic properties for the oxidation of anisole by H2O2 are very different and correlate with their magnetic properties, which reflect their lability/inertness. These different properties most likely depend on the different steric constraints of the methylated pyridyl groups in the two complexes. [ABSTRACT FROM AUTHOR]

Abstract: Alzheimer's disease and oxidative stress are connected. In the present communication, we report the use of a MnII‐based superoxide dismutase (SOD) mimic ([MnII(L)]+, 1+) as a pro‐drug candidate to target CuII‐associated events, namely, CuII‐induced formation of reactive oxygen species (ROS) and modulation of the amyloid‐β (Aβ) peptide aggregation. Complex 1+ is able to remove CuII from Aβ, stop ROS and prevent alteration of Aβ aggregation as would do the corresponding free ligand LH. Using 1+ instead of LH in further biological applications would have the double advantage to avoid the cell toxicity of LH and to benefit from its proved SOD‐like activity. [ABSTRACT FROM AUTHOR]

The magnetic properties of the pentacoordinate [MII(Me4cyclam)N3]+ (Me4cyclam=tetramethylcyclam; N3=azido; M=Ni, Co) complexes were investigated. Magnetization and EPR studies indicate that they have an easy plane of magnetization with axial anisotropy parameters D close to 22 and greater than 30 cm−1 for the Ni and Co complexes, respectively. Ab initio calculations reproduced the experimental values of the zero-field splitting parameters and allowed the orientation of the anisotropy tensor axes with respect to the molecular frame to be determined. For M=Ni, the principal anisotropy axis lies along the Ni−Nazido direction perpendicular to the Ni(Me4cyclam) mean plane, whereas for M=Co it lies in the Co(Me4cyclam) mean plane and thus perpendicular to the Co−Nazido direction. These orientations match one of the possible solutions experimentally provided by single-crystal cantilever torque magnetometry. To rationalize the geometry and its impact on the orientation of the anisotropy tensor axis, calculations were carried out on model complexes [NiII(NCH)5]2+ and [CoII(NCH)5]2+ by varying the geometry between square pyramidal and trigonal bipyramidal. The geometry of the complexes was found to be the result of a compromise between the electronic configuration of the metal ion and the structure-orienting effect of the Me4cyclam macrocycle. Moreover, the orientation of the anisotropy axes is mainly dependent on the geometry of the complexes. [ABSTRACT FROM AUTHOR]

By using complementary experimental techniques and first-principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C2 v, has been investigated. Four complexes have the general formula [Ni(bpy)X2] n+ (bpy=2,2′-bipyridine; X2=bpy ( 1), (NCS−)2 ( 2), C2O42− ( 3), NO3− ( 4)). In the fifth complex, [Ni(HIM2-py)2(NO3)]+ ( 5; HIM2-py=2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazolyl-1-hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM2-py. Analysis of the high-field, high-frequency electronic paramagnetic resonance (HF-HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from −1 to −10 cm−1. First-principles SO-CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3, on one hand, and 4 and 5, on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters ( D (axial), E (rhombic), and gi) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the eg-like orbitals and is due to the difference in the σ-donor strength of NO3− and bpy or HIM2-py, whereas the difference in anisotropy between the two compounds is due to splitting of the t2g-like orbitals; and 2) the anisotropy of complexes 1- 3 arises from the small splitting of the t2g-like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model. [ABSTRACT FROM AUTHOR]

An artificial metalloenzyme based on the covalent grafting of a nonheme FeII polyazadentate complex into bovine β-lactoglobulin has been prepared and characterized by using various spectroscopic techniques. Attachment of the FeII catalyst to the protein scaffold is shown to occur specifically at Cys121. In addition, spectrophotometric titration with cyanide ions based on the spin-state conversion of the initial high spin ( S=2) FeII complex into a low spin ( S=0) one allows qualitative and quantitative characterization of the metal center's first coordination sphere. This biohybrid catalyst activates hydrogen peroxide to oxidize thioanisole into phenylmethylsulfoxide as the sole product with an enantiomeric excess of up to 20 %. Investigation of the reaction between the biohybrid system and H2O2 reveals the generation of a high spin ( S=5/2) FeIII(η2-O2) intermediate, which is proposed to be responsible for the catalytic sulfoxidation of the substrate. [ABSTRACT FROM AUTHOR]

A method for the direct and rare umpolung of the 3 position of indoles is reported. The activation of N-acetylindole with iron(III) chloride allows the CH addition of aromatic and heteroaromatic substrates to the C2C3 double bond of the indole nucleus to generate a quaternary center at C3 and leads regioselectively to 3-arylindolines. Optimization, scope (50 examples), practicability (gram scale, air atmosphere, room temperature), and mechanistic insights of this process are presented. Synthetic transformations of the indoline products into drug-like compounds are also described. [ABSTRACT FROM AUTHOR]