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In this paper we have investigated the efficiency of the intramolecular electron transfer process in a family of RuII–FeII dyads, previously shown to perform the light-driven activation of an iron-bound water molecule. The Ru chromophore and Fe catalytic units are connected at different lengths through a triazole group attached to an alkyl chain containing either three or five carbon atoms. The driving force for Fe oxidation is modified by adding ethyl ester substituents on the bipyridines completing the coordination sphere of a [Ru(bpy)3]2+-like chromophore. Transient absorption measurements and simulations were used to obtain the rate constants of internal charge transfer for the different complexes. The parameters governing the electron transfer process, reorganisation energy λ and electronic coupling HAB were obtained, and the observed variations are discussed within the framework of Marcus theory, which can help in understanding how to optimize the design of molecular photocatalysts. [ABSTRACT FROM AUTHOR]

We report an in-depth study of the reductive activation of O2 by the nonheme [FeII(L 25)(MeCN)]2+ complex carried out by cyclic voltammetry. Experimental evidence is obtained for the slow coordination of dioxygen to the ferrous center yielding an FeII/O2 adduct with a strong FeII–O2 character rather than an FeIII–superoxo one. Electron injection in the FeII–O2 species occurs at a potential of ca. −700 mV vs. SCE, i.e. 200 mV above the O2 to O2˙− reduction, leading to the formation of a FeIII–peroxo intermediate and then FeIII–hydroperoxo upon protonation by residual water. The experimental CVs recorded at variable scan rate or variable FeII concentration are well simulated taking into account a detailed mechanism initiated by the competitive reduction of O2 and the FeII–O2 adduct. Analysis of the concentration of the reaction intermediates generated as a function of the applied potential indicates that the FeIII–peroxo intermediate significantly accumulates at a potential of −650 mV. Oxidative bromination of anisole is assayed under electrolytic conditions at this potential to yield bromoanisole products. The low faradaic yields observed reveal that deleterious reactions such as direct reduction of reaction intermediates likely occur. Based on the detailed mechanism elucidated, a number of improvements to achieve more efficient catalytic reactions can be proposed. [ABSTRACT FROM AUTHOR]

New mononuclear non‐heme FeII complexes derived from a well‐known pentadentate amine/pyridine ligand were synthesized. The new ligand bears an additional OH group on one pyridyl aimed at developing second sphere hydrogen bonds. Two binding modes of the ligand were observed that can be selectively obtained through the choice of the counterion: the OH group develops an intramolecular H‐bond with the 6th exogenous ligand in the classical mode, or an intermolecular one with the unbound triflate counterion in the alternative mode. The complexes were studied by cyclic voltammetry in the reductive activation of O2. The addition of an H‐bond improves the O2 activation potential with a gain of 90 mV in the classical case and 220 mV in the alternative one. The difference is ascribed to the development of stabilizing H‐bonds within the FeIII‐peroxo intermediate with the proximal O atom of peroxo (classical mode) or the distal one (alternative mode). Ultimately, the complex in the alternative mode activates O2 at −0.5 V vs SCE (500 mV gain compared to free O2). We took advantage of this significantly reduced energetic cost to carry out and achieve the electroassisted oxidation of thioanisole by O2 at −0.5 V vs SCE. [ABSTRACT FROM AUTHOR]

Three new amine/pyridine FeII complexes bearing pentadentate ligand with one, two or three electron enriched 4‐methoxy‐3,5‐dimethylpyridine were used as catalysts for the oxidation of small organic molecules by hydrogen peroxide. The distribution of products formed suggests that these ligands are not enough electron donating to promote the O−O heterolytic cleavage of the oxidant in order to generate selective FeV(O) species. Using acetic acid in the reaction mixtures results in a significant increase of the efficiency of these catalytic systems. Our investigations show that the use of AcOH leads to the protonation/dissociation of a pyridyl moiety and the formation of (N4)FeII(OAc)(OH) species. These complexes readily react with excess hydrogen peroxide to yield (N4)FeIII(OAc)(OOH) intermediates. These latter intermediates are proposed to evolve into (N4)FeIV(OAc)(O), which are more efficient than the usual (N4)FeIV(O) and (N5)FeIV(O). [ABSTRACT FROM AUTHOR]

Hydrogen peroxide is a versatile reductant that under the right conditions can react to form dioxygen in an electrochemical reaction. This reaction has a low carbon footprint and applications are being sought for batteries. In this work a computational study is presented on a recently reported nonheme iron(II) complex where we study mechanistic pathways leading to dioxygen formation from H2O2. The work shows that upon reduction of the iron(III)‐hydroperoxo species it rapidly leads through heterolytic cleavage of the dioxygen bond to form iron(IV)‐oxo(hydroxo). The dimerization reaction of two iron(IV)‐oxo(hydroxo) complexes then leads to formation of the dioxygen bond rapidly with small barriers. Dissociation of the dimer expels dioxygen in an exothermic reaction. An alternative mechanism through the formation of a μ‐1,2‐peroxo‐μ‐1,1‐hydroperoxodiiron(II) intermediate was also tested but found to be highly endergonic. These studies highlight the electrochemical feasibilities of nonheme iron(III)‐hydroperoxo complexes. [ABSTRACT FROM AUTHOR]

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]

Oxidation of organic substrates is achieved in nature under mild conditions thanks to metalloenzymes but remains a challenge for chemists. Herein we show by UV-Vis spectroelectrochemistry that when MnIIITPPCl is electrochemically reduced to MnII in CH2Cl2 under O2, a MnIIO2˙ species is generated. Benzoic anhydride reacts with the latter triggering a catalytic current in cyclic voltammetry. Electrolysis on the catalytic wave in the presence of cyclooctene leads to its oxygenation or halogenation depending on the axial ligand present as reported here for the first time. [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]

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]

Activation of hydrogen peroxide by FeII salts (Fenton systems) leads to a myriad of oxidizing agents whose nature, FeIVO, or hydroxyl radicals and FeIII species, is dictated by the reaction conditions, in particular the pH value. Using the non heme FeII complex [FeII(L5²)(CH3CN)]2+ (1) (where L5² is the pentadentate ligand N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) we have observed the simultaneous formation of two reaction intermediates, [FeIV(O)(L5²)]2+ and [FeIII(OOH)(L5²)]2+, in its reaction with excess hydrogen peroxide in the presence of sub-stoichiometric amounts of triethylamine. Kinetic and spectroscopic monitoring of the reaction mixture and of independently prepared [FeIV(O)(L5²)]2+ in the presence of the different constituents of the reaction mixture allows drawing a mechanistic scheme. These two reactive species are formed simultaneously following two independent and competitive pathways. [FeIV(O)(L5²)]2+ is obtained via heterolytic O--O cleavage of the oxidant assisted by the base in a peroxidase-like mechanism whereas [FeIII(OOH)(L5²)]2+ is generated upon homolytic O--O cleavage of hydrogen peroxide. The relative contribution of these two pathways can be tuned by adjusting the amount of base used. [ABSTRACT FROM AUTHOR]

Using light energy and O2 for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O2 by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)3]2+, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV+.. MV+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O2 to O2.− in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways. [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]

We demonstrate that the devised incorporation of an alkylamine group into the second coordination sphere of an FeII complex allows to switch its reactivity with H2O2 from the usual formation of FeIII species towards the selective generation of an FeIV‐oxo intermediate. The FeIV‐oxo species was characterized by UV/Vis absorption and Mössbauer spectroscopy. Variable‐temperature kinetic analyses point towards a mechanism in which the heterolytic cleavage of the O−O bond is triggered by a proton transfer from the proximal to the distal oxygen atom in the FeII‐H2O2 complex with the assistance of the pendant amine. DFT studies reveal that this heterolytic cleavage is actually initiated by an homolytic O−O cleavage immediately followed by a proton‐coupled electron transfer (PCET) that leads to the formation of the FeIV‐oxo and release of water through a concerted mechanism. Don't cleave me this way: A novel synthetic iron(II) complex bearing a second sphere intra‐molecular base efficiently forms an iron(IV)‐oxido species in the presence of H2O2 rather than the usual formation of an FeIII species. Mechanistic studies demonstrate an overall heterolytic cleavage of the O−O bond proceeding by a homolytic cleavage with a concerted proton coupled electron transfer (PCET). [ABSTRACT FROM AUTHOR]