Μ-oxo Dimer Research Articles

Dimerization of Fe(III) Ion in an Aqueous Medium: Mechanistic Modelling and Effects of Ligands.

Aqueous iron solutions generally undergo spontaneous hydrolysis followed by aggregation resulting in the precipitation of nanocrystalline oxyhydroxide minerals. The mechanism of nucleation of such multinuclear oxyhydroxide clusters are unclear due to limited experimental evidence. Here, we investigate the mechanistic pathway of dimerization of Fe(III) ions using density functional theory (DFT) in aqueous medium considering effects of other ligands. Two hydrolyzed monomeric Fe(III) ions in aqueous medium may react to form two closely related binuclear products, the μ-oxo and the dihydroxo Fe2 dimer. Our studies indicate that the water molecules in the second coordination sphere and those co-ordinated to the Fe(III) ion, both participate in the dimerization process. The proposed mechanism effectively explains the formation of dihydroxo and μ-oxo Fe2 dimers with interconversion possibilities, for the first time. Results show, with only water molecules present in the second co-ordination sphere, dihydroxo Fe2 dimer is the thermodynamically and kinetically favored product with a low activation free energy. We calculated the step-wise reaction free energies of dimerization in the presence of nitrate ions in the first and second coordination sphere of Fe(III) ion separately, which shows that with nitrate ions in the second co-ordination sphere, the μ-oxo Fe2 dimer is the kinetically favored product.

Finally: The X-ray crystal structure of the illusive unsubstituted iron(III) phthalocyanine μ-oxo(1) dimer. DFT-predicted Mössbauer quadrupole splitting and antiferromagnetic coupling constants for X-ray geometry

The molecular structure of the unsubstituted iron(III) phthalocyanine [Formula: see text]-oxo(1) dimer ((PcFe)2O) was determined by single crystal X-ray diffraction. In agreement with the earlier speculations, the dimer has a bent (Fe-O-Fe angle is 152.4[Formula: see text]) structure. The interplay between the [Formula: see text]-[Formula: see text] interactions and steric hindrances caused by the isoindole units led to the observed staggering angle of [Formula: see text]24[Formula: see text] between two phthalocyanine ligands. The high-spin iron(III) centers are located significantly above the phthalocyanine N4 planes (0.57–0.58 Å). Several DFT exchange-correlation functionals were used to calculate the absolute value and sign of the Mössbauer quadrupole splitting and antiferromagnetic coupling constant for X-ray determined geometry of (PcFe)2O. It was demonstrated that the hybrid functionals provide the correct sign of the electric field gradient and the magnitude of the antiferromagnetic coupling constant compared to the pure functionals.

Tuning Fluorescence and Singlet Oxygen Quantum Yields of Subporphyrazines by Axial Functionalization.

The axial functionalization of Subporphyrazines (SubPzs) with unreported alkoxy groups, carboxy and carboperoxy rests, as well as sulfanyl, aryl and amino groups, forming B-O, B-S, B-C, and B-N bonds, respectively, has been investigated. The studied oxygen nucleophiles include aromatic and sterically demanding aliphatic alcohols, along with carboxylic acids and peracids. In general, direct substitution of the chloro-SubPz by oxygen nucleophiles of diverse nature proceeds smoothly, with yields of the isolated alkoxy and carboxy-substituted SubPzs ranging from 49 to 100 %. Conversely, direct substitution with sulphur, carbon and nitrogen nucleophiles do not afford the corresponding substituted SubPzs. In these cases, a stepwise procedure involving an axial triflate-SubPz intermediate was employed, affording only the phenyl-SubPz in 8 % yield. The major compound under these conditions was the unreported SubPz μ-oxo dimer, presumably arising from substitution of the triflate-SubPz by the in situ generated hydroxy-SubPz. This result indicates a quite low reactivity of the TfO-SubPz intermediate with carbon, sulphur and nitrogen nucleophiles. All SubPzs prepared in this work exhibited fluorescence at 510-515 nm with quantum yields ranging from 0.1 to 0.24. Additionally, all SubPzs generated singlet oxygen, with ΦΔ values ranging from 0.15 to 0.57, which show no apparent correlation with the axial substituents.

Investigations on the Influence of the Axial Ligand in [Salophene]iron(III) Complexes on Biological Activity and Redox Behavior.

The [N,N'-disalicylidene-1,2-phenylenediamine]iron(III) ([salophene]iron(III)) derivatives 1-4 with anionic axial ligands (A = Cl-, NO3-, SCN-, CH3COO-) and complexes 5 and 6 with neutral ligands (A = imidazole, 1-methylimidazole) as well as the μ-oxo dimer 7 inhibited proliferation, reduced metabolic activity, and increased mitochondrial reactive oxygen species. Ferroptosis as part of the mode of action was identified by inhibitor experiments, together with induction of lipid peroxidation and diminished mitochondrial membrane potential. No differences in activity were observed for all compounds except 4, which was slightly less active. Electrochemical analyses revealed for all compounds a fast attachment of the solvent dimethyl sulfoxide and a release of the axial ligand A. In contrast, in dichloromethane and acetonitrile, ligand exchange did not take place, as analyzed by measurements of the standard potential for the iron(III/II) redox reaction.

Structure of the green heme isolated from allylbenzene-modified chloroperoxidase: A novel heme architecture implicating the mechanisms of CPO inactivation and epoxidation

The chemical identification of the modified heme (the green heme) during chloroperoxidase catalyzed epoxidation of allylbenzene remains unestablished due to its high instability within the protein matrix, the absence of paramagnetically shifted signals, and the difficulty in obtaining crystals of the modified enzyme. We have established the unambiguous structure of the modified prosthetic heme group, which was extracted from the protein matrix using 2D NMR spectroscopy and LC-MS spectrometry. The modified heme was isolated as a µ-oxo dimer that can be quantitatively converted to the corresponding monomer. The depolymerized green heme displayed characteristic NMR signatures of iron porphyrin complexes, but no Nuclear Overhauser Effect was observable to assist in signal assignment. An alternative strategy was applied by removing the iron center of the green heme, resulting in a stable demetallated green porphyrin species. Complete assignment of all the NMR resonances in the demetallated green heme allowed us to establish the molecular architecture of the modified species as a novel N-alkylated heme. Decisive space correlations between the propyl protons of allylbenzene and the γ meso proton coupled with clear dipolar connectivities between the propyl-2H of the substrate and the β proton in the side chain of the propionic acid at carbon-6 of the porphyrin ring, clearly indicate that allylbenzene was covalently attached to the nitrogen atom of the pyrrole ring III of the prosthetic heme. In this study, the mechanism of green CPO formation and its relation to CPO catalyzed chiral transformations are also discussed. It is concluded that the double-phenyl clamp formed by two phenylalanine residues at the distal heme pocket plays a critical role in fine-tuning substrate orientation that determines the outcome of CPO catalyzed epoxidation of substituted styrenes.

Atomic insights into the mechanisms of Al3+ or Cr3+ affecting ferrihydrite nucleation.

Various exotic metal cations commonly coexist with ferrihydrite nanoparticles in natural environments. Lack of knowledge about the metal cations effects on ferrihydrite nucleation and growth greatly blights the deep understanding of ferrihydrite mineralogical properties and reactivities, and thus the fates of associated nutrients, heavy metals/metalloids, and organic pollutants. Here, the nucleation processes and mechanisms of ferrihydrite nanoparticles in the presence of Al3+ or Cr3+ were studied by combining visible spectroscopy, in situ quick X-ray absorption fine structure spectroscopy and quantum chemical calculations. The formation of ferrihydrite can be divided into three stages. In stage 1, Fe(H2O)63+ forms μ-oxo Fe dimers, with the gradual increase of Fe-O bond length (dFe-O) and disappearance of Fe-O multiple scattering. In stage 2, ferrihydrite particles begin to form and grow slowly, during which dFe-O continues to increase and edge- and corner-sharing Fe-Fe bonds appear. In stage 3, ferrihydrite growth rate increases significantly and continues to the end of the reaction, with the decreases of dFe-O. The presence of metal cations at a molar ratio of 0.1 to Fe hinders the formation of μ-oxo dimers by affecting the Fe3+ hydrolysis and polymerization at stage 1 and stage 2, but promotes the conversion of the μ-oxo dimer to the dihydroxo dimer with lower energy barrier and the creation of crystal growth sites and thus enhances the ferrihydrite nucleation and growth at stage 3.

μ-Oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst

Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA) is a water-soluble catalyst capable of electrochemical and photochemical CO2 reduction. Although its catalytic ability has been thoroughly investigated, the mechanism and associated intermediates are largely unknown. Previous studies proposed that Fe-p-TMA enters catalytic cycles as a monomeric species. However, we demonstrate herein that, in aqueous solutions, Fe-p-TMA undergoes formation of a μ-oxo porphyrin dimer that exists in equilibrium with its monomeric form. The propensity for μ-oxo formation is highly dependent on the solution pH and ionic strength. Indeed, the μ-oxo form is stabilized in the presence of electrolytes that are key components of catalytically relevant conditions. By leveraging the ability to chemically control and spectrally address both species, we characterize their ground-state electronic structures and excited-state photodynamics. Global fitting of ultrafast transient absorption data reveals two distinct excited-state relaxation pathways: a three-component sequential model consistent with monomeric relaxation and a two-component sequential model for the μ-oxo species. Relaxation of the monomeric species is best described as a ligand-to-metal charge transfer (τ1 = ∼500 fs), an ionic strength-dependent metal-to-ligand charge transfer (τ2 = 2-4 ps), and finally relaxation of a ligand field excited state to the ground state (τ3 = 5 ps). Conversely, excited-state relaxation of the μ-oxo species proceeds via cleavage of an FeIII-O bond to generate transient FeIV═O and FeII porphyrin species (τ1 = 2 ps) that recombine to the ground-state μ-oxo species (τ2 = ∼1 ns). This latter lifetime extends to timescales relevant for chemical reactivity. It is therefore emphasized that further consideration of catalyst speciation and chemical microenvironments is necessary for elucidating the mechanisms of catalytic CO2 reduction reactions.

Use of Porphyrin Containing Porous Materials in Heterogeneous Catalyst

Due to their potential applications in catalysis, separation, gas storage, drug delivery, and biosensing, porous materials (PMs) such as porous organic polymers (POPs), polymers with intrinsic microporosity (PIMs), porous coordination polymers (PCPs), and metal–organic frameworks (MOFs), have received much attention. Various building blocks have been prepared and demonstrated various functional materials. Among the various organic building blocks, porphyrin has become one of the most important building blocks for the construction of such materials witnessed by a wide range of molecular architectures using porphyrin derivatives with various applications. Mn(III)- and Fe(III)-containing metalloporphyrins are often used to fabricate various functional molecular architectures and to mimic the extraordinary behavior of enzymes in both homogeneous and heterogeneous catalytic systems. In the case of homogeneous catalysis, Mn(III)- and Fe(III)-containing metalloporphyrins have frequent trouble with fast catalytic degradation because of µ-oxo dimer formation or ligand oxidation. To avoid the catalyst degradation, the immobilization and/or site-isolation of homogeneous catalysts using supporters such as polymers, membranes, and MOFs, are widely used strategy. In addition, they are often used to modify the surface of porous silica materials such as SBA-15, MCM-41 and MCM-48, because these porous silica materials exhibit narrow pore size distributions, high thermal stability and easy accessibility.In this presentation, we like to show the use of metalloporphyrins in various porous materials and their use as heterogeneous catalysts.References D. Y. Shin, J. H. Yoon, S. H. Kim, H. Baik, S. J. Lee,* "Immobilization of Porphyrinic Mn(III) Catalyst on a New Class of Silica Support Comprising Three-Dimensionally Interconnected Network with Two Different Sizes of Pores", Catal. Sci. Technol. 2018, 8, 6306-6310.J. Yi, H. Y. Jeong, D. Y. Shin, C. Kim, S. J. Lee,* "Mn(III)-Porphyrin Containing Heterogeneous Catalyst based on Microporous Polymeric Constituents as a New Class of Catalyst Support", ChemCatChem 2018, 10, 3974-3977.J. Yoon, H. M. Choi, S. J. Lee,* "Cu(II)Cl2 containing bispyridine-based porous organic polymer support prepared via alkyne–azide cycloaddition as a heterogeneous catalyst for oxidation of various olefins", New J. Chem. 2020, 44, 9149-9152.H. M. Choi, Y. J. Kim, E. T. Choi, S. J. Lee,* "Selective Photocatalytic Oxidative Detoxification of a Chemical Warfare Agent Simulant by Porphyrin-Containing Polymers of Intrinsic Microporocity." ACS Appl. Polym. Mater.2021, submitted .

Achieving Selectivity for Phosphate over Pyrophosphate in Ethanol with Iron(III)-Based Fluorescent Probes.

Two iron(III)-based molecular receptors employing 1,2-hydroxypyridinone ligands were developed for phosphate recognition and fluorescence sensing via indicator displacement assay (IDA). The tetra- and pentadentate ligands enable anion recognition by the iron(III) center via its remaining one or two open coordination sites. Weak protective coordination of fluorescein at those sites prevents the formation of μ-oxo dimers in aerated solutions. Its rapid and selective displacement by inorganic phosphate results in a 20-fold increase in the fluorescence of the indicator. Both receptors exhibit high affinity for inorganic phosphate and high selectivity over common competing anions, including halides, acetate, carbonate, and, remarkably, pyrophosphate as well as arsenate. Coordination of phosphate to the iron(III) center was confirmed by ATR-IR and 31P NMR spectroscopy.

Front Cover: Manganese (III/IV) μ‐Oxo Dimers and Manganese (III) Monomers with Tetraaza Macrocyclic Ligands and Historically Relevant Open‐Chain Ligands (Eur. J. Inorg. Chem. 19/2022)

The Front Cover shows how the discovery of manganese containing enzymes has prompted a range of synthetic small-molecule complexes to be used as structural and functional mimics of the metalloenzyme chemistries. On the basis of this work, aza-macrocyclic ligands have resulted in a small library of Mn(III) and bis μ-oxo Mn(III)/Mn(IV) complexes, which built on the earlier work established with open-chain ligand scaffolds. The characterization and structural information from these studies has now allowed studies to be expanded to applications beyond mimicking the metalloenzymes that inspired their synthesis, making the transit between monomeric Mn(III) and Mn(III)/Mn(IV) μ-oxo dimer complexes of great interest. More information can be found in the Review by K. N. Green and co-workers.

Manganese (III/IV) μ-Oxo Dimers and Manganese (III) Monomers with Tetraaza Macrocyclic Ligands and Historically Relevant Open-Chain Ligands.

The oxygen-evolving complex (OEC) located in photosystem II (PSII) of green plants is one of the best-known examples of a manganese-containing enzyme in nature, but it is also used in a range of other biological processes. OEC models incorporate two multi-dentate nitrogen-containing ligands coordinated to a bis-μ-oxo Mn(III,IV) core. Open-chain ligands were the initial scaffold used for biomimetic studies, but their macrocyclic counterparts have proven to be particularly appropriate due to their enhanced stability. Dimer and monomer complexes with such ligands have shown to be useful for a wide range of applications, which will be reviewed herein. The purpose of this review is to state with some clarity the different spectroscopic and structural characteristics of the Mn complexes formed with tetraaza macrocyclic ligands both in solution and solid-state that allow the reader to successfully identified the species involved when dealing with similar complexes of Mn.

Iron hexamesityl-5,15-diazaporphyrin: synthesis, structure and catalytic use for direct oxidation of sp3 C-H bonds.

Iron hexamesityl-5,15-diazaporphyrin was synthesized through the cross-coupling reaction of tetrabromodiazaporphyrin. The use of chloroiron(iii) hexamesityl-5,15-diazaporphyrin as a catalyst for oxidation of cyclooctane showed high performance with a total TON up to 731. The introduction of bulky mesityl groups at β-positions prevented the catalyst deactivation via formation of a μ-oxo dimer.

Chemical state of Fe3+ in a Fe3+-type cation exchange resin for the removal and recovery of phosphate ions and the adsorption mechanism of phosphate ion to the resin

To apply the Fe3+-type cation exchange resin, Fe3+-type UBK 10, which is an effective adsorbent of selenite ions, for the removal and recovery of phosphate ions (PO43−), the adsorption mechanism of PO43− to the Fe3+-type UBK 10 resin was investigated. To elucidate the aforementioned mechanism, the chemical state of Fe3+ in the Fe3+-type UBK 10 resin and the adsorption mode of PO43− into the adsorbent were examined via 57Fe Mössbauer spectroscopy and Fe and P K-edge X-ray absorption spectroscopy. The optimum pH for the adsorption of PO43− was found to be approximately 6 and the adsorbed PO43− was rapidly desorbed using 6 mol dm-3 hydrochloric acid (HCl) through the conversion of Fe3+ to FeCl4-, which is practical for the removal and recovery of PO43−. The Fe K-edge X-ray absorption near-edge structure (XANES) spectra showed, through the appearance of three isobestic points, that Fe3+ in the Fe3+-type UBK 10 resin exists as the following four species in equilibrium, depending on the pH: two monomeric hydrolytic species of ferric ions, (-S)2FeOH and (-S)Fe(OH)2, a μ-oxo dimer with corner sharing structure, (-S)2Fe-O-Fe(S-)2, and a dihydroxo dimer with edge sharing structure, (-S)2Fe-(OH)2-Fe(S-)2 (S: SO3 group in UBK 10). It should be emphasized that a PO43− can combine with a (-S)2Fe-(OH)2-Fe(S-)2 site at pH of 6, which is an optimum pH for adsorption of PO43−, forming a bidentate-binuclear configuration, (-S)2Fe-(OH)2(OPO2O)-Fe(S-)2, through the formation of Fe-O-P bonds.

Site Isolation of Catalytically Active Mn(III)-Porphyrin into Porous Materials

Porous materials (PMs) including porous coordination polymers (PCPs), metal–organic frameworks (MOFs), porous organic polymers (POPs), and polymers with intrinsic microporosity (PIMs), have received much attention and they often show novel and/or enhanced performances in wide range of applications such as catalysis, separation, gas storage, drug delivery, biosensing, and imaging. Although they can be successfully obtained by controlling the growth conditions, their fabrication is still a major challenge because of lack of information regarding the crucial factors for the precise control of particles during the synthesis. Furthermore, the translation of building blocks into well-defined structures, whose properties and functions are regulated in the building block level, still remains very challenging. In this regard, metalloporphyrins would be good building blocks for the construction of such functional materials witnessed by a wide range of molecular architectures, such as molecular boxes, self-assembled arrays, CPs and MOFs with various applications. Particularly, Mn(III)- and Fe(III)-containing metalloporphyrins are often used to fabricate various functional molecular architectures and to mimic the extraordinary behavior of enzymes in both homogeneous and heterogeneous catalytic systems.In the case of homogeneous catalysis, these materials have frequent trouble with fast catalytic degradation because of µ-oxo dimer formation or ligand oxidation. To avoid the catalyst degradation, the heterogenization of homogeneous catalysts, such as immobilization and/or site-isolation of homogeneous catalysts using supporters are widely used alternative strategies. In this regard, I like to present the preparations and heterogeneous catalytic uses of Mn(III)–porphyrin in various porous materials (PMs).

O2 Activation by Non-Heme Thiolate-Based Dinuclear Fe Complexes.

Iron centers featuring thiolates in their metal coordination sphere (as ligands or substrates) are well-known to activate dioxygen. Both heme and non-heme centers that contain iron-thiolate bonds are found in nature. Investigating the ability of iron-thiolate model complexes to activate O2 is expected to improve the understanding of the key factors that direct reactivity to either iron or sulfur. We report here the structural and redox properties of a thiolate-based dinuclear Fe complex, [FeII2(LS)2] (LS2- = 2,2'-(2,2'-bipyridine-6,6'-iyl)bis(1,1-diphenylethanethiolate)), and its reactivity with dioxygen, in comparison with its previously reported protonated counterpart, [FeII2(LS)(LSH)]+. When reaction with O2 occurs in the absence of protons or in the presence of 1 equiv of proton (i.e., from [FeII2(LS)(LSH)]+), unsupported μ-oxo or μ-hydroxo FeIII dinuclear complexes ([FeIII2(LS)2O] and [FeIII2(LS)2(OH)]+, respectively) are generated. [FeIII2(LS)2O], reported previously but isolated here for the first time from O2 activation, is characterized by single crystal X-ray diffraction and Mössbauer, resonance Raman, and NMR spectroscopies. The addition of protons leads to the release of water and the generation of a mixture of two Fe-based "oxygen-free" species. Density functional theory calculations provide insight into the formation of the μ-oxo or μ-hydroxo FeIII dimers, suggesting that a dinuclear μ-peroxo FeIII intermediate is key to reactivity, and the structure of which changes as a function of protonation state. Compared to previously reported Mn-thiolate analogues, the evolution of the peroxo intermediates to the final products is different and involves a comproportionation vs a dismutation process for the Mn and Fe derivate, respectively.

Elucidating the Electronic Structure and Magnetic and Conducting Properties of μ-Oxo Mn-phthalocyanine [MnPc(CN)]2O Complex

The μ-oxo phthalocyanine dimer complex [Mn(Pc)(CN)]2O has been characterized as the first single-component molecular conductor exhibiting magnetoresistance at room temperature. Herewith, we report a detailed study of the electronic structure and the magnetic and conducting properties of this μ-oxo MnPc dimer complex. A combined strategy of wave-function-based quantum chemistry calculations on the complex and plane-wave-based periodic calculations on the crystal has been employed together with semiclassical Boltzmann transport theory calculations. Our results provide information about the ligand field parameters of the Mn ions, clarify their spin states on the [Mn(Pc)(CN)]2O complex, and explain the temperature dependence of both the magnetic susceptibility and the effective magnetic moment, taking into account the presence of excited low-lying states with S = 2 and weak intermolecular magnetic interactions, as key ingredients of the magnetic behavior. The analysis of the density of states of the low-spin and high-spin density functional theory + U magnetic solutions and their respective temperature dependence of the electric resistivity give some clues about the conducting properties and the negative magnetoresistance reported for this system.

Axial ligand mediated switchable rotary motions in a ferrocene-bridged diiron(III) porphyrin dimer

In the present work, we have utilized ferrocene-bridged diiron(III) porphyrin dimer and investigate the rotary motions mediated by axial ligands exploiting conformational flexibility of the ferrocene linker. Diiron(III) porphyrin dimer can switch easily between the syn and anti rotamers reversibly based on the axial coordination and thereby forming complexes with different structure and properties for the development of synthetic molecular machines. While the five-coordinate diiron(III) complex with chloride axial ligand and its μ-oxo dimer are in syn-conformation, their six-coordinate complexes are stabilized in anti-conformation. All the complexes are isolated in the solid state and five-coordinate complexes are also structurally characterized. Spectroscopic investigation through UV–visible spectra, solid-state Mössbauer, EPR, and 1H NMR studies confirm the presence of high and low-spin diiron(III) porphyrins, respectively, for five and six-coordinate complexes.

Silicon(IV) Corroles.

Silicon complexes of corrole were obtained for the first time by reaction of the free-base corrole with hexachlorodisilane. The peripheral substituents of corrole strongly influence the nature of the reaction products: β-octaalkyl corrole was mainly isolated as the μ-oxo dimer, while a hydroxo complex was obtained in the case of 5,10,15-tris-(pentafluorophenyl)corrole. In the case of meso-tritolyl corrole, a mixture of monomer/μ-oxo dimer was obtained. The silicon corrole complexes are more stable toward hydrolysis than the corresponding porphyrin derivatives and are endowed with brilliant luminescence properties. The high affinity of silicon for fluoride ion allowed investigation of the ability of an Si corrole to serve as a sensor for F- detection. The strong color variation due to the interaction with the halide ion makes the Si corrole an interesting material for the naked-eye detection of inorganic fluoride.

The First Silicon(IV) Corrole Complexes: Synthesis, Structures, Properties, and Formation of a μ-Oxo Dimer.

The first SiIV corrole complexes were synthesized in good yields by treatment of meso-triarylcorroles with tetrachlorosilane in 1,2-dichloroethane at 60 °C. The central silicon atom possesses a square-pyramidal coordination geometry with slightly longer Si-N bond lengths as compared with those of known triazacorrole SiIV complexes. The SiIV corrole complexes exhibit sharp and blue-shifted absorption spectra and larger fluorescence quantum yields as compared with the corresponding free-base corroles. A μ-oxo dimer of a SiIV corrole was synthesized upon treatment with methanesulfonyl chloride in pyridine at 100 °C. This dimer shows a face-to-face structure with a 90° twist in the solid state. Although the dimer exhibits a blue-shifted Soret band, reflecting the face-to-face geometry, it displays a largely red-shifted and broad fluorescence spectrum with a large Stokes shift, suggesting a large structural change in the S1 state. These intriguing optical properties have been comprehensively studied by magnetic circular dichroism (MCD) spectroscopy, femtosecond transient absorption (fs-TA) measurements, and theoretical calculations.

Hydrogen Evolution by FeIII Molecular Electrocatalysts Interconverting between Mono and Di-Nuclear Structures in Aqueous Phase.

A new FeL/Fe2 L2 manifold, with HL=2-({[di(2-pyridyl)methyl](methyl)amino}methyl)phenol, was prepared in gram scale (>50 % yield) and characterized in solution and solid state. The monomer/dimer interconversion is controlled in aqueous phase, upon varying the pH conditions. The electrocatalytic hydrogen evolution reaction (HER) occurs through the FeL monomer with added trifluoroacetic acid (TFA) and through the Fe2 L2 μ-oxo dimer in acetate buffer (pH 4.9), with an overpotential of about 1 V and faradaic yield up to 75 %. The resulting icat /ip values in the range 15-28 are among the highest reported for Fe-based electrocatalysts (icat is the catalytic current, whereas ip is the current of an Fe-based redox event).