Molecular Electrochemical Catalysis of the CO 2 -to-CO Conversion with a Co Complex: A Cyclic Voltammetry Mechanistic Investigation

International audienceThe electrochemical catalytic reduction of CO2 into CO could be achieved with excellent selectivity and rate in acetonitrile in the presence of phenol with cobalt 2,2′:6′,2″:6″,2‴-quaterpyridine complex [CoII(qpy)(H2O)2]2+ (Co) acting as a molecular catalyst. Upon using cyclic voltammetry at low and high scan rate (up to 500 V/s) two catalytic pathways have been identified. At a low concentration of phenol (<1 M), catalysis mainly occurs after the reduction of Co with three electrons. In that case, the selectivity for CO production is ca. 80% with 20% of H2 as by product, along with a turnover frequency of 1.2 × 104 s−1 for COproduction at an overpotential η of ca. 0.6 V. The triply reduced active species binds to CO2 and the C−O bond is cleaved thanks to the acid. At very large concentration of phenol (3 M), another pathway becomes predominant: the doubly reduced species binds to CO2, while its reductive protonation leads to CO formation. As already shown, this later process is endowed with fast rate at low overpotential (turnover frequency of 3 × 104 s−1 at η = 0.3 V) and 95% selectivity for CO production. By varying the phenol concentration and the scan rate in voltammetry experiments, it was thus possible to identify, activate, and characterize several pathways for the CO2-to-CO conversion and to decipher Co electrochemical reactivity toward CO2

Small-molecule activation with iron porphyrins using electrons, photons and protons: some recent advances and future strategies

International audienceSubstituted tetraphenyl Fe porphyrins are versatile molecular catalysts for the activation of small molecules (such as O2, H+ or CO2), which could lead to renewable energy storage, the direct production of fuels or new catalytic relevant processes. Herein, we review the recent studies of these earth-abundant metal catalysts for the electrochemical activation of dioxygen on the one hand and for the photostimulated reduction of carbon dioxide on the other hand. These two prototype reactions illustrate how mechanistic studies are the only rational approach to gain fundamental insights into the elementary steps that drive the catalysis and for identification of the key intrinsic parameters controlling the reactivity, offering in turn the possibility to rationally tune the structure of the catalysts as well as the catalytic conditions

Reactivity of MnII with Superoxide. Evidence for a [MnIIIOO]+ Unit by Low-Temperature Spectroscopies

International audienceManganese superoxide dismutase cycles between the MnIII and MnII states to produce oxygen and hydrogen peroxide from superoxide. The formation of an adduct has been suggested, but its nature remains questionable because both [MnIIOO-] and [MnIIIOO2-] redox states have been proposed. Study of the reactivity of superoxide with manganese complexes is of current interest. The reaction of [(L)MnII]2+ [L = N-methyl-N,N`,N`-tris(2-pyridylmethyl)ethane-1,2-diamine] with potassium superoxide has been investigated at low temperature in an anhydrous solvent using various techniques. Upon the addition of ca. 2 equiv of potassium superoxide, the [(L)MnII]2+ colorless solution turned blue and the UV-vis spectrum displayed a band at 590 nm (165 M-1 cm-1) and a shoulder at 430 nm (100 M-1 cm-1). Electrospray ionization mass spectrometry showed a peak (m/z = 434.1) assigned to [(L)MnO2]+. The X-band electron paramagnetic resonance spectrum parallel mode displayed a six-line signal separated by 6.6 mT and centered at 86 mT (g = 8.1). These results support the formation of an [MnIIIOO]+ adduct

Reactivity of an Aminopyridine [LMnII]2+ Complex with H2O2. Detection of Intermediates at Low Temperature

International audienceFormation of Mn (hydro)peroxo complexes issued from addition of H2O2 to a [LMnII]2+ acetonitrile solution (L = pentadentate polypyridine ligand) is evidenced by low-temperature UV−visible spectroscopy, ESI-mass spectrometry, and parallel and perpendicular mode detection EPR spectroscopy. The influence of the medium (basicity, water content) on the formation of various species is investigated

A New Mononuclear Manganese(III) Complex of an Unsymmetrical Hexadentate N3O3 Ligand Exhibiting Superoxide Dismutase and Catalase-like Activity: Synthesis, Characterization, Properties and Kinetics Studies

A mononuclear MnIII complex MnL·4H2O (H3L = 1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N′-(2-hydroxybenzyl),N′-(4- methylbenzyl)amino]propan-2-ol) has been prepared and characterized. This complex catalyzes the dismutation of superoxide efficiently, with catalytic rate constant kcat=1.7 × 106M−1 s−1 and IC50 1.26 μM, obtained through the nitro blue tetrazoliumphotoreduction inhibition superoxide dismutase assay, in aqueous solution of pH 7.8. MnL is also able to disproportionate more than 300 equivalents of H2O2 in CH3CN, with initial rate of H2O2 decomposition given by ri=kcat [MnL]2 [H2O2]and kcat = 1.32 (2) mM−2 min−1. The accessibility of the MnIV state (Ep = 0.53 V vs. saturated calomel electrode), suggests MnL employs a high-valent catalytic cycle to decompose O2?− and H2O2.Fil: Ledesma, Gabriela Nanci. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Química Rosario; ArgentinaFil: Eury, Hélène. Centre National de la Recherche Scientifique. Laboratoire de Chimie de Coordination; FranciaFil: Anxolabéhère Mallart, Elodie. Universite de Toulose - Le Mirail; FranciaFil: Hureau, Christelle. Laboratoire de Chimie de Coordination; FranciaFil: Signorella, Sandra Rosanna. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Química Rosario; Argentin

Influence of second sphere hydrogen bonding interaction on a manganese( ii )-aquo complex

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Characterizations of chloro and aqua Mn(II) mononuclear complexes with amino-pyridine ligands. Comparison of their electrochemical properties with those of Fe(II) counterparts.

International audienceThe solution behavior of mononuclear Mn(II) complexes, namely, [(L(5)(2))MnCl](+) (1), [(L(5)(3))MnCl](+) (2), [(L(5)(2))Mn(OH(2))](2+) (3), [(L(5)(3))Mn(OH(2))](2+) (4), and [(L(6)(2))Mn(OH(2))](2+) (6), with L(5)(2/3) and L(6)(2) being penta- and hexadentate amino-pyridine ligands, is investigated in MeCN using EPR, UV-vis spectroscopies, and electrochemistry. The addition of one chloride ion onto species 6 leads to the formation of the complex [(L(6)(2))MnCl](+) (5) that is X-ray characterized. EPR and UV-vis spectra indicate that structure and redox states of complexes 1-6 are maintained in MeCN solution. Chloro complexes 1, 2, and 5 show reversible Mn(II)/Mn(III) process at 0.95, 1.02, and 1.05 V vs SCE, respectively, whereas solvated complexes 3, 4, and 6 show an irreversible anodic peak around 1.5 V vs SCE. Electrochemical oxidations of 1 and 5 leading to the Mn(III) complexes [(L(5)(2))MnCl](2+) (7) and [(L(6)(2))MnCl](2+) (8) are successful. The UV-vis signatures of 7 and 8 show features associated with chloro to Mn(III) LMCT and d-d transitions. The X-ray characterization of the heptacoordinated Mn(III) species 8 is also reported. The analogous electrochemical generation of the corresponding Mn(III) complex was not possible when starting from 2. The new mixed-valence di-mu-oxo [(L(5)(2))Mn(muO)(2)Mn(L(5)(2))](3+) species (9) can be obtained from 3, whereas the sister [(L(5)(3))Mn(muO)(2)Mn(L(5)(3))](3+) species can not be generated from 4. Such different responses upon oxidations are commented on with the help of comparison with related Mn/Fe complexes and are discussed in relation with the size of the metallacycle formed between the diamino bridge and the metal center (5- vs 6-membered). Lastly, a comparison between redox potentials of the studied Mn(II) complexes with those of Fe(II) analogues is drawn and completed with previously reported data on Mn/Fe isostructural systems. This gives us the opportunity to get some indirect insights into the metal specificity encountered in enzymes among which superoxide dismutase is the archetypal model