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Misra S.K.,Concordia University at Montreal | Andronenko S.I.,Kazan Federal University | Thurber A.,Boise State University | Punnoose A.,Boise State University | Nalepa A.,Max Planck Institute For Chemische Energie Konversion
Journal of Magnetism and Magnetic Materials | Year: 2014

EPR studies on two types of nanoparticles of Fe3+ doped, 0.1-10%, ZnO, NL and QJ, prepared using similar chemical hydrolysis methods, in diethylene glycol, and in denatured ethanol solutions, respectively, were carried out at X-band (~9.5 GHz) at 77 K and at Q-band (~34.0 GHz) at 10, 80, and 295 K. To interpret the experimental results, EPR spectra were simulated by exact diagonalization of the spin-Hamiltonian matrix to identify the Fe ions at different magnetically active sites in these samples. The simulation for NL samples revealed that they contained (i) Fe3+ ions, which substituted for Zn ions, the zero-field splitting (ZFS) parameter which has a large distribution over the sample due to oxygen vacancies in the second coordination sphere; (ii) EPR signal from surface oxygen defects; and (iii) ferromagnetically (FM) coupled Fe ions with concentration of Fe more than 1%. The EPR spectra for QJ samples are very different from those for NL samples, exhibiting only rather intense FM EPR lines. The FM and EPR spectra in NL and/or QJ samples are found to vary strongly with differences in the surface morphology of nanoparticles. © 2014 Elsevier B.V. Source

Saracini C.,Johns Hopkins University | Liakos D.G.,Max Planck Institute For Chemische Energie Konversion | Zapata Rivera J.E.,Rovira i Virgili University | Neese F.,Max Planck Institute For Chemische Energie Konversion | And 2 more authors.
Journal of the American Chemical Society | Year: 2014

Irradiation of the copper(II)-superoxide synthetic complexes [(TMG 3tren)CuII(O2)]+ (1) and [(PV-TMPA)CuII(O2)]+ (2) with visible light resulted in direct photogeneration of O2 gas at low temperature (from -40 C to -70 C for 1 and from -125 to -135 C for 2) in 2-methyltetrahydrofuran (MeTHF) solvent. The yield of O2 release was wavelength dependent: λexc = 436 nm, = 0.29 (for 1), = 0.11 (for 2), and λexc = 683 nm, = 0.035 (for 1), = 0.078 (for 2), which was followed by fast O2-recombination with [(TMG3tren)Cu I]+ (3) and [(PV-TMPA)CuI]+ (4). Enthalpic barriers for O2 rebinding to the copper(I) center (∼10 kJ mol-1) and for O2 dissociation from the superoxide compound 1 (45 kJ mol-1) were determined. TD-DFT studies, carried out for 1, support the experimental results confirming the dissociative character of the excited states formed upon blue- or red-light laser excitation. © 2014 American Chemical Society. Source

Gennari M.,CNRS Molecular Chemistry Department | Gennari M.,Pennsylvania State University | Brazzolotto D.,CNRS Molecular Chemistry Department | Pecaut J.,University Grenoble alpes | And 14 more authors.
Journal of the American Chemical Society | Year: 2015

Herein, we describe an uncommon example of a manganese-thiolate complex, which is capable of activating dioxygen and catalyzing its two-electron reduction to generate H2O2. The structurally characterized dimercapto-bridged MnII dimer [MnII2(LS)(LSH)]ClO4 (MnII2SH) is formed by reaction of the LS ligand (2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1,1-diphenylethanethiolate)) with MnII. The unusual presence of a pendant thiol group bound to one MnII ion in MnII2SH is evidenced both in the solid state and in solution. The MnII2SH complex reacts with dioxygen in CH3CN, leading to the formation of a rare mono-μ-hydroxo dinuclear MnIII complex, [(MnIII2(LS)2(OH)]ClO4 (MnIII2OH), which has also been structurally characterized. When MnII2SH reacts with O2 in the presence of a proton source, 2,6-lutidinium tetrafluoroborate (up to 50 equiv), the formation of a new Mn species is observed, assigned to a bis-μ-thiolato dinuclear MnIII complex with two terminal thiolate groups (MnIII2), with the concomitant production of H2O2 up to 40% vs MnII2SH. The addition of a catalytic amount of MnII2SH to an air-saturated solution of MenFc (n = 8 or 10) and 2,6-lutidinium tetrafluoroborate results in the quantitative and efficient oxidation of MenFc by O2 to afford the respective ferrocenium derivatives (MenFc+, with n = 8 or 10). Hydrogen peroxide is mainly produced during the catalytic reduction of dioxygen with 80-84% selectivity, making the MnII2SH complex a rare Mn-based active catalyst for two-electron O2 reduction. © 2015 American Chemical Society. Source

Gerey B.,CNRS Molecular Chemistry Department | Gennari M.,CNRS Molecular Chemistry Department | Goure E.,CNRS Molecular Chemistry Department | Pecaut J.,University Grenoble alpes | And 8 more authors.
Dalton Transactions | Year: 2015

Carboxylate-bridged Mn(ii)-Ca(ii) complexes are potentially relevant for mimicking the first stages of the Oxygen-Evolving Complex (OEC) assembly process. Here, we report on new homonuclear Ca(ii) and heteronuclear Mn(ii)-Ca(ii) complexes with carboxylate-functionalized tripodal tris(2-pyridylmethyl)amine ligands, the heptadentate H3tpaa, previously reported, and the new hexadentate H2tpada, containing respectively three and two carboxylate units. The mononuclear [Ca(Htpaa)(OH2)] (Ca1) and dinuclear [Ca(tpada)(OH2)2]2 (Ca2) calcium complexes, as well as the tetranuclear [{Mn(tpaa)}2{Ca(OH2)5(μ-OH2)}2][Mn(tpaa)]2 (Mn2Ca2·2Mn) and dinuclear [Mn(tpada)ClCa(OH2)2.67(MeOH)2.33]Cl (MnCa) heterometallic species have been structurally characterized; the syntheses of Ca1 and Mn2Ca2·2Mn being previously reported by us (Inorg. Chem., 2015, 54, 1283). The Mn(ii) and Ca(ii) are linked by two μ1,1-bridging carboxylates in MnCa, while only one μ1,3-carboxylate bridge connects each Ca2+ ion to each Mn(ii) in Mn2Ca2. A variable number of water molecules (n = 1 to 7) are coordinated to Ca in all complexes, most of them being involved in hydrogen-bond networks, in analogy to what occurs in the photosystem II. All donor atoms of the tpaa3- and tpada2- ligands are coordinated to the Mn2+ ions, despite the unusually long distance between the Mn2+ ion and the tertiary amine imposed by the constraining nature of the ligands, as supported by theoretical calculations. Solid state EPR spectroscopy, in combination with DFT calculations, has also shown that the Ca2+ ion has an effect on the electronic parameters (zero field splitting) of the linked Mn(ii) in the case of MnCa (μ1,1-carboxylate bridges). In Mn2Ca21,3-carboxylate bridge) the Ca2+ ion induces only slight structural changes in the Mn coordination sphere. This journal is © The Royal Society of Chemistry. Source

Zapata-Rivera J.,Rovira i Virgili University | Caballol R.,Rovira i Virgili University | Calzado C.J.,University of Seville | Liakos D.G.,Max Planck Institute For Chemische Energie Konversion | Neese F.,Max Planck Institute For Chemische Energie Konversion
Chemistry - A European Journal | Year: 2014

The recently described intermolecular O2 transfer between the side-on Ni-O2 complex [(12-TMC)Ni-O2]+ and the manganese complex [(14-TMC)Mn]2+, where 12-TMC and 14-TMC are 12- and 14-membered macrocyclic ligands, 12-TMC=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane and 14-TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long-range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed. The calculations confirm a two-step reaction, with a first rate-determining bimolecular step and predict the exothermic character of the global process. The relative stability of the products and the reverse barrier are in line with the fact that no reverse reaction is experimentally observed. An intermediate with a μ-η1:η1-O2 coordination and two transition states are identified on the triplet PES, slightly below the corresponding stationary points of the quintet PES, suggesting an intersystem crossing before the first transition state. The calculated activation parameters and the relative energies of the two transition sates and the products are in very good agreement with the experimental data. The calculations suggest that a superoxide anion is transferred during the reaction. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

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