Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

Iron cations are essential for the high activity of nickel and cobalt‐based (oxy)hydroxides for the oxygen evolution reaction, but the role of iron in the catalytic mechanism remains under active investigation. Operando X‐ray absorption spectroscopy and density functional theory calculations are used to demonstrate partial Fe oxidation and a shortening of the Fe?O bond length during oxygen evolution on Co(Fe)O_xH_y. Cobalt oxidation during oxygen evolution is only observed in the absence of iron. These results demonstrate a different mechanism for water oxidation in the presence and absence of iron and support the hypothesis that oxidized iron species are involved in water‐oxidation catalysis on Co(Fe)O_xH_y.     

Synthesis and characterization of (cryptand‐222)potassium (2‐methylimidazolato)(meso‐tetraphenylporphinato)ferrate(II)–2‐methylimidazole–tetrahydrofuran (1/1/2)

Metalloporphyrin complexes containing an additional imidazole ligand can provide information about the effect of deprotonation or hydrogen bonding on the axial histidine unit in heme proteins. The title high‐spin five‐coordinate imidazolate‐ligated iron(II) porphyrinate, [K(C₁₈H₃₆N₂O₆)][Fe(C₄H₅N₂)(C₄₄H₂₈N₄)]·C₄H₆N₂·2C₄H₈O, has been synthesized and investigated. The solvated salt crystallizes with one 2‐methylimidazole molecule, two tetrahydrofuran solvent molecules and a potassium cation chelated inside a cryptand‐222 (4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane) molecule. The imidazolate ligand is ordered. The average Fe—Np (Np is a porphyrin N atom) bond length is 2.113 (11) Å and the axial Fe—N_Im (N_Im is an imidazolate N atom) is 2.0739 (13) Å. The out‐of‐plane displacement of the Fe^II atom from the 24‐atom mean plane is 0.6098 (5) Å, indicating an apparent doming of the porphyrin core.     

Système ternaire : H2O–Fe(NO3)3–Co(NO3)2 isotherme : 30 °C

Ternary system: H₂O–Fe(NO₃)₃–Co(NO₃)₂ isotherm: 30 °C. The H₂O–Co(NO₃)₂ binary system has been investigated in the –28 to 50 °C temperature range. The solid–liquid equilibria of the ternary system H₂O–Fe(NO₃)₃–Co(NO₃)₂ were studied by using a synthetic method based on conductivity measurements. One isotherm is established at 30 °C, and the stable solid phases that appear are iron nitrate nonahydrate: Fe(NO₃)₃·9 H₂O, iron nitrate hexahydrate: Fe(NO₃)₃·6 H₂O, cobalt nitrate hexahydrate: Co(NO₃)₂·6 H₂O, and cobalt nitrate trihydrate: Co(NO₃)₂·3 H₂O. To cite this article: B. El Goundali et M. Kaddami, C. R. Chimie 9 (2006).     

Synthesis and Characterization of Iron(III) Complexes of 5‐(8‐Carboxy‐1‐naphthyl)‐10, 15, 20‐tritolyl Porphyrin

5‐(8‐Carboxy‐1‐naphthyl)‐10, 5, 20‐tritolyl porphyrin (H₃CNTTP) and its iron(III) complexes, [Fe(CNTTP)]₂ and [Fe(CNTTP)(N‐MeIm)₂], were synthesized and characterized. X‐ray crystallography revealed that the carboxylate group is “hanging” over the porphyrin plane. The rigid framework makes the distance between the carboxylate oxygen and iron in the same porphyrin too long to form a coordination bond. On the other hand, the carboxylate group is not bulky enough to block the axial binding site. In the presence of OH^–, the carboxylate oxygen is coordinated to iron in the symmetry‐related unit, which led to the dimeric structure, [Fe(CNTTP)]₂. In the presence of excess N‐methylimidazole, a six‐coordinate species, [Fe(CNTTP)(N‐MeIm)₂], was obtained. In such a structure, CH ··· O interactions between the carboxylate group and imidazole probably play an important role to determine the orientation of imidazole plane. Two imidazole planes have relative parallel orientation. For [Fe(CNTTP)(N‐MeIm)₂], ¹H NMR shows pyrrole protons at the region –10 to –25 ppm. EPR shows rhombic spectrum. Those suggest [Fe(CNTTP)(N‐MeIm)₂] is a type II low‐spin iron(III) porphyrinate.     

Copper(II) and cobalt(II) hydroxypyridinecarboxylates: Synthesis, crystal structures, spectral and magnetic properties

Synthesis and characterization of six copper(II) and cobalt(II) octahedral complexes [M(6-OHpic)₂(H₂O)₂] (6-OHpic is 6-hydroxypicolinato), [M(2-OHnic)₂(H₂O)₂] (2-OHnic is 2-hydroxynicotinato), [Cu(6-OHnic)₂(H₂O)₂] (6-OHnic is 6-hydroxynicotinato) as well as [Co(H₂O)₆](6-OHnic)₂ are reported. Their characterization was carried out using elemental analysis, infrared, and magnetic measurements. Based on IR spectra, N,O-coordination of 6-OHpic (via the oxygen atom of the carboxyl group and the nitrogen atom of the pyridine ring), O,O-asymmetrically chelating coordination of the carboxyl groups as well as ionic coordination of 6-OHnic and chelating O,O-coordination (through the oxygen atom of the carboxyl group and the oxygen atom of the amide group) of keto(amide) tautomer of 2-OHnic were supposed. Moreover, crystal structures of 2-OHnicH and the complex [Co(2-OHnic)₂(H₂O)₂]) were determined by X-ray single crystal structure analysis. The system of hydrogen bonds predominantly stabilizes the keto(amide) tautomer of both 2-hydroxynicotinic acid and 2-OHnic anion in the cobalt(II) complex. Intermolecular hydrogen bonds (between the oxygen atom of the amide group and the hydrogen atom of the NH group) interconnect two neighbouring molecules of 2-OHnicH forming dimers. Cobalt(II) in complex [Co(2-OHnic)₂(H₂O)₂] has nearly a regular compressed tetragonal bipyramidal arrangement. Presented at the 1st International Conference “Applied Natural Sciences” on the occasion of the 10th anniversary of the University of Ss. Cyril and Methodius, Trnava, 7–9 November 2007.     

Coordination properties of 4(5)-hydroxymethylimidazole and 4(5)-hydroxymethyl-5(4)-methylimidazole towards cobalt(II) ions

Two new complexes of imidazole alcohols, 4-hydroxymethylimidazole (4-CH₂OHim) and 4-hydroxymethyl-5-methylimidazole (4-CH₂OH-5-CH₃im), with cobalt(II) of formula [CoL₂(H₂O)₂](NO₃)₂ were obtained. These compounds were described through single X-ray diffraction studies, spectroscopic (Ir. Far-IR, UV-Vis-NIR) and magnetic measurements. In the present complexes imidazole ligands are bidentate coordinating the heterocyclic ring through pyridine-like nitrogen and the oxygen atom of the hydroxymethyl group (chromophore CoN₂O₄). The shape of Co(II) coordination polyhedra is that of a distorted octahedron, with the equatorial plane defined by the 4-CH₂OHim (or 4-CH₂OH-5-CH₃im) bidentate ligands and two water molecules occupying axial positions (i.e. trans to each other). Formation of successive cobalt(II) complexes with 4-CH₂OH-5-CH₃im in aqueous solution was followed quantitatively by potentiometry.     

Mössbauer study of paramagnetic relaxation effect in mixed crystals: High spin Fe3+ compounds diluted in diamagnetic hosts

The⁵⁷Fe Mössbauer spectra were measured in mixed crystals with different types of chemical bonding and crystal structure, i.e., (Fe,Al)(acac)₃, (Fe,Co)(acac)₃, K₃[(Fe,Al)(ox)₃]3H₂O, and NH₄(Fe,Al)(SO₄)₂12H₂O. The broadening of Mössbauer linewidth with increasing Fe³⁺–Fe³⁺ distance became less enhanced in the order: (Fe,Al)(acac)₃>(Fe,Co)(acac)₃, or K₃[(Fe,Al)(ox)₃]3H₂O>(Fe,Al)(acac)₃>NH₄(Fe,Al)(SO₄)₂12H₂O. Furthermore, it was found that the broadening of the linewidth was larger in neat tris (-diketonato) iron(III) complexes than in (Fe,Al)(acac)₃. Based on these results, the determining factors of the paramagnetic relaxation time other than Fe³⁺–Fe³⁺ distance and temperature were examined in terms of the Mössbauer linewidth as an indicator.     

Dimeric Complexes of Manganese(II) and Nickel(II) Hexafluoroacetylacetonates with Pyrazole-Containing Nitronylnitroxyl Radicals

Reactions of Ni(II) and Mn(II) hexafluoroacetylacetonates with pyrazole-containing nitronylnitroxyl radicals (L) result in dimeric heterospin complexes [M(hfac)₂L]₂or [M(hfac)₂L]₂· C₆H₆. The paramagnetic ligands in the centrosymmetric binuclear molecules function as a bridge, with the nitroxyl O atom being bound to one metal and the imine N atom of the pyrazole ring to the other metal atom. The metal environment is a cis-octahedron with short M–O_NOdistances, which specifies a strong antiferromagnetic interaction between the lone electrons of paramagnetic centers in M–O–N< exchange clusters.     

Preparation and electrochemical properties of catalysts based on macrobicyclic d-metal tris-dioximate complexes

The use of macrobicyclic tris-dioximate complexes of cobalt, K₃[Co(Dmg)₃] (where DmgH₂=C₄H₈N₂O₂), and iron, (BPh)₂[Fe(Nx)₃] (where NxH₂=C₆H₁₀N₂O₂), and the carbon-supported complexes and their pyrolyzates, as catalysts for oxygen reduction was examined utilizing the combination of thermogravimetry, infrared spectroscopy, mass spectrometry and electrochemical techniques. The results have shown that macrobicyclic cobalt complexes adsorbed on the surface of carbon- based synthetic porous polymers have a pronounced catalytic effect on the reduction of molecular oxygen. A reduction in the oxygen overpotential was observed with heat treatment of the catalysts prepared in an inert atmosphere. The optimum pyrolysis temperature for the electrochemically most active catalysts for oxygen reduction was estimated. Electronic Publication     

mer‐Diaquabis(1H‐imidazole‐κN3)[orotato(2–)]cobalt(II)

In the title complex, mer‐diaqua[2,6‐dioxo‐1,2,3,6‐tetrahydropyrimidine‐4‐carboxylato(2−)]bis(1H‐imidazole‐κN³)cobalt(II), [Co(C₅H₂N₂O₄)(C₃H₄N₂)₂(H₂O)₂], the Co^II ion is coordinated by a deprotonated N atom and the carboxylate O atom of the orotate ligand, two imidazole N atoms and two aqua ligands in a distorted octahedral geometry. The title complex exists as discrete doubly hydrogen‐bonded dimers, and a three‐dimensional network of O—H...O and N—H...O hydrogen bonds and weak π–π interactions is responsible for crystal stabilization.     

Fe(II) and Co(II) pyridinebisimine complexes bearing different substituents on ortho- and para-position of imines: synthesis, characterization and behavior of ethylene polymerization

A series of 2,6-bis(imino)pyridyl iron(II) and cobalt(II) complexes [2,6-(ArNCMe)₂C₅H₃N]MCl₂ (Ar = 2,6-i-Pr₂C₆H₃, M = Fe: 3a, M = Co: 4a; Ar = 2,4,6-i-Pr₃C₆H₂, M = Fe: 3b, M = Co: 4b; Ar = 2,6-i-Pr₂-4-BrC₆H₂, M = Fe: 3c, M = Co: 4c; Ar = 2,4-i-Pr₂-6-BrC₆H₂, M = Fe: 3d, M = Co: 4d) has been synthesized, characterized, and investigated as precatalysts for the polymerization of ethylene in the presence of modified methylaluminoxane (MMAO). The substituents of pyridinebisimine ligands and their positions located significantly influence catalyst activity and polymer property. It is found that the catalytic activities of the iron complexes/MMAO systems are mainly dominated by electronical effect, while those of the cobalt complexes/MMAO systems are primarily controlled by hindering effect.     

Competitive Coordination of 2-[(Diorganylphosphinoyl)-hydroxymethyl)]-1-organylimidazoles with Metal Chlorides

The structure of complexes of bivalent cobalt, copper, zinc, cadmium, and palladium chloride and tetravalent tin chlorides with 1-organyl-2- [(diorganylphosphinoyl)hydroxymethyl]imidazoles, synthesized for the first time, was studied by ¹H NMR and IR spectroscopy. The formation of two types of metal complexes, molecular (Zn, Cd, Pd, Sn) and chelate (Co, Cu), was established. For crystalline 1 : 1 complexes, the structures with imidazolylphosphine oxide playing the role of an N ³,O-bidentate ligand were proposed. In 2L : 1 complexes, these ligands are monodentate and coordinate by the P=O oxygen atom. In DMSO solutions, the molecular complexes undergo dissociation to form coordination compounds in which, depending of the nature of the ligand, either M←N³ or M←O donor-acceptor bond takes place.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 226–232.Original Russian Text Copyright © 2005 by Baikalova, Zyryanova, Chipanina, Sinegovskaya, Ivanova, Mamaseva, Gusarova, Afonin, Trofimov.     

Quantum chemical modeling of the photode-composition of the water dimer in chlorophyll complexes of magnesium and manganese

On the basis of quantum-chemical calculations in the MPNDO/3 approximation, a reaction has been proposed for the photodecomposition of water in complexes M·Chl_a...(H₂O)₂M·Chl_a+O₂+4H⁺+4e^– (M=Mg, Mn), which suggests an interpretation of the primary event of photosynthesis. Calculations of the photosynthetic evolution of oxygen according to the Joliot-Kok model in complexes M·Chl_a...(H₂O)₂ are in good agreement with the experimental data of the thermoluninescence of chlorophyll photosystem II.Kiev Polytechnic Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 33–38, March–April, 1993.     

Molecular orbital calculation of Mössbauer parameters for Fe(CO)4 (C2H4) in low temperature matrix

Molecular orbital calculations have been performed to obtain the electron density and electric field gradient at the iron nucleus of tetracarbonylethene iron Fe(CO)₄(C₂H₄) produced by UV-irradiation of pentacarbonyliron cocondensed homogeneously with ethene in a low temperature matrix, so as to estimate the Mössbauer parameters of the species. Mössbauer isomer shifts and electron densities at iron nuclei (O) of Fe(CO)_n (n=5,4,3,2) as well as Fe(CO)₄(C₂H₄) are discussed: they have fairly good linear relationship to give –0.27 mm/s/a_O ^–3. An isomer of Fe(CO)₄(C₂H₄) produced via thermal reactions of Fe(CO)₄ with ethene in a stratified matrix is discussed by comparing the calculated and observed Mössbauer parameters.     

Mössbauer spectroscopic study of oxo-centered mixed-valence trinuclear iron fumarate

Oxo-centered mixed-valence trinuclear iron dicarboxylic acid complex iron fumarate [Fe₃O(O₂CCH=CHCO₂)₃(H₂O)₃]·nH₂O (n = 18-19), have been synthesized firstly. Variable temperature Mössbauer spectroscopy has been carried out to elucidate the rate of intramolecular electron transfer. It was found that the complex showed a temperature dependent mixed-valence state. At low temperature two quadrupole split doublets were observed corresponding to high spin Fe(III) and high spin Fe(II) state and a complete averaged valence state was observed at about 270 K. At temperatures between 200 and 270 K the spectra showed relaxation effect as the electron transfer rate was comparable to the Mössbauer timescale (-10^–7 s).     

Metal α, ω dicarboxylate complexes. 4. Effect of N-donor substitutions on the polymeric network in cobalt(II) hexanedioate complexes: synthesis and single crystal X-ray investigations

Three cobalt (II)hexanedioate complexes [Co(H₂O)₄(H₂L)]n 1 (H₂L=hexanedioic acid), Co(imidazole)₄ (H₂L)]n 2 and [Co(pyridine)₂ (H₂O)₄][H₂L] 3 are synthesized and structurally characterized to study the effect of N-donor substituents coordinated to the metal center on the polymeric network. Complex 1 is an extended linear polymer; Co(H₂O)₄ units are linked by the monodendate carboxylate from either end of the extended deprotonated hexanedioic acid. There are intra- and interchain H-bonding interactions between the coordinated water molecules and the end carboxylate O atoms, the uncoordinated O atom creates two dimensional hydrogen bonding pattern. Complex 2 also is a linear polymer; Co(imidazole)₄ units are linked by monodentate dibasic acid at the either end but with S shaped conformation of the hexanedioic acid, not as fully extended as in 1. The effect of bulkier N-donor substitution is seen in the distortion of the octahedral coordination polyhedron of Co(II). The noncordinated carboxylate oxygen makes one intra and one interchain H-bonding interaction with the imidazole N–H group making a two-dimensional H-bonded network as in 1. In 3 with the two strong N-donor pyridines coordinated to the metal center, the hexanedioate is out of the coordination sphere and acts as a counter ion. The Co(pyridine)₂(H₂O)₄ units are linked by H-bonding in both the dimensions by extensively folded adipate dianion forming a sheet structure parallel to ab plane. According to our knowledge this is the first example showing a strong H-bonding network in which a tetraaquaCo(II) center forms an eight-membered ring with bidendate H-bonding interactions. None of the coordination polymeric structures form any channels in their molecular packing, even to include a small entity as a water molecule.     

Unstable complexes of sandwich compounds with small molecules. Communication 3. Complexation of metallocenes of the 3d series with molecular oxygen at low temperatures

Conclusions Using the IR spectroscopy method we have shown that at low temperatures metallocenes (⁵-C₅H₅)₂M (M=V, Cr, Mn, Fe, Co, Ni) form complexes with O₂ in which the O₂ molecule is coordinated to the metal atom through one of the oxygen atoms, forming a complex of the M-O-O type.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 58–60, January, 1988.     

Synthesis,crystal structure and infrared spectra of Cu(II) and Co(II) complexes with 4,4′‐dichloro‐2,2′‐[ethylene dioxybis(nitrilomethylidyne)]diphenol

Novel 4,4′‐dichloro‐2,2′‐[ethylenedioxybis(nitrilomethylidyne)]diphenol (H₂L) and its complexes [CuL] and {[CoL(THF)]₂(OAc)₂Co} have been synthesized and characterized by elemental analyses, IR, ¹H‐NMR and X‐ray crystallography. [CuL] forms a mononuclear structure which may be stabilized by the intermolecular contacts between copper atom (Cu) and oxygen atom (O3) to form a head‐to‐tail dimer. In {[CoL(THF)]₂(OAc)₂Co}, two acetates coordinate to three cobalt ions through Co? O? C? O? Co bridges and four µ‐phenoxo oxygen atoms from two [CoL(THF)] units also coordinate to cobalt ions. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and characterization ofpara- andmeta-substituted benzeneseleninato complexes of tin(II), silver(I) and iron(II)

Summary New complexes of tin(II), silver([) and iron(II) withpara- andmeta-substituted benzeneseleninic acids are reported and studied by chemical analyses, spectroscopic methods, conductivity and magnetic measurements. The complexes, of the type Sn(XC₆H₄SeO₂)₂, Ag(XC₆H₄SeO₂) and Fe(H₂O)₂(XC₆H₄SeO₂)₂, were obtained by reaction of SnCl₂, · 2 H₂O. AgNO₃ and Fe(NH₄)₂(SO₄)₂·6 H₂O with the sodium benzeneseleninates, XC₆H₄SeO₂Na (X = H.p-Cl,m-Cl,p-Br,m-Br orp-Me). The i.r. spectral data suggest that the silver and iron complexes contain RSeO ₂ ^– ligands which act as bidentates in O,O-seleninato complexes. All the iron complexes have a distorted octahedralD _4h symmetry. The number of bands in the i.r. spectra of the tin derivatives indicates that there is no equivalence in the manner in which the two seleninato ligands are bonded. We could suggest that the tin atoms are intermolecularly O.O-bonded via one RSeO ₂ ^– group, whereas the second ligand is coordinated to the metal via one oxygen atom forming polymeric -tetrahedral derivatives.