1,721,313 research outputs found
Is the corrolate macrocycle innocent or noninnocent? Magnetic susceptibility, Mossbauer, H-1 NMR, and DFT investigations of chloro- and phenyliron corrolates
No full textan attempt to determine the electron configuration of (anion)iron corrolates, i.e., whether they are S = 1 Fe(IV)-corrolate(3(-)) or S = (3)/(2) Fe(III)-corrolate(2(-.)), with antiferromagnetic coupling between the iron and macrocycle electrons to yield overall S = 1, two axial ligand complexes of an iron octaalkylcorrolate have been studied by temperature-dependent magnetic susceptibility, magnetic Mossbauer, and H-1 NMR spectroscopy, and the results have been compared to those determined on the basis of spin-unrestricted DFT calculations. Magnetic susceptibility measurements indicate the presence of a noninnocent macrocycle (corrolate (2-(.))) for the chloroiron corrolate, with strong antiferromagnetic coupling to the S = (3)/(2) Fe(III) center, while those for the phenyliron corrolate are not conclusive as to the electron configuration. Temperature- and field-dependent Mossbauer spectroscopic investigations of these two complexes yielded spectra that could be simulated with either electron configuration, except that the isomer shift of the phenyliron complex is -0.10 mm/s while that of the chloroiron complex is +0.21 mm/s, suggesting that the iron in the former is Fe(IV) while in the latter it is Fe(III). 1H NMR spectroscopic studies of both axial ligand complexes show large negative spin density at the meso carbons, with those of the chloroiron complex (Cal, S.; Walker, F. A.; Licoccia, S. Inorg. Chem. 2000, 39, 3466) being roughly four times larger than those of the phenyliron complex. The temperature dependence of the proton chemical shifts of the phenyliron complex is strictly linear. DFT calculations are consistent with the chloroiron complex being formulated as S-1 = (3)/(2) Fe(III)-corrolate (2(-.)) S-2 = (1)/(2), with negative spin density at all nitrogens and meso carbons, and a net spin density of -0.79 on the corrolate ring and positive spin density (+0.17) on the chloride ion and +2.58 on the iron. In contrast, the phenyliron complex is best formulated as S = I Fe(IV)-corrolate (3-), but again with negative spin density at all nitrogens and meso carbons of the macrocycle, yet with the net spin density on the corrolate ring being virtually zero; the phenyl carbanion carbon has relatively large negative spin density of -0.15 and the iron +2.05. On the basis of all of the results, we conclude that in both the chloroiron and phenyliron complexes the corrolate ring is noninnocent, in the chloroiron complex to a much larger extent than in the phenyliron complex
Increasing the operation temperature of polymer electrolyte membranes for fuel cells: From nanocomposites to hybrids
No full textAmong the possible systems investigated for energy production with low environtnental impact, polymeric electrolyte membrane fuel cells (PEMFCs) are very promising as electrochemical power sources for application in portable technology and electric vehicles. For practical applications, operating FCs at temperatures above 100 degrees C is desired, both for hydrogen and methanol fuelled cells. When hydrogen is used as fuel, an increase of the cell temperature produces enhanced CO tolerance, faster reaction kinetics, easier water management and reduced heat exchanger requirement. The use of methanol instead of hydrogen as a fuel for vehicles has several practical benefits such as easy transport and storage, but the slow oxidation kinetics of methanol needs operating direct methanol fuel cells (DMFCs) at intermediate temperatures. For this reason, new membranes are required. Our strategy to achieve the goal of operating at temperatures above 120 degrees C is to develop organic/inorganic hybrid membranes. The first approach was the use of nanocomposite class I hybrids where nanocrystalline ceramic oxides were added to Nafion. Nanocomposite membranes showed enhanced characteristics, hence allowing their operation up to 130 degrees C when the cell was fuelled with hydrogen and up to 145 degrees C in DMFCs, reaching power densities of 350 mW cm(-2). The second approach was to prepare Class 11 hybrids via the formation of covalent bonds between totally aromatic polymers and inorganic clusters. The properties of such covalent hybrids can be modulated by modifying the ratio between organic and inorganic groups and the nature of the chemical components allowing to reach high and stable conductivity values up to 6.4 x 10(-2) S cm(-1) at 120 degrees C. (c) 2006 Elsevier B.V. All rights reserved
Crosslinked sulfonated poly(phenylene sulfide sulfone) membranes for vanadium redox flow batteries
No full textIn this work, crosslinked hydrocarbon-based cation exchange membranes have been developed for vanadium redox flow battery applications. By Friedel-Crafts alkylation of sulfonated poly(phenylene sulfide sulfone) (sPSS), membranes with various degrees of crosslinking have been prepared in one step. Among those, the membrane containing 9% of crosslinker (sPSScl9) represents the best solution. Owing to a much higher selectivity, battery self-discharge and capacity fading (tested over 100 charge/discharge cycles at 120 mA cm−2) of sPSScl9 outperform benchmark Nafion of comparable thickness (N212). Furthermore, the crosslinking strategy permits to obtain stable membranes even in highly oxidizing environments, due to a combination of crosslinking, that holds together the polymer chains, and oxidation of sulfides to sulfones that increases the rigidity of the backbone. As a result, sPSScl9 incubated in the presence of V(V) shows unchanged ion exchange capacity and proton conductivity, and a 10× reduction of vanadium permeability with respect to untreated membranes
Iron corrolates: Unambiguous chloroiron(III) (corrolate)2-radical dot π-cation radicals
No full textThe structures, electron configurations, magnetic susceptibilities, spectroscopic properties, molecular orbital energies and spin density distributions, redox properties and reactivities of iron corrolates having chloride, phenyl, pyridine, NO and other ligands are reviewed. It is shown that with one very strong donor ligand such as phenyl anion the electron configuration of the metal is d4 S = 1 Fe(IV) coordinated to a (corrolate)3- anion, while with one weaker donor ligand such as chloride or other halide, the electron configuration is d5 S = 3/2 Fe(III) coordinated to a (corrolate)2-radical dot π-cation radical, with antiferromagnetic coupling between the metal and corrolate radical electron. Many of these complexes have been studied by electrochemical techniques and have rich redox reactivity, in most cases involving two 1-electron oxidations and two 1-electron reductions, and it is not possible to tell, from the shapes of cyclic voltammetric waves, whether the electron is added or removed from the metal or the macrocycle; often infrared, UV-Vis, or EPR spectroscopy can provide this information. 1H and 13C NMR spectroscopic methods are most useful in delineating the spin state and pattern of spin density distribution of the complexes listed above, as would also be expected to be the case for the recently-reported formal Fe(V)double bond, longO corrolate, if this complex were stable enough for characterization by NMR spectroscopy. Iron, manganese and chromium corrolates can be oxidized by iodosylbenzene and other common oxidants used previously with metalloporphyrinates to effect efficient oxidation of substrates. Whether the "resting state" form of these complexes, most generally in the case of iron [FeCl(Corr)], actually has the electron configuration Fe(IV)(Corr)3- or Fe(III)(Corr)2-radical dot is not relevant to the high-valent reactivity of the complex. © 2006 Elsevier Inc. All rights reserved
Cyanide complexes of iron corrolates: Spin delocalization and autoreduction
No full textComplex formation of (7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrolato)iron chloride, [(7,13-Me2Et6C)FeCl], with cyanide ion in dimethylformamide, DMF-d(7), was studied by H-1 NMR spectroscopy. It is found that a bis-cyanide complex is formed initially, in which the electron configuration is a low-spin Fe(III) corrolate(2-.). This complex is not stable, and it is readily reduced with an excess of cyanide in the solution. The reduction occurs at the corrole ring instead of on the iron center giving the monocyanide complex of the low-spin Fe(III) corrole, [(7,13-Me2Et6C)FeCN](-). Thus, this is a case where an axial ligand serves as a reducing agent of the macrocycle and not of the metal
NMR and EPR investigations of iron corrolates: Iron(III) corrolate ¤Ç cation radicals or iron(IV) corrolates?
No full textThe chloroiron corrolates of 2,3,7,8,12,13,17,18-octamethyl- and 7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrole ([(Me8C)FeCl] and [(7,13-Me2Et6C)FeCl], respectively) and their bisimidazole complexes have been investigated by NMR spectroscopy as a function of temperature, and by EPR spectroscopy at 4.2 K. Magnetic susceptibilities were measured by the modified Evans method. It is found that the electron configuration of the chloroiron corrolates is that of a S = 3/2 Fe(III) center coupled to a corrolate ¤Ç radical, where one electron has been removed from the ¤Ç system of the corrolate. This ¤Ç radical is antiferromagnetically coupled to the unpaired electrons of the iron to yield an overall S = 1 complex, as evidenced by the very large positive shifts of the meso-H resonances (183 and 172 ppm). That this antiferromagnetic coupling is very strong is supported by the near-Curie behavior of the 1H chemical shifts. For the chloroiron corrolates in the presence of imidazole, imidazole-d4, and N-methylimidazole at temperatures of -50 ┬░C and below, the mono- and bisligand complexes are formed. The NMR spectra can be assigned on the basis of chemical exchange between the chloroiron(III) parent complex and the bisligand complex at -30 ┬░C, and between the bisligand complex and the monoligand complex at -50 ┬░C. The bisimidazole complexes show pyrrole CH2 and CH3 resonances characteristic of low-spin Fe(III) centers (S = 1/2), but with strongly upfield-shifted meso-H resonances (╬┤ values of -95 and -82.5 ppm for the octamethyl complex and -188 and -161 ppm for the dimethylhexaethyl complex at 203 K) characteristic of the presence of a macrocycle-centered unpaired electron. The magnetic moments of these bisligand complexes are somewhat lower than expected for overall S = 1 systems, and decrease as the temperature is lowered. The lower apparent magnetic moments (2.01.8 ╬╝(B) between -50 and -90 ┬░C) are believed to be caused by a combination of weak or no magnetic coupling between the metal and macrocycle electrons and decreasing solubility of the complex as the temperature is lowered. The non-Curie behavior of the 1H chemical shifts observed in the low-temperature (-50 to -90 ┬░C) NMR spectra likely arises from a combination of the effects of weak antiferromagnetic coupling of metal and macrocycle spins, a low-lying electronic excited state, and ligand binding/loss equilibria at the highest temperatures studied (-50 ┬░C)
NiO-YSZ foams with hierarchical microstructure for SOFC anodes
No full textA technique for the preparation of NiO-YSZ cellular ceramics is presented. The technique is based on the in-situ polymerization of a Polyurethane system loaded with the ceramic powders. After sintering, materials with a total porosity value as large as 70% can be obtained, showing a bimodal distribution of the porosity, related to the open cell structure and the porosity of the scaffold network. In fact, the walls of the foam were not fully densified, permitting obtaining a microstructure with a very large triple phase boundary (TPB)
ATR-FTIR spectroscopic study of the effect of ceramic addition in novel ionoconductor gels for biomedical applications in space
Full text linkEffect of an ormosil-based filler on the physico-chemical and electrochemical properties of Nafion membranes
No full text
- …
