323,214 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
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)
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
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
Sol-gel hybrid Organic/inorganic Nanocomposites by condensation reactions of diphenylsilanediol and organo-alkoxysilanes for photonic applications.
No full textSol-gel derived hybrid organic/inorganic (O/I) nanocomposites are largely exploited in materials technology, due to the synergic merge at the molecular scale of the components characteristics through a large extent of phase interaction. Thanks to their structural flexibility, hybrid materials are being increasingly used for a variety of applications in the field of integrated optics for the development of both passive and active devices.
Since the control of molecular and supramolecular structures is a fundamental issue for properties tailoring, recent research has focused on the synthesis of nanocomposites by assembling nanobuilding blocks (nbb), a technique that allows attaining nanometric control of the final materials. Si-based nanobuilding blocks are nanosized-preformed objects that can build up an organic/inorganic network exploiting the reactivity of organosilanes with functional end-capping organic groups.
We here report a review of our main results on the preparation and characterization of sol-gel hybrid organic/inorganic (O/I) nanocomposites by the assembling of silicon–based nbb, prepared by condensation of diphenylsilanediol with organo-alkoxysilanes and metal alkoxides. Two different methodologies have been followed: the condensation reaction was run both in non-hydrolytic conditions and using a controlled hydrolytic process, by means of in situ water production. Several techniques, such as multinuclear NMR, FTIR, FT-Raman and GPC, have been used to clarify the reaction pathways, with analysis of intermediate species, and to characterize the molecular complexity as well as the ratios between linear and cyclic Si oligomers in the final nbb.
High refractive index hybrid coatings were prepared by nbb assembling in different matrices. The thermal polymerization process and the two-photon induced polymerization (TPIP) process for the preparation of patternable films from methacrylate-based nbb were studied using DSC and FE-SEM analyses. Highly cross-linked networks have been obtained leading to patternable films with well-defined and small structures
Novel rhodium porphyrin derivatives. II. Synthesis and characterization of hexacoordinated Rh(III) porphyrinates
No full textRhodium derivatives of meso-tetraphenyl-, octaethyl- and etioporphyrin(I) react with ligands L (L = DMA, t-buNC, PPh2Me, POMe3) thermally or photo- chemically leading to the corresponding hexacoordinated complexes. The influence of the counter ion on the reaction pathway is discussed. © 1987
Effects of tin phosphate nanosheet addition on proton-conducting properties of sulfonated poly(ether sulfone) membranes
No full textOrganic/inorganic composite membranes were prepared by dispersing nanosheets of layered tin phosphate
hydrate [Sn(HPO4)2·nH2O (SnP)] in sulfonated poly(ether sulfone) (SPES) at SnP contents of 0–40 vol.%.
The stabilities and proton conductivities of SPES/SnP nanosheet (SnP-NS) composite membraneswere investigated
and comparedwith those of SPES/SnP particle (SnP-P) composite membranes. The chemical stabilities as evaluated
by thermogravimetry, differential thermal analysis, and diffuse reflectance Fourier-transform infrared spectroscopy
were improved in both composite membranes. The improvement in the structural stability of SPES/SnP-NS composite
membranes was more evident than that in SPES/SnP-P. The results suggest that exfoliation of SnP increases
the area of the SPES–SnP interface and extends the connectivity of the network of hydrogen bonds. A composite
membrane containing 10 vol.% SnP-NS (SPES/SnP-NS10vol.%) showed a high conductivity of 5.9×10−2 S cm−1
at 150 °C under saturated water vapor pressure. Although less water was present in SPES/SnP-NS10vol.% than in
SPES/SnP-P10vol.% or pure SPES, the conductivity of SnP-NS10vol.% was the highest among these samples at
130 °C under a high relative humidity (RH). However at a low RH, the proton-conducting property was not
improved by changing the composition of the SnP-NS. These results suggest that the hydrogen-bond network
operates effectively for proton conduction at a high RH, but at a low RH, the network fails to conduct as a result
of a decrease in water content accompanied by structural stabilization
Sulfated zirconium oxide as electrode and electrolyte additive for direct methanol fuel cell applications
No full textSulfated zirconium oxide (S-ZrO2) was used as electrode and electrolyte additive for direct methanol fuel cells (DMFCs). Composite Nafion electrolyte membranes and Pt electrocatalysts, both containing S-ZrO2 at different content, were prepared. The morphology and catalytic activity of prepared catalysts were investigated by scanning electron microscopy, and voltammetric technique. Results indicated that Pt/S-ZrO2 catalysts showed enhanced efficiency towards oxygen reduction reaction and increased methanol tolerance as compared to bare platinum. Pt/S-ZrO2-based carbon cloth electrodes were prepared and assembled as cathode in a DMFC, with Nafion/S-ZrO2 as composite electrolyte membrane. With respect to bare platinum and Nafion, higher values of current and power density were recorded at 110 °C. The use of S-ZrO2 both as catalyst and electrolyte additive provided enhanced membrane/electrode interface stability, as revealed by EIS spectra recorded during cell operation. © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved
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)
- …
