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81.
Structure Reactivity Correlations in Coordinatively Unsaturated Chelate Complexes. VI. Synthesis, Adduct Formation, Redox Potentials, and Photochromic Iodine Derivatives of Iron(II) Complexes with Schiff Base Ligands Possessing Electron-Withdrawing Substituents Iron(II) complexes of the type 1 have been prepared by different synthetic methods. In contrast to similar chelates of the “saloph” and “salen” types, the high-spin complexes 1 form stable high-spin diadducts ( 1a – 1d ) and an unusual triadduct ( 1e ) with pyridine. The oxidation potentials of the FeII/FeIII couple as measured by cyclic voltammetry are dependent on the solvent as well as the equatorial ligand substituents. The potentials are more positive in pyridine than in DMF, indicating a stabilization of FeII by pyridine. The redox potentials are discussed with respect to those of other metals in the same ligand environment. The complexes form iodine derivatives which show photochromic behaviour in THF solution. The rate of the reaction with dioxygen in the solid state as well as in pyridine solution decreases in the order 1f > 1a ≈ 1b > 1c ≈ 1d > 1e and correlates with the increasing oxidation potentials. 相似文献
82.
The principles of the method of NMR line narrowing by measurement with spinning of the sample about the magic axis (MAR-NMR) are introduced, with particular emphasis on the effects of internal motion upon the possibilities and limitations of the method. The applications of the method in 1H-NMR studies of polymer structure and dynamics are then reviewed. Due to both theoretical and experimental limitations, narrowing of dipolar broadened NMR lines by MAR can be observed in 1H NMR spectra only in those cases where internal motion is anisotropic, or in heterogeneous systems where line width is limited by differences of magnetic susceptibility. In polymers, both solid and liquid, the method makes possible differentiation between isotropic and anisotropic internal motion. In systems with anisotropic internal motion, MAR-NMR makes possible a characterization of motional codes which normally are obscured by residual dipolar interactions, as well as of geometrical restrictions upon these motions. 相似文献
83.
Cline Schneider Frdric Doucet Stanislav Strekopytov Christopher Exley 《Polyhedron》2004,23(18):3185-3191
Hydroxyaluminosilicates (HAS) are critical secondary mineral phases in the biogeochemical cycle of aluminium. They are formed from the reaction of silicic acid (Si(OH)4) with an aluminium hydroxide template and act as a geochemical control of the biological availability of Al. There are two main forms of HAS which we have called HASA and HASB and which of these will predominate will depend upon the Si(OH)4 to Al ratio in any one environment. In all but the most heavily weathered environments or those undergoing a progressive acidification Si(OH)4 will be present in significant excess to Al and HASB will be the dominant secondary mineral phase. We have tried to determine the solubility of HASB(s) so that its contribution to Al solubility control might be compared with other secondary minerals such as Al(OH)3(gibbsite). In preliminary experiments, the dissolution of HASB(s) was found to be non-congruent with almost no Al being released during 18 months ageing. We then demonstrated that HASB(s) was significantly less soluble than Al(OH)3(s) prepared under identical experimental conditions. We have used this information to describe a solubility expression for HASB(s) at a predefined quasi-equibrium and to calculate a solubility constant.
K*Al2Si2O5(OH)4=[Al2O4+][SiO2]2[OH-]4