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91.
92.
Pb(18‐crown‐6)Cl2 and Hg(18‐crown‐6)I2 are obtained as transparent colourless crystals of needle and hexagonal shape, respectively, by isothermal evaporation of their dichloromethane solutions. Pb(18‐crown‐6)Cl2 crystallizes with the trigonal crystal system [ , no. 148, a = b = 1176.3(2), c = 1191.8(3) pm, V = 1428.2(5) 106·pm3, Z = 3] whereas Hg(18‐crown‐6)I2 crystallizes with the orthorhombic crystal system (Pnma, no. 62, a = 1613.9(2) pm, b = 2822.2(5) pm, c = 841.3(1) pm, V = 3832(1)106·pm3, Z = 8). Both compounds are characterized by linear MX2 (HgI2 or PbCl2) molecular units which are encrypted by the crown ether. In both cases, the divalent metal ion resides in the middle of the crown ether resulting in a hexagonal bipyramidal coordination environment for the metal cations. The molecular symmetry comes close to D3d. Hg(18‐crown‐6)I2 and Pb(18‐crown‐6)Cl2 differ in the way the single MX2@18‐crown‐6 units are packed. Whereas the Hg(18‐crown‐6)I2 molecules are arranged in a (distorted) cubic closest packing, the Pb(18‐crown‐6)Cl2 molecules adopt a hexagonal closest packing.  相似文献   
93.
Treatment of anhydrous YbCl3 with LiN(SiMe3)2 followed by reaction with 1 equivalent of 1,1,3,3,5,5‐hexaphenyl‐1,3,5‐trisiloxanediol afforded the first mono(trisiloxanediolate) complex of a rare earth element. The compound [Ph2Si(OSiPh2O)2]Yb(THF)(μ‐Cl)3Li2(THF)3 ( 1 ) was isolated in the form of colorless crystals in very high yield (93%). A single‐crystal X‐ray diffraction study confirmed the presence of an eight‐membered inorganic ring system containing ytterbium. Coordination of one THF ligand and retention of two equivalents of lithium chloride lead to formation of an “ate” complex.  相似文献   
94.
Structures of Polar Magnesium Organyls: Synthesis and Structure of Base Adducts of Bis(cyclopentadienyl)magnesium Eight donor‐acceptor complexes of bis(cyclopentadienyl)magnesium ( 1 ) with N‐ and O‐donor Lewis bases have been synthesized and characterized by X‐ray structure analysis. With acetonitrile, dimethoxyethane, diethyleneglycoldimethylether, dioxane, and tetramethylethylenediamine simple 1:1 adducts are formed ( 2 – 6 ). In some cases a change of the hapticity of one cyclopentadienylring from η5 to η2 or η1 is observed ( 4 – 6 ). In the adduct with pentamethyldiethylenetriamine ( 7 ) one C5H5‐ring is removed from the magnesium atom forming the cation [Mg(C5H5)(PMDTA)]+ and an uncoordinated five‐ring anion. With the crown ether 15‐crown‐5 the two ionic Mg compounds 8 and 9 are formed which have a [Mg(15‐crown‐5)L2]2+‐cation [L = pyridine, THF] and two uncoordinated cyclopentadienyl anions. Cyclopentadienyl‐methyl‐magnesium reacts with 15‐crown‐5 to the salt [Mg(CH3)(15‐crown‐5)]+ C5H5? ( 10 ) which has also a free cyclopentadienyl anion.  相似文献   
95.
Complexes of the Alkali Metal Tetraphenylborates with Macrocyclic Crown Ethers Alkali metal tetraphenylborates, MB(C6H5)4 (M = Li to Cs), react in tetrahydrofuran with macrocyclic crown ethers to give complexes of the general formula MB(C6H5)4(crown)m(THF)n. Suitable single crystals for X‐ray structure analysis were grown from a solvent mixture of tetrahydrofuran and n‐hexane. The salt like complexes [Li(12‐crown‐4)(thf)][B(C6H5)4] ( 1 ), [Na(15‐crown‐5)(thf)][B(C6H5)4] ( 2 ), and [Cs(18‐crown‐6)2][B(C6H5)4] · THF ( 6 ), the mononuclear molecular complexes [KB(C6H5)4(18‐crown‐6)(thf)] ( 3 ), [RbB(C6H5)4(18‐crown‐6)] ( 4 ), and [CsB(C6H5)4(18‐crown‐6)] · THF ( 5 ), and the compound [CsB(C6H5)4(18‐crown‐6)]2[Cs(18‐crown‐6)2][B(C6H5)4] ( 7 ), which contains a binuclear molecule ([CsB(C6H5)4(18‐crown‐6)]2) beside a [Cs(18‐crown‐6)2]+ cation and a [B(C6H5)4]? anion, are described. All compounds are charactarized by infrared spectra, elemental analysis, NMR‐spectroscopy, and X‐ray single crystal structure analysis.  相似文献   
96.
97.
Teacher-education research lacks a common theoretical basis, which prevents a convincing development of instruments and makes it difficult to connect studies to each other. Our paper models how to measure effective teacher education in the context of the current state of knowledge in the field. First, we conceptualize the central criterion of effective teacher education: “professional competence of future teachers”. Second, individual, institutional, and systemic factors are modeled that may influence the acquisition of this competence during teacher education. In doing this, we turn round the perspective taken by Cochran-Smith and Zeichner (Studying teacher education. The report of the AERA panel on research and teacher education. Lawrence Erlbaum, Mahwah 2005), who mainly take an educational-sociological perspective by focusing on characteristics of teacher education and looking for their effects. In contrast, we take an educational-psychological perspective by focusing on professional competence of teachers and examining influences on this. Challenges connected to an assessment of teacher-education outcomes are discussed as well.  相似文献   
98.
Hydrogenases are H2 converting enzymes that harbor catalytic cofactors in which iron (Fe) ions are coordinated by biologically unusual carbon monoxide (CO) and cyanide (CN) ligands. Extrinsic CO and CN, however, inhibit hydrogenases. The mechanism by which CN binds to [FeFe]-hydrogenases is not known. Here, we obtained crystal structures of the CN-treated [FeFe]-hydrogenase CpI from Clostridium pasteurianum. The high resolution of 1.39 Å allowed us to distinguish intrinsic CN and CO ligands and to show that extrinsic CN binds to the open coordination site of the cofactor where CO is known to bind. In contrast to other inhibitors, CN treated crystals show conformational changes of conserved residues within the proton transfer pathway which could allow a direct proton transfer between E279 and S319. This configuration has been proposed to be vital for efficient proton transfer, but has never been observed structurally.  相似文献   
99.
Simple bis(oxazoline) ligands, especially azabis(oxazolines), can promote the copper(II)-catalyzed Michael addition of indoles to benzylidene malonates with up to >99 % ee (ee=enantiomeric excess), provided that the ligand/metal ratio is tuned meticulously with particular regard to the electronic properties of the substrate. Despite a common paradigm followed in many asymmetric catalyses, an excess of chiral ligand is not always beneficial. In fact any excess of ligand has to be avoided to reach excellent enantioselectivities when electron-rich benzylidene malonates are used. On the contrary, malonates carrying an electron-withdrawing group require an excess of ligand for an optimum ee value. A correlation of optical yields versus the sigma(I) values of several para substituents shows a sigmoid trajectory. In the presence of an additive, such as triflate, the significance of the ligand/metal ratio vanishes and very good enantioselectivities are achieved at any rate--no matter whether electron-donating or withdrawing substituents are present.  相似文献   
100.
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