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991.
Jokic D Boudon C Pognon G Bonin M Schenk KJ Gross M Weiss J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(14):4199-4209
Based on the efficient combination of calixarene spacers and acetylenic porphyrin derivatives, a new generation of cofacial bis-porphyrins has been synthesized. The first crystal structure of a cofacial bis-porphyrin-calixarene conjugate is reported. Their unique architectural features, analogous to those of pac-man-type bis-porphyrins, allow these calixarene-porphyrin conjugates to adapt their shape to the size of bidentate guests, such as diazabicyclo[2.2.2]octane (dabco) and 1,4-pyrazine. The predefined, cofacial arrangement of the porphyrin moieties observed in the solid state and in solution results in extremely high affinities (in the range of 10(9) M(-1)) for these guests. The 1,3-alternate calixarene conformations afford "open-mouth" pac-man structures whose ability to bite on nitrogen bidentates depends on their functionalization. A cone conformer provides a much more flexible structure that exhibits the highest affinity for dabco and pyrazine. 相似文献
992.
Bielawa H Hinrichsen O Birkner A Muhler M 《Angewandte Chemie (International ed. in English)》2001,40(6):969
The cover picture shows an arrangement of ruthenium atoms representing the active site of ruthenium catalysts for ammonia synthesis. This B(5)-type active site consists of three ruthenium atoms in one layer (blue spheres) and two ruthenium atoms in the layer directly above this (red spheres). This arrangement is energetically preferred, because none of the ruthenium atoms is in contact with both of the nitrogen atoms (green spheres) of the adsorbed N(2) molecule. Based on knowledge of this active site a number of catalysts were prepared by using [Ru(3)(CO)(12)] (upper green sphere) as a precursor. These catalysts were characterized (left red sphere and TEM micrograph in the background), tested using a parallel screening procedure (left and lower blue spheres), and kinetically analyzed (right blue sphere). The resulting barium-promoted Ru/MgO catalyst is the most active catalyst for ammonia synthesis to be described (right red sphere). It can be concluded that the B(5)-type site also dominates the activity of the promoted catalysts (lower green sphere). The enormous catalytic potential of Ba-Ru/MgO is described by Muhler et al. on pp. 1061 ff. 相似文献
993.
Crystal Structures of [ReCl4(PhC?CPh)]2 · 2 CH2Cl2 and PPh4[ReOCl4] Single crystals of [ReCl4(PhC?CPh)]2 · 2 CH2Cl2 were obtained by chilling dilute solutions of the solvate [ReCl4(PhC?CPh)POCl3] in CH2Cl2. PPh4[ReOCl4] was formed by the reaction of the diphenyl acetylene complex [ReCl5(PhC?CPh)] with PPh4Cl · H2O in CH2Cl2 solution. [ReCl4(PhC?CPh)]2 · 2 CH2Cl2: space group P21/c, Z = 2, 2244 observed independent reflexions, R = 0.038. Lattice parameters (19°C): a = 987.2 pm; b = 1533.9 pm; c = 1193.8 pm; β = 90.17° The compound forms centrosymmetrical dimeric molecules with ReCl2Re bridges with Re? Cl distances of 241.2 and 267.6 pm. The longer Re? Cl bond is situated in trans-position to the equatorial, side-on coordinated diphenyl acetylene ligand with mean Re? C distances of 200 pm. PPh4[ReOCl4]: space group P4/n, Z = 2, 1487 observed, independent reflexions, R = 0.047. Lattice parameters (19°C): a = b = 1272.0 pm; c = 771.3 pm. The compound crystallizes in the AsPh4[RuNCl4] type; it consists of [ReOCl4]? anions and PPh4+ cations. The anions are tetragonal with C4v symmetry and bond lengths Re? O = 165.4 pm and Re? Cl = 232.6 pm; the bond angle OReCl is 106.7°. 相似文献
994.
Kurt J. Schenk Soraia Meghdadi Mehdi Amirnasr Mohammad H. Habibi Ahmad Amiri Mehdi Salehi Abolfazl Kashi 《Polyhedron》2007,26(18):5448-5457
The synthesis and characterization of a series of cobalt(III) complexes of the general type [Co(N2O2)(L2)]+ are described. The N2O2 Schiff base ligands used are Me-salpn (H2Me-salpn = N,N′-bis(methylsalicylidene)-1,3-propylenediamine) (1–3) and Me-salbn (H2Me-salbn = N,N′-bis(methylsalicylidene)-1,4-butylenediamine) (4–5). The two ancillary ligands L include: pyridine (py) 1, 3-metheylpyridine (3-Mepy) 2, 1-methylimidazole (1-MeIm) 3, 4-methylpyridine (4-Mepy) 4 and pyridine (py) 5. These complexes have been characterized by elemental analyses, IR, UV–Vis, and 1H NMR spectroscopy. The crystal structures of trans-[CoIII(Me-salpn)(py)2]PF6, 1, and cis-α-[CoIII(Me-salbn)(4-Mepy)2]BPh4 · 4-Mepy, 4, have been determined by X-ray diffraction. Examination of the solution and crystalline structures revealed that the outer coordination sphere of the complexes exerts a noticeable influence on the inner coordination sphere of the Co(III) ion. The electrochemical reduction of these complexes at a glassy carbon electrode in acetonitrile solution indicates that the first reduction process corresponding to CoIII–CoII is electrochemically irreversible, which is accompanied by the dissociation of the axial (R-py)–cobalt bonds. It has also been observed that the Co(III) state is stabilized with increasing the flexibility of the ligand environment. 相似文献
995.
M. I. El-Barghouthi M. Schenk M. B. Zughul A. A. Badwan W. F. Van Gunsteren 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):375-377
In this work, the relative free energies of binding individual members of a series of mono- and di-substituted benzenes to
α-cyclodextrin in water have been estimated using the single step perturbation methodology (SSP). The computations involved
were based on 5 ns molecular dynamics simulations of a judiciously chosen unphysical reference state and its complex with
α-CD in water. The thermodynamic integration method (TI) was also used to calculate the relative free energies of binding
for comparison purposes. The results of SSP computations were in good agreement with experimental data. The results also show
that the SSP technique exhibits a potential alternative to more traditional but more expensive free energy computational methods. 相似文献
996.
Wolfdieter A. Schenk 《Journal of organometallic chemistry》1980,184(2):195-204
cis- and trans-tetracarbonyltungsten complexes (R3P)(R′3E)W(CO)4 (R3P,R′3E = i-Pr3P, Ph2-i-PrP, Ph3P, Ph2HP, (Me2n)3P, Ph(i-PrO)2P, (i-PrO)3P, (PhO)3P, Ph3As, Ph3Sb) are obtained from the chlortetracarbonyltungstates [(R3P)W-(CO)4Cl]- by several different routes. a cis—trans equilibrium is attained at 20–30°C with the trans isomer being thermodynamically more stable in nearly all cases. The cis-trans isomerisation of the phosphine—arsine and phosphine—stibine complexes is shown by ligand exchange experiments to occur via dissociation of the arsine and stibine ligand, respectively. The bis(phosphine) complexes, however, isomerise intramolecularly without bond cleavage. 相似文献
997.
Base flipping, the conformational change of a nucleobase to an extrahelical position, is a key step in the enzymatic repair of damaged DNA. An assay that can detect the flipped-out species in free solution without covalent modification of the DNA would be desirable. The design and synthesis of a simple, sensitive, and rapid assay using specific noncovalent binding to pyrimidines by zinc-cyclen and a commonly used fluorescent reporter group, dansyl, is reported. The binding of the zinc-cyclen unit to a flipped-out thymine base results in a change in the fluorescent properties of the dansyl group that is distinct from nonspecific binding to duplex DNA or intercalation into either the flipped-in or flipped-out species. The assay was tested using fluorescence spectroscopy and detection at 533 +/- 5 nm with normal and abasic duplex DNA as negative and positive controls. The data obtained are fitted to a one-site binding model to determine the equilibrium constant for the two-step process involving base flipping and binding to be approximately 10-6 M. 相似文献
998.
DirkJ.A. DeRidder Kees Goubitz Henk Schenk Bert Krijnen JanW. Verhoeven 《Helvetica chimica acta》2003,86(3):812-826
The crystal structures of seven N‐aryltropan‐3‐one (=8‐aryl‐8‐azabicyclo[3.2.1]octan‐3‐one) derivatives 1T1, 2T1, 2T2, 3T2, 5T2, 2T3 , and 3T3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1T2 and 4T2 published previously. The piperidine ring adopts a chair conformation. In all structures, the aryl group is in the axial position, with the plane through the aryl C‐atoms nearly perpendicular to the mirror plane of the piperidine ring. The through‐bond interaction between the piperidine ring N‐atom (one‐electron donor) and the substituted exocyclic C?C bond (acceptor) not only elongates the central C? C bonds of the piperidine ring but also increases the pyrimidalization at C(4) of the piperidine ring. Flattening of the C(2)–C(6) part of the piperidine ring decreases the through‐bond interaction. 相似文献
999.
Since its observation in 1985, nuclear resonance scattering of synchrotron radiation has become an excellent tool to study hyperfine interactions as well as dynamical effects in solids. It has proven to be a complementary method to Mössbauer spectroscopy. Nuclear resonance scattering combines the advantages of both local probe experiments and scattering techniques. It gives valuable information as well on electronic and magnetic structures and on dynamics in solids. Experiments benefit from the high beam quality of third-generation synchrotron radiation sources, as the small beam size and divergence. Besides the standard isotope 57Fe, other Mössbauer isotopes have become more important in nuclear resonant scattering of synchrotron radiation. This article concentrates on the 151Eu isotope. 相似文献
1000.
Elschenbroich C Plackmeyer J Nowotny M Behrendt A Harms K Pebler J Burghaus O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(24):7427-7439
Five new paramagnetic dinuclear complexes containing [5]trovacenyl groups, (eta7-C7H7)V(eta5-C5H4-), have been prepared and characterized, including by single-crystal X-ray diffraction. As intervening spacers, ethenediyl units in the geminal and vicinal (Z)- and (E)-bridging modes as well as methanediyl and ethanediyl units have been included with the aim of studying their propensity to transmit electric and magnetic information. It is found that redox splitting of consecutive electron-transfer steps is resolved for reduction (0-->1- -->2-) only, unsaturation of the C2 bridge not being requisite, since the -CH2CH2- spacer also gives rise to a small redox splitting. Magnetic communication is quantified in terms of the exchange coupling constant J, accessible from the EPR hyperfine pattern in solution and from magnetic susceptometry in the solid state. The results obtained from these methods generally differ; this fact is not surprising in view of conformational differences in the respective states of aggregation. It is concluded that orientation-dependent mechanisms of spin-spin interactions (pi-orbital overlap, hyperconjugation) contribute extensively although, as implied by sizeable J values for -CH2- and -C2H4- linked di[5]trovacenyl groups, coupling mediated by the sigma-orbital chain must also be considered. 相似文献