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  • 1
    ISSN: 0947-6539
    Keywords: alkali metals ; catechol ligands ; NMR spectroscopy ; supramolecular chemistry ; titanium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The spontaneous self-assembly of methylene-bridged dinuclear TiIV complexes M4[L3Ti2] (L = 1-3) from CH2-bridged di(catechol) ligands (1-H4-3-H4) was found to proceed in the presence of lithium or sodium carbonate acting as a base (M = Li, Na). In contrast, only an unspecific mixture of coordination compounds was obtained in the presence of potassium carbonate. This difference in behavior is due to the ability of Li+ or Na+ to stabilize the triple-stranded dinuclear titanium(IV) species [L3Ti2]4-. In the solid state, Li4[(1)3Ti2] exhibits a highly symmetric structure with three lithium cations bound to the meso-helicate tetraanion. 6Li NMR together with 1H NMR studies at variable temperature revealed that, in solution, an unsymmetrical „bowl-shaped“ species is formed with only two Li+ bound to the anion. A methyl substituent on the methylene spacer of the di(catechol) ligand (ligand 2) led to supramolecular systems with new stereocenters and thus a higher content of information. However, only one of the four possible diastereoisomeric metal complexes of [(2)3Ti2]4- was observed. A third type of ligand, with a methyl group attached to one terminus of the linear ligand, was also investigated. The synthesis of such an unsymmetrical ligand, 3-H4, was readily achieved starting from 2,3-dimethoxyben-zyl alcohol (4) and 2,3-dimethoxybenz-aldehyde (6). Upon complexation, the ligand 3-H4 led to a statistical mixture of the two possible isomers of the dinuclear titanium(IV) complex [(3)3Ti2]4-.
    Additional Material: 8 Ill.
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  • 2
    ISSN: 0947-6539
    Keywords: chiral mesophases ; Langmuir-Blodgett films ; liquid crystals ; phthalocyanines ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure and physical properties of optically active, metal-free 2,3,9,10,16,17,23,24-octa (S-3,7-dimethyloctoxy)phthalocyanine ((S)-Pc(8,2)) are reported and compared with those of the phthalocyanine with (R,S) side chains (mixture of 43 stereoisomers). Unlike the latter compound, (S)-Pc(8,2) lacks a crystalline phase. A freshly prepared sample is in a distorted mesophase and reorganizes irreversibly to a more ordered phase above 65 °C. X-ray diffraction and circular dichroism studies indicate that the molecules are stacked in columns which have a hexagonal arrangement and a left-handed helical superstructure, that is, a novel chiral Dh* mesophase. Solid state NMR measurements reveal that the phthalocyanine units in the columns begin to vibrate laterally when the temperature is increased. At 111 °C (Dh* → Dr transition) they start to rotate around their columnar axes and at the same time the side chains become liquidlike. Energy migration is very efficient in the chiral Dh* phase and also in the frozen mesophase below 3 °C, as follows from luminescence spectroscopy. Intracolumnar charge transport, studied by the time-resolved microwave conductivity technique, turns out to be slower in the helically distorted columns than in linear columns. (S)-Pc(8,2) forms a very stable bilayer at the air-water interface, which can be transferred to give a high quality Langmuir-Blodgett film. The fact that this phthalocyanine is mesogenic at room temperature is thought to be responsible for this behavior.
    Additional Material: 14 Ill.
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  • 3
    ISSN: 0947-6539
    Keywords: anthraquinones ; molecular devices ; supramolecular chemistry ; titanium dioxide ; viologens ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Linked viologen-anthraquinone molecules are attached to TiO2 nanocrystallites supported on a conducting glass substrate. The resulting assembly is incorporated as the working electrode in an electrochemical cell. Under opencircuit conditions, band-gap excitation of the semiconductor nanocrystallite results in viologen-mediated electron transfer to anthraquinone. Prior application of a negative potential step, which results in 2e-/2H+ reduction of anthraquinone, permits electron transfer only to viologen. At positive applied potentials, electron transfer following band-gap excitation is largely suppressed. Some implications of these findings for modulation of function in molecular devices are considered.
    Additional Material: 8 Ill.
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  • 4
    ISSN: 0947-6539
    Keywords: cations ; crystal structures ; phenyl rings ; supramolecular chemistry ; tetraphenylphosphonium ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Examination of the Cambridge Structural Database reveals that Ph4P+ cations in crystals associate through phenyl-phenyl nonbonded interactions which are attractive, concerted, and widespread. Intermolecular phenyl-phenyl conformations, which are offset-face-to-face (off), edge-to-face (ef) or vertex-to face (vf), combine in five classes of supramolecular motifs for {Ph4P+}2 pairs, namely the sextuple phenyl embrace (SPE) with (ef)6 and offset sextuple phenyl embrace (OSPE) containing (off)1 (ef)2(ef/vf)2, the translational quadruple phenyl embrace (TQPE) with (ef)4, the parallel quadruple phenyl embrace (PQPE) with (off)1(vf)2, and the double phenyl embrace (DPE) with (off)1. Typical intermolecular attractive energies (kJ per mol of {Ph4P+}2) for these motifs are SPE 85, OSPE 57, TQPE 70, PQPE 41, DPE 34. There is strong interpenetration of the cations in these motifs: 489/770 structures in the CSD have P⃛P≤7 Å (spherical Ph4P+ has a van der Waals diameter of 13.6 Å). Of the 812 instances of P⃛P ≤7 Å, 86% are SPE, 10% are OSPE, 2% are TQPE, and only 2% are unclassified, Average P⃛P separations in the PQPE and DPE are 8.3 Å. Centrosymmetry is prevalent in all except the TQPE, which has implications for the engineering of noncentric crystals.
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 15-19 
    ISSN: 0947-6539
    Keywords: helical structures ; self assembly ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Helical complexes have been much studied as examples of self-assembled supramolecular units. Their structural features are analysed in terms of the metal ion, the binding site on the ligand and the bridging group linking these sites. Understanding these elements allows the controlled assembly of multicomponent systems. Preliminary results show the complexes to have high stability, arising from the inertness of the self-assembled species.
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  • 6
    ISSN: 0947-6539
    Keywords: bipyridines ; copper compounds ; selfassembly ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The newly synthesised ligand 2 combines binding components known to undergo specific and distinct self-assembly processes with Cu1 ions. It complexes Cu1 to form, in almost quantitative yield, a large inorganic architecture 1 made up from four ligand molecules and twelve metal ions. The structure of 1 was ascertained by X-ray radiocrystallography as well as by NMR spectroscopy and electrospray mass spectrometry. It consists of a macrocycle of nanometric dimension with an external diameter of 28 Å; the central cavity has a diameter of 11 Å, which contains four PF-6 anions as well as solvent molecules. The spontaneous formation of 1 results from a self-assembly process based on a “program” combining two assembly subroutines, each specific to one of the ligand subunits. Self-assembly through double or, more generally, multiple subroutines can be used to generate a wide variety of highly complex inorganic supramolecular architectures by combination of two or more assembly processes.
    Additional Material: 5 Ill.
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  • 7
    ISSN: 0947-6539
    Keywords: aromatic stacking ; crystal engineering ; diamondoid networks ; packing model ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ability to predict and subsequently control solid-state structure has been identified as a major challenge in the field of crystal engineering. Here we suggest the concept of constitutive models as a tool for understanding crystal packings and for designing new solid-state structures. Such models are intended to relate molecular interactions and their geometrical constraints with solid-state organization. These models will most likely be of greatest use for crystals consisting of supramolecular networks, that is, infinite assemblies of small molecules associating through strong, directional, and selective noncovalent interactions. The concept of the constitutive packing model is illustrated for interpenetrated diamondoid coordination networks based on crystalline adducts of 4,4′-biphenyldicarbonitrile with silver(I) salts. Observed structural deformations induced by counterions of varying size may be understood in terms of the interference of two supramolecular networks within this system: the diamondoid metal-ligand coordination network and face-to-face aromatic stacks of the organic ligand. The constitutive model developed here has been applied to other diamondoid coordination networks in the literature and is found to be general.
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  • 8
    ISSN: 0947-6539
    Keywords: arginine ; guanidines ; molecular recognition ; receptors ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Bisphosphonates 2 and 3 represent the first artificial receptor molecules for alkylguanidinium ions. They bind to the guanidinium moiety by forming a 1:1 chelate complex, stabilized by a planar network of electrostatic interactions and hydrogen bonds. This hydrogen bonding configuration is identical to the „arginine fork“ postulated by Frankel as a key element in RNA-protein recognition of the AIDS virus. Our guanidinium-bisphosphonate complexes thus constitute the first synthetic model for this important biological interaction and demonstrate that the high binding energy can be a driving force for a conformational change in the receptor (induced fit, e.g., in the RNA). Although binding of monosubstituted alkylguanidines is generally strong (Ka ≈ 10 000 in DMSO), molecular tweezer 3 recognizes N- and C-amide-protected arginine derivatives especially well (Ka ≈ 300 000 in DMSO), because an additional hydrogen bond is formed between the amide and the phosphonate. Since 3 does not bind amines effectively, it is highly selective for arginine, even in the presence of lysine or other amino acids. For di-, tri-, and tetrasubstituted guanidines the association constant remains low (Ka≤1000 in DMSO) reflecting the increase in the steric bulk of the guest.
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  • 9
    ISSN: 0947-6539
    Keywords: chirality ; coordination modes ; macrocyclic ligands ; octahedral complexes ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 10
    ISSN: 0947-6539
    Keywords: calixarenes ; hydrogen bonds ; molecular boxes ; noncovalent assembly ; supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3-bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen-bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen-bonded assembly, but this effect very much depends on the exact positions of the groups.
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