Browsing by Author "Demleitner, B."

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    Enantiomerisation of tetrahedral homochiral [M4L6] clusters: Synchronised four Bailar twists and six atropenantiomerisation processes monitored by temperature-dependent dynamic H-1 NMR spectroscopy
    (Wiley-v C H Verlag Gmbh, 2002)
    Saalfrank, Rolf W.
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    Demleitner, B.
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    Glaser, H.
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    Maid, H.
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    Bathelt, D.
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    Hampel, F.
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    Bauer, W.
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    Teichert, M.
    Temperature-dependent H-1 NMR studies prove homochiral, racemic [(Delta,Delta,Delta,Delta)/(Lambda,Lambda,Lambda,Lambda]-[(NH4)(4)boolean AND[Mg-4(L-1)(6)]} (1) to be kinetically stable on the NMR timescale. Due to steric reasons, rotation around the central C-C single bond in (L-1)(2-) is blocked, which prevents 1 from enantiomerisation. Most interestingly, however, the H-1 NMR spectrum of racemic 2a reveals dynamic temperature dependence. This phenomenon can be explained by simultaneous Bailar twists at the four octahedrally coordinated magnesium centres, synchronised with the sterically unhindered atropenantiomerisation processes around the C-C single bonds of the six ligands (L-2)(2-), leading to the unprecedented enantiomerisation (Delta,Delta,Delta,Delta)-2a reversible arrow (Lambda,Lambda,Lambda,Lambda)-2a. The profound nondissociative rearrangement occurs without the formation of diastereoisomers. Supplementary support for the interpretation of the temperature-dependent dynamic H-1 NMR spectra of 2 a is presented by additional studies of [(Delta,Delta,Delta,Delta)/(Lambda,Lambda,Lambda,Lambda]-{EtNH3)(4)boolean AND[Mg-4(L-2)(6)]} (2b). In 2 a and 2b, the ether methylene protons exhibit identical temperature dependence. However, with addition, the methylene protons of the ethyl ammonium groups of 2b display similar temperature dependence as the ligand ether methylene protons.
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    Synthesis of tetrahemispheraplexes with ammonium, alkylammonium or alkaline metal ions as exohedral guests via self-assembly or guest exchange
    (Wiley-v C H Verlag Gmbh, 2003)
    Saalfrank, Rolf W.
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    Demleitner, B.
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    Glaser, H.
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    Maid, H.
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    Reihs, S.
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    Bauer, W.
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    Maluenga, M.
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    Hampel, F.
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    Teichert, M.
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    Krautscheid, H.
    To study the interplay between the steric demand of the guest and the space available at the host, we have prepared the tetraammonium complexes {(NH4)(4)boolean AND[Mg-4(L-1/2)(6)]} 1, tetrakis(methylammonium) complexes {(Me-NH3)(4)boolean AND[Mg-4(L (1/2))(6)]} 2, tetrakis(ethylammonium) complex {(Et-NH3)(4)boolean AND[Mg-4(L-1)(6)]} 3, and mixed sodium tris(ethylammonium) complex {Na(Et-NH3)(3)boolean AND(Mg-4(L-2)(6)]} 4. The tetrahemispheraplexes 1-4 were available in one-pot syntheses. The tetrakis(alkylammonium) clusters {(R-1-NH3)(4)boolean AND[Mg-4(L-1)(6)]} 5 and the mixed diammonium-bis(p-methoxybenzylammonium) cluster {(NH4)(2)(R-2-NH3)(2)boolean AND[Mg-4(L-1)(6)]} 6, however, were prepared by exchange of the ammonium ions of la by alkylammonium ions, Similarly, exchange of the four ammonium ions of 1b by alkaline metal cations afforded the tetraalkaline metal tetrahemispheraplexes {M(4)boolean AND[Mg-4(L-2)(6)]} 7. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).