Oligomere Harnstoffe - Synthese, Konformation und Anionenbindung

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Urea functions (R-NH-CO-NH-R) are able to form two hydrogen bonds to the same acceptor and have been used as binding sites in various anion receptors of the podand type. If three urea functions are incorporated in a suitably sized macrocycle anions should be bound by six convergent R-NH···X hydrogen bonds as indicated schematically for nitrate. High binding energies are predicted on the basis of first MD simulations for trigonal planar anions such as Nitrate or Carbonate and especially for "tripodal" anions such as Sulfonate and Phosphonate. In the frame of this project various macrocylic triureas of this general type will be synthesised, since subtle changes of the size or shape of the spacer groups will most probably have drastic influences on the complexation behaviour. The design of these ligands will be based primarily on MD simulations, and secondly on reasonable modifications of these "lead structures". Additional substituents extending the outside of the macrocycle will achieve solubility either in apolar or aqueous solutions and/or create chromo- or fluorophoric properties. Binding constants for the range of interesting anions will be determined, using techniques like NMR-titration, UV-Vis spectrometry or conductimetry. Calorimetric measurements will complete the set of thermodynamic data for the complexation. The results thus obtained will be compared again with calculated values, to understand the complexation in detail and to check the validity of the theoretical/mathematical model. In selected cases we will also try to measure the complexation/decomplexation kinetics (dynamic NMR and stoppped-flow techniques). For comparison we will synthesise and study linear analogues (tri- to hexa-ureas), which should assume a helical conformation, eventually induced by the complexed anion(s). These studies might be extended finally to the construction of helical poly-ureas.