title: Cyclic tetraureas with variable flexibility--synthesis, crystal structures and properties.
abstract: Macrocyclic molecules containing several amide or urea functions may serve as anion receptors. We describe the synthesis of 32-membered macrocycles, in which four rigid xanthene units (X) and/or diphenyl ether units (D) as flexible analogues are linked via urea groups. All six possible combinations of these units (XXXX, XXXD, XXDD, XDXD, XDDD and DDDD) were synthesized and two examples were characterised by single-crystal X-ray analyses (DDDD and two structures for XXXD). Both macrocycles showed distinct differences in their overall conformation and consequently in their hydrogen-bonding pattern. Hydrogen-bonded solvent molecules are found for both compounds and intramolecular hydrogen bonds for the two structures of XXXD, but surprisingly no direct intermolecular hydrogen bonds between the macrocyclic tetraurea molecules. The interaction with various anions was studied by (1)H NMR spectroscopy. Stability constants for all tetramers were determined by UV spectroscopy for complexes with chloride, bromide, acetate and dihydrogenphosphate in acetonitrile-THF (3:1). The strongest binding was found for XXXD and acetate (log beta = 7.4 +/- 0.2), the weakest for XXXX and acetate (log beta = 5.1 +/- 0.5). MD simulations in chloroform and acetonitrile boxes show that all molecules except DDDD adopt very similar conformations characterized by an up-down-up-down arrangement of the spacer groups. Clustered solvation shells of acetonitrile molecules around XXXX and DDDD suggest their preorganization for spherical/planar and tetrahedral/bidentate anions, respectively, which in turn was corroborated by simulation of the corresponding complexes with chloride and dihydrogenphosphate.
authors.affiliation: Fachbereich Chemie, Pharmazie und Geowissenschaften, Abteilung Lehramt Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, D-55099, Mainz, Germany.
authors.forename: Denys
authors.initials: D
authors.lastname: Meshcheryakov
authors.forename: Françoise
authors.initials: F
authors.lastname: Arnaud-Neu
authors.forename: Volker
authors.initials: V
authors.lastname: Böhmer
authors.forename: Michael
authors.initials: M
authors.lastname: Bolte
authors.forename: Julien
authors.initials: J
authors.lastname: Cavaleri
authors.forename: Véronique
authors.initials: V
authors.lastname: Hubscher-Bruder
authors.forename: Iris
authors.initials: I
authors.lastname: Thondorf
authors.forename: Sabine
authors.initials: S
authors.lastname: Werner
mesh_terms: D003198:Computer Simulation; D018360:Crystallography, X-Ray; D003500:Cyclization; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry; D014508:Urea / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D014508:Urea; D009584:Nitrogen