Carbamate complexation by urea-based receptors: studies in solution and the solid state

The interactions of a series of urea based neutral hydrogen bond donor anion receptors have been investigated with i) alkylcarbamate anions formed by the reaction of carbon dioxide with primary aliphatic amines and ii) the zwitterionic species formed by the reaction of carbon dioxide with 1,4,5,6-tetrahydropyrimidine. Significant downfield chemical shift changes were observed for the urea NH protons in many cases, consistent with host:anion hydrogen bonding interactions, and thus stabilisation of the carbon dioxide bound species. In the case of the alkylammonium-alkylcarbamate salts, this represents successful competition with electrostatic interactions between the alkylcarbamate and alkylammonium components of the salt. A synchrotron structure of a ternary complex formed by an amide appended diindolylurea, the ammonium carbamate salt formed by 1,3-diaminopropane and CO2 and 18-crown-6, was elucidated and shows the carbamate group bound by six hydrogen bonds (accepting five and donating one) to the functionalised diindolylurea