New branched macrocyclic ligand and its side-arm, two urea-based receptors for anions: Synthesis, binding studies and crystal structure

The synthesis and characterization of the two new hosting molecules for anions are reported. The first one(L1) is a branched tetraazamacrocycle bearing two p-nitrophenylureido groups as side-arms, whereas the second one(L2) has the same linear chain and binding moiety of L1 side-arm. The binding properties of both ligands towards the halides series and acetate anions (G) were investigated by NMR and UV-Vis spectroscopy in a dimethyl sulfoxide–0.5% water solution. Both ligands interact with F-, Cl- and AcO- while Br- and I- did not. The NMR experiments proved that the binding occurs viaH-bond to the ureido fragments. Fluoride anion is basic enough to deprotonate the ureido group of both ligands, thus preventing the determination of the addition constants to both ligands. L1 forms G–L species of 1 : 1 ([GL1]) and 2 : 1 ([G2L1]) stoichiometry while L2 forms only the 1 : 1 [GL2] species. The higher value of the formation constant of the [AcO1]- vs. the [AcOL2]- species (log K = 5.5 vs. 2.8 for the reaction AcO- + L = AcOL-) suggested that both side-arms of L1 cooperate in binding acetate; this does not occur with Cl-.The results confirmed that this tetraaza-macrocyclic base acts as a preorganizing scaffold for side-arms when they are linked to it via an amide function. The crystal structure of L2_H2O is also reported