A new macrocyclic polylactam-type neutral receptor for anions - Structuralaspects of anion recognition

A macrocyclic ligand containing four amide functionalities was synthesised and its complexes with various anions (F-, Cl-, AcO-, H2PO4-, and p-NO2C6H4 O-) were investigated in solution and in the solid state. NMR titration exp eriments (carried out in [D-6]DMSO), X-ray studies and electrospray mass sp ectrometry (ESI MS) were employed for determination of stoichiometry and se lectivity. The results in solution indicated predominant formation of 1:1 c omplexes for all anions studied. However, the existence of a 2:1 complex of 1 with bidentate AcO- anion as a minor species was also detected. X-ray cr ystal structure determination provided evidence that the Cl- anion is too b ulky to be included in the cavity of the 18-membered tetralactam ring [K-as s(Cl-; DMSO) = 65 M-1], but that F- fits well [K-ass(F-; DMSO) = 830 M-1]. The binding mode of AcO- anion consists of formation of four hydrogen bonds to only one of the carboxylate oxygen atoms (employing the syn-anti lone p airs of AcO-). The selectivity of the receptor towards AcO- anion [K-ass(Ac O-; DMSO) = 2640 M-1] is discussed in terms of a favourable arrangement of hydrogen-bond donors. The limited extent of formation of a 2:1 (1/AcO-) com plex is attributed to unfavourable interactions between two receptor molecu les.