RESUMO
Enantiopure alleno-acetylenic ligands, containing two chiral allene moieties, assemble diastereoselectively with zinc(II) ions to form trinuclear triple-stranded helicates, featuring two internal cavity binding sites ("helicages"). The addition of cycloalkanes or heteroalicycles results in inclusion complex formation with two guest molecules bound in one helicate. While no positive allosteric effects were observed, the chiroptical responses increased strongly upon complexation, with guest-induced circular dichroism (ICD) signals reaching up to ΔΔϵ=205 m-1 cm-1 . The highest binding affinity was observed for six-membered 1,4-dichalcogens, with binding constants (K1 ) of up to 2400 m-1 for 1,4-oxathiane in CD3 OD. The X-ray co-crystal structure of 1,4-dioxane bound to a dinuclear triple-stranded helicate confirmed the previously predicted guest orientation inside the helicage, with the two oxygen atoms facing towards the electropositive zinc(II) metal centers.
RESUMO
Enantiopure alleno-acetylenic ligands assemble diastereoselectively upon the addition of a zinc(II) salt to form triple-stranded helicates, which provide a sufficiently large helical cage ("helicage") for the encapsulation of guests. The inclusion complexation of heteroalicycles is confirmed by ROESY and DOSY NMR spectroscopy and quantified in (1) Hâ NMR binding titrations. The ECD spectra of the helicates, which showed strong Cotton effects and exciton coupling, were found to be extremely sensitive to the nature of the guest molecules. Consequently, a series of nonchromophoric, achiral guests of different sizes as well as regioisomers (1,3- and 1,4-dioxane) became distinguishable on the basis of their induced CD (ICD) spectra. Molecular dynamics (MD) simulations show the adaptability of the cavity size to individual guest molecules and support the selective ICD output. Particularly high affinity towards 1,4-dioxane allowed its selective detection at parts-per-million (ppm) levels in aqueous solutions.