Investigation by Mass Spectrometry of Metal Complexes of New Molecular Hosts: Cyclic Oligomer of Sugar Amino Acid and Sugar-Aza-Crown Ethers

Abstract

The affinity of cyclic oligomers of sugar amino acid and sugar-aza-crown ether compounds towards various transition metal cations (Cu(II), Ni(II), Co(II), Fe(II) and Zn(II)) was investigated with positive-ion electrospray mass spectrometry. The binding between the receptors (M) and the different metals (Met) is evidenced mainly by the presence of the [M + Met(II)Cl](+) ion. The experimental results showed that all studied receptors present specificity to Cu(II). An attempt has been made with CuI but no complexation was obtained. The formation of these complexes can be rationalized by considering the presence of two oxygens and two nitrogens on the receptor rim. The lone electron pair can serve as the electron donor to Cu(II). Theoretical calculations were carried out in order to show the structure of the complex and, in particular, to determine if Cu(2+) is situated either on the outer surface, on the rim of the receptor or inside the cavity. Comparison of complex formation was carried out by mixing the four receptors with various amounts of Cu(II) (one equivalent and five equivalents). It appears that the best complexation was obtained with the sugar-aza-crown ethers (amine linker) for both benzylated and methylated compounds. In addition, the stereochemical effects have been investigated.