Argentivorous Molecules with Oxyethylene Chains in Side-Arms: Silver Ion-Induced Selectivity Changes toward Alkali Metal Ions

Argentivorous molecules with mono, di, tri, tetra, and penta-oxyethylene chains in aromatic side-arms were prepared (L1-L5). Titration experiments using proton nuclear magnetic resonance and cold electrospray ionization (cold-spray ionization, CSI) mass spectrometry showed that silver ions were trapped in the cyclen moiety and the arranged oxyethylene chains of the side-arms when two equivalents of silver ions were added. The silver complexes formed by adding one equivalent of silver ion to L2-L5 bind alkali metal ions using the oxyethylene chains; alkali metal ion-induced CSI mass spectral changes of L2-L5 were measured in the absence and presence of silver ions to compare the binding properties of the ligand for Li+, Na+, and K+ ions. As a result, the intensity ratios of [L + H + M](2+)/[L + H](+) in L1-L3 were almost zero or very low. L4 and L5, which have tetra(oxyethylene) and penta(oxyethylene) chains, respectively, bind a larger size of alkali metal ions. On the other hand, in the presence of silver ions, the ratio for [L + Ag + M](2+)/[L + H]+ (M = Li, Na, K) in L2-L5 was increased. The highest [L + Ag + M](2+)/[L + H]+ ratios for K+ were observed in L4 and L5, while selectivity for Na+ was observed in the case of L2 and L3. These results indicate that the increased binding ability and selectivity by L2-L5 are due to the arrangement of oxyethylene chains by the conformational change of the aromatic side-arms. The Ag+-induced carbon-13 nuclear magnetic resonance spectral changes suggested that the second and third oxyethylene units, close to the benzene, are involved in the coordination of the second metal ion.

Automated summary

Argentivorous molecules with aromatic side-arms containing mono, di, tri, tetra, and penta-oxyethylene chains were prepared and their binding properties and selectivity for silver and alkali metal ions were investigated. The results showed that the conformational change of the aromatic side-arms in L2-L5 led to enhanced binding ability and selectivity due to the arrangement of oxyethylene chains.