Synthesis and cation-binding studies of gold(i) complexes bearing oligoether isocyanide ligands with ester and amide as linkers

Abstract

A series of dinuclear gold(I) isocyanide complexes of bis(alkynyl)calix[4]arene was designed and synthesized, and their photophysical and cation recognition properties were studied. Complex 1, [{calix[4]arene-(OCH₂CONH-C₆H₄CC)₂}{Au(CN-C₆H₄O(CH₂CH₂O)₂CH₃)}₂], was found to show a high selectivity towards Al³⁺ in CH₂Cl₂–MeCN (1 : 1 v/v). Upon addition of Al³⁺, drastic changes in the electronic absorption, emission and ¹H NMR spectra were observed. These changes have been attributed to the formation of Au(I)⋯Au(I) interactions induced by the high binding affinity of the amide site for the Al³⁺ ion, instead of the high binding affinity expected of the oligoether site for alkali and alkaline earth metal ions. Further studies with the control complex, [{calix[4]arene-(OOC-C₆H₄CC)₂}{Au(CN-C₆H₄O(CH₂CH₂O)₂CH₃)}₂] (4), indicated that the amide carbonyl oxygen in the flexible pendants is crucial for the binding of Al³⁺.