Synthesis and metal extraction studies of a novel chromogenic 5,17-bisazocalix[4]arenes

Three novel chromogenic cone azocalix4]arenes 5a–c, which have cavity and the azo groups as metal-binding sites and as coloration sites were synthesized. They were studied by the liquid–liquid extraction of selected metal cations (Sr²⁺, Ag⁺, Hg⁺, Hg²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cr³⁺, Al³⁺). Through examination of the extraction, a novel selectivity of these compounds toward Hg²⁺ cations has been determined. Besides, it has been also found that azocalix4]arene 5c is highly sensitive to acid–base titration, which can be detected by the naked eye.