Synthesis and Co-ordination chemistry of Allosteric systems and Sensors for Zinc Metal Ions

Abstract

The ditopic ligand, L1, contains both a bipyridine domain and a tetra-azacrown binding domain. Introduction of one equivalent of Cu2+ ions results in co-ordination of the ion by all four of the available N-donor atoms in the azacrown unit. However, introduction of further Cu2+ ions results in the formation of a variety of species depending on the ratio of Cu2+ ions to ditopic ligands available. The bipyridine site is capable of behaving as a bidentate chelator as co-ordination at the bipyridine unit by a further Cu2+ ion allosterically alters the behavior of the azacrown domain. The co-ordination of a Cu2+ ion at the bipyridine unit causes the unit to adopt a planar conformation, preventing all four available N-donor atoms in the tetra-azacrown from co-ordinating an encapsulated copper ion. Therefore coordination of the bipyridine domain changes the tetra-azacrown unit from a tetradentate to a tridentate N-donor unit. A diamino-functionalised cryptate, L2, was synthesized, which is capable of reacting irreversibly with butanal in aqueous media, when in the presence of an excess of metal ions. Co-ordination of the cryptate unit is required in order for the cyclisation to occur, excess metal ions act as Lewis acids in order to promote the reaction, and also co-ordinate the cryptate ion. Co-ordination of the cryptate ion forces the bipyridine unit into a more planar position, allowing the amine groups to move into closer proximity to each other. Subsequent reaction with butanal induces the ligand to form a cyclised bis-aminal complex, as the amino groups react readily with the aldehyde to form a seven-membered bis-aminal species. The cyclised ligand was found to display metal dependent luminescent properties, with zinc ions producing the most intense increase in emissive properties by a significant margin as well as increasing the luminescent lifetime. A potentially hexadentate ligand, L3, was synthesized, which upon co-ordination with dicationic metal ions forms dinuclear double stranded helicate species. The ligand partitions into two separate tridentate binding domains, consisting of two N-donor atoms from a pyridyl-triazole unit and an O-donor atom from a carbonyl oxygen group on the coumarin unit. The ligand contains two coumarin fluorophores, and the luminescent properties of the dinuclear helicate complexes were investigated. It was found that upon co-ordination of Zn2+ the emissive properties were enhanced in comparison with the parent ligand, however introduction of Co2+, Cu2+, Cd2+ and Hg2+ ions induced fluorescent quenching of varying degrees