Exploring new paths towards biological anion recognition by lanthanide complexes

Anions are ubiquitous in nature. In order to understand them in their natural setting, this thesis focusses on developing lanthanide complexes as hosts for the recognition of biologically relevant anions, particularly chloride and sialic acid. Chapter-I introduces lanthanide chemistry encompassing macrocyclic complexes, luminescence, and MRI. An account of anion recognition and lanthanide complexes used as hosts for biologically relevant anions are reviewed. Chapter-II focuses on developing receptors for chloride recognition. The concept of binuclear lanthanide complexes, bridged by flexible ethane/propane spacer, for chloride recognition is envisioned. The proposed complexes are synthesised, and their anion binding properties are studied by luminescence, NMR spectroscopy, and X-ray crystallography. The study demonstrates the versatility of these binuclear complexes as the strongest chloride binding receptor to function in water and buffers to date. Thus, the work described here, hits a new milestone in anion coordination chemistry, ameliorating detailed exploration. Chapter-III builds on the drawbacks from these neutral complexes towards anion binding, and proposes tryptophan-conjugated cationic binuclear complexes for halide recognition. Initial studies show promising results towards halide recognition in water from this cationic coordination complex. Chapter-IV revisits a ditopic approach of arylboronate-conjugated lanthanide chelates for sialic acid recognition, a tumour biomarker. However, the complexes suffer from protodeboronation, leading to the cleavage of boronic acid which is reported for the first time with structural evidence. The resulting boron free complex binds to L-lactate and D-gluconate. Chapter-V employs the ditopic approach, but with the incorporation of rigid anthracene and dansyl spacers between them, to improve the stability of the arylboronate during complexation. However, the anthracene-based receptor proved synthetically challenging. The success in making the dansyl-functionalised ligand extents the possibility of investigating sialic acid binding. Chapter-VI summarises the work embodied in this thesis and Chapter-VII describes the complete experimental procedures followed in accomplishing the results reported in Chapters II-V.