Flexible podand and metal-based hosts for anion binding and sensing

Abstract

A series of amino-pyridyl and aminomethyl-pyridyl ligands, all with both pyridyl and secondary amine functionalities have been synthesised. A new series of organic and metal- based derivatives of these ligands have been prepared, along with the preparation of an organic viologen host, with previously reported urea arms,(^1) and the properties of these new compounds, and in particular their anion binding behaviour have been studied. The 3-aminomethyl-pyridyl ligands interact with transition metals and anions, in both solution and the solid state. They showed a particular affinity for copper(II) salts, and a nitrate sensing system has been prepared using the Ligands fluorescence, through the addition of copper to a solution of the photo-active aminomethyl-pyridyl ligand. However, the active species has proved difficult to characterise. Ruthenium-based derivatives have been shown to form both 1:1 and 2:1 host: guest complexes with a variety of anions. These compounds have also shown an equilibrium in which anion binding promotes time-averaging of diastereotopic methylene protons. The addition of anions to the fluorescent sensing analogues has shown quenching of the fluorescence. Organic systems involving amino-pyridyl ligands around a di- and tri-substituted aryl core bind anions in solution. The fluorescent pyrene-derived host shows 2:1 host: guest binding in NMR spectroscopic studies, which is in contrast to work on aryl derivatives and previous work on anthracene containing hosts,(^2) which has shown 1:1 host: guest binding. Organic viologen compounds with urea arms and a control compound with pyridyl arms, show a colourimetric response to carboxylates, although in the presence of oxygen, decomposition occurs. These hosts also show an affinity for halide anions, which do not bring about this colourimetric response.