Lower rim amide (1,3) functionalised calix[4]arene amido-thiourea derivatives as dimetallic Zn(II) coordination complexes for anion recognition/sensing

Abstract

The synthesis of two amido-thiourea based receptors/sensors <b>1</b> and <b>2</b>, formed by functionalisation of the lower rim of a calix[4]arene, and the photophysical analysis of these with Zn(II) and various anions is described. The two structures, differing by the substituent on the thiourea moiety, were found to bind Zn(II) effectively in ethanol (EtOH); the recognition giving rise to significant changes in the UV-vis absorption spectra, which was red-shifted for both chemosensors. The changes in the absorption spectra were further analysed using nonlinear regression analysis programme (using the global analysis software ReactLab™ Equilibria), which revealed high binding constants for both systems. The resulting dimetallic complexes <b>(1,2)–Zn</b>_<b>2</b> were (pre-formed and) then used to coordinate anionic species such as AcO⁻, H₂PO₄⁻ and Br⁻ through electrostatic interactions, showing high affinity towards these anions. In all cases, nonlinear regression analysis showed both 1:1 or 1:2 sensor:anion depending on the anion used. Although the <b>2-Zn</b>_<b>2</b> complex showed high affinity for acetate, phosphate and bromide, the <b>1-Zn</b>_<b>2</b> complex showed selectivity towards the recognition of phosphate in EtOH.