Water-soluble β-aminobisulfonate building blocks for pH and Cu2+ indicators

Two water-soluble phenyl beta-aminobisulfonate ligands were synthesised and characterised by spectroscopic techniques including UV-visible absorption, electron paramagnetic resonance (EPR) and H-1 NMR spectroscopy. The acid-base and complexometric binding properties were studied in water and methanol, respectively. Single crystal X-ray crystallography was used to elucidate the solid-state properties. The pK(a)s of the phenyl beta-aminobisulfonate 1 and methoxyphenyl b-aminobisulfonate 2 were evaluated to be 3.1 and 4.4, respectively. UV-visible, EPR and NMR spectroscopy provide direct evidence for complexation of 2 with Cu2+ in methanol due to coordination with the pendant methoxy moiety. The EPR and NMR data of 1 show evidence for some interaction, although no such conclusion could be derived from the UV-visible absorption spectra. The results highlight the potential of phenyl beta-aminobisulfonates as building blocks for developing water-soluble pH and cation chemosensors.