Glyphosate and AMPA binding by two polyamino-phenolic ligands and their dinuclear Zn(II) complexes

We report on the binding abilities towards the popular herbicide N-(phosphonomethyl)glycine (glyphosate or PMG) and its primary metabolite aminomethylphosphonic acid (AMPA), of the two polyamino-phenolic compounds 3,3′-bis[N,N-bis(2-aminoethyl)aminomethyl]-2,2′-dihydroxybiphenyl (L1) and 2,6-bis{[bis-(2-aminoethyl)amino]methyl}phenol (L2). The presence of peculiar biphenol (L1) or phenol (L2) spacers that link two separate dien units confers to ligands interesting anion binding properties but also affords stable Zn(II)-dinuclear complexes (R1 and R2), which act as efficient metallo-receptors for such substrates. Potentiometric studies evidenced the great anion-coordination abilities of L1 and L2, which both preferentially bind PMG over AMPA in a wide range of pH. However, the formation of metal complexes markedly influences their coordination properties, with R1 that displays a maintained capacity to discriminate PMG over AMPA and R2 that shows an opposite binding selectivity. 1H and 31P NMR experiments supported the effective coordination of such substrates by the R systems while fluorescence titrations were performed to assess whether the anion-recognition processes can be detected by fluorescence signalling. This study highlights that the structural characteristics of the metallo-receptors, along with the coordination requirements of their metal centers, are powerful tools that can be finely modulated to achieve suitable systems for the selective recognition/sensing of targeted species in aqueous solution.