The role of polarizability in determining metal ion affinities in polymer-supported reagents: Phosphorylated ethylene glycol

This is the first of a series of reports that probes the polarizabilities of different immobilized ligands in order to build toward a rational design of ion-selective polymer-supported reagents. In this report, the polarizabilities of four ligands on a polystyrene backbone (phosphorylated ethylene glycol (pEG1), ethylene glycol (EG1), sulfonic acid, and dimethylamine (DMA)) are evaluated with Au(III) and Eu(III) as representative of soft and hard ions, respectively. Interactions are followed by the extent of sorption, the effect of acid concentration, band shifts in FTIR spectra, and Langmuir plots. The phosphoryl oxygen, hydroxyl OH, and amine show an affinity of Au(III) > Eu(III) while Eu(III) > Au(III) is evident with the sulfonic acid. The PO stretching frequency shifts from 1263 to 1249 cm−1 upon Au(III) binding. The affinities indicate that oxygen can be considered soft and hard, depending on its microenvironment: neutral oxygen is soft while anionic oxygen may be classified as hard. Though Eu(III) prefers ionic binding with a strong acid ligand, it is also capable of weak binding with a neutral oxygen donor at pH 3 and 4. Sorption is affected by solution acidity despite the absence of ion exchange. Three routes to enhancing ion-interactions thus emerge: preparing single-site ligands whose polarizability matches the targeted metal ion, preparing ligands with multiple binding sites each of which bind weakly on their own, and preparing single-site ligands with dual ion exchange/coordination character.