Through-bond communication between polymer-bound phosphinic acid ligands and trivalent metal ions probed with FTIR spectroscopy

The binding of trivalent metal ions to immobilized ligands is analyzed to determine the principal type of interaction, the extent of ion exchange, and changes due to through-bond communication. FTIR spectroscopy is found to be a readily accessible means of determining through-bond communication and spectra provide support for a molecular level interpretation of the interaction. The affinity of the phosphinic acid ligand for Al(III), La(III), Lu(III), Fe(III), and In(III) is quantified with distribution coefficients from 0.01-6 M nitric acid solutions. High affinities are evident with In(III) > Fe(III) > Lu(III) > La(III) > Al(III). The affinities correlate with the softness parameter and thus the polarizability of the metal ion. The FTIR spectrum of the phosphinic acid has two incompletely separated bands at 1168/1126 cm−1 associated with the PO bond and two similar bands at 967/951 cm−1 due to the PO(H) bond. The bands at 1168/951 and 1126/967 cm−1 are assigned to different hydrogen bonded conformers within the metal-free polymer. Through-bond communication is evident as the extent of metal ion binding increases: the PO and PO bands at 1168 and 951 cm−1, respectively, collapse into a single band between 1040 and 1080 cm−1 representative of PO2(M) while hydrogen bonding among unexchanged P(O)OH ligands gives the bands at 1133 and 981 cm−1.