Binding of Divalent Transition Metal Ions to Immobilized Phosphinic Acid Ligands. Part I. Characterization by Fourier Transform Infrared Spectroscopy

This report introduces a multi-part study on determining the fundamental mechanisms by which a range of phosphorus-based ligands bind metal ions. FTIR spectroscopy is utilized to propose the mechanism; in the subsequent paper, density functional theory is applied to interrogate the spectra and identify a definitive mechanism. In this report, we follow the binding of divalent metal ions (Pb(II), Cd(II), Cu(II), Zn(II), Ni(II)) to phosphinic acid immobilized on crosslinked polystyrene. Important issues include identifying the moieties involved in forming the phosphinate – metal bond, and whether metal ion valency affects the outcome. Phosphinic acid has bands at ≈1100 cm−1and 950 cm−1, each subdivided into two bands. Upon binding the metal ion, two new bands appear at ≈1000 cm−1 while only one band in each of the two sets shifts. An affinity sequence of Pb(II) > Cd(II) > Cu(II) > Zn(II) > Ni(II) follows the order of increasing ionic index indicating a dominant ion-exchange mechanism, unlike trivalent ions which follow an order of decreasing polarizability. FTIR spectra confirm that a higher than expected affinity for Cu(II) is due to a coupling of the ion-exchange mechanism to a redox reaction that results in the formation of Cu(0) and conversion of the phosphinic ligand to phosphonic acid, as indicated by a pronounced band at 926 cm−1 which is also present in the spectra of separately prepared phosphonic acid and Cu-loaded phosphonic acid. The behavior of the FTIR bands indicates that the phosphinic acid ligand follows the same mechanism with divalent and trivalent ions. DFT subsequently addresses: the bonds that give the metal phosphinate band, the lack of movement of bands not involved in the metal phosphinate, and hydrogen bonding among ligands.