Affinity of two highly preorganized ligands for the base metal ions Co(II), Ni(II) and Cu(II): A thermodynamic, crystallographic and fluorometric study

The metal ion complexing properties of 1,10-phenanthroline-2,9-dicarboxylic acid (PDA), with cobalt(II) and nickel(II) are reported. These were determined by UV–Vis spectroscopy in 0.1 M NaClO4 at 25 °C and the log K values are 12.6 ± 0.05 and 13.6 ± 0.05 respectively. These two values are compared to that of the previously reported value of copper(II), as well as the previously reported values of the ligand 2,9-bis(hydroxymethyl)-1,10-Phenanthroline (PDALC). Crystal structures of {[Ni(PDALC)2]Cℓ2}2·5H2O, {[Co(PDALC)2]Cℓ2}2·5H2O and {[Ni(PDAMOD)2](H2O)2}2 are reported, all showing distorted octahedral geometry around the metal centre. Furthermore, all three structures have four nitrogens and two oxygen atoms from alternate ligands coordinated to the metal ion. The crystal structure of the free ligand PDALC is also reported showing π–π stacking, hydrogen bonding with the solvent and also the extent of its preorganization. Lastly, a fluorescence study of divalent metal ions with these two ligands was carried out which included the three metal ions of interest. These results illustrate that Ca(II) produces the largest CHEF (chelation enhanced fluorescence) effects, followed by Mg(II) and Ba(II) in the PDA complexes. Metal ions with non-filled d-subshells produce very large quenching of the fluorescence. For the PDALC complexes, the largest CHEF was with Zn(II), followed by Ca(II), then Mg(II) which is the same as that of Ba(II), then closely followed by Cd(II). Both ligands show quenching effects with Co(II), Ni(II) and Cu(II) which are in the order: Co(II) > Ni(II) > Cu(II) for both ligands.

Stability constants of PDA with Co(II) and Ni(II) are determined. The structures of [Ni-(PDAMOD)2]n+, [Ni-(PDALC)2]n+ and [Co-(PDALC)2]n+ as well as the free ligand PDALC are determined and shown. Fluorescence measurements with a number of M2+ ions are determined and discussed.Figure optionsDownload as PowerPoint slide