Intraligand cooperation in metal-ion binding by immobilized ligands: The effect of bifunctionality



Intraligand cooperation between carbonyl and phosphoryl moieties in ion bonding was studied by the synthesis of β-ketophosphonic acid (βkPh), phosphonoacetic acid (PhAc), and phosphonic acid (Ph) resins. A subsequent reaction gave bifunctional analogues with sulfonic acid as an additional ligand (βkPhs, PhAcs, and Phs resins). Ionic affinities were quantified with dilute solutions of Cu(II), Cd(II), and Pb(II) in 0.10M HNO3. The effect of an increased solution ionic strength was studied with a second set of 0.10M HNO3/0.04M NaNO3 solutions. The results from the monofunctional resins and nitric acid alone showed that βkPh had the highest metal-ion affinities, whereas PhAc had affinities similar to those of Ph. Comparing βkPh to Ph led to the conclusion that the carbonyl group was a strong contributor to binding through intraligand cooperation with the phosphoryl group, whereas comparing PhAc to Ph led to the conclusion that the carbonyl group played no role in the ion binding. These opposing conclusions were reconciled as follows: (1) the extent of complexation from the 0.10M HNO3 solutions increased significantly when the monofunctional resins were sulfonated and (2) a comparison of the results for the monofunctional resins when contacting Cu(II), Cd(II), and Pb(II) in the absence of NaNO3 with those for the bifunctional resins when contacting those ions in the presence of NaNO3 showed that the ionic affinities were comparable for Ph and Phs, greater for PhAcs than for PhAc, and comparable for βkPhs and βkPh. The resins PhAcs and βkPh displayed comparable ionic affinities that were higher than that of Ph. The results were consistent with the conclusion that, in the monofunctional resin, the affinity of the phosphonoacetate ligand for the metal ions was reduced because of intraligand hydrogen bonding. Intraligand cooperation was, therefore, an important variable in enhancing the metal-ion affinities, but its effect could be attenuated by intraligand hydrogen bonding. This attenuation could be eliminated by the introduction of a highly hydrophilic group into the matrix. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 463–468, 2004