An Oxygen-Nitrogen Donor Macrocycle Immobilized on Silica Gel. A Reagent Showing High Selectivity for Copper(II) in the Presence of Cobalt(II), Nickel(II), Zinc(II) or Cadmium(II)

Abstract

A 17-membered O₂N₃-donor macrocycle has been appended to a derivatized silica gel substrate, and the binding and selectivity properties of the functionalized surface towards CO¹¹, Ni¹¹ CU¹¹, Zn¹¹ and Cd¹¹ in aqueous solution have been investigated. Metal uptake was found to be concentration-dependent with the maximun (saturation) loading capacity in all cases being less than the 'theoretical' amount based on the calculated degree of polymer functionalization present. Metal loading and leaching both occurred readily in aqueous media; the loading studies were performed at near-neutral pH whereas metal leaching was carried out by using either dilute (hydrochloric) acid or aqueous ammonia solution. The functionalized silica underwent no degradation during repeated loading/leaching cycles, and, under similar conditions, its binding capacity is highest for CU¹¹ relative to the other ions investigated. The observed binding order was CO¹¹ < Ni¹¹ < CU^II > Zn^IIand this sequence parallels the respective log K values for the complexes of the free macrocycle. However, the loading order of Zn^II > Cd^II which was observed is the opposite to the corresponding log K values for the unattached complexes. Steric effects involving the silica backbone appear to influence the binding in this case. The insolubilized product may be used to remove selectively Cu^II in the presence of the other ions mentioned above.