Transition metal cation separations with a resorcinarene-based amino acid stationary phase

Abstract

A resorcinarene-based macrocyclic ligand functionalized with alanine and undecyl groups (AUA) was synthesized and applied to ion chromatographic separations. The selectivity and separation of transition metal ions on a column packed with AUA adsorbed onto 55% cross-linked styrene–divinylbenzene resin are presented. The upper and lower rims of the resorcinarene were modified with amino acids and –C₁₁H₂₃ alkyl chains, respectively. The four carboxylic acid groups on the upper rim act as cation-exchangers while the four –C₁₁H₂₃ alkyl chains serve to anchor the ligand to the resin surface by the hydrophobic effect. A systematic study of the effect of different eluent components including non-metal-chelating (HNO₃) and chelating acids (oxalic acid, succinic acid, dipicolinic acid, and citric acid) on the retention of transition metal ions was investigated. Six metal ions (Mn²⁺, Co²⁺, Ni²⁺, Cd²⁺, Cu²⁺, and Zn²⁺) were separated on the AUA column within a reasonable time with a single eluent gradient using oxalic acid. The separation is compared to that obtained using a commercial column containing carboxylic acid functional groups. The AUA column containing four preorganized carboxylic acid groups showed selectivity for Cu²⁺ when no chelating eluent was present, a selectivity which was not observed with the comparison column.