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

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.