Unusual specificity of a receptor for Nd³⁺ among other lanthanide ions for selective colorimetric recognition

Abstract

A rare example of a reversible recognition of Nd³⁺ by a newly synthesized molecular receptor (L₁), having a diazo group as the reporter functionality, is reported. Studies revealed that this receptor eventually forms a [Nd³⁺]₂L₁ type complex, through the formation of the intermediate complex [Nd³⁺]· L₁ following a two-step equilibrium process. Respective equilibrium constants for two successive processes were evaluated based on data obtained from the systematic fluorescence titration. Formation of [Nd³⁺]₂L₁ caused a detectable change in color, and associated affinity constants were also evaluated from spectrophotometric titration data. A rather unusual binding mode of L1 for Nd³⁺ ion is established by various spectroscopic studies. The remarkable specificity of L₁ for Nd³⁺, constitutes a rare example of a highly selective receptor for this ion in the presence of excess of all other lanthanide ions.