APTRA-Based Luminescent Lanthanide Complexes Displaying Enhanced Selectivity for Mg²⁺

A series of three europium(III) complexes has been created in which an APTRA moiety has been integrated into the sensitising chromophore (APTRA=o-aminophenol-N,N,N-triacetate). The constitutionally isomeric complexes EuL¹ and EuL² feature the APTRA unit linked to a metal-bound pyridine ring through an alkynyl unit, differing according to the disposition of the APTRA substituents relative to the C≡C unit (para-N and para-O). In EuL³, the APTRA ring is directly bonded to the Eu-coordinated pyridine (para-O). The metal binding affinities for magnesium, calcium and zinc ions have been measured by using emission and excitation spectroscopy. The pyridylalkynylaryl systems, EuL¹ and EuL², offer superior affinity and selectivity for Mg²⁺. The Mg²⁺ affinities are surprisingly very different from prior studies on structurally related systems that incorporate organic fluorophores as reporters, as opposed to the macrocyclic Eu complex moiety. A much-reduced affinity for calcium and zinc—possibly arising from the lower donor ability of the aryl N or O atoms arising from extended conjugation—means that magnesium ion concentrations can be measured directly in serum for the first time, by using such an approach. An apparent dissociation constant for magnesium binding of K_d=2.4 mm was calculated in the serum background.