A Combination Of Factors: Tuning The Affinity Of Europium Receptors For Phosphate In Water

Although recognition of hard anions by hard metal ions is primarily achieved via direct coordination, electrostatic and hydrogen-bonding interactions also play essential roles in tuning the affinity of such supramolecular receptors for their target. In the case of Eu-III hydroxypyridinone-based complexes, the addition of a single charged group (-NH3+, -CO2-, or -SO3-) or neutral hydrogen-bonding moiety (-OH) peripheral to the open coordination site substantially affects the affinity of the metal receptor for phosphate in water at neutral pH. A single primary ammonium increases the first association constant for phosphate in neutral water by 2 orders of magnitude over its neutral analogue. The addition of a peripheral alcohol group also increases the affinity of the receptor but to a lesser degree (21-fold). On the other hand, negatively charged complexes bearing either a carboxylate or sulfate moiety have negligible affinity for phosphate. Interestingly, the peripheral group also influences the stoichiometry of the lanthanide receptor for phosphate. While the complex bearing a -NH3+ group binds phosphate in a 1:2 ratio, those with -OH and H (control) both form 1:3 complexes. Although the positively charged Eu-III-Lys-HOPO has the highest K-a1 for phosphate, a greater increase in luminescence intensity (36-fold) is observed with the neutral Eu-III-Ser-HOPO complex. Notably, whereas the affinity of the Eu-III complexes for phosphate is substantially influenced by the presence of a single charged group or hydrogen-bond donor, their selectivity for phosphate over competing anions remains unaffected by the addition of the peripheral groups.