Impact of varying the phenylboronic acid position in macrocyclic Eu(iii) complexes on the recognition of adenosine monophosphate

The selective recognition of anions in water by artificial receptors remains a significant research challenge. The creation of a receptor selective for adenosine monophosphate (AMP) is particularly difficult due to its similarity in structure with the more negatively charged anions, ADP and ATP. We recently developed a macrocyclic Eu(III) complex that selectively binds AMP in water, by incorporating a sterically demanding quinoline arm that inhibits coordination of commonly interfering anions such as ATP. A phenylboronic acid motif was installed within the ligand to engage the ribose sugar of AMP through reversible covalent bonds. Herein we report two new Eu(III) complexes, [Eu·oBOH₂]⁺ and [Eu·pBOH₂]⁺, to investigate the impact of varying the position of the phenylboronic acid group on the anion binding properties of the Eu(III) receptors. We found that [Eu·pBOH₂]⁺ showed preferential binding to AMP over ATP, but exhibits a lower level of discrimination between AMP and ADP compared with the isomeric complex [Eu·mBOH₂]⁺. Surprisingly, [Eu·oBOH₂]⁺ showed no response to anions but displayed a unique response to pH, ascribed to the direct coordination of the ortho-boronate ester to the Eu(III) centre. Finally, we present first principles computations that offer a promising approach to access the emission spectra of lanthanide complexes, aiding the design of responsive lanthanide probes with specific photophysical properties.