Structural investigation of Fe(III)-salen complexes as "turn-on" fluorogenic probes for selective detection of pyrophosphate ions

In this work, we revisited the disassembly approach (also known as the Zelder's approach) recently proposed for sensing pyrophosphate (PPi) in water and based on the decomposition of metal-salen complexes. A systematic study devoted to the structural optimization of this novel class of PPi-responsive fluorogenic probes was conducted. Screening of eight different vicinal diamines (i.e., bridge of the salen ligand) combined with the use of 8-formyl-7-hydroxycoumarin (i.e., salicylaldehyde derivative) as the fluorescent reporter, has led to a set of novel and fully characterized coumarin-salen Fe(III) complexes. A series of analytical validations helped us to identify that coumarin-salen Fe(III) complexes derived from ethylenediamine and racemic 1,2-propylenediamine backbones exhibit the best and selective PPi-sensing performances (the limits of detection were estimated as 3.15 × 10-6 M and 2.81 × 10-6 M respectively). The implementation of both fluorescence time-course measurements and RP-HPLC-fluorescence analyses has enabled us to gain further insights into the disassembly-based probes' activation mechanism. This study therefore contributes to demonstrate that the disassembly approach is a valuable strategy to achieve fluorogenic activity-based sensing of anions.