Lanthanide Fluorobenzoates as Bio‐Probes: a Quest for the Optimal Ligand Fluorination Degree

Abstract

The thorough study of fluorinated benzoates of lanthanides (Eu, Tb, Nd, Er, Yb, Gd, La, Lu) is reported. Their composition in single crystal and powder state revealed two predominant structural motifs. An in-depth luminescence study has been performed on the reported fluorobenzoates, showing, that terbium and europium complexes in solid state possess high luminescence intensity with the quantum yield of up to 69 %. High solubility in most organic solvents, as well as in water, combined with the high luminescence intensity in water solution and non-toxicity allowed the testing of europium complexes as bioprobes in cellulo. Among all tested fluorobenzoates, europium 2-fluorobenzoate dihydrate combined the best luminescent properties, thermodynamic stability, aqueous solubility, and non-toxicity, and was shown to be a viable bio-marker.

Metal Complexes as a Promising Source for New Antibiotics

“…67,68 It has good luminescent properties which make it suitable for applications in cellular imaging, immunoassays and organic light-emitting diodes. [69][70][71] The zinc complex 26 is related to a class of macrocyclic metal complexes recently shown to have promising activity against Mycobacterium tuberculosis, 72,73 and was synthesized as part of a broader project to develop functionalized macrocycles for biological applications. 74,75 The unusual structure of these compounds makes them interesting scaffolds for further studies into their biological behaviour.…”

On the Structural Features of Substituted Lanthanide Benzoates

“…Studying the possibility to affect nuclearity, it is also important to pay attention to the substituents in the benzoate core that hampers the polymer formation due to certain substituents may also affect luminescence properties. Among the mostly reported are NO 2 group, that dramatically quenches luminescence,, , and N 3 group was recently reported to be equally effective quencher . OH‐ and NH 2 groups are also known to quench lanthanide‐based luminescence …”

“…Pentafluorobenzoic acid (Hcarb 14 ) demonstrates p K a of 1.6–1.8 . This results in the extreme lability of the complexes with fluorobenzoate‐anions: their composition depends not only on the lanthanide ion and the synthesis conditions, but also on the crystallization conditions , , , , , …”

“…Decrease of the number of the fluorine substituents (carb = carb 15 ‐carb 22 ) results in less variability of the single crystal structures , . Analysis of the complexes with the fluorobenzoate anions (Figure ) revealed that most of the studied complexes can be divided into two groups by the type of coordination environment of the lanthanide ion in a crystal (Figure ): with two ortho ‐substituents (n ortho ) in the ligand (Figure a) and with one n ortho (Figure b).…”

“…On the one hand it allows almost endless possibilities to tune their structures, and, hence, their luminescence properties, while on the other hand it dramatically hampers predicting their structures and even composition (e.g. water content) . This paper aims on the accumulating of the existing structural data of the lanthanide benzoates and the revealing of the general structure–property relationship trends based on this data.…”