Enhancing the Anion Affinity of Urea-Based Receptors with a Ru(terpy)₂²⁺ Chromophore

Abstract

Covalent linking of a Ru­(terpy)₂²⁺ substituent improves recognition and sensing properties of the urea subunit toward anions. Urea’s anion affinity is enhanced by the electrostatic attraction exerted by the Ru^II cation and by the electron-withdrawing effect exerted by the entire polypyridine–metal complex. Such an enhancement of the anion affinity, which results from the combination of a through-space and a through-bond effect, is greater than that exerted by the classical neutral electron-withdrawing substituent nitrophenyl. Small yet significant modifications of π–π* and MLCT bands of the Ru­(terpy)₂²⁺ chromophore, detected through UV–vis spectrophotometric titrations, allowed the determination of the constants for the formation of receptor–anion H-bond complexes in diluted MeCN solution. On ¹H NMR titration experiments, carried out under more concentrated conditions, the interaction of a second Cl^– ion was observed, taking place through an outer-sphere mechanism. The Ru­(terpy)₂²⁺ substituent favors the deprotonation of a urea N–H fragment on addition of a second equivalent of fluoride, with formation of HF₂^–