Enhancing the anion affinity of urea-based receptors with a Ru(terpy)2(2+) chromophore

Covalent linking of a Ru(terpy)2(2+) 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)2(2+) 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 (1)H 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)2(2+) substituent favors the deprotonation of a urea N-H fragment on addition of a second equivalent of fluoride, with formation of HF2(-).