Enhancing
the Anion Affinity of Urea-Based Receptors with a Ru(terpy)<sub>2</sub><sup>2+</sup> Chromophore
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Abstract
Covalent linking of a Ru(terpy)<sub>2</sub><sup>2+</sup> 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<sup>II</sup> 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)<sub>2</sub><sup>2+</sup> 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 <sup>1</sup>H NMR titration experiments, carried
out under more concentrated conditions, the interaction of a second
Cl<sup>–</sup> ion was observed, taking place through an outer-sphere
mechanism. The Ru(terpy)<sub>2</sub><sup>2+</sup> substituent favors
the deprotonation of a urea N–H fragment on addition of a second
equivalent of fluoride, with formation of HF<sub>2</sub><sup>–</sup>