Anion-recognition studies of a Re(i)-based square containing the dipyridyl-amide ligand

Abstract

The molecular square [Re(CO)₃Cl(L)]₄ (1) containing the dipyridyl-amide ligand, N,N′-4-dipyridyloxalamide (L), was constructed from Re(CO)₅Cl and L for the purpose of anion-recognition studies. Upon addition of fluoride anions to a THF solution of 1, a remarkable spectral change is observed, and indeed a new absorption band grows at ca. 348 nm. We reasoned that upon addition of fluoride, the hydrogen bonds between F⁻ and –NH groups of L would first form and increase the electron densities of nitrogen atoms. This in turn increases the conjugation throughout the L ligand, which is responsible for the new growing absorption band. Finally, a proton-transfer process occurs upon addition of excess F⁻ anions, corroborated by the ¹H NMR titration experiment due to the occurrence of HF₂⁻. The binding constants based on a 1 ∶ 1 complex (1–X⁻, X⁻ = anions) follow the order: F⁻ > CN⁻ > OAc⁻ > Cl⁻ > Br⁻, PF₆⁻, BF₄⁻, ClO₄⁻, NO₃⁻ and HSO₄⁻. The most electronegative F⁻ anion shows the largest binding constant, followed by CN⁻, OAc⁻ and Cl⁻ anions. The less electronegative Br⁻ anion and bigger PF₆⁻, BF₄⁻, ClO₄⁻, NO₃⁻ and HSO₄⁻ anions do not show any binding affinity with 1. The control titrations carried out using L and the same series of anions showed that the basicity of anions also possibly lends some contribution to the sensing events. However, the binding affinity of 1 toward various anions can be mostly correlated with the electronegativity as well as cavity size of the molecular square, and hence 1 can be expected to be a sensor for F⁻.