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

The molecular square [Re(CO)(3)Cl(L)](4) (1) containing the dipyridyl-amide ligand, N,N'-4-dipyridyloxalamide ( L), was constructed from Re(CO)(5)Cl 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 H-1 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- 4 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-.