Synthesis, Structural Characterization, and Sensing Properties of Clickable Unsymmetrical 1,1′-Disubstituted Ferrocene-Triazole Derivatives

A variety of unsymmetrically 1,1'-disubstituted ferrocenes 3-5 have been prepared by using the tandem click reaction/Staudinger-azaWittig process and their cation complexation properties studied using electrochemical and spectroscopic techniques. The common structural features present in these ligands are the presence of another redox unit, connected to the 1-position of the central ferrocene core through a triazole ring, and a photoactive moiety linked to the 1'-position of such core through an imine group. Receptors 3 and 5 undergo perturbation of the CV in the presence of Pb2+ and Zn2+ cations, whereas receptor 4 undergoes a different type of perturbation in the presence of Ni2+, Cd2+, Zn2+, and Pb2+ cations. The low-energy band of the absorption spectra of receptor 3 is blue-shifted, upon addition of Zn2+, and Pb2+ cations, whereas receptor 4 in the presence of Ni2+ and Cd2+ undergoes a remarkable red shift (Delta lambda = 110 nm) of the LE band which is responsible for the change of color from red to blue. Moreover, receptor 3 underwent a significant increase in its emission band in the presence of Pb2+, whereas in receptor 5, such an increase is notably lower. In conclusion, receptor 4 behaves as a colorimetric: receptor for Ni2+ and Cd2+ and 5 acts as a selective fluorescent receptor for the Pb2+ cation. On the basis of the H-1 NMR titration data and from the DFT theoretical calculations, tentative binding modes have been proposed for the metal complexes formed.