Synthesis, Characterisation, Electrochemistry and Ion-Binding Studies of Ruthenium(II) and Rhenium(I) Bipyridine/Crown Ether Receptor Molecules

A number of molecular receptors containing a ruthenium(II)/rhenium(I) bipyridine moiety as fluorophore and a crown ether, connected to the 4,4'-positions of the bipyridine/phenanthroline unit through >C=O and -HC=N- spacers, as ionophore have been synthesised and characterised. The luminescence and electrochemical properties of these receptors have been studied. The cation-binding properties of these molecules have been investigated with the cations Na+, K+, Rb+, Mg2+, Ca2+, Zn2+, Cd2+, Hg2+, Pb2+ and Cu2+ and the recognition event monitored by H-1 NMR spectroscopy, luminescence, the oxidation potential of the metal ion and UV/Vis absorption studies. The emission intensities of complexes 1-3 are strongly quenched by the addition of Pb2+, Cu2+, Hg2+ and Na+, whereas a significant enhancement of the emission intensities is observed for 4 in the presence Of Cu2+ and Hg2+ and for 5 in the presence of Cd2+ and Hg2+. Luminescence titrations for 1-5 with the above-mentioned metal ions have been carried out. The binding constants (K-s) and stoichiometry of the complexes have, been calculated from titration data, with values of K-s ranging from 9.97 x 10(3) to 1.28 x 10(2) M-1 in the decreasing order Pb2+ > Na+ > Cu2+ > Hg2+ > Cd2+. The stoichiometries found for 1-3 and 5 are in the range 1.85-2.09 and for 4 0.99-1.04. A H-1 NMR spectroscopic scopic study with selected metal ions exhibits significant spectral changes for the protons attached to the crown moiety; a few significant changes are also noted in the aromatic region. Among the ions studied, the highest change is observed with Na+ and Pb2+, whereas K+ does not show any significant change. The electrochemical study exhibits a significant cathodic shift of the Oxidation potentials of the Ru-II centre in the presence of Na+ and Pb2+, thereby indicating complexation of these ions with the crown moiety. The electronic spectra, however, do not show significant spectral changes with various metal ions except for that of the Re-I complex 6, which exhibits some change upon addition of Na+. The results obtained are presented and discussed in light of selectivity and intramolecular energy-transfer. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)