Optical absorption and NMR spectroscopic studies on paramagnetic trivalent lanthanide complexes with 2,2′-bipyridine.: The solvent effect on 4f–4f hypersensitive transitions

Abstract

The optical absorption and NMR studies of trivalent lanthanide chloride complexes with 2,2′-bipyridine (bpy) are presented and discussed. The NMR spectra of paramagnetic complexes exhibit lower as well as higher field shifts of bpy resonances that reflect change in geometry and reveals importance of the factor ( 3 cos 2 θ−1 ) in changing sign of the shift. The paramagnetic shifts recorded have been analyzed and the intramolecular shift ratios suggest that the paramagnetic shift is predominantly dipolar in origin. Electronic spectral studies of the Pr, Nd, Ho, Er and Dy complexes in different solvents (viz. methanol, pyridine, DMSO and DMF), which differ with respect to donor atoms, reveal that the chemical environment around the lanthanide ion has great impact on f–f transitions and any change in the environment results in modifications of the spectra. The oscillator strength for the hypersensitive and non-hypersensitive transitions have been determined and changes in the oscillator strength and band shape with respect to solvent type is rationalized in terms of ligand (solvent) structure and coordination properties.