Lanthanide complexes with n-oxides. Complexes with pyridine 1-oxide, 2,2′-bipyridine 1,1′-dioxide and 2,2′,2″-terpyridine 1,1′, 1″-trioxide

Several complexes of lanthanide(III) thiocyanates, nitrates, perchlorates and trifluoromethanesulphonates with pyridine 1-oxide (pyO) 2,2′-bipyridine 1,1′-dioxide (bipyO2) and 2,2′,2″-terpyridine 1,1′, 1″-trioxide (terpyO3) have been prepared and characterized. The complexes achieve preferentially the eight-coordination; nine- and ten-coordination have been also observed. Complexation of lanthanide(II) cations with these ligands occurs also in aqueous solution. Information on the electron-donating power of the ligands and geometry of the complexes has been obtained from the absorption and emission f-f spectra. Correlation between Sinha's parameter and hypersensitivity has been observed. The intensity of the hypersensitive transitions increases in the order: pyO < terpyO3 < bipyO2 reaching very high and uncommon values for the bipyO2 complexes. The emission spectra of the octakis(pyO) complexes suggest a rather high symmetry for the electrostatic field surrounding the cation. A drop in site symmetry occurs on passing from the pyO to the bipyO2 and terpyO3 complexes. Exceptionally high values for the intensity ratios 5D0 → 7F2/5D0 → 7F1 in europium(III) complexes and 5D4 → 7F5/5D4 → 7F6 in terbium(III) complexes have been determined.