Optical absorption and NMR spectroscopic studies on paramagnetic neodymium(III) complexes with β-diketone and heterocyclic amines: The environment effect on 4f–4f hypersensitive transitions

The optical absorption spectra of [Nd(acac)3(H2O)2]·H2O, [Nd(acac)3bpy] and [Nd(acac)3phen(H2O)2] (where acac = acetylacetone, bpy = 2,2′-bipyridyl and phen = 1,10-phenanthroline) complexes in the visible region, in a series of non-aqueous solvents (methanol, ethanol, isopropanol, chloroform, acetonitrile, pyridine, nitrobenzene and dimethylsulphoxide) have been analyzed. The transition 4G5/2 ← 4I9/2 (Nd-VI) located near the middle of the visible region (17,500 cm−1) is hypersensitive. Its behavior is in sharp contrast to many other typically weak and consistently unvaried, normal 4f–4f transitions. The oscillator strength of this transition for the chelate as well as its adducts with phen and bpy in any of the solvent employed is larger than the oscillator strength of Nd3+ aqua-ion. It is most intense in pyridine for all the complexes studied and, therefore, pyridine is the most effective in promoting f–f spectral intensity. The band shape and oscillator strength of the hypersensitive transitions display pronounced changes as compared to Nd3+ aqua-ion. The band shapes of the hypersensitive transitions show remarkable changes on passing from aqueous solution to various non-aqueous solutions, which is the result of change in the environment about the Nd(III) ion in the various solutions and suggests change in the environment about the Nd(III) ion in the various solutions and suggests coordination of solvent molecule(s), in some cases. A comparative account of hypersensitivity in the present complexes with those of other adducts of Nd(β-diketoenolate)3 with heterocyclic amines is discussed. The NMR signals of heterocyclic amines have been shifted to high fields while the resonances due to acetylacetone moiety have moved to low fields. The paramagnetic shift in the complexes is dipolar in nature.