Hypersensitivity in the 4f-4f absorption spectra of tris (acetylacetonato) neodymium(III) complexes with imidazole and pyrazole in non-aqueous solutions. Effect of environment on hypersensitive transitions

The optical absorption spectra of [Nd(acac)(3)(H2O)(2)] center dot H2O, [Nd(acac)(3)(im)(2)] and [Nd(acac)(3)(pz)(2)] (where acac is the anion of acetylacetone, im is imidazole and pz is pyrazole) complexes in the visible region have been analyzed. The transition (4)G(5/2) <- I-4(9/2) 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. It is overlapped by a less intense transition (4)G(5/2) <- I-4(9/2). The band shapes of the hypersensitive transition show remarkable changes on passing from aqueous solution to various non-aqueous solutions, which is the result of changes in the environment about the Nd(III) ion in the various solutions and suggests coordination of a solvent molecules. Pyridine has been found especially effective in promoting 4f-4f electric-dipole intensity. The DMSO invades the complexes and replaces the water molecules and heterocyclic amines from the coordination sphere. Two DMSO molecules coordinate and the complexes acquire similar structure, [Nd(acac)(3)(DMSO)(2)] in solution. The oscillator strength and the band shape of the hypersensitive transition of all the complexes remains the same in this solvent. The IR spectra and the NMR spectra of the complexes have also discussed. (C) 2011 Elsevier B.V. All rights reserved.