Comparison of optical properties and crystal structures of the praseodymium and europium chloroderivatives of acetates

The praseodymium and europium dichloroacetates were obtained in the form of monocrystals. Crystal structures of the Ln(HCl₂CCOO)₃·2H₂O (Ln=Pr, Eu) compounds were determined by X-ray analysis. Both crystals proved to be isomorphous. They are monoclinic, space group P2₁/n with: a=9.747(6), b=13.857(7), and c=23.595(9) Å, β=95.03(4)°, U=3175(3) Å³, Z=8 for C₆H₇Cl₆O₈Pr and a=9.634(7), b=13.757(11), and c=23.524(14) Å, β=94.84(4)°, U=3107(4) Å³, Z=8 for C₆H₇Cl₆O₈Eu. There are two symmetry independent lanthanide cations, which adopt a nine-coordinate geometry with seven oxygen atoms from carboxylate groups and two oxygen atoms from water molecules. Absorption (Pr³⁺, Eu³⁺), emission and emission excitation (Eu³⁺) spectra of single crystals of Ln(HCl₂CCOO)₃·2H₂O were recorded at room temperature and low temperatures down to 4.2 K. Spectral intensities of the investigated systems are parametrized in terms of the Judd–Ofelt theory and compared to those of lanthanide trichloroacetates and acetates crystals. The relationship between the hypersensitivity and covalency is discussed. The nephelauxetic ratio β and Sinha's parameter δ are calculated based on the absorption spectra. The variation of these parameters and their correlation with the nature of metal–ligand bond is discussed. The bond polarity and bond strength of coordination complex determine the activity and stereospecifity of the catalyst thus the study of these properties are very important because of the application of lanthanide carboxylates as precursors of catalysts. The spectroscopic results are correlated with those from the crystal structure studies, especially with Ln–O distances and the co-ordinating forms of the carboxylate ions. The vibronic coupling in the f–f transitions were analysed. In order to determine the vibronic coupling quantitatively, calculations of the R=I_VIB./I_0-phonon rates were performed from the low temperature absorption spectra. The correlation between the vibronic coupling and covalency is analysed.