Crystal structure and optical properties of Ln(III) octahedral complexes with hexamethylphosphortriamide; [Ln(HMPA)(6)](ClO4)(3)

The neodymium and europium complexes with HMPA (hexamethylophosphortriamide O=P[N(CH3)(2)](3)) of formula Ln(HMPA)(6)(ClO4)(3) were synthesized and obtained as monocrystals from non-aqueous solutions. The structure of the neodymium complex [C36H108N18O18P6Cl3Nd] has been determined by X-ray diffraction. It crystallizes in the cubic space group Fm3m, with the unit cell parameter a=19.222(1) Angstrom. The structure was solved by direct method and refined using the anisotropic full-matrix least-squares method. The final R (wR) factors were 0.042 (0.049) for 468 observed reflections (3098 measured). Each metal ion is coordinated to six HMPA molecules through oxygen atoms of a PO group with Nd-O bond lengths of 2.340(8) Angstrom. Metal ions occupy symmetry centre in ideal octahedral (O-h) symmetry. The three -N(CH3)(2) groups statistically occupy four equivalent positions with an occupancy factor 3/4, which is reflected in the spectral properties of the complex. The compounds were characterized by IR, Raman, luminescence and absorption spectroscopy at 5, 77 and 293 K. Probabilities of electronic transitions for the Nd ion were analysed and compared with those for centrosymmetric single crystals with antipyrine with the formula: Nd(AP)(6)(ClO4)(3). The intensity of the hypersensitive transition of the title compound is lower, moreover a drastic decrease of intensities in the IR region is observed. Attention is focused on a vibronic transition occurring between crystal-field (electronic) levels with simultaneous excitation of phonon or vibrational modes (in the F-7(0)-->D-5(2) excitation spectrum of Eu3+ and I-4(9/2)-->(4)G(5/2), (4)G(7/2)in the absorption spectrum of Nd3+). Using the IR and Raman data of Ln(HMPA)(6)(ClO4)(3) crystals, a preliminary assignment of vibronic lines was made. (C) 1997 Elsevier Science S.A.