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G. Steinhauser, G. Giester, C. Wagner, N. Leopold, J.H. Sterba, B. Lendl, M. Bichler:
"Nitrogen-Rich Compounds of the Lanthanoids: The 5,5'-Azobis[1H-tetrazol-1-ides] of some Yttric Earths (Tb, Dy, Ho, Er, Tm, Yb, and Lu)";
Helvetica Chimica Acta, 92 (2009), 7; 1371 - 1384.

A set of N-rich salts, 3-9, of the heavy lanthanoids (terbium, 3; dysprosium, 4; holmium 5; erbium, 6; thulium, 7; ytterbium, 8; lutetium, 9) based on the energetic 5,5'-azobis[1H-tetrazole] (H2ZT) was synthesized and characterized by elemental anal., vibrational (IR and Raman) spectroscopy, and X-ray structure detn. The synthesis of the lanthanoid salts 3-9 was performed by crystn. from concd. aq. solns. of disodium 5,5'-azobis[1H-tetrazol-1-ide] dihydrate (Na2ZT2 H2O; 1) and the resp. Ln(NO3)35 H2O and yielded large rhombic crystals of the type [Ln(H2O)8]2(ZT)36 H2O in ca. 70% of the theor. yield. The compds. 3-9 are isostructural (triclinic space group P\bar 1) to the previously published yttrium salt 2; they show, however, a clear lanthanoid contraction of several crystallog. parameters, e.g., the cell vol. or the Ln[bond]O bond lengths of the Ln3+ ions and the coordinating H2O mols. The lanthanoid contraction influences the strengths of the H-bonds, which can be obsd. as a red shift by 4 cm-1 in the characteristic IR band, in particular from 3595 cm-1 (3) to 3599 cm-1 (9). In good agreement with previous works, 2-9 are purely salt-like compds. without a coordinative bond between the tetrazolide anion and the Ln3+ cation.