Two novel nitrogen-rich lanthanide compounds of 5,5´-(azobis)tetrazolide (ZT) were synthesized and structurally characterized. The dinuclear, isostructural compounds [Ce₂(ZT)₂CO₃(H₂O)₁₂] · 4 H₂O (1) and [Pr₂(ZT)₂CO₃(H₂O)₁₂] · 4 H₂O (2) were synthesized via two independent routes. Compound 1 was obtained after partial Lewis acidic decomposition of ZT by CeIV in aqueous solution of (NH4)₂Ce(NO₃)₆ and Na₂ZT. Compound 2 was obtained by crystallization from aqueous solutions of Pr(NO₃)₃, Na₂ZT, and Na₂CO₃. By X-ray diffraction analysis at 200 K, it was found that the trivalent lanthanide cations are bridged by a bidentate carbonato ligand and each cation is further coordinated by six H₂O ligands and one ZT ligand thus being ninefold coordinated.

 “Controlling complexation behaviour of early lanthanides via the subtle interplay of their Lewis acidity with the chemical stability of 5,5´-(azobis)tetrazolide”, P. Weinberger, G. Giester, G. Steinhauser, Z. Anorg. Allg. Chem., 646 (2020) 1882–1885.
https://doi.orf/10.1002/zaac.202070231

A series of rare earth element (REE) mixed-anion 5,5′-azobis(1H-tetrazol-1-ide)-carbonate ([REE₂(ZT)₂CO₃(H₂O)₁₀] · 2 H₂O; REE = lanthanides plus yttrium) coordination compounds were synthesized, characterized, and analyzed. Syntheses by simple metathesis reactions under a CO₂ atmosphere were carried out at ambient (La–Gd and Ho) and elevated pressures (55 bar; Tb, Dy, Er, Tm, Yb, and Y). The resulting crystalline materials were characterized principally by single-crystal X-ray diffraction and vibrational spectroscopy (infrared and Raman). All materials are structurally isotypic, crystallizing in the space group C2/c and show nearly identical spectroscopic properties for all the elements investigated. Cell parameters, bond lengths, and bond angles differ marginally, revealing only a slight variation coinciding with the lanthanide (Ln) contraction, that is, the change in the ionic radii of the trivalent rare earth elements.
The herein reported series of rare earth element azobis[tetrazolide]-carbonates represents a remarkable exception as they are a series of isotypic REE coordination compounds with tetrazolide-derived ligands unaffected by the “gadolinium break”.

"Azobis[tetrazolide]‐Carbonates of the Lanthanides - Breaking the Gadolinium Break", Müller, D. , Knoll, C. , Herrmann, A. , Savasci, G. , Welch, J. M., Artner, W. , Ofner, J. , Lendl, B. , Giester, G. , Weinberger, P. and Steinhauser, G., Eur. J. Inorg. Chem., 2018, 1969-1975
https://doi.org/10.1002/ejic.201800218

The crystallization of terbium 5,5’-azobis[1H-tetrazol-1-ide] (ZT) in the presence of trace amounts (ca. 50 Bq, ca. 1.6 pmol) of americium results in 1) the accumulation of the americium tracer in the crystalline solid and 2) a material that adopts a different crystal structure to that formed in the absence of americium. Americium-doped [Tb(Am)(H₂O)₇ZT]₂ZT · 10 H₂O is isostructural to light lanthanide (Ce–Gd) 5,5’-azobis[1H-tetrazol-1-ide] compounds, rather than to the heavy lanthanide (Tb–Lu) 5,5’-azobis[1H-tetrazol-1-ide] (e.g., [Tb-(H₂O)₈]₂ZT₃ · 6 H₂O) derivatives. Traces of Am seem to force the Tb compound into a structure normally preferred by the lighter lanthanides, despite a 10⁸-fold Tb excess. The americium-doped material was studied by single-crystal X-ray diffraction, vibrational spectroscopy, radiochemical neutron activation analysis, and scanning electron microcopy. In addition, the inclusion properties of terbium 5,5’-azobis[1Htetrazol-1-ide] towards americium were quantified, and a model for the crystallization process is proposed.

"Picomolar Traces of Americium(III) Introduce Drastic Changes in the Structural Chemistry of Terbium(III): A Break in the “Gadolinium Break”", J. Welch, D. Müller, C. Knoll, M. Wilkovitsch, G. Giester. J. Ofner, B. Lendl, P. Weinberger, G. Steinhauser, Angew. Chem. Int. Ed.,2017, 56, 13264
https://doi.org/10.1002/anie.201703971