A8Tl11 (A = alkali metal) compounds have been known since the investigations of Corbett et al. in 1995 and are still a matter of current discussions as the compound includes one extra electron referred to the charge of the Tl117− cluster. Attempts to substitute this additional electron by incorporation of a halide atom succeeded in the preparation of single crystals for the lightest triel homologue of the group, Cs8Ga11Cl, and powder diffraction experiments for the heavier homologues also suggested the formation of analogous compounds. However, X-Ray single crystal studies on A8Tl11X to prove this substitution and to provide a deeper insight into the influence on the thallide substructure have not yet been performed, probably due to severe absorption combined with air and moisture sensitivity for this class of compounds. Here, we present single crystal X-Ray structure analyses of the new compounds Cs8Tl11Cl0.8, Cs8Tl11Br0.9, Cs5Rb3Tl11Cl0.5, Cs5.7K2.3Tl11Cl0.6 and K4Rb4Tl11Cl0.1. It is shown that a (partial) incorporation of halide can also be indirectly determined by examination of the Tl-Tl distances, thereby the newly introduced cdd/cdav ratio allows to evaluate the degree of distortion of Tl117− clusters.
Alkali metal thallides have been known since the report of E. Zintl on NaTl in 1932. Subsequently, binary and ternary thallides of alkali metals have been characterized. At an alkali metal proportion of approximately 33% (A:Tl~1:2, A = alkali metal), three different unique type structures are reported: K49Tl108, Rb17Tl41 and A15Tl27 (A = Rb, Cs). Whereas Rb17Tl41 and K49Tl108 feature a three-dimensional sublattice of Tl atoms, the A15Tl27 structure type includes isolated Tl11 clusters as well as two-dimensional Tl-layers. This unique arrangement is only known so far when the heavier alkali metals Rb and Cs are included. In our contribution, we present single-crystal X-ray structure analyses of new ternary and quaternary compounds of the A15Tl27 type structure, which include different amounts of potassium. The crystal structures allow for the discussion of the favored alkali metal for each of the four Wyckoff positions and clearly demonstrate alkali metal dependent site preferences. Thereby, the compound Cs2.27K12.73Tl27 unambiguously proves the possibility of a potassium-rich A15Tl27 phase, even though a small amount of cesium appears to be needed for the stabilization of the latter structure type. Furthermore, we also present two compounds that show an embedding of Tl instead of alkali metal into the two-dimensional substructure, being equivalent to the formal oxidation of the latter. Cs14.53Tl28.4 represents the binary compound with the so far largest proportion of incorporated Tl in the structure type A15Tl27.
Crystals of the two new compounds (NHCtBuAu)3NHCl and [(NHCtBuAu)6(η2‐Si4)]Cl2·7NH3 could be isolated from the reaction of Rb6Cs6Si17 with NHCtBuAuCl in the presence of [2.2.2]‐cryptand in liquid ammonia. Both compounds were characterized by single‐crystal X‐ray diffraction and crystallize trigonally without any alkali metals or chelating ligands. Additionally, the crystal of [(NHCtBuAu)6(η2‐Si4)]Cl2·7NH3 was further interpreted by means of ELF and NBO calculations. In the case of (NHCtBuAu)3NHCl, NMR experiments provided an exceptional insight into the reaction processes in solution and allowed for the detection of sequential precursors. In the class of capped gold triangles (NHCtBuAu)3NHCl impresses with its unique characteristics of being capped by an imide and bound to N‐heterocyclic carbenes as ligands instead of the ubiquitously employed phosphines. The gold capped silicon tetrahedron [(NHCtBuAu)6(η2‐Si4)]Cl2·7NH3 represents the first known silicide‐gold compound, as well as the first known functionalized Zintl anion, crystallized with a cationic central moiety.
A tetragonal distortion of the long-time known NaTl structure at 298 K was observed in different experimental setups, including Zintl’s original procedure of reducing Tl(I)-iodide by sodium liquid ammonia solutions. The powder diffraction pattern obtained by the high temperature synthesis using classical solid-state techniques allowed a model-independent unambiguous structure solution and refinement of tetragonal distorted NaTl (Rp = 0.0179, wRp = 0.0246, R = 0.0477, wR = 0.0527, GooF = 1.24).
In‐depth investigations of the long‐time known Zintl phase NaTl revealed a phase transition of tetragonal NaTl‐tI8 [I41/amd; a = 5.2268(9) Å, c = 7.539(1) Å, V = 205.97(9) Å3] to Zintl's cubic NaTl‐cF16 [Fd3m; a = 7.4697(6) Å, V = 416.79(5) Å3] between 351 and 355 K. This phase transformation was observed for NaTl prepared by two different synthetic routes including Zintl's original procedure. An excess of sodium applied during the synthesis in liquid ammonia also resulted in the formation of NaTl‐tI8. DSC measurements suggest a first order phase transition. In addition to in‐situ temperature dependent powder X‐ray diffraction experiments, DSC measurements and solid‐state NMR investigations, we also performed theoretical DOS and band structure calculations for the cubic and tetragonal phase, respectively. The results suggest Na‐Tl interactions in the second coordination sphere being responsible for the observed tetragonal distortion of Zintl's cubic NaTl.
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