Cs4Ge9·en: a novel compound with [Ge9]4- clusters - synthesis, crystal structure and vibrational spectra

Abstract

Red-orange prismatic crystals of Cs4Ge9·en (en = ethylenediamine) were obtained via metathesis reaction of an en solution of K4Ge9 with a large excess of CsI. The compound is very sensitive to air and moisture. Cs4Ge9·en is not isotypic to Rb4Ge9·en and crystallizes in a new structure type (monoclinic, space group P21/c (No. 14); a = 16.757(3), b = 16.462(3), c = 15.657(2) Å, β = 90.37(1)°; Z = 8; Pearson code mP200). The crystal structure comprises two types (A and B) of discrete [Ge9]4− units arranged in the motif of a distorted hexagonal close-packing (h.c.p.). In this h.c.p. framework, each tetrahedral hole is filled by one Cs atom and each larger octahedral hole is filled by a group of one en and two Cs atoms. The topology of the two crystallographically distinct [Ge9]4− clusters corresponds to distorted, monocapped tetragonal antiprisms with bond lengths varying in the range 2.551–2.848Å (type A cluster) or in the range 2.538–2.975Å (less regular type B cluster). According to the DTA and TG measurements, Cs4Ge9·en decomposes at 378 K under liberation of gaseous ethylenediamine yielding Cs4Ge9. The Raman spectra of the title compound were interpreted based on the idealized C4ν symmetry of an isolated [Ge9]4− cluster. The characteristic breathing mode was localized at 222 cm−1. The results are analyzed and discussed in context with those of the previously reported compounds Cs4Ge9, K4Ge9 and Rb4Ge9·en.