Cs₄Ge₉·en: A novel compound with [Ge ₉]^4- clusters - Synthesis, crystal structure and vibrational spectra

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, mono-capped tetragonal antiprisms with bond lengths varying in the range 2.551-2.848 Å (type A cluster) or in the range 2.538-2.975 Å (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 C_4v 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 Rb ₄Ge₉·en.