First synthesis of group-14 polyanions by extraction of a binary alloy with dmf and a novel conformation of the (Ge₉-Ge₉) ^6- dimer...

Crystals of the title compounds [K₆(Ge₉-Ge ₉)](dmf)₁₂ (1) and [Rb₆(Ge₉-Ge ₉)](dmf)₁₂ (2) have been synthesized by dissolving binary phases of the formal composition K₄Ge₉ and Rb ₄Ge₉, respectively, in N,N-dimethylformamide (dmf) and subsequent cooling to -36 °C. [K_2.5Cs_3.5(Ge ₉-Ge₉)](dmf)₁₂ (3) was obtained as a by-product during the reaction of K₂Cs₂Ge₉ with sulphur in dmf. Single crystal structure analyses of 1, 2 and 3 show that all compounds contain dimeric [Ge₉-Ge₉]^6- units. The dimers are connected via the alkali metal atoms in form of [(A⁺) ₃(μ₂-dmf)₆] chains and build a two-dimensional square net of the composition _∞ ²{A₆(dmf)₁₂[Ge₉-Ge₉]} with A = K and Rb for 1 and 2, respectively. [(Cs⁺-K ⁺-Cs⁺)(μ₂-dmf)₆] linkages bridge the dimers in 3 under formation of _∞²{K _2.5Cs_3.5(dmf)₁₂[Ge₉-Ge ₉]}. 1 crystallized in the tetragonal space group I4 with a = b = 34.003(5), c = 17.366(4) Å, V = 20078(6) ų, Z = 8, compound 2 in the triclinic space group P1 with a = 13.501(3), b = 14.766(3), c = 21.426(4) Å, α = 88.11(3)°, β = 86.76(3)°, γ = 75.54(3)°, V = 4129(1) ų; Z = 2 and 3 in the monoclinic space group C2/c with a = 21.459(4), b = 17.153(3), c = 22.939(5) Å, β = 93.37(3)°, V = 8429(3) ų, Z = 4. The dimer consists of two Ge₉ clusters, whose shapes are best described as C_2ν-distorted monocapped square antiprisms. The clusters are connected via an exo Ge-Ge bond with its orientation either co-linear to the shorter diagonal of the open rectangular of the antiprism or radial with respect to the cluster centre in a new type of [Ge₉-Ge₉] ^6- dimer.