Low-Valent M_xAl₃ Cluster Salts with Tetrahedral [SiAl₃]⁺ and Trigonal-Bipyramidal [M₂Al₃]²⁺ Cores (M=Si/Ge)

Schnockel's [(AlCp*)(4)] and Jutzi's [SiCp*][B(C6F5)(4)] (Cp*=C5Me5) are landmarks in modern main-group chemistry with diverse applications in synthesis and catalysis. Despite the isoelectronic relationship between the AlCp* and the [SiCp*](+) fragments, their mutual reactivity is hitherto unknown. Here, we report on their reaction giving the complex salts [Cp*Si(AlCp*)(3)][WCA] ([WCA](-)=[Al(ORF)(4)](-) and [F{Al(ORF)(3)}(2)](-); R-F=C(CF3)(3)). The tetrahedral [SiAl3](+) core not only represents a rare example of a low-valent silicon-doped aluminium-cluster, but also-due to its facile accessibility and high stability-provides a convenient preparative entry towards low-valent Si-Al clusters in general. For example, an elusive binuclear [Si-2(AlCp*)(5)](2+) with extremely short Al-Si bonds and a high negative partial charge at the Si atoms was structurally characterised and its bonding situation analysed by DFT. Crystals of the isostructural [Ge-2(AlCp*)(5)](2+) dication were also obtained and represent the first mixed Al-Ge cluster.