Synthesis of a low-valent Al4+ cluster cation salt

Low-valent aluminium compounds are very reactive main-group species and have therefore been widely investigated. Since the isolation of a stable molecular Al(I) compound in 1991, [(AlCp*)(4)] (Cp* = [C5Me5](-)), a variety of highly reactive neutral or anionic low-valent aluminium complexes have been developed. By contrast, their cationic counterparts have remained difficult to access. Here, we report the synthesis of [Al(AlCp*)(3)](+)[Al(ORF)(4)](-) (R-F = C(CF3)(3)) through a simple metathesis reaction between [(AlCp*)(4)] and Li[Al(ORF)(4)]. Unexpectedly, the [Al(AlCp*)(3)](+) salt forms a dimer in the solid state and concentrated solutions. Addition of Lewis bases results in monomerization and coordination to the unique formal Al+ atom, giving [(L)(x)Al(AlCp*)(3)](+) salts where L is hexaphenylcarbodiphosphorane (x = 1), tetramethylethylenediamine (x = 1) or 4-dimethylaminopyridine (x = 3). The Al+-Al-Cp* bonds in the resulting [(L)(x)Al(AlCp*)(3)](+) cluster cations can be finely tuned between very strong (with no ligand L) to very weak and approaching isolated [Al(L)(3)](+) ions (when L is dimethylaminopyridine).

Automated summary

Low-valent aluminium compounds, which are highly reactive, have been widely studied in the field of chemistry. In this study, a method for synthesizing [Al(AlCp*)(3)](+)[Al(ORF)(4)](-) was reported, and it was found that the salt forms a dimer in the solid state and concentrated solutions.