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]– (RF = 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)xAl(AlCp*)3]+ salts where L is hexaphenylcarbodiphosphorane (x = 1), tetramethylethylenediamine (x = 1) or 4-dimethylaminopyridine (x = 3). The Al+–AlCp* bonds in the resulting [(L)xAl(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). Although neutral and anionic low-valent aluminium complexes are widespread, their cationic counterparts have remained rare. Now, a salt of [Al(AlCp*)3]+ featuring a formal low-valent Al+ cation has been isolated that dimerizes in concentrated solutions and the solid state, and also forms Al4 clusters on coordinating with Lewis bases.