Synthesis of a low-valent Al 4 + 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* = [C 5 Me 5 ] - ), 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(OR F ) 4 ] - (R F  = C(CF 3 ) 3 ) through a simple metathesis reaction between [(AlCp*) 4 ] and Li[Al(OR F ) 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).