New Synthetic Routes to Molecular Aluminum Clusters

Recent research has uncovered an inverse relationship between burn rate and particle size in nanocrystalline aluminum. This discovery has fueled considerable interest in the pursuit of molecular aluminum clusters, representing the ultimate minimum in particle size. Toward this end, we are exploring new synthetic routes to these clusters. We are employing a high-temperature, solid-state approach in targeting unligated aluminum clusters such as Al₁₃^-, which has been shown to have an unexpectedly high level of stability. This method involves stoichiometric reactions between elemental alkali metals and elemental aluminum in sealed tubes. Products of these reactions are typically characterized through powder X-ray diffraction.

In addition to simple, unligated species, we are interested in generating ligated aluminum clusters through use of a metal atom reactor (Figure 1). Here, aluminum metal is evaporated inside the evacuated reactor through resistive heating, filling the space with gaseous aluminum atoms. During this process, a volatile ligand is introduced into the reactor through an inlet tube. Both species are then deposited on the liquid nitrogen-cooled reactor walls as a frozen matrix. Upon warming, clusters of naked aluminum atoms are "capped" by ligand shells. Ligands currently being investigated include toluene, THF, acetonitrile, pyridine, PEt₃, and NEt₃, as they represent a wide range of coordinating ability, volatility, and melting points. Products obtained from the metal atom reactor have thus far been characterized through powder X-ray diffraction and solid-state IR.  However, the isolation of soluble species will vastly extend our methods of characterization.