HIGHLIGHTS
• Two cyclic aluminum trimers with aluminum I oxidation states were synthesized from a reduction reaction that started with aluminum III diiodides. 
• The cyclic aluminum trimers conducted 2+2 and 2+4 addition reactions with benzene and an acetylene derivative that use oxidative chemistry and not Lewis Acid mechanisms.
• There is excellent potential for using the cyclic aluminum trimers as catalysts toward carbon monoxide and carbon dioxide, which would provide a less costly alternative to expensive noble metals. 

Use of aluminum is continuing to increase in industrial and transportation machinery due to the metal’s high strength-to-weight ratio. End-users are finding that aluminum’s presence in their machinery increases productivity and efficiency and reduces cost. 

The fact that aluminum is readily available (the most abundant metal in the earth’s crust) and relatively low in cost is also motivating researchers to determine how this metal can perform as a catalyst particularly in comparison to more expensive noble metals such as platinum. 

In a previous TLT article,¹ researchers evaluated the possibility of using aluminum as a catalyst to produce hydrogen by splitting water. Aluminum can theoretically produce hydrogen in this manner but issues such as the presence of an oxide coating can inhibit the reaction. An alternative approach was found by pressing the element gallium into an aluminum foil leading to the formation of a composite. The aluminum species present in the composite are nanoparticles. When added to water, the composite triggered the formation of hydrogen gas at room temperature in a reaction that took only 15 minutes to complete. Different sources of aluminum such as beverage cans were found to facilitate hydrogen production.

Dr. Clare Bakewell, senior lecturer in the Department of Chemistry at Kings College London in London, UK, says, “We have been working with various types of aluminum compounds to assess their utility for bond breaking reactions and catalysis. As an example, aluminum compounds have been found to degrade per-and polyfluoroalkyl substances (PFAS) by breaking the highly stable carbon-fluorine bond. Aluminum in this and other applications has displayed the versatility and flexibility to access different oxidations states, and in some cases even mediate redox (reduction oxidation) catalysis.”

The most common oxidation state for aluminum is +3. Based on previous work, Bakewell saw that a reduction reaction was probably occurring in some processes where aluminum forms adducts with organic solvents such as benzene. She says, “We speculated that in starting with aluminum III species, the products isolated, dialumene-benzene adducts, were probably formed by the reaction of a transient, highly reactive, aluminum I intermediate and the aromatic reaction solvent used in the process.”

By working with two distinct aluminum III species, the researchers were able to synthesize a new aluminum I species that also exhibits a neutral charge.

Cyclic trimer reactivity
Aluminum III diiodides prepared with two different aromatic ligands (para-toluene and meta-xylene) were reduced with potassium in hexane at room temperature. The reaction leads to the formation of two crystalline materials with similar chemical structures. 

Single crystal X-ray diffraction identified the chemical structure as a cyclic trimer consisting of three aluminum atoms. A schematic of this cyclic trimer is shown in Figure 1. Bakewell says, “This finding represents the first time a neutral cyclic aluminum I trimer has been synthesized. We were surprised by this result as we expected the transient intermediate to be a species with an aluminum-aluminum double bond.”


Figure 1. Three aluminum atoms form a triangle in the middle of this image, which is a depiction of a solid-state crystal structure for one of the aluminum cyclic trimers that shows potential as a catalyst. Figure courtesy of Kings College London.

The researchers utilized computational methods (density functional theory) to better understand the electronic features of the cyclic aluminum trimer. Bakewell says, “The aluminum atoms are held together with three two-center, two electron bonds. There is no evidence that the cyclic aluminum trimer exhibits any aromatic characteristics.”

None of the aluminum-aluminum bonds were equivalent. The slight differences in the bonds are due to the steric impact of the ligands leading to the presence of two long and one short bond. The cyclic aluminum trimers are stable in aromatic solvents for an extended period of time (greater than one week). Bakewell says, “When isolated as solids, they need to be maintained in an inert atmosphere (dry box) to prevent degradation.”

The cyclic aluminum trimer readily reacts with methyl iodide and hydrogen at room temperature. Both reactions occur at room temperature and take less than 15 minutes to complete. Benzene and bis(trimethylsilyl) acetylene react with the cyclic aluminum trimer at 80℃ for between three and six hours. Bakewell says, “The cyclic trimer carries out 2+2 and 2+4 addition reactions with benzene and the acetylene derivative. All of these reactions use oxidative chemistry, and not the Lewis Acid mechanisms that explain the more well-known aluminum mediated processes such as Friedel-Crafts.”

Treatment of the cyclic aluminum trimer with ethylene yields a series of compounds that involve the sequential insertion of ethylene between one of the aluminum-aluminum bonds. The initial reaction produces a monomeric addition product at room temperature. Further reactivity is observed by adding excess ethylene at room temperature to produce an ethylene dimer as the major product. Spectral analysis suggests that one of the minor products has a structure consistent with a C4 hydrocarbon unit that may form due to coupling of two ethylene dimers. 

Bakewell says, “The novel nature of this reaction is the formation of five- and seven-member aluminum and carbon rings.”

Bakewell feels that there is excellent potential for using the cyclic aluminum trimer to facilitate different chemical processes. She says, “This cyclic molecule can be tailored for use in different reactions by changing the type of ligands attached to the aluminum trimer. Our intention is to evaluate the reactivity of the cyclic aluminum trimer toward carbon monoxide and carbon dioxide. We are hoping this species can be used as a catalyst, but we will need to determine how to regenerate it by converting the aluminum +3 oxidation state through a reduction process to +1.”

Additional information can be found in a recent paper² or by contacting Bakewell at clare.bakewell@kcl.ac.uk.