FEATURE ARTICLE
Modern Main-Group Chemistry:
Base Metals with Noble Intentions
CAMERON JONES Class of 2018–19
Professor and holder of the R.L. Martin Distinguished Chair of Chemistry Monash University Melbourne, Australia
The main-group elements lie on the left and right sides of the periodic table, in what are called the s- and p-blocks. Those elements comprise very reactive base metals (e.g., sodium, Na; magnesium, Mg; and potassium, K) to the left and a mixture of typically “earth abundant” metals (e.g., aluminum, Al), metalloids (e.g., silicon, Si) and nonmetals (e.g., phosphorus, P) to the right. By the mid-twentieth century, it was widely believed that the chemistry of the main-group elements had almost reached the limit of its development and that few major discoveries were left to be made. Main-group elements typically existed in one oxidation state (i.e., charge) in their compounds (e.g., Mg2+, Al3+, Si4+), and as a result, they had very limited application to synthetic processes, such as catalysis, which generally require two or more oxidation states of similar energy to proceed.
In the late twentieth century, several enlightened chemists questioned the entrenched dogma surrounding main-group element chemistry and showed that compounds containing main-group elements in a range of oxidation states, lower than normally exhibited by the element, can be stabilized. They used several chemical “tricks” to achieve that goal, mainly by employing very bulky ligands (i.e., the chemical fragments attached to the element) to protect the main-group element from the decomposition processes they would normally undergo.
That technique has since led to an explosion of interest in main-group chemistry over the last twenty years, which has seen the development of many new compound types that would never have been thought possible, let alone be stored in bottles at room temperature.
Such compounds are of considerable fundamental importance, not least because the study of their properties and reactivity has required a reassessment of long-held views and rules of electronic structure and chemical bonding. More than being solely of fundamental interest, applications of the chemistry of highly reactive low-oxidation-state main-group compounds have rapidly emerged over the last decade. The advances made in this field have been driven largely by the realization that the electronic properties of main-group compounds more closely mimic those of the transition metals (the elements in the middle of the periodic table) than classical, “normal” oxidation-state main-group systems. That is, like transition metals, they can have multiple oxidation states, which are close in energy and thus are chemically accessible. For transition metals, these are the properties that have made their compounds so valuable as catalysts for use in many industrial chemical processes.
WHAT IS A CATALYST? Simply put, it is a compound or material that will alter the kinetic energy barrier to a reaction occurring without affecting the overall change in energy from reactants to products for that reaction. Catalysts are not consumed in the reaction, and so only very small quantities of the catalyst are required to allow a reaction to go to completion. To put the importance of catalysts into perspective, the annual global market for products derived from catalysis is greater than $10 trillion US. Moreover, catalysts are central to nearly every aspect of everyday life, such as producing commodity chemicals, fuels, plastics, fine chemicals, and pharmaceuticals. Even so, most effective catalysts used today are derived from the noble metals, such as palladium (Pd), platinum (Pt), and rhodium (Rh).
That approach is becoming increasingly problematic for several reasons:
- These metals are prohibitively expensive. For example, palladium costs about $2,700 US per ounce.
- Their deposits are dwindling. Palladium makes up about 0.01 part per million in Earth’s crust.
- They are highly toxic.
- Their removal from chemical products (especially pharmaceuticals) is increasingly costly and regulated.
As a result, there is a global drive toward the development of effective but more sustainable alternatives to noble metals in catalytic processes.
CATALYSIS PROFILE
MgMg ORBITAL
Given that the electronic and reactivity properties of low-oxidation-state main-group compounds seem to mimic those of transition metals, the former are excellent candidates for the ultimate replacement of the latter in many catalytic processes. That is especially true for the base main-group elements, which are cheap (an ounce of magnesium costs about 15 cents US), nontoxic, abundant (magnesium makes up about 29,000 parts per million in Earth’s crust), and readily available from domestic sources. The potential that low-oxidation-state main-group compounds have in catalysis has only begun to be realized in the last five years, with more research groups from around the world working on solving this problem. Jones' research group was one of the pioneers in this area, and the work they are carrying out at Monash University, and with collaborators at Texas A&M University, is keeping us at the forefront of this emerging field.
Their approach to installing main-group compounds as replacements for transition metal systems in catalysis is threefold:
- Develop synthetic and stabilization techniques to access previously inaccessible low-oxidation-state main-group metal complexes, at the same time gaining a deep understanding of the electronic structure and bonding in those species.
- Exploit developed main-group compounds for the “transition metal–like” activation of strong and typically inert bonds in catalytically relevant small molecules, such as hydrogen, carbon monoxide, carbon dioxide, and ethylene. Such bond activations represent fundamental steps in many catalytic processes.
- Apply the knowledge gained from small-molecule activation studies to the development of important catalytic protocols, which have previously been achievable only with noble transition metals.
That approach has proved very successful and has led to many high-profile advances in recent times. To give just a few examples from each prong of their approach, they have developed many main-group compounds exhibiting a variety of oxidation states, many of which are beginning to populate chemistry textbooks and are finding use by chemists around the globe. The standout example here is their stabilization of the first compounds to contain magnesium in an oxidation state of +1, all previous examples having the metal in the +2 oxidation state. They have used the compounds to activate catalytically relevant molecules under very mild conditions. For example, magnesium +1 compounds can assemble many carbon monoxide and hydrogen molecules to give value-added organic molecules, in a similar fashion to the transition metal–catalyzed Fischer–Tropsch process for the transformation of carbon monoxide into hydrocarbon fuels. Building on that work, similar reactions have been made “catalytic” in which each main-group molecule can convert hundreds of thousands of small-molecule starting materials to useful organic molecules. The “activity” of these main-group catalysts can actually be greater than that of the noble transition metal catalysts that had to be used previously for such reactions. Although in its infancy, the field of sustainable main-group element catalysis seems certain to grow. More and more main-group replacements are likely to be found for the problematic noble transition metal catalysts that have for decades dominated chemical processes used to produce useful chemical products, both in the industrial and academic settings. They hope to continue at the leading edge of this discipline, which is an aspiration made all the easier through their now embedded collaborations with world-leading chemists in the outstanding Department of Chemistry at Texas A&M.
IN COLLABORATION WITH
Francois Gabbaï, University Distinguished Professor and holder of the Arthur E. Martell Chair of Chemistry, Department of Chemistry, College of Science
Michael Nippe, associate professor, Department of Chemistry, College of Science
David Powers, assistant professor, Department of Chemistry, College of Science
Mohammadjavad Karimi, graduate student, Hagler Institute for Advanced Study Heep Fellow, Department of Chemistry, College of Science
Gabrielle Risica, graduate student, Hagler Institute for Advanced Study Heep Fellow, Department of Chemistry, College of Science
