Monday, March 7, 2005
Carnegie Mellon Scientists Develop New, Green Chemistry by Combining Oxygen with Well-Studied Fe-TAML Catalyst
Scientists at Carnegie Mellon University have successfully synthesized a catalyst, called a Fe-TAML® activator, that reacts avidly with oxygen to transform molecules into substances that play critical roles in laboratory, industrial, medical and environmental settings. (TAML stands for Tetra-Amido Macrocyclic Ligand, and Fe is iron).
The research results, published in the Jan. 29 issue of the Journal of the American Chemical Society, have the potential to extend the use of Fe-TAML activators to remediate environmental problems and to modify industrial processes to make them more efficient and productive.
“This chemistry is, to our knowledge, unprecedented in the scientific literature,” said Terry Collins, the Thomas Lord Professor of Chemistry at Carnegie Mellon. “Oxygen is a natural, safe, cheap and abundant resource, so the fact that this oxidant works so well in our experiments with Fe-TAMLs is quite exciting.” Specifically, the Fe-TAML activator reacts with oxygen to generate a high-valent iron(Fe) complex that can oxidize organic and inorganic substances.
Oxygen is essential for many life processes, including those in which specific enzymes modify compounds via a reaction called oxidative catalysis. Fe-TAML activators TAMLs are made from elements found in nature. While they are synthetic compounds, Fe-TAML activators function like certain enzymes when combined with oxygen.
To date, research with Fe-TAML activated by hydrogen peroxide, already has yielded spectacular results for decontaminating a variety of dangerous environmental pollutants. Using oxygen rather than hydrogen peroxide could expand the known uses of Fe-TAMLs, according to Collins. Moreover, oxygen could eventually replace hydrogen peroxide, a compound that is more expensive and certainly less abundant than oxygen, said Collins.
While nature is quite successful at using oxygen to catalyze many critical chemical processes that sustain life, chemists have found it difficult to replicate these feats, said Collins. Existing synthetic catalysts that are designed to interact with oxygen fail to combine effectively with this molecule, and they do not as yet function well under mild conditions. The new chemistry found by the Carnegie Mellon team opens new ways to harness oxygen for efficient, cost-effective green industrial processes.
By inducing a reaction between oxygen and a Fe-TAML activator whose iron atom was remarkable in the specific chemical state of Fe(III), the state of iron found naturally in rust, the Collins team obtained a very reactive molecule.
Carnegie Mellon chemistry professor Eckard Münck and his graduate student Filipe Tiago de Oliveira studied the reactive species with Mössbauer spectroscopy at temperatures near absolute zero and proposed that it consists of two Fe(IV)-TAMLs linked by one oxygen atom. (Fe(IV) is a very rare oxidation state of iron) This proposal was subsequently confirmed by collaborators at Osaka City University in Japan who determined the structure by X-ray crystallography.
The reaction of Fe(III) with oxygen to yield Fe(IV) is unprecedented. Another unique feature of this enzyme-like active species is its formation under ambient conditions such as room temperature and pressure.
In the presence of oxygen, this new Fe(IV),Fe(IV)-μ-oxo dimer catalyzes several interesting oxidation reactions, found the Carnegie Mellon scientists. For example, benzylic alcohols, which are alcohols commonly used in industrial processes, are oxidized to their corresponding aldehydes, which are themselves highly reactive and useful chemical compounds.
Much of the research was carried out by Anindya Ghosh, who carried out the work as part of his Ph.D. thesis on the Collins lab.