Speeding up chemical reactions is key to improving industrial processes or mitigating unwanted or harmful waste. Realizing these improvements requires that chemists design around documented reaction pathways. Now, a team of Penn State researchers has found that a fundamental reaction called oxidative addition can follow a different path to achieve the same ends, raising the question of whether this new order of events has been occurring all along and potentially opening up new space for chemical design.
A paper describing the research appeared in the Journal of the American Chemical Society.
The reactions of organic compounds - those containing carbon, hydrogen, oxygen and a few other elements - are limited by the bonding patterns and electron arrangements specific to organic elements. More electron arrangements are available in transition metals, another type of element that includes, for example, platinum and palladium. When transition metals interact with organic compounds, this added layer of complexity can modify the electron structure of organic compounds leading to a wider diversity of potential reactions, including breaking chemical bonds and catalyzing reactions not possible among purely organic compounds. Understanding the diversity of ways these chemical reactions can occur could help chemists design ways to exploit transition metals to increase the efficiency of industrial processes or find new solutions that could, for example, help reduce environmental pollutants, according to the researchers.
"Transition metals have properties that allow them to 'break the rules' of organic chemistry," said Jonathan Kuo, assistant professor of chemistry in the Eberly College of Science at Penn State and the leader of the research team. "As an example, even though biological systems are largely considered to be organic, much of the chemistry in cells occurs at active sites, where metallic co-factors actually drive the reactivity. Transition metals are also used to catalyze industrial-scale chemical reactions. General understanding as to how these reactions work is a way to approach the efficiency of nature or even invent reactions that don't have a known analogy in nature."
Chemical reactions occur because the atoms that compose molecules "want" to be in a state that is more stable. This stabilization is accomplished mainly by rearranging electrons amongst orbitals - the cloudlike regions around atomic nuclei where electrons are likely to be located. A hydrogen atom, for example, has only one electron that lives in a "1s" orbital. However, two hydrogen atoms can bond to make dihydrogen (H2), where the two 1s orbitals mix to make two hybrid orbitals. The more stable of the two hybrid orbitals hosts the two electrons, resulting in a net energy savings and more stability. Larger, more complex elements can have multiple s-orbitals with different energy levels as well as p-, d- and f-orbitals, which have varied shapes and capacity, leading to more diversity in electronic structure and more possible types of chemical reactions.
"In nature, a hydrogen atom can only support its electron using its only orbital resource, the 1s orbital," Kuo said. "But two hydrogen atoms can get together and say, 'we have two electrons and two orbital resources, what's the most efficient way to share the burden amongst our resources. Most organic elements have only s- and p-orbitals, but the transition metals add d-orbitals to the mix."
In most descriptions of oxidative addition, transition metals are said to donate their electrons to organic substrates during the binding process. The close proximity of the organic molecule to the transition metal allows the two sets of orbitals to mix, driving many types of reactions. Because of this, there has been much effort to develop transition metal compounds that are electron dense, which would potentially make them more powerful activators.
"It has, however, been noted that some oxidative additions are a little different," Kuo said. "A subgroup are actually accelerated by transition metal compounds that are electron deficient. We were able to identify a plausible explanation, where instead of the transition metal donating elections, the first step in the reaction involved electrons moving from an organic molecule to the transition metal. This type of electron flow, known as heterolysis, is well-known, but had not previously been observed to result in a net oxidative addition."
The research team used compounds containing the transition metals platinum and palladium - which were not electron dense - and exposed them to hydrogen gas. They then used nuclear magnetic resonance spectroscopy to monitor changes to the transition metal complex. In this way, they could observe an intermediate step that indicates hydrogen had donated its electrons to the metal complex, prior to approaching a final resultant state that was indistinguishable from oxidative addition.
"We are excited to add this new play to the transition metal playbook," Kuo said. "Showing that this can occur opens up new and exciting ways we might use transition metal chemistry. I am especially interested in finding reactions that could break down stubborn pollutants."
In addition to Kuo, the research team includes first author Nisha Rao, a graduate student in chemistry at Penn State. The Penn State Eberly College of Science supported this research.
