Uranium—A Promising Catalyst: The Hydrogenation of Ethene to Ethane

Sydney Hirsch | sydney.hirsch@yale.edu March 15, 2020

Uranium—A Promising Catalyst: The Hydrogenation of Ethene to Ethane

In chemical processes, catalysts are molecules that increase the rate of a reaction. At the end of the reaction, the catalyst is replenished, an important property that allows it to be used in a much smaller amount than the reactants. Some of the most common catalysts in organometallic chemistry are complexes of transition metals and alkenes— carbon compounds containing one or more double bonds. A team of scientists at the University of Sussex recently discovered one such transition metal–alkene complex, specifically a uranium–pentalene complex that catalyzes the hydrogenation (breaking of the double bond) of ethene (C2H4) to ethane (C2H6). In the reaction of this uranium(III) complex with ethene, the researchers observed the formation of an intermediate ethene-bridged diuranium complex. In this complex, ethene binds as a ligand to the uranium portion in two places, and the oxidation state of uranium increases from +3 to +4. The scientists were encouraged to investigate this phenomenon further after noting that the intermediate complex reacted readily with hydrogen gas to produce ethane. This means that uranium could serve as a catalyst to convert ethene to ethane, a fundamental but elusive reaction that, if done efficiently on an industrial scale, could revolutionize how we make molecules.

“Uranium in the oxidation state +3 is known to be strongly reducing for polar molecules such as carbon monoxide,” said Richard Layfield, one of the researchers in the study. This means that uranium in this form tends to give electrons to other molecules. However, due to ethene’s low polarity, the researchers assumed that it would be a poor acceptor of these electrons. To show that the uranium complex did indeed fully convert ethene to ethane, they performed additional characterization procedures to confirm the formation of ethane. Natural bond orbital analysis, which looks at bonding orbitals with the greatest electron density, confirmed the formal double reduction of ethene, showing that the uranium-carbon bonding orbitals were strongly polarized towards the carbon. The researchers were able to determine that the C-C bonds of the ligand had lengthened from 1.332 Angstroms, consistent with a double bond, to 1.497 Angstroms, consistent with a single bond. Therefore, it seemed as if the alkene had been transformed into an alkane.

The scientists at Sussex first observed the formation of uranium complex through nuclear magnetic resonance (NMR) spectroscopy, which analyzes the components of mixtures. It does so by applying an external magnetic field and recording the frequencies emitted by the excited electrons; these various recorded emissions yield a spectrum for analysis. The team observed features reminiscent of a dinitrogen-bridged uranium complex, and from there predicted the presence of the complex. After confirming the molecular structure of the complex, the researchers ensured with solution NMR that the solid-state structure did not change in solution. “When you have a picture from the diffraction measurement, it is easy to be tempted into thinking that the molecules look the same when you dissolve them in solvent,” Layfield said. “Our chemical reactivity takes place in solution, so if you make a reactivity prediction in solution based on a solid-state structure you should gather evidence that the structure is not significantly different in the two different environments.”

After a comprehensive analysis, the scientists were able to confirm that this complex is a suitable catalyst for the formation of ethane from ethene. Their research represents the first use of a uranium catalyst for alkene-alkane conversion. The conversion of alkenes into alkanes opens the door to the possibility of upgrading simple molecules into valuable petrochemicals used as energy sources; for instance, ethene specifically is often used in the making of plastics. Also, depleted uranium is generally a feared element, stored in expensive facilities, and only used for military or aircraft purposes. According to Layfield, however, their discovery may allow for its application in synthetic chemistry, slightly enabling the scaling-up of uranium based-catalysts.

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