Taming Elusive Molecules For Energy Conversion, Pollutant Removal, And Generation Of Valuable Resources

Every day we are surrounded by chemical processes that improve our quality of life, while most of us are not even aware that they exist, and those who are aware are often left astonished at how nature can transform molecules.

A well-known example is the conversion of toxic pollutants to nonhazardous products in an automobile emission control catalyst. Or take one of the most important processes of the chemical industry, which is the oxidation of ammonia by oxygen. Each year, more than 30 million metric tons of nitric acid is produced, which essentially feeds the world because it is the precursor to fertilizer. Another fascinating example is the conversion of solar into more useful chemical energy, as exemplified by photosynthesis.

The common feature of these reactions is the activation of very strong chemical bonds. Yet, the reaction mechanisms for these processes are only poorly understood. However, there is a consensus in respect to the importance of the chosen metal catalyst and the presence of reactive intermediate species, which forms on the catalyst surface.

It has been suggested that these key intermediates are able to activate strong bonds due to special structural features, which are highly reactive terminal metal-oxygen or metal-nitrogen bonds.
The synthesis and isolation of such key intermediates are highly desirable in order to understand these important processes and design better catalysts in the future.

Thus, we wanted to find common trends concerning the properties of molecules which stabilize palladium complexes with such terminal oxo and imido groups. Such bonding motives are one of the most important classes of such highly reactive intermediates. We conducted, therefore, a theoretical computational study on the stability and reactivity of such compounds. Thereby aiming at modeling these elusive molecules prior to the synthesis in our laboratories in order to minimize experimental efforts and reduce costs accordingly.

Our calculations showed two possible reactive intermediates, either a palladium oxo compound with the metal in a low oxidation state (II) or a high valent palladium(IV). Because high valent palladium(IV) metal cores are electron deficient, our computations predicted that a high priority is a donation of electron density to the metal. Following those predictions, we believe that the synthetic isolation of palladium terminal oxo compounds is feasible given an appropriate environment surrounding the metal center.

We are currently following these computational predictions experimentally in the laboratory and are pursuing our dream to isolate these elusive compounds. We hope to develop eventually new catalysts for energy conversion and storage as well as the synthesis of valuable base chemicals.

These findings are described in the articles entitled How to tame a palladium terminal imido, recently published in The Journal of Organometallic Chemistry and How to tame a palladium terminal oxo, recently published in the journal Chemical Science. This work was conducted by Annette Grünwald and Dominik Munz from the Friedrich-Alexander Universität Erlangen-Nürnberg.

References

1. Chem. Sci. 2018, 9, 1155.
2. J. Organomet. Chem. 2018, 864, 26.