Acceptorless, Intramolecular, Alkyl Dehydrogenation in the Solid-State in a Rhodium Phosphine Complex; Reversible Uptake of Three Equivalents of H₂ per Molecule

Thomas M. Douglas, Andrew S. Weller, New J. Chem., 2008, DOI: 10.1039/b718615k 

Prof. Andrew Weller and a co-worker at the University of Oxford have developed a remarkable system for the hydrogenation and dehydrogenation of a cyclic alkane. Thanks to their work, this process can now be achieved in the solid state, without any need for a hydrogen acceptor. Andrew Weller speaks to NJC about it.

1.   How did you get interested in this research project?

We had previously reported that the reversible alkyl dehydrogenation of cyclopentyl phosphine occurs readily in solution and we were therefore interested to see if the same process happened in the solid state. Our thoughts were that as only hydrogen is being added and removed this might be a perfectly feasible reaction to attempt in the solid state. This project is part of a larger on-going research theme into low-coordinate cationic rhodium complexes that have application in areas such as C-H activation, catalysis and hydrogen storage. 

2.   What is the most important result in the paper and its implications?

Alkyl (and alkane) dehydrogenation usually needs a sacrificial hydrogen acceptor to drive the reaction. With some of our cyclopentyl phosphine systems an acceptor is not necessary, and once a vacant site is generated on the rhodium metal centre, C-H activation is quickly followed by beta-hydride transfer and loss of dihydrogen. In the work presented in NJC we show that this process occurs in the solid state and is reversible: addition of dihydrogen re-hydrogenates the alkene while simple removal of an H₂ atmosphere results in dehydrogenation. 

Solid state organometallic chemistry is an area that is only sparsely reported, and to show that such a challenging reaction can occur in the solid state is a very interesting result. Moreover, the system actually takes up and releases 3 equivalents of H₂ per molecule, as Rh hydrides are also reversibly generated in the process in addition to hydrogenating the alkene. While these molecules could not be useful for the practicable storage and release of H₂ (too expensive, too low H₂ uptake), the fact that H₂ is being stored in an alkyl group and can be released at room temperature and pressure could well be of relevance to the storage and release of H₂for future energy applications.

3.   Are there any particular challenges facing future research in this area?

As with many solid state reactions, characterising the product is significantly more difficult than with solution chemistry. The real challenge lies in finding ways to reliably characterise and probe the subtleties of structure and reactivity in the solid state in materials that are at best microcrystalline (and so single-crystal X-ray diffraction is not suitable) and then use this information to drive new chemistry in the solid state. One of the exciting facets of solid state organometallic chemistry is that vacant sites generated on a metal centre are not bound with solvent molecules, which can suppress or change reactivity (even dichloromethane binds to sufficiently unsaturated metal centres), and thus the systems may react with incoming substrates in different ways than in the solution phase.