Tag Archives: Hydrogen

Frustrating Fuel Cells.

This is not the first time and probably will not be the last time I post about Frustrated Lewis Pairs (FLPs). Due to the unquenched acidity and basicity of FLPs, these systems present an extraordinary reactivity to the cleavage and activation of small molecules. Unarguably, the most important application is the activation of hydrogen, FLPs are capable to heterolitically cleavage the strong bond of the molecule of dihydrogen resulting in a hydride adduct of the Lewis acid and a protonated Lewis base at room temperature. From the point of view of a synthetic chemist, the activation of hydrogen with FLPs opens the door to a new class of metal-free hydrogenation reactions. However, creativity in the area of FLPs seems to be endless as shown in the great contributions by Andrew Ashley and Gregory G. Wildgoose groups from the Imperial College of London and the University of East Anglia towards the oxidation of hydrogen.

FLP Fuel Cell

In the search for alternatives to fossil fuels, hydrogen has raised as a promising and a clean source for the production of electricity from chemical energy with the use of fuel cell technology. In the absence of a catalyst, the necessary process of oxidation of hydrogen is slow and require large overpotentials. Here is where the FLPs play their role as they considerably reduce the voltage required for the hydrogen oxidation due to the generation of hydride intermediates that are easier to oxidise to protons. Ashley, Wildgoose and co-workers were able to stoichiometrically oxidise hydrogen using one of the most basic FLP system B(C6F5)3/P(tBu)3 but unfortunately the system is not robust enough to complete more than one catalytic cycle. Some improvements were made recently replacing B(C6F5)3 for a borenium cation as a Lewis acid although the system is still lacking enough stability to properly catalyse the oxidation. As a proof of principle, it is indeed possible to oxidise hydrogen with an electrochemical/FLP approach. FLP systems have the advantage of their inherent “tuneability”  and there is still plenty of room for improvement in the way to develop a FLP based fuel technology.

Do you want to know more? Here you are the original papers:

Elliot J. Lawrence, Vasily S. Oganesyan, David L. Hughes, Andrew E. Ashley, and Gregory G. Wildgoose. An Electrochemical Study of Frustrated Lewis Pairs: A Metal-Free Route to Hydrogen Oxidation. J. Am. Chem. Soc., 2014, 136 , 6031–6036.

Elliot J. Lawrence, Thomas J. Herrington, Andrew E. Ashley, Gregory G. Wildgoose. Metal-Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach. Angew. Chem. Int. Ed. 2014, 53, 9922 –9925.

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Frustration to a Good End.

Frustration: “The feeling of being upset or annoyed as a result of being unable to change or achieve something”. It sounds like a very negative feeling that every researcher has felt at some point at their careers. It is not the case for the topic of Frustrated Lewis Pairs, where frustration is actually a good thing. But, what is a Frustrated Lewis Pair (usually abbreviated as FLP)?, well not that fast, let’s start for the beginning.

frustration

One of the basics of chemistry reactivity is that the combination of a Lewis acid and a Lewis base leads to the formation of a classical Lewis adduct as exemplified by the combination of borane with ammonia to form the ammonia-borane adduct NH3.BH3. But, what happens if we introduce steric impediments in both the Lewis acid and the Lewis base? Then, it comes “the frustration” of the adduct. Do not mistake this with the actual frustration of a chemist attempting a reaction that does not work. In this situation, the steric demands preclude formation of simple Lewis acid-base adducts and then is when we have a Frustrated Lewis Pair.

pubications in FLP

In this very particular scenario where both acidity and basicity remain unquenched, FLPs have an extraordinary reactivity towards the cleavage and activation of small molecules such as hydrogen, alkenes, alkynes or CO2 among others. Unarguably the most important and more developed applications are within the fields of activation of hydrogen and catalytic hydrogenations. The number of publications and citations in Frustrated Lewis Pairs chemistry has been increasing since the first  publication in 2006 by Douglas Stephan, one of the “fathers” and most active researchers of the topic.

hydrogenacion

In my opinion there is still a long way to walk and we’ll see in the next years new developments in catalytic asymmetric hydrogenation along with applications in areas such as hydrogen storage, CO2 capture and fuel cells.

If you want to know more there are many good articles and reviews on the topic, these could be a starting point:

Seminal work by Douglas Stephan’s group

Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, Douglas W. Stephan. Reversible, Metal-Free Hydrogen Activation. Science, 2006, 314, 1124-1126.

A couple of reviews for newcomers to the topic

Stephan, Douglas W. “Frustrated Lewis pairs”: a concept for new reactivity and catalysis Organic & Biomolecular Chemistry (2008), 6(9), 1535-1539.

Stephan, Douglas W.; Erker, Gerhard. Frustrated Lewis Pairs: Metal-​free Hydrogen Activation and More. Angewandte Chemie, International Edition (2010), 49(1), 46-76.