Tag Archives: Frustrated Lewis Pairs

Superbases in FLP chemistry

A new post for the series I am crazy about Frustrated Lewis Pairs (FLPs). If you are new to this exciting field I recommend you to read first one of my previous posts: “Frustration to a Good End”.superbases2The extraordinary reactivity of FLPs allows the stoichiometric and catalytic activation of small molecules being the most important application the heterolytic cleavage of hydrogen. The spectrum of Lewis bases used in FLP chemistry is wider compared to the Lewis acid partners. Phosphines, amines, N-heterocyclic carbenes or carbodiphosphoranes have been used, with a few exceptions, combined with boranes such as B(C6F5)3 and derivatives.

strategy

The activation of the strong H-H bond has been commonly achieved using a type of the Lewis bases mentioned above in the presence of strong Lewis acidic boranes. Krempner and co-workers have recently showed that the use of strong Lewis acids is not strictly necessary for the activation of hydrogen. In this new approach denominated as “Inverse” Frustrated Lewis Pairs, organic superbases combined with rather moderate and weak Lewis acids are capable to reversibly cleavage hydrogen. Organic superbases have an enormous proton affinity and are very strong due to the great stability of their conjugated acids once they are protonated. In their recent work Krempner and co-workers activate hydrogen using phosphazene based superbases with BPh3, HBMes2 or 9-BBN and extend the concept of Inverse Frustrated Lewis Pairs to the catalytic hydrogenation of organic molecules using N-Benzylidenaniline as model substrate.catalysisDo you want to know more? Read the original paper

Suresh Mummadi, Daniel K. Unruh Jiyang Zhao, Shuhua Li and Clemens Krempner. “Inverse” Frustrated Lewis Pairs – Activation of Dihydrogen with Organosuperbases and Moderate to Weak Lewis Acids. J. Am. Chem. Soc., 2016, 138 (10), pp 3286–3289.

Are you a newcomer to the topic? Take a look to this papers

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.

Advertisements

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.

An Unusual Lewis Acid Hydrocarbon.

I would like to highlight the definition of hydrocarbon, a very basic concept in organic chemistry but of relevance for this post. According to the IUPAC, hydrocarbons are compounds consisting of carbon and hydrogen only. It doesn’t seem obvious to imagine an organic compound composed only by carbon and hydrogen with Lewis acidity not being a charged species such as a carbocation. Back in 2010 the group of Manuel Alcarazo at Max Planck Institute of Coal Research published a  very creative paper describing the properties of a bisfuorenylallene as a Lewis acid. The natural tendency of the fluorene moites to accept a pair of electrons to gain aromatization gives the central carbon of the allene a carbocationic character and thus the ability to behave as a Lewis acid.

Unsual hydrocarbon lewis acid

A clear experimental evidence of the behaviour of the bisfluorenylallene as a Lewis acid is the formation of classical Lewis acid-Lewis base adducts in the presence of small Lewis bases. In another stroke of genius, Acarazo and co-workers apply the Lewis acid ability of the fluorenyl based allene to the field of Frustrated Lewis Pairs (a field dominated by borane-based Lewis acids) towards the activation of sufur-sulfur bonds.

Do you want to know more? Check this paper out.

Blanca Ins, Sigrid Holle, Richard Goddard, and Manuel Alcarazo. Heterolytic S-S Bond Cleavage by a Purely Carbogenic Frustrated Lewis Pair. Angew. Chem. Int. Ed. 2010, 49, 8389 –8391.

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.