Tag Archives: inorganic chemistry

Grignard Goes Inorganic

Great news for the inorganic chemists!!! A classical strategy widely used for decades in organic chemistry is now available for the synthesis of bimetallic species. A new Mn-Mg bonded compound was recently discovered by the group of Cameron Jones in Australia while trying to synthesise new Mn-Mn complexes. The direct comparison with the well known Grignard reagents gives the new Mn-Mg based complex the qualification of “inorganic Grignard reagent”.

inorganic grignard

Normal Grignard reagents, or maybe more correctly now Organic Grignard reagents, are organomagnesium based strong nucleophiles used mainly but not exclusively for the formation of carbon-carbon bonds. Discovered by French chemist François Auguste Victor Grignard who was awarded the 1912 Nobel Prize in Chemistry, Grignard reagents have become an essential tool in organic synthesis.

reacciones_2

The inorganic version, the so called “Inorganic Grignard reagents”, are not a new concept although their use is yet far from been a general tool for the creation of metal-metal bonds. The new Mn-Mg bonded compound discovered by Jones and co-workers was successfully utilised to transfer the Manganese fragment for the preparation of the first two-coordinate manganese(I) dimer, and the related Mn-Cr hetereobimetallic complex.

Do you want to know more? Take a look to the original paper.

Jamie Hicks, Chad E. Hoyer, Boujemaa Moubaraki, Giovanni Li Manni, Emma Carter, Damien M. Murphy, Keith S. Murray, Laura Gagliardi, and Cameron Jones. A Two-Coordinate Manganese(0) Complex with an Unsupported Mn–Mg Bond: Allowing Access to Low Coordinate Homo- and Heterobimetallic Compounds. J. Am. Chem. Soc., 2014, 136, 5283–5286.

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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.

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.

X-Ray Snapshots. Mechanism Study on Pd-mediated Aromatic Bromination.

The study of the mechanism of a reaction is always a complicated task that usually starts with a draft on paper. You apply your knowledge in chemistry to make a hypothesis that you then need to prove with experimental facts. Even when the analyses performed in the lab strongly support your proposals you will never have complete certainty if you don’t have “the photo” of your “intermediate suspects”. A common strategy for the study of reaction mechanisms involving organometallic species is the synthesis of the proposed intermediates (your suspects) and then try to capture “the photo” with the use of X-Ray crystallography techniques. I mean, you have to pray for your compounds to crystallise…

reaction photosnap

Because I have suffered all these problems in the lab is why I consider so remarkable the mechanistic study of a Palladium-mediated aromatic bromination published by Fujita and co-workers in 2014 in JACS. The strategy carried out by the authors consists of the use of a crystalline flask of ZnI2 and tris(4-pyridyl)triazine in which the Palladium complex object of the study is embedded. The reaction is performed in the crystalline flask and the intermediates captured in-situ by time dependent X-Ray diffraction. The authors literally take “photos” of the catalytic cycle of the reaction and show in-situ cristallography as a great tool for the elucidation of reaction mechanisms.

Do you want to know more? Then go directly to the source.

Ikemoto, Y. Inokuma, K. Rissanen and M. Fujita. X‑ray Snapshot Observation of Palladium-Mediated Aromatic Bromination in a Porous Complex. J. Am. Chem. Soc. 2014, 136, 6982-6985