Helical Silverback

The helicene-NHC-Iridium complex. Notice the twisted nature of the helicene and also the chiral-at-iridium attribute.

I have always been fascinated by the organometallic chemistry of iridium (Ir), and this is a great paper about a new iridium complex from Chem. Comm. [1]. The researchers have reported the first synthesis of a class of helicene- N- heterocyclic carbene-iridium complexes in enantiopure forms. The chirality is due to the helical structure of the helicene (the fused benzene rings) and also the iridium center – so this is what we call a ‘chiral-at-metal’ complex. It is noteworthy that the chiroptical properties of the iridium complex have been investigated by circular dichroism (CD), something very familiar to protein scientists and finding more applications in stereochemistry and supramolecular chemistry in recent years, too.

The complexes were synthesized by a multi-stage approach, for which the last step involved [Cp*IrCl₂]₂, a common starting material for many organoiridium complexes. The complexes were purified by crystallization, because in many cases the undesirable chlorido complexes were also formed.

NMR, molecular rotation, circular dichroism, and X-ray crystallography were among some of the techniques which were used to characterize these novel iridium complexes. X-ray crystal data were particularly informative because that suggested the key cyclometallation occurred at one particular carbon on the helicene (rather than another one) due to steric considerations, meaning that the helical structure was twisted in a particular way.

An important insight gained from all the different experiments was that the formation of the iridium complex led to electronic coupling between the iridium center and the helicenic-NHC ligand. And the researchers believed it was the first example that the properties of the N-heterocyclic carbene have led to impact on the chiroptical properties of the resulting NHC-transition metal complexes.

All in all, this is great research and I love the beautiful architecture of the iridium complex!

by Ed Law

29/6/2016

Reference:

1.  Electronic and chiroptical properties of chiral cycloiridiated complexes bearing helicenic NHC ligands

Nora Hellou,   Claire Jahier-Diallo,   Olivier Baslé,   Monika Srebro-Hooper,   Loïc Toupet,  Thierry Roisnel,   Elsa Caytan,   Christian Roussel,   Nicolas Vanthuyne,   Jochen Autschbach,   Marc Mauduit and Jeanne Crassous
Chem. Commun., 2016, Advance Article

DOI: 10.1039/C6CC04257K

CO2 Medusa

Review on: Matter et. al., 'Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions', Science, 2016, 352, 6291, 1312-1314. [1]

This article [1], just published on the 9^th June issue of Science Magazine, has already received a lot of coverage on the web. Of course, having an intense passion in Chemistry and Geology, I am totally excited by this wonderful work!  It represents the two subjects holding hand-in-hand, contributing towards a better future.

The rising level of CO₂ in the atmosphere has been a serious issue, and many means have been devised to counteract this important factor that leads to global warming. A strategy is known as carbon capture and storage (CCS). It is the process of capturing excess anthropogenic CO₂, and to store and then deposit it to a sink, so that the CO₂ will not enter the atmosphere. The deposition site is usually a geological formation under the ground. Of course, deep ocean storage is not yet a feasible approach, because the introduction of CO₂ into ocean will lead to ocean acidification.

So what rock should we shoot the gas into? Most researchers tend to favor a sedimentary rock, sandstone, as the sink, because of the well-established research experience with this type of rock. Yet, a potential pitfall is that the fissures present in the rock layers of sandstone can lead to the leaching of CO₂ back to the atmosphere.

Thus, the researcher direction in the field has changed.  In this collaborative work, the researchers injected CO₂ into the igneous rock – the extrusive volcanic rock known as basalt. This is a great strategy because the minerals present in basalt can react with CO₂, and through a carbonation reaction, that results in the formation of the mineral calcite, a polymorph of CaCO₃. In this way, the excess CO₂ can thus be mopped up. The reaction rate was much faster than the rates modeled by computational methods. Also, the research represented a nice example of using an isotopic labeling technique (¹⁴C in this case) to characterize the formation of carbonate minerals from CO₂.

This is an impressive idea, yet the researchers also noted a possible obstacle to the generalization of this process is cost. Also, when a scaling up of the process is required, the reaction rate has to be acceptable to compromise the long-term cost. It is quite obvious a better understanding of the mechanism is required to fine-tune the carbonation reaction and to shut down other unproductive pathways – like the sandstone scenarios. To me, it seems that catalysis will be able to contribute to an improvement of the CCS process (Once a chemist, always a chemist?!). Indeed, I have found a paper back in 2013 in ‘Catalysis Science & Technology’, where the authors demonstrated the use of nickel nanoparticles as a catalyst for the mineralization reactions from carbon dioxide. [2] So, it seems that the door has already been opened and there are active research projects towards this direction. If the time course of the carbonation / mineralization reaction can be significantly shortened through the application of a low-cost catalyst, the process will become common place and then this can alleviate the problem of excess CO₂ in the long term.

One final chemical point of view is that we should bear in mind that CO₂ is a potential one-carbon building block. In photosynthesis, CO₂ is used to form the six-carbon sugar, catalyzed by the enzyme complex Rubisco. So, similarly, if CO₂ can be stored efficiently, this can be a great starting substrate for the production of other compounds, often employing organometallic catalysis.

All in all, it is a truly brilliant contribution, and it is more so because it has a great potential to lead to a better future.

by Ed Law

11/6/2016

Reference:

1. Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions.

J. M. Matter et. al., Science, 2016, 352, 6291, 1312-1314.

Further commentaries on:

http://www.sciencemag.org/news/2016/06/underground-injections-turn-carbon-dioxide-stone

http://science.sciencemag.org/content/352/6291/1262

2.  Nickel nanoparticles catalyse reversible hydration of carbon dioxide for mineralization carbon capture and storage.

G. A. Bhaduri and L. Siller, Catal. Sci. Technol., 2013, 3, 1234.