A closer look at molecular bonding
ChemSci Pick of the Week
If you’re not a hardened organometallic chemist, today’s ChemSci Pick of the Week may seem a little impenetrable at first glance. To bring this promising research to a slightly wider audience, we’ve thrown in a handy diagram and a detailed explanation of what’s going on, so do not be deterred!
A Spanish–Indian collaboration of chemists from Indian Institute of Science Education and Research Pune and Universitat de Barcelona have synthesised a molecule previously thought to be unstable – a η6 benzene complex with copper (I). If that means nothing to you, read on!
What this means is that a copper (I) cation (that is a copper atom with a single positive charge) is bonding to a benzene ring – a large, flat molecule consisting of 6 carbon atoms in a ring, each bonded to one hydrogen atom. Benzene is electron-rich, meaning it is happy to interact with the electron-poor cation.
In most molecules electron bonds are 'stationary' links which connect two atoms. Benzene, however, is an 'aromatic' molecule; all of the electron bonds in benzene are delocalised – that is spread around across either side of the molecular ring. Picture a large cloud of electrons hovering over the whole ring, and another one beneath. This is actually an oversimplification, but it’s a useful way to visualise what’s going on. The electron cloud in aromatic molecules is known as a π-system.
Making contact
Because of this delocalised π-system, the benzene ring has a few options for how it can interact with metal cations. It can interact via one, two, or six points of contact. These points of contact are known as hapticities, and are represented by the Greek letter eta, or η. So an η1 complex has a hapticity of one – that is, the benzene ring is interacting with the copper cation via one point of contact. Picture the benzene ring angled end-on, presenting just the carbon atom at the end for bonding to the copper cation.
There could also be an η2 complex, where the benzene ring is still end on, but is presenting its flat end to the copper cation, so that the two carbon atoms at the end can interact with the copper atom.
The final option is an η6 complex, where all six carbon atoms are involved in the bonding, via the delocalised π-system that we’ve discussed before. In other words, it is presenting its flat side to the copper cation. For this to work, there needs to be enough space around the cation to fit the large benzene molecule in, sideways-on.
And because there’s such a large concentration of electrons in the π-system, it is likely to be repelled by the copper cation’s own large electron cloud.
Challenge accepted
Because of these challenges, the η6 complex is harder to achieve than the others, which is exactly why the researchers are trying to synthesise it.
"Being a synthetic chemist, it is always fascinating to challenge formidable targets", says Shabana Khan of the Indian Institute of Science Education and Research Pune. "In a previous communication, it was proposed that eta 6 benzene complex with copper(I) is difficult to isolate and hence, we chose this topic."
To achieve this, the authors have designed a ligand – that is another molecule for the copper cation to bond to on the opposite side from the benzene ring. This ligand is designed in such a way that it is able to accept some of the electron charge from the copper cation, relieving such of the repulsions between the cation’s electron cloud and the benzene π-system.
This complex could have applications as a catalyst in various synthetic reactions.
This article is free to read in our open access, flagship journal Chemical Science: Nasrina Parvin et al., Chem. Sci., 2018. DOI: 10.1039/C8SC00459E
ChemSci Pick of the Week
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