Silabenzene

Silabenzene is one of those chemical curiosities. With little use except to theoretical organic chemists, it’s a benzene molecule with one carbon atom replaced by a silicon “pendant” atom. A heterocyclic aromatic.

For a long time it was believed to be unstable and impossible to isolate, having only been observed in matrix isolation. In fact, chemists tried for years to succesfully synthesise the stuff, only finally succeeding in 2000. They couldn’t even make silabenzene itself (as in my litle diagram here), because of its high reactivity. Instead they needed to use a steric protective group — in other words, they attached a bulky molecule to the silicon to stop anything getting close enough to react with it! The first silicon heterocycle created was 2-silanaphthalene, synthesised three years earlier in 1997.

Now benzene, as you may know, is a very stable molecule, thanks to its big delocalised cloud of π-bonded electrons. That means that in chemical reactions, it’s a lot easier to attach something to a benzene ring than to break it open. Carbon-carbon bonds are tough.

Silicon, on the other hand, is rubbish at forming π-bonds. It’s especially rubbish at forming π-bonds with carbon atoms. Even though silabenzene is actually aromatic, the carbon tends to suck all the electrons away from the silicon, making a highly polar bond which is quite easy to split apart. As a result, Si-C double bonds are highly reactive.

I have to wonder though, if it were possible to create one, how stable a polycyclic molecule like pyrene might be with a silicon atom trapped in the centre of it…

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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13 Responses to Silabenzene

  1. Hambeleleni Kombada says:

    Im am currently working on the reaction of reacting Silabenzene with ozone. i just want some iformation on the reaction on silabenzene and its synthesis

  2. There is some fun chemistry going on on the toxin receptor part of the molecule. The upshot of which is that the Pc part is slightly oxidised and we see this as either a shift in colour or a change in refractive index.

  3. invaderxan says:

    Oh, that sounds interesting… Out of interest (if you don’t mind my asking), what’s the mechanism behind the optical change?

  4. invaderxan says:

    Nice. Thank you. I’ll giveit a proper read when I can. I’ve been thinking recently, I don’t read enough chemistry papers…

  5. Here you go– http://dx.doi.org/10.1021/jo802171t
    I thought it was really cool…maybe you will, maybe you won’t :)

  6. Ahh, right. wasn’t sure if you were just looking to get a silicon stuck in a ring, or if you wanted actual Si-C bonds.
    I’m playing with several phthalocyanines at the moment making pesticide sensors. The phthalocyanine bit gives us an easy to detect optical change, and an addition to the phthalocyanine head gives up toxin specificity.

  7. invaderxan says:

    Actually… Could you give me a little more info on Don Tilley’s paper, please? I’m having a little trouble finding it — and I’d love to read it!

  8. Totally! Nice emissive materials…IIRC Antonio Facchetti has done quite a bit with siloles, and Don Tilley recently had a SUPER COOL paper out on metallafluorenes. ;)

  9. invaderxan says:

    It’s a good idea, but phthalocyanines (and other porphyrin derivatives) are chelating agents. The lone pairs of those central nitrogens form ligands to a central metal atom — and yes, silicon could be chelated this way.
    I’m looking more at organo-silicon compounds here though, with direct carbon-silicon bonds. :)
    Cheers though!

  10. invaderxan says:

    Oh, that’s true. They have potential uses in electronics, don’t they?
    I was concentrating on benzenoids for this post, to be honest. You’re right though, I should write something about siloles sometime! :)

  11. I’m not sure of the chemistry, but phthalocyanines are pretty stable cyclical molecules you can trap metals in – I’m sure (although this is early Sunday morning before my 4th cup of Tea sure) that you can put silicon in there in place of a metal.
    I’ve no idea if this gives the type of chemistry you are looking for.

  12. But what about siloles and silafluorenes? Pretty interesting for the materials people…

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