Posted by: Mark Foreman | July 26, 2011

Titanium complexes which activate methane

Dear Reader,

the other day I was reading Chemistry world and I saw something interesting. It was a paper about a titanium complex which can activate methane gas. Now I might be getting a little jaded but I worry about overselling of chemistry and I am aware that alkane (paraffin) activation is a holy grail of organometallic chemistry.

If we could harness methane at low temperatures and be able to convert it into useful chemicals then it would make a lot of money and save the environment from pollution. So with great interest I choose to read the paper (Jaime A. Flores, Vincent N. Cavaliere, Dominik Buck, Balazs Pinter, George Chen, Marco G. Crestani, Mu-Hyun Baik and Daniel J. Mindiola, Chemical Science, 2011, 2, 1457) which is entitled “Methane activation and exchange by titanium-carbon multiple bonds”.

While on the train before I could get to work and read a copy, I choose to do a search of the Cambridge database for that type of titanium complexes. I searched for the following fragment.

What I found in the first structure was a bit weird looking at first, the carbene carbon seemed to be linear and sp type rather than the sp2 type which I was expecting. I checked the other examples of this type of complex and again they were all weird in this way.

The Ti-C-R angle is 175.5 degrees, while is much bigger than the Cr-C-C angle of 125.5 degrees in (Ethoxy(ferrocenyl)methylidene)-pentacarbonyl-chromium(0). Also the Ti-C distance of 1.867 Å seemed short, I measured 2.084 Å for the chromium complex, while chromium is a different metal to titanium I still thought that the Ti-C distance was rather short so I did a search of all
titanium carbenes. They all seemed to be “short”

I have to admit “I ♥ 18 VE rule”, I applied this rule to the complex and I worked out that the complex has 12 valence electrons so it is 6 short of the magic 18. Based on the shape and the bond lengths I came to a view that it is more like this.

This carbyne complex will have 14 VE which is closer to the ideal of 18 VE, while I have a great love of the 18 VE rule it is important to bear in mind that for the far left of the periodic table there is a problem with packing the required number of ligands around a metal. Thus as a result things like Cp2ZrMe2 (16 VE) is quite stable.


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