Sciencemadness Discussion Board

cyclopentadiene to pentamethyl-cyclopentadiene (Cp to Cp*)

Antwain - 3-11-2007 at 22:11

Soon I will be getting a small amount of dicyclopentdiene. I was wondering if there was an easy way to convert this to pentmethyl-cyclopentadiene?

The only way I can think of that I can actually do is:

1) crack the dimer

2) dissolve in xylene and add sodium to form the cyclopentadienyl ion.

3) react with methyl iodide

4) repeat steps 2 and 3 four more times.

This is doable, but hardy a nice solution and with losses in each step, especially a side reaction during the deprotonation, the yield will not be great. Any ideas?

Edit- changed the name of the thread to something more understandable.

[Edited on 5-11-2007 by Antwain]

Antwain - 4-11-2007 at 15:27

And, nudge! :)

trilobite - 4-11-2007 at 17:47

Using a strong non-nucleophilic base would be a good idea. Maybe LDA, t-BuOK or something like that. Then your methyl iodide would be preferentially reacting with cyclopentadienyl anions instead of the base and you could be using five equivalents of the base from the beginning.

Another possibility is the org. synth. way:
Maybe there is a reason why it isn't made of cyclopentadiene.

yan123 - 5-11-2007 at 02:59

i think it is too hard!!

garage chemist - 5-11-2007 at 03:44

What is your interest in such an obscure material? Does it have some unique properties or applications?

Antwain - 5-11-2007 at 03:49

it forms complexes related to ferrocene. When I get back to lab work proper I will be attempting a major campaign of synthesising sandwich compounds, but first I need to get the reagents.

Nerro - 5-11-2007 at 05:55

The penta-Me variety is not a very interesting compound I think. It will have a higher molar mass than ferrocene but otherwise I don't really see it.

Have you considered Cp-(CH<sub>2</sub>;)<sub>n</sub>-Cp (n = 3, 4, 5, 6)? Sidegroups might be added to facilitate the dissociation of only one of the Cp's.

[Edited on 5-11-2007 by Nerro]

Antwain - 5-11-2007 at 23:40

@ Nerro- From my learning, it is the second most important cyclic pi donor, after Cp. The methyl groups add steric bulk and donate electrons, allowing stronger bonding. From a wiki article about cyclopenadieneyl complexes:

Cp* is an important ligand in organometallic compounds arising from the binding of the five ring-carbon atoms in C5Me5-, or Cp*-, to metals. Relative to the more common cyclopentadiene (Cp) ligand, pentamethylcyclopentadiene (Cp*) offers certain features that are often advantageous.

Being more electron-rich, Cp* is a stronger donor and is less easily removed from the metal. Consequently its complexes exhibit increased thermal stability. Its steric bulk allows the isolation of complexes with fragile ligands. Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures. Its complexes also tend to be highly soluble in non-polar solvents.

It is also interesting that wiki (admittedly an unreliable source) only digresses to this ligand and no other substituted cyclopentadienes

The only downside is that the higher electron density of the ligand also makes the complexes more susceptible to air oxidation.

And what do you mean by the dissociation of only one ligand? Why would I want either of them to dissociate?

I have made ferrocene before at uni which was vaguely interesting. I have never purified a metal complex by sublimation before :P. We also did a friedel-crafts acylation with Ac2O, which changed its colour from orange to very deep red. Apparently Cp*2Fe is yellow :).

The only shame is that iron is the very best metal for this, and basically the only one that isn't air sensitive. I don't know about ruthenium, but I think its too big to fit snugly between the rings. It is still probably the second best metal but I don't have any. Its not exactly cheap either, but its better than the element next to it :D

[Edited on 6-11-2007 by Antwain]

Nerro - 6-11-2007 at 14:16

You'd want one of the CP's to dissociate if you're going to use the complex as a catalyst. I have no idea why you'd want that but I was just basically thinking out loud.

It's true that with Cp* the complex is more stable but why exactly do you want the complex to be more stable? It's already one of the most stable complexes I've ever made. I have a four year old plastic bottle of it standing here without any signs of degradation or formation of oxides. Like you said, it survived being sublimated!

Actually polymeric complexes sound very interesting! Fe's linked by linked Cp rings! Interesting!

Antwain - 6-11-2007 at 15:07

Actually polymeric complexes sound very interesting! Fe's linked by linked Cp rings! Interesting!

Such things do exist, though that is further down the road than I am looking. There are some interesting ones based on diphenyl, or perhaps more technically these days phenyl-benzene and benzene on chromium(1).

Well I have done the ferrocene before as I mentioned but I had to hand that in for assessment, I want it to look pretty in a jar for the moment. Part of the fun is making it and part is having it afterwards. Perhaps I will find a use for it one day.... there are reactions I have seen for displacing Cp with various things. And in that sense, Cp* is less useful. But I will make some anyway, if I can.

There is a great one called the 'club sandwich' [Ni2Cp3]+ :D

nitroglycol - 6-11-2007 at 19:08

Originally posted by Nerro
The penta-Me variety is not a very interesting compound I think. It will have a higher molar mass than ferrocene but otherwise I don't really see it.

I imagine the penta-CF3 would be interesting. The simple compound, without the iron, is reportedly more acidic than nitric acid, according to March's Advanced Organic Chemistry, 4th ed.