Jor
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failure [Cr(en)3]Cl3 ???
I tried the synthesis of tris(ethylenediamine)chromium(III) chloride, following this procedure:
http://wwwchem.uwimona.edu.jm/lab_manuals/c21jexpt.html#1a
So I added 1,30g of chromium(III) chloride (wich became very wet and hard to handle during weighing) to a 100mL RBF followed by 5mL ethylenediamine
(this stuff fumes very bad, like 37% HCl!!), and 5mL methanol. Next 0,45g of granular zinc was added, and the mixture was refluxed for about 50
minutes.
This way i obtained a purple liquid (?). This was poured in an erlenmeyer, and the RBF was rinsed once with 1-2mL of methanol, and this was also added
to the flask. By this time, a lot of purple material crystallised out. I don't understand this, as it is supposed to be yellow!
I was wondering what could have caused this. I was thinking that it could possible be that one or more of the 'en' ligands was replaced by water,
because my chromium chloride was quite wet, but I realised this could't be possible, because even de dry hydrate contains 6 moles of water.
So maybe it was a chromium(II)-compounds? I really doubt it, as the solution was in contact with air for at least 2 minutes, so it should oxidise very
fast and change of color. Besides the purple color was formed even after just mixing the starting materials and it was strong when the solution was
refluxing.
Interesting was that quite some zinc (almost everything I started with, at least that was what i guessed) remained as the metal in the flask. But
according to the link above, at least some zinc should remain.
You guys have any idea? All the chemicals used were lab-grade materials, except the chromium(III)chloride, wich was made by dissolving 99,9+% chromium
in concentrated hydrochloric acid (analytical reagent) followed by evaporation.
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watson.fawkes
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A purple colored species at low concentration would optically mask a yellow. Do you
have a prism or diffraction grating to look at the product with? It would give you some ballpark idea of product ratios.
The zinc seems to be acting as a catalyst, storing chlorine while the ligand attaches. You added zinc last, however. Is it possible you had a reaction
before the zinc was added that led to a byproduct? The Wikipedia page on the cobalt analogue mentions a rare monodentate ligand, which might have more favorable formation conditions if the chlorine atoms bound to the
chromium center have fewer places to go.
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Jor
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Zinc acts as the reducing agent, to reduce Cr(III) tot Cr(II). Cr(II) readily exchanges ligands, while Cr(III) complexes are highly inert. This is why
pure CrCl3 takes years or even longer to dissolve in water, but if a small amount if zinc is present, dissolution is rapid. The Zn does NOT act as a
catalyst, it is not regenerated. That's why I wondered so much zinc remained....
So I think what you say can't be the explanation. However, I noted an immediate change in color when adding the MeOH/H2NCH2CH2NH2 to the chromium
chloride. Maybe some Cr(II) was still present (because the compounds was prepared from HCl and Cr, wich produces Cr(II) as an intermediate:
http://woelen.homescience.net/science/chem/exps/chromium2/in...
I will try to bubble air into the solution using a syringe. It could very well be possible that not all Cr(II) has been oxidised, because i refluxed
very vigorously, wich excludes oxygen, and thus prevents the formation of the desired complex. Maybe I should do this again, and reflux while a
bubbles (passing through O2) is in the solution. Problem is I am out of the hydrated chromium chloride. I am currently making some more (from 3,5g of
99,9+% Cr-powder).
I really wonder if purple is actually masking the yellow crystals. It all looks very, well, purple! 
But maybe you're right. But I do not have a prism.
Or, If I had really bad luck, the ZnO-layer on the zinc prevented the reaction to proceed. Maybe I should have cleaned it with some 0,1M HCl...
[Edited on 12-10-2009 by Jor]
Edit(woelen): Changed link to my website, so that it works again.
[Edited on 26-10-13 by woelen]
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watson.fawkes
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Quote: Originally posted by Jor  | | The Zn does NOT act as a catalyst, it is not regenerated. That's why I wondered so much zinc remained. |
The
reason I had for this was that, per the web page you cited, the initial chromium salt has three Cl per Cr and so does the final complex, so there's no
excess chlorine (in perfect efficiency).
Some less plausible ideas I've had: Perhaps the ethylenediamine is dimerizing to diethylenetriamine, and that this is acting as a ligand. Perhaps the
complex you have is (CrCl3)(en)x, where, if the Cr wasn't dissolved, the ligand was attaching to the molecule as a whole. Or perhaps that the
ethylenediamine was attaching to two Cr centers.
As for your Cr(II) hypothesis, if you have a Cr(II) salt you might just try that directly.
Another thing you might try on a subsequent run is to mix the zinc and chromium chloride first in methanol, adding the ethylenediamine last.
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woelen
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I do not understand at all what is the role of the zinc in this reaction. I can imagine reduction of Cr(3+) to Cr(2+), but what compound does the
reoxidation back to Cr(3+). In the refluxing operation there hardly is any air, which can reach the reaction mixture. Besides that, the redox reaction
and back-oxidation by oxygen from air would make the reaction mixture highly alkaline. So, I am inclined to think that the Zn only is needed for
making a tiny amount of Cr(2+) and when this is formed, then electron transfer goes from one Cr-atom to another. If the latter is the mechanism, then
hardly any zinc will be used up.
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