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I am a second year university student, so I'm afraid my chemistry isn't great. I am looking for an explanation of how the $\ce{Cp-}$ anion acts as a reducing agent.

To contextualise - in a simple synthesis of ferrocene, a hydrated salt of $\ce{FeCl3}$ (the hexahydrate) was mistakenly added to a dry solution of LiCp in THF, which had been prepared by reacting n-BuLi and cyclopentadiene under nitrogen. Clearly anhydrous $\ce{FeCl2}$ was what was supposed to be added.

So far I know that a re-protonation of $\ce{Cp-}$ would occur upon addition of a hydrate to regenerate CpH and thus ruin the reaction.

However, the reaction was allowed to continue and some ferrocene was collected by vacuum sublimation, and therefore I know that something present must have reduced the $\ce{Fe^3+}$ to $\ce{Fe^2+}$ in order for some ferrocene to form. I assume this was the $\ce{Cp-}$ anion as I know this is oxidised by the air. I would expect this to be oxidation under basic conditions because of the LiOH created in the re-protonation step.

I am looking for the half-equation for the oxidation of $\ce{Cp-}.$ What does cyclopentadienyl anion oxidise to? Why are the cyclopentadienyl salts air-sensitive?

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    $\begingroup$ Isn't it better to describe cyclopentadiene $\ce{<=> H+}$ + cyclopentadienyl anion as an simply an acid-base reaction (no change of electron count for cyclopentadiene/cyclopentadienyl anion). The anion stabilized (aromaticity), and -- contrasting to cyclopentadiene -- not so prone to dimerization. On the other side, the cyclopentadienyl cation would be a considerable uphill battle (because of its antiaromaticity). And to be specific, ferrocene is an example of a well known, air stable compound, too (formally $\ce{Fe^{2+}}$ and two units of $\ce{C5H^-_5}$). $\endgroup$
    – Buttonwood
    Commented Jan 8, 2022 at 17:30
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    $\begingroup$ Huh? No, cyclopentadiene ($\ce{C5H6}$), dicyclopentadiene ($\ce{C10H12}$). There is a Wikipedia article about ferrocene via Grignard reaction as if the metal could dissociate (like famously for the anthracene/Mg/THF complex here). Here, this would lead to Li which could reduce Fe(III) to Fe(II). But (and thus hesitant to post this as an answer), the C-Li bonds are even more polarized than C-Mg; this sets the first reaction in question. $\endgroup$
    – Buttonwood
    Commented Jan 8, 2022 at 19:07
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    $\begingroup$ Cyclopentadienyl salts are not air-sensitive. Some complexes are, but it's more of a fault of the central atoms than the ligand afaik. $\endgroup$
    – Mithoron
    Commented Jan 8, 2022 at 20:35
  • $\begingroup$ Could you give an explanation as to why something like TlCp is less air sensitive @Mithoron - as per a couple of sources I've found that say this is sometimes used. What is it about the metal cation $\endgroup$ Commented Jan 9, 2022 at 16:43
  • $\begingroup$ equally why was LiCp suggested for this lab and not NaCp or KCp - why is the lithium Cp used most often $\endgroup$ Commented Jan 9, 2022 at 16:53

1 Answer 1

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Disclaimer: None of this constitutes proof; it's just an informed guess. Any corrections with reputable sources are welcome.


The simplest proposal seems to be single-electron transfer (SET) from $\ce{Cp-}$ to $\ce{Fe^3+}$. This forms the neutral cyclopentadienyl radical as well as $\ce{Fe^2+}$. This $\ce{Fe^2+}$ then reacts with any remaining unreduced $\ce{Cp-}$ to form ferrocene $\ce{FeCp2}$.

All in all for a balanced equation, we would need three equivalents of $\ce{Cp-}$ per equivalent of $\ce{Fe^3+}$. One equivalent for the redox reaction / SET:

$$\ce{Fe^3+ + Cp- -> Fe^2+ + Cp^.}$$

and two equivalents to form ferrocene:

$$\ce{Fe^2+ + 2Cp- ->FeCp2}$$

The radicals probably go off and dimerise, which leads to dihydrofulvalene $\ce{Cp-Cp}$. (Basically, fulvalene, but with the middle double bond changed to a single bond.)

There is some support for this hypothesis, namely the accidental synthesis of ferrocene from $\ce{FeCl3}$ and $\ce{CpMgBr}$ (see Wikipedia), which Buttonwood also mentioned in the comments. This follows almost the same scheme as outlined above. In the present case we have $\ce{CpLi}$ instead of $\ce{CpMgBr}$, and there are definitely differences between Grignards and organolithiums, but to a first approximation I'd expect similar behaviour.

Organolithiums are known to sometimes react as single-electron reducing agents, too (e.g. lithium–halogen exchange sometimes involves SET) so it's hardly implausible that $\ce{CpLi}$ might behave similarly.

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    $\begingroup$ Thank you for being so kind:) That's really interesting. It this how it would oxidise in air as well? I found this on Wikipedia - 'Lithium cyclopentadienide is a colourless solid, although samples often are pink owing to traces of oxidised impurities' $\endgroup$ Commented Jan 8, 2022 at 19:50
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    $\begingroup$ @MatildaWillcox This comment is be even more speculative than my actual post, but... potentially yes? I could see Cp- doing a SET to oxygen (forming superoxide anion). Only difference is that it might react again after that to form oxygenated products. I don't think the pink colour tells us much though. Could be anything. $\endgroup$ Commented Jan 8, 2022 at 20:26
  • $\begingroup$ Given the cited history published in the ACS (see my answer) of the light sensitivity of FeCl3, this answer is well deserving of the provided disclaimer. $\endgroup$
    – AJKOER
    Commented Jan 9, 2022 at 12:45
  • $\begingroup$ I have 4 citations on the photo abilities ascribed to aqueous FeCl3. To deny its possible role here is a losing game. But please, attempt to prove me wrong by adding more air/light exposure to your FeCl3 mix. $\endgroup$
    – AJKOER
    Commented Jan 9, 2022 at 14:32
  • $\begingroup$ Per this source: "The comparison of photocatalytic activity of Fe-salts, TiO2 and TiO2/FeCl3 during the sulfanilamide degradation process", to quote: "Experiments were carried out in the presence of homogeneous Fe-salts (FeCl3 , Fe(NO3)3 , Fe2(SO4)3 , FeSO4 ), heterogeneous compounds (TiO2 ) and a mixture of catalysts (TiO2 /FeCl3 ) under UV-A and solar irradiation. Based on the comparison of several factors... economic and ecological aspects, FeCl3 is proposed as the optimal catalyst." $\endgroup$
    – AJKOER
    Commented Jan 9, 2022 at 15:58

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