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Although many synthesis methods of ferrocene are discussed in literature, I want to know about the black precipitation which occurs when $\ce{FeCl2・4H2O}$ in $\ce{DMSO}$ solution reacts with cyclopentadienyl anion in ether solution. What’s this?

At first, crushed $\ce{KOH}$ ($\pu{20 g}$) and diethyl ether $\pu{100 mL}$ are added into a $\pu{300 mL}$ erlenmeyer flask, then the solution is stirred in $10$ minutes. In addition, cracked cyclopentadiene (monomer) $\pu{5.5 mL}$ is added into this solution. Crushed $\ce{FeCl2・2H2O}$ ($\pu{6.5 g}$) and $\ce{DMSO}$ $\pu{25 mL}$ are added into a $\pu{100 mL}$ beaker, then the solution is stirred. This solution is added drop by drop into the flask. This is the time when the black precipitation forms.

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  • $\begingroup$ The major solvent is dirthyl ether and the other product of the reaction is KCl so my guess is that the solid product is KCl. No idea why is it black, perhaps is contains some Fe(0). $\endgroup$
    – Waylander
    Aug 13, 2021 at 9:41
  • $\begingroup$ Based on some some results I am seeing online (but I am struggling to find primary sources) the opposite effect seems more likely. The iron could be getting oxidized and thus precipitating Fe3O4, maybe also Fe(III) oxide/hydroxide species mixed in. $\endgroup$ Aug 13, 2021 at 13:52

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