# Can ferrocene type sandwich compounds be made with fluorene?

Ferrocene will be well known to made members of this site. It consists of two cyclopentadienyl anions sandwiching an iron-II ion.

During my first year organic chemistry course (about 40 years ago) I remember a friend suggesting that the fluorene anion had a similar stability and it might be possible to make ferrocene type compounds with an iron-II ion sandwiched between two fluorene anions.

A group of us persuaded a sympathetic professor to let us have a go, and as I recall we prepared the anion by adding lithium aluminium hydride to solution of fluorene in ether cooled with dry ice then adding iron-II chloride. The iron chloride immediately went into solution, so some form of complex was formed, but we couldn't get much further as on warming to room temperature we got a hideous mess of many different products and we didn't have time to analyse it thoroughly.

So my question is whether these types of fluorene compound are known, or alternatively whether it is known that they cannot be prepared? I have attempted to Google this, but there is such a vast literature on ferrocene type compounds that I have not been able to find anything relevant.

• You mean sandwich compounds in general? They sure are. – Mithoron Apr 7 '20 at 15:45
• I rather doubt you added LiAlH4 to a solution of flourene in acetone. The reaction between LiAlH4 and acetone is immediate and violent. Have you misremembered? – Waylander Apr 8 '20 at 7:00
• @Waylander It was 40 years ago so I have quite possibly misremembered. Maybe the solvent was ether? – John Rennie Apr 8 '20 at 7:57
• Ether is much more likely, or THF – Waylander Apr 8 '20 at 9:12
• @Waylander the guy who set it up for us kept an eye on us to make sure we didn't harm ourselves or anyone else :-) – John Rennie Apr 8 '20 at 9:14

Yes there are, many in fact. One of the examples is having a complex consisting of a fluorene molecule and of cp altogether with iron, described by Decken in Acta Cryst. E (doi: 10.1107/S1600536804028077):

(source)

You are not limited to complexation with iron, though. Already by the title «Synthesis and characterisation of Ti, Cr, Mo and W bis(fluorene) complexes» by Cloke et al. in J. Organomet. Chem. (source, paywall) you may see that there is a lot of variation possible by either varying the metal, or the second molecule to build the complex.

• Thanks :-) Although I'm a bit surprised that is a neutral fluorene molecule. I assumed any compound would be with the fluorenyl anion. Do you know if that type of compound exists? – John Rennie Apr 7 '20 at 18:12
• @JohnRennie There's lots of kinds of sandwich compounds, this one's still rather "classical" ;) Still, Wikipedia art. shows one of an anion derivative en.wikipedia.org/wiki/Fluorene – Mithoron Apr 7 '20 at 18:25
• @JohnRennie What the image depicts is only a selection of the structure model, the ($\eta^5$-Cyclo­penta­dienyl)($\eta^6$-9H-fluorene)­iron(II) cation. There equally is a hexaphosphate anion, $\ce{PF6^-}$ in the model, too. If you access the publication as .html (journals.iucr.org/e/issues/2004/12/00/hg6105/index.html), the fourth button in the left hand column, just below «cif» opens an interactive 3D view (direct: publcif.iucr.org/cifmoldb/gui/cifjmol.php?cifid=hg6105) -- and this tetragonal bipyramide easily is seen. – Buttonwood Apr 7 '20 at 19:22
• @JohnRennie Regarding the fluorenyl anion alone, formally obtained by giving away one of the two H's at C-9, there equally are complexes with metal cations much less nicely «piled» as ferrocene and the example shown in the answer, too. Ad hoc an example as Mg salt (doi.org/10.1524/ncrs.2012.0243) which may be seen interactively with the remaining hydrogen atoms in the CCDC data base here: ccdc.cam.ac.uk/structures/… – Buttonwood Apr 7 '20 at 19:31
• Thanks again. It looks as if the negative charge is not delocalised as it is in the cyclopentadienyl anion i.e. the metal is coordinated to the C9 carbon not to the centre of the C5 ring. We had anticipated the charge would be delocalised so you'd get a direct analogue of ferrocene. – John Rennie Apr 8 '20 at 9:18