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Cyclopentadienyl $\ce{C5H5^-}$ is a hydrocarbon anion with a 5 fold symmetry. The carbon atoms can be replaced with nitrogen, oxygen, sulfur, etc. to yield diverse aromatic 5-membered rings. For example, because a nitrogen is isoelectronic to a carbon and a hydrogen, replacing CH with nitrogen yields pyrrole, imidazole, triazole, tetrazole, and even pentazole.

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Here is an interesting question. Boron and hydrogen can make a divalent anion $\ce{B12H12^{2-}}$ with an icosahedral symmetry. Since a carbon atom is isoelectronic to a BH, can we replace some or even all BH with C like what we did on cyclopentadienyl? enter image description here

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Probably not an icosahedron. We might have better results with a smaller cluster, such as four atoms.

One major difficulty is the fact that isoelectronic replacement of boron with carbon introduces a positive charge, and so the equivalent to $\ce{B12H12^{2-}}$ would be $\ce{C12H12^{10+}}$ with a high positive charge density. Such a structure would be expected to break up into simpler species to disperse the charge, react even with relatively benign counterions or nucleophiles, or both.

Given this charge difference between isoelectronic species, full interchange of boron and carbon in the cluster is more likely with smaller clusters than the icosahedron. Here the case of a four-atom cluster is considered, as this allows both the carbon-ring cation and the boron-ring anion to have the same charge per atom ($\ce{B4H4^{2-}}$ vs $\ce{C4H4^{2+}}$).

The $4n+2$ rule would predict aromatic coupling of these species were planar, but they apparently are not. Both are instead calculated to have puckered structures. For the boron species, Shen et al.[1] study ions of the form $\ce{B_nH_n^{2-}}$ by computation; for $n=4$ they report the aforementioned puckered structure with $D_{2d}$ symmetry. This work does not directly examine experimental detection, but reports that $\ce{B4H4^{2-}}$ has been seen.

In the case of $\ce{C4H4^{2+}}$, Firme et al.[2] report that the cation has a calculated puckered structure. The calculations find a similar puckering in the tetramethyl-substituted species $\ce{C4(CH3)4^{2+}}$, but a planar structure for the more strongly conjugated phenylated cations. Experimentally Olah and Staral[3] experimentally prepared and studied these species via NMR spectroscopy (tge unsubstituted $\ce{C4H4^{2+}}$ ion is nit yet experimentally known). The experimental detection of both $\ce{B4H4^{2-}}$ and $\ce{C4(CH3)4^{2+}}$ with apparent puckered, $D_{2d}$ ring geometries indicates that with conjugated four-atom clusters full interchange between carbon and boron is indeed possible.

References

  1. Yan-Fang Shen, Chang Xu and Long-Jiu Cheng (2017). "Deciphering chemical bonding in BnHn2− (n = 2–17): flexible multicenter bonding". RSC Adv. 2017,7, 36755-36764. https://doi.org/10.1039/C7RA06811E.

  2. Firme CL, Antunes OA, Esteves PM (2007). "Electronic nature of planar cyclobutenyl dication derivatives". J Phys Chem A. 111(46):11904-7. doi: 10.1021/jp075869j. Epub 2007 Oct 19. PMID: 17948974.

  3. George A. Olah and John S. Staral (1976). "Novel aromatic systems. 4. Cyclobutadiene dications". J. Am. Chem. Soc. 98, 20, 6290–6304. https://doi.org/10.1021/ja00436a037

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    $\begingroup$ Cool, but OP thought about exhanging BH for C, not CH. $\endgroup$
    – Mithoron
    Oct 15, 2023 at 12:33
  • $\begingroup$ Maybe so, but we tend to see more B/C interchange experimentally. $\endgroup$ Oct 15, 2023 at 18:02

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