Imperial College London

Henry Rzepa

Both the cyclopropenium cation and the cyclopentadienide anion are
well-known 4n+2-type aromatic ions, but could the two together form an
ion-pair?  
[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/ion-pair-page001.svg){.aligncenter
.size-large .wp-image-18059 decoding="async"
width="200"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/ion-pair-page001.svg)

A search of the Cambridge structure database reveals 52 instances of the
cyclopropenium cation with a variety of counter-anions, 77
cyclopentadienide anions with a variety of counter-cations and one
(SOWMOG, private communication to CSD) where the two sub-structures are
common. The pyridinium-cyclopropenium fragment is actually a di-cation
stabilized with dimethylamino substituents, with these charges
balanced by two cyclopentadienide anions stabilized with ester
substituents. The stacking distance between the ion-pairs is \~3.5-3.6Å,
a bit larger than normal π-π stacking distances of 3.2-3.3Å

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/SOWMOG-1002x1024.jpg){.aligncenter
.size-large .wp-image-18060 loading="lazy" decoding="async" width="450"
height="460"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/SOWMOG-1002x1024.jpg 1002w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/SOWMOG-293x300.jpg 293w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/SOWMOG-768x785.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/SOWMOG.jpg 1069w"
sizes="(max-width: 450px) 100vw, 450px"}

So could a "pure" cyclopropenium cyclopentadienide ion-pair exist, and
if so what would its π-π stacking distance be?
A ωB97XD/Def2-TZVPPD/SCRF=water calculation (DOI:
[10.14469/hpc/2442](https://doi.org/10.14469/hpc/2442)) provides one
answer to this question; 2.57Å!^‡^ It is a true minimum in the potential
energy surface (all +ve force constants) with a calculated dipole moment
of only 7.57D. This species is "only" 27.1 kcal/mol higher in ΔG than
the neutral hydrocarbon (DOI:
[10.14469/hpc/2443](https://doi.org/10.14469/hpc/2443)), a difference
which is as low as it is because of the gain in aromatic stabilization
of two rings upon ion-pair formation.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/039.jpg){.aligncenter
.size-full .wp-image-18064 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/039.jpg 773w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/039-276x300.jpg 276w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/039-768x834.jpg 768w"
sizes="(max-width: 773px) 100vw, 773px"}

A few posts back, I was considering candidates for the [most polar
neutral compound
synthesized](http://www.ch.imperial.ac.uk/rzepa/blog/?p=17205) and I
suggested a candidate with a dipole moment of \~22D, based as it happens
on cyclopropenium and cyclopentadienide rings directly connected by a
bond. So when this bond is removed and the two rings are allowed to
stack one above the other, we now have an interesting inversion of the
original challenge: [**what is the least-polar ionic organic
compound**]{style="color: #ff0000;"} (ionic in the sense of being an
unconnected ion-pair)?

Here are some more properties of this intriguing "neutral" ion-pair.

1.  It has a number of low-frequency modes with correspond to the two
    rings moving with respect to each other (ν 216 cm^-1^)  
    ![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/ion-pair.gif){.aligncenter
    .size-full .wp-image-18066 decoding="async" width="400"}
2.  The molecular electrostatic potential illustrates the sense of
    polarization, with negative region (orange) residing on the
    5-membered
    ring:![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/040.jpg){.aligncenter
    .size-full .wp-image-18067 decoding="async" width="400"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/040.jpg 744w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/040-252x300.jpg 252w"
    sizes="(max-width: 744px) 100vw, 744px"}
3.  The most stable π-type molecular orbital (below) reminds of the
    [π-complex formed in the benzidine
    rearrangement](http://www.ch.imperial.ac.uk/rzepa/blog/?p=9186) and
    that in fact modelling this ion-pair may require a multi-reference
    (CASSCF) wavefunction, with the single-determinantal one used here
    only being a first
    approximation.![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/041.jpg){.aligncenter
    .size-large .wp-image-18070 decoding="async" width="400"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/041.jpg 614w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/041-271x300.jpg 271w"
    sizes="(max-width: 614px) 100vw, 614px"}
4.  A QTAIM analysis of the electron density topology shows only weak
    "bond" connectors between the two rings, with ρ(r) being typical of
    weak interactions such as hydrogen
    bonds.![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/042.jpg){.aligncenter
    .size-full .wp-image-18073 decoding="async" width="400"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/042.jpg 979w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/042-300x272.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/042-768x697.jpg 768w"
    sizes="(max-width: 979px) 100vw, 979px"}
5.  An ELF (electron localisation function) analysis also holds no
    surprises, with all the electron density basins (purple) confined to
    the two rings, just as expected of an
    ion-pair.![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/043.jpg){.aligncenter
    .size-full .wp-image-18077 decoding="async" width="400"
    srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/043.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/04/043-280x300.jpg 280w"
    sizes="(max-width: 768px) 100vw, 768px"}
6.  I will leave one further question to a future discussion; [what
    happens to the aromaticity and ring
    currents](http://www.ch.imperial.ac.uk/rzepa/blog/?p=9218) of the
    two individual rings as they combine to form this ion-pair? Might
    this property be connected to the very close separation between the
    two rings?

So we have a remarkably "neutral" ionic hydrocarbon to match the "ionic"
neutral organic molecules previously discussed. This ion-pair may yet
prove to have interesting properties, even if is unlikely to be
synthesized without the addition of stabilising substituents.

------------------------------------------------------------------------

^‡^ For example, the stacking distance in
[graphite](https://en.wikipedia.org/wiki/Graphite#Structure) is 3.35Å.

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Both the cyclopropenium cation and the cyclopentadienide anion are well-known 4n+2-type aromatic ions, but could the two together form an ion-pair? A search of the Cambridge structure database reveals 52 instances of the cyclopropenium cation with a variety of counter-anions, 77 cyclopentadienide anions with a variety of counter-cations and one (SOWMOG, private communication to CSD) where the two sub-structures are common.

Messaggi di Rogue Scholar

Cyclopropenium cyclopentadienide: a strangely neutral ion-pair?