Imperial College London

Henry Rzepa

Chemical and engineering news (C&EN) is asking people to
[vote](http://yearinreview.cenmag.org/molecules-of-the-year/) for their
*molecule of the year* from six highlighted candidates. This reminded me
of the history of internet-based "*molecules of the moment*". It is
thought that the concept originated in December 1995 [here at
Imperial](http://www.ch.ic.ac.uk/motm/) and in January 1996 at [Bristol
University](http://www.bris.ac.uk/Depts/Chemistry/MOTM/motm.htm) by Paul
May and we were joined by Karl Harrison at Oxford shortly thereafter.
Quite a few more such sites followed this concept, differentiated by
their time intervals of weeks, months or years. The genre is well suited
for internet display because of plugins or "helpers" such as Rasmol,
Chime, Jmol and now JSmol which allow the three dimensions of molecular
structures to be explored by the reader. Here I discuss a [second
candidate](http://www.ch.imperial.ac.uk/rzepa/blog/?p=16997) from the
C&EN list; a ferrocene-based Ferris
wheel\[cite\]10.1038/nchem.2553\[/cite\],\[cite\]10.5517/CCDC.CSD.CC1JPKYQ\[/cite\]
(DOI for 3D model:
[10.5517/CCDC.CSD.CC1JPKYQ](http://doi.org/10.5517/CCDC.CSD.CC1JPKYQ))
originating from research carried out at Imperial by Tim Albrecht, Nick
Long and colleagues.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/ferris.jpg){.aligncenter
.size-full .wp-image-17145 loading="lazy" decoding="async" width="387"
height="340"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/ferris.jpg 387w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/ferris-300x264.jpg 300w"
sizes="(max-width: 387px) 100vw, 387px"}

The chemical interest was the redox chemistry of the six metal centres,
and the interactions between these centres, expressed more succinctly as
"do the iron centres talk to each other?". The suggestion was that the
charges in the molecules originating from oxidation move between
ferrocene centres at a rate that is fast compared to the electrochemical
timescale. An analogy is drawn to the nanoscale and uniformly charged
conductive rings.

I was interested to compare this system with any similar Fe compounds
that might also be known in the CSD (Cambridge structure database). Here
are some that I found:

1.  CEFDOG\[cite\]10.1021/om50006a013\[/cite\] with two cyclic ferrocene
    units with both neutral Fe and Fe(+) present
2.  EZEVIO\[cite\]10.1021/om0400393\[/cite\],
    3D: [10.5517/CC805N2](https://doi.org/10.5517/CC805N2)  with Fe and
    Ge as the metals.
3.  FULVFE\[cite\]10.1021/ic50079a030\[/cite\] from 1969 with two Fe
    centres.
4.  PETTUD and PETVAL\[cite\]10.1021/om00032a036\[/cite\] with two Fe
    centres.
5.  PETVEP and PETVIT\[cite\]10.1021/om00032a036\[/cite\] with Fe and Zr
    centres
6.  URAFUQ and URAGAX (3D:
    [10.5517/CCDC.CSD.CC1JPKZR](https://doi.org/10.5517/CCDC.CSD.CC1JPKZR)),
    the system shown above.
7.  VOKXOI\[cite\]10.1016/0022-328X(91)80206-Y\[/cite\] with one Fe and
    one Fe^+^.
8.  VOKXUO\[cite\]10.1016/0022-328X(91)80206-Y\[/cite\] with one Fe and
    one Co^+^.
9.  WOJDOQ\[cite\]10.1107/S1600536814017346\[/cite\], 3D :
    [10.5517/CC133PGC](https://doi.org/10.5517/CC133PGC) from 2014 with
    three Fe units.
10. ZECTOQ\[cite\]10.1016/0022-328X(95)00108-3\[/cite\] with one Fe and
    one Th.

Returning to the communication between ferrocene units, the six-unit
ferris wheel noted above has four sets differentiated from the other two
in the solid state, although in solution by NMR they are all seen as
equalised by exchange. The twist angle between four pairs is \~47° (C-C
distance 1.471Å) and for the other two it is \~18° (C-C distance 1.466Å)
which allows a fair measure of π-π conjugation to operate between the
rings. Contrast this with the
smaller WOJDOQ\[cite\]\<10.1107/S1600536814017346\[/cite\], where the
torsions between the rings are closer to 80° (C-C distance 1.486Å) thus
inhibiting π-π conjugation. It would certainly be interesting to compare
*e.g.* the cyclic voltammetry for these two species to see if electronic
communication between the rings is affected by this structural
difference.

WOJDOQ![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/WOJDOQ.jpg){.aligncenter
.size-large .wp-image-17151 loading="lazy" decoding="async" width="436"
height="425"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/WOJDOQ.jpg 436w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/WOJDOQ-300x292.jpg 300w"
sizes="(max-width: 436px) 100vw, 436px"}

In regard to the
D~3~-symmetric WOJDOQ\[cite\]\<10.1107/S1600536814017346\[/cite\], this
is of course chiral and here its chiroptical properties intrigue,^‡^
along with questions of whether the two enantiomers are
configurationally stable at room temperatures. If so, perchance they
might be capable of acting as asymmetric catalysts?

Finally I speculate whether these sorts of rings can be constructed as
Möbius strips or perhaps even as [trefoil
knot](http://www.ch.imperial.ac.uk/rzepa/blog/?p=2046)s. It is certainly
nice to see new molecules that spark all sorts of interesting new ideas!

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

^‡^The calculated optical rotation of WOJDOQ
(TPSSh/6-311G(d,p)/SCRF=dichloromethane) is 427° at 800 nm and 1077° at
589 nm (doi: [10.14469/hpc/1971](https://doi.org/10.14469/hpc/1971));
the VCD (ωB97XD/6-311G(d,p)/SCRF=dcm) is shown below (doi:
[10.14469/hpc/1970](http://doi.org/10.14469/hpc/1970));

[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/3.svg){.aligncenter
.size-large .wp-image-17161 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/3.svg)

the  ECD (doi: [10.14469/hpc/1972](https://doi.org/10.14469/hpc/1972) ):

[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/3-ecd.svg){.aligncenter
.size-large .wp-image-17162 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/3-ecd.svg)

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


Chemical and engineering news (C&amp;EN) is asking people to vote for their
<em>
 molecule of the year
</em>
from six highlighted candidates. This reminded me of the history of internet-based “
<em>
 molecules of the moment
</em>
”. It is thought that the concept originated in December 1995 here at Imperial and in January 1996 at Bristol University by Paul May and we were joined by Karl Harrison at Oxford shortly thereafter.

Messaggi di Rogue Scholar

Molecule of the year (month/week)?