Imperial College London

Henry Rzepa

A celebration of the life and work of the great chemist Paul von R.
Schleyer was held this week in [Erlangen,
Germany](http://schleyersymposium.mgms-ds.de){target="_blank"}. There
were many fantastic talks given by some great chemists describing
fascinating chemistry. Here I highlight the presentation given by Andy
Streitwieser on the topic of organolithium chemistry, also a great
interest of Schleyer\'s over the years. I single this talk out since I
hope it illustrates why people still get together in person to talk
about science.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/sw-2.svg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="250"}

The presentation focused on the structure of the simplest possible
metallocene, lithium cyclopentadienyl and why the calculated structure
showed that the hydrogen atoms attached to the cyclopentadienyl ring
pointed slightly away from the metal rather than towards it (by
\~1-2°).^†^ Various explanations had been put forward, some had waxed
and then waned. It was still basically an open problem. Now, the title
of the symposium was *Theory and Experiment: A Meeting at the
Interface*; Streitwieser had given the theory and whilst listening, I
realised I might be able to help relate this to known experiments,
*i.e.* crystal structure data. I could do so by analysing the known
crystal structures of
metallocenes.\[cite\]10.1021/acs.jchemed.5b00346\[/cite\] So here is the
basic search query, and I will go through it thus:

1.  A general ring is defined (sizes 4,5,6,7,8) and the ring and metal-C
    bonds are all specified as of type \"any\" (it is difficult to know
    how such bonds might be classified, ie delocalised, aromatic, etc,
    so best not to constrain things) and a metal is attached.
2.  **4M** is basically any metal; again the search is unconstrained,
    but one could focus on certain columns of the periodic table if one
    wished.
3.  A ring **centroid** is computed.
4.  **ANG1** is defined as the angle H-C-centroid, the angle of interest
    in Andy\'s talk. The limits were constrained to lie between 140° and
    179°. I did this because when the angle becomes 180°, the torsion
    becomes mathematically undefined and I did not want to risk this
    happening.
5.  **TOR1** is defined as the torsion H-C-centroid-metal. Values of
    180° would indicate that the hydrogen was pointing away from the
    metal; values of 0° would indicate it was pointing towards the
    metal. The absolute value of the torsion is taken to avoid confusion
    induced by its sign.
6.  **ANG2** is one test whether the ring is planar. For an even
    membered ring, it is the angle subtended at the centroid to opposing
    carbon atoms. For odd membered rings it is the angle at the centroid
    involving one carbon and a centroid defined by an opposing pair of
    atoms (see below).
7.  The quality of the crystal structure determination is controlled by
    specifying that the R value be \< 5%, no errors, no disorder. Also,
    the terminal H-positions are normalised (to correct known errors in
    H distances deriving from x-ray diffraction). I would point out that
    in the early days, the actual positions of the hydrogen were often
    not actually determined, but \"idealised\". In this case this would
    mean that the H-C-centroid angle would probably be set to 180°. For
    perhaps the last 20 years or so however, the positions of hydrogen
    atoms have been routinely refined. Unfortunately, I know of no
    search query that can separate the two cases, and so we will have to
    live with the mixture and see what we get.
8.  We define another constraint separately, which is that the
    temperature of the data collection sample is \<140K. This ensures
    that the data will be free of more vibrational/thermal noise and so
    should be rather more accurate.
9.  Finally, a note on the topic of \"research data management\" or RDM.
    I have deposited the files defining the search query in a repository
    and have assigned DOIs both to the overall search
    collection\[cite\]10.14469/hpc/346\[/cite\] and to each individual
    search definition, the DOIs for which are shown below.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/as-sq.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="300"}

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/as-sq1.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="300"}

**The 4-ring case**.\[cite\]10.14469/hpc/347\[/cite\] Here the
temperature constraint was relaxed, since there are few entries. The two
red \"hot-spots\" occur at torsion angles of \~180° (hydrogen pointing
**away** from metal) at bond angle values of between 173-176°. 

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C4.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

**The 5-ring case**.\[cite\]10.14469/hpc/348\[/cite\] This includes the
classic
[ferrocene](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3908){target="_blank"}
example, the first metallocene for which the structure was correctly
identified. There are many more examples, and this search is now
constrained to \<140K. The two hot spots occur at bond angles of very
close to 180°, at which values the torsion itself becomes undetermined.
That the hot spots actually occur at 0° and 180° and are not spread
evenly across the right hand side axis is remarkable given this. There
is a significant tail for the 180° torsion (H pointing away from metal)
down to H-C-centroid angles of about 170°, but there is no evidence of
this tail for torsions of 0°.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C5140.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

One more test must be applied to see if the 5-ring is planar or not. The
deviation from planarity is only 2-3°, and there seems to be no
correlation between lower values of the H-C-centroid bond angle and
non-planarity.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C5-ring.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

**The 6-ring case**.\[cite\]10.14469/hpc/349\[/cite\] There are again
numerous examples of data \<140K for such rings. There is now a very
distinct hotspot at angles of \~170° for the case/torsion where the
hydrogen is pointing **towards** the metal.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C6140.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

This feature persists when the ring planarity is tested, and it occurs
specifically for rings where the angle subtended at the centroid is
\~180° and H-C-centroid angles of \~170°. So this is clear-cut effect
which demands explanation **\#1**.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C6-ring.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

**The 7-ring case**\[cite\]10.14469/hpc/350\[/cite\] again shows a
strong hot spot at \~172° for a torsion corresponding to the hydrogens
pointing towards the metal. This hot spot is matched by angles subtended
at the ring centroid that are close to 180° (*i.e.* planar). This is
clear-cut effect which demands explanation **\#2**.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C7140.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C7-ring.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

**The 8-ring case**\[cite\]10.14469/hpc/351\[/cite\] also shows a hot
spot for hydrogens pointing towards the metal by the strikingly large
degree of \~157°, and this feature is associated with a linear
C-centroid-C angle. This is clear-cut effect which demands
explanation **\#3**.

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C8140.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

![NH3-8](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/04/Metal-C8-ring.jpg){.aligncenter
.size-full .wp-image-15936 decoding="async" width="440"}

**The 9- and 10-ring cases.  **There are no examples!  Time to make
some?

To summarise. 

1.  The above was done during a conference in response to a point made
    by one of the speakers. In fact, it proved possible to show the
    speaker the diagrams above \<18 hours after he gave the talk.^‡^
2.  An immediate question that arose from this discussion was whether
    the hot-spots were artefacts of non-planar rings. So the ANG2 test
    was added to the plots the next day (today) as part of this
    dissemination.
3.  Also discussed (yesterday) was how these conference insights might
    be shared. I suggested the forum here and Professor Streitwieser
    heartily agreed. Another alternative was to write it up as a regular
    journal article. But we both agreed that ..
4.  what you see here is just a statistical analysis. The next stage
    would be to individually inspect all the molecules which make up
    these statistics. You see it might just be that every molecule
    contributing to a \"hot-spot\" cluster might have special
    circumstances which conspire to make it look as if there is an
    interesting chemical effect going on. It is unlikely that such
    coincidences could accrue in such a manner, but the possibility does
    have to be considered.
5.  I think we both felt that a better way was to expose the basic
    effects here, as a sort of open science research project, and anyone
    interested could then (a) try to replicate these plots, which is why
    you will find the DOIs of datasets containing the definition files
    to assist in any such replication and (b) tunnel down to any
    specific hot spot to identify the precise chemical characteristics
    that might give rise to the geometrical effect.
6.  This could then be followed up by computational analysis of the
    electronic properties which might give rise to the effect. This
    would in effect complete the cycle, since this was the starting
    point for Streitwieser\'s original talk. Remember, the theme of the
    celebration was the interplay between theory and experiment, a
    particular favourite of  Schleyer\'s.
7.  Regarding the chemical insights, a distinct trend over the ring
    sizes 4-8 can be seen. The 4-ring shows the hydrogens pointing away
    from the metal, the 5-ring could be said to be largely agnostic
    (remember the error in crystallographic angles is probably in the
    region 1-3°) whilst there is an indication that for the 6-8 rings
    the ring hydrogens tend to point towards the metal. I have
    summarised three key points illustrating this as \#1-3 above.
8.  It is tempting to conclude that a fairly general chemical effect is
    operating here over \#1-3, although of course it could be a number
    of effects specific to each ring which merely look like a general
    trend.

So the chemical interpretation of this project is unfinished, a general
feature of much of science of course. But my aim here was to give a
flavour of how a scientific meeting at its best can bring together like
(or often unlike) minds which can tease out new connections and lead
perchance to new discoveries.

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

^‡^These hours were productively employed by sharing a Franconian
banquet together, and a modicum of sleep, as well as the searches
described above. And in case you see no citations at the bottom of this
post, they too take about 48 hours to propagate through the CrossRef and
DataCite systems. Be patient and they will appear. ^†^In my original
representation, I showed the Hs pointing towards the metal. In fact Prof
Streitwieser has just contacted me reversing this orientation and
correcting my recollection of his lecture.

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


A celebration of the life and work of the great chemist Paul von R. Schleyer was held this week in Erlangen, Germany. There were many fantastic talks given by some great chemists describing fascinating chemistry. Here I highlight the presentation given by Andy Streitwieser on the topic of organolithium chemistry, also a great interest of Schleyer's over the years.

Postagens de Rogue Scholar

Celebrating Paul Schleyer: searching for hidden treasures in the structures of metallocene complexes.