Imperial College London

Henry Rzepa

[n-Butyl
lithium](http://en.wikipedia.org/wiki/N-Butyllithium){target="_blank"
rel="noopener noreferrer"} is
[hexameric](http://www.ch.imperial.ac.uk/rzepa/blog/?p=9778){target="_blank"
rel="noopener noreferrer"} in the solid
state\[cite\]10.1002/anie.199305801\[/cite\] and in cyclohexane
solutions. Why? Here I try to find out some of its secrets.

::: {#attachment_9782 .wp-caption .aligncenter style="width: 254px"}
![SUHBEC. CLICK FOR
3D.](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/SUHBEC.jpg){.wp-image-9782
loading="lazy" decoding="async"
aria-describedby="caption-attachment-9782"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([350,350],'load wp-content/uploads/2013/03/SUHBEC.mol;','c3');"
width="244" height="227"}

SUHBEC. CLICK FOR 3D.
:::

The crystal structure reveals the following points of interest:

1.  Six lithium atoms form a cluster with triangular faces.
2.  An off-centre carbanion caps a triangular lithium face.
3.  Four of the butyl groups are in a fully extended antiperiplanar
    conformation
4.  But two di-axial n-butyl exhibit a gauche conformation.

The lithium cluster has twelve electrons available for bonding; if the
Li is considered as Li^+^, balanced by six C^--^ carbanions, the twelve
electrons come from the six carbon lone pairs pointing towards each of
six triangular faces. An [ELF
analysis](http://hdl.handle.net/10.6084/m9.figshare.653692){target="_blank"
rel="noopener noreferrer"} can help identify how these twelve electrons
are arranged. Shown below is the environment of a single Li-face, with
the ELF basin ringed. It integrates to 2.08 electrons. So each
tetrahedral cluster of three lithiums and one carbanion could be
considered as a *two-electron-four-centre* bond, perhaps a natural
progression from the *two-electron-three-centre* bonding found in a
slightly less electron deficient system such as diborane. 

::: {#attachment_9844 .wp-caption .aligncenter style="width: 275px"}
![ELF basins. Click for
3D](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/n-butyl-ELF.jpg){.wp-image-9844
loading="lazy" decoding="async"
aria-describedby="caption-attachment-9844"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([350,350],'load wp-content/uploads/2013/03/SUHBEC_bas.mol;','c1');"
width="265" height="269"}

ELF basins. Click for 3D
:::

NBOs (natural bond orbitals) reflect this character. An NBO represents a
localised two-electron orbital, and
[analysis](http://hdl.handle.net/10042/24402){target="_blank"
rel="noopener noreferrer"} indeed reveals six such orbitals, each having
the form shown below.

::: {#attachment_9857 .wp-caption .aligncenter style="width: 248px"}
![NBO. Click for
3D.](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/Li_NBO.jpg){.wp-image-9857
loading="lazy" decoding="async"
aria-describedby="caption-attachment-9857"
onclick="jmolInitialize('../Jmol/','JmolAppletSigned.jar');jmolSetAppletColor('white');jmolApplet([350,350],'load wp-content/uploads/2013/03/Li_mo103.cub.xyz;isosurface color green yellow wp-content/uploads/2013/03/Li_mo103.cub.jvxl translucent;','c2');"
width="238" height="218"}

NBO. Click for 3D.
:::

This picture in turn leads us to identify this system as spherically
aromatic\[cite\]10/cppn9t\[/cite\]. The three-dimensional equivalent of
the Hückel rule is that any system with 2(N+1)^2^ σ or π electrons (or
both) in a cluster can be considered aromatic/diatropic. In this case,
N=0 and hence the magic count is 2 for each of the six CLi~3~
tetrahedra. The diatropic ring current might be manifested in the
[computed](http://hdl.handle.net/10.6084/m9.figshare.653904){target="_blank"
rel="noopener noreferrer"} ^1^H NMR chemical shifts of the CH~2~^--^
protons (-0.8ppm). Aromaticity does not immediately spring to mind with
the name n-butyl lithium, but this unprepossessing molecule has six
aromatic regions!

Each lithium atom is in turn hemispherically surrounded by three of
these 2.08 electron basins (below, although the ELF centroid is very
much biased towards the carbon, indicating considerable ionicity). What
wonderful electronic economy! Despite there being only twelve electrons
to be shared amongst six lithium atoms, each lithium manages
nevertheless to surround itself with 6.24 electrons. All crammed into
one half sphere, leaving a nice coordination hole; n-butyl lithium is
after all a highly reactive species (even as a hexamer).

![n-butyl-ELF1](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/n-butyl-ELF1.jpg){.aligncenter
.wp-image-9848 loading="lazy" decoding="async" width="298" height="245"}

I want to finish by exploring the observation that two of the six
n-butyl groups adopt a gauche conformation. In free n-butane itself,
around 31% of the population adopts this shape, which curiously is
around the same proportion as is found in the hexameric structure of
n-butyl lithium. More generally, a search of the Cambridge database for
compounds containing such groups reveals the following distribution;
about 1 in 7.

![Gauche](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2013/03/Gauche.jpg){.aligncenter
.wp-image-9850 loading="lazy" decoding="async" width="294" height="268"}

Well, when you deprive a molecule of electrons, as any species with
lithium must invariably suffer from, it is wonderful how the system
responds. In this sense, a hexameric structure seems a very natural
outcome. And it has brought us the *two-electron-four-centre* bond and
the associated spherical *aromaticity*, both of which are a nice bonus.

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


n-Butyl lithium is hexameric in the solid state[cite]10.1002/anie.199305801[/cite] and in cyclohexane solutions. Why? Here I try to find out some of its secrets. SUHBEC. CLICK FOR 3D.  The crystal structure reveals the following points of interest: Six lithium atoms form a cluster with triangular faces. An off-centre carbanion caps a triangular lithium face.

Publicaciones de Rogue Scholar

The mysterious (aromatic) structure of n-Butyl lithium.