Imperial College London

Henry Rzepa

A recent article reports, amongst other topics, a computationally
modelled reaction involving the capture of molecular hydrogen using a
substituted borane (X=N, Y=C).\[cite\]10.1073/pnas.1709586114\[/cite\]
The mechanism involves an initial equilibrium between **React** and
**Int1**, followed by capture of the hydrogen by **Int1** to form a
5-coordinate borane intermediate (**Int2** below, as per Figure 11).^‡^
This was followed by assistance from a proximate basic nitrogen to
complete the hydrogen capture *via* a **TS** involving H-H cleavage. The
forward free energy barrier to capture was \~11 kcal/mol and \~4
kcal/mol in the reverse direction (relative to the species labelled
**Int1**), both suitably low for reversible hydrogen capture. Here I
explore a simple variation to this fascinating reaction.^∞^

[  
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/H-cap.svg){.aligncenter
.size-large .wp-image-18823 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/H-cap.svg)

This variation involves transposing X and Y such that Y=N^+^ and X=C^--^
to form a carbon ylide such that X=C becomes much more nucleophilic than
the original nitrogen nucleophile. An animation of the full IRC^†^
(intrinsic reaction coordinate computed at ωB97XD/cc-pvtz; FAIR data
doi: [10.14469/hpc/2704](https//doi.org/10.14469/hpc/2704)) is shown
below.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/Ha.gif){.aligncenter
.size-full .wp-image-18827 decoding="async" width="550"}

The profile shows that the reaction is concerted between the species
labelled **React** and **Prod**; no sign of **Int1** and **Int2!**

1.  The region IRC -12 to -5 involves B-C bond cleavage. Because the C
    is so very nucleophilic, the 4-ring species labelled **React**
    becomes very stable and opening it requires a high barrier.
2.  Between IRC -5 and 0, the BH~2~ group rotates, losing its original
    interaction with the C to slowly create an empty acceptor orbital on
    the boron which can then interact with the incoming hydrogen.
3.  At IRC= 0 (the transition state) the hydrogen has been captured by
    the boron to form a 5-coordinate species, in a manoeuvre that
    reminds one of the orbital capture of satellites by planets on their
    way to the outer reaches of the solar system. If the barrier to this
    capture is computed from IRC= -4 (the region of **Int2**) it is very
    much lower than the original
    system\[cite\]10.1073/pnas.1709586114\[/cite\], again a reflection
    of the higher nucleophilicity of X=C^--^.
4.  The fly past continues until IRC= +7, at which point one end of the
    bound hydrogen has become suitably orientated to interact with the
    nucleophilic carbon *via* lone-pair donation into the acceptor
    H-H σ^\*^ orbital, thus helping to break it.
5.  By IRC= +9, the H-H cleavage is complete.
6.  By IRC= +13 the reaction has reached **Prod**, being overall \~ -12
    kcal/mol exothermic.
7.  The overall thermochemistry is dominated by the potent carbon
    nucleophile in the reactant, which in turn makes this modification
    entirely useless for the purposes of a hydrogen-capture system!

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/H.svg){.aligncenter
.size-large .wp-image-18828 decoding="async" width="450"}[  
![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/Hg.svg){.aligncenter
.size-large .wp-image-18836 decoding="async"
width="450"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/Hg.svg)

The evolution of the dipole moment along the IRC shows very non-linear
behaviour (such plots are rarely shown in most published IRC analyses;
they should be!), ending of course with the ionic zwitterion that is the
imminium borohydride **Prod**. Indeed the entire reaction coordinate is
an unusually vivid example of a *non-least motion path*!

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2017/09/HDM.svg){.aligncenter
.size-large .wp-image-18829 decoding="async" width="450"}

This simple atom transposition has given us a very instructive exercise
in reaction paths, by-passing entirely both ** Int1** and **Int2**
(making them *hidden intermediates*), and converting **React** →
**Prod** into a concerted reaction. It would be great to probe this
convoluted journey using reaction dynamics!

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

^∞^Archived as DOI: [10.14469/hpc/3096](https.doi.org/10.14469/hpc/3096)

^‡^ Such a species [can be
seen](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5114) as a *hidden
intermediate* in the mechanism of reduction of a carboxylic acid by
diborane.

^†^None were shown in the original
study.\[cite\]10.1073/pnas.1709586114\[/cite\]

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A recent article reports, amongst other topics, a computationally modelled reaction involving the capture of molecular hydrogen using a substituted borane (X=N, Y=C).[cite]10.1073/pnas.1709586114[/cite] The mechanism involves an initial equilibrium between

 React

and

 Int1

, followed by capture of the hydrogen by

 Int1

to form a 5-coordinate borane intermediate (

 Int2

below, as per

Messaggi di Rogue Scholar

Hydrogen capture by boron: a crazy reaction path!