Imperial College London

Henry Rzepa

Here is a third candidate for the C&EN "molecule of the year"
[vote](http://yearinreview.cenmag.org/molecules-of-the-year/). This one
was shortlisted because it is the first example of a metal-nitrogen
complex exhibiting single, double and triple bonds from different
nitrogens to the same metal\[cite\]10.1039/c5sc04608d\[/cite\] (XUZLUB
has a 3D display available at DOI:
[10.5517/CC1JYY6M](https://doi.org/10.5517/CC1JYY6M)). Since no
calculation of its molecular properties was reported, I annotate some
here.

[![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/CrN123.svg){.aligncenter
.size-large .wp-image-17173 decoding="async"
width="259"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/CrN123.svg)

Firstly, the ^14^N spectra were recorded, and so it is of interest to
see if the chemical shifts reported can be replicated using calculation
(ωB97XD/Def2-TZVPP/SCRF=thf). The method selected is not in the least
"optimised" for this nucleus; it is often the case that various
permutations of functional and basis set must be probed for the best
combination for any particular nucleus. Another limitation of the
calculation is that it has been done without the (rather large)
counterion in place; a full model should certainly include this. The
shifts below are referenced with respect to the internal N≡N signal
reported at 310 ppm. The calculations have DOI:
[10.14469/hpc/1980](https://doi.org/10.14469/hpc/1980) (N~2~) and
[10.14469/hpc/1983](https://doi.org/10.14469/hpc/1983) (CrN123).

|                  |         |      |      |
|------------------|---------|------|------|
| Nucleus          | N-Cr    | N=Cr | N≡Cr |
| N(obs,thf), ppm  | 214     | 560  | 963  |
| N(calc,thf), ppm | 213;223 | 558  | 1093 |

The match is reasonable for three nitrogens; less so for the Cr≡N
variety (DOI:
[10.14469/hpc/1982](https://doi.org/10.14469/hpc/1982)) but no doubt
could be improved by playing with the method as noted above and probably
also correcting for spin-orbit coupling perturbations of the N nucleus
by the Cr nucleus. The ^14^N shifts of quite a number of other
intermediates in the synthesis of this molecule are reported and having
a method to hand which can be used to check if the structural assignment
matches that calculated for it is always useful. 

Next, I have a look at the nature of the Cr-N bonds themselves. The
observed lengths are Cr-N: 1.863 (1.862) 1.881 (1.873); Cr=N 1.736
(1.714); Cr≡N 1.556 (1.518)Å. Calculated values in parentheses. To put
this into context, I show CSD (Cambridge structure database) searches
(search query
DOI:[10.14469/hpc/1981](https://data.hpc.imperial.ac.uk/resolve/?doi=1981&access=)) for
the three types of CrN bond. Firstly the triple bond (65 examples) which
reveals the most probable value of \~1.54Å. This matches fairly well
with the above values.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-triple.jpg){.aligncenter
.size-full .wp-image-17180 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-triple.jpg 919w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-triple-300x110.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-triple-768x282.jpg 768w"
sizes="(max-width: 919px) 100vw, 919px"}

Next, Cr=N (50 examples) with a most probable value of 1.65Å. The value
reported for CrN123 (1.736Å) is quite a bit longer for this bond. Again
a caveat; the searches specified the bond type exactly, and this does
then depend very much on how each entry in the CSD was indexed, by
humans perceiving the structure and assigning the bond type on the basis
of their expert chemical knowledge. It is quite likely that these
integer assignments are at best informed estimates and at worst poor
guesses. 

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/CrN.jpg){.aligncenter
.size-full .wp-image-17181 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/CrN.jpg 925w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/CrN-300x113.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/CrN-768x290.jpg 768w"
sizes="(max-width: 925px) 100vw, 925px"}

Finally, Cr-N (1398 examples) with the most probable value of 2.07Å
which is a fair bit longer than the two values for CrN123. There are
relatively few examples in the region of 1.87Å, which is where the
CrN123 values come.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-N.jpg){.aligncenter
.size-full .wp-image-17182 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-N.jpg 919w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-N-300x108.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/Cr-N-768x276.jpg 768w"
sizes="(max-width: 919px) 100vw, 919px"}

If one repeats this search, but limiting the N atom to carrying two
carbons as well as a bond to Cr (as in NPr*^i^*~2~) one gets the
surprise of a bimodal (perhaps even trimodal) distribution, with an
additional cluster at lengths of 1.82Å, in closer agreement with CrN123.
Again I remind of the caveat that "single" bonds are often assigned by
human curators on the basis of perceived chemistry. It would
nevertheless be interesting to tunnel down to the possible explanation
of this bimodal feature.

![](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/cr-na-1024x717.jpg){.aligncenter
.size-large .wp-image-17189 decoding="async" width="400"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/cr-na-1024x717.jpg 1024w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/cr-na-300x210.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/cr-na-768x538.jpg 768w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2016/12/cr-na.jpg 1686w"
sizes="(max-width: 1024px) 100vw, 1024px"}

These comparisons suggest that in CrN123, the three types of bond are
not isolated but may be interacting electronically in a complex manner
to increase the bond order of the nominal Cr-NR~2~ "single" bonds
(Cr=N(+)R~2~) whilst decreasing that of the nominal Cr=N "double" bond. 

Try try to quantify the bond properties a bit more, I tried the ELF
basin population technique. ELF (electron localization function) is one
method of partitioning the electron density in the molecule into well
defined regions or basins (which we call bonds). The results (DOI:
[10.14469/hpc/1984](https://doi.org/10.14469/hpc/1984)) came out Cr-N
4.05 and 4.01e, each comprising two basins which is often typical of a
bond with significant π character. The integration for a single bond is
of course 2.0. The Cr=N bond was 5.07e in two basins and that for
the Cr≡N **[3.26e]{style="color: #ff0000;"}** (far removed from \~6.0 in
a triple bond). The valence shell total is 16.4e. These values could be
said to be "challenging", perhaps hinting that the bonding and electron
density distribution in this molecule is not quite what it seems.
Certainly worth a more detailed look with other methods of bond
partitioning.

Well, with M123 synthesized, are there any prospects of a M1234 complex
being discovered? (quadruple bonds to N [HAVE been
suggested](http://www.ch.imperial.ac.uk/rzepa/blog/?p=3065)!).

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


Here is a third candidate for the C&amp;EN “molecule of the year” vote. This one was shortlisted because it is the first example of a metal-nitrogen complex exhibiting single, double and triple bonds from different nitrogens to the same metal[cite]10.1039/c5sc04608d[/cite] (XUZLUB has a 3D display available at DOI: 10.5517/CC1JYY6M). Since no calculation of its molecular properties was reported, I annotate some here.

Messaggi di Rogue Scholar

Molecule of the year? “CrN123”, a molecule with three different types of Cr-N bond.