Covalent capture of nitrous oxide by N-heterocyclic carbenes

Article abstract of DOI:10.1002/anie.201106589

A good catch: N-Heterocyclic carbenes (NHCs) form stable adducts with nitrous oxide (N₂O) under mild reaction conditions. The adducts display unique reactivity as evidenced by an alkylation reaction which leads to a rupture of the N-N bond.

Full text of DOI:10.1002/anie.201106589

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Angewandte
Communications
DOI: 10.1002/anie.201106589
Nitrogen Oxides
Covalent Capture of Nitrous Oxide by N-Heterocyclic Carbenes**
Alexander G. Tskhovrebov, Euro Solari, Matthew D. Wodrich, Rosario Scopelliti, and
Kay Severin*
In a recent publication, Ravishankara and et al. conclude that
nitrous oxide will be “the dominant ozone-depleting sub-
stance emitted in the 21st century”.[1,2] N2O is also a green-
house gas which is around 300 times more potent than CO2.[3]
The environmental impact of N2O should be regarded as a
strong incentive to study the basic chemistry of this gas in
more detail. In principle, N2O is a very interesting oxidizing
agent.[3,4] It has a high oxidation potential and it is environ-
mentally benign (side product: N2). However, N2O is kineti-
cally very inert and this has hampered its utilization as an
oxidant or as a building block for more complex molecules. It
is thus not surprising that considerable efforts have been
made to activate N2O chemically.[3,4] Transition-metal com-
plexes have emerged as promising activation agents.[5] Not-
withstanding, N2O is a very poor ligand and structurally
characterized N2O complexes have only been reported
recently.[6] The reaction of N2O with organic compounds
typically requires elevated temperatures or pressures.[3,4]
Olefins, for example, are converted into carbonyl compounds
in the presence of N2O at 150–2508C and pressures of more
than 10 bar.[4c] The oxidation of PPh3 was found to occur at
temperatures below 1008C but supercritical N2O was
employed (p > 100 bar).[7] Only highly reactive molecules
such as triethylborane[8] and certain silicon-containing com-
pounds (e.g., silaethenes or disilenes)[9] are oxidized by N2O
under ambient conditions. An exciting recent finding was the
fact that frustrated Lewis pairs (FLPs) are able to bind N2O.[10]
In contrast to the oxygen-transfer reactions mentioned above,
N2O is bound intact between the Lewis acid (fluoroarylbor-
anes) and the Lewis base (trialkylphosphines) as evidenced by
crystallographic analyses. Herein we report that N-hetero-
cyclic carbenes (NHCs) are also able to capture N2O to give
stable adducts. The adducts display unique reactivity as
evidenced by an alkylation reaction which results in rupture
reactivity of NHCs towards N2O. When a solution of IMes in
THF (33 mm) was subjected to an atmosphere of N2O, the
solution slowly became yellow along with the formation of a
yellow precipitate (1). Isolation (yield: 90%) and analysis of 1
by NMR spectroscopy, mass spectrometry, and elemental
analysis suggested the formation of an N2O adduct
(Scheme 1). This structure was confirmed by a crystallo-
Scheme 1. Synthesis of the N2O adducts 1 and 2.
graphic analysis (see below). In a related fashion, 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene (IPr) reacted with N2O
to give the adduct 2. Purification of 2 required column
chromatography on silica gel which reduced the yield of the
isolated adduct to 41%.[13] The fact that 2 could be purified by
chromatography was a first indication of its high stability.
The adducts 1 and 2 are very soluble in polar organic
solvents (e.g., CHCl3, CH2Cl2, or THF). Crystallization was
achieved by slow evaporation of CH2Cl2/n-hexane (for 1) or
Et2O/n-hexane (for 2) solutions. Single-crystal X-ray diffrac-
tion analyses were performed for both complexes[14] and
graphical representations of the molecular structures are
depicted in Figure 1.
In both adducts, a bent N2O group is connected through
the N atom (N3) to the carbon atom (C1) of the heterocycle.
Overall, the bond lengths and angles are similar for the two
adducts (Table 1). The bonds of the C1 atom to the three
adjacent nitrogen atoms (N1–N3) all have lengths of approx-
imately 1.36 ꢀ. With 1.25 ꢀ, the N4-O1 bond is significantly
shorter than the N3-N4 bond (1.333(2) and 1.352(4) ꢀ for 1
and 2 respectively). This difference is in contrast to what has
ꢀ
of the N N bond.
N-heterocyclic carbenes such as the commercially avail-
able 1,3-dimesitylimidazol-2-ylidene (IMes) are highly Lewis-
basic compounds,[11] which are able to form adducts with the
inert gas CO2.[12] These findings prompted us to explore the
[*] A. G. Tskhovrebov, Dr. E. Solari, Dr. M. D. Wodrich, Dr. R. Scopelliti,
Prof. K. Severin
ꢀ
been observed for N2O adducts of FLPs, for which the N N
ꢀ
bond [(1.25 ꢁ 0.01) ꢀ)] is shorter than the N O bond [(1.33 ꢁ
Institut des Sciences et Ingꢀnierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
0.01) ꢀ].[10] The degree of bending, in contrast, is similar for
FLP/N2O adducts and for 1 and 2 (ca. 1108). The rather long
bond between the two adjacent nitrogen atoms is reminiscent
of what has been observed for imidazolylidene triazines, the
coupling products of NHCs and azides. In these compounds,
E-mail: kay.severin@epfl.ch
[**] This work was supported by funding from the Swiss National
Science Foundation and the EPFL.
ꢀ
the length of the central N N bond is typically on the order of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106589.
1.33–1.37 ꢀ.[15] The plane defined by the bent N2O group is
232
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Angew. Chem. Int. Ed. 2012, 51, 232 –234

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The reactivity of adduct 1 was examined in a series of
experiments. Solutions of 1 are stable towards water and air.
This was evidenced by dissolving 1 in a mixture of [D8]THF
and H2O (10:1) under air. No significant decomposition was
1
observed by H NMR spectroscopy after three days. Cross-
over experiments were performed with THF solutions con-
taining equal amounts of 1 and free carbene IPr, or 2 and free
carbene IMes. After three days at room temperature, transfer
1
of N2O was not observed by H NMR spectroscopy. These
results imply a high stability of the adducts 1 and 2. At
elevated temperatures, 1 was found to decompose into the
urea 3: heating a toluene solution of 1 for 3 hours at 1008C
gave 3 in 60% yield (Scheme 3). In situ NMR experiments in
C2D2Cl4 at 1308C revealed a clean and quantitative 1!3
transformation within 30 minutes. This type of reactivity is in
line with what has been found for FLP/N2O adducts[10] and for
nitrosylimines,[22] both of which decompose thermally with
liberation of dinitrogen.
Figure 1. Molecular structures of 1 (left) and 2 (right) in the crystal.
Thermal ellipsoids shown at the 50% probability level.
Table 1: Selected bond lengths and angles for 1 and 2.
1 (exptl.)
2 (exptl.)
1 (theor.)
2 (theor.)
C1-N1 [ꢁ]
C1-N2 [ꢁ]
C1-N3 [ꢁ]
N3-N4 [ꢁ]
1.366(2)
1.360(2)
1.360(2)
1.333(2)
1.250(2)
109.6(1)
113.1(1)
25.4(2)
1.354(4)
1.368(4)
1.358(4)
1.352(4)
1.250(4)
110.7(3)
112.9(3)
39.5(5)
1.365
1.361
1.323
1.352
1.202
110.9
113.9
24.07
1.361
1.357
1.327
1.346
1.205
110.5
114.1
26.25
N4-O1 [ꢁ]
C1-N3-N4 [8]
N3-N4-O1 [8]
N4-N3-C1-N1 [8]
Scheme 3. Reactivity of the N2O adduct 1.
markedly inclined with respect to the plane defined by the
heterocycle: the dihedral angle N1-C1-N3-N4 found for 1 is
25.4(2)8 and that found for 2 is 39.5(5)8.
The reactivity of 1 towards electrophiles was examined
using methyliodide. When 1 was added to a mixture of
toluene and MeI (1:1) a solution formed which slowly turned
red. The speed of the transformation could be accelerated by
placing the reaction flask next to a mercury lamp.[23] After
5 hours, we were able to isolate the alkylation product 4 in
60% yield (Scheme 3). Compound 4 was examined spectro-
scopically as well as by single-crystal X-ray analysis
(Figure 2).[14] The data reveal the formation of a salt
composed of a guanidinium-type cation and a triiodide
anion. The formation of this salt might proceed through an
alkylation of the central N3 atom with subsequent liberation
of nitrosyl iodine, which decomposes to give I2.[24] A second
To gain further information about the electronic situation
in NHC/N2O adducts, we performed density functional
computations at the M06-2X/cc-pVTZ level of theory.[16–18]
Computational geometric data generally agrees well with the
experimental data, although shorter N4-O1 (by ca. 0.05 ꢀ)
and C1-N3 (by ca. 0.03 ꢀ) bond lengths are predicted.
Additionally, the differences in dihedral angles (N4-N3-C1-
N1) between 1 and 2, which measure the planarity of the N2O
relative to the plane of the heterocycle, are not observed:
both structures are predicted to be 24–278 out of plane. To
assess the resonance picture depicted in Scheme 2, we
computed Hirshfeld-I charges[19] at the M06-2X/6-31G(d)
level of theory as implemented in an in-house version of
QChem.[20,21] Both structures 1 and 2 reveal the N3 atom
(ꢀ0.57, ꢀ0.55) to be more negative than the O1 atom (ꢀ0.32),
which is indicative of a significant contribution from the
resonance structure B.
Figure 2. Molecular structure of 4 in the crystal. Thermal ellipsoids
Scheme 2. Resonance structures of NHC/N2O adducts.
Angew. Chem. Int. Ed. 2012, 51, 232 –234
shown at the 50% probability level.
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methylation would give the cation along with the triiodide
anion. However, a more detailed study is needed to corrob-
orate this mechanistic hypothesis. The formation of 4 is
significant because it demonstrates that NHC-activated N2O
ꢀ
can undergo reactions which lead to a rupture of the N N
bond. One should note that this type of reactivity if very rare
for N2O.[25] The fact that we observe alkylation at N3 also
confirms the importance of the resonance structure B in
agreement with the results of the computational study.
In summary, we have demonstrated that N-heterocyclic
carbenes are able to form stable adducts with N2O under
ambient conditions. The adducts display a bent N2O group,
which is bound to the heterocycle through the N atom.
Alkylation of the N2O adduct 1 by MeI was shown to induce a
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rupture of the N N bond, a rarely observed reactivity pattern
in N2O chemistry. Current efforts in our laboratory aim to
provide a more detailed understanding of the chemistry of
imidazolium-2-diazotates. Additional results will be reported
in due course.
Received: September 16, 2011
Published online: November 16, 2011
Keywords: computational chemistry · heterocycles ·
.
N-heterocyclic carbenes · nitrogen oxides · structure elucidation
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