Adducts of nitrous oxide and N-heterocyclic carbenes: Syntheses, structures, and reactivity

Article abstract of DOI:10.1021/ja4030287

N-Heterocyclic carbenes (NHCs) react at ambient conditions with nitrous oxide to give covalent adducts. In the crystal, all compounds show a bent N ₂O group connected via the N-atom to the former carbene carbon atom. Most adducts are stable at room temperature, but heating induces decomposition into the corresponding ureas. Kinetic experiments show that the thermal stability of the NHC-N₂O adducts depends on steric as well as electronic effects. The coordination of N₂O to NHCs weakens the N-N bond substantially, and facile N-N bond rupture was observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a covalent modification of the terminal O-atom, and cleavage of the C-N₂O bond was observed in a reaction with thionyl chloride. The coordination chemistry of IMes-N₂O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the complexes CuOTf, Fe(OTf)₂, PhSnCl₃, CuCl₂, and Zn(C ₆F₅)₂. Structural analyses show that IMes-N₂O is able to act as a N-donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic changes as evidenced by a bond length analysis.

Full text of DOI:10.1021/ja4030287

Article
pubs.acs.org/JACS
Adducts of Nitrous Oxide and N‑Heterocyclic Carbenes: Syntheses,
Structures, and Reactivity
Alexander G. Tskhovrebov, Basile Vuichoud, Euro Solari, Rosario Scopelliti, and Kay Severin*
́
́ ́ ́
Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: N-Heterocyclic carbenes (NHCs) react at ambient conditions
with nitrous oxide to give covalent adducts. In the crystal, all compounds show
a bent N2O group connected via the N-atom to the former carbene carbon
atom. Most adducts are stable at room temperature, but heating induces
decomposition into the corresponding ureas. Kinetic experiments show that
the thermal stability of the NHC−N2O adducts depends on steric as well as
electronic effects. The coordination of N2O to NHCs weakens the N−N bond substantially, and facile N−N bond rupture was
observed in reactions with acid or acetyl chloride. On the other hand, reaction with tritylium tetrafluoroborate resulted in a
covalent modification of the terminal O-atom, and cleavage of the C−N2O bond was observed in a reaction with thionyl chloride.
The coordination chemistry of IMes−N2O (IMes = 1,3-dimesitylimidazol-2-ylidene) was explored in reactions with the
complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and Zn(C6F5)2. Structural analyses show that IMes−N2O is able to act as a N-
donor, as an O-donor, or as a chelating N,O-donor. The different coordination modes go along with pronounced electronic
changes as evidenced by a bond length analysis.
Scheme 1. Fixation of N2O by FLPs (a) or NHCs (b)
INTRODUCTION
■
Nitrous oxide is a very inert gas, and its chemical activation
under ambient conditions is a challenging task.1 Transition-
metal complexes have been used with good success for this
purpose.2 With few exceptions,3−5 these reactions proceed via
oxygen atom transfer to the metal complex and release of
dinitrogen. In principle, the oxygen atom can be further
transferred to organic substrates, resulting in a metal-catalyzed
oxidation process. However, turnover is difficult to achieve, and
only a few catalytic processes have been reported so far.6 Highly
oxophilic compounds such as low-valent silicon compounds,7
BEt3,8 and PCy39 are also known to react with N2O under mild
conditions, with oxygen atom transfer being the exclusive
reaction mode. The utilization of N2O as a nitrogen atom
transfer agent has been realized in a few cases. Reactions of
lithium−organic10 and calcium−organic11 compounds with
N2O can give azines, hydrazones, or diazo compounds, but the
yields of these transformations are typically poor. Of industrial
importance is the reaction of sodium amide with N2O, which
gives NaN3.12 Deprotonated amines and hydrazines can also
react with N2O in nitrogen atom transfer reactions.13 The
incorporation of all three atoms of N2O was achieved in
reactions with strained alkynes.14
In 2009, the group of Stephan reported the fixation of N2O
by frustrated Lewis pairs (FLPs) (Scheme 1a).15,16 This type of
reaction is remarkable as it leaves the N−N−O unit intact.4 We
have recently reported another example of covalent fixation of
intact N2O.17 The N-heterocyclic carbenes 1,3-dimesitylimida-
zol-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene (IDipp) were shown to react with N2O to
give the stable adducts 1 and 2 (Scheme 1b). Preliminary
studies indicate that the carbene-based adducts are quite
distinct from N2O adducts of FLPs, in terms of both structure
and reactivity.17,18 Detailed investigations about the chemistry
of N-heterocyclic carbene (NHC)−N2O adducts are reported
below.
RESULTS AND DISCUSSION
■
Having established that NHCs with bulky aromatic mesityl
(Mes) or 2,6-diisopropylphenyl (Dipp) groups are able to fix
N2O,17 we were interested in whether other N-substituents
would be tolerated as well. To facilitate the synthetic access, we
decided to generate the carbenes in situ by reaction of the
corresponding imidazolium salts with potassium bis-
(trimethylsilyl)amide (KHMDS) in THF. After filtration of
the resulting potassium salt, the solution of the carbenes was
Received: March 26, 2013
© XXXX American Chemical Society
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stirred at room temperature under an atmosphere of N2O. After
24 h, the volume of the solution was reduced, which resulted in
precipitation of the N2O adducts 1−7 (isolated yields 40−
56%). This simple in situ methodology worked not only for the
known compounds 1 and 2, but also for adducts with aliphatic
substituents at the N-atoms, including small methyl and large
isopropyl groups (Scheme 2).
The solid-state structures of 3−8 were determined by X-ray
diffraction. In addition, we have analyzed a new structure of 1
containing a cocrystallized water molecule. Graphic representa-
tions of the structures are depicted in Figure 1, and selected
Scheme 2. Synthesis of NHC−N2O Adducts from
Imidazolium Salts
The synthesis of a N2O adduct of a saturated carbene proved
to be more challenging. Reaction of 1,3-bis(2,4,6-trimethyl-
phenyl)-2-imidazolidinylidene (SIMes) with N2O in THF
resulted in adduct formation, but the product decomposed
into the corresponding urea (see below), making isolation of a
pure compound difficult. Key to success was the utilization of a
hexane/Et2O mixture (1:2) instead of THF as the solvent.
Under these conditions, the N2O adduct 8 precipitated directly
from the reaction mixture, which allowed its isolation in 62%
yield (Scheme 3).
Scheme 3. Synthesis of the N2O Adduct 8
Figure 1. Ball-and-stick representations of the molecular structures of
the NHC−N2O adducts 1·H2O and 3−8 in the crystal.
The new compounds 3−8 were comprehensively charac-
terized by elemental analysis (C, H, N), high-resolution ESI+
mass spectrometry, UV/vis and NMR spectroscopy, and single-
crystal X-ray diffraction. All compounds are soluble in
dichloromethane and chloroform. Interestingly, the alkyl-
substituted adducts 5−7 are also soluble in water. A noteworthy
feature of the NHC−N2O adducts is their color. Solutions
appear yellow, and the UV/vis spectra show absorption bands
at ∼300 and 450 nm. NMR spectroscopic analyses (CD2Cl2 or
CDCl3) of the adducts 3−6 and 8 revealed an apparent C2v
symmetry. Attempts to “freeze out” rotational isomers were not
successful. The low-temperature NMR spectrum of 1 in
CD2Cl2 at −80 °C was very similar to that recorded at room
temperature. Similar results were observed for the asymmetri-
cally substituted (Me/Et) adduct 7. It should be noted that
rotational isomers have been detected by NMR spectroscopy
for thiazole- and 1,3,4-thiadiazole-2-nitrosamines.19 These
compounds also feature a terminal N2O group. In addition to
the standard 1H and 13C NMR measurements, we have
recorded 15N NMR spectra of the adducts 1 and 6 in CDCl3.
The signals of the N2O group appear at 336.9 and 617.6 ppm
for 1 and at 341.8 and 604.6 ppm for 6. These values are shifted
to lower field compared to what is observed for free N2O (135
and 218 ppm),20 but they are close to those reported for FLP−
N2O adducts (∼380 and 575 ppm).16c
bond lengths and angles are given in Table 1. For comparison,
we have also listed the values for the structures of 1 and 2,
which we had reported earlier.17 All adducts show a bent N−
N−O group (α = 110.19(17)−118.7(4)°) which is connected
via the terminal N-atom to the carbon atom of the heterocycle.
The C−N−N−O unit is in all cases nearly flat, with dihedral
angles between 172.65(7)° and 180.0(3)°. While the lengths
observed for the N−O and C−N(N) bonds are very similar for
the eight compounds, the N−N bond lengths vary between
1.270(5) and 1.33(2) Å. Despite these variations, the N−O
bonds are always shorter than the N−N bonds. This trend is in
contrast to what has been reported for N2O adducts of FLPs,
which feature shorter N−N bonds than N−O bonds.15,16
Large structural differences are found for the dihedral N−C−
N−N angles, which correspond to the angle between the planes
defined by the bent N2O group and the heterocycle. At first
glance, there seems to be a correlation between the size of the
N-R groups and the bending of the N2O group, with smaller
groups resulting in reduced bending. For the N,N′-dimethyl-
substituted adduct 5, for example, the two planes are nearly
coplanar (N−C−N−N = 8.06°), whereas a large inclination of
46.56° is found for adduct 6 with isopropyl groups attached to
B
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1·H2O and 3−8
b
N−N
N−O
C−N(N)
C−N−N−O
N−C−N−N
a
1
1.333(2)
1.328(3)
1.352(4)
1.293(3)
1.312(4)
1.270(5)
1.3039(15)
1.3223(11)
1.3124(17)
1.343(2)
1.250(2)
1.245(3)
1.250(4)
1.273(3)
1.265(3)
1.240(5)
1.2609(14)
1.2644(11)
1.2693(15)
1.247(2)
1.360(2)
1.362(4)
1.358(4)
1.376(4)
1.375(4)
1.355(6)
1.3743(16)
1.3783(12)
1.3771(18)
1.354(2)
175.36(13)
176.0(2)
25.4(2)
31.9(4)
39.5(5)
10.2(5)
29.8(5)
16.4(8)
8.07(18)
46.52(13)
14.3(2)
22.0(3)
1·H2O
a
2
178.4(3)
c
3
178.8(2)
180.0(3)
4
5
6
7
8
178.0(4)
179.86(9)
172.65(7)
179.72(11)
174.78(16)
a
b
c
Data taken from ref 17. The two terminal N-atoms of the N−C−N−N unit are cis to each other. Two independent molecules are found in the
crystal.
the N-atoms. However, a more detailed inspection shows that
there are also large differences of the dihedral angle for
compounds with sterically very similar N-R groups (1 and 3, or
4 and 6). We therefore conclude that the energy potential for a
rotation around the C−N(N) bond is rather flat and packing
effects are likely to contribute to the observed differences in
dihedral angles. The suggestion that rotation around the C−
N(N) bond is facile is in line with the NMR spectra, which
show one set of signals for the two N-R groups. For compound
1·H2O with a cocrystallized water molecule, one can observe a
hydrogen bond between the water and the N2O group.
Interestingly, the bond is formed between the OH group and
the N(C)-atom (N···H = 1.940(19) Å) and not between the
OH group and the terminal O-atom. This observation is
evidence for the basicity of the N(C)-atom. The presence of
the hydrogen bond has only a minor influence on the bond
lengths of the N2O group.
nitrosoimine derived from benzophenone, for example, has a
half-life of around 3 min at room temperature in methanol.24
More stable N-nitrosoimines can be obtained by nitrosation of
heterocyclic imines, but liberation of dinitrogen is still
observed.22−24 Upon irradiation, however, some N-nitro-
soimines were found to release NO.26 As potential NO donors,
N-nitrosoimines have been examined for their antithrombotic,
antiplatelet, and vasodilating activities.23,27 To the best of our
knowledge, crystallographic analyses of N-nitrosoimines have
not been reported so far.
To determine the thermal stability of the NHC−N2O
adducts, we have monitored the decomposition in THF-d8 at
1
60 °C by H NMR spectroscopy. For this study, adducts with
an aromatic mesityl substituent (1), as well as with a large (3)
and small alkyl substituent (5), were chosen. Furthermore, we
have examined adduct 8 with a saturated backbone. In all cases
the solution became colorless, and we observed the clean
decomposition into the corresponding ureas (Scheme 5).
In principle, the structures of NHC−N2O adducts can be
described by three mesomeric forms: the diazotate A, the
nitrosamide B, and the nitrosoimine C (Scheme 4). Structurally
Scheme 5. Thermolysis of NHC−N2O Adducts
Scheme 4. Mesomeric Structures of NHC−N2O Adducts
The decomposition reactions followed first-order kinetics
(see the Supporting Information, Figures S1−S4) and did not
depend on the concentration. These observations are in line
with what has been observed for other N-nitrosoimines,25 and
they suggest that the reactions proceed in an intramolecular
fashion. The highest stability was observed for adduct 5 with
t1/2 = 15.1 h, whereas 8 was the least stable one (t1/2 = 0.4 h).
Compounds 1 and 3 decomposed with t1/2 of 2.3 and 5.6 h,
respectively. The enhanced stability of 1, 3, and 5 over that of 8
can be explained by the presence of the CC bond in the
heterocyclic ring, leading to additional resonance stabilization
and a decreased electrophilicity of the C(NNO)-atom. The
higher stability of the adducts 3 and 5 with aliphatic N-Cy and
N-Me groups over that of adduct 1 with aromatic N-Mes
groups is likely due to electronic effects as well.
characterized diazotates are scarce, but data for the thallium salt
trans-Tl(ONNMe) have been reported.21 In the crystal, the salt
shows a long N−O bond (1.35(4) Å) and a short N−N bond
(1.24(5) Å), similar to what has been reported for N2O adducts
of FLPs.15,16 The opposite trend is observed for the NHC−
N2O adducts. This comparison indicates a smaller contribution
of the mesomeric form A. The relevance of form B, with a
negatively charged N(C)-atom, is underlined by the presence of
a N···HO hydrogen bond in 1·H2O, by the facile rotation
around the N−C bond, and by a computational study that we
had performed earlier on compound 1.17
The mesomeric forms A and B are both zwitterions,
reminiscent of what is found for NHC adducts of heteroallenes
such as CO2 and CS2.22 Mesomeric form C represents an N-
nitrosoimine. N-Nitrosoimines have been known for more than
100 years.23 They can be obtained by nitrosation of imines.24
Generally, N-nitrosoimines decompose rapidly to give dini-
trogen and the corresponding carbonyl compound.25 The N-
As mentioned above, compound 5 is soluble in water.
Surprisingly, heating a solution of 5 in D2O at 100 °C for 60 h
did not lead to a detectable decomposition. Similarly, a solution
of compound 5 in CD3OD could be tempered at 60 °C for 60 h
without significant degradation. A plausible explanation for the
C
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remarkable stability in water and methanol is a selective
stabilization of the ground state by hydrogen bonds. This
hypothesis is supported by the fact that compound 5 displays
solvatochromism (Figure S23, Supporting Information).
Interestingly, a completely different behavior was observed
for the N-nitrosoimine derived from benzophenone, with
decomposition being faster in methanol than in THF.24
The good stability of the NHC−N2O adducts at ambient
temperature provided the opportunity to study the chemical
reactivity of this class of compounds in more detail. Compound
1 was used as a representative example for these investigations.
First, we examined the reactivity of 1 toward acids. Addition of
an excess of aqueous HCl or HOTs to a solution of 1 in
dichloromethane or THF led to the immediate formation of a
colorless solution, from which the guanidinium salt 13 or 14
could be isolated in 45% and 47% yield, respectively (Scheme
6). 2-Fold deprotonation of the guanidinium salt 13 or 14
Figure 2. Ball-and-stick representations of the molecular structures of
14 (left) and 15 (right) in the crystal. Anions are omitted.
Scheme 6. Reactions of 1 with Acids
what has been observed for neutral 2-iminoimidazolines
(1.294(3)−1.298(1) Å).31
The reactions with acids (Scheme 6) and with acetyl chloride
(Scheme 7) both result in cleavage of the N−N bond. These
results corroborate our previous findings17,18 that coordination
to NHCs alters the chemical reactivity of the N2O group
substantially: instead of the commonly observed oxygen atom
transfer, rupture of the N−N bond is now a viable alternative. It
should be noted that N−N bond cleavage of N2O has also been
observed in reactions with metal complexes, but only a very few
examples have been described so far.3
In contrast to small electrophiles, reaction of 1 with tritylium
tetrafluoroborate in CH2Cl2 did not lead to N−N bond
rupture. Instead, the immediate formation of adduct 16 was
observed (Scheme 8).
would give imidazolin-2-iminato ligands. The latter have
emerged as versatile 2σ,4π-electron donors for early transition
metals.28 The reaction of NHC−N2O adducts with acids could
therefore be used as a new pathway for the synthesis of these
important ligands.
A plausible mechanism for the reaction of 1 with acids is a
protonation of the N(C)-atom, followed by heterolytic cleavage
of the N−N bond to give NO+ and a 2-iminoimidazoline. The
latter is immediately protonated to give 13 and 14. The
released NO+ can undergo subsequent reactions with the
solvent THF.29 The formation of NO+ is supported by the
following experiment: addition of aqueous HCl to a solution of
1 in MeOH in the presence of a mixture of sulfanilic acid and 1-
naphthylamine30 gave a red-pink color due to the formation of
an azo dye.
Scheme 8. Reaction of 1 with Ph3CBF4
Reaction of a solution of 1 in THF with an excess of acetyl
chloride (3 equiv) also resulted in N−N bond rupture to give
the acetylguanidinium salt 15 in 84% isolated yield (Scheme 7).
In the product, the Ph3C group is bound to the oxygen atom
of the N2O group as evidenced by a crystallographic analysis
(Figure 3). The C(1)−N(3) (1.392(2) Å) and N(4)−O(1)
(1.3651(17) Å) bonds in 16 are longer than in 1, while the
N(3)−N(4) bond is shorter (1.2572(19) Å). The differences
indicate a larger contribution of the resonance form A in 16
when compared to 1. The dihedral angle N(4)−N(3)−C(1)−
N(2) (24.9(3)°) in 16 is almost the same as in 1 (25.4(2)°).
The angle N(3)−N(4)−O(1) (107.32(13)°), on the other
hand, is slightly smaller than in 1 (113.1(1)°). The close
relation of 16 to N2O adducts of FLPs described by Stephan et
al.15,16 is noteworthy. In 16, the N2O group is sandwiched
between a Lewis-basic NHC and a Lewis-acidic tritylium group,
whereas phosphines and boranes have been used by Stephan.
Not surprisingly, the bond lengths and angles observed for 16
are comparable to what has been reported for FLP−N2O
adducts.15,16
Scheme 7. Reaction of 1 with Acetyl Chloride
The source of the NH proton in 15 is presently not clear. It
could be derived from the solvent or from impurities in the
reagent (AcOH, H2O). NO+ is again a likely side product in
this reaction.
Single-crystal X-ray analyses confirmed the proposed
structures of 14 and 15 (Figure 2). The C−N bond, which
connects the N2O group to the heterocycle, is longer in 15
(1.369(5) Å) than in 14 (1.344(4) Å). This difference can be
attributed to conjugation to the CO bond of the acetyl
group. The C−N bond in 14 is slightly longer compared to
Furthermore, we examined the reactivity of 1 toward the
chlorinating agent thionyl chloride. Treatment of a THF
D
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Scheme 10. Reaction of 1 with Different Transition-Metal
Complexes
Figure 3. Ball-and-stick representations of the molecular structure of
−
16 in the crystal. The BF4 anion is omitted.
solution of 1 with an excess of SOCl2 at ambient temperature
resulted in intensive bubbling and formation of a yellow
solution, from which 17 was isolated in 76% yield (Scheme 9).
Scheme 9. Reaction of 1 with SOCl2
The reaction likely proceeds via liberation of N2 and SO2; the
latter was detected experimentally by a qualitative test with
Na2Cr2O7. Chloroimidazolium chlorides such as 17 are
known.32 They are generally prepared by reaction of mild
chlorinating agents such as CCl4 and C2Cl6 with NHCs.
The formation of 17 by chlorination with SOCl2 is
remarkable because it requires the cleavage of a strong C−N
bond. As a potential mechanism for this reaction, one can
propose the coordination of SOCl+ to the terminal oxygen
atom of the N2O group, followed by an intramolecular
rearrangement to give 17 along with N2 and SO2.
Finally, we were interested to explore the reactivity of 1
toward Lewis-acidic transition-metal complexes. For reactions
with the complexes CuOTf, Fe(OTf)2, PhSnCl3, CuCl2, and
Zn(C6F5)2 we were able to isolate defined adducts (18−23,
Scheme 10). The complexes were isolated in good yields (46−
94%), with the exception of the Fe(II) complex 20, for which
only a few crystals were obtained. The structures of all adducts
were determined by single-crystal X-ray analysis (Figure 4).
The structural analyses revealed that 1 displays remarkably
versatile coordination chemistry. In the solid state, one can
observe η1-coordination via the N(C)-atom (19) or via the
terminal O-atom (18, 20). Furthermore, ligand 1 is able to act
as a chelating ligand with η2-coordination via the N(C)-atom
and the O-atom (20−23). For the Sn(IV) complex 21 and the
Cu(II) complex 22, the N,O-chelate features a short M−O
bond and a long N−M bond, whereas a more equilibrated
situation is found for the chelate in the Fe(II) complex 20 and
the Zn(II) complex 23 (Table 2). The different coordination
modes have a strong influence on the bond lengths in the N2O
group of ligand 1. The nearly planar trans configuration of the
C−N−N−O unit, on the other hand, is conserved for all
complexes. For the O-bound IMes−N2O ligands in 18 and 20,
the C−N and N−O bonds are significantly longer than in free
1, while the N−N bonds are shorter (Table 2). These data
indicate an increased electron density on the O-atom and a
significant contribution of the resonance form A. The bond
lengths in 18 and 20 are close to those found for FLP−N2O
adducts which contain PNNO−M linkages.16 The bond lengths
in the N2O group of chelates 20−23 are close to each other.
The C−N and N−O bonds in the chelates are shorter than
those found for monodentately coordinated 1, but longer than
in free 1. The N−N bonds are shorter than in free 1, but longer
compared to those in η1-coordinated 1, indicating that the
contribution of the resonance form A is more pronounced for
η1-coordination than for η2-coordination. A unique coordina-
tion mode of the IMes−N2O ligand is observed for complex 19.
Here, we observe exclusive coordination via the N(C)-atom. As
expected, the bond lengths in the N2O unit of 19 indicate a
large contribution of the resonance form B (Table 2).
CONCLUSIONS
■
Covalent capture of the inert gas N2O can be achieved at
ambient conditions using NHCs. The latter can conveniently
be prepared in situ by deprotonation of imidazolium salts. The
adducts contain intact N2O groups, which are bound via the
terminal N-atom to the heterocycle. They can thus be described
as imidazolium diazoates or as N-nitrosoimines. Compared to
other N-nitrosoimines reported previously, the NHC−N2O
adducts are remarkably stable. When dissolved in THF, only
slow thermal decomposition into ureas was observed at
elevated temperatures. Further stabilization was achieved by
using protic solvents. The good thermal stability of these N-
nitrosoimines allowed for the first time a detailed structural
characterization of this class of compounds. A distinctive
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via the O-atom. The coordination chemistry of NHC−N2O
adducts was explored using the IMes-derived 1 as a
representative example. Structural analyses of six different
transition-metal complexes revealed a remarkably versatile
coordination behavior: 1 is able to act as a N-donor, as an
O-donor, or as a chelating N,O-donor. The different
coordination modes go along with pronounced electronic
changes as evidenced by a bond length analysis.
Overall, the results presented in this work contribute to the
understanding of the basic chemistry of N2O, an inert gas with
a highly negative environmental impact.33 Furthermore, our
work demonstrates that N-nitrosoimines display a rich
chemistry, which goes far beyond the simple decomposition
into ureas.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and analytical data as well as
crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
kay.severin@epfl.ch
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Ecole Polytechnique Fed
de Lausanne (EPFL) and the Swiss National Science
Foundation.
■
́
er
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ale
REFERENCES
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Figure 4. Ball-and-stick representations of the molecular structures of
18−23 in the crystal. Anions and H-atoms are omitted.
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for
18−23
N−N
N−O
C−N(N) C−N−N−O N−C−N−Nb
18
19
1.261(5)
1.358(8)
1.355(8)
1.318(4)
1.225(8)
1.231(8)
1.294(3)
1.279(3)
1.290(2)
1.274(4)
1.265(2)
1.395(5)
1.369(8)
1.379(8)
1.374(3)
1.384(3)
1.384(3)
1.371(5)
1.362(3)
178.2(3)
173.6(6)
174.6(6)
177.6(2)
179.9(2)
173.75(18)
176.4(3)
175.24(16)
71.9(5)
37.2(1)
27.9(1)
63.7(4)
88.0(3)
30.0(3)
14.4(6)
26.6(3)
a
20 η1 1.268(3)
20 η2 1.300(3)
21
22
23
1.294(3)
1.296(4)
1.301(2)
a
The complex contains two ligands with different metrical parameters.
structural feature of the NHC−N2O adducts is the presence of
long N−N and short N−O bonds. The opposite trend has been
observed for diazotates and N2O adducts of FLPs. The
structural data suggest that the N−N bond of NHC−N2O
adducts is rather weak. This was confirmed in reactions with
acids or with acetyl chloride, which resulted in rupture of the
N−N bond. In contrast, reaction with the bulky Lewis acid
Ph3C+ led to the formation of a stable adduct with coordination
F
dx.doi.org/10.1021/ja4030287 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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