Synthesis of Organic Super-Electron-Donors by Reaction of Nitrous Oxide with N-Heterocyclic Olefins

Article abstract of DOI:10.1021/jacs.9b10660

The reaction of nitrous oxide (N₂O) with N-heterocyclic olefins (NHOs) results in cleavage of the N-O bond and formation of azo-bridged NHO dimers. The latter represent very electron-rich compounds with a low ionization energy. Cyclic voltammetry studies show that the dimers can be classified as new organic super-electron-donors, with a reducing power similar to what is found for tetraazafulvalene derivatives. Mild oxidants are able to convert the neutral dimers into radical cations, which can be isolated. Further oxidation gives stable dications.

Full text of DOI:10.1021/jacs.9b10660

Communication
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Synthesis of Organic Super-Electron-Donors by Reaction of Nitrous
Oxide with N‑Heterocyclic Olefins
†
†
†
‡
†
́
Leonard Y. M. Eymann, Paul Varava, Andrei M. Shved, Basile F. E. Curchod, Yizhu Liu,
†
§
Ophelie M. Planes, Andrzej Sienkiewicz, Rosario Scopelliti,† Farzaneh Fadaei Tirani,†
́
and Kay Severin*,†
†
́
́
́
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne,
Switzerland
‡Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom
§
́
́
́
Institute of Physics, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S
* Supporting Information
Nitrous oxide (N2O) is isoelectronic to CO2 and likewise a
chemically inert compound.7 Despite its inert character, N2O is
able to form stable covalent adducts with NHCs under
ambient conditions (Scheme 1c).8 To date, NHC−N2O
adducts have not been used in the context of catalytic
reactions, but they have been shown to act as mild and
selective oxidants for metal complexes9 and as precursors for
azo dyes.10
ABSTRACT: The reaction of nitrous oxide (N2O) with
N-heterocyclic olefins (NHOs) results in cleavage of the
N−O bond and formation of azo-bridged NHO dimers.
The latter represent very electron-rich compounds with a
low ionization energy. Cyclic voltammetry studies show
that the dimers can be classified as new organic super-
electron-donors, with a reducing power similar to what is
found for tetraazafulvalene derivatives. Mild oxidants are
able to convert the neutral dimers into radical cations,
which can be isolated. Further oxidation gives stable
dications.
The results summarized above prompted us to explore
whether NHOs would also react with N2O. Chemical
activation of N2O with NHOs can indeed be achieved under
mild conditions. Instead of simple 1:1 adducts, we observed
the formation of azo-bridged NHO dimers (Scheme 1d).
These dimers represent new super-electron-donors, as
evidenced by cyclic voltammetry and reactions with aryl
iodides. Details of these investigations are given below.
For our studies, we used NHOs with 2,6-iPr2C6H3 (Dipp),
2,4,6-Me3C6H2 (Mes), and 2,6-Me2C6H3 (Xyl) substituents.11
When concentrated solutions of these compounds in CH3CN
were exposed to an atmosphere of N2O, a gradual color change
to orange/red was observed. After 48 h, strongly colored
precipitates had formed (1−3; Scheme 2), which were isolated
and washed with CH3CN.
-heterocyclic carbenes (NHCs) are able to form covalent
N
adducts with CO2 (Scheme 1a).1,2 The resulting
Scheme 1. Reactions of NHCs and NHOs with CO2 or N2O
Scheme 2. Reaction of NHOs with N2O
imidazolium-2-carboxylates have been used as organocatalysts
for different reactions,3 and they represent easy-to-handle
NHC transfer reagents for the synthesis of metal carbene
complexes.4
N-heterocyclic olefins (NHOs) are alkylidene derivatives of
NHCs.5 The exocyclic CC double bond is strongly
polarized, and the high charge density at the Cα atom makes
NHOs strongly Lewis basic compounds. Similar to NHCs,
NHOs react with CO2 to give zwitterionic covalent adducts
(Scheme 1b). These adducts are of key importance in CO2
sequestration reactions with NHOs.6
Received: October 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b10660
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
The products displayed reduced symmetry compared with
the starting NHOs, as evidenced by the presence of a double
set of NMR signals for the N-aryl groups. This observation
indicated that 1−3 are not simple NHO−N2O adducts.
Analysis of 1 and 3 by single-crystal X-ray crystallography
showed that azo-bridged NHO dimers had formed (for details,
see the Supporting Information (SI)). The formation of these
dimers can be rationalized by assuming that the reactions
proceed via zwitterionic NHO−N2O adducts of type A
(Scheme 2), which can tautomerize to the diazohydroxides
B.12 A condensation reaction with remaining NHO then
provides the dimers 1−3. Presumably, the condensation
reaction is initiated by N−O bond rupture of the
diazohydroxide B, forming either a vinyl diazonium compound
or a diazoalkene.13
tetraazafulvalene derivative C using the same computational
protocol (DFT/ωB97X-D/IEFPCM) gave a value of 3.34 eV,
close to the one obtained for 1.
It is worth discussing the structurally related compound E2+,
F, and G (Figure 1). E2+ was described by Hu
̈
nig and co-
workers.19 Electrochemical investigations showed that the
corresponding neutral form E is formed at E1/2 = −1.00 V,18
but isolation of the neutral compound was not attempted. The
lower reducing power of E compared with 1 is likely related to
the presence of annulated benzene rings.14 The dipnictenes F
and G were recently reported by Ghadwal and co-workers.20
These compounds were obtained by reaction of phenyl-
substituted NHOs with ECl3 (E = P, As) followed by
reduction. Electrochemical investigations of F indicated that
the first oxidation occurs at E1/2 = −1.36 V.20a
Solutions of 1−3 in THF appear dark orange. The
absorption in the visible range is in line with the solid-state
structures of 1 and 3, which both show a coplanar arrangement
of the two heterocycles and the divinyldiazene bridge, allowing
for efficient π conjugation. Linear-response time-dependent
density functional calculations further confirm this observation,
indicating for the main band of 1 a ππ* character located over
the heterocycles and the divinyldiazene bridge (see the SI for
computational details).
The chemical reactivity of the new azo-bridged NHOs was
investigated, again using compound 1 as a representative
example. The large difference between the first and second
oxidation potentials allows for selective one-electron oxidation
of 1. The oxidation can be accomplished using chloroform,
benzyl bromide, or silver triflimide as the oxidant (Scheme 3).
The resulting salts 4a−c were isolated in yields between 40 and
72%.
The reducing power of 1 was assessed by cyclic voltammetry
(CH3CN, 0.1 M NBu4PF6). Two well-separated reversible
redox transitions were observed at E1/2 = −1.34 and −0.73 V,
referenced versus an external Fc/Fc+ redox couple. Similar
values were obtained for the azo compounds 2 and 3 (see the
SI). The first oxidation potential is comparable to what has
been observed for some tetraazafulvalene derivatives, for
example C and D (Figure 1), which are termed organic
super-reducing agents.14−17
Scheme 3. Reactions of 1 with Different Oxidants
In the absence of air, the salts are stable in solution (THF)
and in the solid state.21 Crystallographic analyses of 4a−c
revealed that the single-electron oxidation resulted in length-
ening of the C1−C2 and N1−N1′ bonds and shortening of the
C2−N1 bond (Table 1).
The presence of a radical cation in 4 was confirmed by EPR
spectroscopy (Figure 2a). The complex hyperfine coupling
indicates that the radical is delocalized over the planar π
system. Such delocalization is in accordance with the results of
DFT calculations, which show that the spin density is
distributed over the two heterocycles and the divinyldiazene
bridge (Figure 2b).
Solutions of 4 are strongly colored, and the UV−vis
spectrum (THF) shows absorption bands at 661 and
736 nm in addition to a main band at λmax = 516 nm. The
occurrence of low-energy bands is typical for π-conjugated
radicals.22
Figure 1. Redox potential for the first oxidation of 1 in comparison to
the values for structurally related compounds reported in the
literature. The values are based on CV measurements in CH3CN
(CH2Cl2 for F) with respect to the Fc/Fc+ redox couple.17
Density functional theory (DFT) calculations using the
ωB97X-D exchange−correlation functional18 were performed
to gain further insight into the electronic structure of the new
compounds, using 1 as representative example (for details, see
the SI). The calculations revealed a gas-phase vertical
ionization energy (vIE) of 4.49 eV and an adiabatic ionization
energy (aIE) of 4.16 eV. Inclusion of an implicit solvent model
for CH3CN in the calculation lowered the ionization energies
to 4.19 eV (vIE) and 3.24 eV (aIE). Calculating the aIE of
The addition of 2 equiv of AgBF4, an excess of bromine, or
2,2,3,3-tetrachlorohexafluorobutane to a solution of 1 resulted
in the formation of imidazolium salts 5a−c, which could be
B
DOI: 10.1021/jacs.9b10660
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Journal of the American Chemical Society
Communication
(benzyloxy)benzene (10) was formed in only 43% yield, and
incomplete conversion was observed.
Table 1. Selected Bond Lengths for 1, 3, 4a−c, and 5b As
a
Determined by X-ray Crystallography
In summary, we have examined the reaction of N-
heterocyclic olefins with nitrous oxide. Instead of simple
N2O adducts, we observed N−O bond cleavage and formation
of azo-bridged NHO dimers (1−3). These dimers are very
strong electron donors, which can be converted into stable
radical cations or a dicationic imidazolium salts. The first
oxidation potentials are similar to what is observed for some
tetraazafulvalenes, allowing the reduction of aryl iodides.
Consequently, 1−3 can be classified as new super-electron-
donors. Tetraazafulvalenes have been used as potent reducing
agents in synthetic chemistry,14 and similar applications can be
envisioned for the new diazenes.
compound
C1−C2
C2−N1
N1−N1′
1
3
4a
4b
4c
5b
1.3692(14)
1.3685(14)
1.408(3)
1.4108(18)
1.4109(15)
1.450(3)
1.3703(14)
1.3713(13)
1.324(2)
1.3214(17)
1.3237(14)
1.271(3)
1.2907(16)
1.2902(16)
1.327(3)
1.333(2)
1.3300(18)
1.400(3)
ASSOCIATED CONTENT
a
■
All of the structures show a crystallographic inversion center.
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b10660.
Experimental details, analytical data, and supporting
figures (PDF)
Crystallographic data for 1 (CIF)
Crystallographic data for 3 (CIF)
Crystallographic data for 4a (CIF)
Crystallographic data for 4b (CIF)
Crystallographic data for 4c (CIF)
Crystallographic data for 4d (CIF)
Crystallographic data for 5b (CIF)
Crystallographic data for 6 (CIF)
Figure 2. (a) EPR spectrum of 4a and (b) calculated spin density of
the radical cation.
AUTHOR INFORMATION
Corresponding Author
*kay.severin@epfl.ch
■
isolated in high yields (Scheme 3). Crystallographic analysis of
5b showed that the double oxidation led to further lengthening
of the C1−C2 and N1−N1′ bonds and shortening of the C2−
N1 bond (Table 1).
When a solution of 1 in a mixture of diethyl ether and
hexane (2:1) was exposed to dioxygen, we observed the
formation of a brown-yellow oily solid. Workup allowed
isolation of the salt 6 in 69% yield (Scheme 3). As evidenced
by mass spectrometry and single-crystal X-ray crystallography
(see the SI), the reaction with O2 resulted in oxidation of one
of the Cα atoms of the azo-bridged dimer.23
To qualify as an organic super-reducing agent, a compound
should be able to reduce aryl iodides.14 We examined the
reaction of 1 with aryl iodides 7 and 9 (Scheme 4).24 When a
solution of 7 and 1 in a mixture of DMF and toluene (1:1) was
heated to 100 °C, cyclization to form indoline 8 was
observed.25 The latter could be isolated in 89% yield. The
more challenging substrate 9 could also be reduced. However,
ORCID
Andrzej Sienkiewicz: 0000-0003-3527-7379
Rosario Scopelliti: 0000-0001-8161-8715
Farzaneh Fadaei Tirani: 0000-0002-7515-7593
Kay Severin: 0000-0003-2224-7234
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The work was supported by the Swiss National Science
Foundation and EPFL.
■
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exchange occurred during the crystallization process (glassware).
(24) For reactions of these substrates with tetraazafulvalenes, see:
(a) Jolly, P. I.; Zhou, S.; Thomson, D. W.; Garnier, J.; Parkinson, J. A.;
Tuttle, T.; Murphy, J. A. Imidazole-derived carbenes and their elusive
tetraazafulvalene dimers. Chem.Sci. 2012, 3, 1675−1679. (b) Murphy,
J. A.; Khan, T. A.; Zhou, S.; Thomson, D. W.; Mahesh, M. Highly
Efficient Reduction of Unactivated Aryl and Alkyl Iodides by a
Ground-Sate Neutral organic Electron Donor. Angew. Chem., Int. Ed.
2005, 44, 1356−1360.
(25) A reaction between 7 and 1 in toluene at room temperature
resulted in the precipitation of the radical cation (1•+)(I−) (4d), the
structure of which was characterized by X-ray diffraction. For details,
see the SI.
E
DOI: 10.1021/jacs.9b10660
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX