CAAC-Based Thiele and Schlenk Hydrocarbons

Article abstract of DOI:10.1002/anie.201915802

Diradicals have been of tremendous interest for over a century ever since the first reports of p- and m-phenylene-bridged diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk. Reported here are the first examples of cyclic(alkyl)(amino)carbene (CA

Full text of DOI:10.1002/anie.201915802

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Accepted Article
Title: p- and m-Phenylene Bridged Carbon Centred Diradicals with
CAAC-Scaffolds: Analogues of Thiele and Schlenk Hydrocarbons
Authors: Anukul Jana, Avijit Maiti, Jessica Stubbe, Nicolás I. Neuman,
Pankaj Kalita, Prakash Duari, Carola Schulzke, Vadapalli
Chandrasekhar, and Biprajit Sarkar
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915802
Angew. Chem. 10.1002/ange.201915802
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CAAC-Based Thiele and Schlenk Hydrocarbons
Avijit Maiti,[a] Jessica Stubbe,[b] Nicolás I. Neuman,[b] Pankaj Kalita,[a] Prakash Duari,[a] Carola
Schulzke,*[c] Vadapalli Chandrasekhar,*[a],[d] Biprajit Sarkar,*[b],[e] and Anukul Jana*[a]
Dedicated to Professor Dr. Herbert W. Roesky on the occasion of his 85th birthday.
Abstract: Diradicals have been of tremendous interest for over a
century ever since the first reports of p- and m-phenylene-bridged
diphenylmethylradicals in 1904 by Thiele and 1915 by Schlenk.
Herein, we report the first examples of cyclic(alkyl)(amino)carbene
(CAAC)-analogues of Thiele's hydrocarbon, a Kekulé diradical, and
Schlenk's hydrocarbon, a non-Kekulé diradical without using CAAC
as a precursor. The CAAC analogue of Thiele's hydrocarbon has a
singlet ground state, whereas the CAAC analogue of Schlenk's
hydrocarbon contains two unpaired electrons. The latter forms a
dimer, by an intermolecular double head-to-tail dimerization. Our
disclosed straightforward synthetic methodology is modular and can
be extended for the generation of redox-active organic compounds.
Diradicals, are a class of intriguing reactive compounds having
two unpaired electrons.[1] Such compounds have been of
considerable interest for several reasons. Firstly, their design
and assembly have been
a challenge to the synthetic
chemists.[2] Secondly, their structural and spectroscopic
properties have been of interest both from a theoretical and an
experimental point of view.[3] And thirdly, open-shell compounds
in general as well as diradicals that fall into this category have
been of significant importance for the design of novel functional
materials with applications in a variety of areas including
molecular electronics.[4] Considering their assembly, the two
radical centres in reported diradicals have been connected via
different types of motifs.[5] Following the landmark report on the
formation of the triphenylmethyl radical in 1900 by Gomberg,[6]
Thiele et al. reported in 1904 their diradical I (Scheme 1), aka
Thiele's hydrocarbon, in which the two radical centres are
connected through the p-phenylene bridge.[7] In 1915, Schlenk et
al. reported the diradical II, aka Schlenk's hydrocarbon, in which
the two radical centres are connected through the m-phenylene
bridge.[8] This system constitutes an open-shell triplet diradical
(non-Kekulé diradical) since a direct communication between the
two radical centers of II is not possible in contrast to diradical I.
Scheme 1. Chemical structures I-VI (Ar = 4-MeOC6H3, Dip = 2,6-iPr2C6H3).
Following the initial synthesis of I and II, a variety of other
derivatives[9] including nitrogen analogues such as III[10] and
IV[11] (Scheme 1) were prepared and studied.[12] Ghadwal et al.
reported the N-heterocyclic carbene (NHC)-analogue of Thiele's
hydrocarbons, V.[13] Recent theoretical studies by Munz and
coworkers pointed to CAAC-analogues of Thiele's hydrocarbon,
VI (Scheme 1) as excellent candidates for opto-electronic
applications.[14] However, the synthesis and properties of such a
system has not been reported yet. It has already been noticed
[a]
A. Maiti, P. Duari, Dr. P. Kalita, Prof. V. Chandrasekhar, Dr. A.
Jana
Tata Institute of Fundamental Research (TIFR) Hyderabad,
Gopanpally, Hyderabad-500107, Telangana, India
E-mail: ajana@tifrh.res.in
that replacing the Ph2C·/Ph2C-scaffolds in
I and II with
isoelectronic Ar2N·+/Ar2N+ as in III and IV and with isoelectronic
NHC as in V results in distinct changes in their electronic and
consequently in their spectroscopic properties. Given this
realization, along with the theoretical calculations and, even
more importantly, the wide use of CAAC-scaffolds for the
isolation of a variety of open-shell compounds,[15] we were
interested in designing diradicals containing CAAC-motifs.
Employing the corresponding pyrrolinium salts as
precursors constitutes one of the routes toward CAAC-scaffold
based carbon centred radicals. In order to get the requisite bis-
pyrrolinium salts a strategy based on our recent report[16] was
applied (Scheme 2).[17] The three sequential reactions of 1p or 1m
[b]
[c]
J. Stubbe, Dr. N. I. Neuman, Prof. Dr. B. Sarkar
Institut für Chemie und Biochemie, Anorganische Chemie, Freie
Universität Berlin, Fabeckstraße 34–36,14195, Berlin, Germany
Prof. Dr. C. Schulzke
Institut für Biochemie, Universität Greifswald, Felix-Hausdorff-
Straße 4, D-17487, Greifswald, Germany
E-mail: carola.schulzke@uni-greifswald.de
Prof. V Chandrasekhar
[d]
[e]
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016, India
E-mail: vc@iitk.ac.in
Prof. Dr. B. Sarkar
Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, D-70569, Stuttgart, Germany
E-mail: biprajit.sarkar@iac.uni-stuttgart.de
Supporting information for this article is given via a link at the end
of the document.
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angle [°]: N1‒C1 1.275(3), C1‒C5 1.495(3), C5‒C6 1.391(3), C6‒C7 1.387(3),
C7‒C5’ 1.397(3); N1‒C1‒C2 113.15(21).
with two equivalents of iPrNH2/Et3N, PCl5, and iPrMgCl afforded
the bisimines, 2p and 2m. The final cyclization step of bisimines,
2p and 2m to the corresponding p- and m-phenylene-bridged
pyrrolinium cations, 3p and 3m was accomplished in a one-pot
reaction involving two equivalents of nBuLi, isobutylene oxide,
and Tf2O (Scheme 2).[17]
In 3p the distance between the connecting carbon centres
of the pyrrolinium moiety (C1) and the central phenyl ring (C5) is
1.495(3) Å indicating single bond character (Figure 1).[18] Both C-
and N-centers of the pyrrolinium-moieties adopt a planar
geometry (ƩC1 = 360.00° and ƩN1 = 359.88°). The angle
between the planes involving N1-C1-C2 moiety of the CAAC-
scaffold and the central phenyl ring is 89.006(78)°. Due to
problems of twinning and the presence of disorder, we are not
discussing the metrical parameters of 3m (Figure 2).[18]
Scheme 2. Synthesis of p- and m-phenylene bridged pyrrolinium cations 3p
and 3m.
Compounds 3p and 3m were characterized by multi-nuclear
solution NMR spectroscopy (1H, 13C{1H}, and 19F{1H}) as well as
1
by solid state single crystal X-ray diffraction analysis.[18] The H
NMR spectra of 3p and 3m exhibit only one set of septet at  =
4.33 and 4.32 ppm, respectively and this indicates the fast
rotation of pyrrolinium-scaffold in the NMR-time scale even at
lower temperature (−35 °C). The four aromatic C-H of 3p give
rise to a singlet at  = 7.69 while the corresponding aromatic C-
H of 3m result in three resonances in a 1:1:2 ratio at  = 7.89,
7.79, and 7.72 ppm. These results indicate that in solution
compounds 3p and 3m have a symmetric structure with a C2-axis.
This has also been reflected in their corresponding solid state
structures determined through single crystal X-ray diffraction
(vide infra).
Figure 2. Molecular structure of 3m with thermal ellipsoids at 30 %. All
hydrogen atoms are omitted for clarity.
The cyclic voltammogram of 3p showed only a single
reversible reduction wave at E1/2 = −1.36 V in CH3CN/0.1 M
NBu4PF6 or Bu4NBArF ([BArF ]– = [3,5-(CF3)2-C6H3]4B– (Figure
4
4
3).[17] No other reduction wave was detected within the
solvent/electrolyte window. This result is surprising as the
presence of two pyrrolinium substituents is expected to result in
two reduction steps, as was observed for the corresponding
NHC analogues of the Thiele hydrocarbons, V.[13] A shift of the
second reduction wave beyond the solvent/electrolyte window is
unlikely as such a shift would lead to a potential difference of
more than 1.00 V between the two redox transformations of 3p.
These data, thus indicate that the reduction wave observed at
E1/2 = −1.36 V should be ascribed to two close-lying one-electron
waves or to one two-electron reduction.
Figure 1. Molecular structure of 3p with thermal ellipsoids at 50 %. All
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond
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of two-isomers (4psyn and 4panti). Calculations at TPSSh/def2-
TZVP level of theory point to strong antiferromagnetic coupling
of the spins, leading to a singlet ground state which is stabilized
with respect to the triplet excited state by more than 2000 cm-
1 [17]
.
Accordingly, no EPR signals were observed for the two-
electron reduced compound 4p When a crystalline sample of 4p
was dissolved in C6D6 and a 1H NMR spectrum immediately
recorded, we observed the presence of 4pqanti as the major
isomer as in the solid state molecular structure (vide infra).
1
Time-dependent H NMR shows that the 4pqanti isomer reached
an equilibrium with 4pqsyn isomer in about 190 minutes (Figures
S16 and S17).[17]
In the solid state molecular structure only the anti-isomer,
4pqanti, is present (Figure 5).[18] The distance between C1-C5 is
1.381(2) Å which is similar to that observed in Thiele's
hydrocarbon, I (1.381 Å)[19] but longer than that found in V
(1.375 Å for unsaturated NHC and 1.371 Å for saturated
NHC).[13] The C-C bond lengths in the central phenyl ring reveal
an alternation of short and long bond lengths consistent with the
chemical structure. The sum of the bond angles around C1 and
C1' is 359.54°. Both N1 and N1' also adopt an almost planar
geometry (ƩN = 358.16°). Compound 4p could be reversibly
oxidized to 3p by two equivalents of AgOTf in a 78 % isolated
yield (Scheme 3).
Figure 3. Cyclic voltammograms of 3p (blue), with 0.1 M NBu4BArF4, and
3m(red), with 0.1 M NBu4PF6, in MeCN at 100 mV·s-1.
Additionally, proof for the two-electron nature of this wave
was generated by performing CV and differential potential
voltammogram (DPV) measurements on an equimolar solution
of 3p and 3m. (Figure 4 and see below). The comparative current
heights also imply the two-electron nature of the reduction wave
of 3p. This implies that the electronic coupling between the two
redox centres is very small in the case of the CAAC-scaffold and
that it is motif dependent. The case of the NHC analogue of the
Thiele hydrocarbons V (ΔE1/2 = 0.25 V for unsaturated NHC-
motif and ΔE1/2 = 0.15 V for saturated NHC-motif) supports this
reasoning.[13]
Scheme 3. Synthesis of 4p.
Figure 4. Left: Cyclic Voltammogram of a 0.1 M solution of 3p (blue part) and
3m (red part) with 0.05 M NBu4BArF4 in MeCN at 100 mV·s−1. Right: Cyclic
voltammogram (top) and differential potential voltammogram (DPV) (bottom)
of a 0.1 M solution of 3p and 3m in MeCN at 100 mV·s−1 (CV) and 20 mV·s−1
(dpv) with 0.05 M NBu4BArF
.
4
3p displays a single absorption band at λmax = 267 nm
consistent
with
its
colorless
nature.
Vis/NIR
clean
spectroelectrochemical measurements showed
a
conversion of this compound to a new species with an
absorption maximum at λmax = 431 nm (Figure S41) and this is
very similar to the spectrum of the isolated two-electron reduced
quinoidal form 4p (λmax (ε) = 429 (54062) nm (Lmol-1cm-1)).[17]
In accordance with the cyclic voltammetry experiments, the
reduction of 3p with two equivalents of KC8 leads to 4p in about
70 % yield as a thermally stable but highly air and moisture
sensitive yellow solid (Scheme 3).[17] At room temperature a well
resolved 1H NMR spectrum was obtained, which is consistent
with a diamagnetic singlet ground state. 4p consists of a mixture
Figure 5. Molecular structure of 4pqanti with thermal ellipsoids at 50 %. All
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond
angle [°]: N1‒C1 1.395(2), C1‒C5 1.381(2), C5‒C6 1.452(2), C6‒C7 1.352(2),
C7‒C5’ 1.453(2); N1‒C1‒C2 108.15(10).
In contrast to 3p, the meta-substituted compound, 3m
displays two one-electron reduction steps at E1/2 = −1.61 V and
−1.88 V in CH3CN/0.1 M Bu4NPF6 or Bu4NBArF ([BArF ]– = [3,5-
4
4
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(CF3)2-C6H3]4B–), the first of which is completely reversible
(Figures 3 and S34-S35).[17] The difference between the two
dimethyl substituted m-quinodimethane is also known to
dimerize in an intermolecular double head-to-head fashion under
the formation of m-cyclophane.[20] In benzene, 4m2 dissociates to
4m, a process that can be followed by a slow color change from
light yellow to red.
reduction waves of 270 mV points to
a
reasonable
thermodynamic stability for the one-electron reduced mixed-
valence species. However, the second reduction is not
reversible in CH3CN solvent. The native form of 3m in solution
displays absorption bands only in the UV region. Upon one-
electron reduction new bands appear in the visible region with
maxima at λmax = 364 and λmax = 493 nm (Figure 6). Reversing
the potential to the starting potential led to the quantitative re-
generation of the spectrum corresponding to 3m, thus proving
the reversible nature of the first reduction wave on the Vis-NIR
spectroelectrochemical timescale. The second reduction turned
out to be irreversible in CH3CN as the starting spectrum of the
compound could not be regenerated once two electrons were
added. This implies that 4m, the 2-e reduced compound, reacts
with CH3CN. This was confirmed by an independent reaction of
4m and CH3CN. This is in contrast to the stability of 4m in hexane
(Figure S32).[17] However, the initial changes in the UV/vis
spectrum upon the second reduction of 3m are very similar to
those observed for the one-electron reduced species (Figure 6).
This suggests that there is no electronic coupling between the
two reduced centers in the two-electron reduced form, and the
differences in the redox potentials between the two reduction
waves should be largely assigned to coulombic interactions. No
further bands at longer wavelengths were observed either for the
one- or the two-electron reduced compounds. It was not
possible to investigate the one-electron reduced form of 3m by
EPR spectroscopy. Both EPR spectroelectrochemistry (longer
electrolysis times compared to Vis-NIR spectroelectrochemistry
because of different cell designs), and chemical reduction
proved unsuccessful for this purpose.
The chemically isolated two-electron reduced compound
4m was investigated both as the red sticky compound as well as
in solution in hexane via EPR spectroscopy. In both cases,
strong EPR responses were observed with signals centered at
g-values of 2.0032 and 2.0042 respectively (Figure 7).[17] The
spectrum in solution was nicely simulated by considering
hyperfine coupling to one 14N and to five different 1H nuclei
(Table S2).[17] These findings support a diradical nature of 4m as
required for the subsequent dimerization reaction. Additionally,
the EPR data indicate the existence of two uncoupled electron
spins in 4m. This fact is also supported indirectly by the Vis-NIR
spectroelectrochemistry data, where the “attempted” generation
of the two-electron reduced species delivered a spectrum that
just displayed a doubling of intensity of the spectrum of the one-
electron reduced form (see above). Calculations at TPSSh/def2-
TZVP level of theory indicate a triplet ground state for 4m with
the excited singlet state lying at least 400 cm-1 higher in
energy.[17] A likely explanation for the observation of two
uncoupled spins could be the orthogonal twist of the spin-
containing peripheral groups with respect to the central phenyl-
ring. The spin-spin coupling in this case is likely to be highly
sensitive to the aforementioned twist, and DFT calculations
probably overestimate the coupling because of this sensitivity.
Figure 6. Changes in UV/vis spectrum of 3m in MeCN with 0.1 M Bu4NPF6
during the first reversible (left) and second irreversible (right) reduction.
When the reduction of 3m was carried out with two
equivalents of KC8 in hexane, the formation of a deep-red
coloured solution was observed (Scheme 4). After work up, the
resulting red colored sticky compound was crystallized from
benzene/hexane solution and after one week very few yellow
crystals of 4m2 could be isolated in a reproducible manner, the
molecular structure of which was confirmed by single crystal X-
ray diffraction analysis (Figure 7).[18] Compound 4m2 is the dimer
of the initially formed diradical 4m and is possibly formed by an
intermolecular double head-to-tail dimerization. 4m can be
reversibly “re-oxidized” back to 3m by two equivalents of AgOTf
in hexane in a 58% isolated yield, indirectly indicating the
Scheme 4. Synthesis of 4m and 4m2.
existence of
Quinodimethane, the parent hydrocarbon of the m-quinonoid or
a
di-radical centre in 4m (Scheme 4). m-
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Figure 7. EPR spectra at 25 °C of 4m as a sticky red oil (left) and 4m in hexane
solution (by dissolving the sticky red oil) (right)
Keywords: carbenes • conjugation • diradicals •Thiele’s
hydrocarbon • Schlenk’s hydrocarbon
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Figure 8. Molecular structure of 4m
with thermal ellipsoids at 50 %. All
2
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and bond
angles [°]: N1‒C1 1.391(5), C1‒C5 1.370(6), C5‒C6 1.465(6), C6‒C7 1.341(5),
C7‒C8 1.512(6), C8‒C9 1.501(6), C9‒C10 1.336(6), C10‒C5 1.443(6),
C7‒C11 1.535(6), C8‒C11’ 1.594(5), C11‒N2 1.478(5); N1‒C1‒C2 109.33(30),
N2‒C11‒C12 103.00(26).
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In 4m2, the sum of the bond angles around C1 (ƩC1 =
359.9°) and C11 (ƩC11 = 325.84°) indicate that the respective
centres adopt a planar and a pyramidal geometry, respectively.
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C11-C8'-C7'-C11') in the dimer adopts a chair conformation
(Figure 8).[18] Theoretical calculations show that dimerization of
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In conclusion, we have designed and synthesized the first
CAAC-based Thiele and Schlenk hydrocarbons without the use
of CAAC as a precursor. The CAAC analogue of the Thiele
hydrocarbon displays a singlet ground state (Kekulé diradical)
while the CAAC analogue of the Schlenk hydrocarbon shows
two unpaired electrons (non-Kekulé diradical) and undergoes an
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intermolecular
double
head-to-tail
dimerization.
The
straightforward synthetic methodology revealed in this study will
be instrumental for generating new classes of carbon centres
based di- and poly-radical systems[21] and also for the synthesis
of new class of organic redox systems.
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Acknowledgements
This project was funded by intramural funds at Tata Institute of
Fundamental Research (TIFR) Hyderabad, Gopanpally,
Hyderabad-500107, Telangana, India from the Department of
Atomic Energy (DAE), Government of India, India V.C. is
thankful to the Department of Science and Technology, New
Delhi, India, for a National J. C. Bose fellowship. We are grateful
to the reviewers for their critical insights to improve the quality of
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2
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Entry for the Table of Contents
COMMUNICATION
p- and m-phenylene bridged bis-
pyrrolinium cations A provide access
to CAAC-analogues of Thiele's
hydrocarbon B, a Kekulé diradical,
and to Schlenk's hydrocarbon C, a
non-Kekulé diradical, respectively.
Compound B has a singlet ground
state. Compound C has two unpaired
electrons in the ground state and
forms a dimer by an intermolecular
double head-to-tail dimerization.
Avijit Maiti, Jessica Stubbe, Nicolás I.
Neuman, Pankaj Kalita, Prakash Duari,
Carola Schulzke,* Vadapalli
Chandrasekhar,* Biprajit Sarkar,* and
Anukul Jana*
Page No. – Page No.
CAAC-Based Thiele and Schlenk
Hydrocarbons
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