Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation

Article abstract of DOI:10.1039/d0sc00699h

We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridinylidene) form persistent radical cations, whereas others such as the dimer ofN,N'-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) withN,N'-dimethyl imidazolin-2-ylidene (NHC),N,N'-dimethyl imidazolidin-2-ylidene (saNHC) andN-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2' positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.

Full text of DOI:10.1039/d0sc00699h

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ARTICLE
Aromaticity and Sterics Control Whether a Cationic Olefin Radical
is Resistant to Disproportionation
Julian Messelberger,a,† Annette Grünwald,a,† Stephen J. Goodner,a Florian Zeilinger,a Piermaria
Received 00th January 20xx,
Accepted 00th January 20xx
Pinter,a Matthias E. Miehlich,a Frank W. Heinemann,a Max M. Hansmann,b,c and Dominik Munz*a
DOI: 10.1039/x0xx00000x
We elucidate why some electron rich olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridin-
ylidene) form persistent radical cations, whereas others such as the dimer of N,N’-dimethyl benzimidazolin-2-ylidene
(benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl)(amino) carbenes (CAAC) with N,N’-dimethyl
imidazolin-2-ylidene (NHC), N,N’-dimethyl imidazolidin-2-ylidene (saNHC) and N-methyl benzothiazolin-2-ylidene (btNHC)
are reported. Whereas the olefin radical cations with the saNHC and btNHC are isolable, the NHC compound with a satu-
rated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties
of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experimen-
ts, we propose a general computational method to model the electrochemical potentials and disproportionation
equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental
values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation.
The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly
due to aromaticity. Contrarily, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2’ positions
of the heterocyclic monomers. Rigid tethers increase accordingly the resistance of homodimeric radical cations, whereas
the electronic effects of substituents seem much less important for the disproportionation equilibrium.
S
S
S
S
S
S
S
S
S
S
S
N
N
Introduction
S
1 (TTF)
2
3
4
Electron-rich olefins are an exciting class of organic redox sys-
tems with potentially three stable redox states1-3 and are po-
pular reductants in organic synthesis.4-9 Exciting applications
are in particular associated with the open-shell redox state with
one unpaired electron, which allows for intriguing conductive
and photochemical properties. The arguably most relevant de-
rivative, tetrathiafulvalene (TTF 1, Fig. 1), its saturated congener
2,10, 11 as well as its benzannulated derivative 3 even have been
called “the brick and mortar” of organic materials. They are
now commonly applied in switches, solar cells, or organic field-
effect transistors (OFETs).12-14 Very similar redox properties are
found for olefins derived from biscationic paraquat (methyl vio-
logen, respectively; 1,1'-dimethyl-4,4'-bipyridinylidene) 4, the
related ortho-derivative (1,1’-dimethyl-2,2’-bispyridinylidene)
5, the dimethylaminopyridine (DMAP) dimer 6,15 as well as 4,4'-
bipyrylene 7.16-20
N
N
N
N
N
N
N
O
O
N
6
7
5
8
N
N
S
N
N
S
N
N
S
N
S
N
N
N
N
N
N
N
12
14
9
11
10
S
N
N
N
13
N
S
N
Dipp
Dipp: 2,6-diisopropylphenyl-.
Fig. 1 Homo- (1–13) and heterodimeric (14) electron-rich olefins.
The aza-analogues of TTF, i.e. tetraaminoethylenes or enetetra-
mines, originate from the formal dimerization of unsaturated-
(NHC, 8), saturated- (saNHC, 9) or benzannulated- (benzNHC,
10) N-heterocyclic carbenes (NHCs). Other examples comprise
thiazolin-2-ylidenes (11, 12, 13, Fig. 1).21, 22 However, har-
nessing the exceptional electronic properties of NHC-derived
electron-rich olefins for organic electronics remained challen-
ging due to the undesired dissociation into the free carbenes
† Both authors contributed equally.
a J. Messelberger, A. Grünwald, S. J. Goodner, F. Zeilinger, P. Pinter, M. E.
Miehlich, Dr. F. W. Heinemann, Dr. D. Munz Friedrich-Alexander-Universität
Erlangen-Nürnberg, Lehrstuhl für Allgemeine und Anorganische Chemie,
Egerlandstr. 1, 91058 Erlangen, Germany. E-mail: dominik.munz@fau.de
b Dr. M. M. Hansmann, Georg-August Universität Göttingen, Institut für
Organische und Biomolekulare Chemie, Tammannstraße 2, 37077 Göttingen,
Germany.
(“Wanzlick’s equilibrium”).23-30
Whereas 8 is kinetically
unstable towards dissociation,31-33 10 stands in equilibrium with
c Dr. M. M. Hansmann, Technische Universität Dortmund, Organische Chemie,
Otto-Hahn-Str. 6, 44227 Dortmund, Germany.
its monomers at room temperature.34, 35
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ARTICLE
Journal Name
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E
E
E
E
BF4
BF4
DOI: 10.1039/D0SC00699H
electronic effects,
aromaticity
tether: sterics
N
N
N
N
Dipp
Dipp
Dipp
15
KN(SiMe3)2
toluene, rt
- HN(SiMe3)2,
- KBF4
E
E
H
N
G (E, K)
2
THF, rt
H
+
N
N
E
E
?
E
E
E
E
N
N
17
(quantitative)
18
(quantitative)
E = N, S, CR2 (electronic effects)
16
Scheme 1 Formation of CAAC–NHC adduct 17 and deprotonation to 18.
Fig. 2 What controls whether cationic olefin radicals disproportionate? G, Gibbs
Free Energy; E, electrochemical potential; K, equilibrium constant.
PF6
N
N
N
N
N
Dipp
19
Dipp
Dipp
KN(SiMe3)2
H
N
Thus, we introduced electron-rich triazaolefins.36 Intriguingly,
heterodimer 14, resulting from formal dimerization of a cyclic
(alkyl)(amino) carbene (CAAC)37 with a benzNHC, did not disso-
ciate even upon heating to 100 °C and showed similar redox po-
tentials as TTF (1: E1,2 = +0.32 V, E2,3 = −0.08 V vs. Fc/Fc+).14
Paralleling the rich chemistry of TTF derivatives, subsequent
studies were directed at introducing bridges in order to obtain
mixed-valent compounds and singlet biradicaloids.38-41 Other
investigations explored complementary synthetic approaches42-
benzene, rt
- HN(SiMe3)2,
- KPF6
benzene
rt
H
N
PF6
N
N
21
22 (54%)
20
Scheme 2 Formal CH insertion of 19 and deprotonation afforded 22.
BF4
X
N
Dipp
N
N
S
Dipp
Dipp
15
NEtiPr2
H
H
KN(SiMe3)2
H
N
PhH, rt
- HN(SiMe3)2,
- KX
I
MeCN, rt
S
N
- NEtiPr2·HX
45
or focused on other heterocycles as building blocks.46-48 In
N
S
combination with further related reports,49-52 a considerable
variety of such two-stage redox systems are now available. For
applications as organic materials, it is highly desirable to predict
or at least rationalize the redox potentials of these scaffolds.
One of the authors suggested that the electronic properties of
carbene heterodimers could be understood by the -acceptor
properties of the related free carbenes.47 Yet, it is not under-
stood which combinations of carbenes afford isolable radical
cations upon removal of one electron. It has been proposed
that the delocalization of the radical, heteroatom effects, or
steric bulk might be important.17 Surprisingly and in opposition
to the very stable radical cations derived from TTF, the benzNHC
dimer 10 does not give a persistent radical cation. Instead, only
a two-electron oxidation is observed in the cyclic voltammetry
(CV) experiment with E1,2 = −0.92 V vs. Fc/Fc+.21 The electro-
chemical properties of 9 have not been reported according to
our knowledge.
23
24
X = I, BF4
25
(11%)
Scheme 3 Heterodimer 25 was synthesized using Hünig’s base.
Also the addition of CAAC 19 to the imidazolinium salt 20 gave
21 (Scheme 2). Subsequent deprotonation afforded the satura-
ted heterodimer 22 after workup in 54% overall yield. These
two synthetic approaches were not successful for the sulfur-
containing derivative 25, where instead the homodimer of the
btNHC (13) formed irreversibly. However, slow addition of
Hünig’s base to 15 in presence of excess of 23 led to the
formation of the desired addition product 24 as a mixture with
residual 15 and 13, where the latter could be removed during
the workup. Treatment with KHMDS gave then, after crystalli-
zation at −35 °C, 25 as a yellow, crystalline solid (Scheme 3).53
Electrochemical Properties
Herein, we show which factors determine whether a cationic
olefin radical is stable towards disproportionation into the
parent olefin and biscation. Thereby, we elucidate how elec-
tronic effects of the heteroatoms within the heterocycles, aro-
maticity, tethers and sterics influence the disproportionation
equilibrium (Fig. 2). Based on the experimental properties of
newly and previously synthesized electron rich olefins, we pro-
pose a convenient and general concept to understand, predict
and model in silico this disproportionation equilibrium.
In order to elucidate the electrochemical properties of these
two-stage redox-systems, CV experiments were performed (Fig.
3). All redox events were found to be reversible (Figs. S14, S15,
S16). The CAAC–NHC dimer 18 showed two sequential one-
electron oxidations giving rise to the stable radical cation 18rad
(E2,3 = −1.26 V vs. Fc/Fc+ in THF) and the biscation 18biscat
(E1,2 = −0.66 V vs. Fc/Fc+ in THF). Surprisingly, 22 featured only
one redox wave (E1,2 = −0.69 V). The large separation of the two
halfwaves of E = 0.64 V is indicative of a high reorganization
energy and hence in this particular case of a two-electron oxi-
dation. The btNHC derivative 25 showed two redox events
(E2,3 = −0.50 V, E1,2 = −0.24 V; vs. Fc/Fc+ in THF), which can be
assigned to the oxidation of the olefin to the radical cation 25rad
and the biscation 25biscat, respectively. The halfwave potentials
E1,2 and E2,3 for 25 are shifted to more positive potential in
relation to the CAAC–NHC heterodimer 18.
Results and Discussion
Synthesis
The synthesis of the CAAC–NHC heterodimer 18 followed the
synthetic approach chosen for 14.36 The cyclic iminium salt 15
reacted swiftly at room temperature with 16 to give the
colorless addition product 17 (Scheme 1). Subsequent deproto-
nation by potassium hexamethyldisilazide (KHMDS) afforded 18
quantitatively as a yellow solid.
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OTf
N
N
AgOTf
THF, rt
- Ag
N
N
N
N
Dipp
18rad (quantitative)
Scheme 4 Synthesis of radical cation 18rad through oxidation with AgOTf.
Dipp
18
Fig. 3 CV in 0.1
M
nBu4NPF6, 100 mV s-1, vs. Fc/Fc+ (18, red, E1,2 = −0.67 V,
E2,3 = −1.27 V; 22, green, E1,2 = −0.69 V; 25, blue, E1,2 = −0.24 V, E2,3 = −0.50 V in
THF).
Fig. 5 Experimental and simulated EPR spectra of 18rad in THF.
Fig. 4 CV in 0.1 M nBu4NPF6, 100 mV s-1, vs. Fc/Fc+ (9, orange, E1,2 = −1.27 V in
DMF; 13, purple, E1,2 = −0.45 V, E2,3 = −0.60 V in MeCN).
Fig. 6 Solid-state structures and colors of the two polymorphs A (left) and B (right)
of the radical cation in 18rad
. Ellipsoids are shown at the 50% probability level;
hydrogen atoms and triflate anions are omitted for clarity. Selected bond lengths
[Å] and dihedral angle [°] for A: C1C2 1.446(2), C1N1 1.386(2), C2N2 1.359(2);
N1C1C2N2 117.2(2)°. Selected bond lengths [Å] and dihedral angle [°] for B:
C1C2 1.449(3), C1N1 1.396(2), C2N2 1.357(2); N1C1C2N2 50.3(3)°.
Comparison with the redox potentials of the CAAC–benzNHC
dimer 14 (E1,2 = −0.55 V; E2,3 = −0.89 V vs. Fc/Fc+ in THF)54 re-
veals furthermore a shift to positive potential upon substitution
of the methylamino group by sulfur.36 However, the separation
E of the two redox potentials, i.e. the stability of the
heterodimeric radical 25rad towards disproportionation, is less
affected by the heteroatom substitution (14: E = 0.34 V; 25:
E = 0.26 V). In light of the structural analogies between the
homo- (9, 13) and heterodimers (18, 25) derived from saNHC
and btNHC, we also measured the CV for the two homodimers
9 and 13 (Fig. 4).55 Indeed, 9 showed only a two-electron
oxidation wave in dimethylformamide (E1,2 = −1.26 V vs.
Fc/Fc+),56 while homodimer 13 featured two reversible waves
separated by E = 0.15 V in acetonitrile.57 Unfortunately, no CV
could be obtained for 9 in acetonitrile. However, a survey of
reported potentials of electron rich olefins (Table S1) gra-
tifyingly revealed that solvation effects on the separation of the
two redox waves, subject of research herein, typically amount
to less than 0.1 V.58
Compound 18rad features an EPR spectrum in THF at room tem-
perature with a g-value of 2.0019 and hyperfine coupling to
three non-equivalent nitrogen atoms (a1 = 12.2 MHz; a2 = 13.0
MHz; a3 = 14.1 MHz; Fig. 5). To further corroborate the struc-
ture of the radical cation 18rad, single crystals were obtained by
vapor diffusion of diethyl ether into a saturated solution in
tetrahydrofuran. Single crystals of two polymorphs A (red) and
B (orange) were obtained from the same batch (Fig. 6). These
two polymorphs showed similar structural parameters as had
been reported for 14rad 36
The C1C2 bonds [A: 1.446(2) Å; B:
.
1.449(3) Å] are significantly longer than olefinic double bonds
[1: 1.349(3) Å;59 14: 1.346(2) Å],36 but comparable in length to
the CC bond reported for 14rad [1.439(3) Å].36 Strikingly how-
ever, the dihedral angle between the two carbene moieties
differed considerably for the two polymorphs [N1-C1-C2-N2 for
A: 117.2(2)°; N1-C1-C2-N2 for B: 50.3(3)°]. We conclude that a
discussion of solid-state dihedral angles should be taken with
caution.
Radical Cation Generation: Oxidation of Heterodimer 18
Following the electrochemical studies, we aimed for the iso-
lation of the radical cation 18rad. Treatment of 18 with silver
triflate led indeed to the quantitative formation of deeply red
colored 18rad (Scheme 4).
Computations: What Makes a Cationic Radical Persistent?
Generally, “potential inversion” (i.e., a seemingly two-electron
redox process) is opposed to a “normal ordering” of well-be-
haved one-electron redox events.
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biscation2+
radical+
olefin
°1 / E1,2
°2 / E2,3
Scheme 5 Stepwise reduction to give the electron-rich olefin.
A two-electron transformation occurs in a two-stage redox
system, if the addition of the second electron proceeds with
greater ease than for the first (Scheme 5).60 Despite of continu-
ous improvement in the recent decades,61 the solution phase
thermodynamical data of chemical reactions comprising charge
transfer remain often challenging to predict accurately.62
Indeed, we did not obtain a satisfying correlation between
experiment and computations using computationally efficient
implicit solvation for the two-stage redox systems studied
herein (Fig. S28). Contrarily, conceptual DFT (density functional
theory),63 which relies on the energy of the frontier orbitals,
seems an attractive alternative. Indeed, conceptual DFT has
been applied to predict electrochemical potentials of one-stage
redox systems (Eq. 1).64
Fig. 7 The experimental halfwave potentials E1,2 of the homodimers are linearly
correlated with the calculated chemical potential 1 according to Eq. 2. E1,2 is
given vs. SCE.
Fig. 8 The experimental halfwave potentials E2,3 of the homodimers are linearly
correlated with the calculated chemical potential 2 according to Eq. 1. E2,3 is
given vs. SCE.
 = –  21 (IP + EA) ≈ 12 ( HOMO +  LUMO
)
(Eq. 1)
–
The difference of equations 1 and 2 (Eq. 3 – 5) should hence
describe the separation of the two redox waves E1,2 and E2,3 (E)
in experimental CVs.
There, the chemical potential  of a compound corresponds to
the Mulliken electronegativity .65 The mean of the vertical io-
nization potential (IP) and electron affinity (EA), which defines
the Mulliken electronegativity, can be expressed by the frontier
orbital energies.66, 67, 68 The ionization potential is related with
the eigenvalue, i.e. the energy of the highest occupied mo-
lecular orbital (HOMO)  HOMO. Correspondingly, the electron
affinity is related to the energy of the singly occupied molecular
orbital (SOMO) after addition of one electron (i.e., the vertical
electron affinity). The latter value can be sometimes approxi-
mated by the energy of the lowest unoccupied molecular orbital
(LUMO) or the energy of the lowest unoccupied -orbital in case
of a radical, respectively. Pearson suggested that Eq. 1 should
be replaced by Eq. 2 for an “oxidized” compound without signifi-
cant ionization potential.69 The value of the chemical potential
is then equivalent to the electron affinity (Eq. 2).
E ∝ – = 2 – 1
E ∝ [–EA]biscation
12 [(IP+ EA)]radical
E ∝ [ LUMO biscation
12 [( HOMO +  LUMO)]radical
(Eq. 3)
(Eq. 4)
(Eq. 5)
+
]
–
The chemical potential  (unit: electronvolt eV) is the mo-
lecular equivalent to the molar Gibbs Free Energie G and both
values are interconvertible using Avogadros’s constant and the
elementary charge of an electron (i.e., Faraday’s constant F).
Accordingly, it is directly connected with the electrochemical
potential E (unit: Volt V) as well as the equilibrium constant K
of a reaction via Nernst’s equation (Eq. 6; RT/zF, Nernst Factor;
R, ideal gas constant; T, temperature; z, number of electrons; F,
Faraday constant). A negative sign for  hence indicates an
exergonic reaction, i.e. disproportionation of the cationic
radical to the olefin and biscation.
 = – EA ≈  LUMO
(Eq. 2)
In order to model the electrochemical properties of two-stage
redox systems, we propose the following method: Recon-
sidering the redox processes depicted in Scheme 5, the
chemical potential °1 for the biscations, which are very difficult
to be further oxidized, should be solely related with their
electron affinity according to Eq. 2. Differently, the chemical
potential °2 for the reduction of the cationic radicals should be
approximated by Eq. 1. Importantly, the cationic radicals will
only be resistant towards disproportionation, if the reductive
formation of the radical cation FROM the biscation (°1) is more
favorable than further reduction TO the olefin (°2). Conse-
quently, a persistent radical cation is formed (   if the
first reduction to give the radical cation (1) is more facile than
the second reduction to the olefin (2).
RT
zF
G
zF

lnK
E = E° +
= –
= – 
(Eq. 6)
We performed the quantum chemical calculations on the
B3LYP-D3BJ/def2-TZVPP//B3LYP-D3BJ/def2-SVP level of theory.
Thereby, we were aiming at a straightforward computational
protocol of general applicability. As discussed above, solvation
has sometimes an influence on the redox potentials.56
A
literature survey reveals nevertheless that it affects the
electronic coupling of electron rich olefins by less than 0.1 V (cf.
Table S1). Furthermore note that most electrochemical data
have been reported in various solvents due to solubility,
stability and resolution challenges.
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Fig. 10 SOMOs of 18rad (left) and 25rad (right).
Fig. 9 The thermodynamic stability towards disproportionation of the cationic
radicals (E) is linearly correlated with the difference of the calculated chemical
potentials in the gas phase ( = 2 – 1). Heterodimers are given in blue
circles, homodimers are given in red squares and diamonds. Heterodimers and
homodimers given in red diamonds are not included in the linear regression.
Indeed, the first oxidation step E1,2 for the homodimers was well
(R2 = 0.94) reproduced by calculating the chemical potential 1
using equation 2 (Fig. 7). A worse fit was obtained when appro-
ximating the electron affinity with the respective LUMO ener-
gies (Fig. S19; R2 = 0.88). Accordingly, no satisfying correlation
was obtained with equation 1 (Fig. S20; R2 = 0.60). The second
redox step E2,3 was well (R2 = 0.93) reproduced with equation 1
(Fig. 8), where the energies of the -HOMO and -LUMO were
used for the ionization potential and the electron affinity, res-
pectively.70 Modelling E of the two halfwaves showed an
equally good agreement with the experimental data (Fig. 9;
R2 = 0.94). Also here, reasonable results were only (Figs. S22,
S23, S24) obtained when using Eq. 2 for 1 and Eq. 1 for 2.
Impressively, the accuracy (mean absolute deviation MAD) of
the computational predictions is within 0.06 V deviation from
the experiment (0.07 V if including the further examples dis-
cussed below; Table S2). The calculations predict a large sepa-
ration for TTF 1 (E = 0.4 V;   0.36 eV) and saTTF 2
(E = 0.5 V;   0.45 eV) and two-electron processes for the
NHC- (8), saNHC- (9) and benzNHC (10) homodimers (E = 0 V;
 0). The same trend is obtained for the heterodimers 14,
18, 22, 25 (Fig. S26).
Fig. 11 CAAC–carbene heterodimeric radical cations, where the spin density is
mainly located on the CAAC moiety, form isolable radical cations due to the
aromaticity of the carbene heterocycles as modeled by NICS0zz. Correction for -
effects affords a NICS0zz value of +1.0 ppm for 22.72c
Truly, the NICS0zz values for the carbene moieties connected to
the CAACs in the biscations 14biscat, 18biscat, 25biscat as well as
non-aromatic 22biscat predict that increasing aromaticity (i.e. a
more negative NICS0zz value) increases the stability towards dis-
proportionation (Fig. 11). The same order is found using experi-
mentally determined aromaticity descriptors (Fig. S29)73-76 as
well as NICS scans dissecting -contributions (Figs. S30–S32).72c
The aromaticity effect can be explained by the loss of aromatic
stabilization upon reduction to the olefins. Apparently, for
cationic radicals where the spin density is mainly located on the
CAAC moiety, a large aromatization energy associated with the
other heterocycle renders the second reduction 2 un-
favorable and hence prevents overall the disproportionation to
the olefin and biscation. For the homodimers,77 the aromaticity
seems to be almost irrelevant to the stability of the radical
cations as evidenced by the large separation E obtained for
both aromatic 1 (E = 0.39 V)4 as well as aliphatic 2 (E =
0.45 V).78 Accordingly, the aromatization energy, which origina-
tes in the oxidation of the olefin to its biscation, does not seem
to have an influence on the (in)stability of the homodimeric ra-
dical cations towards disproportionation.
Aromaticity
Nevertheless, E was both experimentally and computationally
slightly larger for the hetero- than for the homodimers. We
attribute this to a systematic influence of the CAAC. We found
previously that the spin density in 14rad is principally located on
the CAAC.36 The same is true for 18rad and 25rad (Fig. 10). This
suggests that the aromaticity of the NHC moieties is more re-
duced in the second reduction step from the radical cations to
the olefins (2) than in the first step from the biscations to the
cationic radicals (1). Consequently, we had a look at the bis-
cations’ nucleus-independent chemical shift (NICS) of the NHC
derived carbene rings.71 NICS is a convenient and straight-
forward method to approximately assess the aromaticity of
planar  rings. The NICS0zz value relates to the calculated out-
of-plane (“zz”) part of the isotropic negative chemical NMR shift
of a dummy hydrogen atom in the center of an aromatic ring.
This method seems well suitable for the molecules studied
herein due to the two diastereotopic faces of the aromatic
rings.72 The more negative the chemical shift of the dummy
atom, the “more aromatic” is the carbene entity.
Steric- and Heteroatom Effects
When comparing TTF (1) with its saturated (2) or benzannula-
ted (3) congener, only a marginal difference was observed in the
predicted chemical potential  and the experimentally deter-
mined separation of the redox waves (E = 0.36, 0.45, 0.38 V;
Fig. 12, left). The dimers 8, 9 and 10 are not stable towards dis-
proportionation and the calculated redox potentials  are
consequently all negative (Fig. 12, middle). The thiazolin-2-
ylidene derivatives 11, 12, and 13 are borderline cases (Fig. 12,
right).
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Journal Name
View Article Online
DOI: 10.1039/D0SC00699H
N
N
N
N
S
S
S
S
N
S
S
N
° [eV]
E [V]
° [eV]
E [V]
° [eV]
E [V]
1: 0.36
2: 0.45
3: 0.38
0.39
0.45
0.33
8: <0
9: <0
0
0
0
11: 0.03
12: 0.02
13: 0.06
0.13
a
/
10:
<0
0.14
Fig. 12 Extending the aromatic system does only have a very small influence on
the stability of the homodimeric radical cations towards disproportionation
(calculated: ; experimentally determined: E). [a] To the best of our know-
ledge, 12 has not been reported in the literature.
Fig. 14 Optimized structural parameters of radicals 9rad (left) and Ph9rad (right).
Side view illustrates the dihedral angle N-C-C-N’.
At first sight contradicting this analysis, the replacement of the
methyl substituents in 9 by phenyl groups leads to the isolable
purple radical cation Ph9rad upon one electron oxidation as evi-
denced by a signal in the EPR spectrum.79, 80 In lack of detailed
electrochemical data, we resynthesized Wanzlick’s Dimer Ph9
and measured the CV (Figs. S11, S12).81 Notably, the CV of Ph
9
showed two overlapping, quasi-reversible redox waves with
E1,2 = −0.63 V and E3,4 = −0.73 V vs. Fc/Fc+ (E = 0.10 V).82 This
is in reasonable agreement with the previously reported equili-
brium constant of K = 25 with K = [radical-cation]2/([olefin] [bis-
cation]).80 Intriguingly, the computationally predicted differ-
ence of the chemical potentials for Ph9 of  = –0.09 eV is very
different to 9 ( = –0.96 eV) and borderline in respect to
forming a persistent radical cation (cf. Fig. 8). Examining the
optimized structural parameters of the cationic radicals 9rad and
Ph9rad (Fig. 14) reveals a considerably larger dihedral angle for
9rad (34°) in reference to Ph9rad (15°). This indicates counter-in-
tuitively that the “bulky” phenyl groups are better accommod-
ated in the planar- than in the twisted conformation due to a
coplanar orientation. Accordingly, the planar conformation of
Ph9rad is stabilized and hence prevents the disproportionation of
the cationic radical. We conclude that Ph9 is a challenging
borderline case, where the computational analysis predicts and
rationalizes the surprising resistance of the cationic radical to
disproportionate very well.
Inspired by the unexpected effect of the phenyl substituents on
stability of the radical cation Ph9rad towards dissociation, we in-
vestigated tethers in the 2,2’ positions of the homodimers (Fig.
15). We find that tethers33, 83 increase the stability towards dis-
proportionation. For example, whereas the disproportionation
of the bisNHC radical 8rad is predicted to be exergonic ( <
0 eV), Pr-teth8rad ( = 0.27 eV) and Et-teth8rad should be
persistent radicals ( = 0.39 eV). Indeed, this is also what had
been found experimentally (E = 0, 0.19, 0.36 V). The same
trend was predicted for 10rad as well as the dimethylamino-
pyridine derivatives 6rad with only one tether. Also here, the cal-
culated potentials are in excellent agreement with the experi-
mental data (e.g., Et-teth6rad:  = 0.18 eV; E = 0.23 V).7 15 In-
triguingly, the resistance of the radicals to disproportionate is
also here controlled by the twist of the cationic olefin radical.
Untethered and hence unperturbed 10rad shows for instance a
twist of 44°, whereas the comparable long propylene linker
allows for 11° and the shorter ethylene linker for only 4°. We
conclude that tethers enforce a more planar, rigid structure
which increases the resistance of the radical cations towards
disproportionation due to enhanced resonance stabilization.
Fig. 13 Optimized structural parameters of radicals 1rad, 2rad, 8rad, 10rad, and 13rad
Side view illustrates the dihedral angle S-C-C-S’ or N-C-C-N’, respectively.
.
Although the two separate potentials E1,2, E2,3 (cf. Figs. 6, 7) vary
considerably, the stability towards disproportionation of the ra-
dical cations is also here not considerably affected. However, it
is intriguing to note that all compounds with a large E value (1,
2, 3, 4, 7) do not feature alkyl substituents in the 2,2’ position
of the parent carbenes. The compounds with one substituent
(5, 11, 12, 13) show moderate E values, while the separation
between the two halfwaves is very small for the 2,2’ disubsti-
tuted heterocycles (Fig. S27). For example, 1rad (0°) and 2rad
(12°) with sulfur atoms in the 2,2’ positions form persistent
radical cations (1rad: E = 0.39 V; 2rad: E = 0.45 V).4, 78 Likewise,
13rad with a calculated dihedral angle of 13° forms a stable
radical cation (E = 0.15 V, vide supra). Contrarily, the diamino-
carbene derived radical cations 8rad and 10rad with significantly
larger dihedral angles of 42° and 44° disproportionate (Fig. 13).
This observation can be qualitatively understood by enhanced
stabilization of a delocalized radical cation, but also quan-
titatively using Eq. 5. An enhanced twist of the radical cations
leads in all cases to an increase of the ionization potential in
reference to the electron affinity of the biscations. Accordingly,
 becomes negative and the radical cations disproportionate
(E = 0 V. Contrarily, for compounds without strong structural
rearrangement, the ionization potential of the radical cations is
larger than the electron affinity of the biscations. This allows
for an overall positive  and hence a stable radical cation
(E > 0 V Following the computational predictions, we find
that the steric bulk in the 2,2’ position favors a twisting of the
cationic radical olefins. Therefore, they disproportionate to the
parent olefins and biscations.
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(alkyl)(amino) carbene (CAAC) in combination with saturated-
View Article Online
(saNHC) and unsaturated N-heterocycliDcOcI:a1r0b.1e0n3e9/sD0(NSCH0C0s6)99aHs
well as benzothiazolin-2-ylidene (btNHC). In order to pinpoint
the differences between heterodimers and their homodimeric
congeners, we also investigated the electrochemical properties
of three previously reported homodimers. Importantly, the he-
terodimer derived from the saturated NHC does not form a per-
sistent radical cation in opposition to the other heterodimers.
We then proposed a computational method based on con-
ceptual density functional theory to understand and predict the
stability of olefin radical cations towards disproportionation.
This method predicts the experimentally determined values
with an accuracy of 0.07 V. We demonstrated that the persis-
tence of CAAC-derived heterodimeric radical cations is due to
aromaticity, which we suggest exploiting further in the future.
Contrarily, the persistence of homodimer-derived cationic radi-
cals is controlled almost exclusively by the steric bulk in the 2,2’
positions of the parent carbenes, which we suggest deliberate-
ly harnessing as well. Neither the introduction of electron-
attracting nor electron-donating groups leads to considerably
increased stability towards disproportionation.
Tethers
however have a huge beneficial effect, because they prevent a
tilt of the carbene moieties, thus enforcing enhanced resonance
stabilization of the olefin radical cation. The positive effect of
sulfur atoms in the 2,2’ position as present in TTF consequently
is not electronic in nature. Instead, substituents in the 2,2’ po-
sition sterically destabilize the cationic olefin radical through
enforcement of
a twisted conformation with reduced
resonance stabilization.
Experimental Section
All reactions were carried out under an atmosphere of dry di-
nitrogen, either in an MBraun dinitrogen filled glovebox or using
standard Schlenk techniques. 1H and 13C NMR spectra were
recorded on JEOL ECX 270, JEOL ECX 400 or Bruker Avance III HD
600 MHz instruments operating at 269.71 MHz, 399.79 MHz
and 600.13 MHz for 1H and at 67.82 MHz, 100.62 MHz and
150.90 MHz for 13C, respectively and at a probe temperature of
25 °C. The solvent residual signals were used as an internal
reference for the 1H NMR and 13C NMR spectra. 1H NMR
multiplicities are abbreviated as follows: s = singlet,
d = doublet, dd = double of doublets, t = triplet, q = quartet,
spt = septet, m = multiplet. All coupling constants J are given in
Hz. Solvents were purified using a two-column solid-state
purification system (Glass Contour System, Irvine, CA).
Hexanes, toluene and benzene were stored over a mirror of
potassium; all other solvents were stored over activated
molecular sieves. Deuterated NMR solvents were obtained dry
and packaged under argon and stored over activated molecular
sieves or a mirror of potassium (C6D6). Melting points were de-
termined using a Bibby Scientific SMP10 melting point
apparatus. Elemental analyses were obtained using Euro EA
3000 (Euro Vector) and EA 1108 (Carlo-Erba) elemental
analyzers. The solution EPR spectrum were recorded on a JEOL
continuous wave spectrometer JESFA200 equipped with an X-
band Gunn diode oscillator bridge, a cylindrical mode cavity,
Et-
Fig. 15 Optimized structural parameters of radicals 6rad
teth8rad Pr-teth8rad, 10rad Et-teth10rad and Pr-teth10rad as well as calculated and experi-
mental CV parameters. Side view illustrates the dihedral angle N-C-C-N’. Two con-
, , ,
Et-teth6rad Pr-teth6rad, 8rad
,
,
Pr-
formers of comparable energy (G < 1 kcal mol1) were calculated for Et-teth8rad
,
teth8rad Et-teth10rad, and Pr-teth10rad; given values relate to the average. [a] To the best
,
of our knowledge, Et-teth8 has not been reported. Instead, the potential for mixed
ethylene/propylene bridged Et,Pr-teth8 is given.84 [b] Hünig and coworkers question
84, 85
the position of E2,3
.
Conclusion
Although key for the design of organic electronics, it has been
unclear for decades why some electron-rich olefins such as te-
trathiafulvalene (TTF) or bispyridinylidenes form persistent and
isolable radical cations, whereas others such as the ene-
tetramine derived from dimerized N,N’-dimethylimidazolidin-2-
ylidenes (saNHC; “Wanzlick’s dimer”) do not. Herein, we
showed by a combined experimental and computational inves-
tigation that sterics and aromaticity determine the stability of
cationic olefin radicals towards disproportionation into the
parent olefins and biscations. More specifically, we reported
three electron-rich carbene heterodimers derived from a cyclic
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Journal Name
and a helium cryostat. The spectrum was obtained on a freshly ethylenediamine was prepared as reported,106 but purified by
View Article Online
prepared solution of 5 mM compound 18rad in THF and simula- column chromatography (EtOAc:hexanDeO/1I::150,.10R3f9=/D00.8SC).006T99hHe
ted using the eviewX and esimX programs written by E. Bill cyclization and subsequent deprotonation was performed
(Max-Planck-Institut für Kohleforschung, Mülheim an der Ruhr, according to the literature procedure for aryl-substituted satu-
Germany). The EPR spectrum was measured under following rated NHC olefins.107 The analytical data were consistent with
conditions: microwave frequency  = 8.953 GHz, modulation the values in the literature.26
width 1.0 mT, microwave power = 0.1 mW, modulation btNHC=btNHC olefin 13 was synthesized according to a
frequency = 100 kHz, time constant = 0.1 s. Electrochemical modified literature procedure108 by deprotonation of the
measurements were carried out at room temperature under benzothiazolium salt 23 using Hünig’s base in MeCN. The
inert atmosphere with an Autolab Type-III potentiostat. They ammonium salt was removed by reaction with tripotassium
were recorded in 0.1 M nBu4NPF6 solution in THF using a glassy phosphate in Et2O.
carbon working electrode and platinum electrodes as counter 3-Dimethylimidazolin-2-ylidene 16. This compound was syn-
and reference electrodes. Ferrocene was added as internal thesized according to a modified literature procedure.109 1,3-
standard and all measurements were referenced as described Dimethyl-1H-imidazol-3-ium iodide (2.00 g, 8.93 mmol, 1.0 eq.)
in the literature. Electrochemical potentials referenced vs. the and KHMDS (1.75 g, 8.75 mmol, 1.0 eq.) were combined in a
Fc/Fc+ redox couple were converted to potentials references vs. Schlenk flask and suspended in Et2O (15 mL). The mixture was
SCE as suggested by Connelly and Geiger (Fc/Fc+: 0.40 V vs. SCE stirred for 20 min at room temperature. The supernatant
in MeCN; Fc/Fc+: 0.45 V vs. SCE in DMF; Fc/Fc+: 0.56 V vs. SCE in solution was separated from the precipitated potassium salt via
THF).86 Electrochemical potentials referenced vs. Ag/AgNO3 a filter cannula. Volatiles were removed in vacuo to afford the
(0.01 M) and Ag/AgCl, KCl (saturated) were converted to poten- carbene 16 as a pale yellow oil, which was stored at −40 °C. The
tials referenced vs. SCE (Ag/AgNO3 (0.01 M): 0.30 V vs. SCE; low yield of only 45% (390 mg) is due to the volatility of 16
Ag/AgCl, KCl (saturated): 0.045 V vs. SCE).87, 88 1,3-Dimethyl- under reduced pressure. The analytical data were consistent
1H-imidazol-3-ium iodide, 3-methylbenzo[d]-thiazol-3-ium with the values in the literature.109
iodide 23, the cyclic iminium salt 15 (1-(2,6-diisopropyl-phenyl)- CAAC–NHC HBF4 salt 17. The salt 15 (1.50 g, 4.0 mmol) and 16
2,2,4,4-tetramethyl-3,4-dihydro-2H-pyrrol-1-ium tetrafluoro- (409 mg, 4.22 mmol, 1.05 eq.) were combined in a Schlenk flask
borate), and free CAAC 19 were synthesized according to litera- and suspended in THF (20 mL). The mixture was stirred for 12 h
ture procedures.37a, 89-92 1,3-Dimethylimidazolinium hexafluo- at room temperature. The supernatant solution was removed
rophosphate 20 was synthesized as described in the literature93 via a filter cannula and the residue was washed with THF
but using NH4PF6 instead of NH4BF4. All other reagents were ob- (30 mL) until the powder was colorless. The solid was dried in
tained from commercial sources and used as is without further vacuo to afford 17 in quantitative yield (1.88 g). M.p.: 201 °C.
purification. All DFT calculations were performed using ORCA 1H NMR (600 MHz, CD3CN)  = 7.38 (s, 1H, NCH imidazole), 7.28
4.0.1.94, 95 The geometry optimizations were performed at the (dd, J1 = 7.7 Hz, J2 = 2.0 Hz, 1H, m-ArCH Dipp), 7.25 (t, J = 7.7 Hz,
B3LYP-D3BJ/def2-SVP level of theory96-99 and applying 1H, p-ArCH Dipp), 7.22 (d, J = 2.0 Hz, 1H, NCH imidazole), 7.12
Grimme’s D3 dispersion correction100 with Becke-Johnson dam- (dd, J1 = 7.3 Hz, J2 = 2.0 Hz, 1H, m-ArCH Dipp), 5.02 (s, 1H, NCH
ping.101, 102 The RIJCOSX103 approximation in combination with pyrrolidine), 4.26 (s, 3H, NCH3), 3.55 (s, 3H, NCH3), 3.45 (spt,
the def2/J auxiliary basis set was used in order to save J = 6.8 Hz, 2H, 2x CH(CH3)2 iPr), 2.43 (d, J = 13.9 Hz, 1H, CH2
computation time. Tighter than default convergence criteria pyrrolidine), 2.29 (d, J = 13.9 Hz, 1H, CH2 pyrrolidine), 1.72 (s,
were chosen for both the optimization (tightopt) of the 3H, NCCH3), 1.37 (d, J = 6.8 Hz, 3H, CHCH3 iPr), 1.36 (s, 3H,
structural parameters as well as the scf iterations (tightscf) and NCCH3), 1.31 (d, J = 6.6 Hz, 3H, CHCH3 iPr), 1.25 (s, 3H,
more accurate than default grid values (Grid5, FinalGrid6) were NCHCCH3), 1.18 (d, J = 6.8 Hz, 3H, CHCH3 iPr), 0.96 (s, 3H,
used. All reported optimized structures were verified as true NCHCCH3), 0.48 (d, J = 6.8 Hz, 3H, CHCH3 iPr) ppm. 13C NMR
minima by the absence of negative eigenvalues in the harmonic (101 MHz, CD3CN)  = 152.2 (NCN), 151.0 (ArC Dipp), 148.2 (ArC
vibrational frequency analysis. The energies of all structures Dipp), 137.8 (ArC Dipp), 129.1 (ArCH Dipp), 127.2 (ArCH Dipp),
were corrected by single-point calculations on the B3LYP- 125.9 (ArCH Dipp), 73.4 (CCHN), 67.7 (NC(CH3)2), 55.2 (CH2
D3BJ/def2-TZVPP level of theory. For the calculation of °1, the pyrrolidine), 44.6 (NCC(CH3)2), 38.7 (NCH3), 37.8 (NCH3), 33.5
energies of the -HOMO and the -LUMO for the optimized (CH3 pyrrolidine), 31.7 (CH3 pyrrolidine), 30.2 (CH3 pyrrolidine),
structural parameters of the cationic radicals were used. For 28.9 (CHCH3 iPr), 28.4 (CHCH3 iPr), 27.1 (CH3 pyrrolidine), 25.9
i
i
i
i
the calculation of °2, the vertical electron affinity was used, i.e. (CHCH3 Pr), 25.8 (CHCH3 Pr), 25.6 (CHCH3 Pr), 24.3 (CHCH3 Pr)
the energy of the SOMO upon addition of one electron for the ppm. Elemental analysis for C25H40F4N3B · 0.6 THF: Calcd.: C
optimized structural parameters of the biscations.
64.19; H 8.81; N 8.20. Found: C 64.01; H 8.69; N 8.06%.
saNHC=saNHC olefin 9. This compound was synthesized accor- CAAC=NHC olefin 18. The salt 17 (1.50 g, 3.19 mmol) and
ding to a modified literature procedure. 10 was deprotonated KHMDS (624 mg, 3.13 mmol, 0.98 eq.) were combined in a
using an equimolar amount KHMDS in benzene. Filtration and Schlenk flask. Toluene (15 mL) was added and it was stirred for
removal of volatiles afforded 9 in quantitative yield. The 12 h at room temperature. The supernatant solution was
analytical data were consistent with the literature.104
separated from the potassium salt via a filter cannula. The
saNHC=saNHC olefin Ph9. This compound was synthesized solvent was removed in vacuo and the residue was washed with
according to a modified literature procedure.105 N,N′-Diphenyl- hexanes. The pale yellow solid was dried in vacuo to afford 18
8 | J. Name., 2012, 00, 1-3
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in 80% yield (970 mg). M.p.: 63 °C. 1H NMR (270 MHz, C6D6)  (CH(CH3)2 iPr) ppm. Elemental analysis for C25H41N3: Calcd.: C
View Article Online
DOI: 10.1039/D0SC00699H
= 7.13 – 7.07 (m, 3H, ArCH Dipp), 5.71 (d, J = 2.1 Hz, 1H, NCH 78.27; H 10.77; N 10.95. Found: C 78.35; H 10.85; N 10.70%.
imidazoline), 5.58 (d, J = 2.1 Hz, 1H, NCH imidazoline), 3.55 (spt, CAAC=btNHC olefin 25. The salt 15 (534 mg, 1.43 mmol,
J = 6.7 Hz, 2H, 2x CH(CH3)2 iPr), 2.52 (s, 3H, NCH3), 1.84 (s, 2H, 1.0 eq.) and 3-methylbenzo[d]thiazol-3-ium iodide (23)
CH2 pyrrolidine), 1.69 (s, 6H, NC(CH3)2), 1.39 (s, 3H, NCH3), 1.37 (594 mg, 2.15 mmol, 1.5 eq.) were suspended in MeCN. Diiso-
i
(d, J = 6.7 Hz, 6H, CH(CH3)2 Pr), 1.28 (d, J = 6.8 Hz, 6H, CH(CH3)2 propylethylamine (278 mg, 2.15 mmol, 1.5 eq.) was added
iPr), 1.13 (s, 6H, NCC(CH3)2) ppm. 13C NMR (68 MHz, C6D6)  = dropwise over the course of 20 min at room temperature. The
150.1 (NCN), 143.2 (ArC Dipp), 142.7 (ArC Dipp), 129.7 (ArCH red suspension was stirred for 8 h and the volatiles were
Dipp), 129.4 (ArCH Dipp), 127.4 (ArC Dipp), 126.8 (CCN), 124.9 removed in vacuo. Dry K3PO4 (3.04 g, 14.3 mmol, 10 eq.) and
(ArCH Dipp), 63.0 (NC(CH3)2), 58.4 (CH2 pyrrolidine), 48.1 (NCH Et2O (20 mL) were added. The mixture was stirred for 24 h and
imidazoline), 44.0 (NCH imidazoline), 42.5 (NCC(CH3)2), 31.1 filtered. The yellow solid was washed with Et2O (3 x 5 mL) and
(NCH3), 29.9 (NCH3), 28.9 (CHCH3 iPr), 25.7 (CHCH3 iPr), 24.7 extracted with MeCN (4 x 5 mL). The solvent was evaporated.
(CHCH3 iPr) ppm (three CH/CH3 signals are superimposed). The yellow solid was suspended in benzene (10 mL), stirred for
Elemental analysis for C25H39N3: Calcd.: C 78.69; H 10.30; N 10 min, filtered and dried (3 x). A solution of KHMDS (285 mg,
11.01. Found: C 78.16; H 10.35; N 10.61%.
1.43 mmol, 1.0 eq.) in benzene (2 mL) was then added to a
CAAC–NHC radical 18rad
.
Silver trifluoromethanesulfonate suspension of the yellow solid in benzene (5 mL) at room
(AgOTf) (134 mg, 0.52 mmol, 1.0 eq.) and 18 (200 mg, temperature. The mixture was stirred for 1 h and the volatiles
0.52 mmol, 1.0 eq.) were suspended in THF (5 mL). The mixture were removed in vacuo. The heterodimer was extracted with
instantly turned deep maroon. The mixture was filtered and the hexanes (3 x 3 mL). The solution was concentrated to 2 mL,
solvent was removed in vacuo to afford 18rad as a red solid in filtered, and cooled to 35 °C upon which crystals formed. The
quantitative yield (276 mg). Single crystals suitable for X-ray mother liquor was decanted after 16 h and the yellow crystals
analysis were obtained by slow diffusion of Et2O into a saturated were washed with cold hexanes (2 x 1 mL). Drying in vacuo
THF solution at room temperature.
afforded 25 in 11% yield (67 mg). M.p.: 190-191 °C. 1H NMR
CAAC–NHC PF6 salt 21. The carbene 19 (395 mg, 1.38 mmol, (400 MHz, C6D6)  = 7.25 (dd, J1 = 8.3 Hz, J2 = 6.9 Hz, 1H, p-ArCH
1.2 eq.) and 1,3-dimethylimidazolinium hexafluorophosphate Dipp), 7.16 – 7.14 (m, 2H, m-ArCH Dipp, superimposed by
(20) (281 mg, 1.15 mmol, 1.0 eq.) were suspended in benzene solvent), 6.88 (dd, J1 = 9.3 Hz, J2 = 7.7 Hz, 2H, ArCH
(10 mL). The mixture was stirred for 16 h at room temperature, benzothiazoline), 6.79 (dd, J1 = 8.8 Hz, J2 = 7.4 Hz, 1H, ArCH
filtered, the residue washed with benzene (2 x 2 mL) and dried benzothiazoline), 6.66 (dd, J1 = 8.8 Hz, J2 = 7.4 Hz, 1H, ArCH
i
in vacuo to give 390 mg of the crude heterodimer. This benzothiazoline), 3.40 (spt, J = 6.9 Hz, 2H, 2x CH(CH3)2 Pr), 2.73
compound was used as is for the next step without further (s, 3H, NCH3), 1.78 (s, 2H, CH2 pyrrolidine), 1.64 (s, 6H,
purification. H NMR (270 MHz, CD3CN):  = 7.28 – 7.20 (m, 3H, NC(CH3)2), 1.47 (d, J = 6.9 Hz, 6H, , CH(CH3)2 iPr), 1.26 (d,
1
i
ArCH Dipp), 4.77 (s, 1H, NCH pyrrolidine), 3.81 – 3.58 (m, 5H, J = 6.9 Hz, 6H, , CH(CH3)2 Pr), 1.13 (s, 6H, NCC(CH3)2) ppm. 13C
CH(CH3)2 iPr and H2CCH2 imidazoline overlapping), 3.31 (spt, NMR (101 MHz, C6D6)  = 152.5 (NCS), 150.3 (ArCS), 148.6 (ArCN
J = 6.5 Hz, 1H, CH(CH3)2 iPr), 2.98 (s, 3H, NCH3), 2.33 (d, benzothiazoline), 138.2 (ArC Dipp), 137.3 (ArC Dipp), 128.4
J = 13.9 Hz, 1H, CH2 pyrrolidine), 2.23 (d, J = 13.9 Hz, 1H, CH2 (ArCH, superimposed by solvent), 125.1 (ArCH), 124.8 (ArCH),
pyrrolidine), 1.68 (s, 3H, NCCH3), 1.32 (d, J = 6.5 Hz, 6H, 124.7 (ArCH), 121.2 (ArCH), 117.4 (ArCH), 114.5 (CCN), 64.1
i
CH(CH3)2 Pr), 1.31 (s, 3H, NCCH3), 1.22 (d, J = 6.8 Hz, 3H, CHCH3 (NC(CH3)2), 58.1 (CH2 pyrrolidine), 49.1 (NCH3), 43.8
iPr), 1.17 (s, 3H, NCHCCH3), 1.16 (s, 3H, NCHCCH3), 1.04 (d, (NCC(CH3)2), 31.3 (CH3 pyrrolidine), 30.0 (CH3 pyrrolidine), 29.0
i
J = 6.8 Hz, 3H, CHCH3 Pr) ppm.
(CH(CH3)2 iPr), 26.0 (CH(CH3)2 iPr), 25.2 (CH(CH3)2 iPr) ppm.
CAAC=saNHC olefin 22. The salt 21 was suspended in benzene Elemental analysis for C28H38N2S · 0.05 KI: Calcd.: C 76.26; H
(2 mL). A solution of KHMDS (132 mg, 0.663 mmol, 0.9 eq. re- 8.68; N 6.35, S 7.26. Found: C 76.22; H 8.37; N 6.25, S 6.84%.
ferring to the heterodimer salt) was added under stirring at
room temperature and solids were then filtered off. Removal
of volatiles afforded 22 in overall 54% yield (238 mg). M.p.
95 °C. 1H NMR (400 MHz, C6D6)  = 7.18 – 7.15 (m, 1H, m-ArCH
Acknowledgements
D.M. and M.M.H. thank the Fonds der Chemischen Industrie for
Dipp, superimposed by solvent), 7.12-7.11 (m, 1H, p-ArCH
Dipp), 7.10 (d, J = 1.7 Hz, 1H, m-ArCH Dipp), 3.54 (spt, J = 6.8 Hz,
2H, CH(CH3)2 iPr), 2.69 – 2.60 (m, 4H, H2CCH2 imidazolidine),
2.58 (s, 3H, NCH3), 1.89 (s, 2H, CH2 pyrrolidine), 1.73 (s, 6H,
NC(CH3)2), 1.63 (s, 3H, NCH3), 1.44 (d, J = 6.7 Hz, 6H, CH(CH3)2
Liebig fellowships. Financial Support by the Boehringer-Ingel-
heim Fonds and the German-American Fulbright commission
are gratefully acknowledged. We thank the RRZE Erlangen for
computational resources. Continuous support by K. Meyer and
M. Alcarazo is gratefully acknowledged.
i
iPr), 1.33 (d, J = 6.7 Hz, 6H, CH(CH3)2 Pr), 1.19 (s, 6H, NCC(CH3)2)
ppm. 13C NMR (151 MHz, C6D6)  = 150.4 (NCN), 142.5 (ArC
Dipp), 141.3 (ArC Dipp), 131.4 (CCN), 127.3 (ArCH Dipp), 124.5
(ArCH Dipp), 62.5 (NC(CH3)2), 58.0 (CH2 pyrrolidine), 54.6 (CH2
imidazolidine), 53.8 (CH2 imidazolidine), 47.5 (NCH3), 45.6
(NCH3), 42.6 (NCC(CH3)2), 30.9 (CH3 pyrrolidine), 29.7 (CH3
pyrrolidine), 29.0 (CH(CH3)2 iPr), 25.8 (CH(CH3)2 iPr), 24.6
Keywords: Carbene • NHC • Radical • Electron Rich Olefin • DFT
• organic electronics
Notes and references
1
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Only sterics and aromaticity control whether radical olefin cations are stable.