Use of Charge-Charge Repulsion to Enhance π-Electron Delocalization into Anti-Aromatic and Aromatic Systems

Article abstract of DOI:10.1002/chem.201606036

A series of 9-fluorenyl cations has been studied and it is shown that increasing charge on a heterocyclic substituent group enhances the anti-aromatic character of the carbocation system. Similarly, a series of dibenzosuberenyl cations has been studied and increasing charge on a substituent group is shown to enhance aromatic character in the carbocation system. These studies include the direct observations of dicationic and tricationic species using stable-ion conditions and low temperature NMR. The structures of these ions were further characterized using DFT calculations, confirming that highly charged organic ions may exhibit unusual distributions of π-electrons and delocalization of electrons in 4n or 4n+2 π-systems.

Full text of DOI:10.1002/chem.201606036

A Journal of
Accepted Article
Title: Use of charge-charge repulsion to enhance π-electron
delocalization into anti-aromatic and aromatic systems
Authors: Akinari Sumita, Makafui Gasonoo, Kenneth J. Boblak,
Tomohiko Ohwada, and Douglas Klumpp
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To be cited as: Chem. Eur. J. 10.1002/chem.201606036
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Supported by

10.1002/chem.201606036
Chemistry - A European Journal
COMMUNICATION
Use of charge-charge repulsion to enhance π-electron
delocalization into anti-aromatic and aromatic systems
Akinari Sumita,[b] Makafui Gasonoo,[a] Kenneth J. Boblak,[a] Tomohiko Ohwada,*[b] and Douglas A
Klumpp*[a]
Dedicated to Professor George A. Olah on the occasion of his 90th birthday
Monocation
Dication
Abstract: A series of 9-fluorenyl cations has been studied and it is
shown that increasing charge on a heterocyclic substituent group
enhances the antiaromatic character of the carbocation system.
Similarly, a series of dibenzosuberenyl cations has been studied and
increasing charge on a substituent group is shown to enhance
aromatic character in the carbocation system. These studies include
the direct observations of dicationic and tricationic species using
stable ion conditions and low temperature NMR. The structures of
these ions were further characterized using DFT calculations,
confirming that highly-charged organic ions may exhibit unusual
distributions of π-electrons and delocalization of electrons in 4n or
4n+2 π-systems.
!, 7.25-7.65
!, 4.97-5.77
1
2
Scheme 1. 1H NMR data for fluorenyl cations 1 and 2.
through molecules and between molecules.5 The fluorenyl
cation (and the related dibenzosuberenyl cation) provide an
ideal opportunity to study structural effects that may influence
The use of organic materials for electronic applications
continues to be a very active field of research.1 Organic
materials offer distinct advantages over traditional inorganic
materials in electronic components. This includes the ease of
synthetic modifications, lower costs for syntheses and materials,
as well as being amenable for quality control and large-scale
processing. Despite the progress made in this area, there
continues to be the need for new methods of controlling the
electronic and magnetic states of organic materials. Several
recent studies of stable organic ions have demonstrated that
charge-charge repulsive effects may enhance π-electron
delocalization in conjugated π-systems, including both 4n or
4n+2 π-electron systems.2 For example, Mills and coworkers
examined the mono- and dicationic fluorenyl ions (1-2, Scheme
1).3 NMR studies showed an increasing degree of anti-aromatic
or paramagnetic ring current in the dication 2 compared to the
monocation 1, as the fluorenyl ring 1H are significantly shifted
charge mobility within ionized structures.
previously used protonated N-heterocycles in the generation of
highly-charged cationic systems.6
Besides enhancing the
Our group has
chemical reactivities within these superelectrophilic systems,
protonated N-heterocycles are also capable of inducing charge
migration in associated π-systems.7 Herein, we describe our
studies of isoelectronic and isosteric monocations, dications,
and trications of the fluorenyl and dibenzosuberenyl cations.
These cationic species are studied by low temperature NMR
using stable ion conditions with further analysis using
computational methods. The results are interpreted according to
charge-charge repulsive effects on both the 4n and 4n+2 π-
electron systems, leading to enhanced paramagnetic and
diamagnetic ring current with charge delocalization.
Results and Discussion
The phenyl-substituted fluorenyl cation (3a) and
dibenzosuberenyl cation (4a) were previously studied by Olah
and colleagues8,9 and these ions are used for comparison to the
more highly charged ions 3b-d and 4b-c generated in this study.
Two routes were considered to prepare the cations: the aryl
ketones (5a-d) should cyclize with dehydration to the fluorenyl
cation while direct ionization of fluorenols (6a-d) or the
dibenzosuberenols (7a-c) should form the respective cations.
upfield with the dication.
Calculated nucleus-independent
chemical shift (NICS) values are also consistent with the 4n π-
system exhibiting greater anti-aromatic character in the dication
2 compared to the monocation 1.4
In the design of organic electronics, a critically important
phenomenon involves the movement of holes, or cationic charge,
Ar
Ar
Ar
HO
O
[a]
[b]
Professor D. A. Klumpp. Mr. M. Gasonoo, and Mr. K. J. Boblak
Department of Chemistry
Northern Illinois University
DeKalb, Illinois, United States
E-mail: dklumpp@niu.edu
Professor T. Ohwada and Mr. A. Sumita
Graduate School of Pharmaceutical Sciences
The University of Tokyo
Tokyo, Japan
E-mail: ohwada@mol.f.u-tokyo.ac.jp
Ar
n
3
4
5
6, n=0
7, n=1
Ar =
c: 2-pyrazinyl
d: 2-pyrimidyl
a: phenyl
b: 2-pyridyl
In FSO3H-SbF5-SO2ClF solution, ionization of aryl ketone
5b and fluorenol 6b provides clean access to the fluorenyl
dication 3b (Table 1). Thus, ion 3b exhibits the expected twelve
13C NMR signals - with a carbocation resonance observed at δ
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201606036
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COMMUNICATION
Table 1. Selected 1H and 13C NMR spectral data for fluorenyl-
and dibenzosuberenyl cations (3a-d and 4a-c).a,b
carbocation 13C resonance is found at δ 183.7.9
With the
pyridyl-substituted dibenzosuberenol 7b, the dicationic 4b is
1
formed in superacid. The H NMR of the dication 4b shows a
224.2 (C+)
7.25, 7.25,
7.55, 7.65
modest downfield shift of ethenediyl signals compared to the
monocation 4a. The 13C NMR reveals a sizable upfield shift, as
the dication 4b possesses a carbocation signal at δ 159.8. The
isoelectronic trication 4c was generated by ionization of
181.9 (C+)
8.15
3a
pyrazinyl-substituted dibenzosuberenol (7c).
The 1H NMR
190.0 (C+)
4a
5.41, 5.49,
spectrum shows further downfield shift of the ethenediyl protons
(δ 8.75) and the 13C NMR shows a carbocation signal with
modest upfield shift compared to the dication 4b.
The increasing positive charge leads to significant changes
in the NMR spectra of both the fluorenyl cations and
dibenzosuberenyl cations. These changes may be understood
5.64, 6.08
HN
159.8 (C+)
HN
3b
8.62
177.3 (C+)
HN
4.86, 4.94,
5.00, 5.69
4b
as
a
consequence of charge-induced enhancement of
NH
152.2 (C+)
HN
paramagnetic and diamagnetic ring current in the respective
cationic systems. Our results are consistent with the findings of
Mills and coworkers and their earlier work with fluorenyl cations.3
That is, an increasing charge on the substituent group leads to a
greater degree of charge delocalization in the fluorenyl cation,
and consequently, increasing anti-aromatic character in the 12π-
electron system. In the case of the dibenzosuberenyl cations,
increasing charge on the substituent group leads to a similar
delocalization of π-electrons, but as a 4n+2 π-system, there is an
increasing aromatic character and diamagnetic ring current
observed in the NMR spectra.
3c
8.75
169.8 (C+)
HN
4.68, 4.80,
4.88, 5.60
NH
4c
HN
3d
aSpectral data obtained from FSO3H-SbF5-SO2ClF solution at
-40 °C (d6-acetone external standard). bData for 3a taken from
reference 6.
190 (see Supporting Information for a complete list of peaks).
This carbocation signal is shifted considerably from the fluorenyl
monocation (3a) reported by Olah, et al.,8 as the carbocation
center in 3a is observed at δ 224 from a FSO3H-SO2ClF solution.
Additionally, the fluorenyl 1H signals are shifted upfield in the
dication 3b (δ 5.41-6.08) compared to the monocation 3a (δ
7.25-7.65). Cyclization of substrate 5b is a convenient means of
generating the fluorenyl dication 3b, so this method of
generation was also used for ions 3c and 3d. Thus, ionization of
the pyrazine-based ketone (5c) leads to the expected fluorenyl
trication 3c. Trication 3c exhibits an even further upfield shift for
the carbocation 13C resonance (δ 177) and upfield shift for the
This chemistry was further studied by theoretical
calculations.11 (Anti-)aromaticity of π-systems is a multiphasic
issue. However, thermochemical evaluations were inappropriate
in our system, because thermodynamic (de)stabilization of the
system included both effects arising from (anti)aromatization and
increase/decrease of charge-charge repulsion. Therefore, we
focused on the evaluation of (anti-)aromaticity in terms of
magnetic and geometrical aspects. The nucleus-independent
chemical shifts (NICS) method has been used previously to
evaluate magnetic properties of the aromaticity and anti-
aromaticity of π-systems, including fluorenyl mono- and
dications.12
systems at the CPCM-B3PW91/6-311++G(2d,p) (solvent
NICS calculations were done on the present
1
fluorenyl H signals (δ 4.86-5.69) – compared to the mono- and
=
dicationic fluorenyl species (3a,b). Likewise, ionization of the 2-
pyrimidyl ketone 5d in superacid provides the fluorenyl trication
3d. This ion also exhibits strong shielding of the fluorenyl ring
protons and shielding of the carbocation 13C resonance. The 1H
signals of the fluorenyl ring are observed at δ 4.68-5.60, while
the carbocation 13C signal is observed at δ 169.8. In a recent
report about the unsubstituted 9-fluorenyl cation, Sander et al.
noted the difficulties associated with generating and observing
the unstable fluorenyl cation.10 Our studies show that acid-
promoted cyclizations of biaryl ketones (5) provide convenient
access to these cationic species.
CF3SO3H) level with the test atom calculated 1 Å above the
center of the 5, 6, and 7 carbon rings of the optimized fluorenyl
and dibenzosuberenyl cation structures. To reduce the influence
of σ-aromaticity, we used NICS(1)zz values, along with
NICS(1)iso values.13 In the case of the fluorenyl cations (3a-d),
the NICS(1)zz values become increasingly positive as the charge
on the heterocycle increases
– a result consistent with
increasing anti-aromatic character (Figure 1).
NH
HN
HN
HN
NH
The dibenzosuberenol substrates 7a-c were also ionized in
FSO3H-SbF5-SO2ClF and studied by low temperature NMR
(Table 1).
Alcohol 7a ionizes cleanly to provide the
3a
3b
3c
+25.3
3d
monocationic species 4a. In the 13C NMR spectrum, the
carbinol 13C resonance of 6a (δ 78.8) disappears and it is
replaced by a new downfield signal at δ 181.9 – a signal
attributed to the dibenzosuberenyl carbocation center. This
observation is in accord with the results from Olah and Liang,
where 4a was observed from a FSO3H solution and the
+13.9
+47.4
+12.5 +26.9
+40.5 +84.9
NICS(1)iso +0.43
+6.82 +21.1
+23.2 +68.6
+10.6
+34.6
+79.9
+3.91
NICS(1)zz
0.84
(0.073)
(0.088)
0.80
-
0.79
(0.088)
(0.120)
-
0.89
(0.059)
(0.053)
HOMA
(EN)
(GEO)
-
-
(0.084)
(0.113)
Figure 1. Calculated NICS values (6 and 5-carbon rings) and
HOMA (6-carbon rings) values for cations (3a-d).
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10.1002/chem.201606036
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COMMUNICATION
Besides generating anti-aromatic character, delocalization
of the π-electrons in ions 3a-d should lead to decreasing
aromaticity in the benzenoid rings. Aromatic character may be
evaluated using the calculated harmonic oscillator model of
aromaticity (HOMA) index – a computational model based on
the variance of ring-perimeter bond lengths (see Supporting
Information).14,15 HOMA index can be divided into two terms; a
bond weakening/strengthening term (EN) and a measured bond
alternation term (GEO). When the HOMA index values are
calculated for the series of ions 3a-d, a consistent increase in
the GEO values is seen in the benzenoid ring as the charge
fluorenyl ring carbons (i.e., C3 and C4) and hydrogen atoms
become increasingly positive (Table 2). The complete list of
NPA charges is found the Supporting Information. These data
are consistent with the previous results (NICS(1)zz and HOMA
studies) revealing greater delocalization of the cationic charge
throughout the fluorenyl ring with the more highly charged ions.
The increasing NPA charges on the fluorenyl ring hydrogen
atoms also suggest electronic effects involving both π- and σ-
electrons.
delocalized π-electrons, there is also an inductive electron effect
involving the C-H σ-bonds. However, because the anti-
While charge-charge repulsion clearly leads to
1
increases at the heterocycle
–
verifying large-scale
aromaticity is strong, H signals of dication (3b) or trication (3c,
3d) appeared upfielded (Table 1). It is also notable that the
summation of carbon and hydrogen NPA charge increases at
the fluorenyl ring - proceeding from 0.756 for 3a to 1.118 for 3d.
This sizable increase in positive charge may be interpreted as
evidence for two effects: donation of some electron density from
the phenyl group to the fluorenyl ring system in 3a and removed
of some electron density from the fluorenyl ring and towards the
highly charged heterocyclic substituents in 3b-d.
delocalization of the π-electrons of the fluorenyl cation.
The dibenzosuberenyl cations were also examined by
NICS calculations and the results are in agreement with the
trends observed from low temperature NMR studies (Figure 2).
Y
NH
HN
X
HN
4
6
5
7
3
6'
4'
2
8
The NPA charges were also calculated for the
dibenzosuberenyl cations (Table 2) and the results are
consistent with the previous data – increasing charge on the
substituent heterocyclic rings enhance delocalization of the
carbocation charge center. Thus, the calculated NPA charge on
C5 decreases from 0.299 to 0.163 to 0.101 through the series
4a to 4c. Likewise, the NPA charges increase on the atoms of
the central seven carbon ring (i.e., C-4’) and the fused
benzenoid rings (i.e., C1 and C3). These data reveal an
increasing delocalization of positive charge throughout the
dibenzosuberenyl cation as the charge increases on the
heterocyclic substituent. The results are in accord with the
experimental NMR data and the calculated NICS and HOMA
values.
1'
9'
9
1
10
11
4a
-7.40
4b
4c
HOMA (EN, GEO) [C15 Ring]
-11.7 -8.05
-31.9 -21.6
NICS(1)iso
NICS(1)zz
-11.8 -8.33
-32.1 -22.5
-11.4
-31.1
4a 0.72 (0.11, 0.17)
-19.7
4b 0.75 (0.10, 0.15)
0.63 0.41
(0.16) (0.40)
(0.21) (0.19)
0.59 0.48
(0.17) (0.34)
(0.24) (0.14)
0.60 0.46
(0.16) (0.35)
(0.23) (0.18)
HOMA
(EN)
(GEO)
0.77 (0.09, 0.14)
4c
Figure 2. Calculated NICS values and HOMA values for cations
(4a-c).
Thus, the NICS(1)zz values decrease through the series 4a -> 4b
-> 4c, an observation consistent with increasing aromatic
character. Above the benzenoid rings, the NICS(1)zz value (six-
carbon ring) does not change from 4a to 4c. On the other hand,
analysis of the NICS(1)zz properties above the seven-carbon ring
reveals a consistent decrease from -19.7 in the monocation (4a),
to -21.6 in the dication (4b), to -22.5 in the trication (4c). The
HOMA values were also calculated for the dibenzosuberenyl
cations 4a-c. Several notable observations may be made
regarding ions 4a-c. First, the HOMA values for the six-carbon
ring decreases in the series 4a -> 4b -> 4c. This is consistent
with greater delocalization of the carbocation charge as a result
of the charge-charge repulsive effects. In the same series, there
is a modest increase in HOMA values in the seven-carbon ring
and in the fifteen-carbon perimeter. This trend may also be
Table 2. CPCM-MP2/6-31G+(d) (CF3SO3H) calculated NPA
charges of selected fluorenyl ring and dibenzosuberenyl ring
carbon atoms and ring hydrogen atoms (hydrogen charges in
parentheses).a
3a
3b
3c
3d
Ar
9
C3 -0.112 -0.062 -0.040 -0.028
(C3-H) (0.264) (0.269) (0.273) (0.275)
8
1
8'
1'
7
2
C4'
C9
0.027
0.438
0.756
0.058
0.362
0.973
0.035 0.053
0.289 0.209
1.074 1.118
5' 4'
6
3
5
4
Summation of total NPA
charge (C,H) on fluorenyl
ring system
4a
4b
-0.225 -0.224
(0.262) (0.267) (0.271)
4c
understood to be
a consequence of increasing charge
C1
(C1-H)
-0.247
Ar
delocalization – less character of a triarylmethyl cation and more
character of a 4n+2 π-system. Interestingly, the changes in
NICS(1)zz and HOMA values are comparatively small in the
dibenzosuberenyl cations 4a-c, when compared to the changes
in NICS(1)zz and HOMA values for the fluorenyl cations 3a-d.
In order to further evaluate charge-charge repulsive effects
in these ions, natural population analysis (NPA) of atomic
charges was calculated (CPCM-MP2/6-31G+(d) level; solvent =
CF3SO3H). The results show a clear trend within the series of
fluorenyl ions 3a -> 3d: as the amount of positive charge
increases on the heterocyclic substituent, the NPA charge at the
carbocation center (C9) becomes more negative, while the other
6
4
5
7
3
6'
4'
C4'
0.029
0.031
-0.210 -0.184
0.040
8
2
C3
-0.239
1'
9'
9
1
(C3-H) (0.265) (0.271) (0.275)
10 11
C5
0.299
0.968
0.163
1.034
0.101
1.093
Summation of total NPA charge
(C,H) on dibenzosuberenyl
ring system
aComplete list of charges provided in Supporting Information
The above data present a consistent picture: the highly-charged
heterocyclic substituents enhance charge delocalization in the
fluorenyl cations (3) and dibenzosuberenyl cations (4). Thus,
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COMMUNICATION
charge-charge repulsive effects tend to favor the delocalized
resonance forms of trications 3c and 4c. This is seen in the
greater anti-aromaticity and aromaticity of the respective π-
systems.
external standard.
previously cooled NMR instrument/probe.
NMR analysis is then done using a
Acknowledgements
HN
HN
HN
Support from the U. S. National Science Foundation is gratefully
acknowledged (1300878). A.S. thanks Japan Society for the
Promotion of Science for Research Fellowship for Young
Scientists (16J08260). The computations were performed at the
Research Center for Computational Science, Okazaki, Aichi,
Japan. We thank the computational facility for generous
allotments of computer time.
NH
NH
NH
3c
HN
HN
HN
NH
NH
NH
4c
Conclusions
Keywords: aromatic • anti-aromatic • dication • trication •
In this study, we have shown that cationic 4n+2 or 4n π-
electron systems may show enhanced aromaticity or anti-
aromaticity through charge-charge repulsive effects. Evidence
for the charge-induced π-electron delocalization includes
observing by NMR greater paramagnetic ring current in the
fluorenyl cation system (4n π-system) and diamagnetic ring
current in the dibenzosuberenyl cation system (4n+2 π-system).
These results were further supported by theoretical calculations,
as NICS calculations confirmed increased anti-aromatic
character as charge increased adjacent to the fluorenyl cation.
Likewise, charge was correlated to increased aromatic character
in the dibenzosuberenyl cation series. Both the NICS and
HOMA calculations revealed an increasing aromatic character
as the charge increased adjacent to the dibenzosuberenyl cation.
These results have several implications. They confirm that
highly-charged organic ions possess extensive delocalization of
π-electrons. Moreover, the results suggest that charge-charge
repulsive interactions may be a useful tool in the design and
manipulation of the electronic and magnetic states of organic
superacid
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materials.
Charged N-heterocycles are conveniently
a) R. R. Naredla, C. Zheng, S. O. Nilsson Lill, D. A. Klumpp J. Am.
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incorporated into organic structures and the results show that
they may be effectively used to control the distribution of
electrons in organic structures.
[8]
[9]
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Wallingford, CT, 2013.
The biaryl ketones 3b-d were prepared by the reactions of 2-
lithiobiphenyl with the corresponding hetrocyclic nitriles, using a
method described previously.16 The dibenzosuberenols 4a-c
were prepared from the reactions of aryllithium reagents with 9-
dibenzosuberenone following a literature procedure.17
[12] a) R. Gershoni-Poranne, A. Stanger Chem. Soc. Rev. 2015, 44, 6597-
6615. b) R. Das, A. Chakraborty, S. Pan, P. K. Chattaral Curr. Org.
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Low temperature NMR studies:
The magic acid solution
is first prepared by dissolving ca. 0.5 g SbF5 in 1 mL of FSO3H in
a test tube (sealed with a cork). After thorough mixing (Vortex
mixer), the tube is placed in a dry ice-acetone bath maintained
at -40 °C. To this solution is added 0.5 mL of pure SO2ClF and
the acidic media is kept cold. The desired substrate (50-70 mg)
is then added to the superacid solution and this solution is
thoroughly mixed (while keeping the tube cold). When most of
the substrate has dissolved, 0.5 mL of the solution is transferred
into a cold NMR tube (-40 °C) and the NMR tube is quickly
sealed with a coaxial insert containing d6-acetone as the
[14] T. M. Krygowski, M. K. Cyranski Chem. Rev. 2001, 101, 1385-1419.
[15] NICS(1)zz parameters are not largely be affected by change of the
charge of the fluorenyl cation, or the number of hetero atoms in the
hetero-ring (See SI).
[16] K. N. Boblak, M. Gasonoo, Y. Zhang, D. A. Klumpp J. Org. Chem. 2015,
80, 11948-11952.
[17] F. J. Villani, C. A. Ellis, R. F. Tavares, C. Bigos J. Med. Chem., 1964, 7,
457–460.
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10.1002/chem.201606036
Chemistry - A European Journal
COMMUNICATION
This article is protected by copyright. All rights reserved.

10.1002/chem.201606036
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
Take charge attitude.
Akinari Sumita, Makafui Gasonoo,
Kenneth J. Boblak, Tomohiko Ohwada,
and Douglas A Klumpp*
NH
HN
HN
Page No. – Page No.
1H NMR, !:
8.15
8.62
8.75
NH
Use of charge-charge repulsion to
enhance π-electron delocalization in
anti-aromatic and aromatic systems
HN
HN
1H NMR, !:
4.86, 4.94,
5.00, 5.69
7.25, 7.25,
7.55, 7.65
5.41, 5.49,
5.64, 6.08
This article is protected by copyright. All rights reserved.