Synthesis, Structures, and Properties of Neutral and Radical Cationic S,C,C-Bridged Triphenylamines

Article abstract of DOI:10.1021/acs.orglett.9b04575

A structurally constrained S,C,C-bridged triphenylamine was synthesized, and the corresponding radical cation was obtained as a hexachloroantimonate by chemical oxidation. An X-ray crystallographic analysis revealed an almost planar structure for this rad

Full text of DOI:10.1021/acs.orglett.9b04575

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis, Structures, and Properties of Neutral and Radical Cationic
S,C,C-Bridged Triphenylamines
Shin-ichiro Kato,* Takanori Matsuoka, Shuichi Suzuki, Motoko S. Asano,§
,
†
†
‡
Toshitada Yoshihara,§ Seiji Tobita,§ Taisuke Matsumoto, and Chitoshi Kitamura
¶
†
†Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka-cho, Hikone, Shiga
5
22-8533, Japan
‡
Department of Chemistry, Graduate School of Engineering Science, 1-3, Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
§
Division of Molecular Science, Faculty of Science and Technology, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515,
Japan
¶
Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, 6-1, Kasuga-koh-en, Kasuga, Fukuoka 816-8580,
Japan
*
S Supporting Information
ABSTRACT: A structurally constrained S,C,C-bridged triphenylamine was
synthesized, and the corresponding radical cation was obtained as a
hexachloroantimonate by chemical oxidation. An X-ray crystallographic
analysis revealed an almost planar structure for this radical cation, which
thus represents the first example of a planar, para-unsubstituted triphenyl-
amine radical cation analogue with a sulfur bridge. The electronic properties
of the radical cation were examined by UV−vis−NIR and ESR spectroscopy
as well as DFT calculations.
riarylamine derivatives usually exhibit a trigonal planar
T
geometry, wherein the three aryl rings adopt a propeller-
shaped structure. These compounds represent an important
class of π-electron donors, and they have been investigated as
1,2
components for organic semiconductors. Triarylamines have
also attracted significant interest as redox-active units or spin
sources in organic mixed-valence systems and in the context of
3
,4
molecular magnetism. It is therefore hardly surprising that
organic chemists have devoted continuous efforts to the
development of synthetic routes to various triarylamine radical
cations and evaluation of their structures and physicochemical
Figure 1. Internally bridged triphenylamines 1−3, which provide
stable radical cations upon chemical oxidation.
5
14,15
as these generate stable radical cations.16−22 In
properties. Among the triarylamines that have been reported,
(NPPTZ),
some para-substituted triphenylamines form stable radical
cations, e.g., the commercially available tris(4-bromophenyl)-
aminium hexachloroantimonate, which is also known as “magic
the radical cationic state, the π-plane of the benzene ring at the
N-position is almost perpendicular to that of the phenothiazine
moiety and, as a result, the spin and positive charge are
inherently localized on the phenothiazine moiety. Conse-
quently, the electronic spin-structures of NPPTZ radical
cations are different from those of triphenylamine radical
cations. We envisaged that structurally constraining the
benzene ring into the same plane as the phenothiazine moiety
in NPPTZ could represent a reasonable strategy to generate
and isolate stable para-unsubstituted triphenylamine radical
cations: the spin density of the resulting molecule in the radical
cationic state should be delocalized over the entire conjugated
6
blue”. However, examples of isolable triphenylamine radical
7
cations without para substituents remain scarce, and only the
structurally constrained, bridged triphenylamine derivatives 1−
3, which contain three methylene (1), three oxygen (2), or two
oxygen bridges (3), provide isolable, stable radical cations
8
−12
upon chemical oxidation (Figure 1).
Thus, the develop-
ment of stable triphenylamine radical cations without para
substituents remains a challenging task. The exceptionally high
•+
•+
stability of radical cations 1 −3 has been attributed to their
•
+
•+
framework, as in the case of 1 −3 . Herein, we report the
synthesis, structure, and properties of planar triphenylamine 4,
high-to-moderate planarity, which promotes efficient π-
13
conjugation and spin delocalization.
During our challenging program to develop the chemistry of
para-unsubstituted triphenylamines, we focused on N-
arylphenothiazine derivatives such as N-phenylphenothiazine
Received: December 20, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04575
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Organic Letters
Letter
which is bridged by one sulfur and two carbon atoms (S,C,C-
•+
bridged triphenylamine; Figure 2), and its radical cation 4 .
Figure 2. S,C,C-bridged triphenylamine 4 and ketone 5.
To the best of our knowledge, 4• is the first isolated para-
unsubstituted triphenylamine radical cation with a sulfur
bridge that has been structurally characterized. Furthermore,
we describe the structure and properties of the unexpected
byproduct 5, which was obtained during the synthesis of 4.
Notably, the structures of 4 and 5, in which one or two bulky
dimethylmethylene groups of 1 are replaced by a sulfur atom
or a sulfur atom and a carbonyl group, respectively, are
expected to form assembled structures with intermolecular
interactions in the crystalline state.
+
Compound 4 was synthesized in three steps from 1,3-
23
dimethyl 2-iodobenzene-1,3-dicarboxylate (6), as shown in
Scheme 1. An Ullmann coupling of 6 with phenothiazine in the
Scheme 1. Synthesis of 4 and 5
Figure 3. X-ray structures of (a) 4, (b) 5, and (c) 4• with thermal
ellipsoids at 50% probability. Methyl groups and hydrogen atoms are
omitted for clarity. Only one of the two independent molecules for 4
+
•+
is shown. (d) Dimeric structure of 4 .
•
+
Table 1. Selected Bond Lengths (Å) of 4, 4 , and 5 as
a
Determined from the X-ray Crystallographic Analysis
b
4•+
bond
4
5
N1−C1
N1−C2
N1−C3
C1−C4
C2−C5
S1−C4
S1−C5
1.442(3)
1.428(3)
1.425(3)
1.403(4)
1.403(4)
1.741(3)
1.749(3)
1.421(2)
1.406(2)
1.447(2)
1.421(2)
1.422(2)
1.7072(18)
1.441(2)
1.415(2)
1.411(3)
1.406(3)
1.405(3)
1.7393(19)
1.742(2)
1.7016(17)
a
b
Atomic labeling is shown in Figure 3. Values for one of the two
presence of Cu, CuI, and K CO furnished diester 7 in 55%
2
3
independent molecules in the unit cell.
yield. The nucleophilic addition of MeMgBr to 7 followed by
hydrolysis provided diol 8 in 83% yield. Finally, a 2-fold ring
closure of 8 via a Friedel−Crafts alkylation in 85% phosphoric
acid furnished 4 as a pale yellow solid in 62% yield together
with ketone 5, which was isolated as an orange solid in 2%
partial double-bond character of the N1−C2 and N1−C3
bonds (Scheme S1). In the crystal, molecules of 5 form one-
dimensional columns with an antiparallel arrangement via π−π
stacking interactions (Figure S4). These interactions are much
less pronounced in crystals of 4.
2
4
yield.
The detailed solid-state structures of 4 and 5 were obtained
from single-crystal X-ray diffraction analyses (Figures 3 and
S1−S4). In 4 and 5, the dihedral angles between rings A and C
and between B and C are 5.5−24.9°, suggesting effective
conjugation between the phenothiazine moiety and ring C due
to the planarization of the methylene bridge. Both 4 and 5
exhibit a slightly bent but relatively planar structure with
moderate dihedral angles of 19.9° between rings A and B. The
N1−C2 and N1−C3 bonds in 5 are shorter than the N1−C1
bond and the corresponding bonds in 4 (Table 1). This was
attributed to intramolecular charge-transfer (ICT) from the
nitrogen atom to the carbonyl group in 5, which results in a
The UV−vis spectrum of 4 in CH Cl exhibited an
2
2
absorption band with a longest-wavelength maximum (λmax)
of 334 nm (Figure 4a). The λmax of 4 is red-shifted relative to
that of NPPTZ (320 nm), clearly indicating the extension of
the π-conjugation by the methylene bridge. DFT calculations
25
at the B3LYP/6-311+G(d,p) level of theory indicated that
the HOMO and LUMO of 4 are delocalized over the entire π-
framework (Figure 4b). This distribution differs from that of
NPPTZ, whose HOMO is localized on the phenothiazine
moiety (Figure S10). The UV−vis spectrum of 5 is quite
different from that of 4, i.e., it shows a broad absorption at λ
max
B
DOI: 10.1021/acs.orglett.9b04575
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Organic Letters
Letter
while the C1−C4 and C2−C5 bonds elongate (Table 1). This
change in the bond lengths likely reflects the antibonding
character of the N−C and S−C bonds and the bonding
character of the C−C bonds in the HOMO of 4. In the crystal
structure, 4• forms antiparallel dimers in which the two
phenothiazine moieties overlap slightly (Figures 3d and S6).
The intermolecular distance in the dimer (3.35 Å) is somewhat
shorter than the sum of the van der Waals radii for C−C
+
(
∼3.40 Å) and C−S (∼3.50 Å), implying some associative
interactions, e.g., SOMO−SOMO interactions (Figure S14
and Table S15).
•
+
The UV−vis−NIR spectrum of 4 [SbCl ] in CH Cl
6
2
2
exhibited an absorption at λ
= 1058 nm (Figure 5a).
max
Figure 4. (a) UV−vis spectra of 4, 5, and NPPTZ in CH Cl .
2
2
HOMOs and LUMOs of (b) 4 and (c) 5 calculated at the B3LYP/6-
3
11+G(d,p) level of theory.
=
447 nm. The HOMO of 5 is delocalized, albeit that it is
almost completely localized on the acridone moiety (Figure
c), which indicates that the absorption band at ∼450 nm
4
corresponds to the ICT transition. Notably, unlike NPPTZ, 4
and 5 are fluorescent in solution (Figure S7 and Table S1)
with absolute fluorescence quantum yields of 0.02 and 0.45,
respectively.
The cyclic voltammogram of 4 in CH Cl exhibits two
reversible oxidation waves at +0.21 V and +1.00 V (vs Fc/Fc ),
indicating that its cationic species are electrochemically stable
2
2
+
•+
Figure 5. (a) UV−vis−NIR spectra of 4 and in-situ-generated
NPPTZ• in CH
+
Cl . (b) HOMO and SOMO of 4 calculated at the
•+
2 2
(
Figure S9 and Table S2). The first oxidation potential of 4 is
UB3LYP/6-311+G(d,p) level of theory.
by 0.05 V negatively shifted relative to that of NPPTZ (+0.26
V). Based on DFT calculations, the HOMO level (−5.11 eV)
of 4 is higher than that of NPPTZ (−5.25 eV). This result
most likely reflects the antibonding character of the N−C bond
connecting the nitrogen atom of the phenothiazine moiety and
the carbon atom of the remaining benzene ring in the HOMO
of 4 (Figures 4a and S10). This finding is consistent with the
increased negativity of the first oxidation potential of 4
compared to that of NPPTZ. Compound 5 exhibits one
reversible oxidation wave at +0.57 V. The oxidation potential
of 5 is somewhat positively shifted relative to that of NPPTZ
owing to the presence of an electron-withdrawing carbonyl
group in 5.
Based on TD-DFT calculations (Figure 5b and Table S6), this
broad NIR absorption, which extends to 1200 nm, most likely
involves HOMO−SOMO transitions. The distribution of the
•
+
SOMO of 4 , which is similar to that of the HOMO of neutral
4, is delocalized over the entire π-framework. The λ value of
max
•
+
4
is significantly red-shifted compared to that of in-situ-
formed NPPTZ (887 nm), and the molar absorptivity of
•
+
12
•
+
•+
4
in the NIR region is more than twice that of NPPTZ .
These results confirm a higher degree of delocalization of the
•
+
•+
spin and/or positive charge in 4 than in NPPTZ . The
•
+
radical cation salt 4 [SbCl ] is quite stable under ambient
6
conditions, both the solid state and in solution, and the UV−
vis−NIR spectrum of its CH Cl solution remained virtually
Motivated by the results of the electrochemical measure-
ments, we attempted the isolation of a radical cation salt of 4
by chemical oxidation. Treatment of a CH Cl solution of 4
2
2
unchanged after 1 day (Figure S8).
•
+
The ESR spectrum of 4 [SbCl ] in CH Cl at room
2
2
6
2
2
with 1 equiv of magic blue afforded the desired radical cation
temperature exhibits a three-line splitting with g = 2.005
(Figure 6a). The well-resolved hyperfine structure originates
from hyperfine coupling interactions of the unpaired electron
with the N nucleus (I = 1; 99.6%); the observed ESR
spectrum was also closely reproduced by a spectral simulation.
•
+
4
[SbCl ] in 74% yield as a dark purple solid. A single-crystal
6
X-ray diffraction analysis revealed the detailed structure of
4
•
+
14
[SbCl ] (Figures 3c and S5). Similar to the crystal
6
1
5
structures of other phenothiazine derivatives, the one-
electron oxidation of 4 increases the planarity of the
phenothiazine moiety, i.e., the dihedral angle between rings
A and B of 4• is 4.6°. Notably, the dihedral angles between
rings A and C (4.9°) and between B and C (1.2°) are still
smaller than the corresponding values for neutral 4. The sum
The hyperfine coupling constant of 4• (a = 0.72 mT) is
+
N
•
+
distinctly smaller than those reported for 1 (0.94 mT) and
+
•+
7,8
•+
2
(0.90 mT), indicating that the spin density in 4 is less
•
+
•+
localized on the nitrogen atom relative to that in 1• and 2 .
+
•+ •+
Subsequently, we calculated spin density maps for 4 , 1 , 2 ,
•
+
•+
of the C−N−C bond angles is 360°. Overall, 4 exhibits a
fully planar structure except for the methyl groups, and it can
thus be regarded as an analogue of planar triphenylamine
radical cations without para-substitution. Upon oxidation, the
N1−C1, N1−C2, S1−C4, and S1−C5 bond lengths contract,
and NPPTZ (Figures 6b, S11, and S12). The Mulliken
•
+
atomic spin density (MSD) value on the N atom of 4
(+0.294) is smaller than those of 1• (+0.376) and 2
+
•+
•
+
(+0.311), which is consistent with the a value trend (4
<
N
•
+
•+
•+
2
< 1 ). The MSD value on the N atom of 4 is
C
DOI: 10.1021/acs.orglett.9b04575
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Synthesis and characterization of all new compounds,
spectral data, details of computational studies, and
crystallographic data (PDF)
Accession Codes
CCDC 1949205−1949207 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: kato.s@mat.usp.ac.jp.
ORCID
Figure 6. (a) Experimentally observed ESR spectrum of 4• in
+
Shin-ichiro Kato: 0000-0001-5310-0062
Shuichi Suzuki: 0000-0001-6294-9084
Motoko S. Asano: 0000-0003-2398-8747
Toshitada Yoshihara: 0000-0002-7960-0740
Seiji Tobita: 0000-0001-8024-7318
Notes
CH Cl (g = 2.005; a = 0.72 mT; green) and the simulated spectrum
2
2
N
•+
•+
(
black). (b) Spin density map of 4 and NPPTZ calculated at the
UB3LYP/6-311+G(d,p) level of theory.
•
+
comparable to that of NPPTZ (+0.280) and the value on the
S atom of 4• (+0.184) is smaller than that of NPPTZ
+
•+
The authors declare no competing financial interest.
(
4
+0.241; Figure S11). Notably, the spin density distribution in
•
+
exhibits a clear contribution from ring C (Figure 6b), while
ACKNOWLEDGMENTS
•
+
■
the corresponding benzene ring in NPPTZ contributes very
little to the spin density distribution. Thus, the methylene
This work was financially supported by a Grant-in-Aid for
Scientific Research (18K05089) and a research grant from
Takahashi-zaidan (09-003-145). This work was performed
within the Cooperative Research Program “Network Joint
Research Center for Materials and Devices” (Kyushu
University and Osaka University). The authors thank Mr.
Ken Kobori (Gunma University) and Ms. Tomoko Amimoto
(Hiroshima University: N-BARD) for the measurement of
ESR and mass spectra.
bridges in 4•+ allow the spin density to delocalize from the
phenothiazine moiety to the connected benzene moiety.
•
+
Consequently, the spin density distribution of 4 is similar
•
+
to those of the other bridged triphenylamine radical cations 1
and 2• (Figure S12). Given that the Mulliken charge (MC)
+
•
+
value on the N atom of 4 (+0.345) is smaller than that in
NPPTZ• (+0.739; Figure S13), and the MC values on the C
+
•
+
atoms adjacent to the N atom of 4 (+0.122/+0.433) are
higher than those of NPPTZ (−0.159/+0.143), the positive
charge is also more effectively delocalized in 4 than in
•+
•
+
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•+
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24) The mechanistic origin for the generation of ketone 5 is still
unclear, although we observed almost no transformation of 4 into 5 in
phosphoric acid.
E
DOI: 10.1021/acs.orglett.9b04575
Org. Lett. XXXX, XXX, XXX−XXX