1,3,5-Benzenetriamine double- and triple-decker molecules

Article abstract of DOI:10.1002/anie.201203451

Double- and triple-decker oligoarylamines have been prepared and characterized by X-ray crystallography for the first time (see picture; C black, N blue). The N-dianisylaminophenyl-substituted double decker exhibited the multiredox activity, and the oxidi

Full text of DOI:10.1002/anie.201203451

Angewandte
Chemie
DOI: 10.1002/anie.201203451
Macrocycles
1,3,5-Benzenetriamine Double- and Triple-Decker Molecules
Daisuke Sakamaki, Akihiro Ito,* Kazuyoshi Tanaka, Ko Furukawa, Tatsuhisa Kato, and
Motoo Shiro
Triarylamine-based macromolecular systems are widely
accepted as an appropriate electroactive material for the
study of electronic, magnetic, and optical properties, and in
fact oligoarylamines have been utilized as excellent hole-
transport materials in photoconductors, light-emitting devi-
ces, and so forth.[1] This is mainly due to four factors: 1) low
oxidation potentials; 2) stability of generated radical cations;
3) relatively small inner reorganization energy;[2] and
4) advances in palladium-catalyzed amination reactions,[3]
which make it possible to synthesize structurally rigid and
two-dimensionally expanded polymacrocyclic oligoaryla-
mines.[4,5]
One-dimensional polyarylamines and their oligomers
with meta-phenylene-linkage and/or alternating meta,para-
Scheme 1. A hypothetical multiply stacked poly(arylamine) and 1,3,5-
benzenetriamine double- and triple-deckers 1a, 1b, and 2 as oligomer
model compounds.
phenylene-linkage in the polymer backbones can be consid-
ered as promising candidates for high-spin polymers.[6] How-
ever, in spite of various studies on polymers and/or oligomer
model compounds, the generated spins in the oxidized state
have been unfortunately shown to be mainly uncoupled
except for the existence of minor high-spin components,[7]
probably because of unexpected defects owing to insufficient
oxidation and also interruption of p conjugation between
spin-containing and ferromagnetic coupling units owing to
undefined conformations of the one-dimensional polymer
chain hamper the robust ferromagnetic exchange coupling
pathway through the polymer backbone. Contrary to poly-
arylamines with one-dimensional connectivity, the retention
of high-spin alignment has been accomplished for oligoaryl-
amines incorporating rigid macrocyclic architectures.[5,8]
Inspired by the extensive work on multilayered cyclo-
phanes from Misumiꢀs group,[9] cylindrical cage-structured
polyarylamines with increased connectivity among macro-
cycles (Scheme 1) are potentially intriguing to prevent the
above-mentioned obstacles.[10] The hypothetical polymer
shown in Scheme 1 can be regarded as a 1,3,5-benzenetri-
amine multiple-decker linked by three meta-phenylenes, or in
other words, a cofacially stacked benzenes connected by three
poly(m-aniline) strands. Herein we report the synthesis and
characterization of the first double- and triple-decker aryl-
amines 1 and 2 corresponding to dimeric and trimeric model
compounds.[11] Compounds 1a and 2 have anisyl groups at all
N-positions to lower the oxidation potentials, and moreover,
in 1b, dianisylaminophenyl groups are attached at all N-
positions to stabilize the oxidized state.[12]
1,3,5-benzenetriamine double- and triple-decker 1 and 2
were successfully obtained by repeatedly using Buchwald–
Hartwig cross-coupling reaction[3] (Scheme 2).[13] In the syn-
thesis of 1a (and 1b), the substituted benzenetriamines 3 (or
7) and the corresponding tribromides 4 (and 8) were coupled
in a one-pot manner to make the cylindrical cage structure.
On the other hand, the triple-decker compound 2 was
prepared by the cross-coupling reaction between a dendritic
oligoarylamine with six preorganized reaction points (11) and
two equivalent of 1,3,5-tribromobenzene.
[*] D. Sakamaki, Dr. A. Ito, Prof. Dr. K. Tanaka
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: aito@scl.kyoto-u.ac.jp
Dr. K. Furukawa
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
Colorless blocklike (1a) and platelike (2) single crystals
were grown by slow evaporation of a dilute mixed solution
(CH3OH/CH2Cl2 for 1a; THF/n-hexane for 2) and their
molecular structures were unequivocally determined by X-
ray crystallography (Figure 1; Supporting Information, Table
S1).[20] As shown in Figure 1a, 1a adopts a C3-symmetrical
double-decker structure, where the two benzene rings as the
decks were cofacially separated by 4.77 ꢁ, and the dihedral
angles between the cofacial 1,3,5-benzenetriamine planes and
pillar-like three meta-phenylene planes were around 76–808,
thus demonstrating that the two cofacial benzene moieties
Prof. Dr. T. Kato
Institute for the Promotion of Excellence in Higher Education
Kyoto University
Yoshida-Nihonmatsu, Sakyo-ku, Kyoto 606-8501 (Japan)
Dr. M. Shiro
Rigaku Corporation, X-ray Research Laboratory
Matsubaracho 3-9-12, Akishima, Tokyo, 196-8666 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203451.
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Scheme 2. Synthetic routes for 1a, 1b, and 2. DPPF=1,1’-bis(diphenylphosphanyl)ferrocene, dba=trans, trans-debenzylideneacetone, Boc=tert-
butoxycarbonyl, 4-DMAP=4-dimethylaminopyridine, TFA=trifluoroacetic acid.
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potential increased with repetitive potential cycling for 1a
and 2 (Supporting Information, Figure S2). This clearly
indicates the electrochemical polymerization on the electrode
surface owing to the instability of the oxidized species, though
the detailed mechanism remains unclear. After 10 cycles, two
discrete oxidation waves were observed at approximately 0.35
and 0.67 V vs. Fc0/+ for the triple-decker species 2, while single
oxidation wave observed at 0.67 V vs. Fc0/+ for double-decker
compound 1a. In contrast, the introduction of dianisylami-
nophenyl groups at all the N-positions of 1a resulted in the
recovery of reversible multiredox behavior, which is probably
due to the existence of six para-phenylene diamine (PD)
moieties as redox centers. As shown in Figure 2, the redox
Figure 1. ORTEP representations of a) 1a and b) 2. Ellipsoids are set
at 50% probability. Crystallization solvent molecules, methoxy groups,
and hydrogen atoms are omitted for clarity. Nitrogen atoms are
colored in black.
Figure 2. Differential pulse voltammogram (DPV) of 1b in CH2Cl2
(0.1m nBu4NBF4) measured with a Pt electrode (surface area 2 mm2)
at a sweep rate of 0.02 Vsꢀ1 at 298 K. The broken line indicates the
simulated DPV (see text for details).
were not totally eclipsed but slightly twisted with each other
(ca. 308). On the other hand, although 2 also has a C3-
symmetrical triple-decker structure, distances between the
cofacial benzene rings were estimated to be slightly longer
than that of 1a (4.84 ꢁ), and moreover, the meta-phenylene
pillars were nearly perpendicular to the 1,3,5-benzenetri-
amine planes. As a consequence, the three cofacial benzene
moieties were confirmed to be nearly eclipsed with each other
(Figure 1b).
To gain insight into the electronic structures of 1a and 2,
we carried out density functional theory (DFT) calculations
(at the B3LYP/6-31G* level) on the model molecules 1a’ and
2’[14] in which all the methoxy groups are omitted. The
optimized structures were in good agreement with the X-ray
structures. The frontier MOs comprised of HOMO and (HO-
1)MO (Supporting Information, Figure S1) were virtually
triply degenerate for 1a’ (and quasi triply degenerate for 2’).
Moreover, those frontier MOs for 1a’ and 2’ were found to be
distributed mainly over the three meta-phenylenediamine
linkers, and therefore, the present double- and triple-decker
molecules can be viewed as three oligo(meta-aniline) moieties
that are linked by 1,3,5-benzentriyl decks (Supporting Infor-
mation, Figure S1).
behavior of 1b can be roughly estimated to be comprised of
first two-electron, second four-electron, and third six-electron
transfer processes, which means that double-decker 1b is
oxidizable up to a dodecacation. From the simulation of the
observed differential pulse voltammogram (DPV),[15] the first
and second oxidation processes of 1b correspond to the
generation of dication (quasi two-electron transfer: ꢀ0.12
(1e) and ꢀ0.06 (1e)) and hexacation (quasi four-electron
transfer: + 0.03 (4e)), respectively, and the third oxidation
process can be judged to be quasi six-electron transfer (+ 0.44
(1e), + 0.49 (1e), + 0.51 (1e), + 0.56 (1e), + 0.60 (1e), and
+ 0.61 (1e)). Therefore, these findings suggest that the
diradical dication and hexaradical hexacation of 1b can be
generated by using appropriate oxidizing agents.
In conjunction with the fact that the same dianisylamino-
phenyl groups are substituted at all of the N-positions, the
optical absorption spectroscopic changes of 1b in CH2Cl2
during the course of the oxidation process were similar to
those of the previously studied tetraaza[14]metacyclophane[12]
(Figure 3). As shown in Figure 3a, a p–p* band with the lmax
of 333 nm in the neutral species 1b was changed into three
new bands corresponding to the oxidized species from 1b+ to
1b2+ (lmax = 415, ca. 600, ca. 1000 nm) with an isosbestic point
at 366 nm. The observed lowest energy band (lmax ꢁ 1000 nm)
The redox behavior of 1a and 2 was measured by cyclic
voltammetry (CV) in CH2Cl2 (0.1m tetra-n-butylammonium
tetrafluoroborate (nBu4NBF4)) at room temperature. Upon
multisweep oxidation, both the peak current and the peak
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can be considered as the charge-resonance (CR) intervalence
band (IV) band owing to the the semiquinone radical cation
of the para-phenylenediamine (PD) moiety,[16] and slightly
shifted hypsochromically with increasing oxidation number,
whereas the next-lowest-energy band (lmax ꢁ 600 nm) exhib-
ited a slight bathochromic shift. Further oxidation of 1b2+
showed continuous increase in absorbance for the CR IV
band, with a slight hypsochromic shift (lmax = 920 nm), and
the intensity reached up to thrice that of 1b2+, thus strongly
indicating that 1b6+ was formed by oxidation of all the PD
moieties of 1b (Figure 3b). Note that the fact that the
decrease in intensity observed in lower-energy region of the
CR IV band with an isosbestic point at 1113 nm (Figure 3b,
inset) on going from 1b2+ to 1b6+, demonstrates the existence
of the charge transfer between the neutral and oxidized PD
moieties through the meta-phenylene and 1,3,5-benzenetriyl
linkers, as have been often observed in the mixed valence
compounds with meta-phenylene and/or 1,3,5-benzenetriyl
coupling units.[16c,17] Finally, the intense CR IV band
decreased rapidly when 1b6+ is further oxidized, and two
new bands with the lmax of 524 and 723 nm grew with some
isosbestic points (345, 450, and 796 nm), corresponding to
a conversion from the semiquinoidal PD to the quinoidal PD
(Figure 3c).
The reversibility of the multistage oxidation processes
prompted us to determine the spin multiplicity of the
polycationic species of 1b. We have measured the electron
spin transient nutation (ESTN) spectra based on the pulsed
ESR spectroscopy[18] for the oxidized samples in a rigid glass
of CH2Cl2 at 5 K. The stepwise chemical oxidation were
carried out by adding 2 and 6 molar equivalents of tris(4-
bromophenyl)aminium
hexachloroantimonate
(Magic
Blue)[19] at 195 K. As shown in Figure 4a and Table 1, the
Figure 3. UV/Vis-NIR absorption spectra of the stepwise electrochem-
ical oxidation of 1b in CH2Cl2 (0.1m/nBu4NBF4) at 298 K: a) 1b to
Figure 4. 2D ESTN spectra of 1b in CH2Cl2 at 5 K after the addition of
a) 2 equiv and b) 6 equiv of oxidant.
1b2+; b) 1b2+ to 1b6+; c) further oxidation process from 1b6+
.
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Table 1: Spectroscopic data of the ESTN spectroscopy for 1b2+ and 1b6+
.
[3] a) J. P. Wolfe, S. Wagaw, J. F. Mar-
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Species
Observed nutation frequency [MHz][a]
Transition assignment
pffiffi
1b2+
24.4 (wt = 2wd)
j1, ꢂ1i,j1, 0i
(doublet impurity)
17.8 (wd)
j1/2, +1/2i,j1/2, ꢀ1/2i)
j3/2, ꢂ3/2i,j3/2, ꢂ1/2i
j3/2, +1/2i,j3/2, ꢀ1/2i
j1, ꢂ1i,j1, 0i
pffiffi
1b6+
19.5 (wq,1
=
3wd)
22.6 (wq,2 =2wd)
pffiffi
(triplet impurity)
(doublet impurity)
16.6 (wt = 2wd)
11.0 (wd)
j1/2, +1/2i,j1/2, ꢀ1/2i
[4] X. Z. Yan, J. Pawlas, T. Goodso-
n III, J. F. Hartwig, J. Am. Chem.
Soc. 2005, 127, 9105.
[a] d=doublet, t=triplet, q=quartet.
[5] a) A. Ito, Y. Yamagishi, K. Fukui, S.
nutation frequency of 24.4 MHz observed after addition of 2
molar equiv of oxidant can be assigned to the spin-triplet state
of 1b2+, judging from the nutation frequency owing to the
spin-doublet impurity. This result is in good accordance with
the DFT results that two electrons are removed from the
degenerate HOMOs (Supporting Information, Figure S1).
On the other hand, after oxidation of 1b with 6 molar equiv of
oxidant, two kinds of new signals at 19.5 and 22.6 MHz were
detected along with those for the impurities with the spin-
doublet (11.0 MHz) and spin-triplet (16.6 MHz) multiplicities
(Figure 4b), and consequently, these nutation signals were
assignable to the spin-allowed transition for the spin-quartet
state from the frequency ratio. Therefore, this result suggests
that 1b treated with 6 equiv of oxidant is in spin-quartet state.
However, the electronic structure of this high-spin state
remains obscure at the present stage, and further examination
is needed for elucidation of electronic structure for hexaca-
tion of 1b.
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In summary, we have succeeded in the preparation and
characterization of 1,3,5-benzenetriamine double- and triple-
decker compounds, which are the first examples of layered
oligoarylamines stacked cofacially through multiple arene
linkers. Furthermore, N-dianisylaminophenyl-group-substi-
tuted double-decker 1b revealed reversible multiredox activ-
ity, and consequently, the polycationic species, which are
generated by treatment of 2 and 6 equiv of oxidant, were
found to be in high-spin state, as is reflected in the highly
degenerate frontier MOs. Understanding the electronic
structures of the polycationic species of the present com-
pounds and the further attempts to create the related
derivatives, which leads to the more robust high-spin poly-
cationic species, which are currently being pursued in the
context of exploitation of spin-polarized molecular wires.
[12] A. Ito, S. Inoue, Y. Hirao, K. Furukawa, T. Kato, K. Tanaka,
Chem. Commun. 2008, 3242.
[13] Synthetic details for 1a, 1b, and 2 are described in the
Supporting Information. As pointed out by one of the referees,
an improvement in overall yields is needed for possible
applications of these compounds.
[14] The full geometry optimizations of 1a’ and 2’ in C3 symmetry
were carried out by using Gaussian09 progaram package:
Gaussian09 (RevisionC.01), M. J. Frisch et al.
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obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: May 4, 2012
Published online: && &&, &&&&
Keywords: arylamines · cyclophanes · electrochemistry ·
.
EPR spectroscopy · radical ions
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Communications
Macrocycles
Double- and triple-decker oligoarylamines
have been prepared and characterized by
X-ray crystallography for the first time
(see picture; C black, N blue). The N-
dianisylaminophenyl-substituted double-
decker exhibited the multiredox activity,
and the oxidized species were confirmed
to be in high-spin states on the basis of
pulsed ESR spectroscopy.
D. Sakamaki, A. Ito,* K. Tanaka,
K. Furukawa, T. Kato,
M. Shiro
&&&& — &&&&
1,3,5-Benzenetriamine Double- and
Triple-Decker Molecules
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