Stable triplet-state di(cation radicals) of a meta-para aniline oligomer by 'acid doping'

Article abstract of DOI:10.1021/ja9616591

We describe the formation of a stable triplet-state oligoaniline di(cation radical) via a proton-triggered redox reaction between N,N'-bis[4-(phenylamino)phenyl]-1,3-benzenediamine (1) and N,N'-bis[4-(phenylimino)cyclohexa-2,5-dienylidene]-1,3-benzenediam

Full text of DOI:10.1021/ja9616591

10626
J. Am. Chem. Soc. 1996, 118, 10626-10628
Stable Triplet-State Di(Cation Radicals) of a Meta-Para
Aniline Oligomer by “Acid Doping”
Martijn M. Wienk and Rene´ A. J. Janssen*
Contribution from the Laboratory of Organic Chemistry, EindhoVen UniVersity of Technology,
P.O. Box 513, 5600 MB EindhoVen, The Netherlands
ReceiVed May 17, 1996X
Abstract: We describe the formation of a stable triplet-state oligoaniline di(cation radical) via a proton-triggered
redox reaction between N,N′-bis[4-(phenylamino)phenyl]-1,3-benzenediamine (1) and N,N′-bis[4-(phenylimino)-
cyclohexa-2,5-dienylidene]-1,3-benzenediamine (2). In this reaction 1 is oxidized, while protonated 2 is reduced,
both yielding the same di(cation radical) 12•2+. The di(cation radical) is characterized with UV/visisble/near-IR and
ESR spectroscopy (D ) 118 MHz; E ≈ 0 MHz). Variable-temperature ESR measurements are consistent with a
triplet ground state for 12•2+. The high stability of 12•2+ under ambient conditions demonstrates that alternating
meta and para oligoanilines are interesting building blocks for future polaronic ferromagnets.
Introduction
Scheme 1
A promising strategy toward high-spin polymers is that of a
polaronic ferromagnetic chain, in which dopable segments are
interlinked by ferromagnetic coupling units.1 Doping of such
a polymer may result in ferromagnetically coupled unpaired
electrons, localized within these segments. A number of
polymers have been prepared that demonstrate the feasibility
of this strategy.2 The concept of a polaronic ferromagnet bears
a strong resemblance to the redox doping of conducting
polymers. A special type of doping in conducting polymers is
the “acid doping” of the emeraldine base of polyaniline to the
conducting emeraldine salt.3 The emeraldine base consists of
an alternating sequence of fully oxidized p-quinodiimine and
fully reduced p-phenylenediamine units linked via p-phenylenes.
Protonation of the polyaniline emeraldine base at the imine
nitrogen atoms induces an internal redox reaction in which one
electron is transferred from each p-phenylenediamine to a
p-quinodiiminium unit. In the resulting conducting emeraldine
salt, all aniline units have the same intermediate oxidation state
and all nitrogen atoms have become equivalent. This internal
redox reaction produces one unpaired electron per two aniline
units. In principle the same proton-induced spin unpairing can
also be used to prepare high-spin polyanilines. Several high-
spin oligoradicals prepared by oxidation of arylamines have been
described.4 Recently, we have shown that oxidation of a meta-
para aniline oligomer results in a triplet di(cation radical) that
combines chemical stability from the p-phenylenediamine units
with ferromagnetic interaction via a m-phenylene ring.5 Here
we demonstrate that the concept of acid doping can be used to
prepare high-spin di(cation radical) oligoanilines. For this
purpose we have prepared the fully reduced 1 and fully oxidized
2 forms of an oligoaniline, in which two para-substituted rings
are linked by m-phenylene. A 1:1 mixture of 1 and 2 has the
required overall oxidation state and by simply adding a drop of
acid a redox reaction is initiated that produces a stable di(cation
radical) as the single product (Scheme 1).
X Abstract published in AdVance ACS Abstracts, October 1, 1996.
(1) (a) Fukotome, H.; Takahasi, I.; Ozaki, M. Chem. Phys. Lett. 1987,
133, 34. (b) Kaisaki, D. A.; Chang, W.; Dougherty, D. A. J. Am. Chem.
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1995, 72, 225.
(2) (a) Murray, M. M.; Kaszynski, P.; Kaisaki, D. A.; Chang, W.;
Dougherty, D. A. J. Am. Chem. Soc. 1994, 116, 8152. (b) Bushby, R. J.;
Ng, K. M. Chem. Commun., 1996, 659.
Results and Discussion
(3) (a) Huang, W. -S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem.
Soc., Faraday Trans. 1 1986, 82, 2385. (b) Wudl, F.; Angus, R. O.; Lu, F.
L.; Allemand, P. M.; Vachon, D. J.; Nowak, M.; Liu, Z. X.; Heeger, A. J.
J. Am. Chem. Soc. 1987, 109, 3677. (c) MacDiarmid, A. G.; Epstein, A. J.
Faraday Discuss. Chem. Soc. 1989, 88, 317.
(4) (a) Torrance, J. B.; Oostra, S.; Nazzal, A. Synth. Met. 1987, 19, 709.
(b) Yoshizawa, K.; Tanaka, K.; Yamabe, T.; Yamauchi, J. J. Chem. Phys.
1992, 96, 5516. (c) Stickly, K. R.; Blackstock, S. C. J. Am. Chem. Soc.
1994, 116, 11576. (d) Ito, A.; Ohta, K.; Tanaka, K.; Yamabe, T.; Yoshizawa,
K. Macromolecules 1995, 28, 5618. (e) Ito, A.; Saito, T.; Tanaka, K.;
Yamabe, T. Tetrahedron Lett. 1995, 48, 8809. (f) Sasaki, S.; Iyoda, M.
Chem. Lett. 1995, 1011. (g) Nakamura, Y.; Iwamura, H. Bull. Chem. Soc.
Jpn. 1993, 66, 3724. (h) Okada, K.; Imakura, T.; Oda, M.; Murai, H.;
Baumgarten, M. J. Am. Chem. Soc. 1996, 118, 3047.
Tetraamine 1 was synthesized from N-phenyl-1,4-benzene-
diamine as described in Scheme 2. Diacetylation with acetic
anhydride and reaction with 0.5 equiv of 1,3-dibromobenzene
in an Ullmann coupling with copper(I) iodide as a catalyst,
afforded the tetra-N-acetyl oligomer, which was hydrolyzed to
tetraamine 1. Tetraimine 2 was prepared from 1 by oxidation
with PbO2 (Scheme 1).
Cyclic voltammetry of 1 under acid conditions (acetonitrile,
10-3 M perchloric acid, 0.1 M tetrabutylammonium hexafluo-
(5) Wienk, M. M.; Janssen, R. A. J. Chem. Commun. 1996, 267.
S0002-7863(96)01659-9 CCC: $12.00 © 1996 American Chemical Society

Stable Triplet-State Oligoaniline Di(Cation Radical)
J. Am. Chem. Soc., Vol. 118, No. 43, 1996 10627
Figure 1. UV/visible/near-IR spectra of a 1:1 mixture of 1 and 2:
(‚‚‚) in MeCN; (s) in MeCN/TFA (99:1).
Scheme 2
Figure 2. X-Band ESR spectrum of 12•2+ in CH2Cl2/TFA recorded at
110 K. The central line is due to some doublet impurity. Inset: ∆Ms
) (2 transition recorded at 4.0 K.
1
2•2+, while at the same time the tetraamine 1 is oxidized to
exactly the same species. In acidic solution this triplet-state
di(cation radical) 12•2+ is stable at room temperature for several
days. Reduction of 12•2+ with hydrazine monohydrate regener-
ates 1 in quantative yield, as evidenced from the UV/visible/
near-IR spectrum showing the band of tetraamine 1 at 4.09 eV
with double intensity as compared to the initial spectrum.
The zero-field splitting parameters (D ) 118 MHz; E ≈ 0
MHz) were determined by simulation of the ∆Ms ) (1
transitions of the ESR spectrum. Assuming a point-dipole
approximation for the zero-field splitting, D ) 118 MHz
corresponds to an average distance between the unpaired
electrons of 8.7 Å. This is consistent with the separation of
9.8 Å between the centers of the two p-phenylenediamine units,
as estimated from standard bond lengths, assuming a planar
geometry.
In order to determine the ground state of 12•2+, variable
temperature ESR measurements were carried out. The signal
intensity of the ∆Ms ) (1 and ∆Ms ) (2 transitions follows
Curie’s law (I ) C/T) between 4 and 100 K and is completely
reversible. The Curie behavior reveals that no extra population
or depopulation of the triplet state occurs in this temperature
region. This can be interpreted in two possible ways.8 Either
the triplet state is the ground state and separated from the singlet
state by a substantial energy gap of a few hundreds of calories
per mole or the singlet and triplet states are exactly degenerate.
In either case, however, the triplet state corresponds to the lowest
energy state.
rophosphate as supporting electrolyte) reveals two chemically
reversible two-electron oxidation/reductions at E1° ) 0.63 and
E2° ) 0.89 V vs SCE. These values are similar to those reported
for N,N′-diphenyl-1,4-benzenediamine.6 Apparently, up to four
electrons can be removed reversibly from oligoaniline 1 under
acid conditions.
When equimolar amounts of tetraamine 1 and tetraimine 2
are dissolved in acetonitrile, no reaction occurs. The UV/visible/
near-IR spectrum of the mixture (Figure 1) is a superposition
of the absorptions of 1 (4.09 eV) and 2 (2.74 and 4.05 eV), and
no ESR signal is observed. Upon addition of 1% (v/v) of
trifluoroacetic acid the electronic spectrum changes completely.
The initial absorption bands disappear and two new peaks at
1.75 and 3.17 eV emerge, attributed to singly oxidized p-
phenylenediamine units.6a,7
Di(cation radical) 12•2+ has also been prepared by chemical
oxidation of tetraamine 1 in dichloromethane/tetrafluoroacetic
acid (1:1), using thianthrenium perchlorate9 or phenyliodine-
(III) bis(trifluoroacetate)10 as oxidizing agent. Addition of <1
equiv of oxidant affords monocation radical 1•+, being a mixed-
valence molecule.11 The ESR spectrum of 1•+ at room
temperature exhibits a well-resolved complex pattern dominated
by hyperfine interaction with two 14N nuclei and two amine
protons. The isotropic couplings of Aiso(N) ) 5.5 G and
The ESR spectrum of the acid-doped mixture, recorded at
110 K (Figure 2), reveals the zero-field splitting characteristic
for a high-spin molecule. Unambiguous proof for a high-spin
state is provided by a ∆Ms ) (2 transition at half-field. These
ESR signals are attributed to the triplet-state di(cation radical)
1
2•2+ resulting from a proton-triggered redox reaction between
tetraamine 1 and tetraimine 2. In this process the protonated
tetraimine is reduced to the corresponding di(cation radical),
(6) (a) Wolf, J. F.; Forbes, C. E.; Gould, S.; Shacklette, L. W. J.
Electrochem. Soc. 1989, 136, 2887. (b) Moll. T.; Heinze, J. Synth. Met.
1993, 55-57, 1521.
(8) (a) Racja, A. Chem. ReV. 1994, 94, 871. (b) Iwamura, H.; Koga, N.
Acc. Chem. Res. 1993, 26, 346. (c) Kanno, F. Inoue, K. Koga, N; Iwamura,
H. J. Phys. Chem. 1993, 97, 13267. (d) Ling, C., Lahti, P. M. J. Am. Chem.
Soc. 1994, 116, 8784.
(9) Murata, Y.; Shine, H. J. J. Org. Chem. 1969, 34, 3368.
(10) Eberson, L.; Hartshorn, M. P.; Persson, O. Act. Chem. Scand. 1995,
49, 640.
(7) The optical absorption spectra of neutral tetraamine 1 and its di-
(cation radical) 12•2+ are found to be identical to those of N,N′-diphenyl-
1,4-benzenediamine and its monocation radical. This suggests that the non-
resonant m-phenylene ring effectively separates the electronic transitions
in 1 and 12•2+
.

10628 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Wienk and Janssen
(330K, CDCl3) δ 23.74 (CH3), 125.46 (C-4 + C-6), 126.65 (C-2),
127.44, 127.54, 127.87 and 128.06 (C-2′ + C-6′, C-3′ + C-5′, C-2′′ +
C-6′′, and C-4′′), 129.61 (C-3′′ + C-5′′), 129.91 (C-5), 140.72 and
141.86 (C-1′ + C-4′), 143.13 (C-1′′), 143.83 (C-1 + C-3), 170.01 and
170.10 (CO). Anal. Calcd for C30H26N4: C, 74.74; H, 5.61; N, 9.17.
Found: C, 74.30; H, 4.89; N, 9.39.
Aiso(H) ) 6.5 G are in good agreement with values reported
for N,N′-diphenyl-1,4-benzenediamine.6a Hyperfine interaction
with only two nitrogen nuclei demonstrates that the unpaired
electron is localized within half of the molecule. Further
oxidation of 1•+ leads to the formation of the di(cation radical)
1
2•2+, with electronic and ESR spectra identical to those obtained
N,N′-Bis[4-(phenylamino)phenyl]-1,3-benzenediamine (1). A so-
lution of tetraamide 4 (1.22 g, 2 mmol) and sodium hydroxide (1.0 g,
40 mmol) in EtOH/H2O (1:1, 40 mL) was heated under reflux for 48
h. During the reaction the initially homogeneous solution gave a liquid/
liquid phase separation and a precipitate was formed. After cooling,
the precipitate was filtered off, washed with water and dried under
reduced pressure. Recrystallization from benzene gave pure 1 (0.73
g, 83%) as a white crystaline solid: mp 211 °C (dec); 1H NMR (CDCl3)
δ 5.52 (2H, br s, NH), 5.56 (2H, br s, NH), 6.48 (2H, dd, J ) 8.0 and
2.2 Hz, H-4 + H-6), 6.59 (1H, t, J ) 2.2 Hz, H-2), 6.86 (2H, tt, J )
7.3 and 1.1 Hz, H-4′′), 6.97 (4H, m, H-2′′ + H-6′′), 7.05 (8H, s, H-2′,
H-3′, H-5′ + H-6′), 7.09 (1H, t, J ) 8.0 Hz, H-5), 7.23 (4H, m, H-3′′
+ H-5′′); 13C NMR (CDCl3) δ 103.75 (C-2), 108.29 (C-4 + C-6),
116.45 (C-2′′ + C-6′′), 120.15 (C-4′′), 120.79 , 121.3 (C-2′ + C-6′
and C-3′ + C-5′), 129.34 (C-3′ + C-5′), 130.19 (C-5), 136.92 (C-1′ or
C-4′), 137.25 (C-1′ or C-4′), 144.26 (C-1′′), 145.58 (C-1 + C-5); ES-
MS m/z (M+) calcd 442.2, obsd 442.4. Anal. Calcd for C30H26N4: C,
81.42; H, 5.92; N, 12.66. Found: C, 81.54; H, 5.85; N, 12.71.
N,N′-Bis-[4-(phenylimino)-cyclohexa-2,5-dienylidene)]-1,3-ben-
zenediamine (2). To tetraamine 1 (110 mg, 0.025 mmol) in chloroform
(5 mL) was added PbO2 (0.6 g, 2.5 mmol). The mixture was stirred at
room temperature for 15 min. The dark red solution was filtered over
silica, and the solvent was removed by evaporation. The product (98
mg, 90%) was isolated as a mixture of three cis-trans isomers (2a,
2b, 2c). These isomers could not be separated by column chromatog-
raphy since during this process isomerization takes place: 1H NMR
(CDCl3) δ 6.37, 6.41, 6.46 ((0.2 + 0.5 + 0.3)H, 3 t, J ) 1.9 Hz, H-2a-
c), 6.67 (0.4H, dd, J ) 8.0 and 1.9 Hz, H-4a + H-6a), 6.69,6.70 (1H,
2 d, J ) 8.0 H-4b and H-6b), 6.72 (0.6H, dd, J ) 8.0 and 1.9 Hz,
H-4c and H-6c), 6.75-7.15 (12H, m, H-2′, H-3′, H-5′, and H-6′), 7.20
(2H, m, H-4′′), 7.31-7.43 (9H, m, H-5 and H-2′′ + H-6′′); 13C NMR
(CDCl3) δ 112.22 (C-2), 117.09, 117.16, 117.19, 117.22 (C-4 + C-6),
120.49, 120.57 (C-2′′ + C-6′′), 124.45, 124.49, 124.72, 125.07, 125.11,
125.40 (syn-(C-2′ + C-3′ + C-5′ + C-6′)), 125.20, 125.23 (C-4′′),
128.91, 128.95 (C-3′′ + C-5′′), 129.53, 129.58, 129.64 (C-5), 136.26,
136.30, 136.77, 137.44, 137.48, 137.98 (anti-(C-2′ + C-3′ + C-5′ +
C-6′)), 149.92, 149.94 (C-1′′), 150.74, 150.77, 150.82 (C-1 + C-3),
158.20, 158.22, 158.28, 158.31 (C-1′), 158.67, 158.74 (C-4′); ES-
MS m/z (M + H+) calcd 239.2, obsd 239.4.
in the acid doping experiment.
Conclusion
We have demonstrated that, in addition to redox doping, acid
doping can be used to generate polaronic high-spin molecules.
The chemical stability of the di(cation radical), 12•2+ confirms
the feasibility of alternating meta and para oligoanilines as
building blocks for future polaronic ferromagnetic polymers.
Experimental Section
General Methods. Commercial grade reagents were used without
further purification. Solvents were purified following standard pro-
cedures. NMR spectra were recorded on a Bruker AM-400 spectrom-
eter, chemical shifts are relative to TMS for 1H and 13C NMR spectra.
Cyclic voltammograms were obtained with 0.1 M tetrabutylammonium
hexafluorophosphate as supporting electrolyte using a Potentioscan
Wenking POS73 potentiostat.
N-Phenyl-N,N′-1,4-phenylenebis(acetamide) (3). Acetic anhydride
(3.8 mL, 40 mmol) was added slowly to N-phenyl-1,4-benzenediamine
(3.68 g, 20 mmol) in acetic acid (20 mL). After the addition was
complete, the reaction mixtrure was heated to 70 °C for 2 h. The acetic
acid was removed by distillation under reduced pressure. Column
chromatography (SiO2, EtOAc) and recrystallization from hexane/
EtOAc (1:1) provided 3 (3.83 g, 87%) as a white crystalline solid: mp
1
137 °C; H NMR (330 K, CDCl3)12 δ 2.09 (3H, s, CH3), 2.16 (3H, s,
CH3), 7.18 (2H, d, J ) 8.7 Hz, H-2 and H-6), 7.20-7.25 (3H, m, H-4′
and H-2′ + H-6′), 7.30 (1H, bs, N-H), 7.33 (2H, dd , J ) 8.4 Hz and
6.9 Hz, H-3′ + H-5′), 7.43 (2H, bd, J ) 8.7 Hz, H-3 + H-5); 13C
NMR (330 K, CDCl3) δ 23.45 (CH3), 23.86 (CH3), 120.80 (C-3 +
C-5), 126.87 (C-4′), 127.23 (C-2 + C-6 or C-2′ + C-6′), 127.60 (C-2
+ C-6 or C-2′ + C-6′), 129.19 (C-3′ + C-5′), 137.4 (C-4), 138.61
(C-1), 143.15 (C-1′), 168.73 (CO), 170,53 (CO). Anal. Calcd for
C16H16N2O2: C, 71.62; H, 6.01; N, 10.44. Found: C, 72.14; H, 5.94;
N, 10.54.
N,N′-Bis[4-(N-phenylacetylamino)phenyl]-1,3-phenylenebis(aceta-
mide) (4). Diamide 3 (2.69 g, 10 mmol), 1,3-dibromobenzene (1.18
g, 10 mmol), K2CO3 (1.38 g, 10 mmol), and CuI (0.05 g, 0.25 mmol)
in ethoxyethyl ether (25 mL) were heated to reflux for 24 h. The hot
reaction mixture was filtered over Hyflo and the filtrate thoroughly
washed with EtOAc. The combined organic fractions were concentrated
by vacuum distillation, and the crude product was purified by column
chromatography (SiO2, CHCl3/MeOH 9:1) and recrystallization from
hexane/EtOAc, providing 4 (3.11 g, 51%) as a white solid: mp 160-
165 °C;13 1H NMR (330 K, CDCl3) δ 1.98 (6H, s, CH3), 1.99 (6H, s,
CH3), 7.06 (2H, d, J ) 8.1 Hz, H-4 and H-6), 7.16 (4H, d, J ) 8.8 Hz,
H-2′′ and H-6′′), 7.2-7.3 (12H, m, H-2, H-5, H-2′ + H-6′, H-3′ +
H-5′, and H-4′′), 7.35 (4H, t, J ) 7.7 Hz, H-3′′ + H-5′′); 13C NMR
Electron Spin Resonance. ESR spectra were recorded using a
Bruker ER200D SRC spectrometer, operating with an X-band standard
cavity and interfaced to a Bruker Aspect 3000 data system. Temper-
ature was controlled by a Bruker ER4111 variable-temperature unit
between 100 K and room temperature or by an Oxford 3120 temperature
controller combined with an ESR900 continuous flow cryostat in the
range 3.8-100 K. Saturation of the ESR signal during variable
temperature experiments on the ∆Ms ) (1 and ∆Ms ) (2 transition
was avoided by using low microwave powers, well within the range
where signal intensity is proportional to the square root of microwave
power at 4.2 K.
(11) Related work on mixed valence in organic molecules see Ref. 4c
and (a) Racja, S.; Racja, A. J. Am. Chem. Soc. 1995, 117, 9172. (b)
Utamapanya, S.; Racja, A. J. Am. Chem. Soc. 1991, 113, 9242. (c)
Bonvoisin, J.; Launay, J.-P.; Rovira, C.; Veciana, J. Angew. Chem. Int. Ed.
Engl. 1994, 33, 2106. (d) Matsushita, M.; Nakamura, T.; Momose, T.; Shida,
T.; Teki, Y.; Takui, T.; Kinoshita, T.; Itoh, K. J. Am. Chem. Soc. 1992,
114, 9172.
UV/visible/near-IR Spectrometry. Absorption spectra in the UV/
visible/near-IR region were recorded using a Perkin-Elmer Lambda 900
spectrophotometer with a sealed 10 mm cuvette.
Acknowledgment. We thank Professor E. W. Meijer and
Dr. E. E. Havinga for helpful discussions and valuable
comments.
(12) The appearance of the 1H NMR spectrum turned out to be very
sensitive towards temperature and concentration.
(13) Between 160 and 165 °C a liquid crystalline phase was observed
by polarization microscopy.
JA9616591