M-phenylene-linked aromatic poly(aminium cationic radical)s: Persistent high-spin organic polyradicals

Article abstract of DOI:10.1021/ol0346281

(Matrix presented) High molecular weight and networked aromatic polyamines prepared by palladium-catalyzed polycondensation were oxidized to yield the corresponding poly(aminium cationic radical)s, which displayed a substantial stability and an intramolecular ferromagnetic interaction of eight or nine spins.

Full text of DOI:10.1021/ol0346281

ORGANIC
LETTERS
2003
Vol. 5, No. 12
2165-2168
m-Phenylene-Linked Aromatic
Poly(aminium cationic radical)s:
Persistent High-Spin Organic
Polyradicals
Tsuyoshi Michinobu, Jun Inui, and Hiroyuki Nishide*
Department of Applied Chemistry, Waseda UniVersity, Tokyo 169-8555, Japan
nishide@mn.waseda.ac.jp
Received April 11, 2003
ABSTRACT
High molecular weight and networked aromatic polyamines prepared by palladium-catalyzed polycondensation were oxidized to yield the
corresponding poly(aminium cationic radical)s, which displayed a substantial stability and an intramolecular ferromagnetic interaction of eight
or nine spins.
High-spin polyradicals have been extensively investigated
as possible candidates for organic ferromagnetic materials.1
The design of such molecules is based on the precise con-
nectivity of robust spin sources and nondisjointed couplers
in a π-conjugated polymer. It was recently reported that the
highly cross-linked polyradical with a triarylmethine radical
as a spin source showed a very large magnetic moment and
magnetic ordering at low temperature.2 However, the tri-
arylmethine radical can survive only below 200 K, so
chemically stable radical species are desired in such a
networked polyradical to raise the magnetic ordering tem-
perature and provide feasibility as a material. Aromatic
polyamines are an attractive material for this purpose because
the aromatic amines are expected to be a prominent spin
carrying site with both a substantial chemical stability even
at room temperature and large spin polarization after a one-
electron removal.3 A series of oligoradicals have been
synthesized to evaluate the radicals,4,5 and p-phenylenedi-
amine-based cationic radicals were found to be more stable
than the triarylaminium cationic radicals.6 The attempted
integration of the p-phenylenediamine-based cationic radical
into linear and two-dimensional polyradicals was tried;7
however, there has been no report on a successful radical
generation to yield the chemically stable and high-spin poly-
(aminium cationic radical)s. By adjusting the monomer
geometry and the feed ratio during the polycondensation and
utilizing the effect of trifluoroacetic acid (TFA) on aromatic
polyamines, we now prepared, for the first time, two-
dimensional networked and persistent polyradicals, in which
p-phenylenediamine-based cationic radicals effectively in-
teracted through 1,3-phenylene or 1,3,5-benzenetriyl couplers
(1) (a) Lahti, P. M. Magnetic Properties of Organic Materials; Marcel
Dekker: New York, 1999. (b) Itoh, K.; Kinoshita, M. Molecular Magnetism;
Kodansha and Gordon and Breach: Tokyo and Amsterdam, 2000. (c) Miller,
J. S.; Drillon, M. D. Magnetism: Molecules to Materials II; Wiley-VCH:
Weinheim, Germany, 2001. (d) Rajca, A. Chem. Eur. J. 2002, 8, 4834.
(2) (a) Rajca, A.; Rajca, S.; Wongsriratanakul, J. J. Am. Chem. Soc. 1999,
121, 6308. (b) Rajca, A.; Wongsriratanakul, J.; Rajca, S. Science 2001,
294, 1503.
(3) (a) Bushby, R. J.; Ng, K. M. Chem. Commun. 1996, 659. (b) Bushby,
R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. J. Mater. Chem. 1997, 7, 2343.
(c) Bushby, R. J.; Gooding, D. J. Chem. Soc., Perkin Trans. 2 1998, 1069.
(d) Higuchi, M.; Shiki, S.; Yamamoto, K. Org. Lett. 2000, 2, 3079. (e)
Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H. Nature 2002,
415, 509.
10.1021/ol0346281 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/14/2003

to bring about their innate high-spin character with an
average spin quantum number (S) of 8/2 to 9/2.
The networked aromatic polyamines were prepared by
using the palladium-catalyzed amination of two multi-
functionalized monomers (Scheme 1). The bifunctionalized
polymer with a preferably higher molecular weight and
bearing all the terminal bromide groups.8 Although the
reaction of N,N′-diaryl-p-phenylenediamine and 1,3,5-tri-
bromobenzene gave a yellow gel and trace amounts of a
soluble fraction with the number average molecular weight
(Mn) of <3.0 × 103 measured by GPC (THF eluent,
calibrated by polystyrene standards), that of the trifunction-
alized sec-amine monomer and the aryl dibromide yielded
solvent-soluble but high molecular weight polymers, poly-1
and poly-2, with Mn (degree of dispersion) values of 1.6 ×
104 (4.8) and 6.5 × 103 (1.5), respectively. This result
suggests that the monomer geometry is also one of the most
important factors in producing a soluble polymer.
Scheme 1a
The IR and NMR spectra of these soluble polymers
substantiated their structures. In the 13C NMR spectrum of
poly-1, 9 very weak absorptions ascribed to the terminal
triarylamine moieties appeared beside the 15 well-defined
ones in the region of the quaternary carbon adjacent to a
nitrogen atom, which indicated the presence of three kinds
of terminal structures under chemically different environ-
ments (see Supporting Information). The MALDI-TOF mass
spectrum provided further evidence. All observed signals
corresponded to the proton adduct of the polymer with all
bromide terminal groups but not sec-amines, and some of
them were consistent with the formation of cyclic products,
which would lead to the restricted conformation of the
polymer chain. This result supported the idea of three kinds
of terminal structures for this polymer and the formation of
the anticipated networked polymer.
Cyclic voltammetry of the polymers in CH2Cl2 showed
completely reversible oxidation and reduction waves at room
temperature (Figure 1). The generated cationic radicals were
stable enough even in the polymer framework to be further
one-electron oxidized from each p-phenylenediamine site.
a Reagents and conditions: (a) Pd2(dba)3, P(t-Bu)3, NaOtBu,
toluene, 100 °C, 24 h.
monomer and the trifunctionalized monomer were reacted
in toluene at 100 °C for 24 h with Pd2(dba)3 and P(t-Bu)3 as
the catalyst and ligand, respectively. The monomer feed ratio
was maintained at [sec-amine]/[bromide] ) 1/2 so as not to
include any sec-amines as a terminal group whose corre-
sponding radicals are known to be unstable, but to yield a
(4) (a) Yoshizawa, K.; Chano, A.; Ito, A.; Tanaka, K.; Yamabe, T.; Fujita,
H.; Yamauchi, J.; Shiro, M. J. Am. Chem. Soc. 1992, 114, 5994. (b) Stickley,
K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994, 116, 11576. (c) Sasaki,
S.; Iyoda, M. Chem. Lett. 1995, 1011. (d) Sasaki, S.; Iyoda, M. Mol. Cryst.
Liq. Cryst. 1995, 272, 175. (e) Wienk, M. M.; Janssen, R. A. J. Chem.
Commun. 1996, 267. (f) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Takui,
T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. J. Am. Chem.
Soc. 1997, 119, 6607. (g) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor,
N. J. Chem. Soc., Perkin Trans. 2 1997, 1405. (h) Louie, J.; Hartwig, J. F.
Macromolecules 1998, 31, 6737. (i) Selby, T. D.; Blackstock, S. C. J. Am.
Chem. Soc. 1999, 121, 7152. (j) Ito, A.; Taniguchi, A.; Tamabe, T.; Tanaka,
K. Org. Lett. 1999, 1, 741. (k) Selby, T. D.; Blackstock, S. C. Org. Lett.
1999, 1, 2053. (l) Hauck, S. I.; Lakshmi, K. V.; Hartwig, J. F. Org. Lett.
1999, 1, 2057. (m) Michinobu, T.; Takahashi, M.; Tsuchida, E.; Nishide,
H. Chem. Mater. 1999, 11, 1969. (n) Ito, A.; Ono, Y.; Tanaka, K. Angew.
Chem., Int. Ed. 2000, 39, 1072. (o) Selby, T. D.; Stickley, K. R.; Blackstock,
S. C. Org. Lett. 2000, 2, 171. (p) van Meurs, P. J.; Janssen, R. A. J. J. Org.
Chem. 2000, 65, 5712. (q) Ito, A.; Ino, H.; Tanaka, K.; Kanemoto, K.;
Kato, T. J. Org. Chem. 2002, 67, 491.
Figure 1. Cyclic voltammograms and differential pulse voltam-
mograms of poly-1 (1 mM/radical unit) in CH2Cl2 with 0.1 M
(C4H9)4NBF4 at room temperature, at 100 mV s-1 scan rate: (a)
without TFA, (b) with 1 vol % of TFA, and (c) with 5 vol % of
TFA. Potential vs Ag/AgCl.
(5) Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc. 1997, 119, 4492.
(6) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem. 1997,
62, 448.
(7) (a) Goodson, F. E.; Hauck, S. I.; Hartwig, J. F. J. Am. Chem. Soc.
1999, 121, 7527. (b) Bushby, R. J.; Gooding, D.; Vale, M. E. Philos. Trans.
R. Soc. London A 1999, 357, 2939.
2166
Org. Lett., Vol. 5, No. 12, 2003

Differential pulse voltammetry of the polymers displayed
unequivocal waves step by step. For poly-1, the oxidation-
reduction potentials E°′(1) - E°′(3) at 0.78, 0.93, and 1.03
V (vs Ag/AgCl) are attributed to the redox between the amine
and the aminium cationic radical, and their potential differ-
ence reflects a strong spin-exchange interaction between
adjacent radicals. The potentials E°′(4) and E°′(5) at 1.33
and 1.42 V, respectively, represent the subsequent oxidation
from the aminium cationic radical to the bication (Figure
1a). The UV/vis spectra of the electrolytic solution indicated
the generation of the poly(aminium cationic radical) without
any side reaction for all polymers and any deposition of the
poly(bication) for poly-2 (see Supporting Information). The
E°′(4) of poly-2 (1.16 V) was so low that it was easy to
produce the soluble poly-2 containing aminium cationic
radical and some bication moieties, which enabled us to
examine the magnetic property of the polyradical with
various electronic states as will be discussed below.
ported up to now. ESR of the polyradicals at low temperature
clearly showed a ∆Ms ) (2 forbidden transition with a fine
structure, indicating multiplet states at least higher than a
triplet. The temperature dependence of these signals revealed
the ferromagnetic interaction between adjacent radicals and
the multiplet ground state for the polyradicals (Figure 2).
The exact assignment of the potentials to the polymer
structures in CH2Cl2 was ascertained by the addition of TFA
to the electrolyte solution. TFA is known to undergo protona-
tion at the aromatic ring of non-Kekule´ type arylamines.5,9
For poly-1 and poly-2, the addition of 1 vol % of TFA re-
sulted in the anodic shift of E°′(2) and E°′(3), while E°′(1)
did not shift (Figure 1b). The 1,3,5-benzenetriamine is sup-
posed to be protonated prior to the 1,3-benzenediamine moi-
ety because it has more tautomeric intermediates in the pro-
tonated form and has an essentially high nucleophilicity. The
oxidation potential of an amine group adjacent to the proto-
nated moiety would anodically shift due to an electrostatic
repulsion. The addition of 5 vol % of TFA brought about
the anodic shift of the E°′(1) potential besides E°′(2) and
E°′(3) to yield a fused unimodal oxidation wave: This also
supports the protonation of the 1,3-benzenediamine moiety
(Figure 1c). Thus the redox at E°′(1) in CH2Cl2 results from
the p-phenylenediamine residue (1) interposed between two
m-phenylene couplers, and E°′(2) and E°′(3) in CH2Cl2 are
attributable to the stepwise redoxes of the p-phenylenedi-
amines (2-4) connected to 1,3,5-benzenetriamine. This
was also supported by the fact that the overall potentials of
poly-3 and poly-4, which have only 1,3,5-benzenetriyl
couplers, anodically shifted with the addition of only 1 vol
% of TFA.
Figure 2. Temperature dependence of the ∆Ms ) (2 signal
intensity for the polyradicals of poly-1 (O) and poly-2 (4) in
CH2Cl2/CF3COOH/(CF3CO)2O (91/8.5/0.5 vol %). Inset: ESR ∆Ms
) (2 spectrum for the polyradical of poly-2 at 5.6 K.
The magnetization and static magnetic susceptibility of
the polyradicals in the frozen media including TFA were
measured with a SQUID magnetometer. The magnetization
normalized with saturated magnetization, M/Ms, plots for the
polyradicals of poly-1 and poly-2 with a spin concentration
of 0.66 and 0.75, respectively, were very close to the
theoretical Brillouin curve for S ) 10/2 at low fields and
lay on the curve for S ) 7/2 at high fields (Figure 3).11 The
The chemical oxidation of poly-1 and poly-2 with NOPF6
solubilized in CH2Cl2 with a minimum amount of 18-crown-6
also gave the corresponding poly(aminium cationic radical)s.
The ESR spectra at room temperature were composed of a
broad singlet line at g ) 2.003, which is characteristic of
aminium cationic radicals. The addition of a small amount
of TFA to the CH2Cl2 solution dramatically improved the
spin concentration and chemical stability of the polyradi-
cals.10 The half-life of the polyradical of poly-1 estimated
by ESR at room temperature was 1 week in a CH2Cl2/TFA
(95/5 vol %) solution under nitrogen, which is extremely
persistent among the π-conjugated organic polyradicals re-
Figure 3. Normalized plots of magnetization (M/Ms) vs the ratio
of magnetic field and temperature (H/(T - θ)) for the polyradical
of poly-2 with a spin concentration of 0.75 in CH2Cl2/CF3COOH/
(CF3CO)2O (91/8.5/0.5 vol %) at T ) 2.5 (b), 3 (O), and 5 (9) K
and theoretical curves corresponding to the S ) 2/2, 4/2, 6/2, 8/2,
and 10/2 Brillouin functions. Inset:
ømolT vs T plots for the
polyradicals of poly-1 (O) and poly-2 (4) with a spin concentration
of 0.66 and 0.75, respectively.
(8) Flory, P. J. J. Am. Chem. Soc. 1941, 63, 3083.
(9) Glatzhofer, D. T.; Allen, D.; Taylor, R. W. J. Org. Chem. 1990, 55,
6229.
Org. Lett., Vol. 5, No. 12, 2003
2167

spin concentration was determined by the saturated magne-
tization of the weighed polyradical. Plots of the product of
the unit molar magnetic susceptibility (ømol) and temperature
(T) vs T for these polyradicals are shown in the inset of
Figure 3. The ømolT values were much higher than the
theoretical value of 0.375 for S ) 1/2 below 180 K and
increased below 50 and 100 K for the polyradicals of poly-1
and poly-2, respectively, reaching 1.30 for S ) 8.4/2 at 5 K.
This indicates a ferromagnetic coupling, on average, between
eight or nine unpaired electrons in these polymers at low
temperature. The values at higher temperature are supposed
to reflect the effective extension of a π-conjugated system
in each polyradical framework.12
However, the S value seems to be overestimated, taking
into account the cross-conjugated structure of the polyradicals
and the spin concentration. Janssen et al. reported the specific
change in the intervalence band during the oxidation of
N,N′,N′′-tris[4-(diphenylamino)phenyl]-N,N′,N′′-triphenyl-
1,3,5-benzenetriamine or the constituent of the present
polymers,5 from which we can find the possibility that the
corresponding mono- or bis(aminium cationic radical) tends
to disproportionate into the corresponding triradical. If this
is also the case for the 1,3,5-benzenetriamine moiety in
poly-1 and poly-2, the generated radicals will assemble
together to form local spin clusters in the polyradicals. The
electrochemical measurement of the polymers in the presence
of 5 vol % of TFA definitely supported the polyradical
generation via multielectron oxidation in one step. Further-
more, the cyclic structure in the networked polymers would
effectively act to compensate the radical defects with multiple
pathways. To verify these assumptions, 0.4 equiv of NOPF6
was used in the oxidation of poly-2 under the same
conditions. The resulting polyradical showed as high an S
value of 8/2 to 9/2 even with a spin concentration of 0.33 as
the same sample with a spin concentration of 0.75.13 This
result strongly supports the idea of radical assembly to
provide the observed high S value for the polyradicals.
Acknowledgment. This work was partially supported by
Grants-in-Aid for COE Research “Practical Nano-Chemistry”
and “Molecular Nano-Engineering” from MEXT, Japan.
T.M. acknowledges the Research Fellowships of the Japan
Society for the Promotion of Science for Young Scientists.
Supporting Information Available: Synthesis and char-
acterization for the monomers and the polymers, UV/vis
spectra during electrolytic oxidation of the polymers, and
corrections of the magnetic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL0346281
(12) Atomic force microscopy measurement of poly-1 on mica at room
temperature clearly revealed that an average vertical distance of the single
polymer (about 500 samples) decreased from 1.43 to 0.83 nm after the
chemical oxidation, indicating the higher π-coplanarity for the polyradical.
(13) The S value of the polyradical oxidized with 2.0 equiv of NOPF6
in the presence of 9 vol % of TFA reasonably decreased to 3/2 with a spin
concentration of 0.38. The addition of TFA is essential for the present results
and discussion, because the polyradical with a low spin concentration
oxidized in the absence of TFA only displayed S ) 1/2 - 2/2, consistent
with the electrochemical measurements. However, the possibility of the
magnetic impurities in TFA was ruled out from the magnetic measurement
of the background media including TFA.
(10) The chemical oxidation of the polymers in CH2Cl2 often resulted
in the production of a dark gel with a short half-life (e.g., 3 h for the
polyradical of poly-1) even if <1.0 equiv of the oxidant was added. The
position protonated by TFA seems to be overlapped with the spin density
distribution of the aminium cationic radical, which prevents the intra- or
intermolecular dimerization of the radicals.
(11) For a dispersed spin system, the M/Ms plots tend to decrease at
high fields due to low S fractions.
2168
Org. Lett., Vol. 5, No. 12, 2003