2,2′:6′,2″:6″,6-Trioxytriphenylamine: Synthesis and properties of the radical cation and neutral species

Article abstract of DOI:10.1002/anie.200500397

A flat radical: Oxygen-bridged triphenylamine 1 and its radical cation 2 were prepared. The neutral compound has a shallow bowl structure, whereas the radical cation is planar. The properties of the highly stable radical cation were clarified; such compou

Full text of DOI:10.1002/anie.200500397

Communications
Heterocyclic Chemistry
2,2’:6’,2’’:6’’,6-Trioxytriphenylamine: Synthesis
and Properties of the Radical Cation and Neutral
Species**
Masato Kuratsu, Masatoshi Kozaki, and Keiji Okada*
A planar stable triphenylamine radical cation is a fascinating
p-electron system that is potentially applicable to electronic
and magnetic materials. Hellwinkel and co-workers synthe-
sized an interesting compound 1,[1] from which the radical
Scheme 1. Reaction conditions: a) 2-bromo-3-methoxyphenol, NaH/
DMSO, 1308C; b) 2-bromo-3-fluorophenol, NaH/DMSO, 1308C; c) hy-
drazine hydrate, Pd/C, p-bromophenol/ethanol, reflux; d) NaOtBu, [Pd-
(dba)2], P(tBu)3/toluene, reflux; e) BBr3/CH2Cl2, ꢀ788C!room tem-
perature; f) K2CO3/DMF, room temperature. dba=trans,trans-dibenzyl-
ideneacetone.
+
cation 1C was generated in concentrated sulfuric acid[1a] or by
oxidation with lead tetraacetate in trifluoroacetic acid.[1c] The
+
triphenylamine framework of 1C is most likely planar. The
dimethylmethylene bridges contribute to its stabilization;
however, they disrupt the CT-type intermolecular interaction
that is crucial for the construction of electronic and magnetic
materials. To overcome this disadvantage and to improve
stability, we designed a synthetic route to the new oxygen-
bridged analogue 2.[2,3] Quite recently, Livant and co-workers
described a product fraction showing a molecular ion of m/z =
287 in the thermolysis of tris(2,6-dimethoxyphenyl)amine.
They assumed the structure 2 for the MS peak. However, the
compound showed no NMR spectroscopic signal.[4] Herein,
we report the preparation, structures, and properties of the
substitution of 7 in DMF with K2CO3 as a base proceeded
efficiently under remarkably mild conditions to give the
desired compound 2 in good yield. Compound 2 had a
reversible oxidation wave at + 0.59 V versus SCE in DMF
(SCE = saturated calomel electrode). The chemical oxidation
of 2 was performed by using tris(p-bromophenyl)aminium
hexafluorophosphate as an oxidant in methylene chloride.
+
ꢀ
The salt 2C ·PF6 can be recrystallized from acetonitrile/
diethyl ether.
+
neutral 2 and the radical cation 2C .
Figure 1 shows the molecular structures determined by X-
ray crystallographic analysis of 2 and 2C+.[6] The neutral
compound 2 has a shallow bowl structure, whereas the radical
The synthesis of 2 is outlined in Scheme 1. Sequential
aromatic nucleophilic substitution reactions of 2,6-difluoro-
nitrobenzene with 2-bromo-3-methoxyphenolate and then
with 2-bromo-3-fluorophenolate gave 4 (71% yield in two
steps). The reduction of 4 proceeded selectively in the
presence of p-bromophenol (10 equiv) to avoid a competing
reduction of aromatic bromide. Intramolecular cyclization of
5 was performed under Pd0-mediated cross-coupling reaction
conditions[5] to afford 6 in 35% yield. Treatment of 6 with
BBr3 gave 7 in good yield. Intramolecular nucleophilic
Figure 1. Molecular structures of 2 and 2C+ drawn at 50% ellipsoid
level; hydrogen atoms are removed for clarity. Selected bond lengths
[ꢀ] and angles [8]. a) For 2 measured at 123 K: a=1.408(2),
b=1.403(3), c=1.412(3); a=115.3(2), b=115.6(2), g=115.7(2).
b) For 2C+ measured at 113 K: a=1.376(3), b=1.377(3), c=1.378(3);
a=120.1(2), b=119.9(2), g=120.0(2).
[*] M. Kuratsu, Dr. M. Kozaki, Prof. K. Okada
Graduate School of Science, Osaka City University
Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan)
Fax: (+81)6-6690-2709
+
ꢀ
cation 2C has a planar structure. The C N bond lengths
become shorter and the C-N-C bond angles approach 1208 in
the radical cation 2C . The bond-length difference is in
qualitative accordance with the HOMO shape of 2; that is,
the C N and C O bonds have an antibonding nature in the
HOMO.
+
E-mail: okadak@sci.osaka-cu.ac.jp
[**] This work was partially supported by a research fund from Chisso
Corporation, Japan. We thank Professor Stephen F. Nelsen for
valuable discussions.
ꢀ
ꢀ
Triphenylamine radical cations without para substituents
are generally unstable because of the large spin densities of
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4056
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200500397
Angew. Chem. Int. Ed. 2005, 44, 4056 –4058

Angewandte
Chemie
+
the para positions.[7] For 2C , neither dimerization tendency in
strength of 0.01 in both cases); Figure 4).[8,9] The observed
the crystal structure nor oxygenation under aerated condi-
tions in solution was observed. The spin delocalization in a
whole molecule involving the oxygen atoms (phenoxazine
absorptions with a rather broad slope in the region of 500–
700 nm can be attributed to vibrational fine structure of the
degenerate band. The peak separation of 1370 cmꢀ1 is close to
that observed for the 10-phenylphenoxazine radical cation
(1270 cmꢀ1).
+
radical cation structure) contributes to the stability of 2C . The
+
radical cation 2C exhibited a well-resolved EPR spectrum.
+
The hyperfine coupling constants for 2C (g = 2.0031, aN =
0.90, aH = 0.26, and aH = 0.06 mT) in butyronitrile at room
p
m
+
temperature were slightly smaller than those for 1C (g =
2.0027, aN = 0.95, aH = 0.30, and aH = 0.07 mT).[1c] The
p
m
smaller spin density values for 2C+ are clearly related to the
positive spin density of the oxygen atoms. The calculated spin-
density map and values are shown in Figure 2.[8]
+
Figure 4. MOs and the energy diagram (UHF) of 2C .
These studies clearly establish that 1) the neutral com-
pound 2 is a good electron donor with a shallow bowl
structure and 2) the radical cation 2C is a highly stable planar
Figure 2. The spin-density map (black: positive spin, white: negative
spin) and values (UB3LYP/6-31G*) for 2C+.
+
species. Moreover, the neutral compound 2 can form stable
CT complexes with several electron acceptors, suggesting
applicability to electronic and magnetic materials. We are
particularly interested in forthcoming projects to follow up
this work. The introduction of stable radicals at the para
positions followed by the formation of CT complexes with
electron acceptors would provide a magnetic CT-salt mate-
rial.[10] Furthermore, the small geometrical change between
the neutral and radical cation states may allow photostimu-
lated phase transition to the photoconductive state 22C+.[11] The
preparation of such materials is in progress.
+
The radical cation 2C had broad absorptions in the visible
region (l = 709 and 785 nm, Figure 3). These absorptions can
be related to the electronic transitions from the occupied
Experimental Section
The synthesis of compounds 2–7 and the EPR spectrum for 2C+ with its
simulation are described in the Supporting Information.
2C+·PF6ꢀ: Compound 2 (25.0 mg, 0.0870 mmol) was dissolved in
dichloromethane (20 mL). A solution of tris(p-bromophenyl)ami-
nium hexafluorophosphate (54.8 mg, 0.0874 mmol in CH2Cl2
(10 mL)) was added dropwise at room temperature with stirring in
a glove box. After 30 min, the solvent was evaporated under vacuum
and the crude product was recrystallized from acetonitrile/diethyl
ether to give 2C+ as deep green plates (29.4 mg, 78%); m.p. ꢁ 278.08C
(decomp). MS: m/z (%): FAB+ (m-NBA) 287 (100) [C18H9NO3+],
FABꢀ (m-NBA) 145 (100) [PF6ꢀ]. Elemental analysis calcd (%) for
Figure 3. UV/Vis absorptions of 2 (a) and 2C+ (c) in dichlorome-
thane.
+
ꢀ
2C ·PF6 (C18H9F6NO3P): C 50.02, H 2.10, N 3.24; found: C 50.12,
H 2.00, N 3.21.
Received: February 2, 2005
Published online: May 31, 2005
MOs delocalized on the outer electron-rich trioxytris(1,3-
phenylene) moieties to the delocalized unoccupied orbital
involving the nitrogen cation. The theoretical calculation
(TD-DFT/6-31G*) roughly reproduced this situation (lcalcd
=
729 nm as a degenerate transition with the main contribution
( ꢁ 90%) of HOMO-1(b), HOMO(b)!LUMO(b) (oscillator
Keywords: amines · EPR spectroscopy · heterocycles · oxidation ·
radical ions
.
Angew. Chem. Int. Ed. 2005, 44, 4056 –4058
www.angewandte.org
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4057

Communications
[1] a) D. Hellwinkel, M. Melan, Chem. Ber. 1974, 107, 616 – 626;
b) D. Hellwinkel, M. Melan, Chem. Ber. 1971, 104, 1001 – 1016;
c) S. Bamberger, D. Hellwinkel, F. A. Neugebauer, Chem. Ber.
1975, 108, 2416 – 2421; d) D. Hellwinkel, W. Schmidt, Chem. Ber.
1980, 113, 358 – 384; e) J. E. Field, D. Venkataraman, Chem.
Mater. 2002, 14, 962 – 964.
[2] M. Kuratsu, M. Kozaki, K. Okada, Chem. Lett. 2004, 33, 1174 –
1175.
[3] A bowl-shaped trioxatriphenylphosphine analogue has been
reported: a) F. C. Krebs, P. S. Larsen, J. Larsen, C. S. Jacobsen, C.
Boutton, N. Thorup, J. Am. Chem. Soc. 1997, 119, 1208 – 1216;
b) G. K. H. Madsen, F. C. Krebs, B. Lebech, F. K. Larsen, Chem.
Eur. J. 2000, 6, 1797 – 1804.
[4] P. D. Livant, D. J. D. Northcott, Y. Shen, T. R. Webb, J. Org.
Chem. 2004, 69, 6564 – 6571.
[5] J. F. Hartwig, M. Kawatsura, S. I. Hauck, K. H. Shaughnessy,
L. M. Alcazar-Roman, J. Org. Chem. 1999, 64, 5575 – 5580.
[6] Crystallographic data for 2: crystal size 0.40 ꢀ 0.30 ꢀ 0.20 mm,
monoclinic, space group P21 (#4), a = 10.0772(13) ꢁ, b =
6.8136(6) ꢁ,
c = 10.0712(13) ꢁ,
b = 119.964(4)8,
V=
599.08(12) ꢁ3, Z = 2, 1calcd = 1.592 gcmꢀ3, 2qmax = 55.08, MoKa
radiation (0.71070 ꢁ), T= 123 K, m = 1.10 cmꢀ1, Tmin = 0.977,
T
max = 0.989, 4450 measured reflections, 1412 unique reflections,
1367 observed reflections (I > ꢀ10s, all data), 208 parameters,
R1(I>2s) = 0.035, Rw(I>2s) = 0.044, GOF = 1.004; Crystallo-
graphic data for 2C+·PF6ꢀ: crystal size 0.30 ꢀ 0.20 ꢀ 0.10 mm,
orthorhombic, space group Pbca (#61), a = 15.5415(10) ꢁ, b =
13.4237(8) ꢁ, c = 15.5653(10) ꢁ, V= 3247.3(4) ꢁ3, Z = 8, 1calcd
=
1.768 gcmꢀ3, 2qmax = 55.08, MoKa radiation (0.71070 ꢁ), T=
113 K, m = 2.58 cmꢀ1, Tmin = 0.953, Tmax = 0.980, 25720 measured
reflections, 3708 unique reflections, 2763 observed reflections
(I > ꢀ10s, all data), 271 parameters, R1(I>2s) = 0.043, Rw-
(I>2s) = 0.047, GOF = 1.030. CCDC 262144 (2) and CCDC
262145 (2C+) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
[7] E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S. Marcoux, D. W. Leedy,
R. N. Adams, J. Am. Chem. Soc. 1966, 88, 3498 – 3503.
[8] The calculation was performed at the optimized geometry
starting from the X-ray crystallographic structure with the
Gaussian03 program package.
[9] A better lcalc value (753 nm) was obtained by using the NCG
methodology with B3LYP/6-31G*: S. F. Nelsen, A. E. Konrads-
son, J. P. Telo, J. Am. Chem. Soc. 2005, 127, 920 – 925, and
references therein.
[10] S. Hiraoka, T. Okamoto, M. Kozaki, D. Shiomi, K. Sato, T. Takui,
J. Am. Chem. Soc. 2004, 126, 58 – 59.
[11] M. Chollet, L. Guerin, N. Uchida, S. Fukaya, H. Shimoda, T.
Ishikawa, K. Matsuda, T. Hasegawa, A. Ota, H. Yamochi, G.
Saito, R. Tazaki, S. Adachi, S. Koshihara, Science 2005, 307, 86 –
89.
4058
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2005, 44, 4056 –4058