Remarkable structure deformation in phenothiazine trimer radical cation

Article abstract of DOI:10.1021/ol048698z

(Chemical Equation Presented) A trimeric phenothiazine and its radical cation were prepared, and their structures were elucidated. In contrast to a largely twisted structure in the neutral species, the radical cation had a unique structure deformation that allowed charge-transfer-type conjugation from the outer phenothiazine rings to the central phenothiazine radical cation.

Full text of DOI:10.1021/ol048698z

ORGANIC
LETTERS
2004
Vol. 6, No. 20
3493-3496
Remarkable Structure Deformation in
Phenothiazine Trimer Radical Cation
Toshihiro Okamoto, Masato Kuratsu, Masatoshi Kozaki, Ken Hirotsu,
Akio Ichimura, Toshio Matsushita, and Keiji Okada*
Department of Chemistry, Graduate School of Science, Osaka City UniVersity,
Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
okadak@sci.osaka-cu.ac.jp
Received July 8, 2004
ABSTRACT
A trimeric phenothiazine and its radical cation were prepared, and their structures were elucidated. In contrast to a largely twisted structure
in the neutral species, the radical cation had a unique structure deformation that allowed charge-transfer-type conjugation from the outer
phenothiazine rings to the central phenothiazine radical cation.
Delocalization and localization of electrons of redox states
in oligomers and dendrimers are important subjects in
materials science.1,2 Many donor- and acceptor-based mo-
lecular assemblies have been synthesized, and in some cases
their redox states have been well characterized using
spectroscopic methods such as UV-vis, EPR, and NMR for
diamagnetic species.1-5 Few studies, however, have provided
clear insight into electron (de)localization in relation to
structure terms for redox states of molecular assembly
systems.6,7 In connection with our previous studies of
phenothiazine radical cations,5a-c we have been interested
in those of higher oligomers. We report that phenothiazine
trimer radical cation 13+ reveals remarkable structure defor-
mation showing a new charge transfer (CT) band in the
visible region.
(5) For oligophenothiazines: (a) Okada, K. In Molecular Magnetism;
Itoh, K., Kinoshita, M., Eds.; Kodansha-Gordon and Breach: Tokyo, 2000;
pp 264-277. (b) Okada, K.; Imakura, T.; Oda, M.; Murai, H.; Baumgarten,
M. J. Am. Chem. Soc. 1996, 118, 3047-3048. (c) Okada, K.; Imakura, T.;
Oda, M.; Kajiwara, A.; Kamachi, M.; Sato, K.; Shiomi, D.; Takui, T.; Itoh,
K.; Gherghel, L.; Baumgarten, M. J. Chem. Soc., Perkin Trans. 2 1997,
1059-1060. (d) Sun, D.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc.
2004, 126, 1388-1401. (e) Kra¨mer, C. S.; Zeitler, K.; Mu¨ller, T. J. J.
Tetrahedron Lett. 2001, 42, 8619-8624. (f) Tsujino, Y. Tetrahedron Lett.
1968, 2545-2550. (g) Diudea, M. V. Tetrahedron Lett. 1982, 23, 1367-
1370.
(6) X-ray structure analyses for lower oligomer or dendrimer charged
or spin systems have been reported; for example: (a) Iyoda, M.; Ogura,
E.; Hara, K.; Kuwatani, Y.; Nishikawa, H.; Sato, T.; Kikuchi, K.; Ikemoto,
I.; Mori, T. J. Mater. Chem. 1999, 9, 335-337. (b) John, D. E.; Moore, A.
J.; Bryce, M. R.; Batsanov, A. S.; Leech M. A.; Howard, J. A. K. J. Mater.
Chem. 2000, 10, 1273-1279. (c) Hotta, S.; Kobayashi, H. Synth. Met. 1994,
66, 117-122. (d) Graf, D. D.; Duan, R. G.; Campbell, J. P.; Miller, L. L.;
Mann, K. R. J. Am. Chem. Soc. 1997, 119, 5888-5899. (e) Kozaki, M.;
Yonezawa, Y.; Okada, K. Org. Lett. 2002, 4, 4535-4538. (f) Fre`re, P.;
Allain, M.; Elandaloussi, E. H.; Levillain, E.; Sauvage, F.-X.; Riou, A.;
Roncali, J. Chem. Eur. J. 2002, 8, 784-792. (g) Yamashita, Y.; Tomura,
M.; Zaman, M. B.; Imaeda, K. Chem. Commun. 1998, 1657-1658. (h) Sedo´,
J.; Ventosa, N.; Ruiz-Molina, D.; Mas, M.; Molins, E.; Rovira, C.; Veciana,
J. Angew. Chem., Int. Ed. 1998, 37, 330-333.
(1) (a) Mu¨llen, K., Wegner, G., Eds. Electronic Materials: The Oligomer
Approach; Wiley-VCH: New York, 1998. (b) Newkome, G. R.; Moorefield,
C. N.; Vo¨gtle, F. Dendrimers and Dendorons; Wiley-VCH: New York,
2001.
(2) For electronic interaction: (a) Nelsen, S. F. Chem. Eur. J. 2000, 6,
581-588. (b) Nelsen, S. F.; Ismagilov, R. F.; Trieber, D. A., II. Science
1997, 278, 846-849. (c) Lambert, C. L.; No¨ll, G. J. Am. Chem. Soc. 1999,
121, 8434-8442.
(3) Geometrical change during oxidation of amino-olefines and -arenes
has been discussed in terms of oxidation potentials: (a) Fritsch, J. M.;
Weingarten, H.; Wilson, J. D. J. Am. Chem. Soc. 1970, 92, 4038-4046.
(b) Nelsen, S. F.; Clennan, E. L.; Echegoyan, L.; Grezzo, L. A. J. Org.
Chem. 1978, 43, 2621-2628.
(4) For oligoanilines: (a) Blackstock, S. C.; Selby, T. D. In Magnetic
Properties of Organic Materials; Lahti, P., Ed.; Marcel Dekker: New York,
1999; pp 165-178. (b) Ito, A.; Ino, H.; Tanaka, K.; Kanemoto, K.; Tato,
T. J. Org. Chem. 2002, 67, 491-498.
(7) X-ray structure analysis for oligo(amino-substituted electron do-
nors): (a) Nelsen, S. F.; Ramm, M. T.; Ismagilov, R. F.; Nagy, M. A.;
Trieber, D. A., II; Powell, D. R.; Chen, X.; Gengler, J. J.; Qu, Q.; Brandt,
J. L.; Pladziewicz, J. R. J. Am. Chem. Soc. 1997, 119, 5900-5907. (b)
Nelsen, S. F.; Ismagilov, R. F.; Gentile, K. E.; Powell, D. R. J. Am. Chem.
Soc. 1999, 121, 7108-7114.
10.1021/ol048698z CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/03/2004

8
Phenothiazine trimer 13 was selectively synthesized
through a cross-coupling reaction9 of Boc-protected 3 with
phenothiazine (Scheme 1).
a
Scheme 1. Synthesis of 13
a Reagents and conditions: (a) (Boc)2O, DMAP/CH3CN, reflux.
(b) Phenothiazine (2 equiv), Pd(dba)2-P(tBu)3-NaOtBu/toluene,
reflux. (c) CH3CO2H, reflux. (d) Bromobenzene (1.1 equiv),
Pd(dba)2-P(tBu)3-NaOtBu/toluene, reflux.
Figure 1. Molecular structure of neutral 13 at 50% ellipsoid level
measured at 173 K. Selected bond lengths (Å) and dihedral angles
(°): a,a′, 1.765(3), 1.763(3); b,b′, 1.401(4), 1.420(4); c,c′,
1.411(4), 1.420(4); A/B ) 31.00, A/E(C8-C9a′′-C10a′′) ) 87.33,
B/F(C2-C9a′-C10a′) ) 86.14, C/D(C4a-C5a-C11) ) 86.09.
The cyclic voltammogram of 13 showed three reversible
oxidation waves, E1/2(1) ) +0.21, E1/2(2) ) +0.50, and
E1/2(3) ) +0.78 V vs Ag/AgCl in benzonitrile. The E1/2(1)
value of 13 was lower than that of a reference of 10-
phenylphenothiazine 6 (+0.38 V) by ∼0.17 V, suggesting a
stabilizing interaction in the radical cation.
Figure 1 shows the molecular structure of neutral 13
determined by X-ray structure analysis.10 The three pheno-
thiazine rings had butterfly structures.11 The averaged bond
angles around the nitrogen atoms deviated slightly from 120°;
∆Rav ) 1.3, 2.3, and 1.7° for the central, left, and right
nitrogen atoms, respectively, indicating p-character for the
nitrogen lone pair orbitals.12 These values are close to that
of 6 (∆Rav ) 0.8°).11b Large dihedral angles were observed
between planes A(B) and E(F), where plane E(F) is defined
by three nitrogen-attached carbon atoms: C8, C9a′′, and
C10a′′ (C2, C9a′, and C10a′). The angles were ∼87° for A/E
and ∼86° for B/F, sufficiently large to prohibit conjugation
between the phenothiazine rings. In fact, the UV-vis
spectrum of 13 was very similar to that of 6 but with ca.
triple the intensity (λmax ) 256 nm, ꢀ × 10-4 M-1 cm-1:
4.45 for 6, 15.2 for 13), indicating that 13 behaves like three
noninteracting molecular assemblies in the neutral state.
In contrast to the neutral state, the oxidized state showed
unique characteristics. We isolated 13+ using the method of
Giffard and co-workers13 through counterion exchange using
(8) Compound data for 13 and 13+. 13: yellow needles; mp 277.0-279.0
°C; 1H NMR (400 MHz, C6D6) δ 6.19 (d, 2H, J ) 8.8 Hz), 6.27 (d, 4H, J
) 7.3 Hz), 6.51 (dd, 2H, J ) 8.8, 2.4 Hz), 6.56-6.64 (m, 9H), 6.73 (d,
2H, J ) 2.2 Hz), 6.92 (dd, 4H, J ) 7.1 Hz, 1.7 Hz), 7.01-7.11 (m, 4H);
IR (KBr, cm-1) 1595, 1570, 1460, 1441, 1298, 1259, 1234, 1128, 1119,
1043, 924, 745, 698, 669, 546; HRMS (FAB) m/z calcd for C42H27N3S3
669.1367, found 669.1382. 13+‚PF6-‚0.5CH3CN: black plates, mp (de-
comp.) ca. 257 °C; EPR g ) 2.0041 (ν0 ) 9.2455 three broad lines with
aN ) 0.55 mT in butyronitrile at room temperature); IR (KBr, cm-1) 1595,
1568, 1474, 1460, 1445, 1281, 1263, 1246, 1165, 1128, 1076, 1032, 843,
766, 737, 698, 669, 637, 557; MS (FAB) 669 (C42H27N3S3+); Anal. Calcd
for 13+‚PF6-‚0.5CH3CN: C 61.82, H 3.44, N 5.87. Found C 62.01, H 3.33,
N 5.90.
-
TBA+PF6 . The formula for the salt obtained from crystal-
+
-
lization from acetonitrile was determined to be 13 ‚PF6 ‚
0.5CH3CN from the elemental analysis.
Figure 2 shows the UV-vis absorption spectrum of 13 .
+
A strong absorption band newly appeared at λmax ) 945 nm
(ꢀ ) 2.83 × 104). The spectrum is totally different from that
of 6+ (ꢀ517 ) 9.54 × 103),14 suggesting a new electronic state
(9) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.;
Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580.
+
of 13 on the basis of the trimeric structure.
(10) Crystallographic data for 13‚CH2Cl2: colorless plate, triclinic, space
group P-1 (#2), a ) 10.713(1) Å, b ) 12.332(2) Å, c ) 15.377(3) Å, R )
82.11(1)°, â ) 82.15(1)°, γ ) 62.851(8)°, V ) 1784.2(4) Å3, Z ) 2, Fcalcd
) 1.383 g/cm3, 2θmax ) 55.0°, Mo KR radiation (0.71070 Å), T ) 173 K,
17 451 measured reflections, 7649 unique reflections, 7428 observed
Figure 3 shows the molecular structure determined by
X-ray analysis.10 The acetonitrile solvent could not be
-
observed because of large disorders in the PF6 counterion
+
parts (not shown). The conformation of 13 was distinctly
reflections (I > -10σ, all data), 489 parameters, R1(I > 2σ) ) 0.078, Rw(I
+
+
> -10σ) ) 0.173, GOF ) 0.926. Crystallographic data for 13
‚
different from that of 13. The central phenothiazine of 13
PF6-‚0.5CH3CN: black plate, monoclinic, space group C2 (#5), a ) 27.60-
(2) Å, b ) 9.129(8) Å, c ) 15.39(1) Å, â ) 94.65(1)°, V ) 3864.7(6) Å3,
Z ) 4, Fcalcd ) 1.400 g/cm3, 2θmax ) 55.0°, Mo KR radiation (0.71070 Å),
T ) 113 K, 15 203 measured reflections, 4636 unique reflections, 4515
observed reflections (I > -10σ, all data), 562 parameters, R1(I > 2σ) )
0.087, Rw(I > -10σ) ) 0.234, GOF ) 0.934. CCDC 239333 (13) and
CCDC 239334 (13+).
was almost flat, whereas the outer two rings had butterfly
structures as observed in the neutral 13, indicating that the
radical cation is localized mainly on the central ring.15 The
(13) Giffard, M.; Mabon, G.; Leclair, E.; Mercier, N.; Allain, M.;
Gorgues, A.; Molinie´, P.; Neilands, O.; Krief, P.; Khodorkovsky, V. J. Am.
Chem. Soc. 2001, 123, 3852-3853.
(14) Wagner, E.; Filipek, S.; Kalinowski, M. K. Monatsh. Chem. 1988,
119, 929-932.
(11) (a) McDowell, J. J. H. Acta Crystallogr. 1976, B32, 5-10. (b)
Moerman, B.; Ouwerkerk, M.; Kroon, J. Acta Crystallogr. 1985, C41,
1205-1208.
(12) Nelsen, S. F. J. Org. Chem. 1984, 49, 1891-1897.
3494
Org. Lett., Vol. 6, No. 20, 2004

Figure2. UV-visspectrumof13+: solidlinefor13+‚PF6-‚0.5CH3CN
-
and dashed line for 6+‚PF6 in methylene chloride.
averaged bond angle around the central nitrogen atom was
120° (∆Rav ) 0.0°), whereas those of the outer nitrogen
atoms were a little higher, ∆Rav ) 1.3° for the left and 0.2°
for the right nitrogen atoms. More importantly, the dihedral
angles between planes A(B) and E(F) were remarkably small
+
for 13 , 11° for A/E and 5° for B/F (Figure 3), enabling
orbital interaction between the p-orbitals of the C2(C8) atoms
and the lone pair orbitals of the nitrogen atoms (Figure 3,
side view). The advantage of this rather surprising structure
+
is conjugated stabilization of 13 without severe steric
Figure 4. Molecular orbitals of HOMO (â) and LUMO (â) for
+
repulsion between the phenothiazine rings. The conceivable
radical cation localized on the outer ring(s) obviously cannot
achieve this stabilization because of the large dihedral angle
between the outer phenothiazine radical cation and the benzo-
ring of the central phenothiazine, as recognized from the large
dihedral angle between planes C and D (C4a-C5a-C11
plane). Electron removal from the central phenothiazine ring
13 (TD-UB3LYP/6-31G*).
must be facilitated by the simultaneous or rapid bond rotation
around the C2,8-N(outer phenothiazines), giving stabilized
+
13 , compatible with the lower and reversible oxidation
potential of 13.
The structural feature described above suggests that the
observed new absorption at 945 nm is an intramolecular CT
band from the outer phenothiazines to the inner radical cation
through the nitrogen lone pair orbitals. To clarify this, we
performed theoretical calculations [TD-UB3LYP/6-31G*]
using the Gaussian 03 program package.16 The geometry was
taken from the X-ray analysis. The calculated absorption
maximum was 954 nm with an oscillator strength f ) 0.204
for the first excited state; the absorption had a main
(15) (a) Singhabhandhu, A.; Robinson, P. D.; Fang, J. H.; Geiger, W.
E., Jr. Inorg. Chem. 1975, 14, 318-323. (b) Von Dreele, R. B.; Harris, J.
Acta Crystallogr. 1983, C39, 170-173.
(16) In the TD calculation, 50 highest occupied and 50 lowest vacant
orbitals were considered. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.;
Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;
Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision B.01; Gaussian, Inc.: Pittsburgh, PA, 2003.
+
Figure 3. Molecular structure of 13 at 50% ellipsoid level
determined at 113 K. Selected bond lengths (Å) and dihedral angles
(°); a,a′, 1.744(9), 1.727(10), b,b′, 1.411(13), 1.424(12), c,c′,
1.395(11), 1.398(11); A/B ) 2.76, A/E(C8-C9a′′-C10a′′) ) 10.85,
B/F(C2-C9a′-C10a′) ) 5.39, C/D(C4a-C5a-C11) ) 82.44.
Org. Lett., Vol. 6, No. 20, 2004
3495

contribution of HOMO (â) f LUMO (â) transition with a
coefficient of 0.91 (Figure 4), compatible with the assignment
of the CT-band from the outer to inner phenothiazine rings.
In summary, we have demonstrated that the neutral
trimeric phenothiazine 13 can be approximated as three
noninteracting phenothiazines, whereas the oxidized 13
receives a surprisingly large geometrical change and behaves
as a conjugated trimeric phenothiazine. Further study,
including higher oligomers and dendrimers and the role of
13 structure in those assemblies, is in progress.
+
Supporting Information Available: Synthetic proce-
+
dure for 13 and 13 and X-ray crystallographic data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
+
OL048698Z
3496
Org. Lett., Vol. 6, No. 20, 2004