Synthesis and characterization of carbazole-based expanded thiaporphyrins

Article abstract of DOI:10.1002/chem.201202573

Carbazole-based porphyrins: Oligothiophene-bridged carbazole dimers and trimers were synthesized as new expanded thiaporphyrins through a sequential coupling and annulation strategy. The structures and electronic properties of these new macrocycles were investigated by spectroscopic methods and DFT calculations and they have been found to exhibit NIR absorption upon NOSbF ₆ oxidation (see figure). Copyright

Full text of DOI:10.1002/chem.201202573

COMMUNICATION
DOI: 10.1002/chem.201202573
Synthesis and Characterization of Carbazole-Based Expanded
Thiaporphyrins
Motoki Masuda and Chihiro Maeda*[a]
Expanded porphyrins, composed of more than four pyr-
Among these, the tetrabenzo-fused core-modified porphyrin
1b exhibits distinct aromaticity and NIR absorption due to
extended p conjugation over the entire macrocycle. Follow-
ing this work, we investigated the ring expansion of such
core-modified porphyrins in order to obtain expanded por-
phyrins with extended conjugation systems. Herein, we
report the synthesis and characterization of oligothiophene-
bridged carbazoles as novel carbazole-based expanded thia-
porphyrins.
The synthetic procedures used in this work are shown in
Scheme 2. A combination of both coupling and annulation
reactions played a key role in the synthesis of these oligo-
thiophene-bridged species. Bisthiophene-bridged carbazoles
were synthesized through Stille coupling. A solution of 3,6-
di-tert-butyl-1,8-dibromocarbazole (9)[13] and bis(tributyl-
stannyl)bisthiophene in toluene in the presence of a catalyt-
roles or other equivalent subunits, have received considera-
ble attention as a result of their unique properties.[1] Of par-
ticular interest is the extended p conjugation of these mac-
rocycles, as compared to regular porphyrins, which may lead
to the application of such compounds as photosensitizers[2]
and near-infrared (NIR) dyes[3] as well as in photodynamic
therapy[4] and nonlinear optical devices.[5] The conjugation
system within a porphyrin may be extended by the introduc-
tion of ethynyl groups[6] or by the fusion of aromatic rings to
the porphyrin core.[7] However, such peripheral modifica-
tions have rarely been applied to expanded porphyrins.[8]
Carbazole-based materials have also been extensively
studied due to their unique properties. They can be highly
emissive and electron conducting and are also both chemi-
cally stable and capable of polymerization and metal-cata-
lyzed cross-coupling.[9] Because a carbazole is simply a ben-
zene-fused pyrrole, the incorporation of carbazole units into
fused porphyrins presents interesting possibilities. Despite
the potential usefulness of such carbazole-based porphyri-
noids, only a few examples have been reported to date.[10–12]
Recently, we reported a multiple annulation strategy, which
allows the synthesis of novel porphyrinoids from 1,3-buta-
diyne-bridged cyclic carbazole oligomers (Scheme 1).[10a]
ic amount of [PdACHTNUGTRNEG(UN PPh3)4] was heated to reflux for 12 h.
After separation by gel permeation chromatography, the bis-
thiophene-bridged carbazole dimer 3 and the trimer 4 were
obtained in 12 and 7.6% yields, respectively. Trithiophene-
bridged carbazoles, however, were prepared in a manner
similar to the synthesis of compound 1a. The Glaser cou-
pling reaction of 3,6-di-tert-butyl-1,8-bis(5-ethynylthien-2-
yl)carbazole (10) and a subsequent annulation reaction with
Na2S[14] provided the trithio-
phene-bridged carbazole dimer
5 and the trimer 6. In addition,
the carbazole dimers 7 and 8,
incorporating both thiophene
and bisthiophene or trithio-
phene linkages, were also pre-
pared. The intramolecular cycli-
zation of compounds 11 and 12
were achieved by Glaser cou-
Scheme 1. Synthesis of the carbazole-based porphyrins 1a, 1b, and 2.
pling under dilute conditions,
resulting in either compound 13
or compound 14, respectively. Finally, the annulation reac-
tions of 13 and 14 with Na2S provided compounds 7 and 8.
The structures of these compounds were characterized by
both spectroscopic and X-ray diffraction analyses. The
1H NMR spectra of these macrocycles were fully consistent
with their proposed structures, with the exception of com-
pound 5. At room temperature, the NMR spectrum of this
compound consists of rather broad, unresolved signals,
whereas at 1408C in [D2]tetrachloroethane, a single set of
peaks is evident (see the Supporting Information). These re-
[a] M. Masuda, Dr. C. Maeda
Department of Applied Chemistry
Faculty of Science and Technology
Keio University
Kohoku-ku, Yokohama 223-8522 (Japan)
Fax : (+81)45-566-1551
E-mail: cmaeda@applc.keio.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202573.
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The UV/Vis absorption and
fluorescence spectra of com-
pounds 1a, 2, and 3–8 are pre-
sented in Figure 2 and summar-
ized in Table 1. The regions of
the maximum UV/Vis absorp-
tion for compounds 1a, 3, and 5
are relatively similar, although
the emission maxima of those
compounds show a distinct red
shift going from 1a to 3 to 5 as
the number of thiophene
spacers increases. In the case of
the carbazole trimers, both ab-
sorption and fluorescence spec-
tra show a red shift going from
2 to 4 to 6. The spectrum of
compound 7 is slightly red shift-
ed and broadened in compari-
son to that of compound 1a,
whereas that of compound 8 ex-
hibits an additional red-shifted
band at l=455 nm. The quan-
tum yields are reduced going
from 1a to 3 to 5, whereas the
other products simply display
moderate yields with no distinct
pattern.
Cyclic voltammetry of these
products was carried out and
the resulting oxidation poten-
tials are summarized in Table 2.
The cyclic voltammograms ex-
hibited quasi-reversible oxida-
tion potentials at approximately
Scheme 2. Synthesis of compounds 3–8.
sults suggest restricted rotation of the single bond linkages
in 5 at ambient temperature and free rotation at higher tem-
peratures. Slow vapor diffusion of methanol into a solution
of 3 in dichloromethane provided crystals qualitatively suita-
ble for analysis. X-ray diffraction analysis provided unam-
biguous elucidation of the structure of compound 3, indicat-
ing that the four sulfur atoms are oriented towards the inte-
rior of the macrocycle (Figure 1a).[15a] The lattice contains
two independent molecules of compound 3, which are coor-
dinated through CH–p interactions (Figure S29 in the Sup-
porting Information). The dihedral angles between the two
carbazole planes are 37.1 and 13.28, respectively. The struc-
ture of the cyclic compound 13 in the solid state was also
confirmed by X-ray diffraction analysis (Figure 1b).[15b] This
lattice also contains two independent molecules of com-
pound 13, although the molecules are stacked through p–p
interactions (Figure S30 in the Supporting Information). The
dihedral angles between the two carbazole planes in this
species are 16.9 and 16.28 and the mean plane deviations are
0.197 and 0.115 ꢁ, indicating a highly planar structure.
Figure 1. X-ray crystal structures of a) compound 3 and b) compound 13;
top view (top) and side view (bottom). tert-Butyl groups and hydrogen
atoms (except for NH protons) are omitted for clarity. The thermal ellip-
soids are at the 50% probability level.
0.5 V except for compound 8, which shows a lower potential
of 0.26 V. DFT calculations were performed to further clari-
fy the structures and electronic properties of these com-
pounds.[16] Product 8 possesses the highest HOMO level,
consistent with the cyclic voltammetry results. In addition,
the LUMO level of 8 is exceptionally stabilized, resulting in
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Chem. Eur. J. 2013, 19, 2971 – 2975

Carbazole-Based Expanded Thiaporphyrins
COMMUNICATION
Table 2. First oxidation potentials versus ferrocene/ferrocenium (Fc/Fc+)
(Eox1),[a] calculated HOMO and LUMO levels,[b] and optical HOMO–
LUMO gaps (DEopt
)
Eox1
[V]
HOMO
[eV]
LUMO
[eV]
DEHOMO—LUMO
[eV]
DEopt
[eV]
1a 0.47
À4.78
À5.03
À4.94
À4.99
À4.87
À4.88
À4.92
À4.76
À1.56
À1.76
À1.59
À1.77
À1.85
À1.95
À1.74
À2.08
3.22
3.27
3.35
3.22
3.02
2.93
3.18
2.68
3.08
3.21
3.05
3.10
2.96
3.00
3.05
2.76
2
3
4
5
6
7
8
0.56
0.49
0.51
0.49
0.35
0.64
0.26
[a] Determined by cyclic voltammetry; solvent: CH2Cl2, supporting elec-
trolyte: Bu4NPF6 (0.10m), scan rate: 0.05 VsÀ1. [b] Calculated at the
B3LYP 6-31G* levels.
Supporting Information). The electron coefficients for the
LUMO levels of compounds 7 and 8 are practically localized
at the bisthiophene moiety and the trithiophene moiety, re-
spectively. These electrochemical results indicate that the
broadened bands in the absorption spectrum of product 7,
as well as the additional band at l=455 nm observed for
compound 8, may be considered as resulting from intramo-
lecular electron transfer.
Finally, the chemical oxidation of the dicarbazoles 3, 5, 7,
and 8 was investigated. In contrast to earlier results with 1a,
which was oxidized by MnO2 to give 1b, no changes were
observed following the attempted oxidation of compounds
3, 5, 7, and 8 with MnO2.[17] Treatment of 1a, 3, 5, 7, and 8
with excess of NOSbF6, however, produced dramatic
changes, with the reaction mixtures displaying NIR absorp-
tion bands in the region of l=1000–2000 nm (Figure 3). In
Figure 2. UV/Vis absorption (left axis) and fluorescence (right axis) spec-
tra in CH2Cl2. a) Compounds 1a, 3, and 5. b) Compounds 2, 4, and 6.
c) Compounds 1a, 7, and 8.
Table 1. Selected photophysical properties of compounds 1a and 2–8 in
CH2Cl2
[b]
lA
lF
[nm][a]
FF
[nm]
1a
2
3
4
5
6
7
8
315, 403
320, 386
329, 338, 407
312, 356, 400
318, 368,[d] 419
322,[d] 413
448
432
0.271[c]
0.144[c]
0.144
0.311
0.065
0.207
0.370
0.139
469, 498
464, 494
503, 532
494, 518
502
318, 406
339, 380, 408, 455[d]
513, 541[d]
[a] Excitation wavelengths are l=360 nm. [b] Fluorescence quantum
yields were determined with reference to the value of 1a (0.271) in
CH2Cl2. [c] Absolute fluorescence quantum yields with the excitation at
l=360 nm. [d] Shoulder.
Figure 3. UV/Vis/NIR absorption spectra with NOSbF6 in CH2Cl2. The
inset shows the ESR spectrum of compound 3 with NOSbF6 in CH2Cl2.
The asterisk marks vibrational overtones of the solvent.
a narrow calculated HOMO–LUMO gap, which is also con-
sistent with the results of absorption spectroscopy. The
HOMO levels of these core-modified porphyrins exhibit
electronic coefficients on the whole macrocycles, whereas
the LUMO levels exhibit electronic coefficients predomi-
nantly on the thiophene moieties, which becomes more
clear as the number of thiophene units increases, suggesting
that such oligothiophenes will become stronger acceptors as
the number of thiophene units increases (Figure S25 in the
this case, the spectrum of 1a is similar to that obtained pre-
viously for 1b, exhibiting a Q-like band, whereas the spectra
of compounds 3, 5, 7, and 8 are instead relatively broad. Im-
portantly, the spectrum resulting from 1a with NOSbF6
agrees with that obtained for a combination of 1b with tri-
fluoroacetic acid (TFA) (Figure S27 in the Supporting Infor-
mation), indicating that reaction with NOSbF6 results in the
two-electron oxidation of 1a to afford the protonation state
of 1b.[18] The observation that the ESR spectrum of the re-
Chem. Eur. J. 2013, 19, 2971 – 2975
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2973

M. Masuda and C. Maeda
[7] a) S. Fox, R. W. Boyle, Tetrahedron 2006, 62, 10039, and references
therein; b) H. S. Gill, M. Harmjanz, J. Santamarꢃa, I. Finger, M. J.
Scott, Angew. Chem. 2004, 116, 491; Angew. Chem. Int. Ed. 2004, 43,
485; c) O. Yamane, K. Sugiura, H. Miyasaka, K. Nakamura, T. Fuji-
moto, K. Nakamura, T. Kaneda, Y. Sakata, M. Yamashita, Chem.
Lett. 2004, 33, 40; d) A. N. Cammidge, P. J. Scaife, G. Berber, D. L.
Hughes, Org. Lett. 2005, 7, 3413; e) E. Hao, F. R. Fronczek,
M. G. H. Vicente, J. Org. Chem. 2006, 71, 1233; f) K. Kurotobi, K. S.
Kim, S. B. Noh, D. Kim, A. Osuka, Angew. Chem. 2006, 118, 4048;
Angew. Chem. Int. Ed. 2006, 45, 3944; g) N. K. S. Davis, A. L.
Thompson, H. L. Anderson, J. Am. Chem. Soc. 2011, 133, 30.
[8] a) H. Hata, H. Shinokubo, A. Osuka, Angew. Chem. 2005, 117, 954;
Angew. Chem. Int. Ed. 2005, 44, 932; b) Y. Inokuma, T. Matsunari,
N. Ono, H. Uno, A. Osuka, Angew. Chem. 2005, 117, 1890; Angew.
Chem. Int. Ed. 2005, 44, 1856; c) P. K. Panda, Y.-J. Kang, C.-H. Lee,
Angew. Chem. 2005, 117, 4121; Angew. Chem. Int. Ed. 2005, 44,
4053; d) W. Maes, J. Vanderhaeghen, W. Dehaen, Chem. Commun.
2005, 2612; e) M. Suzuki, A. Osuka, J. Am. Chem. Soc. 2007, 129,
464; f) S. Mori, K. S. Kim, Z. S. Yoon, S. B. Noh, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2007, 129, 11344; g) D. Wu, A. B. Descalzo, F.
Emmerling, Z. Shen, X.-Z. You, K. Rurack, Angew. Chem. 2008,
120, 199; Angew. Chem. Int. Ed. 2008, 47, 193; h) T. K. Chandrashe-
kar, V. Prabhuraja, S. Gokulnath, R. Sabarinathan, A. Srinivasa,
Chem. Commun. 2010, 46, 5915.
action solution of compound 1a was silent further suggests
that this is the case. Conversely, the ESR spectra of com-
pounds 3, 5, 7, and 8 with NOSbF6 confirmed the presence
of radical species with g values of 2.0022, 2.0020, 2.0024, and
2.0033, respectively (Figure 3 and Figure S28 in the Support-
ing Information), suggesting radical cations. Although these
observations are typical for oligothiophenes,[19] the absorp-
tion seen here is extremely red shifted, indicating that the p-
conjugated network of compounds 3, 5, 7, and 8 is effective-
ly delocalize over the entire macrocycles in the oxidized
state.
In summary, we have successfully synthesized carbazole-
based expanded thiaporphyrins utilizing a coupling and an-
nulation strategy. The structures and electronic properties of
these macrocycles have been investigated by both spectro-
scopic methods and DFT calculations. These compounds
were observed to undergo oxidation by NOSbF6, following
which they exhibited significantly shifted NIR absorption.
Further investigations involving novel carbazole-based por-
phyrinoids and their metal complexes are currently under-
way in our laboratory.
[9] a) J. V. Grazulevicius, P. Strohriegl, J. Pielichowski, K. Pielichowski,
Prog. Polym. Sci. 2003, 28, 1297; b) J. F. Morin, M. Leclere, D. Ades,
A. Siove, Macromol. Rapid Commun. 2005, 26, 761; c) N. Blouin, M.
Leclerc, Acc. Chem. Res. 2008, 41, 1110.
[10] a) C. Maeda, T. Yoneda, N. Aratani, M.-C. Yoon, J. M. Lim, D. Kim,
N. Yoshioka, A. Osuka, Angew. Chem. 2011, 123, 5809; Angew.
Chem. Int. Ed. 2011, 50, 5691; b) C. Maeda, N. Yoshioka, Org. Lett.
2012, 14, 2122.
[11] a) P. Piatek, V. M. Lynch, J. L. Sessler, J. Am. Chem. Soc. 2004, 126,
16073; b) L. Arnold, H. Norouzi-Arasi, M. Wagner, V. Enkelmann,
K. Mꢄllen, Chem. Commun. 2011, 47, 970.
Acknowledgements
This work was supported by Grants-in-Aid for Young Scientists (No.
23750231 (B)) from MEXT, Japan. We thank Prof. A. Osuka and Dr. N.
Aratani (Kyoto-university) for X-ray diffraction analyses, and Prof. T.
Hasobe, Dr. T. Miura, and Dr. H. Sakai (Keio-university) for UV/Vis/
NIR absorption measurements.
[12] Cabazole-incorporated porphyrins were formed as a consequence of
a
Bergman cyclization of b,b’-diethynylated porphyrins. a) H.
Aihara, L. Jaquinod, D. J. Nurco, K. M. Smith, Angew. Chem. 2001,
113, 3547; Angew. Chem. Int. Ed. 2001, 40, 3439; b) M. Nath, J. C.
Huffman, J. M. Zaleski, J. Am. Chem. Soc. 2003, 125, 11484; c) M.
Nath, M. Pink, J. M. Zaleski, J. Am. Chem. Soc. 2005, 127, 478.
[13] M. S. Mudadu, A. N. Singh, R. P. Thummel, J. Org. Chem. 2008, 73,
6513.
Keywords: carbazoles · NIR absorption · pi interactions ·
porphyrinoids · thiophenes
[14] For the transformation of 1,3-butadiyne to the thiophene, see: a) J.
Lagan, S. K. Arora, J. Org. Chem. 1983, 43, 4317; b) J. Krçmer, I.
Rios-Carreras, G. Fuhrmann, C. Musch, M. Wunderlin, T. Debaerde-
maeker, E. Mena-Osteritz, P. Bꢂuerle, Angew. Chem. 2000, 112,
3623; Angew. Chem. Int. Ed. 2000, 39, 3481; c) N. Sumi, H. Naka-
nishi, S. Ueno, K. Takimiya, Y. Aso, T. Otsubo, Bull. Chem. Soc.
Jpn. 2001, 74, 979; d) M. J. OꢅConnor, M. M. Haley, Org. Lett. 2008,
10, 3973.
[1] a) A. Jasat, D. Dolphin, Chem. Rev. 1997, 97, 2267; b) H. Furuta, H.
Maeda, A. Osuka, Chem. Commun. 2002, 1795; c) J. L. Sessler, D.
Seidel, Angew. Chem. 2003, 115, 5292; Angew. Chem. Int. Ed. 2003,
´
´
42, 5134; d) M. Ste˛pien, N. Sprutta, L. Latos-Graz˙ynski, Angew.
Chem. 2011, 123, 4376; Angew. Chem. Int. Ed. 2011, 50, 4288; e) S.
Saito, A. Osuka, Angew. Chem. 2011, 123, 4432; Angew. Chem. Int.
Ed. 2011, 50, 4342.
[2] a) A. Kay, M. Grꢂtzel, J. Phys. Chem. 1993, 97, 6272; b) L. Schmidt-
Mende, W. M. Campbell, Q. Wang, K. W. Jolley, D. L. Officer,
Md. K. Nazeeruddin, M. Grꢂtzel, ChemPhysChem 2005, 6, 1253.
[3] P. C. Ford, Acc. Chem. Res. 2008, 41, 190.
[4] a) R. Bonnett, Chem. Soc. Rev. 1995, 24, 19; b) J. Seenisamy, S. Ba-
shyam, V. Gokhale, H. Vankayalapati, D. Sun, A. Siddiqui-Jain, N.
Streiner, K. Shin-ya, E. White, W. D. Wilson, L. H. Hurley, J. Am.
Chem. Soc. 2005, 127, 2944.
[5] a) M. Pawlicki, H. A. Collins, R. G. Denning, H. L. Anderson,
Angew. Chem. 2009, 121, 3292; Angew. Chem. Int. Ed. 2009, 48,
3244; b) G. S. He, L.-S. Tan, Q. Zheng, P. N. Prasad, Chem. Rev.
2008, 108, 1245.
[6] a) V. S.-Y. Lin, S. G. DiMagno, M. J. Therine, Science 1994, 264,
1105; b) T. E. O. Screen, K. B. Lawton, G. S. Wilson, N. Dolney, R.
Ispasoiu, T. Goodson III, S. J. Martin, D. D. C. Bradley, H. L. Ander-
son, J. Mater. Chem. 2001, 11, 312; c) T. Chandra, B. J. Kraft, J. C.
Huffman, J. M. Zaleski, Inorg. Chem. 2003, 42, 5158; d) T. V.
Duncan, K. Susumu, L. E. Sinks, M. J. Therien, J. Am. Chem. Soc.
2006, 128, 9000.
[15] a) Crystallographic data for compound 3: formula: C56H54N2S4;
¯
Mw =883.25; triclinic; space group: P1; a=13.1318(4), b=
18.7704(7), c=20.6485(6) ꢁ; a=97.2537(19), b=95.2711(18), g=
95.3477(19)8; V=4999.2(3) ꢁ3; Z=4, 1calcd =1.174 gcmÀ3
;
T=
À1808C; 14207 measured reflections; 6329 unique reflections (Rint
=
0.1191); R1 =0.0994 (I>2s(I)); wR2 =0.2296 (all data); GOF=
1.035. The contribution to the scattering arising from the presence
of the disordered solvents in the crystals was removed by use of the
utility SQUEEZE in the PLATON software package (see ref. [20]);
b) Crystallographic
data
for
compound
13:
formula:
4(C52H50N2S2)·2.54(CCl2)·2(O)·1.06(CO); Mw =842.77; triclinic;
¯
space group: P1; a=13.6379(3), b=19.9820(4), c=20.0082(4) ꢁ, a=
112.6846(8), b=97.8256(8), g=105.9883(9)8; V=4652.00(17) ꢁ3;
Z=4; 1calcd =1.203 gcmÀ3; T=À1808C; 15722 measured reflections;
8532 unique reflections (Rint =0.0964); R1 =0.0927 (I>2s(I)); wR2 =
0.2428 (all data); GOF=1.038. CCDC 884500 (3) and 884501 (13)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
2974
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Chem. Eur. J. 2013, 19, 2971 – 2975

Carbazole-Based Expanded Thiaporphyrins
COMMUNICATION
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
[18] Although we tried the NMR studies of the protonation of com-
pound 1b and the oxidation of compound 1a, both spectra in CDCl3
were similarly broad.
[16] Spartan’10, Wavefunction, Inc. Irvine, CA, see: Y. Shao, L. F.
Molnar, Y. Jung, J. Kussmann, C. Ochsenfeld, S. T. Brown, A. T. B.
Gilbert, L. V. Slipchenko, S. V. Levchenko, D. P. OꢅNeill, R. A. DiS-
tasio, Jr., R. C. Lochan, T. Wang, G. J. O. Beran, N. A. Besley, J. M.
Herbert, C. Y. Lin, T. Van Voorhis, S. H. Chien, A. Sodt, R. P.
Steele, V. A. Rassolov, P. E. Maslen, P. P. Korambath, R. D. Adam-
son, B. Austin, J. Baker, E. F. C. Byrd, H. Dachsel, R. J. Doerksen,
A. Dreuw, B. D. Dunietz, A. D. Dutoi, T. R. Furlani, S. R. Gwaltney,
A. Heyden, S. Hirata, C.-P. Hsu, G. Kedziora, R. Z. Khalliulin, P.
Klunzinger, A. M. Lee, M. S. Lee, W. Z. Liang, I. Lotan, N. Nair, B.
Peters, E. I. Proynov, P. A. Pieniazek, Y. M. Rhee, J. Ritchie, E.
Rosta, C. D. Sherrill, A. C. Simmonett, J. E. Subotnik, H. L. Wood-
cock III, W. Zhang, A. T. Bell, A. K. Chakraborty, D. M. Chipman,
F. J. Keil, A. Warshel, W. J. Hehre, H. F. Schaefer, J. Kong, A. I.
Krylov, P. M. W. Gill, M. Head-Gordon, Phys. Chem. Chem. Phys.
2006, 8, 3172.
[19] a) D. D. Graf, R. G. Duan, J. P. Campbell, L. L. Miller, K. R. Mann,
J. Am. Chem. Soc. 1997, 119, 5888; b) T. Nishinaga, A. Wakamiya,
D. Yamazaki, K. Komatsu, J. Am. Chem. Soc. 2004, 126, 3163; c) C.
Lin, T. Endo, M. Takase, M. Iyoda, T. Nishinaga, J. Am. Chem. Soc.
2011, 133, 11339; d) F. Zhang, G. Gçtz, E. Mena-Osteritz, M. Weil,
B. Sarkar, W. Kaim, P. Bꢂuerle, Chem. Sci. 2011, 2, 781; e) Y. Ie, M.
Endou, S. K. Lee, R. Yamada, H. Tada, Y. Aso, Angew. Chem. 2011,
123, 12186; Angew. Chem. Int. Ed. 2011, 50, 11980.
[20] Squeeze-Platon: a) PLATON,
A Multipurpose Crystallographic
Tool, A. L. Spek, Utrecht (The Netherlands), 2005; b) P. van der
Sluis, A. L. Spek, Acta Crystallogr. Sect. A 1990, 46, 194.
[17] The two-electron oxidation did not occur by other oxidants such as
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), Ag2O, and
PbO2.
Received: July 19, 2012
Revised: January 1, 2013
Published online: January 30, 2013
Chem. Eur. J. 2013, 19, 2971 – 2975
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2975