Metal-mediated synthesis of antiaromatic porphyrinoids from a BODIPY precursor

Article abstract of DOI:10.1002/anie.201006314

(Figure Presented) Metallic route to antiaromaticity: The synthesis of a butadiyne-bridged cyclic BODIPY dimer (see structure; C white, B orange, N blue, F green) and trimer was achieved through Pd-catalyzed Stille coupling and Cu-mediated Glaser-type coupling. These cyclic BODIPYs were found to be stable 24π- and 36π-antiaromatic porphyrinoids with planar conformations, and their structures, electronic states, and reactivity were explored.

Full text of DOI:10.1002/anie.201006314

Communications
DOI: 10.1002/anie.201006314
Antiaromaticity
Metal-Mediated Synthesis of Antiaromatic Porphyrinoids from a
BODIPY Precursor**
Takafumi Sakida, Shigeru Yamaguchi, and Hiroshi Shinokubo*
Porphyrinoids have been extensively investigated both in
fundamental and applied chemistry. The synthesis of por-
phyrinoids largely relies on the acid-promoted condensation
reaction of pyrroles.[1] Although metal-mediated reactions are
a powerful tool in current synthetic chemistry, they have
rarely been employed for the synthesis of porphyrinoid
skeletons. Boron dipyrrin (BODIPY) dyes have also received
much attention for materials such as labeling reagents,
chemosensors, light-harvesting systems, and dye-sensitized
solar cells.[2] These two important functional p-systems are
structurally related, but the synthesis of porphyrinoids from a
BODIPY precursor has largely remained unexplored.
Dehydroannulenes have received long-lasting attention in
not only the basic aspect of aromaticity of large p-systems but
also supramolecular chemistry and materials science.[3] For
the synthesis of these p-conjugated macrocycles, facile
synthetic methods have been established on the basis of
transition-metal-catalyzed cross-coupling reactions. The ethy-
nylene group in the macrocycles allows extension of p conju-
gation and offers rigidity for highly ordered structures. These
properties promise the creation of macrocycles possessing
unique structural and electronic features. Herein we report
the incorporation of BODIPYunits into dehydroannulenes to
furnish stable antiaromatic porphyrinoids by transition-
metal-mediated synthesis.[4]
Scheme 1. Synthesis of butadiyne-bridged BODIPY oligomers. Condi-
ꢀ
tions: a) Bu3SnC CSiMe3 (2.4 equiv), [Pd(PPh3)4] (2.5 mol%), toluene,
reflux, 4 h, 63% yield. b) CuCl (1.7 equiv), DMSO, under air, 608C,
24 h. Ar=3,5-di-tert-butylphenyl.
The synthesis of novel porphyrinoid 3 was commenced
with introduction of trimethylsilylethynyl groups to a,a’-
dichloro BODIPY 1 via Stille coupling (Scheme 1).[5,6] Direct
homocoupling by sila-Glaser coupling with CuCl in DMSO
provided the target molecule 3 in 27% yield along with
acyclic BODIPY oligomers (Scheme 1).[7] Normal Glaser
coupling with terminal alkynes was hampered by the low
solubility of diethynyl BODIPY after desilylation of 2.
Macrocycle 3 can be considered to possess a cyclic 24p-
electron conjugation. In general, 4np porphyrinoids take on
highly distorted structures to avoid destabilization due to
antiaromaticity.[8] Thus, elaboration to preserve the planarity
of the macrocycle is often required to achieve distinct
antiaromaticity.[9] In the case of 3, the rigid butadiyne linker
and the BF2 unit should enforce the whole macrocycle into a
flat structure. In fact, the single-crystal X-ray diffraction
analysis unambiguously elucidated the planar and rectangular
structure of 3, for which the mean plane deviation is only
0.058 ꢀ (Figure 1).[10] Accordingly, distinct antiaromaticity of
3 was confirmed by NMR spectroscopic analysis. In the
1H NMR spectrum of 3, a set of two peaks at d = 5.01 and
4.53 ppm was assigned as signals of the b-pyrrolic protons,
which are significantly upfield-shifted in comparison to those
of acyclic BODIPY 2 (d = 6.82 and 6.61 ppm), indicating the
existence of paratropic ring current. The paratropic ring
current effect also shifts the 19F signal for the inner BF2
moiety downfield (Dd = 8.9 ppm). In addition, the largely
positive nucleus-independent chemical shift (NICS)[11] value
(d = + 18.6 ppm) supports strong antiaromaticity of 3. Fur-
thermore, antiaromaticity of 3 induced substantial bond
length alternation around the meso position in 3 in compar-
ison to BODIPY monomer 2’ (Ar= mesityl) and the
[*] T. Sakida, S. Yamaguchi, Prof. Dr. H. Shinokubo
Department of Applied Chemistry
Graduate School of Engineering, Nagoya University
Nagoya 464-8603 (Japan)
Fax: (+81)52-789-5113
E-mail: hshino@apchem.nagoya-u.ac.jp
[**] We are indebted to Prof. Kazuyuki Tatsumi and Prof. Yasuhiro Ohki
(Nagoya University) for help with X-ray analysis and Prof. Atsuhiro
Osuka and Prof. Naoki Aratani (Kyoto University) for help with NIR
fluorescence measurements. This work was supported by MEXT
(Japan) Grants-in-Aid for Scientific Research (Nos. 21685011 and
21108510 “pi-Space”) and the Global COE Program in Chemistry of
Nagoya University. H.S. also acknowledges the Asahi Glass
Foundation for financial support. S.Y. appreciates the JSPS Research
Fellowships for Young Scientists. BODIPY=boron dipyrrin.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006314.
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Figure 1. X-ray crystal structures showing a) top view and b) side view
of 3, c) top view and d) side view of 4’ (Ar=mesityl), and e) top view
and f) side view of 5. meso-Aryl substituents are omitted for clarity.
The thermal ellipsoids are scaled to the 50% probability level.
Figure 2. a) UV/Vis/NIR absorption spectra of 2 (black), 3 (red), and 4
(blue) in CH2Cl2. b) UV/Vis absorption (solid lines) and emission
spectra (dashed lines) of 5 (green) and the parent meso–3,5-di-tert-
butylphenyl BODIPY (gray) in CH2Cl2 with excitation at the longest
absorption maxima.
BODIPY unit in 5 (see below and Figure S24 in the
Supporting Information,).
Another striking feature of an ethynylene linkage is
tolerance to bending. In fact, the angle of C1-C10-C11 in
BODIPY dimer 3 is rather large (165.38). This feature also
allowed the formation of larger macrocycle 4 with three
BODIPY units in 10% yield (Scheme 1). Crystallographic
analysis of 4 was difficult, but we obtained a nice crystal of 4’
(Ar= mesityl).[10] The structure of 4’ is slightly distorted
planar with pseudo C3 symmetry (mean plane deviation =
0.18 ꢀ; Figure 1). This 36p porphyrinoid 4 also exhibits
antiaromaticity in spite of its large p conjugation, as judged
from its upfield-shifted signals for b protons and downfield-
shifted signals for inner fluorine atoms in the 1H and 19F NMR
spectra. A positive NICS value (d = + 4.0 ppm) calculated on
the solid-state structure also supports this assignment.[12]
The spectral features of the UV/Vis/NIR absorption of 3
are quite similar to those of known antiaromatic porphyr-
inoids: a broad Soret-like band at 522 nm and a weak
absorption band in the near infrared region (936 nm) are
observed (Figure 2a). BODIPY trimer 4 also exhibits a weak
absorption band at 849 nm, and the longest absorption
maxima of 3 and 4 are red-shifted by 365 and 278 nm relative
to that of 2 as a result of cyclization. This result is an
indication of small HOMO–LUMO gaps of 3 and 4.
Interestingly, trimer 4 has a large molar extinction coefficient,
which is much larger than those of aromatic expanded
porphyrins with six pyrrole units. This can be accounted for
by the hyperchromic effect by alkynyl moieties at the
a positions as well as attenuated antiaromaticity of 4.[13]
Porphyrinoids 3 and 4 are nonfluorescent.[14] This fact is
rather remarkable in view of the highly emissive nature of
BODIPY derivatives. This lack of fluorescence is probably
due to the small HOMO–LUMO gap of antiaromatic
porphyrinoids, which results in enhancement of the rate of
nonradiative decay.[15]
Electrochemical properties of 3 and 4 were investigated
by cyclic voltammetry, both of which exhibited two reversible
reduction waves and one reversible oxidation wave. The
HOMO–LUMO gaps of 3 and 4 are determined to be 1.44
and 1.45 eV, respectively (Figure S27 in the Supporting
Information). These electrochemical HOMO–LUMO gaps
are roughly matched with optical HOMO–LUMO gaps (1.32
and 1.46 eV for 3 and 4) derived from the longest wavelength
absorption maxima. It is also noteworthy that the splitting
between the first and second reduction potentials of 3
(42 mV) is larger than that of 4 (25 mV), indicating more
effective delocalization of the resultant radical anions of 3 on
the macrocycles.
Finally, the chemical reactivity of 3 was briefly inves-
tigated. Hydrogenation of 3 in the presence of Pd/C occurred
selectively at butadiyne units without touching the pyrrole
moiety to furnish butylene bridged BODIPY dimer 5 in good
yield (Scheme 2). X-ray crystallographic analysis of BODIPY
dimer 5 revealed its steplike structure, which is fabricated by
two parallel BODIPY units (Figure 1e,f).[10] The breaking of
p-conjugation retrieves bright green fluorescence as
a
BODIPY dye at 543 nm (Figure 2b). The absorption spec-
trum of 5 consists of a blue-shifted strong absorption band at
488 nm and a red-shifted weak one at 534 nm as a shoulder, in
comparison to the parent meso-mesityl BODIPY mono-
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Communications
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mer.[16] The excitation spectra of 5 monitored at 543 and
577 nm exhibited identical spectra including the shoulder
absorption, indicating that this minor peak is also the
absorption band of 5 (Figure S26). This spectral feature may
be interpreted as follows by exciton coupling theory:[17] the
exciton coupling between the two almost parallel transition
dipoles along the long axis of the BODIPY units generates
one high-energy transition and one low-energy forbidden
transition, which could be assigned to the absorption bands at
488 and 534 nm, respectively.[18]
´
´
e) M. Stꢁpien, B. Szyszko, L. Latos-Graz˙ynski, J. Am. Chem. Soc.
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´
´
g) M. Ste˛pien, L. Latos-Graz˙ynski, N. Sprutta, P. Chwalisz, L.
In conclusion, we have demonstrated the usefulness of
transition-metal-mediated reactions to synthesize novel
stable antiaromatic porphyrinoids. Incorporation of the
BODIPYunit into porphyrin-like cyclic p-conjugation greatly
alters its photophysical properties. The prospective properties
of antiaromatic porphyrinoids for NIR materials have been
also demonstrated. Crossover between porphyrin and
BODIPY research will offer chances to create novel porphyr-
inoids as well as cyclic BODIPY arrays with fascinating
characteristics. Further exploration of synthesis of novel
porphyrinoids by transition-metal-mediated reactions is cur-
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[10] Crystal data for 3: C56H52B2Cl6F4N4, Mw = 1091.34, triclinic,
ꢀ
space group P1 (No. 2), a = 9.421(4), b = 10.307(5), c =
15.049(7) ꢀ, a = 83.34(2), b = 75.43(2), g = 72.636(16)8, V=
1348.6(11) ꢀ3, Z = 1, Dcalcd = 1.344 gcmÀ3, T= 173(2) K, R =
0.0921 (I > 2.0s(I)), Rw = 0.3209 (all data), GOF = 1.142 (I >
2.0s(I)). Crystal data for 4’: C136H94B6Cl12F12N12O2, Mw =
ꢀ
Received: October 8, 2010
485.21, triclinic, space group P1 (No. 2), a = 18.358(14), b =
Revised: November 26, 2010
Published online: February 2, 2011
18.505(15), c = 20.125(14), a = 95.249(19), b = 91.390(19), g =
94.952(14)8, V= 6779(9) ꢀ3, Z = 2,
D
calcd = 1.297 gcmÀ3
,
T=
173(2) K, R = 0.1053 (I > 2.0s(I)), Rw = 0.3499 (all data),
GOF = 0.957 (I > 2.0s(I)). Crystal data for 5:
27.5H33BCl1.26F2N2, Mw = 485.21, triclinic, space group P1 (No.
2), a = 9.745(5), b = 10.728(5), c = 13.170(5) ꢀ, a = 87.222(5),
b = 78.102(5), g = 81.505(5)8, V= 1332.3(11) ꢀ3, Z = 2, Dcalcd
Keywords: annulenes · aromaticity · cross-coupling ·
.
ꢀ
C
dyes/pigments · porphyrinoids
=
1.210 gcmÀ3, T= 153(2) K, R = 0.0780 (I > 2.0s(I)), Rw = 0.2251
(all data), GOF = 1.032 (I > 2.0s(I)). CCDC-795975 (2’), 795976
(3), 795977 (4’), and 795978 (5) 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.
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[13] Alkynyl substituents at the a positions of BODIPYs enhance
molecular absorption coefficients. The molar absorption coef-
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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ficient of bis(trimethylsilylethynyl) BODIPY
105 mÀ1 cmÀ1, while that of the parent meso-3,5-di-tert-butyl-
phenyl BODIPY is only 6.0 ꢂ 104 mÀ1 cmÀ1
2
is 1.2 ꢂ
[16] The Stokes shift of 5 is 311 cmÀ1 while that of meso-3,5-di-tert-
butylphenyl BODIPY is 772 cmÀ1
[17] M. Kasha, H. R. Rawls, M. A. El-Bayoumi, Pure Appl. Chem.
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[18] DFT calculations for 5 at the B3LYP/6-31G(d) level suggested
the involvement of through-bond interactions between molec-
ular orbitals of two BODIPY units (Figure S33).
.
.
[14] No emission peak was detected in the range 1000–1500 nm.
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