Oxocyclohexadienylidene-substituted subporphyrins

Article abstract of DOI:10.1002/anie.201006449

Quinonoidal subporphyrin analogue 2 was synthesized by oxidation of meso-tris(3,5-di-tert-butyl-4-hydroxyphenyl)subporphyrin 1 (see scheme). Compound 2 was readily deprotonated to generate anionic species 3. These three macrocycles were structurally characterized, including the full delocalization of the anionic charge in 3. Copyright

Full text of DOI:10.1002/anie.201006449

DOI: 10.1002/anie.201006449
Porphyrinoids
Oxocyclohexadienylidene-Substituted Subporphyrins**
Shin-ya Hayashi, Jooyoung Sung, Young Mo Sung, Yasuhide Inokuma, Dongho Kim,* and
Atsuhiro Osuka*
A subporphyrin is a genuine ring-contracted porphyrin that
has a 14p electron aromatic system and a bowl-shaped
structure. The chemistry of subporphyrins began with the
synthesis of tribenzosubporphine in 2006,[1] and the synthesis
of meso-aryl-substituted subporphyrins was achieved later by
two groups independently.[2] Subporphyrins are characterized
by distinct aromaticity, a porphyrin-like intense absorption,
and green fluorescence. Large effects of meso-aryl substitu-
ents on the electronic properties of subporphyrin owing to
their free rotation have been demonstrated for meso-4-
aminophenyl-substituted subporphyrins,[3a] meso-oligo-1,4-
phenyleneethynylene-substituted
meso-oligo-2,5-thienylene-substituted
subporphyrins,[3b]
and
subporphyrins.[3c]
Other chemical modifications, such as peripheral modifica-
tions[4a,b] and meso-alkyl substitution,[4c] have been also
developed. Despite these efforts, the chemistry of subpor-
phyrins still remains at its infant stage, and exploration of
novel subporphyrins is highly desirable to expand their
chemistry.
meso-Alkenylidenyl-substituted porphyrins hold a unique
position in porphyrin chemistry in view of extended conju-
gation, perturbed absorption spectra,[5a] O2 reduction sys-
tems,[5b,c] solvatochromism,[5d] and anion binding.[5e] In many
cases, their structures in solution are uncertain because they
exist as a variety of tautomers.[6] For example, 3,5-di-tert-
butyl-4-hydroxyphenyl-substituted porphyrin is readily oxi-
dized to oxocyclohexadienylidene (OCH)-substituted por-
phyrin, which has an extended quinonoid conjugated struc-
ture.[6] To the best of our knowledge, however, none of meso-
alkenylidene-substituted subporphyrins has been reported to
date. Herein, we present OCH-substituted subporphyrin as
the first example of meso-alkenylidene-substituted subpor-
phyrin.
First, we synthesized meso-tris(3,5-di-tert-butyl-4-hydrox-
yphenyl)subporphyrin 1 based on the condensation of pyri-
dine-tri-N-pyrrolylborane[4b] and 3,5-di-tert-butyl-4-hydroxy-
benzaldehyde with the yield of isolated product being 1.9%.
A high-resolution electrospray ionization (HR-ESI) mass
measurement revealed an intense borenium cation peak
at m/z 855.5446 (calcd for C57H69B1N3O3 = 855.5436
1
[MÀOMe]+). The H NMR spectrum had a singlet at d =
8.16 ppm for the six b-pyrrolic protons and a single set of
signals that are due to the meso-aryl substituents, and a singlet
at d = 0.90 ppm for the B-axial methoxy protons. The bowl-
shaped structure of 1 was unambiguously confirmed by single-
crystal X-ray diffraction analysis (Figure 1).[7] The dihedral
angles of the meso-aryl substituents towards the subporphyrin
core are 34.18, 48.08, and 57.68, respectively, and the bowl
depth, defined as the distance from the central boron atom to
the mean plane of peripheral six b-carbon atoms, is 1.37 ꢀ.
These structural features are common to those of usual meso-
aryl-substituted subporphyrins.[2a]
Subporphyrin 1 was readily oxidized by MnO2 to OCH-
substituted subporphyrin 2 in 50% yield. It is worth noting
that 2 can be reduced to 1 using NaBH4. In contrast to the
usual subporphyrin cases, the HR-ESI mass spectrum of 2 did
not exhibit a borenium cation peak in the positive-ion mode
but displayed a characteristic intense anionic parent peak at
m/z 882.5383 (calcd for C58H69B1N3O4 = 882.5396 [MÀH]À) in
the negative-ion mode. The 1H NMR spectrum of 2 was
[*] S. Hayashi, Dr. Y. Inokuma, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
J. Sung, Y. M. Sung, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
[**] This work was supported by Grants-in-Aid (nos. 22245006 (A) and
20108001 “pi-Space”) from MEXT. The work at Yonsei University
was supported by World Class University Program (R32-2008-000-
10217-0) and the Star Faculty program from the Ministry of
Education and Human Resources Development, Korea, and
AFSOR/AOARD grant (no. FA 2386-09-1-4092).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006449.
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3253

Communications
tion, which results in a deeper bowl depth of 1.47 ꢀ.
Interestingly, intermolecular hydrogen-bonding interactions
between the B-axial hydroxy group and quinone carbonyl,
and the B-hydroxy and phenol hydroxy group, are observed in
the crystal structure (Supporting Information, Figure S2-2).
Figure 3 shows UV/Vis absorption and fluorescence
spectra of 1 and 2 in CH2Cl2. Subporphyrin 1 has absorption
and fluorescence spectra that are typical for meso-aryl-
substituted subporphyrins, with some bathochromic shifts
and an enhanced fluorescence quantum yield (FF = 0.52). In
sharp contrast, 2 exhibits a remarkably perturbed and red-
shifted absorption spectrum that consists of three major bands
at 400, 501, and 740 nm, and is virtually non-fluorescent.
Figure 1. X-ray crystal structure of 1. Ellipsoids set at 50% probability;
solvent molecules are omitted for clarity.
consistent with its non-symmetric structure, differentiating
two quinonoidal substituents (two broad singlets at d = 8.33
and 8.21 ppm) from a hydroxyphenyl substituent (a sharp
singlet at d = 7.63 ppm). The former two broad peaks
coalesced at 508C, indicating restricted rotation of the
quinonoidal substituents or proton transfer among meso-
substituents. A singlet for the B-axial methoxy protons was
observed at d = 2.97 ppm, indicating dearomatization, which
is consistent with the upfield shifts of its b-pyrrolic protons
observed at d = 7.20, 7.11, and 6.87 ppm, respectively.
Single crystals suitable for X-ray diffraction analysis were
grown as its B-axial hydroxy derivative 2-OH (Figure 2) from
a solution of 2 in a mixture of acetone/acetonitrile (1:2).[7] The
distortion toward the quinonoidal form is obvious as shown in
Figure 3. UV/Vis absorption (c) of 1, 2, and 3 and fluorescence
spectrum of 1 (a) in CH2Cl2.
À
C O bonds (1.23 and 1.24 ꢀ in the quinonoid moieties, 1.37 ꢀ
À
in the hydroxyphenyl substituent) and Cmeso Cipso bonds (1.38
In the next step, anionic species 3 was generated by the
addition of triethylamine (10 equiv) to a solution of 2 in
CH2Cl2. The color of the solution immediately changed from
dark green to blue and the UV/Vis absorption spectrum
showed three peaks at 381, 499, and 643 nm (Figure 3).
Importantly, the 1H NMR spectrum revealed a C3-symmetric
structure by a singlet at d = 8.20 ppm for the quinonoidal
protons, a singlet at d = 6.85 ppm for b-pyrrolic protons, a
singlet at d = 3.12 ppm for B-axial methoxy protons, and a
singlet at d = 1.44 ppm for tert-butyl protons, indicating its
non-aromatic feature and full delocalization of anionic charge
over the molecule.
and 1.39 ꢀ in the quinonoid moieties, 1.45 ꢀ in the hydrox-
yphenyl substituent). Reflecting the quinonoidal structure,
the dihedral angles toward the subporphyrin mean plane are
quite small (9.58 and 10.18), but that of the hydroxyphenyl
substituent is normal (39.88). The bowl-shaped structure is
preserved, but the pyrrole ring, which is hampered by the two
quinonoid substituents, is bent to mitigate the steric conges-
The structure of 3 was unambiguously determined by X-
ray diffraction analysis on single crystals of 3-OH obtained
from slow crystallization from a mixture of pyridine/octane
(Figure 4).[7] The countercation is a protonated pyridine,
which is located just below the bowl of 3-OH (Supporting
Information, Figure S2-3). All meso substituents exhibit
distinct structural distortions owing to nontrivial contribution
À
of a quinonoidal form. The C O bond lengths (1.23, 1.24, and
À
1.26 ꢀ) are in the range of a quinone C O double bond, and
À
the Cmeso Cipso bonds (1.38, 1.40 and 1.40 ꢀ) show double-
bond character. The dihedral angles are all small (0.88, 4.98,
and 6.68). These structural features give rise to severe bending
of all of the pyrrole units to avoid the steric congestion,
causing a large bowl depth (1.56 ꢀ). As a whole, the
Figure 2. X-ray crystal structure of 2-OH. Ellipsoids set at 50%
probability; solvent molecules are omitted for clarity.
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transfer leads to a shorter excited-state lifetime more
significantly than neutral species 2.
The oxidation and reduction potentials of 1–3 were
measured by cyclic voltammetry in CH2Cl2 containing 0.10m
Bu4NPF6 as a supporting electrolyte (Supporting Information,
Figure S5). Subporphyrin 1 exhibited two oxidation potentials
at 0.27 and 0.54 V and one reduction potential at À2.12 V.
OCH-substituted subporphyrin 2 exhibited one reversible
oxidation wave at 0.55 V and a reduction wave at À1.23 V,
while anionic species 3 showed two reversible oxidation
waves at 0.10 and 0.58 V, and a reduction wave at À1.22 V.
The second reversible oxidation wave of 1 was considered to
be identical to the first reversible oxidation wave of 2.
Subporphyrin 2 and 3 showed quite similar reduction waves,
which probably arises from the common quinonoidal struc-
ture, but anionic species 3 is apparently more susceptible to
oxidation. MO calculations on 1–3 were performed at the
B3LYP/6-31G(d) level (Supporting Information, Figure S6
and S7).[9] Subporphyrin 1 has a2u-like HOMO, a1u-like
HOMOÀ1, and a couple of degenerate eg-like LUMO and
LUMO + 1. Compound 2 has quite similar but stabilized
HOMOÀ1, LUMO + 1, and LUMO + 2 compared to the
HOMOÀ1, LUMO, and LUMO + 1 of 1, while the HOMO
and LUMO of 2 are totally different from the MOs of 1 and
have a small energy gap. This observation is consistent with
the experimental results. Molecular orbitals of 3 are quite
similar to 2, but characterized by destabilization and deloc-
alization.
Figure 4. X-ray crystal structure of 3-OH. a) Top view and b) side view.
Ellipsoids set at 50% probability; solvent molecules, counterion, and
tert-butyl groups are omitted for clarity.
subporphyrin 3-OH has a planar extended structure with
almost C3 symmetry.
The absorption spectrum of 2 is dependent on solvent
polarity: it is red-shifted in nonpolar solvents (dichloro-
methane, toluene, and hexane) and blue-shifted in polar
solvent (methanol, pyridine, and DMSO; Supporting Infor-
mation, Figure S3-1). Interestingly, the latter spectra are
almost the same as that of 3, suggesting that the subporphyrin
2 exists as a deprotonated species in these polar solvents, as
the anion 3 is considerably stabilized owing to the effective
delocalization of the negative charge.
In summary, we have synthesized the novel subporphyrin
analogue 2 by the oxidation of 1. OCH-substituted subpor-
phyrin 2 exhibits quite different electronic and structural
properties from typical subporphyrins. Deprotonation of 2
proceeds smoothly to provide the anionic species 3 that
exhibits an almost planar C3-symmetric structure. Exploration
of more elaborate molecular systems incorporating this
switching unit is now being actively pursued in our labora-
tories.
To gain further insight into the effect of the OCH
substituent on the electronic system, we investigated the
excited-state dynamics. Many porphyrinoids with quinonoidal
substituents at meso positions have short excited-state life-
times and low fluorescence quantum yields.[5] Specifically,
Milgrom et al. have shown that meso-tetrakis(3,5-di-tert-
butyl-4-oxocyclohexadienylidene)porphyrin has a sub-pico-
second S1 state lifetime and low fluorescence quantum yield,
while meso-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)por-
phyrin shows a few nanosecond S1 state lifetime.[5c,8] Similarly,
we observed that quinonoidal subporphyrin system 2 exhibits
a short S1-state lifetime (ca. 40 ps; Supporting Information,
Figure S4-1), while meso-tris(3,5-di-tert-butylphenyl)subpor-
phyrin has a 3 ns lifetime.[4b] Furthermore, even a shorter S1-
state lifetime (ca. 10 ps; Supporting Information, Figure S4-2)
is observed for 3. Owing to the electron-donating ability of
the phenoxide anion and coupling between the phenoxide
anion and subporphyrin p and p* molecular orbitals, intra-
molecular charge transfer occurs in anionic species 3. These
interactions increase electron density over the subporphyrin
system and raise the energy of p* orbital, leading to a blue-
shift in the absorption spectra. The promotion of electron
density to subporphyrin core by the intramolecular charge-
Received: October 14, 2010
Revised: December 20, 2010
Published online: March 2, 2011
Keywords: conjugated systems · delocalization ·
.
oxoporphyrinogen · porphyrinoids · subporphyrins
[1] a) Y. Inokuma, J. H. Kwon, T. K. Ahn, M.-C. Yoon, D. Kim, A.
Osuka, Angew. Chem. 2006, 118, 975 – 978; Angew. Chem. Int. Ed.
2006, 45, 961 – 964; b) T. Torres, Angew. Chem. 2006, 118, 2900 –
2903; Angew. Chem. Int. Ed. 2006, 45, 2834 – 2837.
[2] a) Y. Inokuma, Z. S. Yoon, D. Kim, A. Osuka, J. Am. Chem. Soc.
2007, 129, 4747 – 4761; b) N. Kobayashi, Y. Takeuchi, A. Matsuda,
Angew. Chem. 2007, 119, 772 – 774; Angew. Chem. Int. Ed. 2007,
46, 758 – 760.
[3] a) Y. Inokuma, S. Easwaramoorthi, Z. S. Yoon, D. Kim, A. Osuka,
J. Am. Chem. Soc. 2008, 130, 12234 – 12235; b) Y. Inokuma, S.
Easwaramoorthi, S. Y. Jang, K. S. Kim, D. Kim, A. Osuka, Angew.
Chem. 2008, 120, 4918 – 4921; Angew. Chem. Int. Ed. 2008, 47,
4840 – 4843; c) S. Hayashi, Y. Inokuma, A. Osuka, Org. Lett. 2010,
12, 4148 – 4151.
Angew. Chem. Int. Ed. 2011, 50, 3253 –3256
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3255

Communications
[4] a) E. Tsurumaki, S. Saito, K. S. Kim, J. M. Lim, Y. Inokuma, D.
[7] Crystallographic data for 1: C58H72B1N3O4·C8H18, Mr = 1000.22,
monoclinic, space group P21/a (no. 14), a = 18.016(5), b =
18.660(4), c = 19.494(4) ꢀ, b = 116.063(9)8, V= 5887(2) ꢀ3, T=
123 K, 1calcd = 1.128 gcmÀ1, Z = 4, R1 = 0.0706 (I > 2s(I)), Rw =
0.1978 (all data), GOF = 1.029. 2: 4(C57H68B1N3O4)·1.46-
(C3H6O1)·C2H3N1, Mr = 3605.43, monoclinic, space group P21/a
(no. 14), a = 14.919(3), b = 24.058(6), c = 18.377(5) ꢀ, b =
114.996(9)8, V= 5978(3) ꢀ3, T= 123 K, 1calcd = 1.002 gcmÀ1, Z =
1, R1 = 0.0824 (I > 2s(I)), Rw = 0.2130 (all data), GOF = 1.041. 3:
2(C57H67B1N3O4)·2(C5H6N1)·13(C5H5N1), Mr = 2926.41, ortho-
rhombic, space group Ama2 (no. 40), a = 29.305(5), b =
Kim, A. Osuka, J. Am. Chem. Soc. 2008, 130, 438 – 439; b) E.
Tsurumaki, Y. Inokuma, S. Easwaramoorthi, J. M. Lim, D. Kim,
A. Osuka, Chem. Eur. J. 2009, 15, 237 – 247; c) S. Hayashi, Y.
Inokuma, S. Easwaramoorthi, K. S. Kim, D. Kim, A. Osuka,
Angew. Chem. 2010, 122, 331 – 334; Angew. Chem. Int. Ed. 2010,
49, 321 – 324.
[5] a) I. M. Blake, H. L. Anderson, D. Beljonne, J.-L. Brꢁdas, W.
Clegg, J. Am. Chem. Soc. 1998, 120, 10764 – 10765; b) T. G.
Traylor, K. B. Nolan, R. Hildreth, J. Am. Chem. soc. 1983, 105,
6149 – 6151; c) L. R. Milgrom, C. C. Jones, A. Harriman, J. Chem.
Soc. Perkin Trans. 2 1988, 71 – 79; d) J. P. Hill, A. L. Schumacher,
F. DꢂSouza, J. Labuta, C. Redshaw, M. R. J. Elsegood, M. Aoyagi,
T. Nakanishi, K. Ariga, Inorg. Chem. 2006, 45, 8288 – 8296; e) A.
Shundo, J. P. Hill, K. Ariga, Chem. Eur. J. 2009, 15, 2486 – 2490.
[6] a) L. R. Milgrom, Tetrahedron 1983, 39, 3895 – 3898; b) J. P. Hill,
I. J. Hewitt, C. E. Anson, A. K. Powell, A. L. McCarry, P. A. Karr,
M. E. Zandler, F. DꢂSouza, J. Org. Chem. 2004, 69, 5861; c) Y. Xie,
J. P. Hill, A. L. Schumacher, P. A. Karr, F. DꢂSouza, C. E. Anson,
A. K. Powell, K. Ariga, Chem. Eur. J. 2007, 13, 9824 – 9833.
18.062(4), c = 31.395(8) ꢀ, V= 16618(6) ꢀ3, T= 123 K, 1calcd
=
1.170 gcmÀ1
,
Z = 4, R1 = 0.0655 (I > 2s(I)), Rw = 0.1394 (all
data), GOF = 1.003. CCDC 794968 (1), 794969 (2), and 794970
(3) 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.
[8] M. A. Bergkamp, J. Dalton, T. L. Netzel, J. Am. Chem. Soc. 1982,
104, 253 – 259.
[9] See the Supporting Information for the full citation.
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