Tribenzosubporphines: Synthesis and characterization

Article abstract of DOI:10.1002/anie.200503426

(Figure Presented) Keeping it in the family: A one-pot condensation with a boric acid template leads to the synthesis of subporphineboron(III) complexes (see example; B green, O red, N blue, C turquoise, H white), ring-contracted congeners of porphyrins.

Full text of DOI:10.1002/anie.200503426

Angewandte
Chemie
Template Synthesis
DOI: 10.1002/anie.200503426
Tribenzosubporphines: Synthesis and
Characterization**
Yasuhide Inokuma, Jung Ho Kwon, Tae Kyu Ahn,
Min-Chul Yoo, Dongho Kim,* and Atsuhiro Osuka*
Subphthalocyanines 1 possess a unique position in the
chemistry of phthalocyanines as a ring-contracted congener
of phthalocyanine. Since the first synthesis of subphthalocya-
nines by Meller and Ossko in 1972,[1] they have continued to
gather much interest in light of their 14p-electron aromatic
circuit, triangular bowl-shaped structures, intense fluores-
cence, and nonlinear optical properties.[2] In sharp contrast,
subporphyrin, which is a porphyrinic counterpart of 1,
remains a missing player, thus leaving its study totally
untouched, despite recent extensive reports of various ring-
expanded and ring-contracted porphyrins.[3] Subporphyrins
are expected to be promising macrocycles from both basic and
[*] J. H. Kwon, T. K. Ahn, M.-C. Yoo, Prof. Dr. D. Kim
Center for Ultrafast Optical Characteristics Control and
Department of Chemistry
Yonsei University
Seoul 120-749 (Korea)
Fax: (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Y. Inokuma, Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science
Kyoto University
and
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
[**] This work was partly supported by a Grant-in-Aid from the Ministry
of Education, Culture, Sports, Science, and Technology, Japan,
(No. 15350022) and the 21st Century COE on Kyoto University
Alliance for Chemistry (to A.O.), as well as the National Creative
Research Initiatives Program of the Ministry of Science and
Technology of Korea (to D.K.).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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applied viewpoints, as judged from the various interesting
attributes of subphthalocyanines.[2] Thus, a reliable synthetic
route to subporphyrins is highly desirable. Herein we report
the synthesis and characterizations of [14]tribenzo-
subporphine(1.1.1)boron(iii) complexes 2a–f. To the best of
our knowledge, this is the first report on the synthesis of
subporphyrins.
The hydroxyboron complex of tribenzosubporphine 2a
was synthesized by a modified protocol developed by Gouter-
man and co-workers[4] for the synthesis of a tetrabenzopor-
phine using 2-(3-oxo-2,3-dihydro-1H-isoindol-1-yl)acetic acid
(3) and with replacement of the template from a ZnII to a
BIII ion.[1,2] Compound 3 and boric acid were mixed, ground
into a fine powder, and subsequently heated to 3508C in an N2
atmosphere for 3.5 h. TLC analysis of the resulting black melt
indicated formation of many products, from which a fraction
eluting at Rf = 0.30 on a silica gel TLC plate with a 1:2:2
mixture of diethyl ether/hexane/dichloromethane was iso-
lated. This product was identified as 2a on the basis of its
parent ion signal by MALDI-TOF mass analysis and its
characteristic strong green fluorescence. Repeated separa-
tions by chromatography on silica gel gave 2a as an orange
solid in 1.4% yield. Phenylboronic acid or 4-methoxyphenyl-
boronic acid can also be used as the template and give 2a in
comparable yields. The subporphyrin 2a can also be prepared
in 1.2% yield under irradiation with a domestic microwave
oven (output: 500 W) for 30 minutes. Although the yields are
low, the syntheses of 2a is quite reproducible.
Reflecting its C3 symmetry, the subporphyrin 2a exhibits a
very simple 1H NMR spectrum that consists of a singlet at d =
9.44 ppm for the meso positions, a pair of double doublets for
the benzo protons at d = 8.86 and 7.88 ppm, and a signal at d =
À2.60 ppm for the hydroxy proton. The observed chemical
shifts indicate a diatropic ring current in line with its 14p-
electronic network. The 13C NMR spectrum also supports the
structure of 2a, with signals observed at d = 137.0 and
131.6 ppm for the pyrrolic carbon atoms, signals at d = 127.2
and 121.5 ppm for the fused
Figure 1. Crystal structures of a) 2a and b) 2e at 50% probability of
thermal ellipsoids.
reaction has also been observed on dissolving 2a in methanol,
which immediately gave rise to the appearance of a new spot
of 2b on the TLC plate which became the single product upon
refluxing the solution. The methoxy protons of 2b are
1
observed at d = 0.81 ppm in the H NMR spectrum because
of the diatropic ring current. Similarly, refluxing 2a in 2-
propanol afforded isopropoxyboron complex 2c, which gives
rise to signals corresponding to the isopropoxy protons at d =
0.63 (1H) and À0.42 ppm (6H) in the 1H NMR spectrum. The
subporphyrin 2c is easily hydrolyzed to 2a by a small amount
of adventitious water in solution, while 2b is rather stable
under basic or neutral conditions, but is readily hydrolyzed in
acidic aqueous solution.
The axial hydroxy group of 2a can be also exchanged by a
carboxy group by stirring it in the presence of an excess
amount of carboxylic acid, thus providing 2d, 2e, and 2 f
through the equilibrium shown in Scheme 1. The equilibrium
benzo carbon atoms, and a
signal at d = 97.3 ppm for the
meso carbon atom. The
11B NMR spectrum exhibits
a singlet at d = À14.6 ppm,
which is slightly less shielded
relative to those of subphtha-
locyanines which appear in
the range of d = À17.7 to
Scheme 1. Equilibria between 2a and 2d–f.
À19.6 ppm.[2a] The high reso-
lution electrospray ionization
time-of-flight (HRESI-TOF) mass spectrum of 2a also
supports the structure, with a signal at m/z 409.1387 (calcd
for [C27H16BN3O]+: m/z 409.1386). The structure of 2a has
been confirmed by X-ray diffraction analysis to be a bowl-
shaped tribenzosubporphine structure (Figure 1a).[5]
A solution of 2a in diethyl ether was stirred in the
presence of H218O for 6 h at room temperature. ESI-TOF
mass analysis of this reaction mixture detected two main
signals at m/z 409.14 and 411.14, thus suggesting a facile
replacement of the axial hydroxy group. A similar exchange
between 2a and 2d could be easily followed by 1H NMR
spectroscopy and allowed for the determination of an
equilibrium constant of 3.2 (Supporting Information). Stirring
a solution of 2a in the presence of an excess amount of
carboxylic acid or continuous removal of water from the
reaction mixture gave the carboxyboron complexes in high
yields. The 11B NMR spectra of 2d–f exhibit characteristic,
extremely broad signals in the range of d = À13.7 to
À14.6 ppm, and contrast the relatively sharp signals of the
hydroxy- and alkoxyboron complexes 2a–c.
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Furthermore, X-ray diffraction analysis shows that in the
solid-state the macrocycles 2b, 2c, and 2e have bowl-shaped
structures, with a variable curvature that depends on the axial
group (see Figure 1b and the Supporting Information).[5] The
degree of curvature can be characterized by the depth of the
bowl, which is defined as the distance from the lowest edge of
the macrocycle (the mean plane of the peripheral six carbon
atoms of the benzene rings) to the boron atom and is 2.33 Š
for 2a, 1.70 Š for 2b, 2.10 Š for 2c, and 2.02 Š for 2e. The
bowl depths of the subporphyrins are less than that (2.55 Š)
of subphthalocyanine 1 (L = OH),[2c] and 2b is the shallowest
bowl in the series.[6] The average N-B-N angles are 105.18 for
2a, 105.58 for 2b, 105.28 for 2c, and 108.48 for 2e, and the
determined by the time-correlated single-photon counting
technique. The lowest triplet excited state (T1) of 2a has been
detected in CH2Cl2 by nanosecond time-resolved transient
absorption spectroscopy to have a broad absorbance in the
range 350–700 nm whose decay follows a single exponential
function with t = 54 ms. Furthermore, femtosecond time-
resolved transient absorption anisotropy measurements
revealed that the anisotropy value is initially about 0.12 and
decays with t = 46 Æ 0.5 ps. This decay has been assigned to its
rotational diffusion time which is distinctly shorter than that
(130 ps) of ZnIITPP,[10] thus reflecting the smaller molecular
size of 2a compared with that of ZnIITPP. Overall, the optical
properties of the 14p-electron conjugated aromatic subpor-
phyrins can be regarded as analogous to those of 18p-electron
conjugated porphyrins, despite differences that reflect the
structural difference in the molecular symmetry and molec-
ular size.
The electrochemical properties of 2a and 2b in acetoni-
trile have been examined by cyclic voltammetry using
tetraethylammonium perchlorate as a supporting electrolyte.
The first and second one-electron oxidation potentials were
determined to be 0.30 and 0.92 V and 0.31 and 0.93 V, as
reversible and irreversible waves, versus the ferrocene/
ferrocenium ion couple for 2a and 2b, respectively, while
the reduction-associated waves could not be detected up to
À2.0 V. Thus, it may be concluded that subporphyrins are
better electron donors than subphthalocyanines.[2a,11]
In summary, the first synthesis of
À
average B N bond lengths are 1.51 Š for 2a, 1.50 Š for 2b,
À
1.51 Š for 2c, and 1.48 Š for 2e, and the B O bond lengths
are 1.446(4) Š for 2a, 1.445(5) Š for 2b, 1.435(3) Š for 2c,
and 1.506(5) Š for 2e, respectively. These structural param-
eters suggest 2e has a different electronic nature from those
À
of 2a–c. A possible explanation may be that the B O bond for
2e has a larger ionic character than those of 2a–c. Overall the
bowl-shaped structures of the subporphyrins are similar to
those of subphthalocyanines.[7]
The absorption spectrum of 2a exhibits a sharp Soret band
at 355 nm with a shoulder at 337 nm, and Q bands at 480 and
514 nm, which are blue-shifted relative to those of metal-
loporphyrins mainly because of the reduced p-conjugation
pathway (Figure 2). The Q bands show a vibronic structure
tribenzosubporphyrin 2 has been ach-
ieved through the one-pot condensa-
tion of 3 in the presence of boric acid as
a template. Detailed comparative stud-
ies of the optical and electrochemical
properties of 2 with those of porphyr-
ins, phthalocyanines, and subphthalo-
cyanines will generate a deeper under-
standing of the electronic nature of
porphyrins and phthalocyanines, and
possible applications of 2 to dye-based
devices will be worthy of further inves-
tigations.
Figure 2. a) Absorption and fluorescence spectra of 2a in CH2Cl2, and calculated absorption
bands, and b) the fluorescence decay of 2a.
Received: September 28, 2005
Published online: December 28, 2005
similar to those of subphthalocyanines and benzoporphyrins
with large molar extinctions of the Q band of 9 ” 104 mÀ1 cmÀ1,
Keywords: aromaticity · boron · macrocycles · porphyrinoids ·
which is more than three times larger than the value (2.97 ”
104 mÀ1 cmÀ1) of ZnIITPP (TPP = tetraphenylporphyrin). The
strong and sharp Soret bands with an extinction coefficient of
1.64 ” 105 mÀ1 cmÀ1 differ significantly from the rather broad
Soret bands of subphthalocyanines around 300 nm and are
similar to the Soret bands of porphyrins. This similarity has
been confirmed by calculation of the molecular orbitals
(MOs) at the B3LYP/6-31G* level (see the Supporting
Information). The p–p* transitions calculated by time-depen-
dent density functional theory (TD-DFT) match well with the
absorption spectrum of 2a (indicated by vertical bars in
Figure 2).[8] Interestingly, 2a exhibits intense green fluores-
cence at 517 nm with FF = 0.41,[9] whose decay has been found
to obey a single exponential function with t = 3.4 Æ 0.2 ns, as
.
template synthesis
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Communications
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[5] Crystallographic data for 2a: C27H16BN3O·H2O, Mr = 409.24,
monoclinic, space group C2/c (no. 15), a = 24.535(5), b =
11.708(3), c = 13.940(3) Š, b = 91.192(14)8, V= 4003.5(15) Š3,
T= 123 K, 1calcd = 1.358 gcmÀ3, Z = 8; R1 = 0.0681 (I > 2s(I)),
Rw = 0.2176 (all data), GOF = 1.181. Crystallographic data for
2b: C28H18BN3O, Mr = 423.26, orthorhombic, space group Pnma
(no. 62), a = 16.272(2), b = 17.852(2), c = 6.8503(9) Š, V=
1990.0(4) Š3, T= 90 K, 1calcd = 1.413 gcmÀ3, Z = 4; R1 = 0.0947
(I > 2s(I)), Rw = 0.1970 (all data), GOF = 1.335. Crystallo-
graphic data for 2c: C30H22BN3O, Mr = 451.32, monoclinic,
space group P21/c (no. 14), a = 9.8657(11), b = 13.8409(15), c =
17.1556(18) Š, b = 104.831(7)8, V= 2264.6(4) Š3, T= 123 K,
1calcd = 1.324 gcmÀ3, Z = 4; R1 = 0.0584 (I > 2s(I)), Rw = 0.1765
(all data), GOF = 1.005. Crystallographic data for 2e:
C29H15BN3O2F3, Mr = 505.25, monoclinic, space group P21/n
(no. 14), a = 9.5441(11), b = 15.8360(17), c = 15.4303(18) Š, b =
102.245(7)8, V= 2279.1(4) Š3, T= 123 K; 1calcd = 1.473 gcmÀ3
,
Z = 4; R1 = 0.0722 (I > 2s(I)), Rw = 0.2223 (all data), GOF =
1.242; CCDC-281754–281757 contains the supplementary crys-
tallographic 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.
[6] Crystal-packing diagrams are pairwise dimeric forms for 2a, 2c,
and 2e, whereas J-aggregation-type packing is seen for 2b (see
the Supporting Information). This situation may lead to the
shallowest bowl depth in 2b.
À
[7] In 1 (L = OH), the average N-B-N angle, B N bond length, and
À
B O bond length are reported in Ref. [2c] to be 102.98, 1.49 Š,
and 1.438(14) Š, respectively.
[8] K. A. Nguyen, R. Pachter, J. Chem. Phys. 2001, 114, 10757.
[9] Relative to FF = 0.37 for 8-amino-1-naphthalenesulfonic acid in
ethanol; G. Weber, L. B. Young, J. Biol. Chem. 1964, 239, 1415.
[10] H. S. Cho, H. Rhee, J. K. Song, C.-K. Min, M. Takase, N. Aratani,
S. Cho, A. Osuka, T. Joo, D. Kim, J. Am. Chem. Soc. 2003, 125,
5849.
[11] B. del Ray, U. Keller, T. Torres, G. Rojo, F. Agullꢀ-Lꢀpez, S.
Nonell, C. Martí, S. Brasselet, I. Ledoux, J. Zyss, J. Am. Chem.
Soc. 1998, 120, 12808.
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