Axial ligand exchange reactions of meso-aryl subporphyrins - Axially fluoro-substituted subporphyrin and a μ-oxo dimer and trimer of subporphyrins

Article abstract of DOI:10.1021/ic900880b

High reactivity of the boron atom of mesoaryl subporphyrins enables the introduction of a broad range of functional groups to its axial position. Axially fluoro-substituted subporphyrins were easily synthesized upon treatment of axially hydroxyl-substitut

Full text of DOI:10.1021/ic900880b

Inorg. Chem. 2009, 48, 7885–7890 7885
DOI: 10.1021/ic900880b
Axial Ligand Exchange Reactions of meso-Aryl Subporphyrins;Axially Fluoro-
Substituted Subporphyrin and a μ-Oxo Dimer and Trimer of Subporphyrins
Soji Shimizu, Atsushi Matsuda, and Nagao Kobayashi*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received May 6, 2009
High reactivity of the boron atom of meso-aryl subporphyrins enables the introduction of a broad range of functional
groups to its axial position. Axially fluoro-substituted subporphyrins were easily synthesized upon treatment of axially
hydroxyl-substituted subporphyrins with BF3 OEt2. Homogeneous μ-oxo dimers of subporphyrins were formed by
3
heating monomers in the presence of triethylamine under a high vacuum. A heterogeneous subporphyrin-
phthalocyanine-subporphyrin trimer was selectively formed from the respective monomers under similar reaction
conditions. Structures of these molecules were elucidated by 1H NMR spectra and a single-crystal X-ray diffraction
analysis, and the interactions between neighboring chromophores in the dimeric systems were estimated from
absorption and magnetic circular dichroism spectra.
Introduction
to its intense fluorescence and unusual nonlinear optical
properties.6 Subporphyrins known to date exhibit an intense
Soret band and rather weak, broad Q bands, both in the
shorter wavelength region compared to porphyrins due to
their contracted 14π-electron conjugation systems. Most of
these subporphyrins comprise a bowl-shaped structure in
which a boron atom is coordinated at the center of the
molecule in tetrahedral fashion by three nitrogen atoms of
pyrrole rings and one axial ligand such as a halogen atom,
hydroxy group, or carboxy group.2,3 Due to this domelike
structure, free rotation of the aryl groups at meso positions is
allowed in most meso-aryl subporphyrins, leading to a much-
enhanced coplanarity of the meso-aryl substituents and
subporphyrin core compared to the case of meso-aryl por-
phyrins. These structural features result in a large pertur-
bation effect of the aryl groups on the optical properties
and electronic structures of the subporphyrins.2 Recently,
Osuka et al. utilized this characteristic well to modify two-
photon absorption properties and emission behavior by
introducing various functional groups to the para positions
of meso-phenyl substituents via transition-metal-catalyzed
substitution reactions of meso-p-bromophenyl subporphyr-
ins.7 The coplanarity is also known to facilitate efficient
energy transfer from dendritic carbazole moieties at meso
positions to subporphyrin cores.8
Recent growth of interest in expanded and contracted
porphyrin analogues stems largely from their unique proper-
ties, which originate from their novel larger and smaller
macrocyclic π-conjugation systems compared to the well-
known 18π-electron conjugation system of porphyrins.1
Among these, subporphyrin is a comparatively new entry
in this class of molecules,2-4 and it had been anticipated for
many years as a real contracted congener of porphyrins. In
contrast, its phthalocyanine counterpart, subphthalocya-
nine, has been known for more than 30 years5 and has been
investigated in a variety of fields as a functional molecule, due
*To whom correspondence should be addressed. Tel./fax: +81-22-795-
7719. E-mail: nagaok@mail.tains.tohoku.ac.jp.
(1) (a) Sessler, J. L.; Gebauer, A.: Weghorn, S. J. The Porphyrin Hand-
book; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 1999; Vol 2, Chapter 9. (b) Sessler, J. L.; Weghorn, S. J. Expanded,
Contracted & Isomeric Porphyrins; Elsevier Science Ltd: New York, 1997. (c)
Furuta, H.; Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795–1804. (d) Sessler,
J. L.; Seidel, D. Angew. Chem., Int. Ed. 2003, 42, 5134–5175. (e) Torres, T.
Angew. Chem., Int. Ed. 2006, 45, 2834–2837.
(2) (a) Kobayashi, N.; Takeuchi, Y.; Matsuda, A. Angew. Chem., Int. Ed.
2007, 46, 758–760. (b) Takeuchi, Y.; Matsuda, A.; Kobayashi, N. J. Am. Chem.
Soc. 2007, 129, 8271–8281. (c) Makarova, E. A.; Shimizu, S.; Matsuda, A.;
Luk'yanets, E. A.; Kobayashi, N. Chem. Commun. 2008, 2109–2111.
(3) (a) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoon, M.-C.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 961–964. (b) Inokuma, Y.; Yoon, Z.
S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2007, 129, 4747–4761. (c) Tsurumaki,
E.; Saito, S.; Kim, K. S.; Lim, J. M.; Inokuma, Y.; Kim, D.; Osuka, A. J. Am.
Chem. Soc. 2008, 130, 438–439. (d) Saito, S.; Kim, K. S.; Yoon, Z. S.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2007, 46, 5591–5593. (e) Inokuma, Y.; Osuka,
A. Dalton Trans. 2008, 2517–2526.
(7) (a) Inokuma, Y.; Easwaramoorthi, S.; Jang, S. Y.; Kim, K. S.; Kim,
D.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 4840–4843. (b) Inokuma, Y.;
Easwaramoorthi, S.; Yoon, Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2008,
130, 12234–12235. (c) Inokuma, Y.; Hayashi, S.; Osuka, A. Chem. Lett. 2009,
38, 206–207.
(8) (a) Xu, T.; Lu, R.; Liu, X.; Chen, P.; Qiu, X.; Zhao, Y. Eur. J. Org.
Chem. 2008, 1065–1071. (b) Liu, X.; Lu, R.; Xu, T.; Xu, D.; Zhan, Y.; Chen, P.;
Qiu, X.; Zhao, Y. Eur. J. Org. Chem. 2009, 53–60.
_ ꢀ
(4) Mysliborski, R.; Latos-Grazynski, L.; Szterenberg, L.; Lis, T. Angew.
Chem., Int. Ed. 2006, 45, 3670–3674.
(5) Meller, A.; Ossko, A. Monatsh. Chem. 1972, 103, 150–155.
ꢀ
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r
2009 American Chemical Society
Published on Web 07/20/2009
pubs.acs.org/IC

7886 Inorganic Chemistry, Vol. 48, No. 16, 2009
Shimizu et al.
Another feature of the subporphyrin is the high reactivity
of the central boron atom, in which axial ligand exchange
reactions take place easily. This reactivity should be useful
both for fine-tuning of the properties of subporphyrins and
construction of molecular assemblies by using a μ-oxo
dimerization reaction. Similar approaches have been under-
taken in subphthalocyanine chemistry.9,10 Torres and co-
workers recently reported the substitution reaction of
an axial hydroxy group with fluoride and an intriguing
columnar packing diagram in the crystal structures of axially
fluoro-substituted subazaporphyrin due to a reduction in
steric hindrance and a favorable interaction between sub-
azaporphyrin units.11 Ng and Xu tested heterodimer forma-
tion of subphthalocyanine and either porphyrin or
phthalocyanine and revealed complexation processes in de-
tail.12 Inspired by these pioneering studies from the chemistry
of phthalocyanine, our recent research along this direction
revealed easy replacement of an axial hydroxy substituent
with fluoride and the rather less polar nature of axially
fluoro-substituted subporphyrin in silica gel columns with
only slight changes in other properties such as aromaticity
and absorption and fluorescence spectra. This lower polar
nature made isolation of the subporphyrins by silica gel much
more facile, since axially hydroxy species tend to be adsorbed
on silica gel. The axially fluorinated subporphyrin can be
converted to the original hydroxy species upon treatment
with trifluoroacetic acid followed by water. A μ-oxo dimeri-
zation reaction of axially hydroxyl-substituted subporphyr-
ins was first reported by us in 2007,2b and recently we found
that the conversion yields are totally dependent on the
electron-donating properties of meso-aryl substituents. This
reaction was further utilized to construct a subporphyrin-
phthalocyanine-subporphyrin heterotrimer system.
derived using the SHELXS-97 program and refined on F2 using
the SHELXL-97 program.
Syntheses. General Procedure for the Syntheses of Axially
Fluoro-Substituted Subporphyrins (2a-2c). Axially hydroxyl-
substituted subporphyrin (6.2 μmol) was dissolved in CH2Cl2
(3 mL), and an excess amount of BF3 OEt2 (50 μL, 0.40 mmol)
3
was added. The solution was stirred for 30 min at room
temperature, and then the solvent was removed. Axially
fluoro-substituted subporphyrin was purified using silica gel
chromatography (CHCl3) in yields of 95% for 2a, 92% for 2b,
and 97% for 2c. Further purification was performed by recrys-
tallization from CH2Cl2/MeOH.
Axially Fluoro-Substituted meso-Phenylsubporphyrin (2a).
1H NMR (CDCl3): δ 7.63 (t, J=7.2 Hz, 3H, phenyl-p), 7.72
(m, 6H, phenyl-m), 8.07 (d, J=7.2 Hz, 6H, phenyl-o), 8.18 (s,
6H, pyrrole-β) ppm. HRMS (ESI-FT-ICR) calcd for C33H21-
BN3FNa ([M + Na]+): 512.1705. Found: 512.1701. UV/vis
(CHCl3): λmax (ε) 456 (12 000), 369 (157 000) nm.
Axially Fluoro-Substituted meso-Anisylsubporphyrin (2b).
1H NMR (CDCl3): δ 4.00 (s, 9H, OCH3), 7.26 (d, J=8.6 Hz,
6H, anisyl-m), 8.00 (d, J=8.6 Hz, 6H, anisyl-o), 8.15 (s, 6H,
pyrrole-β) ppm. HRMS (ESI-FT-ICR) calcd for C36H27-
BN3O3FNa ([M + Na]+): 602.2022. Found: 602.2019. UV/vis
(CHCl3): λmax (ε) 494 (13 000), 462 (11 000), 377 (160 000) nm.
Axially Fluoro-Substituted meso-(4-Trifluoromethylphenyl)-
1
subporphyrin (2c). H NMR (CDCl3): δ 8.00 (d, J = 4.3 Hz,
6H, aryl-m), 8.18 (d, J=4.3 Hz, 6H, aryl-o), 8.19 (s, 6H, pyrrole-
β) ppm. HRMS (ESI-FT-ICR) calcd for C36H18BN3F10Na ([M
+ Na]+): 716.1326. Found: 716.1323. UV/vis (CHCl3): λmax (ε)
456 (13 000), 369 (155 000) nm.
General Procedure for Syntheses of μ-Oxo Dimers of Sub-
porphyrins (3a-3d). Axially hydroxyl-substituted subporphyrin
(10 μmol) was dissolved in 3 mL of 100:1 CHCl3/triethylamine,
and the solvent was removed. The residue was heated at 85 °C in
vacuo for 12 h and subjected to gel permeation chromatogra-
phy-high-performance liquid chromatography (GPC-HPLC)
using 1000:1 CHCl3/triethylamine as an eluent to give 3a-3d as
the first fractions (55% for 3a, 6% for 3b, 90% for 3c, and 84%
for 3d).
μ-Oxo Dimer of meso-Phenylsubporphyrin (3a). 1H NMR
(CDCl3): δ 7.52 (m, 6H, phenyl-p), 7.56 (m, 12H, phenyl-m),
7.64 (s, 12H, pyrrole-β), 7.78 (d, J = 7.2 Hz, 12H, phenyl-o)
ppm. HRMS (ESI-FT-ICR) calcd for C66H42B2N6ONa ([M +
Na]+): 979.3498. Found: 979.3501. UV/vis (CHCl3): λmax (ε)
469 (9900), 364 (140 000) nm.
Experimental Section
Instrumentation. Electronic absorption spectra were recorded
with a Hitachi U-3410 and a JASCO V-570 spectrophotometers.
Magnetic circular dichroism (MCD) spectra were recorded with
a JASCO J-725 spectrodichrometer equipped with a JASCO
electromagnet, which produces magnetic fields of up to 1.09 T
(1T = 1 tesla) with both parallel and antiparallel fields. The
magnitudes were expressed in terms of molar ellipticity per tesla
([θ]M/deg dm3 mol-1 cm-1 T-1). The fluorescence spectrum was
μ-Oxo Dimer of meso-Anisylsubporphyrin (3b). 1H NMR
(CDCl3): δ 7.12 (d, J = 8.8 Hz, 12H, anisyl-m), 7.62 (s, 12H,
pyrrole-β), 7.75(d, J=8.8 Hz, 12H, anisyl-o) ppm. UV/vis (CHCl3):
1
measured with a Hitachi F-4500 spectrofluorimeter. H NMR
spectra were recorded on a JEOL ECA-600 spectrometer
(operating as 594.17 MHz) and a Bruker AVANCE 400 spectro-
meter (operating as 400.33 MHz) using the residual solvent as
the internal reference (δ=7.260 ppm for CDCl3). High-resolu-
tion mass spectra were recorded on a Bruker Daltonics Apex-III
spectrometer. X-ray crystallographic studies were carried out on
a Rigaku Saturn CCD spectrometer with graphite monochro-
λmax (ε) 503 (12 000), 473sh (10 000) and 370 (135 000) nm.
μ-Oxo Dimer of meso-(4-Trifluoromethylphenyl)subporphyrin
(3c). 1H NMR (CDCl3): δ 7.62 (s, 12H, pyrrole-β), 7.80 (m, 24H,
aryl-o, m) ppm. HRMS (ESI-FT-ICR) calcd for
C72H36B2F18N6ONa ([M
+
Na]+): 1387.2741. Found:
1387.2854. UV/vis (CHCl3): λmax (ε) 467 (14 000), 364
(199 000) nm.
˚
matized Mo KR radiation (λ=0.71073 A). The structures were
μ-Oxo Dimer of meso-(3-Pyridyl)subporphyrin (3d). 1H NMR
(CDCl3): δ 7.58 (m, 6H, aryl-m), 7.73 (s, 12H, pyrrole-β), 8.10 (d,
J=8.0 Hz, 6H, aryl-o), 8.83 (d, J=4.9 Hz, 6H, aryl-p), 9.03 (s,
6H, aryl-o0) ppm. HRMS (ESI-FT-ICR) calcd for
C60H36B2N12ONa ([M + Na]+): 985.3213. Found: 985.3209.
UV/vis (CHCl3): λmax (ε) 465 (7700), 363 (109 000) nm.
(9) (a) Geyer, M.; Plenzig, F.; Rauschnabel, J.; Hanack, M.; del Rey, B.;
Sastre, A.; Torres, T. Synthesis 1996, 1139–1151. (b) Kobayashi, N.; Ishizaki,
T.; Ishii, K.; Konami, H. J. Am. Chem. Soc. 1999, 121, 9096–9110. (c) Fukuda,
T.; Olmstead, M. M.; Durfee, W. S.; Kobayashi, N. Chem. Commun. 2003, 1256–
1257.
ꢀ
(10) (a) Gonzalez-Rodrıguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.;
Echegoyen, L. Org. Lett. 2002, 4, 335–338. (b) Iglesias, R. S.; Claessens, C. G.;
Torres, T.; Aminur Rahman, G. M.; Guldi, D. M. Chem. Commun. 2005, 2113–
2115.
Results and Discussion
The axial fluoro-substitution reaction of meso-phenyl
subporphyrin 1a,2a which we were motivated to investigate
by the recent report on subazaporphyrin and subphthalo-
ꢀ
(11) Rodrıguez-Morgade, M. S.; Claessens, C. G.; Medina, A.; Gonzalez-
ꢀ
Rodrıguez, D.; Gutierrez-Puebla, E.; Monge, A.; Alkorta, I.; Elguero, J.;
Torres, T. Chem.;Eur. J. 2008, 14, 1342–1350.
(12) Xu, H.; Ng, D. K. P. Inorg. Chem. 2008, 47, 7921–7927.
cyanine by Torres et al.,11 was carried out using BF3 OEt2 in
3

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7887
Scheme 1. Synthesis of Axially Fluoro-Substituted Subporphyrins
2a-2c
CH2Cl2 at room temperature (Scheme 1). This reaction
proceeded almost quantitatively, to give axially fluoro-sub-
stituted meso-phenyl subporphyrin 2a in 95% yield. Con-
sidering the higher temperature (110 °C) needed for the
reaction of subphthalocyanine and the lower yields of both
subazaporphyrin and subphthalocyanine, the reactivity of a
boron atom in subporphyrin is higher than that of subaza-
porphyrin and subphthalocyanine. Since quantitative con-
version was achieved in the case of subporphyrins having
either electron-donating anisyl substituents (1b)2b or elec-
tron-withdrawing trifluoromethyl substituents (1c),2b aryl
groups at meso positions do not appear to have any effect
on the yields. Axially fluoro-substituted species can be easily
hydrolyzed, to give the original axially hydroxyl-substituted
species upon treatment with trifluoroacetic acid in CH2Cl2
followed by water. During this investigation, we observed a
less polar nature of the fluoro species 2a-2c in silica gel
compared to the hydroxyl-substituted species (see the Sup-
porting Information). As was reported elsewhere,2 axially
hydroxyl-substituted subporphyrins tend to be adsorbed on
silica gel, and the isolated yields were apparently lower than
synthetic yields. The less polar nature of the axially fluoro-
substituted species prompted us to test the axial ligand
exchange procedure prior to separation of the reaction
mixture by silica gel column. Tripyrrolyl borane was reacted
with arylaldehyde in refluxing propionic acid for 3-4 h. After
removing the solvent, the resultant tar was dissolved in
CH2Cl2, and BF3 OEt2 was added. The mixture was stirred
3
at room temperature for 30 min, and complete conversion of
axially hydroxyl-substituted subporphyrins to fluorinated
species was monitored by thin-layer chromatography
(TLC) analysis. Separation by silica gel column provided
axially fluoro-substituted subporphyrins as the first fraction
with some tailing due to partial decomposition into hydroxyl-
substituted species in the silica gel. Apart from the different
behaviors in silica gel, changes in other properties are fairly
modest. In the 1H NMR spectra of 2a, 2b, and 2c, a signal due
to the protons at β positions is shifted to a lower magnetic
field of 8.18, 8.15, and 8.19 ppm, respectively (see the
Supporting Information). This slight downfield shift of β
protons implies an enhancement of the aromaticity of the
subporphyrin core, probably due to an increase in planarity,
which was predicted for optimized structures calculated by
DFT methods using the B3LYP hybrid functional with
6-31G(d) basis sets (see the Supporting Information). In
the UV/vis absorption spectra, slight blue shifts of both the
Soret band and Q bands by 3-4 nm were observed, but the
absorption and MCD spectra were essentially identical in
Figure 1. UV/vis (bottom) and MCD (top) spectra of (a) 2a, (b) 2b, and
(c) 2c (solid lines) and the corresponding axially hydroxyl-substituted
species (1a-1c; broken lines) in CHCl3.
shape (Figure 1). Furthermore, a blue shift of the fluore-
scence was observed, and the fluorescence quantum yields
were increased (Table 1 and see the Supporting Information).
μ-Oxo bond formation is a useful reaction for building
molecular assemblies of subporphyrins. This possibility was
first suggested by us with a face-to-face μ-oxo dimer of meso-
3-pyridyl subporphyrin,2b and recently Osuka and Inokuma
reported an axially linked slipped-stack subporphyrin dimer
by using coordination of carboxy groups of meso-phenyl
substituents and control of the conformation of the biphenylene-
bridged subporphyrin dimer upon μ-oxo bond formation.13
(13) (a) Inokuma, Y.; Osuka, A. Chem. Commun. 2007, 2938–2940. (b)
Inokuma, Y.; Osuka, A. Org. Lett. 2008, 10, 5561–5564.

7888 Inorganic Chemistry, Vol. 48, No. 16, 2009
Shimizu et al.
Table 1. Absorption and Fluorescence Properties
absorptiona
λ
em (nm)b ΦF
c
compound
1ad
1bd
1cd
2a
2b
2c
3a
3b
3c
486 (8000), 461 (12000), 373 (151000)
496 (14000), 466 (11000), 379 (154000)
480 (7000), 460 (13000), 373 (155000)
456 (12000), 369 (157000)
492 (13000), 462 (11000), 377 (160000)
456 (13000), 369 (155000)
469 (9900), 364 (140000)
516
536
510
507
530
505
533
546
539
0.12
0.11
0.10
0.15
0.19
0.16
0.08
0.10
0.07
503 (12000), 370 (135000)
467 (14000), 364 (199000)
a λ/nm (ε/M-1 cm-1) data recorded in CHCl3. b Data recorded in
benzene. c Relative to 9,10-diphenylanthracene in benzene (ΦF = 0.84
excited at 366 nm).16 d See ref 2b.
Figure 2. 1H NMR spectra of μ-oxo dimer (3a) (bottom) and the
corresponding monomer (1a) (top) in CDCl3.
Scheme 2. Synthesis of μ-Oxo Dimers 3a-3d
Figure 3. X-ray crystal structure of 3a, top view (a) and side view
(b). The thermal ellipsoids are scaled to the 50% probability level.
the meso-aryl substituents. In the 1H NMR spectrum of 3a,
all of the peaks shifted to higher magnetic field due to the
aromatic ring current effect of the neighboring subporphyr-
ins (Figure 2). Δδ values of the β and ortho protons are large
(0.48 ppm for β protons and 0.29 ppm for ortho protons)
compared to those of meta and para protons. Similar changes
in the spectra were also observed for other dimers, 3b-3d
(see the Supporting Information). The structure of 3a was
finally revealed by single-crystal X-ray diffraction analysis
(Figure 3).14 Two subporphyrin units are linked through a
bent B-O-B bond with an angle of 139.6(3)°. The subpor-
phyrinunits are twistedby ca. 33° toeachothertoavoid steric
congestion between the meso-phenyl substituents. The bowl
depth, definedby the height of the boron atom from the mean
plane defined by six β-pyrrolic carbon atoms, became deeper
Several reaction conditions under which μ-oxo dimers of
subphthalocyanines, porphyrins, and phthalocyanines were
formed were tested to synthesize subporphyrin dimers,9 but
no reaction was found suitable for meso-aryl subporphyrins.
μ-Oxo dimers of subporphyrins were formed under certain
reaction conditions such that, after the addition of a small
amount of triethylamine, the starting monomers were heated
at 85 °C under a high vacuum for 12 h (Scheme 2). This
reaction produced μ-oxo dimers in 6-90% yield along with
recovery of the starting subporphyrin monomers. The start-
ing monomers (1a-1d)2a,2b and μ-oxo dimers (3a-3d) were
easily separated by recycling GPC-HPLC. The yields were
apparently dependent on the meso-aryl substituents. In
the case of electron-donating phenyl and anisyl substituents,
the yields were apparently low, 55% (3a) and 6% (3b), while
subporphyrins having electron-withdrawing substituents
such as p-trifluoromethylphenyl and 3-pyridyl groups tended
to form dimers in high yields of 90% (3c) and 84% (3d),
respectively. The higher yields in the case of subporphyrins
having electron-withdrawing substituents can be explained
by an increased Lewis acidity of the boron atom as a result of
than that of the corresponding subporphyrin monomer (1.40
3b
˚
˚
and 1.46 A for 3a and 1.29 A for 1a ). The UV/vis absorp-
tion spectra of 3a-3d all exhibited slight blue shifts of the
Soret bands by ca. 10 nm, while the tail of the Soret bands
extended to 430 nm, which cannot be observed for the
respective monomers (Figure 4). In contrast, the tail of Q
bands and emission maximum shifted slightly to the red. The
fluorescence quantum yields were slightly decreased, 0.12 for
1a and 0.08 for 3a (Table 1). These results can be basically
accounted for in terms of exciton coupling theory, which was
first applied to porphyrin compounds in the case of a μ-oxo
dimer of scandium porphyrins by Gouterman et al. in 1977.15
(14) Crystallographic data for 3a: C68H44B2N6OCl6, MW = 1195.41,
monoclinic, space group P21/n (no. 14), a=18.966(5), b=12.408(3), c=
3
26.138(6) A, β=110.0886(10)°, V=5777(2) A , Z=4, Fcalcd=1.374 g/cm3, T=
-100 °C, 12 893 measured reflections, 5272 unique reflections,
R=0.0755, Rw=0.2319 (all data), GOF=0.959.
˚
˚
(15) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977, 25,
139–153.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7889
Figure 5. 1H NMR spectra of 1a (a), 4 (b), and 5 as synthesized (c) in
CDCl3.
vacuum, and formation of the subporphyrin-phthalo-
cyanine-subporphyrin heterotrimer 5 was confirmed by
MALDI-TOF mass (see Supporting Information) and
1
1H NMR spectra (Figure 5). In the H NMR spectrum,
Figure 4. UV/vis (bottom) and MCD (top) spectra of μ-oxo dimers
(3a-3d; solid lines) and the corresponding monomers (1a-1d; broken
lines) in CHCl3.
upfield shifts of thesignals due to protons at theβ positions of
1a and R-benzo positions of 4 were observed. The 2:3
integration ratio of signals due to the R-benzo and β-pyrrolic
protons surely characterized this compound as the target
heterotrimer. Unfortunately, however, isolation attempts,
either by silica gel or GPC column, led to decomposition of
the trimer into the original monomers and subporphyrin-
phthalocyanine dimer. Despite the instability of the B-O-Si
bond, it is worth noting that the heterotrimer species was
selectively formed instead of the formation of subporphyr-
in-subporphyrin and phthalocyanine-phthalocyanine
homodimers. This can be ascribed to the difference in
reactivity of silicon and boron in μ-oxo bond formation.
The μ-oxo dimer of scandium porphyrins showed a similar
blue shift and red tail of the Soret band and a red shift of the Q
bands. The former was explained by exciton coupling inter-
actions, while the latter was attributed to broadening of the
band by solvent effects. The red tail of the Soret band is
attributable to a forbidden lower-energy band created by
exciton coupling, which becomes partially allowed by sym-
metry lowering caused by molecular vibration. Since the
strength of the dipole moment of the Q band is smaller than
that of the Soret band, exciton coupling interactions should be
less significant for the Q band, so that inhomogenous solvent
broadening due to dimerization is the main factor for the red
shift of the Q band. This illustration can also be applied to the
μ-oxo dimers of subporphyrins. Since the blue shifts of the
Soret band (ca. 10 nm for 3a) and the decrease of fluorescence
quantum yields were small, the extent of exciton coupling
interaction can be assumed to be less significant in the μ-oxo
dimers of subporphyrins. This would be reasonable consider-
ing their nonplanar structures and bent B-O-B bonds.
We further extended this reactivity of the boron atom to
construct a heterotrimer system. Phthalocyanines are well-
known UV/vis absorbing dyes, exhibiting intense Q bands at
around 700 nm. Moreover, similar μ-oxo oligomerization
reactivity was seen for silicon phthalocyanines. A heterotri-
mer system of meso-aryl subporphyrins and silicon phthalo-
cyanine is of interest because the resultant trimer system can
absorb a wide range of light. Under similar reaction condi-
tions to those of the μ-oxo dimerization reaction, 1a and octa-
butoxy silicon phthalocyanine 4 were reacted at 120 °C in a
2:1 molar ratio in the presence of triethylamine under a high
Conclusions
Axial ligand exchange reactions were utilized in order to
synthesize novel axially fluoro-substituted subporphyrins
(2a-2c), μ-oxo dimers (3a-3d), and the μ-oxo subporphyr-
in-phthalocyanine-subporphyrin hetero timer (5). Axially
fluoro-substituted subporphyrins exhibited slightly enhanced
aromatic character, probably due to rigid structures of the
fluorinated species. A drastic change of behavior in silica gel
made the column separation of subporphyrin species easier
by substituting the axial ligands with fluorine atoms prior to
column separation. In our previous work, isolation of sub-
porphyrins from the reaction mixture should be improved as
a result of its severe adsorption on silica gel. By using this
fluorination reaction, the subporphyrin species was easily
isolated. μ-Oxo dimers of subporphyrins were easily synthe-
sized fromthe respective monomers by heating at85°C under
a vacuum. Exciton coupling interactions between the sub-
porphyrins were revealed by UV/vis absorption spectra, but
the extent was fairly modest, reflecting the nonplanar struc-
ture and bent B-O-B bond. A heterogeneous μ-oxo trimer
of subporphyrin and silicon phthalocyanine was selectively
(16) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229–235.

7890 Inorganic Chemistry, Vol. 48, No. 16, 2009
Shimizu et al.
formed from the respective monomers, which was confirmed
by the MALDI-TOF mass and 1H NMR spectra. Unfortu-
nately, however, successful isolation of this trimer has not yet
been achieved due to facile decomposition into the subpor-
phyrin monomer and μ-oxo dimer of subporphyrin and
silicon phthalocyanine. Molecular assembly by using the μ-
oxo oligomerization reaction is one of the most promising
ways to develop new properties from these simple compo-
nents, but the stability of the μ-oxo bonding must be
controlled. Investigations in this direction are currently being
undertaken in our laboratory.
(No. 20108001, “pi-Space”) from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan
and a Grant-in-Aid for Young Scientists (B) (No.
20750025) from Japan Society for the Promotion of
Science (JSPS). The authors thank Dr. Chizuko Kabuto
for helpful discussion about X-ray analyses and Prof.
Masahiro Hirama and Dr. Shuji Yamashita for MALDI-
TOF mass measurements.
Supporting Information Available: A picture of TLC devel-
opment, H NMR and fluorescence spectra, optimized struc-
1
tures of 1a and 2a, MALDI TOF mass spectrum of 5, and a CIF
file of 3a. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. This work was partly supported by a
Grant-in-Aid for Scientific Research on Innovative Areas