Dipyrrin-porphyrin hybrids: Potential π-conjugated platform to fabricate coordination oligomers

Article abstract of DOI:10.1246/cl.2005.1150

Bis-dipyrrin-appended porphyrin derivatives, potential building blocks for supramolecular coordination architectures, were synthesized and exhibited complexation at the two dipyrrin sites with BF₂ and Rh(CO) ₂. Copyright

Full text of DOI:10.1246/cl.2005.1150

1150
Chemistry Letters Vol.34, No.8 (2005)
Dipyrrin–Porphyrin Hybrids: Potential ꢀ-Conjugated Platform
to Fabricate Coordination Oligomers
Hiromitsu Maedaꢀ and Miki Ito
Department of Bioscience and Biotechnology, Faculty of Science and Engineering,
Ritsumeikan University, Kusatsu 525-8577
(Received June 13, 2005; CL-050758)
Bis-dipyrrin-appended porphyrin derivatives, potential
building blocks for supramolecular coordination architectures,
were synthesized and exhibited complexation at the two dipyrrin
sites with BF2 and Rh(CO)2.
Metal coordination enables organic ligands to form potential
oligomers and nanospace applicable for materials science.1
Among a variety of coordination assemblies, porphyrin platform
is used as a functional ꢀ-conjugated system for versatile pur-
poses.2 Porphyrin squares and prism fabricated by meso-pyridyl
units were achieved by coordination to metal cations like PdII.3
Contrasted with neutral ligands such as 2,20-bipyridines, dipyrrin
(dipyrromethene), a dye consisting of two pyrroles bridged by
conjugated linkage, is an essential bidentate monoanionic ligand
for metal ion in natural and artificial systems (Figure 1).4 Dipyr-
rin forms metal complexes in flexible coordination modes ac-
cording to metal cations, due to the acyclic structure. Therefore,
dipyrrin, a promising planar scaffolding for supramolecular ar-
chitectures, would give discrete neutral coordination oligomers.
Recently, Lindsey et al. reported dipyrrin metal complexes as
bridging units for energy transfer systems consisting of porphy-
rins.5 Here synthesis of dipyrrin-substituted porphyrins as build-
ing units for promising coordination space is reported. Dipyrrin
units chelate metal cations and analogs such as difluoroboron
(BF2) and Rh(CO)2 moieties.
Scheme 1. Synthesis of m-formylphenyl-substituted porphyrin
2.
Figure 2. X-ray single crystal structure of porphyrin 2, (a) top
view (ORTEP drawing) and (b) ꢀ-stacking diagram from the
side view. Thermal ellipsoids are scaled to the 50% probability
level.
Under acidic conditions using TFA,8 formyl derivative (2)
was transformed into dipyrromethane-appended porphyrins (3a
and 3b) as major products using ꢁ-free- and ꢁ-methylpyrrole,9
respectively (see the Supplemental Information). The derivative
with unsubstituted pyrroles readily suffers the polymerization,
possibly due to the remaining ‘‘free’’ pyrrolyl ꢁ- and ꢂ-posi-
tions. Without purification, 3a and 3b were converted to bisdi-
pyrrin-porphyrin hybrids (4a and 4b) by p-chloranil oxidation.
The formation of 4b was suggested by 1H NMR, but the forma-
tion of other derivatives 3a, 3b, and 4a, rather unstable com-
pounds, was just inferred by MALDI-TOF-MS. For the efficient
synthesis of stable dipyrromethane and dipyrrin derivatives,
were examined two strategies: (i) metal coordination in the core
to avoid trapping protons in the cavity and (ii) complete substi-
tution of dipyrrin units to increase the stability. First, complex-
ation of diamagnetic NiII, resistible to demetallation, was per-
formed by the treatment with Ni(acac)2 hydrate in refluxing tol-
uene for 8 h to afford 2-Ni in 87% yield. Second, to obtain the
derivative substituted totally except for N-positions, condensa-
tion of 2-Ni with 2,3,4-trimethylpyrrole10 resulted in the rather
stable dipyrromethane 3c-Ni (Scheme 2). In 1H NMR spectrum
in CDCl3, the signal of formyl proton of 2-Ni at 10.28 ppm com-
pletely disappeared, and, instead, that ascribable to meso-CH
proton of 3c-Ni appeared at 5.66 ppm. Dipyrromethane 3c-Ni,
gradually decomposed on silica gel chromatography, was stabi-
Figure 1. Frameworks of 2,20-bipyridine (left) and dipyrrin
(right) as coordination ligands.
5,15-Di(m-formylphenyl)porphyrin (2), a key precursor for
synthesis of dipyrrin-substituted porphyrin, was obtained in
13% yield by acid-catalyzed [2 þ 2] condensation of protected
aldehyde6 and meso-pentafluorophenyl (C6F5) dipyrromethane
using methanesulfonic acid (MSA) and oxidation by DDQ, fol-
lowed by deprotection of porphyrin 1 using TFA (Scheme 1). In
order to avoid fragmentation and scrambling by acid catalyst
under cyclization, electron-withdrawing substituent was used
in the dipyrromethane unit. m-Formyl-substitution, which could
be more useful and suitable for the construction of coordination
space than p- and o-formyl substitutions, was derived from the
corresponding protected aldehyde. The structures of porphyrins
1
1 and 2 were confirmed by H NMR and FAB-MS. Although
conformation of the two formyl moieties of 2 in solution was
not determined, the moieties in the solid state orient in the anti
conformation, not the syn, which was elucidated by X-ray single
crystal structure analysis.7 The ꢀ-plane of 2 stacks at edge-to-
ꢀ
edge of each other with the plane distance of 3.897 A (Figure 2).
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.8 (2005)
1151
Zn(OAc)2 is found to show the MALDI-TOF-MS suggesting
the formation of dimer (m=z ¼ 2738, corresponding to 2 ꢁ
4c-Ni þ 2 ꢁ Zn2þ; see the Supplemental Information) as a
major product and trimetric and tetrametric structures as traces,
which infers that the dipyrrin moieties behave as potential scaf-
folds for supramolecular coordination polymers. Further investi-
gations to fabricate nanostructures based on the dipyrrin metal
complexes are now in progress.
This paper was supported by the ‘‘Academic Frontier’’
Project for Private Universities, namely the matching fund sub-
sidy from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), 2003–2008. The authors thank Prof.
Atsuhiro Osuka and Mr. Soji Shimizu, Kyoto University, for
the X-ray measurement and Prof. Hitoshi Tamiaki, Ritsumeikan
University for helpful discussions.
Scheme 2. Transformation of formyl groups into dipyrrome-
thane and dipyrrin units, and coordination at the dipyrrin sites
of 4c-Ni.
References and Notes
1
a) ‘‘Transition Metals in Supramolecular Chemistry,’’ ed. by J.-P.
Sauvage, John Wiley & Sons, Chichester (1999). b) M. Fujita, K.
Umemoto, M. Yoshizawa, N. Fujita, T. Kusukawa, and K. Biradha,
Chem. Commun., 2001, 509.
´
2
3
a) J. Wojaczynski and L. Latos-Grazynski, Coord. Chem. Rev., 204,
˙
113 (2000). b) T. Imamura and K. Fukushima, Coord. Chem. Rev.,
198, 133 (2000).
a) P. J. Stang, J. Fan, and B. Olenyuk, Chem. Commun., 1997, 1453.
b) N. Fujita, K. Biradha, M. Fujita, S. Sakamoto, and K. Yamaguchi,
Angew. Chem., Int. Ed., 40, 1718 (2001). c) M. L. Merlau, M. P. Mejia,
S. T. Nguyen, and J. T. Hupp, Angew. Chem., Int. Ed., 40, 4239 (2001).
a) H. Falk, ‘‘The Chemistry of Linear and Oligopyrroles and Bile
Figure 3. UV–vis absorption spectra of 2-Ni (solid line), 4c-Ni
(dotted line), and BF2 complex 5 (broken line) in CHCl3.
lized by dehydrogenation using p-chloranil to give a rather polar
dipyrrin 4c-Ni in 84% yield from 2-Ni. The resonance of sp3-CH
proton between dipyrromethane pyrroles in 1H NMR (CDCl3)
was vanished and that of dipyrrin NH proton was observed at
the downfield at 12.22 ppm, due to intramolecular hydrogen
bonding. UV–vis absorption spectrum of 4c-Ni in CHCl3 shows
a Soret-band at 412.0 nm and a large absorption at 519.5 nm,
possibly due to the electronic interaction with dipyrrin units
(Figure 3).
4c-Ni is the first example of phenylene-bridged bis-dipyrrin-
appended porphyrin. So far, various porphyrin derivatives with a
dipyrrin moiety have been reported by using long chain like phe-
nylethynyl spacer,11 and only two reports on the phenylene-
bridged mono-dipyrrin derivatives have been reported.5,12 As
mentioned above, the phenylene-bridged dipyrrin-porphyrin hy-
brids are less stable and, in order to avoid the decomposition,
need the protection of pyrrole moieties in two dipyrrin units seen
in 4c-Ni.
4
Pigments,’’ Springer-Verlag, Vienna (1989). b) C. Bruckner, Y. Zhang,
¨
S. J. Rettig, and D. Dolphin, Coord. Chem. Rev., 263, 279 (1997). c) Y.
Zhang, A. Thompson, S. J. Rettig, and D. Dolphin, J. Am. Chem. Soc.,
120, 13537 (1998). d) A. Thompson, S. J. Rettig, and D. Dolphin,
Chem. Commun., 1999, 631. e) Q. Chen, Y. Zhang, and D. Dolphin,
Tetrahedron Lett., 43, 8413 (2002). f) T. E. Wood, N. D. Dalgleish,
E. D. Power, A. Thompson, X. Chen, and Y. Okamoto, J. Am. Chem.
Soc., 127, 5740 (2005).
L. Yu, K. Muthukumaran, I. V. Sazanovich, C. Kirmaier, E. Hindin,
J. R. Diers, P. D. Boyle, D. F. Bocian, D. Holten, and J. S. Lindsey,
Inorg. Chem., 42, 6629 (2003).
5
6
7
H. Shinmori, T. Kajiwara, and A. Osuka, Tetrahedron Lett., 42, 3617
(2001).
Crystal data for 2 (from CH2Cl2/CH2ClCH2Cl/hexane): C46H20N4-
F20O2, MW ¼ 850:67, monꢂoclinic, a ¼ 6:692ð4Þ, b ¼ 11:255ð7Þ,
ꢀ
ꢀ 3
c ¼ 24:50ð2Þ A, ꢂ ¼ 91:02ð2Þ , V ¼ 1845ð2Þ A , T ¼ 123ð2Þ K, space
group P21=c (No. 14), Z ¼ 4, Dc ¼ 1:531 g/cm3, ꢃðMo KꢁÞ ¼
0:1299 mmꢃ1, 17530 reflections measured, 17040 unique (Rint
¼
0:131), R ¼ 0:090, Rw ¼ 0:126, GOF ¼ 1:136 ðI > 3ꢄðIÞÞ. Crystallo-
graphic data for 2 has been deposited with Cambridge Crystallographic
Data Centre as supplementary publication No. CCDC-274878. Copy of
the data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge, 12, CB2 1EZ, UK; fax: +44 1223
336033; or deposit@ccdc.cam.uk).
Up to date, BF2 complexes of dipyrrins have been reported
as energy donor units seen in the synthetic photosynthetic mod-
els.13 In order to examine the chelation behavior, metal coordi-
nation chemistry of 4c-Ni at the dipyrrin units were examined.
.
When 4c-Ni was treated with excess BF3 OEt2 (ca. 30 equiv.)
8
9
H. Furuta, H. Maeda, T. Furuta, and A. Osuka, Org. Lett., 2, 187
(2000).
R. L. Hinman and S. Theodoropulos, J. Org. Chem., 28, 3052 (1963).
in CHCl3 with 5% Et3N at reflux temperature, bis-BF2 complex
5 was obtained in 52% yield (Scheme 2). 1H NMR signals of di-
pyrrin NH in CDCl3 disappeared. UV–vis absorption spectrum
in CHCl3 is found to show the Soret band at 411.5 nm and a large
absorption at 527.5 nm, which is red-shifted at 8 nm as compared
to that of metal-free 4c-Ni (Figure 3), and fluorescence quench-
ing infers the possibility of energy transfer from dipyrrin to
porphyrin. Moreover, [RhCl(CO)2]2 (1 equiv.) and NaOAc (10
equiv.) in refluxing CH2Cl2 gave bis-RhI complex 6 in 38%
yield.14 MALDI-TOF-MS exhibits the peak of the species miss-
ing the carbonyl units. In contrast to 5 and 6, treatment with
10 G. G. Kleinspehn, J. Am. Chem. Soc., 77, 1546 (1955).
11 a) F. Li, S. I. Yang, Y. Ciringh, J. Seth, C. H. Martin, III, D. L. Singh,
D. Kim, R. R. Birge, D. F. Bocian, D. Holten, and J. S. Lindsey,
J. Am. Chem. Soc., 120, 10001 (1998). b) F. D’Souza, P. M. Smith,
M. E. Zandler, A. L. McCarty, M. Itou, Y. Araki, and O. Ito, J. Am.
Chem. Soc., 126, 7898 (2004).
12 M. Koepf, A. Trabolsi, M. Elhabiri, J. A. Wytko, D. Paul, A. M.
Albrecht-Gary, and J. Weiss, Org. Lett., 7, 1279 (2005).
13 J. Karolin, L. B.-A. Johansson, L. Strandberg, and T. Ny, J. Am. Chem.
Soc., 116, 7801 (1994).
14 M. W. Roomi, Tetrahedron Lett., 15, 1131 (1974).
Published on the web (Advance View) July 16, 2005; DOI 10.1246/cl.2005.1150