Directly Pd(II)-bridged porphyrin belts with remarkable curvatures

Article abstract of DOI:10.1021/ja1046654

A β,β′-doubly 2,6-pyridylene-bridged porphyrin dimer and trimer were prepared by Suzuki-Miyaura coupling reactions and confirmed to have largely bent structures. These oligoporphyrins were readily metalated via meso-C-H bond activation with the assistance of the pyridyl nitrogen atoms to produce the corresponding Pd(II) complexes, which display even larger bent structures and larger TPA values at 800 nm.

Full text of DOI:10.1021/ja1046654

Published on Web 08/10/2010
Directly Pd(II)-Bridged Porphyrin Belts with Remarkable Curvatures
Jianxin Song,† Naoki Aratani,*,†,‡ Ji Haeng Heo,§ Dongho Kim,*,§ Hiroshi Shinokubo,*,| and
Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan,
PRESTO, Japan Science and Technology Agency, Spectroscopy Laboratory for Functional π-Electronic Systems and
Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, and Department of Applied Chemistry,
Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 463-8603, Japan
Received May 28, 2010; E-mail: aratani@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr; hshino@apchem.nagoya-u.ac.jp;
osuka@kuchem.kyoto-u.ac.jp
Scheme 1. Synthesis of Porphyrin Belts
Abstract: A ꢀ,ꢀ′-doubly 2,6-pyridylene-bridged porphyrin dimer
and trimer were prepared by Suzuki-Miyaura coupling reactions
and confirmed to have largely bent structures. These oligopor-
phyrins were readily metalated via meso-C-H bond activation
with the assistance of the pyridyl nitrogen atoms to produce the
corresponding Pd(II) complexes, which display even larger bent
structures and larger TPA values at 800 nm.
Electronically interactive multiporphyrinic systems are promising
in light of their π-conjugated properties that are favorable for
applications such as optoelectronic devices, sensors, photovoltaic
devices, nonlinear optical (NLO) materials, and photodynamic
therapy (PDT) pigments.1 To realize the electronic and photophysi-
cal properties required for such applications, exploitation of novel
means to manipulate the interporphyrinic interaction is highly
desirable. Here we report the synthesis of ꢀ,ꢀ′-doubly 2,6-
pyridylene-bridged Ni(II)-porphyrin arrays that can accommodate
Pd(II) metal in the cavity created by two pyridyl moieties.2
Interestingly, these porphyrin arrays exhibit remarkably bent
conformations due to constraints arising from ꢀ,ꢀ′-double 2,6-
pyridylene bridges. More importantly, the formation of the direct
meso-to-meso metal bridge triggers activation of the electronic
interaction between the neighboring porphyrins, hence causing
effective delocalization of the π-electronic system.
ꢀ,ꢀ′-Dipyridyl Ni(II) porphyrin 2 was synthesized by a
Suzuki-Miyaura coupling reaction of ꢀ,ꢀ′-diboryl Ni(II) porphyrin
13 and 2,6-dibromopyridine.4 Double Suzuki-Miyaura coupling
reaction of 2 with 1 afforded 3 in 56% yield (Scheme 1).5
Diporphyrin 3 has a preorganized central cavity which would be
suitable for metal insertion. In fact, treatment of 3 with Pd(OAc)2
in the presence of NaOAc in CH2Cl2/methanol at room temperature
resulted in facile metalation to afford 3-Pd as an air and moisture
36° and that between two porphyrins to 87.3°, thus inducing further
stable brown solid in 85% yield. The structures of 3 and 3-Pd have
curving of the dimer. The C-Pd-C and N-Pd-N angles in 3-Pd
1
been assigned by their H NMR and high-resolution electrospray
were 161° and 157°, respectively.
ionization time-of-flight (HR-ESI-TOF) mass spectra.5 The final
The UV-vis absorption spectrum of 3 exhibits a slightly broad
Soret band at λmax ) 421 nm (Figure 2) as compared with that of
the monomer. In sharp contrast, that of 3-Pd shows a remarkably
structural confirmation was obtained from single-crystal X-ray
diffraction analysis (Figure 1).5 Interestingly, the diporphyrin 3
displays a gable structure with a diporphyrin dihedral angle of
perturbed absorption spectrum that spreads over the whole visible
106.5°. The pyridyl bridges are held to the neighboring pyrroles
region with peaks at 450, 500, 579, and 673 nm. Density functional
theory (DFT) calculations revealed distinct differences between the
molecular orbitals of 3 and 3-Pd (Figure S10). One of the
with dihedral angles of ca. 47°. In 3-Pd, Pd(II) insertion decreased
the dihedral angle between the pyridyl and porphyrin planes to ca.
degenerated unoccupied MOs (LUMO+1) of 3 is well hybridized
† Kyoto University.
‡ PRESTO.
with a Pd vacant orbital, forming a highly stabilized LUMO of
3-Pd delocalized over palladium, meso-carbon, and pyridine
nitrogen. In contrast, HOMO-2 of 3 which has a2u character with
§ Yonsei University.
| Nagoya University.
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11868 J. AM. CHEM. SOC. 2010, 132, 11868–11869
10.1021/ja1046654  2010 American Chemical Society

C O M M U N I C A T I O N S
the anti-form by 3.2 kcal/mol (Figure S11).5 The structure of 4
was unambiguously determined by X-ray diffraction analysis to
be a remarkably bent syn-form that has a large hollow (Figure 1).
All the constituent Ni(II) porphyrins take on ruffled conformations
with neighboring porphyrins with dihedral angles of 88 and 89°.
Metalation of 4 with Pd(OAc)2 in CH2Cl2 furnished 4-Pd in 75%
yield. The absorption spectra of 4 and 4-Pd are similar to those of
3 and 3-Pd, suggesting nonconjugative and conjugative features
of the arrays, respectively.
The two-photon absorption (TPA) cross section values of the
porphyrin belts were measured by using an open-aperture Z-scan
method.5,6 Laser excitation at 800 nm was chosen to completely
eliminate the contribution from one-photon absorption. TPA values
were determined for 3 (7000 GM), 3-Pd (15 700 GM), 4 (17 400
GM), and 4-Pd (24 000 GM), respectively. In line with the
conjugative nature of 3-Pd and 4-Pd, their TPA values are larger
than those of 3 and 4, indicating the substantial contribution of the
direct Cmeso-Pd-Cmeso linkage in the overall π-electronic conjuga-
tion.1
In summary, a ꢀ,ꢀ′-doubly 2,6-pyridylene-bridged porphyrin
dimer and trimer with largely bent structures were constructed by
consecutive Suzuki-Miyaura coupling reactions. These oligopor-
phyrins were readily metalated via double meso-C-H bond
cleavage with the assistance of the pyridine coordination. The
Pd(II)-bridged porphyrin belts display even larger bent structures
and larger TPA cross section values at 800 nm. These features are
particularly advantageous for future applications in optical devices
working in the near-IR region. Recognition of fullerenes by these
bent oligoporphyrins7 and further extension of this synthetic strategy
toward the truly cyclic porphyrin tubes are currently being explored
in our laboratories.
Figure 1. X-ray crystal structures of porphyrin belts. (a) Top view of 3,
(b) side view of 3, (c) top view of 3-Pd, (d) side view of 3-Pd, (e) skew
view of 4, and (f) side view of 4. The thermal ellipsoids are 50% probability
level. tert-Butyl groups, solvent molecules, and hydrogen atoms are omitted
for clarity.
Acknowledgment. This work was partially supported by Grants-
in-Aid for Scientific Research (Nos. 22245006 (A), 21685011, and
20108001 “pi-Space”) from MEXT, Japan, and PRESTO program
from JST, Japan. H.S. gratefully acknowledges financial support
from the Toray Science Foundation. The work at Yonsei University
was supported by World Class University (R32-2009-000-10217-
0) Programs from MEST, Korea, and the Fundamental R&D
Program for Core Technology of Materials funded by the Ministry
of Knowledge Economy, Korea.
Supporting Information Available: Preparation and analytical data
for samples, and crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 2. UV-vis absorption spectra of 3, 3-Pd, 4, and 4-Pd in CH2Cl2.
Inset: Open-aperture Z-scan traces of 4-Pd.
a large MO coefficient on meso-carbons is strongly interactive with
the filled dzx orbital on Pd, raising the orbital over the original a1u
HOMO of 3. The coplanarization of pyridine and porphyrin also
helps this situation, thus resulting in a substantial decrease of the
HOMO-LUMO gap by metalation.5 These features are supported
by electrochemical analysis. The first oxidation potential (vs Ag/
Ag+) is decreased from 0.75 V for 3 to 0.52 V for 3-Pd, and the
first reduction potential is increased from -1.52 V for 3 to -1.40
V for 3-Pd, respectively.5
References
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In order to expand this synthetic strategy, the cross-coupling
reaction of tetraborylporphyrin 53 and 2 equiv of 2 was attempted
and indeed produced trimer 4 in 13% yield. HR-ESI-TOF mass
(3) Hata, H.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2005, 127, 8264.
(4) Song, J.; Kim, P.; Aratani, N.; Kim, D.; Shinokubo, H.; Osuka, A.
Chem.sEur. J. 2010, 16, 3009.
1
(5) For details, see Supporting Information.
and H NMR spectra of 4 are fully consistent with the expected
structure.5 Triporphyrin 4, judging from the bent structure of 3,
should take a syn- or an anti-form, which would be difficult to
distinguish based on the spectroscopic data. DFT calculations
(B3LYP/631SDD) indicate that the syn-form is more stable than
(6) Sheik-Bahae, M.; Said, A. A.; Wei, T.-H.; Hagan, D. G.; van Stryland, E. W.
IEEE J. Quantum Electron. 1990, 26, 760.
(7) The cavities of these curved systems are suitable for binding fullerenes. The
details will be reported elsewhere.
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