Synthesis of the porphyrin-fused porphyrin, [2]porphyracene

Article abstract of DOI:10.1039/a906073a

A novel porphyrin-fused porphyrin is formed by an oligomerization reaction of a (porphyrinato)nickel complex with TeCl₄; the isolated compound, [2]porphyracene, was characterized by spectroscopic methods and direct observation using scanning tu

Full text of DOI:10.1039/a906073a

Synthesis of the porphyrin-fused porphyrin, [2]porphyracene
Ken-ichi Sugiura,* Takuya Matsumoto, Satoshi Ohkouchi, Yasuhisa Naitoh, Tomoji Kawai, Yoshio Takai,
Kantaro Ushiroda and Yoshiteru Sakata*
The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Japan.
E-mail: sakata@sanken.osaka-u.ac.jp
Received (in Cambridge, UK) 27th July 1999, Accepted 24th August 1999
A novel porphyrin-fused porphyrin is formed by an oligo-
merization reaction of a (porphyrinato)nickel complex with
TeCl4; the isolated compound, [2]porphyracene, was charac-
terized by spectroscopic methods and direct observation
using scanning tunnelling microscopy; the excitation energy
value of 13460 cm21 (743 nm) is the lowest value reported for
porphyrin dimers.
a preliminary molecular geometry calculation [15.5 3 8.8 Å;
Fig. 2(c)].
Aromatic protons of 1a (Hb, Hc, Hd, He, Hf and Hmeso) were
upfield shifted by 0.18–0.67 ppm compared with the corre-
Porphyrins hold a unique position among organic compounds
since they have a small HOMO–LUMO gap and can incorpo-
rate almost all types of metal ions in the central cavity.1
Reflecting these characteristics, porphyrins are present not only
in various biological systems, but are applied in many advanced
materials. Recently much efforts has been focused on the
expansion, contraction, isomerization and modification of the
porphyrin chromophore to lead to more elaborated func-
tions.2–7
In the course of our synthetic study on chalcogen substituted
porphyrins for advanced materials,8 we serendipitously dis-
covered a novel reaction between [5,15-bis(3,5-di-tert-butyl-
phenyl)porphyrinato]nickel 2 with tellurium(iv) tetrachloride
(TeCl4) to give a highly conjugated porphyrin-fused porphyrin.
Here we report the new expanded porphyrin 1a, denoted
[2]porphyracene (porphyrin + acene; the number [2] implies the
number of porphyrin nuclei) since this condensation manner is
an acene type.
To prepare 3 which has a TeCl3 group at a meso-position, the
reaction of 2 with 1 equiv. of TeCl4 in dry CH2Cl2 at room
temperature9 was carried out. The obtained reaction mixture
was analyzed by matrix assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectrometry and was
found to give a series of peaks from M+ = 1483 up to 6000 with
equal mass spacings of ca. 744. This indicates clearly, contrary
to the initial expectation, the formation of oligomeric products
of 2, from the dimer to the octamer. Separation of the dimer
fraction from higher oligomers was successfully achieved using
gel permeation liquid chromatography. The dimer fraction was
further separated into three components A, B and C having M+
= 1483, 1716 and 1949, respectively, using silica-gel flash
chromatography. In a typical reaction, 48.3 mg of 1 and 8.8 mg
of TeCl4 gave 15.0 mg of A, 1.0 mg of B, 0.7 mg of C, and 20.0
Fig. 1 Compounds 1–3 along with 1H NMR assignments for 1a.
1
mg of higher oligomers. The H NMR spectrum of the main
product A is different from those of porphyrin dimers directly
connected at meso–meso or meso-b positions.6 On the basis of
1
detailed H NMR analysis including truncated driven differ-
ential nuclear Overhauser effect measurements and MALDI-
TOF mass spectra, the structure of A was assigned to be 1a (Fig.
1) where the two porphyrin rings are fused at b- and meso-
positions. The structures of B and C were assigned to be 1b and
1c, respectively, in a similar manner.
To confirm the structural assignment, we performed a direct
observation of 1a using scanning tunnelling microscopy (STM)
(Fig. 2).† The STM image of 1a adsorbed on a Cu(100) surface
shows four lobes arranged in a parallelogram which are
attributable to 3,5-di-tert-butylphenyl groups which are STM
active substituents.10 The size of the parallelogram is 13 3 9 Å
which is consistent with the molecular size of 1a estimated by
Fig. 2 STM image of 1a: (a) top view, (b) perspective view, (c) molecular
geometry of 1a. The image shows a four-lobed pattern in a parallelogram
arangement. The observed size of the parallelogram is 13 3 9 Å.
Chem. Commun., 1999, 1957–1958
This journal is © The Royal Society of Chemistry 1999
1957

edge-fused oligomerization reaction of the nickel porphyrin
complex takes place to give a double connection. Since the
corresponding free base of 2 did not give oligomeric products,
increased reactivity of the nickel porphyrin 2 at its b-positions
may be responsible for the reaction.6e As shown in Scheme 1,
the dimerization reaction of 2 probably takes place through a
cyclic intermediate 4,11 which has not, so far, been detected.
The final detelluration reaction is similar to that of bis(ar-
yl)tellurium dichloride to give aryl coupling products.9 The
isolation of 1b and 1c suggests that these are the intermediates
of [4]porphyracene and higher oligomers.
In conclusion, a novel and highly conjugated [2]porphyr-
acene has been isolated. In addition to the expanded p-system of
the porphyrin, [2]porphyracene has two central cavities for
homo- or hetero-metallation. These characteristics will allow us
to use porphyracene for the preparation of advanced materials.
We are currently investigating detailed properties of [n]por-
phyracenes including characterization of higher oligomers.
This work was supported in part by a Grant-in-Aid for
Scientific Research on Priority Area (#11136222 “Metal-
assembled Complexes” to K-i.S and #10146103 “Creation of
Characteristic Delocalized Electronic Systems” to Y. S.) from
the Ministry of Education, Science, Sports and Culture, Japan.
We appreciate the technical assistance provided by the
Materials Analysis Center of ISIR, Osaka University.
Fig. 3 Absorption spectra of [2]porphyracene 1a (solid line) and 2 (dotted
line) in CHCl3.
sponding protons of 2 indicating the electron donating nature of
the fused porphyrin ring. On the other hand, aromatic protons
Ha and Hg showed downfield shifts of 0.09 and 0.34 ppm,
respectively, owing to the ring current effect of the adjacent
macrocycle and the compression effect from the sterically
crowded protons.
The electronic spectra of 1a and 2 are shown in Fig. 3. The
spectrum of 1a is essentially composed of three bands, band I,
Q-band-like bands IIa and IIb and Soret-band-like band III.
Reflecting the highly conjugated structure, band I appeared at
13460 cm21 (743 nm) i.e. at a much longer wavelength than
those of a porphyrin dimer connected by an acetylene linkage
reported by Therien and coworkers (lmax 14640 cm21; 683
nm)2 or fused dimers reported by Crossley and Burn (lmax
14180 cm21; 705 nm)4 and by Smith and coworkers (lmax
15340 cm21; 652 nm).5 The other absorption bands of 1a, bands
IIa, IIb and III, were at quite similar wavelengths to those of 2,
although the intensity of band III was decreased and those of
bands IIa and IIb were increased. These phenomena suggest a
lowering of the symmetry and new p-electronic system for
[2]porphyracene.
Notes and references
† The STM image was observed with a home-made ultrahigh vacuum
apparatus. An atomically clean single crystal Cu(100) surface was used as
the substrate onto which 1a was sublimed. Details of STM experiments will
be published elsewhere.
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Scheme 1 A proposed reaction mechanism for dimerization.
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Chem. Commun., 1999, 1957–1958