COMMUNICATION
teractions were observed in the absorption and fluorescence
spectra, which vary upon the pyrrolic N-substituent.
been reactivated in light of its relevance to antiaromatic por-
phyrins.[12–14]
Here we wish to report the efficient synthesis of meso-to-
meso 2,5-pyrrolylene-bridged porphyrin nanorings 3–5 from
porphyrin 1 and pyrrole 2 through Suzuki–Miyaura coupling
(Scheme 2). Interestingly, resonance contributors of these
nanorings can be regarded as ([18]annuleno)3[30]annulene,
Synthesis of pyrrole-bridged porphyrin rings 3H–5H is
straightforward through Suzuki–Miyaura cross-coupling of
5,10-diaryl-15,20-dibromoporphyrin 1[2d] and 2,5-diborylpyr-
role 2[15] (Scheme 2). Separation by gel-permeation chroma-
tography (GPC) and silica-gel chromatography afforded pyr-
role-bridged cyclic porphyrin 3-mer 3H (13%), 4-mer 4H
(5.3%), and 5-mer 5H (2.7%). Notably, the reaction repre-
sents a rare example of one-pot synthesis of such large
cyclic arrays from a monomeric porphyrin precursor without
template molecules.
([18]annuleno)4[40]annulene,
and
([18]annule-
no)5[50]annulene for 3–5, respectively. Annulenoannulenes
that consist of multi-annulated aromatic annulenes have
been extensively investigated in terms of their possible over-
all aromaticity.[11] In these three cases, the central resonance
contributors (shown in gray in Scheme 1) correspond to so-
called expanded isophlorins. Isophlorin (N,N’-dihydropor-
phyrin) is a reduced 20p-electron congener of porphyrin, in
which the macrocyclic conjugation surrounds the carbon pe-
riphery. In recent years, the chemistry of isophlorins has
Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectra of the cyclic porphyrin arrays
displayed the parent ion peaks at m/z 2250.2995 for 3H
(calcd for C156H166N15 =2250.3413 [M+H]+), 2999.7303 for
4H (calcd for C208H220N20 =2999.7902 [M]+), and 3750.2339
for 5H (calcd for C260H275N25 =3750.2415 [M]+) (see also
1
the Supporting Information). The H NMR spectrum of 3H
in CDCl3 was quite simple, exhibiting two singlet and two
doublet peaks for the b-protons at 10.44, 9.85, 9.12, and
8.88 ppm, a singlet for the bridging-pyrrole NH protons at
10.62 ppm, a singlet for the inner NH protons at À2.25 ppm,
and
a doublet for the bridging-pyrrole b-protons at
8.00 ppm, while no meso-proton signal was observed. Inter-
estingly two ortho-protons of aryl substituents were distin-
guished, because of restricted rotation of the pyrrole bridges
at room temperature. The electronic system is potentially at-
tractive in view of their structural analogy to that of ([18]an-
nuleno)3[30]annulene; however, the 1H NMR exhibited no
ring current arising from 30p circuit. This can be probably
ascribed to strongly localized porphyrin p circuits. The twist
between the planes of the bridging pyrroles and the por-
phyrins (specified later as 42–768) might contribute towards
the localization of the porphyrin p circuits. The 1H NMR
spectra of 4H and 5H displayed similar chemical shifts for
3H, but the one doublet peak of ortho-protons was ob-
served, indicating their more flexible conformations in solu-
1
tion that lead to the simple H NMR spectra (Supporting In-
formation). Consequently, the spectral data were in line
with the highly symmetric cyclic structures of 3H–5H in so-
lution. Again, 4H and 5H hold the formal octaphyrin and
decaphyrin structures in their cores, but exhibit no specific
ring current effects.
Free-base cyclic arrays 3H–5H were converted into the
corresponding zinc(II) and nickel(II) complexes 3Zn–5Zn,
and 3Ni–5Ni upon treatment with ZnACHTNUGTRENNUG(OAc)2 and NiACHTUNGTRENNUNG(OAc)2,
respectively. Single crystals of 3Ni suitable for X-ray diffrac-
tion analysis were grown by vapor slow diffusion of 2-propa-
nol and methanol into a solution of 3Ni in chlorobenzene
(Figure 1).[16] The X-ray crystal analysis revealed a symmet-
ric solid-state structure in which three of the pyrrole units
point into the core. The pyrrole moieties are tilted by 42–
768 relative to the porphyrin mean planes, which consist of
four core nitrogen atoms. The total molecular conformation
is a dome shape, and the center-to-center distances between
the nickel ions are 10.8, 10.9, and 11.4 ꢁ. The bond lengths
Scheme 2. Synthesis of pyrrolylene-bridged porphyrin rings.
Chem. Eur. J. 2010, 16, 13320 – 13324
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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