Rearrangements of a [36]octaphyrin triggered by nickel(II) metalation: Metamorphosis to a directly meso-β-linked diporphyrin

Article abstract of DOI:10.1002/anie.201105809

Breaking and entering: A figure-of-eight dinickel(II) complex and skeletal rearrangement products including a pyrrole-ring-cleaved product, a directly meso-β-linked diporphyrin (see scheme), and a dearylative oxygenation product have been formed in modera

Full text of DOI:10.1002/anie.201105809

Communications
DOI: 10.1002/anie.201105809
Porphyrinoids
Rearrangements of a [36]Octaphyrin Triggered by Nickel(II)
Metalation: Metamorphosis to a Directly meso-b-Linked
Diporphyrin**
Yasuo Tanaka, Hirotaka Mori, Taro Koide, Hideki Yorimitsu, Naoki Aratani, and
Atsuhiro Osuka*
In memory of Emanuel Vogel
In recent years, increasing attention has been focused on
expanded porphyrins, which consist of more than five pyrrolic
subunits.[1] These macrocycles show large conjugated p sy-
stems, multiple oxidation states, rich metal-coordination
properties, conformational flexibilities, large nonlinear opti-
cal properties, and anion-binding abilities. They have been
particularly suitable for the realization of Mçbius aromatic
and antiaromatic systems.[2,3] Beside these features, the
metalation of expanded porphyrins has triggered unique
skeletal rearrangements. As the first well-characterized
examples, Vogel et al. disclosed in 2003 a rearrangement of
5,24-dioxo-octaphyrin(1.1.1.0.1.1.1.0) to a spirodiporphyrin
triggered by nickel(II) metalation as well as the formation of a
linked diporphyrin, which was serendipitously found during
the study on the nickel(II) metalation of 1.
A solution of 1 in acetonitrile was heated at reflux in the
presence of 10 equivalents of [Ni(acac)2] and NaOAc for 6 h.
After usual workup and chromatographic separation, four
products 2, 3, 4, and 5 were isolated in 6, 21, 9, and 12% yields,
respectively (Scheme 1). When 100 equivalents of [Ni(acac)2]
different
spirodiporphyrin
from
a
[34]octaphyrin-
(1.1.1.0.1.1.1.0) upon its metalation with palladium(II)
ions.[4] Later, we found that a dicopper(II) complex of
meso-pentafluorophenyl-substituted
[36]octaphyrin-
(1.1.1.1.1.1.1.1) (1) was cleanly split into two copper(II)
porphyrins with perfect material balance upon heating.[5] The
same splitting reaction proceeded quantitatively with a CoII/
CuII hybrid octaphyrin complex, while the metalation of 1
with Pd(OAc)2 produced several rearranged products.[3a,5d]
Curiously, similar metathesis-like splitting reactions have
been found with a BIII/CuII hybrid heptaphyrin, which gave a
copper(II) porphyrin and a boron(III) subporphyrin.[6] These
reactions can be regarded as metamorphoses of expanded
porphyrins.[7] Other related examples include rearrangement
of [26]hexaphyrin to doubly N-confused [26]hexaphyrin
induced by copper(I) metalation[8a] and the rearrangement
of [32]heptaphyrin to a palladium(II) complex of N-confused
porphyrin triggered by palladium(II) metalation.[8b] Herein
we report a novel metamorphosis of 1 to a directly meso-b-
Scheme 1. Metalation of 1 with [Ni(acac)2]. Ar=2,3,4,5,6-pentafluoro-
phenyl. Acac=acetylacetonate.
was used, the yields of 2, 4, and 5 were increased to 22, 12, and
20%, respectively, while the yield of 3 was reduced to 6%.
The high-resolution electrospray ionization time-of-flight
(HR-ESI-TOF) mass spectrum of 2 indicated a parent ion
signal at m/z = 2062.9559 ([M+H]+, calcd for C88H17N8F40Ni2,
2062.9602). The 1H NMR spectrum of 2 showed a C2-
symmetric species with a singlet at d = 14.31 ppm and a
doublet at d = 8.83 ppm arising from Ha and Hb (see
Scheme 1), respectively. Signals corresponding to the rest of
the pyrrolic b protons were observed at d = 6.78, 6.34, 6.28,
6.06, and 5.61 ppm, and a signal corresponding to the outer
NH protons was observed at d = 6.82 ppm. This is quite
similar to the spectrum of the structurally well-characterized
[*] Dr. Y. Tanaka, H. Mori, T. Koide, Prof. Dr. H. Yorimitsu,
Dr. N. Aratani, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: osuka@kuchem.kyoto-u.ac.jp
Dr. N. Aratani
PRESTO (Japan) Science and Technology Agency (Japan)
[**] This work was supported by Grants-in-Aid (nos. 22245006 (A) and
20108006 “pi-Space”) from MEXT. Y.T. and T.K. acknowledge a JSPS
Fellowship for Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105809.
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Scheme 2. Compounds 6, 7, 8, and 9. Ar=2,3,4,5,6-pentafluorophenyl
and Ar’=3,5-di-tert-butylphenyl.
dipalladium(II) complex 6,[3a] which showed the signals for Ha
and Hb at d = 17.14 and 8.60 ppm, those for the rest of the
pyrrolic b protons in the range d = 6.09–5.10 ppm, and an NH
signal at d = 6.45 ppm (Scheme 2). These data indicated a
weak paratropic ring current for 2, as seen for 6, in line with its
36p electronic circuit lying in a figure-of-eight doubly twisted
Hꢀckel conformation.
The HR-ESI-TOF mass spectrum of 3 showed the signal
for the parent ion at m/z = 2002.0271 ([M], calcd for
C88H15N8F40Ni = 2002.0173), thus indicating its constitution
as the mononickel(II) complex of 1. The structure of 3 was
solved by X-ray diffraction analysis (Figure 1).[9] The nick-
el(II) atom is bound to the three nitrogen atoms of pyrrole
rings A, B, and C as well as the nitrogen atom N(8), which
originates from the cleaved pyrrole ring and is also connected
to the a-carbon atom C(16) of pyrrole ring D. The nitrogen
atom N(4) of pyrrole ring D is transannularly connected with
C(38). The other tetrapyrrolic segment consisting of pyrrole
rings E, F, and G is metal-free, and pyrrole ring G is fused with
the adjacent pyrrole ring to form a pyrrolizine (azapentalene)
Figure 1. Crystal structure of 3. Top: top view. Bottom: side view. The
thermal ellipsoids are scaled to the 50% probability. meso-Aryl
substituents and solvent molecules are omitted for clarity.
[36]octaphyrin, 2) the 19F NMR spectrum exhibited only 14
ortho-F, 7 para-F, and 14 meta-F signals. Thus, these data
indicated a loss of one pentafluorophenyl substituent (M =
167), 3) the 1H NMR spectrum revealed ten doublets and one
singlet (Ha) at d = 9.88 ppm, which nicely resembled that of
the dinickel(II) complex of directly meso-b-linked dipor-
phyrin 8, which showed the Ha signal at d = 9.24 ppm,[10] and
finally 4) the UV/Vis absorption spectrum of 4 showed
features characteristic of directly meso-b-linked diporphyrins,
such as a moderately broad Soret band arising from exciton
coupling and relatively unperturbed Q bands (Figure 2).[10] To
1
system. Consistent with the crystal structure, the H NMR
spectrum recorded in CDCl3 shows a singlet corresponding to
the pyrrolic NH proton at d = 10.52 ppm, a double of doublets
for Ha (designated in Scheme 1) at d = 8.37 ppm, a singlet for
Hb at d = 8.14 ppm, and 13 doublets for the pyrrolic b protons
in the range d = 6.52–4.97 ppm. The considerably low chem-
ical shift of Hb is ascribed to effective intramolecular hydro-
gen bonding with N(5) of pyrrole ring E. It is apparent that
the formation of 3 from 1 requires a large skeletal rearrange-
ment involving cleavage of a pyrrole ring. In a possible
reaction route from 1 to 3 (see the Supporting Information),
the NNNN-coordinated mononickel(II) complex 7 is postu-
lated as an intermediate to 3. This putative complex 7, which
was not detected in this reaction, might undergo a cascade
reaction sequence as a result of severe strain around the
nickel(II) coordination site.
To our surprise, product 4 has been assigned as being a
dinickel(II) complex of a directly meso-b-linked diporphyrin
on the basis of the following data: 1) the HR-ESI-TOF mass
spectrum showed the parent ion signal at m/z = 1894.9603
([M+H]+, calcd for C82H16N8F35Ni2 = 1894.9603), which was
168 mass units lower than that of the dinickel(II) complex of
Figure 2. UV/Vis absorption spectra of 4 (solid line) and 8 (dotted
line) in CH2Cl2.
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Communications
units than that of the dinickel(II) complex of
[36]octaphyrin. The two nickel(II) ions are each
bound to the three nitrogen atoms of the pyrrole
rings and the oxygen atoms of the resultant
carbonyl groups in a square-planar manner. Pyr-
role rings B, C, D, E, and F are almost coplanar,
with a small mean plane deviation of 0.191 ꢁ,
while pyrrole ring H is significantly tilted relative
to the above-mentioned plane with a dihedral
angle of 69.08. Pyrrole rings D and H, which do not
coordinate to the nickel(II) ion, are both amino
types. These NH protons are hydrogen bonded
with a water molecule in the crystal. The 19F NMR
spectrum of 5 is simple, exhibiting six signals for
the ortho-fluorine atoms, six signals for the meta-
fluorine atoms, and three signals for the para-
fluorine atoms, thus indicating an averaged Cs-
symmetric structure and the restricted rotation of
the meso substituents. The 1H NMR spectrum
exhibited eight pyrrolic b-CH signals in the range
d = 7.21–5.81 ppm and two pyrrolic NH signals at
d = 11.7 and 15.3 ppm. The oxygen atoms at the
meso positions are considered to be derived from
residual water in acetonitrile, since the corre-
sponding experiment in the presence of H218O
yielded the 18O-substituted product (see the
Supporting Information). The observed dearyla-
tive oxygenation is rare and its mechanism is
unclear at present, but may involve the nucleophilic attack of
water at the meso position followed by the elimination of a
Scheme 3. Directly meso-b-linked diporphyrins. Ar=2,3,4,5,6-pentafluorophenyl,
Ar’=2,3,5,6-tetrafluoro-4-isopropylaminophenyl. TFA=trifluoroacetic acid.
obtain stronger structural evidence, 4 was converted into free-
base diporphyrins 10 and its metal complexes 11 and 12
(Scheme 3). Although these diporphyrins displayed absorp-
tion spectra that are consistent with the structural assignment
(see the Supporting Information), their crystallizations turned
out to be difficult. Thus, the diporphyrin 10 was further
derivatized by an aromatic nucleophilic substitution reaction
with isopropylamine[11] to give 13, which was subsequently
metalated with Zn(OAc)2 to form diporphyrin 14. Gratify-
ingly, crystals of 14 suitable for X-ray diffraction analysis were
obtained by slow diffusion of hexane vapor into a solution of
14 in CH2Cl2, and the directly meso-b-linked diporphyrin
structure was determined unambiguously (Figure 3).[9] The
À
two porphyrin rings are connected through a C(meso) C(b)
bond of 1.507 ꢁ and a dihedral angle of 69.98. The formation
of directly meso-b-linked diporphyrin 4 from 1 is interesting
from the viewpoint of “chemical connections” in the porphyr-
inoid family.[7] A possible route to 4 may start from the
putative NNNN-coordinated dinickel(II) complex 9 (see the
Supporting Information), although this complex was not
detected in the reaction mixture. The formation of two
aromatic porphyrin segments is probably a driving force for
this rearrangement.
Finally, the structure of 5 was determined by X-ray
diffraction analysis.[9] The basic octaphyrin(1.1.1.1.1.1.1.1)
connection is preserved, but two pentafluorophenyl-substi-
tuted methine carbon atoms adjacent to pyrrole ring D are
both replaced by carbonyl groups (Figure 4). In line with the
structure, the HR-ESI-TOF mass spectrum showed the
parent ion signal of 5 at m/z = 1762.9780 ([M+H]+, calcd for
C76H19N8F30Ni2O2 = 1762.9815), which was lower by 300 mass
Figure 3. Crystal structure of 14. Top: top view. Bottom: side view. The
thermal ellipsoids are scaled to the 20% probability. meso-Aryl
substituents and solvent molecules are omitted for clarity.
11462 www.angewandte.org
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Figure 4. Crystal structure of 5. Top: top view. Bottom: side view. The
thermal ellipsoids are scaled to the 50% probability. meso-Aryl
substituents and solvent molecules are omitted for clarity.
pentafluorophenyl group. In fact, when the nickel(II) metal-
ation was conducted with 100 equivalents of [Ni(acac)2] under
strictly anhydrous conditions, the formation of 5 was almost
suppressed, and the yields of 2 and 4 were increased to 14 and
32%, respectively. Under these conditions, the formation of 3
was also suppressed, probably because of the enhanced
second nickel(II) metalation.
Taken together, the nickel(II)-metalation products of 1
can be understood as follows: 1) the nickel(II) metalation can
occur at either an NNNC or NNNN ligand, depending on the
conformation of 1, to produce 2 and postulated intermediates
7 and 9, respectively, 2) the NNNC-coordinated dinickel(II)
complex 2 is chemically stable and thus accumulates, 3) the
putative NNNN-coordinated mono- and dinickel(II) com-
plexes 7 and 9 undergo further reactions to form 3, 4, and 5.
The formation of 3 and 4 are plausibly driven by the
transannular electronic interactions enhanced by nickel(II)
metalation. The present results underscore the potential of
expanded porphyrins for undergoing unprecedented reac-
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investigation is currently underway in our laboratory.
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[9] a) Crystallographic data for 3: C88H16F40N8Ni·(heptane)·-
ꢀ
(CHCl3)3 (Mr = 2459.08), triclinic, space group P1 (no. 2), a =
14.9820(12), b = 18.6023(15), c = 20.7235(16) ꢁ, a = 99.544(2),
b = 109.7090(10), g = 113.2920(10)8, V= 4685.7(6) ꢁ3, Z = 2,
1calcd = 1.743 gcmÀ3, T= 90(2) K, R1 = 0.0761 (I > 2s(I)), Rw =
0.2359 (all data), GOF = 1.022; b) crystallographic data for 5:
C76H18F30N8Ni2O2·(toluene)2·(water)(Mr=1962.67), orthorhom-
bic, space group Pbcn (no. 61), a = 19.080(3), b = 39.277(5), c =
20.860(3) ꢁ, V= 15633(4) ꢁ3, Z = 8, 1calcd = 1.668 gcmÀ3, T=
123(2) K, R1 = 0.0870 (I > 2s(I)), Rw = 0.1869 (all data), GOF =
1.024; c) crystallographic data for 14:
C103H71F28N15Zn2·-
(hexane)0.42·(CH2Cl2)0.58 (Mr = 2261.06), triclinic, space group
ꢀ
P1 (no. 2), a = 13.9016(4), b = 19.0975(5), c = 19.8416(6) ꢁ, a =
Received: August 17, 2011
Published online: October 4, 2011
110.6778(13)
b = 101.9521(14),
g = 90.9518(11)8,
V=
4798.3(2) ꢁ3, Z = 2, < Grr> calcd = 1.565 gcmÀ3
,
T= 93(2) K,
R1 = 0.0984 (I > 2s(I)), Rw = 0.3068 (all data), GOF = 1.077.
CCDC 836925 (3), 836924 (5), and 836923 (14) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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123, 10304; c) Y. Nakamura, N. Aratani, A. Tsuda, A. Osuka, K.
Furukawa, T. Kato, J. Porphyrins Phthalocyanines 2003, 7, 264;
Keywords: diporphyrin · metalation · octaphyrin ·
porphyrinoids · rearrangement
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