Gram-scale synthesis of nickel(II) norcorrole: The smallest antiaromatic porphyrinoid

Article abstract of DOI:10.1002/anie.201204395

Small is beautiful: A ring-contracted sister of porphyrin, norcorrole, has been synthesized efficiently as a stable molecule by a nickel-templated strategy. The norcorrole complex is stable but exhibits a distinct antiaromatic character according to the Hückel rule. Oxidation of the norcorrole complex provides an aromatic oxacorrole complex.

Full text of DOI:10.1002/anie.201204395

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Chemie
DOI: 10.1002/anie.201204395
Antiaromatic Porphyrins
Gram-Scale Synthesis of Nickel(II) Norcorrole: The Smallest
Antiaromatic Porphyrinoid**
Tomohiro Ito, Yosuke Hayashi, Soji Shimizu, Ji-Young Shin, Nagao Kobayashi,* and
Hiroshi Shinokubo*
Porphyrins and their related polypyrrolic conjugated macro-
cycles, porphyrinoids, have been targets of intensive research
from basic science to practical applications. Among them,
corrole[1] and subporphyrin[2] are the representative ring-
contracted porphyrins, both of which exhibit distinct aromatic
characters owing to 18 and 14 p-electrons on the conjugation
pathway, respectively (Scheme 1). Norcorrole, which lacks
two meso carbon atoms from a regular porphyrin, would be
another attractive ring-contracted porphyrin. In spite of its
simple and beautiful structure, the synthesis of norcorrole has
been quite challenging owing to structural strain.
the nature of norcorrole. Determination of the structure and
possible antiaromaticity of norcorrole have not yet been
accomplished.
Herein we present unambiguous characterization of an
antiaromatic nickel(II)–norcorrole complex, which can be
prepared easily in a gram scale by a metal-templated syn-
thesis. The high efficiency in the preparation of an antiar-
omatic porphyrinoid is rather remarkable, as the synthesis of
antiaromatic compounds is inherently difficult owing to their
unstable nature. Furthermore, reactions for the synthesis of
porphyrinoids themselves by acid-promoted condensation–
oxidation of pyrroles with aldehydes, are generally low-
yielding. However, the metal-mediated synthesis offers an
efficient route to novel porphyrinoids. Furthermore, the ready
availability of antiaromatic norcorroles would lead to prac-
tical use of antiaromatic compounds for further applications,
such as organic opto-electronic materials.
The synthesis of NiII norcorrole 2 is depicted in Scheme 2,
in which a,a’-dibromodipyrrin NiII complex 1 was employed
as a precursor.[5] We attempted a Ni0-mediated intramolecular
Scheme 1. Structures of subporphyrin, norcorrole, and corrole. The
bold lines indicate each conjugation circuit.
On the basis of the Hꢀckel rule, the 16p-electronic system
of norcorrole is expected to exhibit antiaromatic character. In
2005, however, the DFT calculations by A. Ghosh et al.
predicted a nonaromatic polyenic character rather than
antiaromaticity of norcorrole.[3] This intriguing theoretical
prediction gave a rise to the necessity for the experimental
support. In 2008, Brçring et al. reported the first observation
of a norcorrole–iron(III) complex, which was unfortunately
not isolable owing to rapid dimerization.[4] The instability of
the norcorrole–iron complex prevented further elucidation of
Scheme 2. Nickel-mediated reductive homocoupling of dipyrrin com-
plex 1 to NiII norcorrole 2.
homocoupling[6] of 1 and obtained the formation of NiII
norcorrole 2 in 90% yield as a green solid. Rather unexpect-
edly, the product 2 is quite stable under air at room
temperature and is easily isolatable by silica gel column
purification, similar to normal porphyrin complexes. Owing to
the facile synthetic procedure, the preparation of compound 2
on a gram scale was easily accomplished. Compound 2 was
characterized on the basis of ESI-TOF mass analysis and
[*] T. Ito, Y. Hayashi, Prof. Dr. J.-Y. Shin, Prof. Dr. H. Shinokubo
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Aichi, 464-8603 (Japan)
E-mail: hshino@apchem.nagoya-u.ac.jp
Prof. Dr. S. Shimizu, Prof. Dr. N. Kobayashi
Department of Chemistry, Graduate School of Science
Tohoku University, Sendai 980-8578 (Japan)
E-mail: nagaok@m.tohoku.ac.jp
1
[**] The work at Nagoya University was supported by Grants-in-Aid for
Scientific Research (Nos. 24350023 and 23108705 “pi-Space”) from
MEXT (Japan). The work at Tohoku University was supported by
Grants-in-Aid for Scientific Research (Nos. 23350095 and 20108007
“pi-Space”) from MEXT (Japan). H.S. also acknowledges Yazaki
Science Foundation for financial support.
NMR spectroscopy. The H NMR spectrum of 2 proposed
a highly symmetric structure, of which sharp signals suggest
that the nickel atom takes a low spin state in a square-planar
geometry. More importantly, the 1H NMR spectrum of 2
displays two sets of doublet peaks for pyrrole b-protons in the
far upfield region from d = 1.45 to 1.60 ppm, owing to
a paratropic ring current effect, which clearly indicates
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204395.
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distinct antiaromaticity of NiII norcorrole 2. Recently, anti-
aromatic porphyrins and expanded porphyrins have been
successfully characterized,[7] but this is the first example of
isolation of antiaromatic ring-contracted porphyrins.
3 at 1008C in anhydrous DMF under an O2 atmosphere
provided 3 in 47% yield in 5 days (Scheme 3). Furthermore,
treatment of 2 with m-chloroperoxybenzoic acid (mCPBA) at
ꢀ308C also afforded 3 in 51% yield in 3 h. As no formation of
3 was observed in wet DMF in the absence of O2, we
The structure of NiII norcorrole 2 was unambiguously
elucidated by X-ray diffraction analysis (Figure 1a,b). The
main skeleton of 2 is perfectly planar, with the mean plane
deviation of 0.041 ꢁ. The flat structure containing 16 p-
Scheme 3. Oxidation of NiII norcorrole 2 to NiII oxacorrole 3.
concluded that the oxygen source for the formation of
1
oxacorrole 3 is molecular oxygen, not water. The H NMR
spectrum of oxacorrole 3 revealed its aromatic character,
showing doublet peaks of pyrrole b-protons in the aromatic
region from d = 7.5 to 7.9 ppm. The X-ray diffraction analysis
elucidated the structure of NiII oxacorrole 3 (Figure 1c,d).
The aromaticity of 3 can be explained by 18 p-electrons,
including the lone pair on the oxygen atom at the meso
position.
The UV/Vis absorption spectrum of NiII norcorrole 2
exhibits a substantially different shape from those of typical
porphyrins. Characteristically, the spectrum of 2 has a broad
absorption band at 431 nm and weak absorption bands
ranging from 600 to 1100 nm, which can be considered as
diagnostic for antiaromatic porphyrinoids (Figure 2a).[7c] In
contrast, NiII oxacorrole 3 has a Soret band at 389 nm and Q
bands at 680 and 553 nm (Figure 2b). To elucidate detailed
features of these absorption spectra, we further measured
magnetic circular dichroism (MCD) spectra of 2 and 3.
Generally, absorption spectra of porphyrinoids with 18p-
conjugation systems are interpreted in terms of Goutermanꢂs
four-orbital theory,[10] which specifies that the allowed nature
of Soret bands and the forbidden nature of Q bands are
derived from configuration interactions of four HOMO!
LUMO transitions. As these transitions are associated with
changes of orbital angular momenta, the Soret and Q bands
can be observed as clear CD signals using MCD spectroscopy.
Michl et al. successfully predicted MCD signal patterns of
aromatic molecules by applying perimeter models of 4n + 2-
electron cyclic polyenes.[11] In the MCD spectrum of 3
(Figure 2b), negative-to-positive MCD signals, which are
recognized as Faraday B terms in MCD theory,[12] are
observed at 677 and 555 nm, assigning the split Q bands of 3
at these wavelengths, whereas similarly intense MCD signals
are observed corresponding to the Soret band at 389 nm. The
absorption and MCD spectra of 3 are typical of aromatic
porphyrinoids with lower molecular symmetries. The perim-
eter model was further extended to 4n-electron antiaromatic
molecules, in which six frontier orbitals (hꢀ, h+, sꢀ, s+, lꢀ, l+)
derived from HOMOs, SOMOs, and LUMOs are involved in
the p–p* transitions.[13] In the case of an antiaromatic
compound with a closed-shell ground state,[13c] the lowest-
Figure 1. X-ray crystal structures of NiII norcorrole 2 (a) top view,
b) side view) and NiII 10-oxacorrole 3 (c) top view, d) side view). meso-
Aryl substituents are omitted for clarity. Ellipsoids are set to 50%
probability . e) Bond length alternations for the crystal structures 2 and
3.
electrons on the conjugation pathway results in a strong
antiaromaticity. The NICS (nucleus-independent chemical
shift) value calculated at the point “a” (indicated in Fig-
ure 1a) is d = 39.4 ppm, supporting the antiaromaticity of 2.[8]
Reflecting its antiaromaticity, compound 2 displays significant
bond length alternation (Figure 1e), which is unusual for
ꢀ
aromatic porphyrins such as compound 3. The C C bond
length between two dipyrrin moieties is measured to be
ꢀ
1.49 ꢁ, which is somewhat longer than a typical C C bond
between two sp2 carbon atoms.
Although NiII norcorrole 2 is stable at room temperature
both in the solution and in the solid state, we found that
compound 2 was slowly oxidized under air at high temper-
atures to afford NiII 10-oxacorrole 3.[9] Heating the solution of
2
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determined to be 1.08 and 2.05 V, respectively. The substan-
tially narrow HOMO–LUMO gap of 2 well matches with the
general trend in antiaromatic compounds.
In conclusion, we have achieved a facile synthesis of NiII
norcorrole 2 through intramolecular reductive homocoupling
of a bis(a,a’-dibromodipyrrinato) Ni complex 1. The highly
planar structure and distinct 16p-antiaromaticity of NiII
norcorrole 2 have been unambiguously unveiled for the first
time. In spite of the antiaromatic destabilization and the
strained core structure, the NiII norcorrole is unexpectedly
stable at room temperature but converted into 18p-aromatic
NiII oxacorrole 3 with molecular oxygen at high temperatures.
This interesting reactivity of the oxygen insertion reaction
implies that norcorrole could be a prospective precursor for
various heteroatom-substituted porphyrinoids. Further inves-
tigation into the reactivity of norcorrole is currently underway
in our laboratory.
Received: June 6, 2012
Published online: && &&, &&&&
ꢀ
Keywords: aromaticity · C C bond formation ·
magnetic circular dichroism · nickel · porphyrinoids
.
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Antiaromatic Porphyrins
T. Ito, Y. Hayashi, S. Shimizu, J.-Y. Shin,
N. Kobayashi,*
H. Shinokubo*
&&&& — &&&&
Gram-Scale Synthesis of Nickel(II)
Norcorrole: The Smallest Antiaromatic
Porphyrinoid
Small is beautiful: A ring-contracted
sister of porphyrin, norcorrole, has been
synthesized efficiently as a stable mole-
cule by a nickel-templated strategy. The
norcorrole complex is stable but exhibits
a distinct antiaromatic character accord-
ing to the Hꢀckel rule. Oxidation of the
norcorrole complex provides an aromatic
oxacorrole complex.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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