Confusion and neo-confusion: Corrole isomers with an NNNC core

Article abstract of DOI:10.1002/anie.201100429

More confused than ever: Three types of N-confused corrole isomers (NCCs) were synthesized, and the structures of these isomers were revealed by single-crystal X-ray crystallography. The position of the confused pyrrole ring in the NCC has a pronounced ef

Full text of DOI:10.1002/anie.201100429

DOI: 10.1002/anie.201100429
Porphyrinoids
Confusion and Neo-Confusion: Corrole Isomers with an NNNC Core**
Keitaro Fujino, Yasuyuki Hirata, Yasunori Kawabe, Tatsuki Morimoto, Alagar Srinivasan,
Motoki Toganoh, Yugo Miseki, Akihiko Kudo, and Hiroyuki Furuta*
Corrole is a class of contracted porphyrin with an NNNN
donor core and containing a direct pyrrole–pyrrole link in the
molecular skeleton.[1] Since the first synthesis by Johnson and
Kay in 1965,[2] the unique properties of corrole, and especially
its metal coordination chemistry, which is different from that
of regular porphyrin, has received much attention.[3] Because
of the recent improvement on the triaryl corrole synthesis,
corrole chemistry, and in particular the corrole-based appli-
cations, have been progressively developed.[4]
Our group has been involved in the porphyrin analogue
chemistry, and especially that of confused porphyrinoids,
since the discovery of the porphyrin isomer, N-confused
Scheme 1. Synthesis of N-confused corroles.
porphyrin (NCP).[5] This porphyrin mutant possesses an
NNNC core owing to the presence of a,b’-linked (so-called
confused) pyrrole ring in the framework, which dramatically
alters its properties, reactivity, and coordination chemistry.[6]
The unexpectedly rich chemistry of NCP inspired us to
explore other confused porphyrinoids, which we call the
“confusion approach”.[6b] Until now, various confused
expanded porphyrinoids, such as sapphyrin, pentaphyrin,
and hexaphyrin, have been successfully synthesized and their
peculiar chemistry revealed.[6c] The chemistry of contracted
tetrapyrrolic macrocycle, on the other hand, remains unex-
plored owing to the presence of a set of confused isomers
(NCC1-4) which have an NNNC core with different arrange-
ment of the confused pyrrole ring.[7] This time we have
succeeded in synthesizing two of them (NCC2 and NCC4),
and an unprecedented N-linked isomer (NCC5), to which the
trivial name norrole is given (Scheme 1). Herein, the details
of the structures and properties are described.
work well, intramolecular oxidative cyclization reactions of
N-confused bilanes were examined. Thus, the reaction of 1
with pyrrole provided 7-aza-22-carbabilane (3) in 89% yield,
and the reaction of N-confused dipyrromethane monocarbi-
nol (4) with dipyrromethane (5) afforded 1-aza-21-carbabi-
lane (6) in 69% yield. Subsequent oxidation of bilane 3 with
2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ)
in
CH3CN afforded NCC2-C6F5 in 5% yield. Meanwhile, the
oxidation of bilane 6 with DDQ provided NCC4-C6F5 in 18%
yield, together with NCC5-C6F5 in 1% yield. The addition of a
halide anion did not improve the yields, in contrast to the case
of regular corrole.[8]
1
In the H NMR spectrum of NCC2-C6F5 (NCC4-C6F5) in
CDCl3, a relatively sharp singlet owing to the interior CH
proton was observed at d = À0.91 (1.84) ppm, and broad
resonances attributed to the interior NH protons were
observed at d = 2.61 (4.12) and 5.72 (8.04) ppm, while the
peripheral NH proton appears at d = 9.38 (9.24) ppm. The
1H NMR data suggest that both isomers are aromatic and
adopt the inner-3H form (with three hydrogen atoms inside
the core, as shown in Scheme 2) in CDCl3 solution; this
observation is consistent with the theoretical estimation on
the relative stability of NH tautomers.[9,10] The 13C NMR
signal that is ascribable to the inner CH of NCC4-C6F5 was
observed in an up-field region at d = 91.24 ppm, which is
much higher than other peripheral CH signals observed at d >
100 ppm, reflecting the ring current effect.[5a] One character-
istic difference between NCC5-C6F5 and the other NCCs is
the number of pyrrolic CH protons in the1H NMR spectra:
Details of the synthesis are outlined in Scheme 2. Because
the initial attempt for condensation reactions of N-confused
dipyrromethane dicarbinol (1) with 2,2’-bipyrrole (2) did not
[*] K. Fujino, Y. Hirata, Y. Kawabe, T. Morimoto, Dr. A. Srinivasan,
Dr. M. Toganoh, Prof. Dr. H. Furuta
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu University
Fukuoka 819-0395 (Japan)
Fax: (+81)92-802-2865
E-mail: hfuruta@cstf.kyushu-u.ac.jp
Y. Miseki, Prof. Dr. A. Kudo
Department of Applied Chemistry, Faculty of Science
Science University of Tokyo
nine 13C signals for C H groups, rather than eight in all other
À
Tokyo 162-8601 (Japan)
cases, were observed in the DEPT90 measurement; eight of
the signals appear between d = 6.27–7.90 ppm, while the
unique signal is at d = 1.21 ppm.
[**] The present work was supported by the Grant-in-Aid for Scientific
Research (21750047 and 21108518) and the Global COE Program
“Science for Future Molecular Systems” from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
The explicit structures of NCC2-C6F5, NCC4-C6F5, and
NCC5-C6F5 were elucidated by the X-ray crystallographic
analyses (Figure 1).[11] All of the carbon and nitrogen atoms
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100429.
Angew. Chem. Int. Ed. 2011, 50, 6855 –6859
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6855

Communications
(that is, amino type). In all
NCCs, the confused (a,b’-
linked) and neo-confused
(N,b-linked) pyrrole rings
are tilted significantly from
the corrole plane composed
of the remaining 18 heavy
atoms: namely 29.68 (NCC2-
C6F5), 39.28 (NCC4-C6F5),
and 36.48 (NCC5-C6F5), and
the remaining tripyrrole
units adopt a nearly planar
conformation,
a
situation
that is different from rela-
tively planar COR-C6F5.[12]
The larger distortion of the
planarity of NCC4-C6F5
could explain its weaker aro-
maticity than NCC2-C6F5.
The absorption and
emission spectra of NCCs in
CH2Cl2 are shown in
Figure 2, and the numerical
data are summarized in
Table 1. Compared to the
parent COR-C6F5, signifi-
cant red-shifts in the absorp-
tion spectra are observed for
NCCs. In particular, the red-
shift of as much as 130 nm is
observed for NCC2-C6F5.
Nevertheless, the absorption
coefficients of the Q-type
bands are similar to each
Scheme 2. Structures of porphyrin and corrole isomers.
other.
Meanwhile,
the
Soret-type bands of NCCs
are split, unlike in COR-
C6F5. In the fluorescence
spectra, a distinct emission
was observed in a series of
NCCs with comparable or
smaller absolute emission
quantum
yields
(Fem).
Among the NCCs, NCC5-
C6F5 shows the highest Fem
of 5.7% in CH2Cl2, and
NCC2-C6F5 and NCC4-C6F5
show the Fem of 1.0% and
1.4%, respectively.
A re-
Figure 1. X-ray structures of corrole isomers: a,b) NCC2-C6F5, c,d) NCC4-C6F5, e,f) NCC5-C6F5. Thermal
ellipsoids are set at 30% probability; C6F5 groups are omitted for clarity in views (b,d,f). N blue, C black,
F green.
markable feature of NCCs
is indicated by their large
Stokes
shifts
(774–
1445 cmÀ1) compared to the
can be clearly distinguished from the differences in electron
density, and all of the hydrogen atoms are found in the Fourier
maps, which imply the inner-3H forms of NCCs, being
consistent with the C-N-C bond angles, in the solid state.
Consequently, the nitrogen atoms of outward-pointing con-
fused pyrroles of NCC2-C6F5 and NCC4-C6F5 are protonated
corresponding regular corrole (221 cmÀ1). Because no signifi-
cant difference is recognized in their molecular orbitals (see
below), the large Stokes shifts would not be responsible in
intramolecular charge transfer but could be due to the
flexibility of macrocycles. Furthermore, upon protonation
with CF3CO2H, large red-shifts were observed in the Q-bands
6856 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 6855 –6859

Figure 2. Solution colors and spectra of the corrole and corrole
isomers in CH2Cl2: a) solution color, b) absorption, c) emission. (Black
line COR-C6F5, green line NCC2-C6F5, blue line NCC4-C6F5, red line
NCC5-C6F5).
Table 1: Optical properties of NCCs in CH2Cl2.
[a]
[b]
[c]
[d]
[e]
Structure
labs
[nm]
DHL
[eV]
lem
[nm]
Fem
Stokes shift
labs
[nm]
[cmÀ1
]
COR-C6F5
NCC2-C6F5
NCC4-C6F5
NCC5-C6F5
634[f]
764
2.53
1.85
2.14
2.23
643
812
732
705
0.070
0.010
0.014
0.057
221
774
1445
990
622
792
782
735
Figure 3. Kohn–Sham orbitals and orbital energy levels [eV] of corrole
and NCCs.
662
659[f]
[a] The lowest energy absorption maxima. [b] Theoretical HOMO–LUMO
gaps. [c] The wavelength of emission maxima. Excited at the Soret-like
band. [d] Absolute quantum yields. [e] The values for NCCs monoproto-
nated with CF3CO2H. [f] Shoulder.
molecular charge transfer nature in the HOMO–LUMO
transitions. In contrast, slight localization is observed in
LUMO + 1. Compared to nearly degenerated HOMO and
HOMOÀ1 of COR-C6F5, degeneracy is lost in NCCs, which
would account for the split Soret-like bands of NCCs.
of NCCs: 28 nm (NCC2-C6F5), 120 nm (NCC4-C6F5), and
76 nm (NCC5-C6F5), while a small blue-shift (12 nm) was
observed for COR-C6F5. Such red-shifts could be rationalized
by a loss of planarity owing to the absence of intramolecular
hydrogen bonding and also the steric congestion inside the
NCC core.[13]
The aromaticity of NCCs was evaluated with the aid of
1H NMR chemical shifts, nucleus-independent chemical shift
(NICS)[14] values, and harmonic oscillator model of aroma-
1
ticity (HOMA)[15] indices (Table 2). In the H NMR spectra,
maximum difference between the chemical shifts of periph-
eral CH moieties and those of interior CH or NH moieties
(DdCH-CH or DdCH-NH) was utilized to evaluate the strength of
the aromatic ring current. Because the chemical shifts of the
NH protons are influenced by intramolecular hydrogen
bonding, the DdCH-CH values would be more reliable, though
the DdCH-NH values are described owing to the absence of
Molecular orbitals and HOMO–LUMO orbital energies
calculated at the B3LYP/6-31G** level are shown in Figure 3.
The theoretical HOMO–LUMO energy gaps of NCCs (1.85–
2.23 eV) are considerably smaller than that of the regular
corrole (2.53 eV) and in good agreement with their absorp-
tion spectra (Table 1). Narrow HOMO–LUMO energy gaps
of NCCs are attributable mainly to the rise in the HOMO
energy level. In the HOMO of COR-C6F5, a large contribu-
tion is observed at the four inner nitrogen atoms. Confusion of
COR-C6F5 represents replacement of one inner nitrogen
atom by the carbon atom without significant change in the
shape of HOMO. As a result, the HOMOs of NCCs would be
destabilized owing to less electronegativity of the carbon
atom than the nitrogen atom. In all of the HOMOs and
LUMOs, no significant localization of the molecular orbital
coefficients is recognized, which suggests negligible intra-
Table 2: Evaluation of aromaticity in NCCs.
Structure
DdCH-CH
[ppm]
DdCH-NH
[ppm]
NICS
[ppm]
HOMAall
HOMA18
POR-C6F5
NCP-C6F5
COR-C6F5
NCC2-C6F5
NCC4-C6F5
NCC5-C6F5
–
14.2
–
9.3
5.7
6.7
11.9
11.5
11.3
5.8
3.5
6.8
À13.4686
À12.1474
À12.1363
À6.8208
À4.8230
À7.2753
0.642
0.654
0.726
0.631
0.646
0.669
0.809
0.807
0.789
0.707
0.645
0.711
Angew. Chem. Int. Ed. 2011, 50, 6855 –6859
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
6857

Communications
interior CH moiety in the regular porphyrin. Both POR-C6F5
the former.[16,18] The distance between ClÀ and the nearest
C6F5 group is significantly shorter in the case of NCC2-C6F5
(3.611 ꢁ) than that of NCC4-C6F5 (7.821 ꢁ), which partially
supports this proposal. Furthermore, interaction between ClÀ
and the neighboring C6F5 group in NCC2-C6F5 was realized by
the 19F NMR spectra of NCC2-C6F5, in which the large shifts
of the signals assignable to one of the C6F5 groups were
observed by the addition of Bu4NCl, whereas no significant
change was detected with NCC4-C6F5.[10]
In summary, we have synthesized three types of NCC
isomers, and their structures and properties are revealed. The
position of the confused pyrrole ring in NCC affects the
optical and anion-binding properties distinctly. Owing to the
presence of a carbon atom inside the core of NCCs, rich
coordination chemistry could be expected for NCCs like the
case of NCP.[19] Furthermore, the finding of an N-linked
isomer, norrole, could open a way to a new porphyrinoid
chemistry. Further studies, including the metal coordination
chemistry, is now underway.
(d = 11.9 ppm) and COR-C6F5 (d = 11.3 ppm) shows the large
DdCH-NH values, indicating strong aromaticity. Similarly, NCP-
C6F5 also shows strong aromaticity (DdCH-CH: 14.2 ppm, DdCH-
NH: 11.5 ppm). NCC2-C6F5, NCC4-C6F5, and NCC5-C6F5 have
smaller DdCH-CH (DdCH-NH) values of 9.3 (5.8) ppm, 5.7
(3.5) ppm, and 6.7 (6.8) ppm, respectively. Thus NCCs are
moderately aromatic from the viewpoint of 1H NMR spectra.
In accordance with the Dd values, the NICS values of NCC2-
C6F5 (À6.8208 ppm) and NCC5-C6F5 (À7.2753 ppm) indicate
moderate aromaticity, and that of NCC4-C6F5 (À4.8230 ppm)
indicates weak aromaticity. On the other hand, the HOMA
indices, the structural aspects of aromaticity, for [18]annulenic
moiety (HOMA18) seem also applicable in the present system,
although the indices for all the corrole or porphyrin core
(HOMAall) are not informative. The HOMA18 of around 0.8
(POR-C6F5, NCP-C6F5, COR-C6F5) represents strong aroma-
ticity and of around 0.7 (NCC2-C6F5, NCC5-C6F5) moderate
aromaticity. The HOMA18 of 0.645 for NCC4-C6F5 denotes
weak aromaticity.
Received: January 18, 2011
Revised: March 21, 2011
Published online: June 14, 2011
As shown above, the photophysical properties and the
aromaticity of NCCs differ appreciably depending on the
position of the confused pyrrole ring. Such a difference is also
reflected in the anion-binding behavior (Figure 4). The
Keywords: anion binding · aromaticity · confused porphyrinoids ·
.
corrole · density functional calculations
[1] a) J. L. Sessler, S. J. Weghorn, Expanded, Contracted, and
Isomeric Porphyrins, Pergamon, Oxford, 1997; b) R. Paolesse
in The Porphyrin Handbook, Vol. 3 (Eds.: K. M. Kadish, K. M.
Smith, R. Guilard), Academic, San Diego, 2000, chap. 11,
pp. 201 – 232; c) C. Erben, S. Will, K. M. Kadish in The
Porphyrin Handbook, Vol. 3 (Eds.: K. M. Kadish, K. M. Smith,
R. Guilard), Academic, San Diego, 2000, chap. 12, pp. 233 – 300.
[2] A. W. Johnson, I. T. Kay, J. Chem. Soc. 1965, 1620 – 1629.
[3] I. Aviv-Harel, Z. Gross, Chem. Eur. J. 2009, 15, 8382 – 8394.
[4] a) S. Nardis, D. Monti, R. Paolesse, Mini-Rev. Org. Chem. 2005,
2, 355 – 372; b) I. Aviv, Z. Gross, Chem. Commun. 2007, 1987 –
1999.
Figure 4. Anion binding behavior of NCCs. Values of the binding
constant KCl are given in LmolÀ1. The arrows indicate the direction of
the dipole moment m (values in Debye) of anion-free NCCs. See the
Supporting Information for details.
[5] a) H. Furuta, T. Asano, T. Ogawa, J. Am. Chem. Soc. 1994, 116,
´
767 – 768; b) P. J. Chmielewski, L. Latos-Graz˙ynski, K. Rachle-
wicz, T. Głowiak, Angew. Chem. 1994, 106, 805 – 808; Angew.
Chem. Int. Ed. Engl. 1994, 33, 779 – 781.
neutral NCC2-C6F5 and NCC4-C6F5 possess the peripheral
NH moieties that can bind anions through hydrogen bonding,
as observed for NCP,[16] whereas NCC5-C6F5 and COR-C6F5
lack such an NH moiety at the periphery. Thus, in the
1H NMR spectra of NCC2-C6F5 (NCC4-C6F5) in CDCl3, the
peripheral NH signal exhibits a significant low-field shift from
d = 9.38 (9.24) to 14.40 (13.39) ppm upon an addition of
excess amount of Bu4NCl, while no characteristic spectral
change was observed with NCC5-C6F5 and COR-C6F5. The
[6] a) H. Furuta, H. Maeda, A. Osuka, Chem. Commun. 2002,
1795 – 1804; b) A. Srinivasan, H. Furuta, Acc. Chem. Res. 2005,
38, 10 – 20; c) M. Toganoh, H. Furuta in Handbook of Porphyrin
Science: With Applications to Chemistry, Physics, Materials
Science Engineering, Biology and Medicine, Vol. 2 (Eds.: K. M.
Kadish, K. M. Smith, R. Guilard), World Scientific, New Jersey,
2010, chap. 10, pp. 295 – 367.
[7] A different type of a,b-linked corrole isomer, corrorin, has been
previously reported: H. Furuta, H. Maeda, A. Osuka, J. Am.
Chem. Soc. 2001, 123, 6435 – 6436.
binding constant for ClÀ (KCl) was determined to be KCl
=
3.2 ꢀ 103 LmolÀ1 (NCC2-C6F5) and KCl = 1.6 ꢀ 103 LmolÀ1
(NCC4-C6F5), respectively, from the absorption spectral
changes during the titration with Bu4NCl in CH2Cl2. Interest-
ingly, the anion affinity is much larger than that of NCP (KCl =
580 LmolÀ1),[16b] which is ascribable to the preferred inner-3H
form of NCCs that provides a peripheral NH. Moreover, the
KCl value of NCC2-C6F5 is larger than that of NCC4-C6F5,
which is inconsistent with the estimated ion–dipole interac-
tion,[17] and strongly infers the effective anion–p interaction in
[8] R. A. Decrꢂau, J. P. Collman, Tetrahedron Lett. 2003, 44, 3323 –
3327.
[9] The inner-3H type tautomers are 8 kcalmolÀ1 more stable than
the inner-4H tautomers.
[10] See the Supporting Information.
[11] Crystal data for NCC2: green prisms, C37H11F15N4, Mr = 796.50,
¯
triclinic, space group P1, a = 8.551(3), b = 14.231(5), c =
14.501(5) ꢁ, a = 116.864(6), b = 100.597(7), g = 95.456(7)8, V=
1515.0(9) ꢁ3, Z = 2, T= 223 K, R = 0.0774, wR = 0.1872 (all
data),
GOF = 1.010.
NCC4:
dark
green
blocks,
6858
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 6855 –6859

C37H11F15N4·C7H16, Mr = 896.70, monoclinic, space group P21/n,
a = 15.413(2), b = 10.4586(14), c = 24.102(4) ꢁ, b = 92.159(2)8,
V= 3882.5(9) ꢁ3, Z = 4, T= 296(2) K, R = 0.0670, wR = 0.1621
(all data), GOF = 1.015. NCC5: violet prisms, C37H11F15N4·2
(CH2Cl2), Mr = 966.35, monoclinic, space group P21/c, a =
13.6542(8), b = 13.4057(7), c = 21.1649(11) ꢁ, b = 104.684(1)8,
V= 3747.6(4) ꢁ3, Z = 4, T= 223(2) K, R = 0.0735, wR = 0.2229
(all data), GOF = 1.025. CCDC 719319 (NCC2), 719320
(NCC4), and 719321 (NCC-5) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao, N. J. R.
van Eikema Hommes, J. Am. Chem. Soc. 1996, 118, 6317 – 6318.
[15] J. J. Kruszewski, T. M. Krygowski, Tetrahedron Lett. 1972, 13,
3839 – 3842.
[16] a) H. Maeda, A. Osuka, H. Furuta, J. Inclusion Phenom.
Macrocyclic Chem. 2004, 49, 33 – 36; b) H. Maeda, T. Morimoto,
A. Osuka, H. Furuta, Chem. Asian J. 2006, 1, 832 – 844; c) D.-H.
Won, M. Toganoh, H. Uno, H. Furuta, Dalton Trans. 2009, 6151 –
6158.
[17] The energies for the ion–dipole interaction were estimated to be
À5.86 and À6.13 kcalmolÀ1 for NCC2-C6F5 and NCC4-C6F5,
respectively.
[12] a) Z. Gross, N. Galili, L. Simkhovich, I. Saltsman, M. Botoshan-
sky, D. Blꢃser, R. Boese, I. Goldberg, Org. Lett. 1999, 1, 599 –
602; b) T. Ding, J. H. Harvey, C. J. Ziegler, J. Porphyrins
Phthalocyanines 2005, 9, 22 – 27.
[18] B. L. Schottel, H. T. Chifotides, K. R. Dunbar, Chem. Soc. Rev.
2008, 37, 68 – 83.
[19] Recently, a silver(III) complex of carbacorrole was reported: J.
´
Skonieczny, L. Latos-Graz˙ynski, L. Szterenberg, Chem. Eur. J.
[13] M. Meot-Ner, A. D. Adler, J. Am. Chem. Soc. 1975, 97, 5107 –
5111.
2008, 14, 4861 – 4874.
Angew. Chem. Int. Ed. 2011, 50, 6855 –6859
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6859