A non-fused mono-meso-free pentaphyrin and its rhodium(i) complex

Article abstract of DOI:10.1039/c2cc32054a

5,10,15,20-Tetrakis(pentafluorophenyl) [22]pentaphyrin(1.1.1.1.1) 7 was synthesised and its bis-rhodium(i) complex 12 has been revealed to be a non-fused, yet planar pentaphyrin with an inverted pyrrole. Both 7 and 12 are aromatic, showing sharp Soret-like bands and diatropic ring currents. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc32054a

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COMMUNICATION
A non-fused mono-meso-free pentaphyrin and its rhodium(I) complexw
Tomoki Yoneda,a Hirotaka Mori,a Byung Sun Lee,b Min-Chul Yoon,b Dongho Kim*b and
Atsuhiro Osuka*a
Received 21st March 2012, Accepted 10th May 2012
DOI: 10.1039/c2cc32054a
5,10,15,20-Tetrakis(pentafluorophenyl) [22]pentaphyrin(1.1.1.1.1)
7 was synthesised and its bis-rhodium(I) complex 12 has been
revealed to be a non-fused, yet planar pentaphyrin with an inverted
pyrrole. Both 7 and 12 are aromatic, showing sharp Soret-like
bands and diatropic ring currents.
A recent surge in the chemistry of expanded porphyrins is due
largely to their attractive optical, electrochemical, and coordination
properties arising from their large p-conjugation frameworks.1
Structures of expanded porphyrins depend on many factors
such as the number of pyrrole units, the bridges, the b- or
meso-substituents, the coordinated metals, the availability of
hydrogen bonding interactions, and the degree of protonation.
Among these, pentaphyrins(1.1.1.1.1) that bear five pyrroles
regularly connected through meso-carbons, should contain
structural frustration because of the addition of a pyrrole
and a meso-carbon to the planar porphyrin skeleton. While
b-alkylated pentaphyrins 12 and 2,3 and doubly N-confused
mono-oxygenated pentaphyrin 34 were shown to be non-fused
macrocycles, meso-aryl-substituted pentaphyrin 4, formed
from our modified Rothemund–Lindsey protocol,5 was
forced to take an N-fused conformation with two inverted
pyrroles, because of severe steric congestion (Chart 1).6 This
Chart 1 Various conformations of pentaphyrins and meso-free
expanded porphyrins.
removal of the meso-pentafluorophenyl substituent would cause
large conformational and stability changes of pentaphyrins. In this
paper, we report the synthesis of mono-meso-free pentaphyrin 7
that takes a non-fused and planar conformation with an inverted
pyrrole.
We planned to synthesise mono-meso-free pentaphyrin 7 by
the cross-condensation of tripyrrane 1110 and the meso-free
dipyrromethane dicarbinol 10 (Scheme 1). Along with this
plan, stepwise diacylation11 of dipyrromethane 8 produced 9
N-fused pentaphyrin provided a Mobius aromatic molecule as
¨
the minimum size of expanded porphyrins7 and multiply
fused molecules upon phosphorus insertion.8 Beside these,
conformational studies of the regular pentaphyrins(1.1.1.1.1)
should be a useful step in better understanding the structure–
conformation relationship of expanded porphyrins. Recently,
we revealed that the removal of two meso-pentafluorophenyl
substituents from regular [26]hexaphyrin 5 to 5,10,20,25-
tetrakis(pentafluorophenyl) substituted hexaphyrin 6 led to a drastic
conformational change from a rectangular to a spectacle shape
owing to the substantial steric relief and favorable intramolecular
hydrogen bonding interactions.9 We thought that the similar
a Department of Chemistry, Graduate School of Science,
Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
E-mail: osuka@kuchem-u.ac.jp; Fax: +81-75-753-3970
b Spectroscopy Laboratory for Functional p-electronic Systems and
Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: dongho@yonsei.ac.kr; Fax: +82-2-364-7050
w Electronic supplementary information (ESI) available: General
experimental methods, HR-ESI-TOF mass spectra, UV/vis absorption
spectra, NMR spectra, X-ray crystal structures, results of DFT calcula-
tions and TPA spectra. CCDC 864560. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc32054a
Scheme 1 Synthesis of mono-meso-free pentaphyrin 7 and its bis-
rhodium(I) complex 12.
c
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Chem. Commun., 2012, 48, 6785–6787 6785

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in 67% yield, which was reduced with NaBH4 to afford 10
quantitatively (Scheme 1). A solution of 10 and 11 in CH2Cl2
was treated with 0.1 equiv. p-toluenesulfonic acid for 30 min
and the resulting solution was oxidized with 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) for 30 min. After usual
work-up, we identified a spot corresponding to mono-meso-
free pentaphyrin 7 on TLC of the reaction mixture. We
attempted to purify this compound by repeated chromatography
over gel permeation and silica gel columns but a particular
chemical instability of 7 hampered its complete purification.
Nevertheless, almost pure 7 gave the parent negative ion peak
at m/z = 1050.0775 (calcd for C49H13N5F20 = 1050.0768
[M–H]ꢀ) in the high-resolution electrospray ionisation time-
of-flight mass spectroscopy (HR-ESI-TOF MS), and exhibited
highly upfield shifted singlets for the meso-proton at ꢀ6.14 ppm
and for the inner b-protons at ꢀ1.82 ppm, and downfield shifted
signals due to the outer protons at 8.91, 9.09, and 9.66 ppm
(ESIw). These data indicate a symmetric non-fused aromatic
pentaphyrin structure with an inverted pyrrole. The UV/vis
absorption spectrum of 7 exhibits a sharp Soret band at 509 nm
and Q bands at 645, 694, 754, and 831 nm as a typical feature
of aromatic expanded porphyrins (ESIw).
In next step, the crude mixture that contained 7 was reacted
with [RhCl(CO)2]2 in CH2Cl2 in the presence of sodium
acetate to produce bis-rhodium(I) complex 12 in 10% yield
(three steps from 9).6b The chemical stability of 12 was
considerably improved, allowing its full characterisation even
under the aerobic conditions. The HR-ESI-TOF MS indicated
the parent positive ion peak of 12 at m/z = 1367.8656 (calcd for
Fig. 1 X-Ray crystal structure of 12: (a) top view and (b) side view.
Thermal ellipsoids represent 50% probability and meso-aryl substituents
are omitted for clarity in side view.
1
C53H12N5F20O4Rh2 = 1367.8675 [M + H]+). The H NMR
spectrum again indicated a highly symmetric feature, exhibiting
a singlet due to the meso-proton at ꢀ5.11 ppm, a singlet due to
the inner b-protons at ꢀ2.84 ppm, signals due to the outer
b-protons at 6.53, 7.98, 8.27, and 8.89 ppm. The chemical
shift difference between the inner and outer b-protons is large,
11.7 ppm. The structure of 12 was unambiguously determined
by single crystal X-ray diffraction analysisz to be a non-fused
mono-meso-free symmetric pentaphyrin with an inverted
pyrrole (Fig. 1). The two rhodium ions are coordinated over
the inward-pointing two pyrroles with bond distances of
Rh(1)—N(4), Rh(1)—N(5), Rh(2)—N(1), and Rh(2)—N(2)
of 2.054(5), 2.052(5), 2.040(5), and 2.056(5) A, respectively.
The pentaphyrin skeleton is roughly planar with the mean-plane
deviation of 0.330 A. Importantly, the rhodium coordination
nicely shields the reactive meso-free position, hence improving
the distinct chemical stability of 12. The harmonic oscillator
model of aromaticity (HOMA)12 of 12 was calculated to be 0.56.
The UV/vis absorption spectrum of 12 is similar to that of 7,
exhibiting a strong Soret band at 539 nm and Q bands at 683,
722, and 794 nm (Fig. 2).
Fig. 2 UV/vis absorption spectra of 12 and 4 (Ar = C6F5).
be less than 100 GM at the Q-band region, the maximum TPA
cross-section value of 12 was estimated to be 1200 GM at 1300 nm
which is similar to N-fused [22]pentaphyrin 4 (see Fig. S12w).6
These results are thought to reflect their planar structures,
elongated p-conjugation length and strong aromaticities.14
To explore the excited-state dynamics of 12, femtosecond
transient absorption measurements were carried out (Fig. S13w).
In the TA spectrum of 12, intense ground-state bleach signals
around 500–570 nm are observed that are accompanied by an
excited-state absorption band in the 570–670 nm spectral region.
The excited state lifetime of 12 was estimated to be 21 ps with a
residual long-lived component. Such a long-lived component of
12 is similar to those of other transition metal complexes like
rhodium complexes, which arises from facile intersystem crossing
process to populate the triplet state giving rise to a lack of
fluorescence.15 As a result of the transition metal effect, we could
not detect any fluorescence from 12.
We have already reported the TPA properties of various
porphyrin derivatives including expanded porphyrins which
exhibit relatively large TPA cross section values due to their
enlarged p-conjugation pathways.13 In this report, we have
measured the TPA values of 12 by photoexcitation using a
wavelength-scanning open-aperture Z-scan method in the
wavelength range of 1200–1500 nm at which one-photon
absorption contribution is negligible. While the maximum
TPA cross-section value of porphyrin monomer is known to
c
6786 Chem. Commun., 2012, 48, 6785–6787
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l(Cu-Ka) = 7.470 mmꢀ1, r = 1.917 Mg mꢀ3, T = ꢀ180(2) 1C, Total
reflections 8531, unique 5726 (Rint = 0.1217). R1 = 0.0675 [I 4 2s(I)]
and wR2 = 0.1754 (all data) GOF = 1.022.
1 (a) J. L. Sessler, A. Gebauer and S. J. Weghorn, in The
Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 1999, vol. 2, p. 55;
(b) A. Jasat and D. Dolphin, Chem. Rev., 1997, 97, 2267;
(c) T. D. Lash, Angew. Chem., Int. Ed., 2000, 39, 1763;
(d) H. Furuta, H. Maeda and A. Osuka, Chem. Commun., 2002,
1795; (e) J. L. Sessler and D. Seidel, Angew. Chem., Int. Ed., 2003,
42, 5134; (f) T. K. Chandrashekar and S. Venkatraman, Acc.
Chem. Res., 2003, 36, 676; (g) A. Ghosh, Angew. Chem., Int. Ed.,
Fig. 3 Optimized structure of 7 (a) top view and (b) side view. meso-
Aryl substituents are omitted for clarity in side view.
2004, 43, 1918; (h) M. Ste˛pien, N. Sprutta and L. Latos-Grazynski,
´ ´
_
Angew. Chem., Int. Ed., 2011, 50, 4288; (i) S. Saito and A. Osuka,
Angew. Chem., Int. Ed., 2011, 50, 4342; (j) A. Osuka and S. Saito,
Chem. Commun., 2011, 47, 4330.
2 (a) H. Rexhausen and A. Gossauer, J. Chem. Soc., Chem. Com-
mun., 1983, 275; (b) A. K. Burrell, G. Hemmi, V. Lynch and
J. L. Sessler, J. Am. Chem. Soc., 1991, 113, 4690.
3 C. Bruckner, E. D. Sternberg, R. W. Boyle and D. Dolphin, Chem.
Commun., 1997, 1689.
4 A. Srinivasan, T. Ishizuka, H. Maeda and H. Furuta, Angew.
Chem., Int. Ed., 2004, 43, 2951.
5 J.-Y. Shin, H. Furuta, K. Yoza, S. Igarashi and A. Osuka, J. Am.
Chem. Soc., 2001, 123, 7190.
6 (a) J.-Y. Shin, H. Furuta and A. Osuka, Angew. Chem., Int. Ed.,
2001, 40, 619; (b) S. Mori, J.-Y. Shin, S. Shimizu, F. Ishikawa,
H. Furuta and A. Osuka, Chem.–Eur. J., 2005, 11, 2417.
7 J. K. Park, Z. S. Yoon, M.-C. Yoon, K.-S. Kim, S. Mori,
J.-Y. Shin, A. Osuka and D. Kim, J. Am. Chem. Soc., 2008,
130, 1824.
8 T. Higashino and A. Osuka, Chem. Sci., 2012, 3, 103.
9 T. Koide, G. Kashiwazaki, M. Suzuki, K. Furukawa, M.-C. Yoon,
S. Cho, D. Kim and A. Osuka, Angew. Chem., Int. Ed., 2008,
47, 9661.
10 (a) P. D. Rao, S. Dhanalekshmi, B. J. Littler and J. S. Lindsey,
J. Org. Chem., 2000, 65, 7323; (b) J.-W. Ka and C.-H. Lee,
Tetrahedron Lett., 2000, 41, 4609.
11 H. Mori, N. Aratani and A. Osuka, Chem.–Asian J., 2012, DOI:
10.1002/asia.201100919, in press.
To obtain further insight into the stable conformations and
aromatic properties of 7 and 12, we performed DFT calculations
(B3LYP/6-31G(d)/LANL2DZ).16 The optimized structure of 7
thus calculated has four inward-pointing pyrroles and one out-
ward-pointing pyrrole, which is essentially the same as that of 12
except the large bending of the pyrrole C (Fig. 3). Importantly, this
conformation is exceedingly more stable than other conformers
(ESIw). This structure well explains the 1H NMR spectral data
of 7, and is reminiscent of the similar conformation of
5,10,15,20-tetraphenylsapphyrin.17 In sharp contrast, the optimized
structure of non-fused meso-pentakis(pentafluorophenyl)
substituted pentaphyrin(1.1.1.1.1) was calculated to be a
conformer with two-inverted pyrrole rings, which is prone to
undergo an N-fusion reaction (ESIw). These calculations also
revealed nearly degenerate HOMO/HOMO–1 and LUMO/
LUMO+1, and large HOMO–LUMO gaps of 2.01 and 2.15 eV
for 7 and 12, respectively. The nucleus-independent chemical shift
(NICS) values18 at the centre of the macrocycles were calculated to
be ꢀ17.4 and ꢀ21.8 ppm for 7 and 12. These results are in line with
the strong aromaticity of 7 and 12 (ESIw).
In summary, we synthesised mono-meso-free pentaphyrin 7
and its rhodium(I) complex 12 as the first example of a b-
unsubstituted non-fused pentaphyrin, which showed aromaticity
arising from 22p-electronic circuit along the relatively planar
skeleton with an inverted pyrrole ring. The free base 7 is
susceptible to oxidative damages because of the free meso-position
but the chemical stability of 12 is considerably improved by the
shielding of the free meso-position by the coordinated rhodium
metals. The large TPA value of 12 (1200 GM) has been ascribed
to its planar structure, elongated p-conjugation length and strong
aromaticity.
12 (a) T. M. Krygowski and T. M. Cryan
52, 1713; (b) T. M. Krygowski and T. M. Cryan
1996, 52, 10255.
´
ski, Tetrahedron, 1996,
´
ski, Tetrahedron,
13 J.-Y. Shin, K. S. Kim, M.-C. Yoon, J. M. Lim, Z. S. Yoon,
A. Osuka and D. Kim, Chem. Soc. Rev., 2010, 39, 2751.
14 M. Pawlicki, H. A. Collins, R. G. Denning and H. L. Anderson,
Angew. Chem., Int. Ed., 2009, 48, 3244.
15 (a) Y.-L. Tung, S.-W. Lee, Y. Chi, L.-S. Chen, C.-F. Shu, F.-I. Wu,
A. J. Carthy, P.-T. Chou, S.-M. Peng and G.-H. Lee, Adv. Mater.,
2005, 17, 1059; (b) J. A. Barron, S. Bernhard, P. L. Houston,
H. D. Abruna, J. L. Ruglovsky and G. G. Malliaras, J. Phys. Chem.
A, 2003, 107, 8130; (c) J.-P. Collin, A. Hamiman, V. Heitz, F. Odobel
and J.-P. Savage, J. Am. Chem. Soc., 1994, 116, 5679; (d) A. Antipas
and M. Gouterman, J. Am. Chem. Soc., 1983, 105, 4896.
This work at Kyoto was supported by Grants-in-Aid
(nos. 22245006 (A) and 20108006 ‘‘pi-Space’’) from MEXT.
The work at Yonsei University was supported by the Midcareer
Researcher Program (2010–0029668), an AFSOR/APARD
grant (no. FA2386-09-1-4092) and the World Class University
(R32-2010-000-10217) Programs of MEST of Korea.
16 GAUSSIAN 09 (Revision A.2), Gaussian, Inc., Wallingford, CT,
2009. Full citation is provided in ESIw.
17 (a) K. Rachlewicz, N. Sprutta, L. Latos-Grazynski, P. J. Chmielewski
_
and L. Szterenberg, J. Chem. Soc., Perkin Trans. 2, 1998, 959;
´
_
(b) K. Rachlewicz, L. Latos-Grazynski, A. Gebauer, A. Vivian and
J. L. Sessler, J. Chem. Soc., Perkin Trans. 2, 1999, 2189;
´
_
(c) P. J. Chmielewski, L. Latos-Grazynski and K. Rachlewicz, Chem.–
´
Eur. J., 1995, 1, 68.
Notes and references
18 (a) P. von R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao and
N. J. R. v. E. Hommes, J. Am. Chem. Soc., 1996, 118, 6317;
(b) Z. Chen, C. S. Wannerere, C. Corminboeuf, R. T. Puchta and
P. von R. Schleyer, Chem. Rev., 2005, 105, 3842.
z Crystal data for 12ꢁCH2Cl2; C54H13Cl2F20N5O4Rh2 M = 1452.41,
monoclinic, space group C2/c (No. 15), Z = 8, a = 26.9166(5), b =
14.9711(3), c = 24.9775(4) A, b = 90.1129(8)1, V = 10065.2(3) A,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6785–6787 6787