Synthesis and guest recognition ability of 2,3-dimethoxy-1,4-phenylene- containing porphyrinoids

Article abstract of DOI:10.1021/ol801888d

(Chemical Equation Presented) The acid-catalyzed condensation of the bispyrrolylbenzene derivative and benzaldehyde yielded macrocycles 1 and 2 bearing three and four dipyrrin units which are connected by 2,3-dimethoxy-1,4-phenylene rings. The cationic gu

Full text of DOI:10.1021/ol801888d

ORGANIC
LETTERS
2008
Vol. 10, No. 20
4601-4604
Synthesis and Guest Recognition Ability
of 2,3-Dimethoxy-1,4-phenylene-Containing
Porphyrinoids
Chusaku Ikeda, Naoya Sakamoto, and Tatsuya Nabeshima*
Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba,
Ibaraki 305-8571, Japan
nabesima@chem.tsukuba.ac.jp
Received August 13, 2008
ABSTRACT
The acid-catalyzed condensation of the bispyrrolylbenzene derivative and benzaldehyde yielded macrocycles 1 and 2 bearing three and four
dipyrrin units which are connected by 2,3-dimethoxy-1,4-phenylene rings. The cationic guest recognition ability of 1 was investigated by
1
UV-vis absorption, H NMR spectroscopic techniques, and ab initio calculations (HF/3-21G(*)). The tris-dipyrrin macrocycle 1 was found to
recognize alkali metals in the O6 binding cavity.
Conjugated macrocycles have attracted significant at-
tention in recent years due to their distinctive physical
properties and potential applications. Among the various
macrocycles, the conjugated oligopyrrolic ones, such as
the expanded porphyrins,1 are particularly interesting
because the conjugation along the macrocyclic skeleton
can be changed by a redox reaction,2 protonation,3
conformational change,4 or metal complexation.5 If the
conjugation structure of the macrocycle can be modulated
by external stimuli such as a guest, it would lead to an
interesting supramolecular host, which shows a drastic
change in the properties based on a change in the
conjugation upon guest binding. To date, the recognition
of an anion,6 alcohol,7and carboxylic acid8 has been achieved
by the porphyrinoids, and the changes in the conjugation
structure were shown in some cases.
(1) Sessler, J. L.; Gebauer, A.; Vogel, E. In The Porphyrin Handbook;
Kadish, K. M.; Smith, K. M.; Guilard, R., Eds.; Academic Press: San Diego,
1999; Volumne 2, Chapter 8.
(2) Neves, M. G. P. M. S.; Martins, R. M.; Tome´, A. C.; Silvestre,
A. J. D.; Silva, A. M. S.; Fe´lix, V.; Drew, M. G. B.; Cavaleiro, J. A. S.
Chem. Commun. 1999, 385.
(4) Ste¸pien´, M.; Latos-Graz˙yn´ski, L.; Sprutta, N.; Chwalisz, P.; Szter-
enberg, L. Angew. Chem., Int. Ed. 2007, 46, 7869.
(3) Lash, T. D. Eur. J. Org. Chem. 2007, 5461.
10.1021/ol801888d CCC: $40.75
Published on Web 09/13/2008
2008 American Chemical Society

As another class of porphyrinoids, novel macrocycles con-
taining nonpyrrolic bridging groups have been reported, and
the effective binding with the anion9 and carboxylic acid10 has
been accomplished. However, the macrocyclic conjugation is
often interrupted by the nonpyrrolic bridging groups.
Among the various bridging groups, the 1,4-phenylene ring
is highly attractive because the phenylene ring is known to
contribute to the macrocyclic conjugation depending on the
tilting angle with the neighboring aromatic rings.4,11 Further-
more, various substituents that bind guests can be introduced
into the phenylene ring.
We reported the synthesis of macrocyclic polyimine ligands
containing 2,3-dihydroxy-1,4-phenylene moieties.12,13 The metal
complexes of these ligands recognize other cationic guests
in the O6 binding site in the cavity.13,14 In this context, we
have synthesized the oligopyrrole macrocycles 1 and 215
containing 2,3-dimethoxy-1,4-phenylene linkages because the
guest recognition using the oxygen atoms may tilt the
phenylene rings and thus induce a change in the optical
properties based on the conjugation structure (Scheme 1).
2 by 1H NMR spectroscopy and X-ray crystallography to clarify
the effect of the conformation and the rotation freedom of the
phenylene moieties on the guest binding properties.
Macrocycles 1 and 2 were synthesized via the acid-catalyzed
condensation of 1,4-bis(2-pyrrolyl)-2,3-dimethoxybenzene and
benzaldehyde followed by oxidation with DDQ. Purification
by silica gel chromatography and gel permeation chromatog-
raphy gave 1 and 2 in 24% and 1.5% yields, respectively.
1
In the H NMR spectra of 1 and 2, the pyrrolic-ꢀ protons
appeared as two sets of doublets at 6.48 and 6.72 ppm for 1
and 6.69 and 6.97 ppm for 2. These chemical shifts are roughly
the same as those of the corresponding dipyrrin monomer, 1,9-
bis(2,3-dimethoxyphenyl)-5-phenyldipyrrin 5 (δꢀ-pyrrole ) 6.65
and 7.00 ppm, see Figure S1, Supporting Information). These
results showed that the macrocyclic conjugation of 1 and 2 is
negligibly small. The methoxy protons and aryl protons of the
phenylene rings of 1 and 2 were observed as a sharp singlet at
room temperature but broadened by lowering the temperature
to 188 K. This result shows that the flipping of the phenylene
rings is fast on the NMR time scale at room temperature.
The crystallographic analysis of 1 revealed a planar triangle
structure, in which the six pyrrole rings are nearly coplanar with
the maximum mean plane deviation of 0.421 Å (Figure 1).16
Scheme 1. Structures of Macrocycles 1 and 2
This paper describes the recognition of cationic guests by
the oligopyrrole macrocycle 1 and the large bathochromic shift
upon guest binding. We also examined the structures of 1 and
(5) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata,
N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; Park, J. K.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 681.
(6) (a) Kra´l, V.; Furuta, H.; Shreder, K.; Lynch, V.; Sessler, J. L. J. Am.
Chem. Soc. 1996, 118, 1595. (b) Sessler, J. L.; Camiolo, S.; Gale, P. A.
Coord. Chem. ReV. 2003, 240, 17.
(7) Sessler, J. L.; Mody, T. D.; Lynch, V. J. Am. Chem. Soc. 1993,
115, 3346.
(8) Lintuluoto, J. M.; Nakayama, K.; Setsune, J. Chem. Commun. 2006,
3492.
Figure 1. Crystal structure of of 1·3.5CHCl3 (ORTEP, 50%
probability). (a) and (b) show the top and side view, respectively.
The solvent molecules are omitted for clarity.
(9) Sessler, J. L.; Maeda, H.; Mizuno, T.; Lynch, V. M.; Furuta, H.
J. Am. Chem. Soc. 2002, 124, 13474.
(10) Setsune, J.; Watanabe, K. J. Am. Chem. Soc. 2008, 130, 2404.
(11) (a) Mu¨llen, K.; Unterberg, H.; Huber, W.; Wennerstro¨m, O.;
Norinder, U.; Tanner, D.; Thulin, B. J. Am. Chem. Soc. 1984, 106, 7514.
(b) Ste¸pien´, M.; Latos-Graz˙yn˙ski, L. J. Am. Chem. Soc. 2002, 124, 3838
.
The three dimethoxyphenylene rings are tilted with respect to
the six-pyrrolic plane with tilting angles of 18°, 0.8°, and 144°
(12) Akine, S.; Taniguchi, T.; Nabeshima, T. Tetrahedron Lett. 2001,
42, 8861
.
(13) Akine, S.; Sunaga, S.; Taniguchi, T.; Miyazaki, H.; Nabeshima, T.
Inorg. Chem. 2007, 46, 2959
(14) Nabeshima, T.; Miyazaki, H.; Iwasaki, A.; Akine, S.; Saiki, T.;
Ikeda, C.; Sato, S. Chem. Lett. 2006, 35, 1070
(15) (a) Ikeda, C.; Sakamoto, N.; Nabeshima, T. 12th International
Symposium on Novel Aromatic Compounds, Awaji Island, Japan, 22-27
July, 2007, p 376. Very recently, the rhodium complex of the analogue to
1 was reported. See: (b) Setsune, J.; Toda, M.; Yoshida, T. Chem. Commun.
2008, 1425.
.
.
(16) Crystallographic data for 1·3.5CHCl3: C72.5H57.5C110.5N6O6, M )
1480.97, triclinic, a ) 14.85(2) Å, b ) 14.88(2) Å, c ) 17.50(3) Å, R )
101.00(6)°, ꢀ ) 108.81(7)°, γ ) 104.85(6)°, U ) 3376(9) Å3, T ) 120(2)
K, space group P-1 (no. 2), Z ) 2, 26847 reflections measured, 11868 unique
(Rint ) 0.0549). R1 ) 0.1110 (I > 2σ(I)), wR2 ) 0.2559 (all data), GOF
(F2) ) 1.067.
4602
Org. Lett., Vol. 10, No. 20, 2008

for rings A, B, and C, respectively.17 The tilted orientations of
the phenylene rings forced the four methoxy groups inward and
the other two groups outward.
spectroscopy. Figure 3 shows the changes in the UV-vis
spectrum of 1 in toluene-acetonitrile (95:5) upon the
On the other hand, the X-ray crystallographic study of 2
revealed a twisted figure-eight structure that is located on
the crystallographic 2-fold axis (Figure 2).18 The pyrrole
Figure 3. Absorption spectra of 1 (2.1 µM) recorded in toluene-
acetonitrile (95:5) under various concentrations of RbTFPB. Inset
shows the binding isotherm at 512 nm (circle) and 566 nm (triangle)
analyzed by a nonlinear least-squares regression (calculated lines
is shown in solid lines).
addition of rubidium tetrakis(3,5-bis(trifluoromethyl)phe-
nyl)borate (RbTFPB).
The addition of Rb+ caused a decrease in the absorption
intensity at 512 nm along with a concomitant increase in
the absorption at 566 nm. Notably, a smaller broad absorption
band at 650 nm in the absence of the guest cation shifted to
ca. 780 nm with increasing intensity by the addition of Rb+.
Compound 1 also showed the interaction with K+ and Cs+
since significant spectral changes are also observed (Figures
S2 and S3, Supporting Information).
The bathochromic shifts observed in the peak at 512 nm
were 56, 54, and ca. 50 nm19 for K+, Rb+, and Cs+,
respectively. The observed large bathochromic shifts may
be attributed to the elongation of the effective π-conjugation
along the macrocycle due to the conformational change upon
the complexation with the cationic guests. Analysis of the
observed spectral changes by the nonlinear least-squares
regression supported the 1:1 stoichiometry with the associa-
tion constants (Ka) of (2.1 ( 0.2) × 106, (2.3 ( 0.2) × 106,
and (2.5 ( 0.3) × 106 M-1 for K+, Rb+, and Cs+,
respectively.
The Hartree-Fock calculation suggested that all the
methoxy groups face inside and the guest cation locates
in the center of the triangle core, which is stabilized by a
cation-dipole interaction between the guest cation and
the six oxygen atoms (Figure 4 for Rb+ guest). The
calculation showed that 1 also adopts a suitable conforma-
tion with K+ or Cs+ by maintaining the coordination
distances between the cation and methoxy oxygens by
changing the tilting angles of the phenylene rings. The
calculated average distances between the cation and
Figure 2. Crystal structure of 2·2CH2Cl2 (ORTEP, 50% probability).
(a) and (b) show the top and side views, respectively. Schematic view
of (a) is also shown. The solvent molecules are omitted for clarity.
rings E, F, and the dimethoxyphenylene ring D are nearly
coplanar with a maximum mean plane deviation of 0.08 Å.
But the A, B, and C rings are not coplanar. The dihedral
angles between rings A and B, and B and C are 29.2° and
19.4°, respectively. These distortions form a large cleft
structure with the N(3)sN(3)* distance of 10.19 Å.
Since 1 has a rigid triangle framework with rotatable
phenylene moieties bearing oxygen substituents, the interac-
tion of 1 with alkali metal ions was examined by UV-vis
(17) The tilting angle is defined as the angle between the six-pyrrolic
plane and the dimethoxyphenylene ring. A 0° angle denotes the in-plane
geometry of the phenylene ring with the two oxygen atoms located inside
the cavity.
(18) Crystal data for 2·2CH2Cl2: C94H76C14N8O8, M ) 1587.43, orthor-
hombic, a ) 22.06(3) Å, b ) 12.69(2) Å, c ) 29.55(4) Å, U ) 8275(21)
Å3, T ) 120(2) K, space group Pbcn (no. 60), Z ) 4, 57643 reflections
measured, 7268 unique (Rint ) 0.1178). R1 ) 0.0807 (I > 2σ(I)), wR2 )
0.2099 (all data), GOF (F2) ) 0.995.
(19) Appropriate value due to the broad peak shape. See Figure S3 in
the Supporting Information.
Org. Lett., Vol. 10, No. 20, 2008
4603

Figure 5.
1H NMR spectra of (a) 1 and (b) 1 + RbTFPB recorded
in toluene-d8–CD3CN (95:5) at room temperature. Concentrations
of 1 and Rb+ in (b) are 0.1 and 0.4 mM, respectively.
by the interaction with Rb+. The three singlets also agreed
with the expected Cs symmetry of the proposed cation-
binding structure.
Figure 4.
Proposed Rb+-binding structure of 1 generated by
Hartree-Fock calculations at the 3-21G (*) level. Calculation was
carried out without the phenyl groups. Key: C, gray, H, white, N,
blue, O, red, Rb, green.
In summary, novel macrocyclic hosts containing the
dipyrrin moieties were synthesized from the bis-pyrrolyl-
benzene derivative. The spectroscopic studies and ab initio
calculation suggested that the conjugation along the macro-
cycle 1 is changed by the recognition of the alkali metals in
the cavity. The results obtained in this study may be useful
to construct a supramolecular host that can drastically switch
its conjugation upon guest binding. The synthesis and
properties of the corresponding demethylated derivatives with
different binding properties are the subject of the current
study.
oxygen atoms were 2.95, 3.04, and 3.22 Å for K+, Rb+,
and Cs+, respectively, and the average tilting angles of
the phenylene rings are 32.4°, 34.5°, and 36.9° for K+,
Rb+, and Cs+, respectively. The small angles contribute
to the bathochromic shifts of the absorption spectra upon
cation binding probably due to a more efficient π-conju-
gation in the rigid conformation with the small tilting
angles between the aromatic rings than that of the flexible
1 in the absence of the cationic guest.
The expected conformational change was further sup-
ported by the 1H NMR spectra of 1 in toluene-d8–CD3CN
(95:5) (Figure 5). The methoxy protons of 1 appeared as
a singlet at 4.17 ppm in the absence of the guest cations.
However, the protons appeared as three singlets at 4.08,
4.11, and 4.23 ppm in the presence of 4 equiv of Rb+.
This result indicates that the flipping of the dimethoxy-
phenylene rings are suppressed on the 1H NMR time scale
Acknowledgment. This work was supported by Grants-
in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan.
Supporting Information Available: Synthetic procedure
and spectral data of 1 and 2, calculated structures with K+
1
and Cs+, and H NMR of 1 in the presence of the guest
cations. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL801888D
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Org. Lett., Vol. 10, No. 20, 2008