9,10,19,20-Tetraarylporphycenes

Article abstract of DOI:10.1021/ol802351r

(Chemical Equation Presented) meso-Tetraarylporphycenes, structural isomers of mesotetraarylporphyrins, were synthesized by the acid-catalyzed oxidative coupling reaction for the first time which is an alternative synthetic methodology to the traditional McMurry coupling reaction. The structure of the macrocycle and Ni(II) complex are characterized by single-crystal X-ray diffraction analyses where both form 1-D supramolecular assembly.

Full text of DOI:10.1021/ol802351r

ORGANIC
LETTERS
2008
Vol. 10, No. 24
5545-5548
9,10,19,20-Tetraarylporphycenes
K. S. Anju,† S. Ramakrishnan,† Ajesh P. Thomas,† Eringathodi Suresh,‡ and
Alagar Srinivasan*,†
Photosciences and Photonics Section, Chemical Sciences and Technology DiVision,
National Institute for Interdisciplinary Science and Technology (NIIST-CSIR),
TriVandrum-695019, Kerala, India, and Analytical Science Discipline, Central Salt and
Marine Chemical Research Institute (CSMCRI-CSIR),
BhaVnagar-364002, Gujarat, India
indiansrini@gmail.com
Received October 10, 2008
ABSTRACT
meso-Tetraarylporphycenes, structural isomers of meso-tetraarylporphyrins, were synthesized by the acid-catalyzed oxidative coupling reaction
for the first time which is an alternative synthetic methodology to the traditional McMurry coupling reaction. The structure of the macrocycle
and Ni(II) complex are characterized by single-crystal X-ray diffraction analyses where both form 1-D supramolecular assembly.
Porphycene (2, 3), an isomer of porphyrin (1) with C20H14N4
in its macrocyclic core, is a potential candidate in the area
of photodynamic therapy (PDT) due to immense absorption
in the far red region of the visible spectrum.1 It was first
reported by Vogel and co-workers as a planar, aromatic
macrocycle formally known as [18]porphyrin-(2.0.2.0).2a
Since then, a series of porphycene derivatives1,2 such as
tetraalkyl2b/tetraaryl (2a)2f-h /octaalkyl (2b)2d,e and their
metal complexes3 have been reported. The substituents are
confined to the ꢀ-position of the pyrrole rings, and these
compounds are considered as isomers of octaethylporphyrin
(OEP, 1a). In addition, meso-tetraalkylporphycenes (3) were
also synthesized by Vogel et al.2c All of the derivatives were
† National Institute for Interdisciplinary Science and Technology.
‡ Central Salt and Marine Chemical Research Institute.
(1) (a) Sanchez-Garcia, D.; Sessler, J. L. Chem. Soc. ReV. 2008, 37,
215–232. (b) Stockert, J. C.; Canete, M.; Juarranz, A.; Villanueva, A.;
Horoin, R. W.; Borrell, J. I.; Teixido, J.; Nonell, S. Curr. Med. Chem. 2007,
14, 997–1026.
(3) (a) Gisselbrecht, J. P.; Gross, M.; Kocher, M.; Lausmann, M.; Vogel,
E. J. Am. Chem. Soc. 1990, 112, 8618–8620. (b) Oertling, W. A.; Wu, W.;
Lopez-Garriga, J. J.; Kim, Y.; Chang, C. K. J. Am. Chem. Soc. 1991, 113,
127–134. (c) Li, Z.-Y.; Huang, J.-S.; Che, C.-M.; Chang, C. K. Inorg. Chem.
1992, 31, 2670–2672. (d) Kadish, K. M.; Van Caemebecke, E.; Boulas, P.;
D’Souza, F.; Vogel, E.; Kisters, M.; Medforth, C. J.; Smith, K. M. Inorg.
Chem. 1993, 32, 4177–4178. (e) Lausmann, M.; Zimmer, I.; Lex, J.; Lueken,
H.; Wieghardt, K.; Vogel, E. Angew. Chem., Int. Ed. Engl. 1994, 33, 736–
739. (f) D’Souza, F.; Boulas, P.; Aukauloo, A. M.; Guilard, R.; Kisters,
M.; Vogel, E.; Kadish, K. M. J. Phys. Chem. 1994, 98, 11885–11891. (g)
Canete, M.; Ortiz, A.; Juarranz, A.; Villanueva, A.; Nonell, S.; Borrell,
J. I.; Teixido, J.; Stockert, J. C. Anti-Cancer Drug Des. 2000, 15, 143. (h)
Flower, C. J.; Sessler, J. L.; Lynch, V.; Waluk, J.; Gebauer, A.; Lex, J.;
Heger, A.; Zuniga-y-Rivero, F.; Vogel, E. Chem.sEur. J. 2002, 8, 3485–
3496.
(2) (a) Vogel, E.; Kocher, M.; Schmickler, H.; Lex, J. Angew. Chem.,
Int. Ed. Engl. 1986, 25, 257–259. (b) Vogel, E.; Balci, M.; Pramod, K.;
Koch, P.; Lex, J.; Ermer, O. Angew. Chem., Int. Ed. Engl. 1987, 26, 928–
931. (c) Vogel, E.; Kocher, M.; Lex, J.; Ermer, O. Isr. J. Chem. 1989, 29,
257–266. (d) Vogel, E.; Koch, P.; Hou, X.-L.; Lex, J.; Lausmann, M.;
Kisters, M.; Aukauloo, M. A.; Richard, P.; Guilard, R. Angew. Chem., Int.
Ed. Engl. 1993, 32, 1600–1604. (e) Guilard, R.; Aukauloo, M. A.; Tardieux,
C.; Vogel, E. Synthesis 1995, 1480–1482. (f) Nonell, S.; Bou, N.; Borrell,
J. I.; Teixido, J.; Villanueva, A.; Juarranz, A.; Canete, M. Tetrahedron Lett.
1995, 36, 3405–3408. (g) Gavalda, A.; Borrell, J. I.; Teixido, J.; Nonell,
S.; Arad, O.; Grau, R.; Canete, M.; Juarranz, A.; Villanuea, A.; Stockert,
J. C. J. Porphyrins Phthalocyanines 2001, 5, 846–852. (h) Arad, O.; Morros,
J.; Batllori, X.; Teixido, J.; Nonell, S.; Borrell, J. I. Org. Lett. 2006, 8,
847–850.
10.1021/ol802351r CCC: $40.75
Published on Web 11/17/2008
2008 American Chemical Society

synthesized through McMurry coupling reactions, which is
the only known methodology available to date.1,2 However,
to the best of our knowledge, meso-tetraarylporphycene, the
structural isomer of meso-tetraarylporphyrin (TPP, 1b), is
not known. Herein, we wish to report the synthesis, structural
characterization and coordination studies of meso-tetraarylpor-
phycene. Instead of the traditional McMurry coupling
reactions, we adopted acid-catalyzed oxidative couplings4
for the synthesis of meso-tetraaryl derivatives and the
structural features favor the formation of divalent metal
complexes.
to three and (2) simple and straightforward syntheses of
precursors and target macrocycles. In support of this, meso-
tetratolylporphycene (7) was synthesized from the respective
5,6-ditolyldipyrroethane (5) in 3% yield. Both macrocycles
(6, 7) are highly soluble in most of the organic solvents which
is not very common in the ꢀ-alkyl derivatives2a,b and the
reported yields are comparable to 3.2c Both 6 and 7 are air-
stable and afford violet diamond-shaped crystals from CHCl3/
n-hexane.
1
The H analysis of 4 and 5 shows signals corresponding
to half the linear chain, suggesting the symmetrical nature
in which the pyrrole rings are opposite to each other. The
structure of 4 is further confirmed by single-crystal X-ray
analysis, where the pyrrole rings are trans to each other.5
The pyrrole and the phenyl rings are arranged in a rectangular
fashion to form the two-dimensional supramolecular as-
sembly due to the C-H···πpyrrole interaction; the distance and
the angles are 2.81 Å and 140.1°, suggesting that the target
macrocycle will adopt a rectangular arrangement in the solid
state.
The FAB mass spectral analyses of 6 and 7 exhibit a
molecular ion signal at m/z 614 and 670, respectively,
indicating the exact composition of the macrocycles. The
electronic absorption as well as the emission spectrum of 6
in CH2Cl2 is shown in Figure 1, along with the respective
The synthesis of the target macrocycle and its metal
complexes is depicted in Scheme 1, where the synthesis
Scheme 1. Syntheses of 6-9
involved three steps. The initial step is the quantitative
conversion of benzoin and/or benzil to 1,2-diphenylethane-
1,2-diol by using NaBH4 reduction.5 The second step is the
synthesis of hitherto unknown 5,6-diphenyldipyrroethane (4).
Here, the direct conversion of diol to 4 is unsuccessful in
the presence of various acid catalysts such as borontrifluoride
etherate (BF3·OEt2) and trifluoroacetic acid (TFA). Hence,
we modified the synthetic route by converting diol into
mesylated product using mesyl chloride followed by heating
with pyrrole at 40 °C for 2 days to afford 4 in 35% yield.5
In the final step, the acid-catalyzed oxidative coupling
reaction of 4 using p-toluenesulfonic acid (p-TSA) as the
acid catalyst followed by oxidation in presence of 2,3-
dichloro-5,6-dicyano-p-benzoquinone (DDQ) afforded 6 as
a blue fraction in 5% yield. It is pertinent to point out here
that all the synthetic methods reported to date involve six to
eight steps.1,2 Hence, the main features of this new synthetic
protocol are (1) a reduction in the number of steps from eight
Figure 1. Electronic absorption (blue) and emission (red) spectrum
of 6 in CH2Cl2. Emission intensity is normalized to the absorption
spectrum.
colors. The absorption spectrum of 6 shows the Soret band
at 382 nm and Q-bands between 584 to 653 nm. On
incorporation of the phenyl rings at the meso-positions, a
24 and 9 nm red shift of the Soret band with respect to the
parent porphycene2a and 3c2c was observed. When compared
to 1b, 6 shows promising features, where the molar extinction
coefficient for the Soret band is 3-fold less intense and
substantially blue-shifted. In addition, the Q bands appeared
at longer wavelength with 1 order of magnitude higher.
Furthermore, the emission spectrum of 6 shows a main band
at 659 and a weaker band at 715 nm with the quantum yield
of ΦF 0.23 in argon-saturated solutions. Hence, the value is
relatively lower than that of 3c (ΦF 0.30) and 2-fold higher
than 1b (ΦF 0.11).
(4) (a) Paolesse, R.; Licoccia, S.; Spagnoli, M.; Boschi, T.; Khoury,
R. G.; Smith, K. M. J. Org. Chem. 1997, 62, 5133–5137. (b) Narayanan,
S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Angew. Chem.,
Int. Ed. 1998, 37, 3394–3397. (c) Sessler, J. L.; Seidel, D.; Lynch, V. J. Am.
Chem. Soc. 1999, 121, 11257–11258. (d) Shimizu, S.; Taniguchi, R.; Osuka,
A. Angew. Chem., Int. Ed. 2005, 44, 2225–2229.
(5) See the Supporting Information.
5546
Org. Lett., Vol. 10, No. 24, 2008

The 1H spectrum of 6 in CD2Cl2 shows aromatic features
of porphycene, where the pyrrolic ꢀ-CH protons resonate
as an AB pattern at 9.39 and 8.32 ppm while the o-phenyl
protons are observed at 7.65 and the m,p-phenyl protons
resonate at 7.32 ppm as multiplets (Figure 2). Unlike 1b,
through a possible free rotation along C5-C5′ of 4.5 The
two amino and two imino pyrroles are connected through
the meso-phenyl-substituted vinylene and/or ethylene bridge,
and the amino pyrrolic hydrogen is localized on that
particular nitrogen (Figure 3a). It is pertinent to point out
here that the amino pyrrolic hydrogens are not localized in
any of the porphycene units reported in the literature due to
the disorder in the N-H···N hydrogen bonds.2a-d There are
two strong intramolecular hydrogen bonding interactions with
the N-H···N distances and angles of 1.49 Å and 159.6°. As
observed in 4,5 the macrocycle adopts a rectangular arrange-
ment with N1-N2 and N1-N2′ distances of 2.87 and 2.55
Å, respectively. The distances reported here are similar to
3c in which the corresponding distances are 2.90 and 2.53
Å.2c One of the interesting observations is the presence of
intermolecular interactions through C-H···πpyrrole with the
distance and angle of 2.99 Å and 136° (Figure 3c). It is
formed between the hydrogen of the meso-phenyl rings with
the N1 and N1′ of the pyrrolic π cloud, which generates a
one-dimensional supramolecular assembly in the solid state.
The meso-phenyl rings are almost perpendicular to the mean
macrocyclic plane, and the deviations are 79 and 89.6°,
respectively. The pyrrole rings are hardly deviated from the
mean plane with the maximum deviation of 1.16° with
planarity retained (Figure 3b).
Figure 2.
1H spectra of 6 and 8 in CD2Cl2.
the pyrrolic NH proton is shifted to the downfield region
and resonates as a broad signal at 5.88 ppm, which suggests
that the inner NH protons are in strong intramolecular
hydrogen-bonding interaction with the opposite imine ni-
trogens. This is further confirmed by IR spectral analysis,
where broad stretching vibration is observed from 3600 to
3300 cm-1. The presence of NH protons is assigned by
CD2Cl2/D2O experiment, where the respective NH signal
disappeared. Further, the absence of two R-CH protons in
the pyrrolic rings which are observed in 4, proved by 1H-1H
COSY spectral analysis of 6,5 where the correlation between
the pyrrolic NH proton and the ꢀ-CH protons suggests the
formation of the macrocycle.
The explicit structural detail of the macrocycle 6 is derived
from the single-crystal X-ray diffraction analysis (Figure
3a-c),6 where 6 is a centrosymmetric molecule with C2h
symmetry. Consistent with the above observations, R and
R′ positions of the pyrroles in one dipyrroethane unit are
coupled with another unit to form the macrocycle 6. The
first pyrrole-pyrrole bond is formed from the similar
conformation of 4, and the required planar conformation for
the formation of second pyrrole-pyrrole link is achieved
The metal coordination chemistry of 6 was investigated
using Ni(II) and Pd(II) salts. When 6 was treated with
nickel(II) acetylacetonate, in CH3COOH, blue compound 8
with violet crystals was obtained in quantitative yield. In
the presence of palladium(II) acetate, 9 was synthesized in
55% yield (Scheme 1). The metal complexes 8 and 9 were
confirmed by FAB mass spectral analysis with its parent ion
peak at m/z 670 and 719, respectively. The electronic
absorption spectrum of 8 and 9 showed the Soret band at
393 nm, which was 11 nm red-shifted. The number of
Q-bands were reduced to two from three, and the bands were
blue-shifted and appeared between 584 and 635 nm as
compared to 6. The slightly upfield shifted pyrrolic ꢀ-CH
and the o- and m,p-phenyl protons with the disappearance
1
of NH signal from the H analysis of 8 (Figure 2) and 9
clearly suggested the insertion of Ni(II) and Pd(II) in 6.
The final proof of the proposed metal complex (8) came
from the single-crystal X-ray structure as shown in Figure
Figure 3. Single-crystal X-ray structures of 6 and 8. (a, d, b, e): top and side views of 6 and 8. (c, f): one-dimensional array of 6 and 8.
meso-Phenyl groups and the solvent molecules are omitted for clarity in the side view as well as in the 1D array, which is not involved in
the H-bonding interaction.
Org. Lett., Vol. 10, No. 24, 2008
5547

3d-f. As predicted from the spectral analyses, Ni(II) is
incorporated inside the macrocyclic ring and the geometry
around the metal center is square planar with N1-Ni-N2
and N1-Ni-N2′ angles of 86.8 and 93.2°, respectively
(Figure 3d). Conversion of 6 into 8 slightly rearranges the
rectangular fashion with N1-N2 and N1-N2′ distances of
2.59 and 2.76 Å, respectively. This leads to a decrease in
the N1-N2 distance by 0.28 Å and an increase in the
N1-N2′ distance by 0.21 Å. A similar trend was observed
in 3c,2c where the respective distances are 2.60(0.30) and
2.74(0.21) Å. Insertion of nickel to the porphycene core
retains a one-dimensional array with C-H···πpyrrole intermo-
lecular interactions with the distance and angle of 3.02 Å
and 135° (Figure 3f). This is generated between the hydrogen
of the meso-phenyl rings with N2 and N2′ of the pyrrolic π
cloud. Further, the maximum deviation of the pyrrole rings
is 1.51° and thus maintains the planarity (Figure 3e).
In conclusion, we have demonstrated the syntheses,
spectral properties, and one-dimensional supramolecular
assembly of 9,10,19,20-tetraarylporphycenes and their metal
complexes for the first time. The acid-catalyzed oxidative
coupling reaction for porphycene synthesis is an alternative
route to the traditional McMurry coupling reaction, used
since the discovery of porphycene in 1986.2a The aryl groups
at the meso-position can be properly tuned and find immense
application in the area of PDT.1b Studies are in progress in
this direction in our group.
Acknowledgment. Dr. A.S. thanks DST, New Delhi, and
the Director NIIST-CSIR for financial support. A.K.S. thanks
CSIR for the fellowship. We thank Dr. Babu Varghese, SAIF,
IIT-Chennai, for solving the crystal structure of 4. We greatly
acknowledge Mrs. Viji and Sowmini, NIIST-CSIR, for
recording the FAB and NMR spectra.
(6) (a) Crystal data for 6: C46H32Cl6N4, Mw ) 853.46, triclinic, space
group P-1, a ) 9.2285(14) Å, b ) 9.6234(15) Å, c ) 11.8311(18) Å, R )
78.780(3)°, ꢀ ) 76.834(3)°, γ ) 73.187(3)°, V ) 970.0(3) Å3, Z ) 1, Dcalc
) 1.461 mg/m3, T ) 100(2) K, R1 ) 0.0774, wR2 ) 0.1435. (b) Crystal
data of 8: C46H30Cl6N4Ni, Mw ) 910.15, triclinic, space group P-1, a )
9.276(2) Å, b ) 9.527(3) Å, c ) 11.884(3) Å, R ) 80.079(5)°, ꢀ )
76.283(5)°, γ ) 74.095(5)°, V ) 974.8(3) Å3, Z ) 1, Dcalc ) 1.550 mg/
m3, T ) 100(2) K, R1 ) 0.0897, wR2 ) 0.1614. The single-crystal X-ray
diffraction data were collected on a Bruker SMART Apex diffractometer
equipped with CCD area detector at 100(2) K for 6 and 8. Good quality
single crystals were grown by slow evaporation of n-hexane into CHCl3
solutions of 6 and 8. CCDC-694578 and CCDC-694577 contain the
supplementary crystallographic data for 6 and 8, respectively.
Supporting Information Available: Synthetic procedures
and spectral data for all new compounds. Crystal data for 4,
6, and 8 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
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Org. Lett., Vol. 10, No. 24, 2008