meso-substituted aromatic 34π core-modified octaphyrins: Syntheses, characterization and anion binding properties

Article abstract of DOI:10.1002/chem.200204537

Modified octaphyrins with 34π electrons have been synthesized and characterized following a simple synthetic methodology. An acid-catalyzed α,α coupling of tetrapyrranes containing furan, thiophene and selenophene rings resulted in the formation of the re

Full text of DOI:10.1002/chem.200204537

FULL PAPER
meso-Substituted Aromatic 34p Core-Modified Octaphyrins:
Syntheses, Characterization and Anion Binding Properties
[a]
[a]
Venkataramanarao G. Anand, Sundararaman Venkatraman, Harapriya Rath,[a]
Tavarekere K. Chandrashekar,*[a] Weijie Teng,[b] and Karin Ruhlandt-Senge[b]
Abstract: Modified octaphyrins with
34p electrons have been synthesized
and characterized following a simple
synthetic methodology. An acid-cata-
lyzed a,a coupling of tetrapyrranes
containing furan, thiophene and seleno-
phene rings resulted in the formation of
the respective octaphyrins in relatively
good yield. Solution studies by 1HNMR
and 2D NMR methods and single crystal
X-ray structural characterization reveal
an almost flat structure with two heter-
ocyclic rings inverted. Specifically, in 14
two selenophene rings (one on each
biselenophene unit) are inverted while
in 15 two furan rings (one on each
bifuran unit) are inverted when the
meso substituent are mesityl groups.
On changing the mesityl substituent to
m-xylyl group as in 19, the location of
ring inversion shifts to pyrrole rings (one
on each bipyrrole unit) indicating the
dependence of structure on the meso
substituents. UV/Vis studies, both in
freebase and protonated forms reveal
typical porphyrinic character and the
aromatic nature of the octaphyrins. The
phyrins bind TFA anion in a 1:2 ratio.
The TFA anions are located one above
and below the plane of the octaphyrin
macrocycle and they are held by weak
electrostatic N-H-O interactions similar
to that observed for protonated rubyr-
ins. However, in the present case, there
is an additional non-electrostatic C-H-O
interaction involving b-CHof the in-
verted heterocyclic ring and the carbon-
yl oxygen of the TFA. Furthermore,
inter molecular interactions between the
1
Dd values evaluated by HNMR spec-
troscopy also support their aromatic
nature. The protonated forms of octa-
À
C Hof the meso-mesityl group and the
fluorine of CF3 group of bound TFA
leads to the formation of one-dimen-
sional supramolecular arrays with inter-
planar distance of 13 ä between two
octaphyrins.
Keywords:
aromaticity
anion
¥ porphirinoids
binding
¥
¥
supramolecular array ¥ 34p electrons
Introduction
Expanded porphyrins continue to attract attention of syn-
thetic chemists due to their diverse applications.[1] Apart from
N
H
N
H
N
H
26π
N
N
host of diverse applications[2 6] they can also be used as a tool
N
N
22π
H
H
H
N
H
N
for accessing higher aromatic systems[7
unknown to date.
10]
N
N
Expanded porphyrins have more than 18p electrons in the
conjugated pathway either due to increased number of
heterocyclic rings or due to multiple meso carbon bridges.
Sapphyrin (22p), 1, and rubyrin (26p),[11] 2, are examples of
expanded porphyrins, which display physical properties
1
2
[a] Prof. Dr. T. K. Chandrashekar, Prof. Dr. V. G. Anand,
S. Venkatraman, H. Rath
similar to that of the parent 18p porphyrin. Replacement of
the nitrogens by other chalcogens such as O, S or Se yield core
modified expanded porphyrins.[12] meso-Aryl substitutions on
expanded porphyrins revealed structural diversities such as
mono-[12a] and bis-[13] ring inversions leading to inverted
3 7 (see below). These molecules
have been synthesized by efficient synthetic methods and
characterized through various spectroscopic methods. The
Department of Chemistry
Indian Institute of Technology
Kanpur 208016 (India)
Fax : (‡91)512-2597282/2597436/2590007
E-mail: tkc@iitk.ac.in
expanded porphyrins[14]
[b] W. Teng, Prof. Dr. K. Ruhlandt-Senge
Department of Chemistry, Syracuse University
Syracuse, New York 13244-4100 (USA)
Fax : (‡1)315-443-4070
higher analogues of expanded porphyrins were found to be
18]
non-aromatic,[15
until we reported the first aromatic 30p
Supporting information for this article is available on the WWW under
http://www.chemeurj.org/ or from the author.
modified heptaphyrins,[19] 6 and 7, bearing seven heterocyclic
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DOI: 10.1002/chem.200204537
Chem. Eur. J. 2003, 9, 2282 2290

2282 2290
Ar
Ar
Ar
Ar
X
N
N
N
N
N
N
O
O
N
NH
HN
H
S
S
Y
Y
N
N
Ar
Ar
Ar
Ar
S
S
NH
N
HN
N
N
4
3
N
X = S/Se/O/N-H/N-Me
Y = S/Se
8
8a
Ar
Ar
X
Mes
Mes
Mes
S
Mes
N
N
H
NH
N
N
NH HN
S
S
N
N
H
S
HN
N
N
HN
S
S
N
NH
S
S
H
N
X
Ar
Ar
Mes
Mes
N
NH HN
5
6
X = S or Se
Mes
Mes
9
10
Se
Mes
Mes
Se
NH HN
N
N
HN
N
Mes
S
S
NH
N
Mes
S
NH HN
7
11
units. Isomers of heptaphyrins are synthesized by either
through an acid-catalyzed condensation or oxidative coupling
reaction of appropriate precursors.[20] Later, Sessler and co-
workers reported an all aza isomer of 30p heptaphyrin[21] that
exhibits a flat structure in solution, while a figure eight
conformation in the solid state.
properties of oxa and selena octaphyrins. These octaphyrins
also exhibit ring inversions where the heterocyclic rings
experience a 1808 ring flipping both in solution and solid state.
Furthermore, the change of meso mesityl substituent to m-
xylyl group results in the inversion of pyrrole rings rather than
the selenophene or furan rings. The X-ray structural studies
on the TFA bound octaphyrins reveal formation of 1:2 anion
complex, with different binding modes. The details of these
studies are reported in this contribution.
Octaphyrins, expanded porphyrins with eight heterocyclic
units, with 32p,[15] 34p,[18] 36p[17] and 38p[22] (see above)
reported in literature were found to exhibit both planar and
twisted figure eight conformations, but are non-aromatic in
nature. In a recent preliminary communication,[23] we have
devised a one-pot synthesis for aromatic 34p tetrathia
octaphyrin, 10, through acid catalyzed oxidative coupling
reaction of modified tetrapyrranes. This molecule exhibits an
absolute flat structure as analyzed by X-ray analysis, in sharp
contrast to the figure eight conformations of other octaphyr-
ins known so far. Further confirmation for the aromatic
nature came from the observation of the signals in the high
field region for the b-CHprotons of the inverted heterocyclic
rings and the NHprotons of the pyrrole rings. Even though
Franck and co-workers[9] have reported a 34p vinylogous
porphyrin, only solution-state studies proved the aromatic
behavior. In a recent report,[24] Sessler and co-workers have
reported a cyclo[8]pyrrole, 11, which has eight pyrrole rings
linked to each other without any meso carbon bridges. This
molecule was also found to be flat as analyzed in solid state,
but was accounted to have only 30p electrons in its conjugated
pathway. The cavity of octapyrrole was found very compatible
to bind a sulphate ion in the protonated form. In this paper we
wish to report syntheses, characterization and anion binding
Results and Discussion
Syntheses: Octaphyrin syntheses have been achieved by
various methods involving appropriate precursors.[17, 18, 22, 25]
Cyclisation of simple molecules to huge macrocycles either
through acid-catalyzed condensation or oxidative coupling is
an attractive methodology to access novel higher order
aromatic expanded porphyrins. But all such methods have
been fruitful only in the syntheses of nonaromatic and
nonplanar octaphyrins.[15] Generally it has been observed that
polycyclopyrroles undergo a twisting, which is responsible for
the loss of planarity and hence aromaticity. Due to such
curling, the molecule inspite of following the H¸ckel×s
(4n‡2) p rule does not exhibit aromatic features. Therefore,
arresting the twist in the molecule is a key step in sustaining
the planar form. Also an observation of the previous
syntheses on octaphyrins, revealed steric hindrance due to
bulky substituents on the b-positions of the pyrrole rings.[26]
Chem. Eur. J. 2003, 9, 2282 2290
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T. K. Chandrashekar et al.
FULL PAPER
Such a strain can be minimized if the steric strain is shifted to
the meso carbons, thereby making the meso substituents more
orthogonal to the porphyrin plane. Also the presence of
bigger atoms in the core of the macrocycle should be an added
advantage in further affecting the desired modifications. This
idea stimulated from the retrosynthetic analysis (Scheme 1)
for designing the planar octaphyrin. The 34p system is twisted
to form the non-aromatic molecule. The planar form can be
envisaged by hindering the molecule from the twist. The
untwisted octaphyrin could be split into two equal halves,
which will contain a quarter pyrrole unit with two meso
carbons. Theoretically, bridging the two tetrapyrranes directly
by oxidative coupling reaction should yield the desired
octaphyrin. Based on this, one can expect that the linear
tetrapyrrole could be coupled to get the all aza non-twisted
octaphyrin. But according to a recent report,[26] the bipyrrole
diol dissociates to bipyrrole and benzaldehyde under acidic
conditions. Hence the above approach could not be applied
for the synthesis of all aza isomer of octaphyrin. On the other
hand, these quarter pyrroles can be modified into the
requirements as mentioned above by incorporating the
requisite bulky meso substituents. In the process the required
precursor turns out to be the known modified tetrapyrrane,[27]
which has been, used extensively in the synthesis of various
expanded porphyrins. The modified tetrapyrranes can be
synthesized from the modified diols, which can be obtained
from the corresponding biselenophene/bifuran.
groups on the meso carbons. The modified tetrapyrranes, 12
and 13 on treatment with trifluoroacetic acid (TFA) and
further oxidation by chloranil under reflux conditions yielded
the planar octaphyrins, 14 and 15 in 9.6 and 4% yield,
respectively (Scheme 2). The variation of acid concentration
within the range of one equivalent did not affect the reaction,
while acid concentration of two equivalents yielded more of
corresponding rubyrin probably due to the acidolysis of the
tetrapyrromethanes.[20]
Mes
Mes
N
N
H
Mes
Mes
X
X
X
i) TFA/CH2Cl2
ii) Chloranil
X
34π
HN
NH
X
X
H
N
N
12: X = Se
13: X = O
Mes
Mes
14: X = Se
15: X = O
Scheme 2. Synthesis of octaphyrins.
The formation of octaphyrin at very less concentrations of
TFA indicates the a,a coupling[13] of tetrapyrrane with the
formation of two direct pyrrole pyrrole links to form an
octaphyrinogen intermediate, which on subsequent oxidation
with chloranil yielded the 34p octaphyrin. The probable
mechanism is shown in Scheme 3.
The protonation of tetrapyrrane leads to two intermediates
I and II in which the protonation occurs at a and b positions,
respectively. Based on the product formed, intermediate II is
preferred to form intermediate III through an intermolecular
electrophilic attack. Further rear-
A similar process, as reported earlier for the synthesis of
tetrathia octaphyrin,[23] has been employed for the synthesis of
tetraselena and tetraoxa octaphyrins. When the same reaction
was attempted with meso phenyl derivative, the formation of
octaphyrin was restricted, indicating the necessity of bulkier
rangement of III followed by oxi-
dation with chloranil results in the
formation of octaphyrin. The in-
ability to isolate the intermediate
III before oxidation reveals the
unstable nature of the octaphyri-
nogen relative to the correspond-
ing aromatic congener.
R
R
R
N
H
N
HN
N
N
R
R
N
H
N
H
R
R
R
HN
N
N
NH
HN
HN
NH
NH
TWIST
+
NH
N
NH
N
The understanding that the pres-
ence of bulkier groups on the
second and sixth positions of the
meso phenyl rings facilitates the
formation of the planar octaphyr-
ins further prompted us to check
such validity. Instead of the mesityl
substituent, a m-xylyl group was
chosen, since it also has methyl
groups on the ortho positions. The
required precursor was synthesized
in a modified way from bithio-
phene dialdehyde, 16. The dialde-
hyde was further reduced to the
corresponding diol 17 by Grignard
reagent prepared from bromoxy-
lene (Scheme 4).
R
H
N
H
N
H
N
HN
N
R
R
R
R
R
R
R
N
N
X
X
H
H
NH
HN
HN
X = S/Se/O
NH
R
R
+
X
X
X
N
HO
OH
H
X
Scheme 1. Retrosynthetic analysis.
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34p Core Modified Octaphyrins
2282 2290
m-Xylyl
m-Xylyl
H
N
Mes
X
Mes
X
N
S
S
S
S
m-Xylyl
m-Xylyl
NH
i) TFA/CH2Cl2
ii) Chloranil
HN
S
S
34π
HN
NH
H
18
N
N
H
Mes
Mes
Mes
Mes
m-Xylyl
m-Xylyl
X
X
X
X
+
H
H
NH
NH
19
HN
HN
II
I
Scheme 5. Synthesis of m-xylyl substituted tetrathiaoctaphyrin 19.
H
H
Mes
H
Mes
Mes
X
X
X
Mes
X
X
X
X
NH
H
COSY correlations are marked in the spectrum. There are
five signals in the region 9.8 11.3 ppm, which are assigned to
the protons of bipyrrolic and bifuran protons. The multiplet at
10.8 ppm is correlated to two doublets at 10.05 and 9.83 ppm
confirming the assignment of those to bipyrrolic protons [b, c
and b', c']. The other two doublets at 11.3 and 10.2 ppm are
correlating among themselves and hence assigned to b-CH
proton of non-inverted furan rings [a, a']. The conformation
for the ring inversion came from the COSY analysis of the
signals in the upfield region. In the upfield region, two
doublets at À4.8 and À5.1 ppm were found along with two
broad signals at À2.6 and À3.15 ppm. The doublets were
found to be correlated amongst themselves indicating their
origin from the same heterocyclic ring, which are inverted.
The signals at À3.1 and À2.6 ppm did not show any
correlations and hence were assigned to the NHof the
pyrrole rings. The Dd value of 16.4 ppm justifies the aromatic
character of the octaphyrin.
N
H
H
NH
NH
HN
H
H
H
H
H
H
H
NH
HN
H
N
X
Mes
Mes
Mes
Mes
III
Mes
Mes
Mes
Mes
X
X
X
N
N
H
H
H
NH
HN
X
X
H
H
H
H
NH
X
X
HN
H
N
N
X
Mes
Mes
Mes
Mes
IV
Scheme 3. Mechanistic scheme for the a,a coupling of tetrapyrranes under
acidic conditions.
Tetraselena derivative 14 ex-
hibited only broad signals in the
up-field and low-field region.
Temperature variation from
Mg
Me
Me
Me
Me
MgBr
S
+
333 to 210 K went futile in
resolving the 1HNMR spec-
trum. At room temperature,
two signals at 10.41 and
9.85 ppm corresponding to the
protons of the biselenophene
unit and two signals at 9.52 and
8.77 ppm corresponding to pyr-
role protons were observed. A
broad signal for the protons of
inverted selenophene rings was
observed at À2.06 ppm, clearly
indicating the ring current ef-
m-Xylyl
Br
m-Xylyl
N
S
S
+
H
OH
OH
17
0.1 equiv TFA
O
O
S
H
H
16
m-Xylyl
m-Xylyl
S
S
HN
NH
m-Xylyl =
Me
18
Me
Scheme 4. Synthesis of m-xylyl substituted bithiophene diol and tetrapyrrane.
Compound 17 on subsequent reaction with pyrrole in
presence of acid yielded the required tetrapyrrane 18.
Oxidation of 18 under acidic conditions yielded the desired
planar and aromatic octaphyrin, 19 in 5% yield (Scheme 5).
This reaction clearly indicates the necessity of bulkier groups
on the ortho positions of the meso phenyl rings in stabilizing
the planarity of the molecule.
fect on the protons present inside the cavity of the macrocycle.
These are expected due to ring inversions of one of the
heterocyclic ring in the biselenophene unit as observed in the
case of tetra thia octaphyrin.[23]
m-Xylyl substituted octaphyrin 19 also exhibits aromaticity
as observed by spectroscopic analysis. Based on 1HNMR
spectrum, 19 also exhibit ring inversions due to the observa-
tion of the signals in the upfield region. In the upfield region,
two broad signals are observed at À3.2 and À4.2 ppm for the
b-CHof the inverted heterocyclic rings. In the low field region
(Figure 2), two sets of doublets and a broad singlet at
14.1 ppm are observed. Two doublets at 11.9 and 11.0 ppm
1
1H NMR spectroscopy: The HNMR spectrum of tetraoxa
octaphyrin 15 was well resolved in free base form. A typical
spectrum observed for free base of 15 shown in the Figure 1
1
and the corresponding assignments which is based on H,1H
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T. K. Chandrashekar et al.
FULL PAPER
trum. While the broad singlet at 14.1 ppm does not show any
correlation with any other signal in the spectrum. These
results lead to the understanding that pyrrole ring of the
bipyrrole is inverted rather than the thiophene rings. Such
inversion of pyrrole from the bipyrrolic units has been
observed earlier in di-oxa rubyrins.[12c]
The protonated derivatives of [14 ¥ (TFA)2, 15 ¥ (TFA)2, 19 ¥
(TFA)2] showed only broad resonances in the temperature
333 to 210 K and hence no attempt was made to analyze the
spectra in detail.
Spectral characterization: Electronic absorption spectra of
octaphyrins 14 and 15 display typical porphyrinoid character-
istics. Compound 14 exhibits an intense Soret-like absorption
at 645 nm (e ˆ 2.8  105) and weak (Q-band like) absorptions
in the region 745 910 nm (Figure 3). Upon protonation, the
Figure 1. 1HNMR spectrum of 15 in CDCl3 at 298 K. The assignments are
1
marked based on the H,1HCOSY spectrum.
Figure 3. Electronic absorption spectra of 1.35 Â 10À5 m of 14 (6.75 Â 10À6
m
for protonated) in CH2Cl2. The diprotonated species was generated by
addition of a dilute solution of TFA in CH2Cl2.
Soret-like absorption is shifted to 639 nm (e ˆ 2.1  105), while
a lone Q-band like absorption is observed at 862 nm. In case
of 15, a strong split Soret-like absorptions at 600 nm (e ˆ
17.8  104) and 633 nm (e ˆ 14.4  104) along with Q-band like
absorptions in the region 790 885 nm were observed. On
protonation the Soret-like absorption is shifted to 636 nm
(e ˆ 27.98  104) while the Q-band like absorptions are further
red shifted to lower energy region (855 933 nm). The
electronic absorption spectrum of 19 does not vary much
from that of the tetrathiophene congener 10. Both in the
freebase form and in the protonated form, they resemble very
much with each other. These observations reveal that
octaphyrins have the electronic properties similar to that of
the parent porphyrin.
Crystallographic characterization: Octaphyrin 10, displays a
planar geometry even in solid state. Also two different
Figure 2. 1HNMR spectrum of 19 in CDCl3 at 298 K in the low field
tautomers were identified in the solid state based on the
1
region. The assignments are marked based on the H,1HCOSY spectrum.
[23]
position of the NHprotons.
It was observed that the
The inset shows the up field region.
location of NHadjacent to the non-inverted thiophene helped
the formation of weak N-H-S hydrogen bonds, while, the
b-CHof the inverted thiophene ring formed weak C-H-N
hydrogen bonds with the adjacent pyrrole nitrogens. This
helped the molecule to sustain a flat structure. In the case of
corresponding to one proton each and other two doublets at
11.1 and 10.2 ppm corresponding to two protons each show
1
correlations amongst themselves in the H,1HCOSY spec-
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Chem. Eur. J. 2003, 9, 2282 2290

34p Core Modified Octaphyrins
2282 2290
the other tautomer, the presence of b-CHand the pyrrole NH
created a minor steric hindrance leading to the tilt of the
thiophene ring away from planarity.
A similar structure has been observed even for the
octaphyrins 14 and 15 and also it was found that these
octaphyrins are able to bind anions when they are crystallized
in acidic conditions (Table 1). Since the octaphyrins have been
crystallized under acidic conditions, there is no possibility of
isolating two tautomers. In acidic conditions both the
octaphyrins bind two TFA molecules in 1:2 ratio, one above
and below the plane of the macrocycle. These octaphyrins
revealed for the first time novel interactions between the host
and guest apart from the usual electrostatic interactions. The
TFA anion complex of 14 ¥ (TFA)2 displays a fairly flat
structure of the macrocycle similar to that as observed for
tetrathia octaphyrin (Figure 4). The planarity of the macro-
cycle is slightly disrupted probably due to the binding of the
anions. Four hydrogen bonding interactions were found to be
responsible for the binding of each TFA to the macrocycle.
They are i) N2-H-O1 (2.232 ä, 143.678), ii) N2-H-O1
(2.139 ä, 146.648), iii) C11-H-O1 (2.444 ä, 171.048) and
iv) C34-H-F2 (2.323 ä, 146.228). Of the above four interac-
tions, the first two are typical electrostatic interactions. The
protonation of the imino type nitrogens creates a positive
charge, which attracts the counter anion (CF3COOÀ). The
other two interactions are induced probably due to the
geometric features of the interaction between the anion and
the host macrocycle. The b-CHof the inverted selenophene
ring forms a strong hydrogen bond with the oxygen of the -
OHgroup in TFA. The proximity for the interaction is clearly
evident from the near linear bond angle, which is one of the
strong reasons for the existence of this hydrogen bond. Such
an interaction could be comprehended only due to the
complete inversion of the selenophene ring. Due to such
geometry, the b-CHof the inverted ring comes in plane with
the -CO of the TFA as supported by the bond angle of 171.048.
Figure 4. A) Top view of the TFA bound complex of octaphyrin, 14.
B) Side view; the meso-mesityl rings and noninteracting hydrogens which
are not involved in the hydrogen bonding are omitted for clarity.
Even though, expanded porphyrins with inverted heterocyclic
rings are known to bind anions, this is the first report of the
solid-state evidence for the involvement of the b-CHin
binding a guest molecule to the macrocycle. The carbonyl
group oxygen of TFA was disordered and has 50% proba-
bility, hence it was considered for any type of interactions with
the octaphyrin. Another important interaction observed was
between the fluorine of the TFA and the hydrogen of the
methyl group on the meso-mesityl ring between the non-
inverted heterocyclic rings. Two such interactions are ob-
served due to the binding of two anions to the macrocycle.
Such type of intramolecular hydrogen bonding between the
meso-substituent of the macrocycle and the guest molecule
defines novel binding modes between the host and the guest in
expanded porphyrins. (For relevant bond angles and the
important bond lengths refer Tables S1 and S2 in the
Supporting Information.)
Further analysis on hydrogen bonding revealed that
octaphyrins form one-dimensional supramolecular array (Fig-
ure 5) through intermolecular C-H-F hydrogen bonds (C-H-
F: 2.55 ä; 149.438). This array is formed through the involve-
ment of the aromatic CHfrom the meso-mesityl rings
adjacent to the inverted selenophene rings and one of the
fluorine from the TFA. As described schematically, two TFA
molecules are sandwiched between two octaphyrins, which
Table 1. Crystal data for TFA complexes of 14 and 15.
Se4N4 ¥ (TFA)2
O4N4 ¥ (TFA)2
solvent of crystallization
empirical formula
T [K]
CH2Cl2/TFA/hexane
C72H62N4Se4
88(2)
CH2Cl2/TFA/hexane
C72H62N4O4
86(2)
crystal System
space group
V [ä3]
a [ä]
b [ä]
orthorhombic
Pbca
monoclinic
P2(1)/c
8176.5(8)
23.6220(14)
13.4931(8)
25.6532(15)
90.00
3610.4(3)
9.8383(5)
24.1836(13)
15.3442(8)
90.00
c [ä]
a [8]
b [8]
b [8]
Z
1calcd [gcmÀ3
90.00
90.00
4
98.5330(10)
98.5330(10)
4
]
1.241
1552
refls coll./unique
F(000)
7189/5567
3080
6349/4604
1552
limit. indicies
À 28 ꢀ h ꢀ 28
À 16 ꢀ k ꢀ 16
À 30 ꢀ l ꢀ 30
1.105
À 11 ꢀ h ꢀ 11
À 28 ꢀ k ꢀ 28
À 18 ꢀ l ꢀ 18
1.030
GoF (F2)
inal R indicies {I > 2s(I)}
R1 ˆ 0.0651
R1 ˆ 0.0692
wR2 ˆ 0.1492
wR2 ˆ 0.1822
Chem. Eur. J. 2003, 9, 2282 2290
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2287

T. K. Chandrashekar et al.
FULL PAPER
Figure 5. A view of the one-dimensional array formed by 14 ¥ (TFA)2. The
hydrogen bonding interactions are shown in the dotted lines.
Figure 6. A) Top view of the TFA bound complex of octaphyrin, 15. The
disordered mesityl rings are omitted for clarity. B) Side view; the meso-
mesityl rings and non-interacting hydrogens which are not involved in the
hydrogen bonding are omitted for clarity.
are separated by a distance of 13 ä approximately. To the first
octaphyrin, the TFA is bound to pyrrolic nitrogens, inverted
ring proton and the methyl group of the mesityl ring. At the
other end it has two fluorine atoms free. One of them forms a
weak hydrogen bond with the aromatic CHof the mesityl ring
from the second octaphyrin. A complementary interaction
between the TFA from the second octaphyrin to the first one
makes the interaction stronger. Such interactions continue to
add up over each octaphyrin molecule, thus forming the
supramolecular array in one dimension.
while the C-H-O is a nonelectrostatic interaction due to the b-
CHof the inverted furan ring. The mode of TFA binding to 14
and 15 differs in the way the b-CHof the inverted ring
interacts with the guest molecule (Scheme 6). In case of 14,
the b-CHof the inverted ring binds the TFA bound to the
bipyrrolic unit adjacent to it. On the other hand, in 15, the b-
CHof the inverted ring binds to TFA bound to the non-
adjacent bipyrrol unit. The difference observed is credited to
the modifications observed in the bifuran unit. The non-
inverted ring is slightly pushed away from the center of the
cavity, while the inverted ring is pushed more inside and
Tetraoxaoctaphyrin, 15, also binds two TFA molecules, one
above and below the plane of the molecule (Figure 6). The
distortion to the octaphyrin plane was found to be very
minimal in comparison to 14
and the macrocycle exhibits a
planar structure. In this case,
the TFA and one of the mesityl
rings are disordered and hence
those groups have been exclud-
O
Se
O
Se
H
O
H
H
H
ed while considering the intra-
and intermolecular interac-
tions. Both the TFA molecules
are bound to the macrocycle by
three hydrogen bonding inter-
actions. They are i) N-H-O
(2.142 ä, 142.438), ii) N-H-O
(2.057 ä, 139.668), iii) C-H-O
(2.335 ä, 149.38). The first two
are electrostatic interactions,
N H
N H
H N
H N
F
F
F3C
CF3
N H
N H
H N
H N
O
F
F
O
O
O
O
O
F
F
O
H
H
H
H
O
Se
O
Se
15•(TFA)2
14•(TFA)2
Scheme 6. Different modes of TFA binding to octaphyrins 14 and 15.
2288
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Chem. Eur. J. 2003, 9, 2282 2290

34p Core Modified Octaphyrins
2282 2290
electron density and hole size agree well with the solvent being TFA.
Further details about the refinements are outlined in the Supporting
Information.
towards the adjacent furan unit. This makes the b-CHof the
inverted furan unit to come in close proximity with the TFA
bound to the nonadjacent bipyrrol unit. This could be
probably due to the short size of the oxygen in furan unit,
which experiences more strain on cyclisation and probably
more in the protonated form.
CCDC-196155 (14) and 196156 (15) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crys-
tallographic Centre, 12 Union Road, Cambridge CB21EZ, UK; Fax: (‡44)
1223 336033; or deposit@ccdc.cam.ac.uk).
Chemicals: All NMR solvents were used as received. Solvents like
dichloromethane, tetrahydrofuran and n-hexane were purified and distilled
by standard procedures. Bifuran and biselenophene diols and tetrapyrranes
were synthesized according to the published procedure and stored under
inert atmosphere.[27]
Conclusion
The syntheses of meso aryl aromatic octaphyrins with differ-
ent heteroatoms have been achieved by a simple synthetic
methodology. These molecules are aromatic in nature and are
largest aromatic molecules to be characterized in solid state to
date. Oxidative coupling of appropriate precursors under
acidic conditions has become an attractive strategy for the
synthesis of higher order aromatic expanded porphyrins.
These flat molecules bind anions in acidic conditions in 1:2
ratio. It is expected that by having appropriate meso
substituents it is possible to stack long arrays of planar
molecules, which lead to the formation of channels of varied
dimensions.
Syntheses
5,5'-Bis-(xylylhydroxymethyl)-2,2'-bithiophene (17): To Magnesium turn-
ings (0.35 g, 14.4 mmol) in dry tetrahydrofuran (THF) (5 mL) under
nitrogen was added m-bromoxylene (1.8 mL, 14.4 mmol) in dry THF
(2 mL). The mixture was stirred overnight for the formation of the
Grignard reagent. Then bithiophene dialdehyde 16 (1 g, 4.5 mmol) in dry
THF (10 mL) was added over a period of 30 minutes at 08C. The reaction
was further stirred for 4 h and later quenched with dilute hydrochloric acid
(10 mL). The organic layer was separated and the aqueous layer was
extracted with ether. The combined organic layer was washed with brine
(0.1n) and then dried over anhydrous Na2SO4. The solvent was evaporated
under reduced pressure to obtain a brown semi solid. This was taken
directly for the preparation of tetrapyrrane 18.
5,14-Dixylyl-20,21-dithiatetrapyrrane (18): A known procedure for the
synthesis of tetrapyrrane 18 was followed using diol 17 (0.4 g, 0.92 mmol)
and pyrrole (2.6 mL, 36.8 mmol). Purification by column chromatography
(silica gel, 100 200 mesh, petroleum ether/ethyl acetate 98:2 gave a light
brown colored solid identified as 18 (0.32 g, 65%). 1HNMR (400 MHz,
CDCl3): d ˆ 7.81 (brs, 2NH), 7.06 6.96 (m, 6H), 6.85 (d, J ˆ 3.8 Hz, 2H),
6.64 (d, J ˆ 3.6 Hz, 2H), 6.61, (m, 2H), 6.11(m, 2H), 6.02 (s, 2H), 5.99 (s,
Experimental Section
Instrumentation: Electronic spectra were recorded on a Perkin Elmer
Lambda20 UV/Vis spectrophotometer. Proton NMR spectra were ob-
tained on a 400 MHz JEOL spectrometer in CDCl3. FAB-MS spectra were
obtained on a JEOL-SX-120/DA6000 spectrometer.
‡
2H), 1.5 (s, 12H); FAB MS: m/z (%): 532 (100) [M ].
General synthesis of octaphyrins
X-ray crystallographic studies: X-ray quality crystals for all compounds
were grown as described in the experimental section. The crystals were
removed from the tube and covered with a layer of viscous hydrocarbon oil
(Paratone N, Exxon). A suitable crystal was selected with the aid of a
microscope, attached to a glass fiber, and immediately placed in the low
temperature nitrogen stream of the diffractometer. The intensity data sets
for all compounds were collected using a Siemens SMART system,
complete with 3-circle goniometer and CCD detector operating at À548C.
The data sets for compounds 14 ¥ (TFA)2, and 15 ¥ (TFA)2 were collected at
88 K and 86 K, respectively, using a custom build low temperature device
from Professor H. Hope (UC Davis). In all cases graphite monochromated
MoKa radiation (l ˆ 0.71073 ä) was employed. The data collections
nominally covered a hemisphere of reciprocal space utilizing a combination
of three sets of exposures, each with a different F angle, and each exposure
covering 0.38 in w. Crystal decay was monitored by repeating the initial
frames at the end of the data collection and analyzing the duplicate
reflections. In all cases, no decay was observed. An absorption correction
was applied for all compounds utilizing the program SADABS.[29] The
crystal structures of all compounds were solved by Direct Methods, as
included in the SHELXTL-Plus program package.[30] Missing atoms were
located in subsequent difference Fourier maps and included in the
refinement. The structures of all compounds were refined by full-matrix
least-squares refinement on F2 (SHELX-93).[31] Hydrogen atoms were
placed geometrically and refined using a riding model, including free
5,14,23,32-Tetramesityl-35, 36, 39, 40-tetraselenaoctaphyrin (14): Tetra-
pyrrane 12 (0.25 g, 0.4 mmol) was dissolved in dry CH2Cl2 and stirred under
nitrogen atmosphere in absence of light for 15 minutes. TFA (0.03 mL,
0.4 mmol) in dry CH2Cl2 was added and stirring continued for another one
hour. Reaction mixture was opened to air. chloranil (0.1 g, 0.4 mmol) was
added, and the reation mixture was heated under reflux for one hour on a
preheated oil bath. The solvent was removed under reduced pressure. The
residue obtained was purified by chromatography on a basic alumina
column. A blue color band eluted with petroleum ether/CH2Cl2 (3:2) gave a
brownish metallic solid identified as 14 (0.05 g, 9.6%). Recrystallization
from CH2Cl2/n-hexane afforded gold colored crystals. M.p. 2258C (de-
comp); 1HNMR (400 MHz, CDCl 3, 298 K): d ˆ 10.42 (s, 2H), 9.85 (s, 2H),
9.58 9.48 (br, 4H), 8.77 8.72 (br, 4H), 7.57 (s, 4H), 7.42 (s, 4H), 2.75 (s,
6H), 2.66 (s, 6H), 2.23 (s, 12H), 2.08 (s, 12H), À2.1 (brs, 4H); FAB MS: m/
‡
z (%): 1300 (30) [M ‡2]; UV/Vis (CH2Cl2): lmax (e  10À4 mÀ1 cmÀ1): 646
(28), 745 (1.3), 818 (2.07), 908 nm (3.1); (CH2Cl2/TFA): lmax (e Â
10À4 mÀ1 cmÀ1): 638 (21), 862 nm (2.07); elemental analysis calcd (%) for
C72H62N4Se4: C 66.56, H4.81, N 4.30; found: C 66.89, H4.70, N 4.19.
5,14,23,32-Tetramesityl-35,36,39,40-tetraoxaoctaphyrin (15): A proce-
dure similar as described above was followed by using tetrapyrrane 13
(0.25 g, 0.47 mmol), TFA (0.03 mL, 0.4 mmol) and chloranil (0.116 g,
0.47 mmol). Upon chromatographic separation with basic alumina, a blue
colored band eluted with petroleum ether/CH2Cl2 (1:1) gave a brownish
metallic solid identified as 15 (0.02 g, 4%). 1HNMR (400 MHz, CDCl
,
3
À
rotation about C C bonds for methyl groups with Uiso constrained at 1.2 for
298 K): d ˆ 11.28 (d, J ˆ 3.6 Hz, 2H), 10.88 (m, 4H), 10.21 (d, J ˆ 3.6 Hz,
2H), 10.04 (d, J ˆ 3.6 Hz, 2H), 9.9 (d, J ˆ 4 Hz, 2H), 7.8 (s, 4H), 7.6 (s, 4H),
2.99 (s, 6H), 2.84 (s, 6H), 2.25 (s, 12H), 2.18 (s, 12H), À2.54 (brs, NH),
À3.08 (brs, NH), À4.8 (d, J ˆ 4 Hz, 2H), À5.1 (d, J ˆ 4 H z, 2H ); FAB MS:
non-methyl groups, and 1.5 for methyl groups times Ueq of the carrier
carbon atom. The crystallographic programs used for structure refinement
and solution was installed on a Silicon Graphics Indigo2 R10000 Solid
Impact or a PC clone. Scattering factors were those provided with the
SHELX program system. All non-hydrogen atoms were refined anisotropi-
cally. Disorder was typically handled by including split positions for the
affected groups, and included in the refinement of the respective
occupancies. Complex 14 ¥ (TFA)2 contains a particularly disordered TFA
unit situated in holes in the crystalline network. The TFA molecules have
been removed from the structure so that a better refinement could be
obtained using the Squeeze program, as implemented in Platon.[32] The
‡
m/z (%): 1046 (100) [M ]; UV/Vis (CH2Cl2): lmax (e  10À4 mÀ1 cmÀ1): 600
(16.12), 633 (14.38), 792 (1), 885 nm (4.8); (CH2Cl2/TFA) lmax (e Â
10À4 mÀ1 cmÀ1): 636 (28), 784 (0.5), 855 (0.9), 965 nm (3.4); elemental
analysis calcd (%) for C72H62N4O4: C 82.57 H4.95, N 5.34; found: C 82.88,
H6.12, N 5.25.
5,14,23,32-Tetraxylyl-35,36,39,40-tetrathiaoctaphyrin (19):
A similar
procedure as mentioned above was employed using tetrapyrrane 18,
Chem. Eur. J. 2003, 9, 2282 2290
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2289

T. K. Chandrashekar et al.
FULL PAPER
(0.25 g, 0.47 mmol) and TFA (0.04 mL, 0.47 mol) and chloranil (0.115 g,
0.47 mmol). Upon chromatographic purification with basic alumina, a blue
colored band eluted with petroleum ether/CH2Cl2 (7:3) gave a brownish
metallic solid identified as 19 (0.025 g, 5%). Recrystallization from CH2Cl2/
S. J. Mahajan, T. K. Chandrashekar, R. Roy, P. Ramamurthy, J. Chem.
Soc. Perkin Trans. 2 1999, 961 968; b) A. Srinivasan, S. Mahajan, S. K.
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Chandrashekar, M. O. Senge, K.-I. Sugiura, Y. Saakata, Eur. J. Org.
Chem. 2000, 2357 2360.
1
n-hexane afforded gold colored crystals. M.p. 2508C (decomp); HNMR
(400 MHz, CDCl3, 298 K): d ˆ 14.15 (brs, 2NH), 11.98 (d, J ˆ 4 Hz, 2H),
11.17 (m, 4H), 11.04 (brs, 2H), 10.25 (brs, 4H), 8.54 8.17 (m, 4H), 8.15
8.05 (m, 4H), 8.02 (m, 4H), 2.24 (s, 12H), 2.1 (s, 12H), À3.2 (brs, 2H),
[13] S. J. Narayanan, B. Sridevi, T. K. Chandrashekar, A. Vij, R. Roy, J.
Am. Chem. Soc. 1999, 121, 9053 9068.
‡
¬
À4.2(brs, 2H); FAB MS: m/z (%): 1055 (30%) [M ‡H]; UV/Vis
[14] a) P. J. Chmielewski, L. Latos-Graz«ynski, K. Rachlewicz, Chem. Eur. J.
(CH2Cl2): lmax (e  10À4 mÀ1 cmÀ1): 596 (6.28), 750 (0.7), 833 (1.10), 907 nm
(0.6); (CH2Cl2/TFA): lmax (e  10À4 mÀ1 cmÀ1): 645 (17.53), 792 (0.5), 868
(0.7), 981 nm (2.36).
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Acknowledgement
We thank the Department of Science & Technology and the Council of
Scientific and Industrial Research, New Delhi, Government of India for the
research grants provided. H.R. is thankful to the CSIR for her fellowship.
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Received: October 29, 2002 [F4537]
2290
¹ 2003 Wiley-VCHVerlag GmbH& Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2003, 9, 2282 2290