Organic Letters
Letter
2
+
−
2+
macrocycle with a C axis of symmetry, it could be concluded
shift values of the aromatic 9a ·2[SbCl ] and 9a ·
2
6
−
that the isolated molecule does not represent the plausible
asymmetrical structure of heptaphyrin 9b. Quantum computa-
tional calculations further revealed that 9b was higher in
energy than 9a or 9c, by more than 14 kcal/mol (see Table S1
in the Supporting Information). Yet, NMR spectroscopy could
field region between δ 12.0−14.5 ppm (see Figure S22 in the
Supporting Information). In contrast to the paratropic shifts in
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2+
the H NMR spectrum of 9a, the dication 9a , being aromatic
(see Figure S27 in the Supporting Information) unequivocally
supported the aromatic feature of the dicationic species. Single-
crystal XRD studies of triflate salt revealed the retention of the
same macrocyclic geometry in the oxidized state (Figure 3C).
not distinguish the C axis of symmetry between 9a and 9c.
2
Finally, the molecular structure was unambiguously
established from single-crystal X-ray diffraction (XRD)
1
analysis. Consistent with the H NMR spectrum, it revealed
a slightly bent geometry and confirmed the structure of
2+
−
The dicationic macrocycle, 9a ·2[CF SO ] , was flanked by
two triflate counteranions with each macrocycle, confirming
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3
the oxidation of the heptaphyrin.
18
In a parallel reaction, condensation of furan diol, 8b, with
equiv of trithiophene, 7, also yielded a brown-colored
2
heptaphyrin solution, 10. It was identified from MALDI-TOF/
TOF mass spectrometry of the reaction mixture, which
displayed m/z value of 1273.9043 and further isolated from
column chromatography. This brown-colored solution in
dichloromethane exhibited a twin absorption maximum
(
Figure 2C) at 498 nm (88 900) and 440 nm (77 900). In
1
its H NMR spectrum at room temperature, it displayed well-
(
see Figure S29 in the Supporting Information). Two doublets
at δ 5.68, 5.85 ppm along with a multiplet at 6.22 ppm and a
singlet at δ 5.83 ppm, corresponded to an equal number of
protons in the high-field region. Two low-field doublets at δ
9
.43 and 10.01 ppm also corresponded to the same number of
Figure 3. Molecular structure of 9a [(A) top view and (B) lateral
protons as for the individual signals in the high-field region.
The presence of both shielded and deshielded signals implied
the existence of ring inverted heterocyclic units in the
macrocycle. The NMR spectra revealed the paratropic ring
current effect observed in the 32π antiaromatic macrocyle.
Since the same number of signals are noted as for 9a, it can be
expected that 10 also adopted a C symmetry geometry.
Hence, among the three possible structural isomers (recall
Scheme 2), 10a−10c, the unsymmetrical 10b is not in sync
with the observed H NMR spectrum. Quantum computa-
tional calculations further revealed that 10b was higher in
energy than by 10a and 10c, by more than 29 kcal/mol.
Unfortunately, all our efforts to crystallize 10 were futile and
could not decisively decide the structure between 10a and 10c.
In resemblance to 9a, 10 also exhibited exhibit reversible
two-electron oxidation with Meerwein salt. Upon oxidation, its
blue-colored solution in dichloromethane exhibit a red-shifted
and single intense absorption at 633 nm (414 000), followed
by low-energy bands at 874 (22 500), 914 (23 300), and 1041
nm (59 600) (Figure 2B). The addition of a base reduced the
formed dication to its free base, as monitored using electronic
m/2 peak at 636.9548 in its HRMS (see Figure S36 in the
Supporting Information), suggesting the formation of a
dication. Cyclic voltammetry (CV) studies (Figure 2D) also
supported two reversible one-electron oxidations at +0.53 and
+0.61 V, respectively. Further support for the formation of 30π
2+
−
2+
−
view], (C) [9a] ·2[CF SO ] , and (D) [10a] ·2[SbCl ] deter-
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mined from single-crystal XRD studies.
of the thiophene rings were facing the center of the
macrocycle, and the phenyl rings were close to orthogonal
orientation, with respect to the plane of the molecule defined
by the bridging carbon atoms. Furthermore, the estimated
2
15
Nucleus Independent Chemical Shift (NICS) value of δ +
1
9
1
6
(
ACID) plot (see Figure S20 in the Supporting Information)
support the antiaromatic characteristic of heptaphyrin, 9a. The
absence of 9c can be attributed to the steric hindrance arising
from the quarterthiophene unit within the macrocyclic
framework.
Being a 4nπ macrocycle, 9a was easily oxidized by Meerwein
1
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salt to its corresponding aromatic 30π dicatioinc species,
2
+
9
a . Upon oxidation, the color of its solution dramatically
changed from brown to deep blue in dichloromethane. A
similar color change was observed upon the addition of triflic
acid to a solution of 9a in dichloromethane. In either case, the
solution displayed an absorption maxima red-shifted by more
than 100 nm (Figure 2A) and a new intense band appeared at
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16 nm (417 000), followed by low energy absorption at 988
nm (35 300). Few weaker absoprtions were also observed
between 800 and 900 nm. Observation of m/2 signal from HR-
MS (see Figure S21 in the Supporting Information) further
supported the two-electron oxidation of 9a. Upon addition of a
base, such as triethyl amine, the dication, 9a , was reduced
back to its free base 9a, confirming the reversible two-electron
redox process. From cyclic voltammetric studies, we were able
1
dicationic species was obtained from H NMR spectroscopy.
Even though the pattern of NMR was retained in the oxidized
state, the position of signals in 10 were dramatically inverted
for the 30π dication, 10 ·2[SbCl ] . The downfield
resonated at δ −4.23 and −5.33 ppm (see Figure S37 in the
2+
2
+
−
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to identify the two reversible oxidations at +0.47 and +0.56 V
1
(
Figure 2C). In their respective H NMR spectra, the chemical
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Org. Lett. 2021, 23, 3481−3485
Anand, Venkataramanarao G.
