Synthesis, Structure, and Optical Properties of Di-m-benzihexaphyrins (1.1.0.0.0.0) and Di-m-benziheptaphyrins (1.0.1.0.0.0.0): Blackening of m-Phenylene-Linked Dicarbaporphyrinoids by Simple ?-Expansion

Article abstract of DOI:10.1021/acs.joc.0c00754

Acid-catalyzed condensation of a newly prepared di-m-benzipentapyrrane with appropriate mono- A nd diheterocyclic dialcohols selectively produced stable di-m-benzihexaphyrins and di-m-benziheptaphyrins with only two meso-carbon bridges. Single-crystal X-ray diffraction analyses reveal planar conformation with slight distortion of bridged phenylene rings. Despite the presence of m-phenylene units interrupting the global delocalization, the presence of bithiophene units in di-m-benziheptaphyrins 3a-b exhibits altered optical features covering the entire visible region (ca. 250-720 nm), exhibiting a black dye property as a metal-free porphyrinoid.

Full text of DOI:10.1021/acs.joc.0c00754

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Article
Synthesis, Structure and Optical Properties of Di-m-benzihexaphyrins
(1.1.0.0.0.0) and Di-m-benziheptaphyrins (1.0.1.0.0.0.0): Blackening of
m-Phenylene Linked Dicarbaporphyrinoids by Simple p-Expansion
Thondikkal Sulfikarali, Jayaprakash Ajay, Cherumuttathu H
Suresh, Padinjare Veetil Bijina, and Sabapathi Gokulnath
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c00754 • Publication Date (Web): 10 May 2020
Downloaded from pubs.acs.org on May 14, 2020
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Synthesis, Structure and Optical Properties of Di-m-benzihexaphyrins
(1.1.0.0.0.0) and Di-m-benziheptaphyrins (1.0.1.0.0.0.0): Blackening of
m-Phenylene Linked Dicarbaporphyrinoids by Simple -Expansion
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Thondikkal Sulfikarali,a Jayaprakash Ajay,a Cherumuttathu H. Suresh,b Padinjare V. Bijina,b and
Sabapathi Gokulnath*,a
aSchool of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Kerala-695551, India. E-
mail: gokul@iisertvm.ac.in,
bInorganic and Theoretical Chemistry Section, Chemical Science and Technology Division, CSIR-National Institute of
Interdisciplinary Science and Technology, Trivandrum - 695019, Kerala, India
Supporting Information Placeholder
ABSTRACT: Acid catalyzed condensation of newly prepared di-m-benzipentapyrrane with appropriate mono- and diheterocyclic
dialcohols selectively produced stable di-m-benzihexaphyrins and di-m-benziheptaphyrins with only two meso-carbon bridges.
Single-crystal X-ray diffraction analyses reveal planar conformation with slight distortion of bridged phenylene rings. Despite the
presence of m-phenylene units interrupting the global delocalization, the presence of bithiophene unit in di-m-benziheptaphyrins 3a-b
exhibit altered optical features covering the entire visible region (ca. 250 - 720 nm) exhibiting black dye property as a “metal-free”
porphyrinoid.
tautomerism by appropriate substitutions on the benzene ring
(Figure 1). On the other hand, p-benziporphyrins often show
INTRODUCTION
Expansion of the porphyrin core through the introduction of
various heterocyclic units or benzene units in place of one of
the pyrrole rings may have interesting redox and optical
properties.1 Benziporphyrins are an important class of
carbaporphyrinoids that encompass N3C core with unique
ability to form organometallic species unlike regular
porphyrins.2 Majorly, two types of benziporphyrins namely
para-benziporphyrin and meta-benziporphyrin are well
explored.3 However, the chemistry of ortho-benziporphyrins
and their expanded analogues is being explored recently.4
Meanwhile, benziporphyrinoids comprising one or more arene
linkages in a meta-ortho (m-o), meta-ortho-meta (m-o-m) and
para-ortho-para (p-o-p) fashion also known recently.4d-e The
research groups of Latos-Grażyński and Lash have extensively
investigated the synthetic methodologies and studied their
spectral and electrochemical properties.5 While m-
benziporphyrin systems as in I are typically nonaromatic,
aromaticity of such systems can be realized through keto-enol
aromatic or antiaromatic features depending on the π-electrons
involved in the annulenic conjugation.6 Tetraphenyl meta-
benziporphyrin II forms organometallic complexes with Pd2+
and Pt2+ salts. Upon addition of AgOAc, II undergoes selective
acetoxylation at the internal carbon atom instead of silver
metallation as inferred from X-ray crystallographic analysis.7 In
an earlier report, Ravikanth et al. reported a nonaromatic 24π
dithia m-benzisapphyrin III through acid catalyzed
condensation of benzitripyrrane with bithiophene diol.3a
Structural analogue of porphyrin IV and its bimetallic
complexes bearing m-phenylene unit with the alternating meso
positions was synthesized by Corriu et al.8 Recent interests into
this area established several ways in tuning the optical and
redox properties via reducing the meso-carbon bridges,
introducing fused building blocks, and by incorporating
phenylene units onto the macrocyclic core.9 During such
modifications, the presence of meso-carbons in these cyclic
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structures often dictates their structure and conformational
properties.3b-c,10 A survey of
centres. This is in sharp contrast to our recent ‘black-dye’ via
an expanded porphyrin hetero-bis-metal (AuIII-PdII) complex
with absorption capabilities covering visible to near-IR
region.14b
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RESULTS AND DISCUSSION
Synthesis. The m-phenylene incorporated macrocycles 1-3
were synthesized employing a sequence of steps, as shown in
Scheme 1 and 2. Firstly, 2,5-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-1H pyrrole 5 was synthesized following a
reported method.17 Then, 5 was coupled with commercially
available 1-bromo-3-iodobenzene 4 using 10 mol% Pd(PPh3)4
affording 2,5-bis(3-bromophenyl)-1H-pyrrole 6. Subsequently,
compound 6 was further coupled with (1-(tert-butoxycarbonyl)-
1H-pyrrol-2-yl)boronic acid 7 using a similar coupling method
as described for 6 to yield bis-N-boc protected tripyrrole-like
precursor 8 in 86% yield. In the next step, deprotection of 8 was
successfully performed in the presence of NaOMe at 0 °C to
provide a hitherto unknown tripyrromethane-like precursor 9 as
a stable brown powder in 95% yield. These compounds were
characterized by 1H and 13C NMR spectroscopic methods. The
heterocyclic diols, such as furan diol 10a, thiophene diol 10b,
and bithiophene diols 11a-b, were prepared by following the
reported procedures.18
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Scheme 1. Synthesis of m-phenylene linked key building blocks 6
and 9.
Figure 1. (a) Selected examples of benziporphyrinoids and (b) -
extended chromophores exhibiting ‘black-dye’ property.
literature on carbaporphyrinoids reveals that expanded di-m-
benziporphyrins are ideal candidates that can provide altered
optical properties through skeletal modifications.11 Herein, we
have attempted to further explore the relationship between the
nature of conjugation with the optical and redox properties of
these unique di-m-phenylene incorporated porphyrinoids.12 We
have devised a strategy to reduce the meso-carbon bridges by
incorporating phenylene units in the place of two meso-
positions of a porphyrin and sapphyrin derivatives, thereby
restricting them to near planar conformation.13 Recent interest
in obtaining broad absorption bands in visible to near-IR has
drawn significant attention due to the potential application with
the aim of effective harvesting of solar energy.14 Blackening of
Zn2+-porphyrins V using peripherally fused , ’-quinono rings
was reported by Krautler et al. and recent report by Furuta and
Shimizu on blackening of aza-BODIPYs VI by simple
dimerization unambiguously elucidated the significant role of
linking units in achieving panchromatic absorption.15 In order
to push the absorption band towards near-IR region, Wu et al.
prepared various porphyrin-fused BODIPY dye VII which
displayed enhanced NIR absorption, due to the extended
conjugation of porphyrin with fused BODIPY cores.16
Interestingly, we found our bithiophene incorporated di-m-
benziheptaphyrins 3a-b exhibiting the optical absorption that
covers the entire the UV to visible region with a ‘black-color’
in solution by simple -expansion without involvement of metal
To accomplish the synthesis of di-m-benzihexaphyrins 1-2,
equimolar amounts of 9 with appropriate heterocyclic diols
10a-b were condensed in the presence of a catalytic amount of
para-toluenesulfonic acid (p-TsOH) under inert atmosphere for
3 h, followed by an oxidation with 1.0 equiv of DDQ in open
air at room temperature (Scheme 2) provided the macrocycles
1
and 2 in 10 and 21% yields, respectively. Initial
characterization of 1 and 2 was done using MALDI-TOF
analysis and proved the exact composition of these macrocycles
(Figure S1-4 to S1-7). Then, similar condensation reaction with
biheterocyclic dicarbinols such as 11a-b18b did not provide the
expected macrocycles 3a-b. However, to our surprise, changing
the acid catalyst to BF3•Et2O and using 2.0 equivalents of DDQ
provided the target macrocycles 3a-b in 8 and 3% yields,
respectively. Although the yields of the macrocycles 3a-b are
low, the reaction was found to be reproducible. Attempts to
reduce macrocycles 1 and 2 using NaBH4 did not yield any
reduced products, whereas macrocycles 3a-b underwent
reduction as indicated by sharp color changes. However, they
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instantaneously reverted back to their oxidized forms during
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3
4
column chromatographic purification.
2b). Overall, the spectral features in all these macrocycles
resemble typical non-aromatic characteristics, as observed in m-
benziporphyrinoids3b and their expanded analogues.19
Scheme 2. Synthesis of di-m-benzihexaphyrins 1-2 and di-m-
benziheptaphyrins 3a-b.
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Crystallographic Studies. The structures of 2 and 3a have
been confirmed by single crystal X-ray diffraction analysis as
shown in Figure 3, and crystallographic data are given in Table
S3-1. Suitable single crystals of 2 were obtained by slow
diffusion of methanol into chloroform solution of 2 at room
temperature. Similarly, slow diffusion of n-hexane into the
chloroform solution of 3a provided X-ray quality crystals of 3a.
Both crystallize in monoclinic crystal lattice with P21/n space
group. Structures of 2 and 3a consist of a slightly twisted
tripyrrin part directly connected with phenylene-pyrrole-
phenylene linkage in an almost symmetric manner as shown in
Figures 3a-b. The crystal structure 2 revealed that two
molecules are present in the asymmetric unit. The heterocyclic
rings and two benzene rings (ring A-F) were deviated with
dihedral angles of 19.53°, 18.51°, 16.09°, 8.20°, 16.48°, and
13.31° with respect to the mean plane (C6-C9-C22-C36), thus
giving a curved conformation. Similar heterocyclic rings and
benzene rings (ring A-G) deviations were observed in 3a with
dihedral angles of 27.52°, 15.94°, 11.51°, 14.02°, 17.70°,
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1
NMR Characterization. The H NMR spectrum of 2 in
CDCl3 showed two sharp singlets at 9.08 and 6.68 ppm for ring
A and D, respectively (Figure 2a). A broad singlet at 11.06 ppm
for the NH proton suggests the C2- symmetric structure
preserved in solution. Interestingly, two sets of doublets
between 7.41 to 7.69 ppm and a triplet at 7.43 ppm were
observed and found to correlate with each other in 1H-1H COSY
spectrum due to the outer β-CHs of ring B (Figure S2-7).
Signals corresponding to ring C of imino type pyrroles exhibit
two well resolved doublets at 7.10 and 6.69 ppm. Similar
spectral pattern was observed for 3a with bithiophene
incorporated macrocycle bearing two meso-mesityl rings
(Figure
Figure 3. Single crystal X-ray structures of 2 with top (a) and side
view (b) and 3a with top (c) and side view (d). For clarity, solvent
molecule and mesityl rings have been omitted in the side views.
Ellipsoids are shown at 50% probability level.
Figure 2. Comparison of 1H NMR spectra of macrocycles (a) 2 and
(b) 3a recorded in CDCl3. Selected region is shown. * denotes
residual CHCl3 signal.
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17.79°, and 1.98° with respect to the mean plane (C5-C15-C25-
Page 4 of 9
responsible for covering the absorption to the whole visible
region, thus giving the ‘black-dye’ property of these
macrocycles in solution.15 In the drop-cast film state, both 3a
and 3b exhibit slight broadening and red-shift of the main along
with the bathochromically shifted absorption is due to the
effective interchromophore interactions via the bithiophene
moiety in coplanar conformations. These features could be
absorption bands, denoting the formation of the π-stacked
aggregates (Fig. S4-6).
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C30), thus giving a saddle-like conformation (Figure S3-3 and
S3-4, SI). The decreased ring deviation of one of the benzene
rings can be attributed to the minimized steric congestion
between the adjacent pyrrole rings. The N---N distance inside
the macrocyclic core of 2 is 5.717 Å, which is larger in 3a
having 7.903 Å due to the core expansion by the incorpora tion
of bithiophene units. Significantly, the DFT optimized
structures of both 2 and 3a resemble closely to those of single
crystal X-ray structures without ring inverted conformations
(Figure S6-7 and S6-8, SI).
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Redox Properties. The estimation of the HOMO-LUMO
energy gap for all these macrocycles were investigated via
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Optical properties. The UV-Vis absorption spectrum of 1
in CH2Cl2 exhibits the Soret-like and a Q-like band at 337 and
497 nm (Figure 4), respectively, which are blue-shifted in
comparison to those of 2 with the Soret-like band at 339 nm and
Q-like band at 532 nm. On the other hand, the fluorescence
emission of 1 shows two maxima at 594 and 630 nm on
excitation at 337 nm in CH2Cl2. Upon protonation using dilute
CF3COOH in CH2Cl2, 1 and 2 showed a decreased intensity of
the bands at ca. 330 nm with appearance of new featureless
bands from 400–600 nm (Figure S4-2). On the other hand, the
absorption features of bithiophene incorporated macrocycle 3a
and 3b are perturbed in different ways. 3a displays two broad
bands at 323 and 429 nm with a low energy band at 570 nm.
Similarly, 3b exhibits absorption spectral features as that of 3a.
The additional band at 429 nm in 3a and 414 nm in 3b
cyclic voltammetry in dichloromethane using 0.1 M
tetrabutylammonium hexafluorophosphate as a supporting
electrolyte. The estimated electrochemical HOMO−LUMO gap
of 1.59 V for 1 and 1.67 V for 2 (Figure 5 and S5-1 in SI) were
obtained from one irreversible oxidation wave at 0.71 V, a
reversible reduction peak at -0.88 V for 1, whereas for 2, two
irreversible waves for oxidation process at 0.80 V and 1.00 V
with a reversible reduction peak at ca. −0.88 V and a quasi-
Figure 5. Cyclic voltammograms of 2 and 3a (top to bottom; 1
mM in CH2Cl2, 0.1 M n-Bu4NPF6, scan rate 0.1 mV s-1, glassy
carbon
electrode).
Respective
Differential
Pulse
Voltammograms are shown in red trace.
Table 1. Electrochemicala and optical datab for macrocycles 1, 2,
3a and 3b
E1/2
E1/2
E1/2
E1/2
red.2
b
Eopt
EDFT
ox.2
ox.1
red.1
[V]
[V]
[V]
[V]
[eV]
[eV]
4.11
3.87
4.07
3.94
1
2
0.86c
0.99
1.22
1.23
0.71c
0.79
0.70
0.81
-0.88d
-0.88
-0.95
-0.78d
-
2.49
2.33
2.17
2.20
-1.01
-1.01
-
3a
3b
Figure 4. Comparison of absorption spectra of macrocycles (a) 1
(b) 2 (c) 3a and (d) 3b and their respective protonated forms (green
trace) recorded in CH2Cl2. Inset: Solution colors of respective
macrocycles in CH2Cl2. The solution colors of 3a-b and their
diprotonated forms are not distinguishable.
aPotentials[V] vs ferrocene/ferrocenium ion. Scan rate 0.1 Vs–1;
working electrode, glassy carbon; counter electrode, Pt wire;
supporting electrolyte, 0.1 M nBu4NPF6; reference electrode,
saturated calomel electrode. cDetermined by differential pulse
voltammetry. dtwo-electron reduction process.
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reversible peak at -1.01 V for the reduction process are
indicated that the heterocyclic rings were slightly deviated from
planarity as obtained from single crystal X-ray structure (Figure
3). Analysis of selected FMOs of macrocycle 1 and 2 indicated
that the HOMOs in both were localized mainly on the di-m-
benzipyrrole core (Figure 6). The LUMOs were densely
distributed on furan or thiophene core, leaving no orbital
densities on meso substituents, whereas it is only distributed on
di-m-benzipyrrole core of 3b due to the electron withdrawing
of meso-C6F5 groups (Figure S6-9). Strikingly, LUMOs for
both 3a and 3b were localized on bithiophene and the adjacent
pyrrole rings.
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observed. On the other hand, macrocycle 3a, exhibits two
reversible waves for reduction process at ca. -0.95 V and -1.01
V while two irreversible oxidation peaks are observed at 0.7 V
and 1.22 V with an electrochemical HOMO-LUMO gap of 1.65
V (Table 1 and Table S5-1 in SI). Compound 3b exhibit similar
redox profile providing a slight reduction in the HOMO-LUMO
gap of 1.59 V suggesting that no significant change in the redox
properties was realized on going from electron rich mesityl
rings to the electron withdrawing pentafluorophenyl
substituents.
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DFT Calculations.21 The analysis of frontier molecular
orbitals (FMOs) reveals that the HOMO of macrocycle 1 and 2
are comparable, whereas the LUMO of 2 is found to be
stabilized to some extent as compared to that of 1 (Figure 6).
This leads to slight decrease in the band gap (ΔE = 3.87 eV) for
macrocycle 2 compared to that of 1 (ΔE = 4.10 eV), support the
red shift in the absorption spectra of 2. Further, comparison of
the molecular orbital energies for the ring expanded derivatives
3a and 3b reveals that both the HOMO and LUMO of 3b are
CONCLUSIONS
We herein presented three di-m-phenylene incorporated
hexaphyrin and heptaphyrin-like structures exhibiting non-
aromatic features. Effective localization is not realized due to
the interrupted conjugation at the phenylene moieties as
supported by their spectral and theoretical investigations. Our
current results demonstrate that the replacement of thiophene or
furan with bithiophene unit enable the effective tuning of the
absorption characteristics. This synthetic strategy could be
Figure 6. Energy level diagrams of 1, 2, 3a, 3b with selected molecular orbitals
stabilized as compared to 3a. However the band gap is
comparable (4.07 eV for 3a vs 3.95 eV for 3b), which is
consistent with their optical and redox characteristics. The
NICS(0) values for 1 and 2 were found to be 2.2 and 2.6 ppm,
respectively (Figures S6-1 and S6-2). In addition, the random
current density flow of the ACID plots of 1 and 2 validate the
nonaromatic character of the molecules (Figure S6-5). Similar
low positive NICS(0) values (1.9 ppm for 3a-b) and ACID plots
confirm the nonaromaticity in these macrocycles (Figure S6-3,
S6-4 and S6-6). The S0 (ground) state optimized geometries are
presented in Figure S6-7. The DFT optimized structures for 3a
successfully used for further modification of the porphyrin
structure or for assembly of larger covalent porphyrin arrays.
The atypical optical and electronic properties of these
macrocycles are likely to be systematically tunable by varying
the linkages between the m-arene and heterocyclic subunits.
Presently, activating the inner CH’s of the phenylene rings of
these macrocycles towards organometallic complexes is
underway in our laboratory.
EXPERIMENTAL SECTION
Materials and Methods
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The reagents and materials for the synthesis were used as obtained
2,5-bis(3-bromophenyl)-1H-pyrrole (6). A flame-dried flask was
charged with 522 (1.0 g, 3.13 mmol), 1-bromo-3-iodobenzene (4)
(1.19 g, 9.39 mmol), 2M K2CO3 (4.31 g, 31.3 mmol), Pd(PPh3)4
(0.36 g, 0.31 mmol, 10 mol%). The flask was evacuated and
backfilled with argon and degassed anhydrous THF (20 mL) was
added. The reaction mixture was refluxed for 3 h using an oil bath
with constant stirring. The reaction was allowed to cool to room
temperature and quenched by addition of NaHCO3 (30 mL). The
layers were separated, and the aqueous layer was extracted with
CH2Cl2 (5 × 20 mL). The organic extracts were dried over MgSO4,
filtered, and the solvent was removed in vacuo. The crude product
was purified via flash column chromatography on silica gel (15%
CH2Cl2/Hexane) provided 6 (0.701 g, 60% yield) as a stable white
solid.
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8
from Sigma-Aldrich and Alfa-Aesar chemical suppliers. All
solvents were purified and dried by standard methods prior to use.
Silica gel column chromatography was performed on Wakogel C-
200 and C-300. Alumina column chromatography was performed
on Active alumina (basic). Thin-layer chromatography (TLC) was
carried out on aluminum sheets coated with silica gel 60 F254
(Merck 5554). The visualization of spots was achieved using UV
light (λmax = 254 nm and 366 nm) or a KMnO4 staining solution (3.0
g KMnO4, 20 g K2CO3, 5.0 mL (5.0%) NaOH, 300 mL H2O).
Recrystallized samples of porphyrinoids were utilized for all the
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spectroscopic measurements. 1H NMR (500 or 700 MHz) and 13
C
NMR (125.8 MHz) spectra were recorded on Bruker 400
(AVANCE III) and INOVA 500 (Varian) spectrometer. Chemical
shifts are reported in parts per million (ppm, δ = scale) relative to
the signal of tetramethylsilane (δ = 0.00 ppm). 1H NMR spectra are
referenced to tetramethylsilane as an internal standard or the
residual solvent signal of the respective solvent: CDCl3: δ = 7.26
ppm, CD2Cl2: δ = 5.32 ppm. 13C NMR spectra are referenced to the
following signals: CDCl3: δ = 77.26 ppm. The following
abbreviations for multiplicities were used: singlet (s), broad singlet
(br), doublet (d), triplet (t), quartet (q), multiplet (m) and
combinations thereof, i.e. doublet of doublets (dd). Coupling
constants (J) are given in Hertz [Hz]. Structural assignments were
made with additional information from COSY experiments. HRMS
spectra were measured on a Thermo Fisher Scientific inc. Exactive
or LCQ Advantage via electron spray ionization (ESI) or
atmospheric pressure chemical ionization (APCI) with an orbitrap
analyser. Cyclic and differential-pulse voltammetric measurements
were performed on a PC-controlled electrochemical analyzer (CH
instruments model CHI620C) using a conventional three-electrode
cell for samples (1 mM) dissolved in dry CH2Cl2 Containing 0.1 M
n-Bu4NPF6 (TBAPF6) as the supporting electrolyte. Measurements
were carried out under an argon atmosphere. A glassy carbon
working electrode, a calomel reference electrode and platinum wire
counter electrode were used in all electrochemical experiments.
The potential were calibrated using the ferrocenium/ferrocene
couple. The optical absorption spectra were recorded on a
Shimadzu (Model UV-3600) spectrophotometer. Concentrations of
solutions are ca. to be 1 x 10-6 M (porphyrin Soret band) and 5 x
10-5 M (porphyrin Q-bands). Single crystal X-ray intensity data
were collected on a Bruker KAPPA APEXII diffractometer in
omega and phi scan mode, MoKα = 0.71073 Å at 298 K.
Analytical data for 6:1H NMR (500 MHz, CDCl3): δ = 8.56 (s, 1H),
7.68 (s, 2H), 7.47 (d, J = 7.7 Hz, 2H), 7.38 (d, J = 7.9 Hz, 2H), 7.28
(d, J = 7.7 Hz, 2H), 6.61 (s, 2H);13C{1H} NMR (126 MHz, CDCl3)
δ = 134.4, 132.4, 130.7, 129.6, 127.0, 123.4, 122.7, 109.3; HRMS
(ESI+):
375.9331 [M+H]+, found 375.8978.
m/z
calcd.
for
C16H12Br2N+
Di-tert-butyl
phenylene))bis(1H-pyrrole-1-carboxylate) (8). A flame-dried
flask was charged with (0.5 g, 1.33 mmol), N-(tert-
2,2'-((1H-pyrrole-2,5-diyl)bis(3,1-
6
butoxycarbonyl)-pyrrole-2-boronic acid 723 (0.84 g, 4 mmol), 2 M
K2CO3 (3.67 g, 26.6 mmol), Pd(PPh3)4 (0.15 g, 0.13 mmol, 10
mol%). The flask was evacuated and backfilled with argon and
degassed anhydrous THF (20 mL) was added. The reaction mixture
was refluxed for 4 h using an oil bath with constant stirring. The
reaction was allowed to cool to room temperature and quenched by
addition of H2O (30 mL). The layers were separated, and the
aqueous layer was extracted with CH2Cl2 (5 × 20 mL). The organic
extracts were dried over MgSO4, filtered, and the solvent was
removed in vacuo. The crude product was purified via flash column
chromatography on silica gel (40% CH2Cl2/Hexane) provided 8
(1.1 g, 76% yield) as a stable off-white solid.
1
Analytical data for 8: H NMR (500 MHz, CDCl3): δ = 8.72 (s,
1H), 7.48 (s, 2H), 7.43 (d, J = 7.6 Hz, 2H), 7.36 (s, 2H), 7.30 (t, J
= 7.6 Hz, 2H), 7.16 (d, J = 7.4 Hz, 2H), 6.56 (s, 2H), 6.21 (s, 4H),
1.33 (s, 18H). 13C{1H} NMR (126 MHz, CDCl3) δ = 149.6, 135.3,
134.9, 133.2, 132.0, 128.3, 127.4, 124.8, 122.8, 122.7, 114.7,
110.8, 108.1, 83.9, 27.8; HRMS (ESI+): m/z calcd. for C34H36N3O4
+
550.2700 [M+H]+, found 550.2687; m/z calcd. for C34H35N3O4Na+
572.2525 [M+Na]+, found 572.2507.
Synthetic Manupulations
Synthesis of 2,5-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
1H-pyrrole22 and N-(tert-Butoxycarbonyl)-pyrrole-2-boronic acid
(N-Boc-Py-B(OH)2)23 were prepared by slightly modifying the
previously reported procedures.
2,5-Bis(3-(1H-pyrrol-2-yl)phenyl)-1H-pyrrole (9). When
a
methanolic-THF solution of 8 (0.25 g, 0.45 mmol) was treated with
NaOMe (0.49 g, 9.1 mmol) at 0 °C for 1 h, smooth deprotection
was observed by monitoring the disappearance of the starting
material by TLC. The reaction was quenched by adding ice-cold
water (20 mL). The organic layer was extracted using diethyl ether
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(2 x 20 mL) followed by the removal of solvent under vacuum
128.3, 126.9, 126.6, 126.2, 122.7, 107.3, 21.3, 20.2. UV/Vis/NIR
(CH2Cl2): λmax/nm (log ε [mol–1 dm3 cm–1]): 339 (4.92), 532 (4.24).
MALDI-TOF: m/z calcd for C48H41N3S: 691.286 [M+2H]+; found:
691.308.
1
2
3
4
afforded 9 in a quantitative yield. This product seems sensitive to
heat and light. Hence, this key building block was used
instantaneously in the subsequent condensation step.
5
6
7
8
1
Analytical data for 9: H NMR (500 MHz, CDCl3): δ = 8.73 (s,
Di-m-benziheptaphyrins (3a): An oven dried flask was charged
1H), 8.54 (s, 2H), 7.58 (s, 2H), 7.32 (s, 4H), 7.27 (s, 2H), 6.83 (s,
2H), 6.57 (d, J = 12.9 Hz, 4H), 6.30 (s, 2H); 13C{1H} NMR (126
MHz, CDCl3) δ = 133.7, 133.4, 133.2, 132.1, 129.7, 122.2, 122.0,
119.7, 119.3, 110.4, 108.4, 106.5; HRMS (ESI-): m/z calcd. for
with
9 (0.1 g, 0.286 mmol) and [2,2'-bithiophene]-5,5'-
diylbis(mesitylmethanol) 11a18b (0.13 g, 0.286 mmol). The flask
was evacuated and backfilled with argon and degassed CH2Cl2 250
mL. Then, BF3·OEt2 (36 L, 0.286 mmol) was added using a
microsyringe and the mixture was stirred for 3 h at room
temperature followed by the addition of DDQ (0.13 g, 0.572 mmol).
After stirring for an additional 1 h in open air, the mixture was
treated with 2-3 drops of Et3N and filtered through short alumina
column until the eluent becomes colorless. The solvent was then
removed under vacuum. Column chromatography (silica gel,
hexane/CH2Cl2: 1/0 to 1/6) afforded and the reasonably pure
product. The resulting solid was recrystallized from
CH2Cl2/Hexane to give macrocycle 3a as dark green solids in 8%
yield (16 mg).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
C24H18N3+ [M-H]+
348.1495 found 348.1335.
,
Di-m-benzihexaphyrins (1): 9 (0.10 g, 0.286 mmol) and furan-
2,5-diylbis(mesitylmethanol) 10a18a (0.10 g, 0.286 mmol) was
dissolved in dry CH2Cl2 (250 mL). After stirring for 5 min under a
N2 atmosphere, para-toluenesulfonic acid (p-TsOH) (0.016 g,
0.085 mmol) was added and the mixture was stirred for 2 h at room
temperature, and then DDQ (0.065 g, 0.286 mmol) was added.
After stirring for an additional 1 h, the mixture was treated with 2-
3 drops of Et3N and filtered through short alumina column until the
eluent becomes colorless. The solvent was then removed under
vacuum and the crude product was purified by silica gel column
chromatography using a mixture of CH2Cl2 in hexane (1:1). The
resulting solid was recrystallized from CH2Cl2/Hexane afforded 1
in 10% (19 mg) yield.
1
Analytical data for 3a: H NMR (500 MHz, CDCl3): δ = 9.45 (s,
1H, NH), 8.48 (s, 2H), 7.73 (d, J = 7.8 Hz, 2H), 7.63 (d, J = 7.6 Hz,
2H), 7.45 (t, J = 7.7 Hz, 2H), 7.16 (d, J = 4.0 Hz, 2H), 7.04 (d, J =
4.6 Hz, 2H), 6.96 (s, 4H), 6.73 (d, J = 4.0 Hz, 2H), 6.67 (d, J = 4.5
Hz, 2H), 6.62 (s, 2H), 2.38 (s, 6H), 2.14 (s, 12H); 13C{1H} NMR
(126 MHz, CDCl3) δ = 171.3, 153.6, 148.2, 141.5, 141.3, 138.0,
137.1, 136.5, 135.7, 135.6, 135.3, 134.2, 129.2, 128.9, 128.3,
127.6, 127.4, 125.7, 121.6, 109.9, 21.4, 20.2. UV/Vis/NIR
(CH2Cl2): λmax/nm (log ε [mol–1 dm3 cm–1]): 323 (4.86), 430 (4.72),
570 (4.42). MALDI-TOF: m/z calcd for C52H42N3S2: 772.274
[M+H]+; found: 772.312.
1
Analytical data for 1: H NMR (500 MHz, CDCl3): δ = 11.06 (s,
1H, NH), 9.12 (s, 2H), 7.63 (d, J = 7.8 Hz, 2H), 7.48 (d, J = 7.6 Hz,
2H), 7.36 (t, J = 7.7 Hz, 2H), 7.22 (s, 2H), 7.16 (d, J = 4.7 Hz, 2H),
6.83 (s, 4H), 6.77 (d, J = 4.7 Hz, 2H), 6.59 (d, J = 2.3 Hz, 2H), 2.25
(s, 6H), 2.11 (s, 13H); 13C{1H} NMR (126 MHz, CDCl3) δ = 171.9,
158.6, 154.9, 138.3, 138.2, 137.4, 134.9, 134.1, 133.7, 133.5,
132.0, 129.1, 128.4, 128.2, 126.7, 125.3, 123.5, 123.1, 107.1, 32.2,
30.0, 29.6, 22.9, 21.4, 20.6, 14.4. UV/Vis/NIR (CH2Cl2): λmax/nm
(log ε [mol–1 dm3 cm–1]): 337 (4.68), 351 (4.67), 497 (4.07).
MALDI-TOF: m/z calcd for C48H40N3O: 674.309 [M+H]+; found:
674.349.
Di-m-benziheptaphyrins (3b): Compound 3b was synthesized
using
9 (0.1 g, 0.286 mmol) and [2,2'-bithiophene]-5,5'-
diylbis((perfluorophenyl)methanol)11b18b (0.16 g, 0.286 mmol) by
following the above procedure for compound 3a. Yield 3% (6 mg).
Di-m-benzihexaphyrins (2): The above mentioned procedure was
1
Analytical data for 3b: H NMR (500 MHz, CDCl3) δ = 9.50 (s,
followed
diylbis(mesitylmethanol)
TsOH (0.016 g, 0.085 mmol) and then DDQ (0.065 g, 0.286 mmol).
The dark orange colored fraction was eluted
by
using 9 (0.1
g, 0.286 mmol), thiophene-2,5
1H, NH), 8.50 (s, 2H), 7.80 (d, J = 7.8 Hz, 2H), 7.67 (d, J = 7.6 Hz,
2H), 7.51 (t, J = 7.7 Hz, 2H), 7.29 (s, 2H), 7.18 (d, J = 4.7 Hz, 2H),
6.84 (d, J = 3.7 Hz, 2H), 6.78 (d, J = 4.5 Hz, 2H), 6.66 (s, 2H);
13C{1H} NMR (126 MHz, CDCl3) δ = 173.9, 155.0, 148.3, 140.3,
135.7, 134.9, 134.8, 134.3, 134.0, 129.8, 129.7, 129.4, 128.0,
126.2, 124.7, 122.1, 110.2. UV/Vis/NIR (CH2Cl2): λmax/nm (log ε
[mol–1 dm3 cm–1]): 329 (4.88), 414 (4.69), 563 (4.40). MALDI-
TOF: m/z calcd for C46H20F10N3S2: 868.086 [M+H]+; found:
868.108.
10b18a (0.10 g, 0.286
mmol), p
with 60% CH2Cl2/Hexane afforded 2 as dark violet solids, yield 41
mg (21%).
1
Analytical data for 2: H NMR (500 MHz, CDCl3): δ = 10.46 (s,
1H, NH), 9.08 (s, 2H), 7.68 (d, J = 7.7 Hz, 2H), 7.58 (d, J = 7.6 Hz,
2H), 7.43 (t, J = 7.7 Hz, 2H), 7.10 (d, J = 4.7 Hz, 2H), 6.91 (s, 4H),
6.68 (d, J = 3.2 Hz, 4H), 6.61 (d, J = 2.3 Hz, 2H), 2.34 (s, 6H), 2.10
(s, 12H);13C{1H} NMR (126 MHz, CDCl3) δ = 172.6, 154.8, 148.7,
141.4, 138.2, 138.0, 137.0, 136.02, 134.9, 133.3, 133.0, 129.4,
Reduction of 3a: A flame dried flask was charged with 3a (20 mg,
0.026 mmol), degassed 6 ml THF and 3 ml methanol. Then, NaBH4
(9.90 mg, 0.260 mmol) was added, completion of the reaction was
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Confused Benzihexaphyrin Macrocycle. Org. Lett. 2009, 11,
3930–3933.
monitored using TLC. The reaction was quenched with H2O (20
1
2
3
4
5
6
mL). The organic phase was extracted with CH2Cl2 (2 x 20 ml) and
the solvent was removed under reduced pressure to afford a dark
blue semi-solid. The crude mixture was subjected to column
chromatography, but it rapidly reverts back to its oxidized form.
Hence, further characterization was hampered.
(4) (a) Chen, F.; Hong, Y. S.; Shimizu, S.; Kim, D.; Tanaka, T.;
Osuka, A. Synthesis of a Tetrabenzotetraaza[8]circulene by a
“Fold‐In” Oxidative Fusion Reaction. Angew. Chem. Int. Ed.,
2015, 54, 10639–10642. (b) Pawlicki, M.; Garbicz, M.;
Szterenberg, L.; Latos-Grażyński, L. Oxatriphyrins(2.1.1)
Incorporating an ortho‐Phenylene Motif. Angew. Chem. Int. Ed.
2015, 54, 1906–1909. (c) Adinarayana, B.; Thomas, A. P.;
7
8
9
Yadav,
P.;
Mukundam,
V.;
Srinivasan,
A.
Reduction of 3b (20 mg, 0.023 mmol) was carried out by following
the above procedure using NaBH4 (8.80 mg, 0.260 mmol), giving
similar results as the reduced forms are not stable.
Carbatriphyrin(3.1.1)−A Distinct Coordination Approach of BIII
to Generate Organoborane and Weak C−H···B Interactions.
Chem. -Eur. J. 2017, 23, 2993−2997. (d) Chen, F.; Kim, J.;
Matsuo, Y.; Hong, Y.; Kim, D.; Tanaka, T.; Osuka, A. ortho-
Phenylene-Bridged Hybrid Nanorings of 2,5-Pyrrolylenes and
2,5-Thienylenes, Asian. J. Org. Chem. 2019, 8, 994–1000. (e)
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ASSOCIATED CONTENT
Supporting Information
Das,
M.;
Adinarayana,
B.;
Murugavel,
M.;
Nayak, S.; Srinivasan, A. Di-(m-m-m)terphenyl-Embedded
Decaphyrin and Its Bis-Rh(I) Complex. Org. Lett. 2019, 21,
2867-2871.
CCDC 1979328 (2), 1979329 (3a) contain the supplementary
(5) (a) Lash, T. D. Benziporphyrins, a Unique Platform for
Exploring the Aromatic Characteristics of Porphyrinoid
Systems. Org. Biomol. Chem. 2015, 13, 7846–7878. (b) Lash, T.
D. Recent Advances on the Synthesis and Chemistry of
Carbaporphyrins and Related Porphyrinoid Systems. Eur. J. Org.
Chem. 2007, 33, 5461–5481. (c) Stȩpień, M.; Latos-Grażyński,
L. Tetraphenyl-p-Benziporphyrin: A Carbaporphyrinoid with
Two Linked Carbon Atoms in the Coordination Core. J. Am.
Chem. Soc. 2002, 124, 3838–3839.
crystallographic data for this paper. These data can be obtained free
of
charge
via
www.ccdc.cam.ac.uk/data_request/cif.
Characterization data (including HRMS, 1H, and 13C NMR
spectra, further cyclic voltammograms and further computational
results) absorption and single-crystal X-ray analyses of 2 and 3a.
The Supporting Information is available free of charge on the ACS
Publications website.
(6) Lash, T. D.; Chaney, S. T.; Richter, D. T. Conjugated
Macrocycles Related to the Porphyrins. 12.1 Oxybenzi- and
Oxypyriporphyrins: Aromaticity and Conjugation in Highly
Modified Porphyrinoid Structures. J. Org. Chem. 1998, 63,
9076–9088.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gokul@iisertvm.ac.in
(7) (a) Berlin, K.; Breitmaier, E. Benziporphyrin, A Benzene
Containing, Nonaromatic Porphyrin Analogue. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1246−1247. (b) Stȩpień, M.; Latos-
ORCID
S. Gokulnath: 0000-0001-9299-5217
Grażyński, L. Tetraphenylbenziporphyrin - A Ligand for
Organometallic Chemistry. Chem. - A Eur. J. 2001, 7, 5113–
5117.
Notes
The authors declare no competing financial interest.
(8) Corriu, R. J. P.; Bolin, G.; Moreau, J. J. E.; Vernhet, C. A Facile
Synthesis of New Tetrapyrrole Macrocyclic Derivatives.
Formation of Bimetallic Transition Metal Complexes. J. Chem.
Soc., Chem. Commun. 1991, 211−213.
ACKNOWLEDGMENTS
S.G. thanks IISER Thiruvananthapuram for the facilities and SERB
(Startup Grant No. SB/FT/CS-094/2014) and DST (Inspire Faculty
Support IFA-CH-74) for funding. S.T. thanks CSIR and A.J. thanks
DST-INSPIRE for their research fellowships. We thank Alex
Andrews for solving the X-ray structures of 2 and 3a.
(9) Ishida, M.; Kim, S. J.; Preihs, C.; Ohkubo, K.; Lim, J. M.; Lee,
B. S.; Park, J. S.; Lynch, V. M.; Roznyatovskiy, V. V.; Sarma,
T.; Panda, P. K.; Lee, C. H.; S. Fukuzumi, Kim, D.; Sessler, J. L.
Protonation-Coupled Redox Reactions in Planar Antiaromatic
meso-Pentafluorophenyl-Substituted
Annulated Rosarins. Nat. Chem. 2013, 5, 15–20.
o-Phenylene-Bridged
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