Synthesis of Selenium and Tellurium Core-Modified Azuliporphyrinogens and Benziporphyrinogens and Corresponding Carbaporphyrinoids

Article abstract of DOI:10.1021/acs.inorgchem.8b00648

The synthesis of selenium and tellurium core-modified carbaporphyrinogens was carried out by the reaction of functional selenophene/tellurophene diols with azulene or a benzitripyrrane in the presence of acid. The products were obtained in moderate yields and were characterized by using ¹H and ¹³C NMR, UV-vis, FT-IR, CV, and HRMS spectroscopic techniques. Further, oxidation of the obtained core-modified carbaporphyrinogens in the presence of DDQ in CHCl₃ afforded the corresponding carbaporphyrins in good yields. Benziporphyrins showed no indication of a ring current or macrocyclic aromaticity as confirmed by using proton NMR spectroscopy, but the addition of TFA gave rise to the formation of weakly diatropic dications.

Full text of DOI:10.1021/acs.inorgchem.8b00648

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis of Selenium and Tellurium Core-Modified
Azuliporphyrinogens and Benziporphyrinogens and Corresponding
Carbaporphyrinoids
Sohail Ahmad,† Anchal Singhal,*,† Kharu Nisa, and S. M. S. Chauhan*
Department of Chemistry, University of Delhi, Delhi 110007, India
S
* Supporting Information
ABSTRACT: The synthesis of selenium and tellurium core-modified carbaporphyrinogens
was carried out by the reaction of functional selenophene/tellurophene diols with azulene or
a benzitripyrrane in the presence of acid. The products were obtained in moderate yields
1
and were characterized by using H and 13C NMR, UV−vis, FT-IR, CV, and HRMS spec-
troscopic techniques. Further, oxidation of the obtained core-modified carbaporphyrinogens
in the presence of DDQ in CHCl3 afforded the corresponding carbaporphyrins in good
yields. Benziporphyrins showed no indication of a ring current or macrocyclic aromaticity as
confirmed by using proton NMR spectroscopy, but the addition of TFA gave rise to the for-
mation of weakly diatropic dications.
azuliporphyrin derivatives, such as meso-free,26 meso-aryl,17 core-
INTRODUCTION
■
modified,27,28 ring-expanded,29 and contracted30 azuliporphyrins,
Carbaporphyrinoids1 are porphyrin analogues that involve the
as well as their metal complexes,31,32 adj-diazuli,13 opp-
replacement of one2−6 or more7−13 of the nitrogen atoms with
CH units. This internal carbon atom normally belongs to a carbo-
or heterocyclic ring substituting one of the pyrroles (Scheme 1).
diazuliporphyrins,33,34 and tetraazuliporphyrin tetracation,35
have been reported in the literature. Meanwhile, benziporphyrins
are a family of porphyrin analogues in which one of the pyrrole
units get replaced by a benzenoid ring.36,37 Benziporphyrins act
Scheme 1
as very good ligands for organometallic complexes. Different prop-
erties including aromaticity, reactivity, and tautomeric equilibria
can be tailored accordingly by applicably altering the structure of
benziporphyrins. The first example of a benziporphyrin was
obtained by reacting isophthalaldehyde with tripyrrane using
the 3 + 1 methodology.6 The benzene unit interrupted the por-
phyrinoid conjugative pathway, and the proton NMR spectrum
showed no indication of macrocyclic aromaticity.36 The 3 + 1
MacDonald condensation is the most convenient method for the
synthesis of benziporphyrins. Various diverse structures have been
synthesized in the literature using this method, but core-modified
The introduction of azulene14−16 and benzene6,17−19 moieties to
meso-substituted carbaporphyrinogens and the corresponding
porphyrinoid frameworks 1 and 2 is of particular interest
because of the unusual electronic, aromatic, and coordination
properties20,21 of these systems as well as their potential utility
in medicinal applications.22
Azulene-containing porphyrinoids have many interesting prop-
erties including aromatic characteristics that fall midway between
porphyrins and nonaromatic macrocycles23 and have been
shown to generate organometallic derivatives under mild condi-
tions. Azulene is a special carbocycle where the five-membered
rings behave similarly to heterocycles and both the 1 and 3
positions are highly reactive toward electrophilic substitution.24
The 10 π-electron system of azulene is a combination of two
cyclic rings, cyclopentadiene and cycloheptatriene.25 Various
carbaporphyrinoids have not been explored much as of yet.
Herein, we report the synthesis of a parallel library of sele-
nium and tellurium core-modified azuliporphyrinogens and
benziporphyrinogens and their oxidation to afford the corre-
sponding porphyrins.
EXPERIMENTAL SECTION
■
Melting points were determined on a capillary melting point
1
apparatus and are uncorrected. The H NMR and 13C NMR spectra
Received: March 12, 2018
© XXXX American Chemical Society
A
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Scheme 2
were recorded on Jeol (400 MHz, 75 MHz) spectrometers at room
temperature using TMS as an internal standard. The chemical shifts
(δ ppm) are referenced to the respective solvents, and splitting pat-
terns are designed as s (singlet), d (doublet), t (triplet), m (multiplet),
dt (double triplet), br (broad), and brs (broad singlet). The mass spec-
tra of selected compounds were recorded on an Agilent Tech. high-
resolution Q-TOF mass spectrometer. Cyclic voltammograms were
measured on 1 mM solutions of the complexes in dichloromethane with
tetra-n-butylammonium perchlorate (TBAP, 0.1M) as a supporting
electrolyte using a glassy carbon as a working electrode, a Pt wire as a
counter electrode, and Ag/AgCl as a reference electrode. Column
chromatography was carried out using silica gel (100−200 mesh).
The solvents used were of analytical grade and were dried before use.
Pyrrole was distilled before use. All other chemicals were purchased in
reagent quality and were used as received. All processes and reactions
were carried out under argon and protected from light. The detailed
synthesis procedures and the characterization of compounds are pro-
vided in the Supporting Information.
δ 7.78 ppm. The appearance of two separate singlets for the
inner CH 21, 23 azulene protons and nonequivalent 21, 31, 121,
131 and 22, 32, 122, 132 protons suggested that 5a and 5b adopt
unsymmetrical conformations.
The meso-substituted core-modified ditellura/diselena azuli-
porphyrinogens (5c−h) were synthesized in a similar manner
by the reaction of an equimolar ratio of azulene and the corre-
sponding diols (4c−h) in dry CH2Cl2 (4.1 M) in the presence
of BF3·CH3OH. meso-Tetramethyl 5c and 5d were afforded in
low yield (6 and 7% yields, respectively), while meso-octamethyl
5g and 5h were obtained in moderate yield (31 and 25% yields,
respectively). The relative orientations of methyl units attached
to the meso positions of 5c and 5d resulted in the formation of
these inseparable isomers. The presence of isomers was confirmed
1
by H NMR analysis. For instance, three distinctive multiplet
selenophene-CH resonance signals at δ 6.85, 6.88, and 6.90 ppm
generated by 5d reflected the presence of stereoisomers
1
(Supporting Information). In the H NMR spectra of 5c and
RESULTS AND DISCUSSION
■
5d, the inner azulene CH 21, 23 protons were found to be
identical and appeared as a singlet at δ 7.58 ppm, while four
meso-methyl (CH3) protons appeared as two singlets at δ 1.68
and 1.66 ppm indicating two different meso-CH3.
Ditelluradiazuliporphyrinogen 5e with phenyl groups was
found to be more stable and isolable compared to corresponding
diselenadiazuliporphyrinogen 5f. Compound 5f was only detected
by TLC and could not be isolated, as it was readily oxidized to the
corresponding diazuliporphyrins during the workup procedure.
The presence of isomers of 5e can easily be observed in the
1H NMR spectrum of 5e (Supporting Information).
Oxidation of Ditellura/Diselena Azuliporphyrinogens.
Attempts to oxidize porphyrinogens 5a−f with DDQ in CHCl3
at room temperature for 10 min led to complete decomposition,
and no desired product was detected; treatment with 0.5%
aqueous FeCl3 solutions for longer exposure times also failed
to give the oxidation product. Hence, the oxidation reac-
tion was investigated using 2 equiv of chloranil for 10 min, and
to our delight, porphyrinogens 5e and 5f were successfully oxi-
dized to corresponding diazuliporphyrins 6e and 6f in 7 and
8% yields, respectively (Scheme 3). Extension of the reaction
times (30 min), or the use of excess chloranil (3−5 equiv), led
to the formation of dication 62+, which could be reduced to 6 by
using SnCl2 in THF. Dication 62+ was also easily obtained by
reacting 6 with bromine in dry dichloromethane, followed by
precipitation with the addition of n-pentane to the reaction mix-
ture. Br2 led to oxidation of the macrocycle, which incorporates
Synthesis of Ditellura/Diselena Azuliporphyrinogens.
Desired unsubstituted and substituted tellurophene and selen-
ophene diols 4a−h were synthesized in good yields following
the reported procedure38 using different diynediols and an
aqueous solution of Na2Te/Na2Se in the presence of AgOAc.
The meso-unsubstituted core-modified ditellura/diselena
azuliporphyrinogens (5a, 5b) were prepared by the condensa-
tion of 1 equiv of azulene with 1 equiv of the corresponding
diols (4a, 4b) in the presence of 20% BF3·methanol using
dichloromethane as a solvent (4.1 M) at 0 °C for 20 min, fol-
lowed by stirring the reaction mixture at 50 °C for 45 min. The
crude porphyrinogens were purified by silica gel column
chromatography to afford the blue solids 5,10,15,20-meso-
octahydro-22,24-ditelluradiazuliporphyrinogen (5a) and
5,10,15,20-meso-octahydro-22,24-diselenadiazuliporphyrinogen
(5b) in 7 and 10% yields, respectively (Scheme 2). The telluro-
phene and meso-methylene protons (meso-CH2) appeared at
δ 7.22 and δ 4.53 ppm as two sharp singlets, respectively, in the
1H NMR spectrum of 5a in CDCl3 (Supporting Information),
while the selenophene and meso-methylene protons (meso-
CH2) appeared at δ 6.91 and δ 4.54 ppm as two sharp singlets,
1
respectively, in the H NMR spectrum of 5b (Supporting
Information). The 23 and 123 protons of azulene appeared as a
triplet at δ 7.52 ppm (J = 10 Hz), while 21, 31, 121, 131 and 22,
32, 122, 132 azulene protons appeared as a multiplet at δ 8.24
(J = 9.6 Hz) and δ 7.06 ppm, respectively. The inner 21, 23
CH azulene protons in 5a and 5b appeared as two singlets at
B
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changed from dark red (6e) to dark blue (6e2+) on the
addition of Br2 in the solution of 6e in CH2Cl2 (Figure 2).
Scheme 3
two azulene subunits, and resulted in the generation of
dicationic form 6e2+. This kind of multielectron redox system
was reported for 5,10,15,20-meso-tetraphenyl-22,24-dithiadia-
zuliporphyrin with the addition of bromine.35 Corresponding
porphyrins 6a−d of porphyrinogens 5a−d resulting from oxi-
dation with chloranil could only be detected by TLC and in
high-resolution mass spectrometry. They were not isolated, as
they were unstable and decomposed during the workup
procedure.
Figure 2. UV−vis spectra of 6e in CH2Cl2 (black) and upon the
addition of Br2 (red).
The UV−vis spectra of the diazuliporphyrin with a selenium
core (6f) in CH2Cl2 were observed as quite broad and blue-
shifted compared to those of 6e and appeared at 473 nm
(Figure 3). On the addition of Br2 in the solution of 6f, the
absorption peak was bathochromically shifted to 569 nm with
the decrease in relative intensity, and a new absorption peak
also appeared at 383 nm.
1
In the H NMR spectrum of 5,10,15,20-meso-tetraphenyl-
22,24-ditelluradiazuliporphyrin (6e, Supporting Information),
the chemical shift was found to be similar to that of dithia-
diazuliporphyrin.35 The tellurophene proton appeared as a sin-
glet at δ 10.69 ppm, and the 21, 31, 121, 131 protons appeared
as a doublet at δ 7.88 ppm. The 23 and 123 protons of azulene
appeared as a triplet at δ 6.73 ppm (J = 9.2 Hz), while the
azulene 22, 32, 122, 132 protons appeared as a triplet at δ 6.31 ppm.
The 21,23-H resonance is located at 7.08 ppm, confirming the
absence of macrocyclic aromaticity. On the other hand, the inner
21,23-H protons were found to be shifted upfield (δ 1.49 ppm)
The one-electron oxidation of diazuliporphyrin (6·+, Scheme 3)
was investigated by using the quantitative addition of Br2
(Figures 2 and 3). UV−vis spectra showed a new absorption
peak at 862 nm which disappeared with the further addition of
bromine. During titration, the color of the solution changed
from red (6e), through violet (6e•+), to dark blue (6e2+). On the
other hand, monocation 6f•+ appeared at 853 nm, and the color
of the solution changed from dark pink (6f), through purple
(6f•+), to blue (6f2+).
1
in the H NMR spectrum of 6e2+ (Figure 1), showing that
a diatropic ring current is present in the dicationic macrocycle
(6e2+). The electronic absorption spectrum of ditelluradiazu-
liporphyrin (6e) was recorded in CHCl3, and a strong broad
absorption was observed at 466 nm. The electronic absorption
band was split upon the addition of Br2, and absorption peaks
appeared at 431 and 589 nm. The color of the solution
The redox system monocation and dication were further inves-
tigated using cyclic voltammetry (Supporting Information).
It signified that 6e and 6f undergo two successive oxidations.
The first and second oxidations potentials were measured to
be +0.211 V and +0.409 V for 6e and +0.221 and +0.415 V for
6f, respectively. These potentials are very low, which accounts
1
Figure 1. H NMR spectrum of dication 6e2+ in CD3CN.
C
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The electronic absorption spectrum of 6,11,16,21-tetraphenyl-
24-tellurabenziporphyrin 9e was recorded in CH2Cl2, and two
broad bands were observed at 378 and 671 nm. The spec-
troscopic features of 9e resembled those of nonaromatic
6,11,16,21-tetraphenyl-m-benziporphyrin.40 In the UV−vis
spectrum of 9e in 1% TFA/CH2Cl2 solution (Scheme 5),
the absorption peaks were bathochromically shifted to 449 and
771 nm (shifted by 71 nm) with the increase in relative
intensity (Figure 4). The proton NMR spectrum of 6,11,16,21-
tetraphenyl-24-tellurabenziporphyrin 9e in the presence of
TFA showed the presence of recognizable diatropic ring cur-
rent in 10e, as the internal CH proton was shifted upfield to
6.01 ppm from the aromatic region (Figure 5) while the outer
benzene protons resonated further downfield as a 1H triplet at
δ = 7.80 ppm and a 2H doublet of doublets at δ = 7.97 ppm.
The pyrrolic and tellurophene protons further shifted downfield
compared with the values of tellurabenziporphyrin in the absence
of TFA. The presence of a +2 charge on the system contributed
to the downfield shifts, and the data was found to be consistent
with the emergence of weak diatropic character over the
macrocycle as has been noted for related structures.39,41−48
The selenium-containing benziporphyrin was synthesized
from diol 4f (2,5-bis(hydroxy(phenyl)methyl)selenophene)49
and benzitripyrrane (7) having a concentration of 3.2 mM and
afforded 6,11,16,21-tetraphenyl-24-selenabenziporphyrin (9f)
in 8% yield. The proton NMR spectrum of 6,11,16,21-
tetraphenyl-24-selenabenziporphyrin 9f in CDCl3 (Supporting
Information) showed the outer benzene protons (2,4-H) and
internal CH proton (22-H) in the region 7.0−7.38 ppm, and
the appearance of the internal CH proton in the aromatic
region indicated the absence of a macrocyclic ring current.
Selenabenziporphyrin 9f in CH2Cl2 gave a characteristic
absorption maximum at 416 nm and a broad absorption at
623 nm (Figure 6). In the UV−vis spectrum of 9f in 1% TFA/
CH2Cl2, the absorption peak was bathochromically shifted to
477 and 682 nm (shifted by 60 nm) with the increase in rela-
tive intensity. The bright-green solution of 9f turned orange,
and the color change was clearly visible to the naked eye.
The proton NMR spectrum of 6,11,16,21-tetraphenyl-24-
selenabenziporphyrin 9f in the presence of TFA showed the
presence of a recognizable diatropic ring current as 10f, and
the internal CH (22-H) shifted upfield to 5.42 ppm from the
aromatic region (Figure 7). The pyrrolic and selenophene protons
further shifted downfield compared with the values of selena-
benziporphyrin in the absence of TFA. The presence of a +2
charge on the system contributed to the downfield shifts. From
the proton NMR spectrum of 10e and 10f (Figures 5 and 7), the
22-H internal hydrogen of 10f appeared at 5.42 ppm in com-
parison to that of 10e, which appeared at 6.01 ppm. This indi-
cates that 10f has a stronger diatropic ring current than the ring
Figure 3. UV−vis spectra of 6f in CH2Cl2 (black) and upon the
addition of Br2 (red).
for the easy accessibility of the oxidized forms.33 Both first and
second oxidation potentials of 6e were shifted to lower values
in comparison to those of the selenium system, which indicates
that the tellurium atoms make the system more electron-rich
due to their softness and their possession of good donor capa-
bilities compared to those of selenium atoms.38
Synthesis of Ditellura/Diselena Benziporphyrinogens
and Benziporphyrins. The tellurophene and selenophene
diols (4) were further utilized in the synthesis of selenium- and
tellurium-core-atom-modified carbaporphyrinogens embedded
with benzene (selenium/tellurium benziporphyrinogens).
6,11,16,21-Tetraphenyl-24-tellurabenziporphyrin 9e was syn-
thesized by the condensation of benzitripyrrane39 7 and 2,5-
bis(hydroxy(phenyl)methyl)tellurophene (4e) in CH2Cl2 in
the presence of BF3·Et2O solution under a nitrogen atmo-
sphere (Scheme 4). Initially, the formation of benziporphyri-
nogen stereoisomers 8e was noticed as detected on TLC and
HRMS, but due to the instability of porphyrinogen, isolation
was not facilitated. Further, this species was immediately oxi-
dized with DDQ and stirred for another 30 min. The mixture
was washed with water, and the solvent was evaporated under
reduced pressure. The residue was chromatographed on an alu-
mina column and eluted with dichloromethane, and the prod-
uct was collected as a bright-green band. Evaporation of the sol-
vent afforded 6,11,16,21-tetraphenyl-24-tellurabenziporphyrin
9e in 11% yield. The proton NMR spectrum of 6,11,16,21-
tetraphenyl-24-tellurabenziporphyrin 9e recorded in CDCl3
(Supporting Information) showed the outer benzene protons
(2,4-H) and internal CH proton in the 7.2−7.41 ppm region.
The appearance of the internal CH proton in the aromatic
region indicated the absence of a macrocyclic ring current.26,39
Scheme 4
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Scheme 5
Figure 6. UV−vis spectra of 6,11,16,21-tetraphenyl-24-selenabenzi-
porphyrin, 9f, in 1% Et3N/CH2Cl2 (A) and in 1% TFA/CH2Cl2 (B).
Figure 4. UV−vis spectra of 6,11,16,21-tetraphenyl-24-tellurabenzi-
porphyrin, 9e, in 1% Et3N/CH2Cl2 (A) and in 1% TFA/CH2Cl2 (B).
The diatropic character of the selena- and tellurabenzipor-
phyrins was compared with that of thiabenziporphyrins,39
specifically internal benzene proton 22-H, and absorption
spectra with and without TFA were examined (Table 1).
On the basis of the upfield shifts for 22-H, the thiabenziporphyrin
dications39 showed the largest diatropic ring currents while the
tellurabenziporphyrins showed the smallest effects. The
decrease in the macrocyclic ring current is attributable to the
current of 10e. Furthermore, the internal carbon resonances of
9e and 9f in 13C NMR spectra appeared at 113.9 and 113.5 ppm,
respectively, but in TFA/CDCl3, the peaks for the corresponding
dications shifted upfield to values of 104.0 and 101.1 ppm. The
upfield shifts of the internal carbons presumably relate to
changes in the electronic structure, as similar shifts were noted
for the previously reported oxa- and thiabenziporphyrins.39
1
Figure 5. H NMR spectrum of 6,11,16,21-tetraphenyl-24-tellurabenziporphyrin, 10e, with 2 equiv of TFA in CDCl3.
E
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1
Figure 7. H NMR spectrum of 6,11,16,21-tetraphenyl-24-selenabenziporphyrin, 10f, with 2 equiv of TFA in CDCl3.
Table 1. UV−Vis and Chemical Shifts (ppm) for Heterobenziporphyrin with and without TFA
heterobenziporphyrin
UV (nm)
UV (nm) (with TFA)
1H NMR 22-H
1H NMR 22-H (with TFA)
a
a
a
a
a
thiabenziporphyrin
335, 411, 643
416, 623
378, 671
346, 390, 475, 708
477, 682
449, 771
7.04
5.29
5.42
6.01
selenabenziporphyrin
tellurabenziporphyrin
7.09
7.26
a
Ref 39.
Detailed synthesis procedures; 1H NMR, 13C NMR, and
mass spectra; and cyclic voltammogram data (PDF)
presumed decrease in planarity for heterobenziporphyrins due
to the larger size of selenium and tellurium atoms.
CONCLUSIONS
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: singhal.anchal@gmail.com.
*E-mail: smschauhanchemistry@gmail.com.
ORCID
■
High-yielding parallel libraries of tellurium and selenium core-
modified carbaporphyrinogens were synthesized by the reac-
tion of different selenophene and tellurophene diols with azulene
or a benzitripyrrane. The reaction of diols with equimolar
amounts of azulene in the presence of acid gave the desired
azuliporphyrinogens in moderate yields. Similarly, benzipor-
phyrinogens were synthesized using an equimolar amount of
selenophene or tellurophene diols with benzitripyrrane in the
presence of acid. The oxidation of core-modified azuli and
benziporphyrinogens was carried out in the presence of DDQ
in CHCl3. Dications of azuliporphyrins were easily obtained by
reacting the neutral compounds with bromine, and hence,
these results demonstrate the formation of multielectron redox
systems. Selena- and tellurabenziporphyrins showed no indica-
tion of a ring current or macrocyclic aromaticity by proton
NMR spectroscopy, but the addition of TFA gave rise to the
formation of weakly diatropic dications.
Anchal Singhal: 0000-0002-7992-5844
Author Contributions
†S.A. and A.S. are equal first authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the Council of Scientific
and Industrial Research (CSIR) for financial assistance.
We also thank the University Science Instrumentation Center
(USIC) for providing the facility for the characterization of
compounds.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.8b00648.
■
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