Synthesis of Selenium and Tellurium Core-Modified

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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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

Inorg. Chem. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/07/18. For personal use only.

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 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.

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azuliporphyrin derivatives, such as meso-free,26 meso-aryl,17 coremodified,27,28 ring-expanded,29 and contracted30 azuliporphyrins, as well as their metal complexes,31,32 adj-diazuli,13 oppdiazuliporphyrins,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 as very good ligands for organometallic complexes. Different properties 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 porphyrinoid 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 meso-substituted carbaporphyrinogens and the corresponding carbaporphyrinoids have not been explored much as of yet. Herein, we report the synthesis of a parallel library of selenium and tellurium core-modified azuliporphyrinogens and benziporphyrinogens and their oxidation to afford the corresponding porphyrins.

INTRODUCTION Carbaporphyrinoids1 are porphyrin analogues that involve the replacement of one2−6 or more7−13 of the nitrogen atoms with CH units. This internal carbon atom normally belongs to a carboor heterocyclic ring substituting one of the pyrroles (Scheme 1). Scheme 1

The introduction of azulene14−16 and benzene6,17−19 moieties to 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 properties including aromatic characteristics that fall midway between porphyrins and nonaromatic macrocycles23 and have been shown to generate organometallic derivatives under mild conditions. 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 © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Melting points were determined on a capillary melting point apparatus and are uncorrected. The 1H NMR and 13C NMR spectra Received: March 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00648 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 2

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 azuliporphyrinogens (5c−h) were synthesized in a similar manner by the reaction of an equimolar ratio of azulene and the corresponding 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 by 1H 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 (Supporting Information). In the 1H NMR spectra of 5c and 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 1 H 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 reaction was investigated using 2 equiv of chloranil for 10 min, and to our delight, porphyrinogens 5e and 5f were successfully oxidized 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 mixture. Br2 led to oxidation of the macrocycle, which incorporates

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 patterns are designed as s (singlet), d (doublet), t (triplet), m (multiplet), dt (double triplet), br (broad), and brs (broad singlet). The mass spectra of selected compounds were recorded on an Agilent Tech. highresolution 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 provided in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis of Ditellura/Diselena Azuliporphyrinogens. Desired unsubstituted and substituted tellurophene and selenophene 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 condensation 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, followed 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-mesooctahydro-22,24-ditelluradiazuliporphyrinogen (5a) and 5,10,15,20-meso-octahydro-22,24-diselenadiazuliporphyrinogen (5b) in 7 and 10% yields, respectively (Scheme 2). The tellurophene and meso-methylene protons (meso-CH2) appeared at δ 7.22 and δ 4.53 ppm as two sharp singlets, respectively, in the 1 H NMR spectrum of 5a in CDCl3 (Supporting Information), while the selenophene and meso-methylene protons (mesoCH2) appeared at δ 6.91 and δ 4.54 ppm as two sharp singlets, respectively, in the 1H 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


Article

Inorganic Chemistry

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-dithiadiazuliporphyrin with the addition of bromine.35 Corresponding porphyrins 6a−d of porphyrinogens 5a−d resulting from oxidation 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. In the 1H NMR spectrum of 5,10,15,20-meso-tetraphenyl22,24-ditelluradiazuliporphyrin (6e, Supporting Information), the chemical shift was found to be similar to that of dithiadiazuliporphyrin.35 The tellurophene proton appeared as a singlet 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) in the 1H NMR spectrum of 6e2+ (Figure 1), showing that a diatropic ring current is present in the dicationic macrocycle (6e2+). The electronic absorption spectrum of ditelluradiazuliporphyrin (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

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 blueshifted 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. 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+). The redox system monocation and dication were further investigated 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

Figure 1. 1H NMR spectrum of dication 6e2+ in CD3CN. C


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Inorganic Chemistry

The electronic absorption spectrum of 6,11,16,21-tetraphenyl24-tellurabenziporphyrin 9e was recorded in CH2Cl2, and two broad bands were observed at 378 and 671 nm. The spectroscopic 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 6,11,16,21tetraphenyl-24-tellurabenziporphyrin 9e in the presence of TFA showed the presence of recognizable diatropic ring current 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 6,11,16,21tetraphenyl-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 relative 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-24selenabenziporphyrin 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 selenabenziporphyrin 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 comparison to that of 10e, which appeared at 6.01 ppm. This indicates 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 capabilities 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 synthesized by the condensation of benzitripyrrane39 7 and 2,5bis(hydroxy(phenyl)methyl)tellurophene (4e) in CH2Cl2 in the presence of BF3·Et2O solution under a nitrogen atmosphere (Scheme 4). Initially, the formation of benziporphyrinogen 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 oxidized 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 alumina column and eluted with dichloromethane, and the product was collected as a bright-green band. Evaporation of the solvent afforded 6,11,16,21-tetraphenyl-24-tellurabenziporphyrin 9e in 11% yield. The proton NMR spectrum of 6,11,16,21tetraphenyl-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

D


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Inorganic Chemistry Scheme 5

Figure 4. UV−vis spectra of 6,11,16,21-tetraphenyl-24-tellurabenziporphyrin, 9e, in 1% Et3N/CH2Cl2 (A) and in 1% TFA/CH2Cl2 (B).

Figure 6. UV−vis spectra of 6,11,16,21-tetraphenyl-24-selenabenziporphyrin, 9f, in 1% Et3N/CH2Cl2 (A) and in 1% TFA/CH2Cl2 (B).

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

The diatropic character of the selena- and tellurabenziporphyrins 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

Figure 5. 1H NMR spectrum of 6,11,16,21-tetraphenyl-24-tellurabenziporphyrin, 10e, with 2 equiv of TFA in CDCl3. E


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Inorganic Chemistry

Figure 7. 1H 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 a

thiabenziporphyrin selenabenziporphyrin tellurabenziporphyrin

UV (nm) 335, 411, 643 416, 623 378, 671

UV (nm) (with TFA) a

346, 390, 475, 708 477, 682 449, 771

1H NMR 22-H

a

1

H NMR 22-H (with TFA)

a

5.29a 5.42 6.01

7.04 7.09 7.26

a

Ref 39.

presumed decrease in planarity for heterobenziporphyrins due to the larger size of selenium and tellurium atoms.

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CONCLUSIONS High-yielding parallel libraries of tellurium and selenium coremodified carbaporphyrinogens were synthesized by the reaction 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, benziporphyrinogens 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 indication 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.

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Detailed synthesis procedures; 1H NMR, 13C NMR, and mass spectra; and cyclic voltammogram data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

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.

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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.

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ASSOCIATED CONTENT

S 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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