Synthesis of 5,10,15,20-meso-unsubstituted and 5,10,15,20-meso-substituted-21,23-ditellura/diselena core-modified porphyrinogens: oxidation and detection of mercury(II)

Abstract

Tellurium and selenium incorporated 5,10,15,20-meso-unsubstituted-21,23-ditellura/diselena core-modified porphyrinogens (N₂Te₂ and N₂Se₂), 5,10,15,20-meso-unsubstituted-21-tellura/selena core-modified porphyrinogens (N₃Te and N₃Se) and fully substituted meso-carbons porphyrinogens (N₂Te₂, N₂Se₂ and higher analogs) are synthesized by 3 + 1 condensation of tellurophene/selenophene dipyrranes and their corresponding diols in the presence of BF₃–etharate or BF₃–methanol. The meso-unsubstituted and substituted porphyrinogens were oxidized with chloranil/0.1% aqueous FeCl₃ in CHCl₃ at room temperature to obtain the corresponding porphines and porphyrins which are further reduced to corresponding chlorin and bacteriochlorin, whereas the fully meso-substituted porphyrinogens were found to be good ligands for Hg²⁺. The structures of the products were characterized by IR, ¹H, ¹³C, ¹²⁵Te, ⁷⁷Se NMR, CHN analysis, mass spectrometry and single-crystal XRD.

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Introduction

Porphyrinogens (calixpyrroles) are a class of tetrapyrrolic macrocycles composed of four pyrrole rings linked through the meso-positions by carbon atoms. The β- and meso-positions in porphyrinogens are reactive positions at which suitable substituents can be introduced to tune the properties of the porphyrinogens for specific applications. Fully meso-substituted porphyrinogens are stable and have been used in anion binding,¹ detection of various explosive^1f and self-assembly,¹ while meso-unsubstituted porphyrinogens have remained unexplored due to unstability and complexity in synthesis.² Meso-unsubstituted porphyrinogens are highly desirable synthetic precursors for the construction of meso-unsubstituted porphines^3,4 and other complex systems with special physical and chemical properties.^3b,5 Porphines/porphyrins of porphyrinogens are used as a starting material for various newer porphyrins.⁵ These are further utilized for the synthesis of longer-wavelength absorbing^5c–e and fluorescing molecules such as chlorins, bacteriochlorins and purpurins which are potentially used as fluorescence imaging or phototherapeutic agents.^5f–i

A wide variety of porphyrinogens with donor atoms (O, S, N and P) containing nonpyrrolic arene units,^6–9 have been designed and synthesized to modify the binding ability of the porphyrinogens. The 5,10,15,20-meso-octamethyl-21,23-dithiaporphyrinogen (N₂S₂) and 5,10,15,20-meso-octamethyl-21-thiaporphyrinogen (N₃S) ligands containing nitrogen–sulfur donor atoms are highly selective complexing agents towards heavy transition metal ions (Hg^II, Cd^II and Ag^I).¹⁰ Therefore, a similar development is anticipated for the porphyrinogen with heavier chalcogen donor atoms such as selenium and tellurium, due to their ‘softness’ and better sigma-donor capacity as compared to the lighter group 16 congeners.¹¹ The synthesis of selenium and tellurium 21- and 21,23-core-modified porphyrins,^12,3b in which one or more heteroatoms replace some of the four nitrogen atoms, has been already reported with altered core sizes, metal ion binding properties,¹² reactivity¹² and redox potentials.¹²

Herein we report the synthesis of novel meso-unsubstituted Te/Se core-modified porphyrinogens and meso-substituted Te/Se core-modified porphyrinogens via acid catalyzed 3 + 1 condensation of corresponding tellurophene/selenophene dipyrranes and diols. The meso-unsubstituted porphyrinogens are further oxidized to their corresponding porphyrins and porphines, whereas the meso-substituted porphyrinogens were utilized for the complexation of Hg²⁺.

Results and discussion

The desired 2,5-bis(2-pyrrolylmethyl)tellurophene (dipyrrane) 4a was synthesized in three steps starting from prop-2-yn-1-ol (1a). The reaction of prop-2-yn-1-ol 1a with CuI and NiCl₂·6H₂O, followed by the addition of TMEDA in THF under aerobic condition,¹³ proceeded smoothly to afford the diynediol 2a in 93% yield. The solution of diynediol 2a and AgOAc in MeOH was added to aqueous solution of Na₂Te¹⁴ to afford tellurophene diol 3a in 85% yield. The tellurophene diol 3a reacted with excess pyrrole in the presence of BF₃–etharate to afford dipyrrane 4a in 75% yield. The other dipyrranes (4b–h) were synthesized via similar route (Scheme 1).

Scheme 1 Synthesis of tellurophene and selenophene dipyrranes (4a–h).

N₂Te₂ and N₂Se₂ 5,10,15,20-meso-unsubstituted and 5,10,15,20-meso-substituted-21,23-ditellura/diselena core-modified porphyrinogens (5a–i) were synthesized by following the two methods A and B (Scheme 2).

Scheme 2 Synthesis of core-modified porphyrinogens.

The meso-unsubstituted and meso-tetramethyl core-modified porphyrinogens (5a–d) were prepared by 3 + 1 condensation of one equivalent of dipyrranes (4a–d) with one equivalent of corresponding diols (3a–d) in the presence of BF₃–methanol using dichloromethane as a solvent (3.2 mM) at 0 °C for 1 h, followed by stirring the reaction mixture to room temperature for 45 min (method A). The crude porphyrinogens were purified by silica gel column chromatography to afford white solids 5a in 15%, 5b in 11%, 5c in 27% and 5d in 22% yields respectively. The appearance of meso-methylene protons (meso-CH₂ for 5a, 5b and meso-CH for 5c, 5d) at about δ 3.92–4.04 ppm as a sharp singlet in the ¹H NMR spectra and a corresponding m/z peak in the mass spectra confirmed the formation of ditellura/diselena core-modified porphyrinogens (5a–d). An attempt to collect X-ray diffraction data was unsuccessful because of the poor quality of crystals. Four unsubstituted meso-carbons porphyrinogens (5a and 5b) are highly unstable and are very difficult to handle and become dark at room temperature in comparison to 5c and 5d porphyrinogens. Interestingly the above reaction conditions favor the formation of cyclic tetramers but their higher cyclic oligomers and N-confused products were not obtained. To obtain the higher analogue, the reaction of dipyrranes (4a–d) with corresponding diols (3a–d) in higher reactant concentration (5.3 mM and 16 mM) in dichloromethane or acetonitrile at 0 °C were carried out and it afforded a polymeric oxidized products along with small amount of porphyrinogens. In most cases, the N-confused cyclic tetramer, hexaphyrinogen and octaphyrinogen could be detected only on TLC as they are readily oxidized to the dyes without isolation. Porphyrinogens with phenyl group (5e–f) were detected only on TLC and were not isolated, as they are readily oxidized to corresponding porphyrins during the workup procedure and mixtures of porphyrinogens and porphyrins were obtained. Even isolated porphyrinogens from column purification are readily oxidized to corresponding porphyrins.

The porphyrinogens 5a–f were oxidized with chloranil in CHCl₃ at room temperature for 10 min to obtain the corresponding porphines and porphyrins or by shaking the porphyrinogens solution with 0.1% aqueous FeCl₃ solution for 10 min (Scheme 3). Prolonged shaking or stirring the reaction mixture caused decomposition of the product, whereas the porphyrinogens with 0.1% aqueous FeCl₃ solution in shorter exposure times were not oxidized to the product. However, attempts to oxidize the porphyrinogens 5a–f with DDQ led to complete decomposition. The high resolution mass spectrometry showed corresponding m/z peak to porphyrin/porphine along with porphyrinogen peak (see ESI†). The corresponding porphyrins of porphyrinogens 5a–c are unstable and readily undergo to decomposition at room temperature. Porphyrinogen 5b on oxidation with 0.1% aqueous FeCl₃ solution gave unstable isolable novel 21,23-diselenaporphine in 5% yield, while oxidation with chloranil gave 2% of porphine. In the ¹H NMR spectrum of the 21,23-diselenaporphine, the meso proton appeared at δ 10.49 ppm, while pyrrolic and selenophene protons were appeared at δ 9.36 ppm and 10.29 ppm, respectively. The electronic absorption spectra of 21,23-diselenaporphine was recorded in CHCl₃ and a soret along with Q bands were observed at 415, 474, 504 and 535 nm. Porphyrinogen 5d on oxidation with chloranil gave stable novel 5,10,15,20-meso-tetramethyl-21,23-diselenaporphyrin in 76% yield. In the ¹H NMR spectrum of the 5,10,15,20-meso-tetramethyl-21,23-diselenaporphyrin, the meso methyl group (CH₃) appeared at δ 4.5 ppm, while pyrrolic and selenophene protons were appeared as singlet at δ 9.2 ppm and 10.1 ppm, respectively.¹⁵ The electronic absorption spectra of 5,10,15,20-meso-tetramethyl-21,23-diselenaporphyrin was recorded in CHCl₃ and four Q bands along with one intense soret band were observed at 693.5, 630.0, 554.1, 521.8 and 437.8 nm. The electronic absorption spectra of corresponding porphyrins of porphyrinogens 5a–d are given in ESI.† Porphyrinogens with phenyl group (5e and 5f) were oxidized with DDQ to corresponding porphyrins in 86–90% yield, which were characterized and matched with the spectrum of 5,10,15,20-meso-tetraphenyl-21,23-ditellura/diselenaporphyrin.¹²

Scheme 3

The N₃Te and N₃Se porphyrinogens with four meso-unsubstituted carbons can be used in constructing highly desirable novel 21-tellura/selenaporphyrinogens systems. The N₃Te porphyrinogens 9a (Scheme 4) were synthesized from the unsubstituted diols (3a) and 2,5-bis(2-pyrrolylmethyl)pyrrole (tripyrrane) in the concentration of a 3.2 mM scale (method A), affording 9a (7% yield) and 9b (8% yield). 2,5-Bis(2-pyrrolylmethyl)pyrrole (tripyrrane) was obtained by formylation of pyrrole followed by condensation with pyrrole of the resulting 2,5-bis(hydroxymethyl)pyrrole in the presence of acid.^2a In the ¹H NMR spectrum of 9a in CDCl₃ at room temperature, meso-methylene protons connected with a tellurophene ring and connected with a pyrrole ring appeared at δ 3.99 and δ 3.86 ppm as a sharp singlet, respectively. The N₃Se porphyrinogen 10a was prepared in the same manner, affording 10a (11% yield) and 10b (traceable amount). An attempt to obtain 21-tellura/selenaporphine by oxidation of 9a and 10a were carried out by shaking the porphyrinogens (9a and 10a) solution with 0.1% aqueous FeCl₃ or addition of chloranil directly to the reaction mixture at room temperature for 10 min, but it led to the formation of a complex mixture and even in HRMS no peak corresponding to 21-tellura/selenaporphine were detected.

Scheme 4

The absence of steric hindrance in meso-unsubstituted selenaporphine and meso-tetramethylselenaporphyrin offers the further utilization of these compounds. The reduction of selenaporphine/porphyrin were carried out by using p-toluenesulfonhydrazide in the presence of pyridine and potassium carbonate to synthesize novel phototherapeutic model compounds diselenachlorin and diselenabacteriochlorin (Scheme 5). Reaction of the 5,10,15,20-meso-tetramethyl-21,23-diselenaporphyrin with 10 equivalent p-toluenesulfonhydrazide and 10 equivalent K₂CO₃ in pyridine for 24 h, gave meso-tetramethyldiselenachlorin and meso-tetramethyldiselenabacteriochlorin in 5% and 2% yield respectively. The corresponding chlorin and bacteriochlorin of meso-tetramethylselenaporphyrin are unstable and readily undergo decomposition at room temperature. The formation of meso-tetramethyldiselenachlorin was confirmed by the appearance of reduced pyrrolic proton at about δ 4.29 ppm and δ 4.60 ppm as triplet in the ¹H NMR spectra. The selenophene and pyrrole protons of diselenachlorin are upfield shifted with respect to selenaporphyrin. The selenophene protons were appeared as doublets at δ 9.27 and 9.64 ppm (J = 4.8 Hz) while pyrrolic protons appeared as singlet at δ 9.07 ppm. The electronic absorption spectra of meso-tetramethyldiselenachlorin (soret and Q bands were observed at 437.9, 522.6, 550.8, 631.7 and 689.0 nm) was more similar to the starting material. However, the longest wavelengths absorption (689 nm) is more intensified and 4 nm hypsochromically shifted compared to the starting meso-tetramethylselenaporphyrin. The ¹H NMR spectrum of meso-tetramethyldiselenabacteriochlorin is much simplified due to symmetrical structure of bacteriochlorin. The selenophene signal is downfield shifted and appeared at δ 8.92 ppm as singlet. A comparative shift for the reduced pyrrolic protons was also observed. The UV-vis spectrum of the meso-tetramethyldiselenabacteriochlorin, shows three-bands at 408, 535, 739 nm, in which the longest wavelengths absorption is significantly more red-shifted (739 nm) as compared to the corresponding chlorin (689 nm) and selenaporphyrin (693 nm).

Scheme 5

An attempt to obtain chlorin by reduction of 21,23-diselenaporphine (meso-unsubstituted porphyrin) using p-toluenesulfonhydrazide, led to the decomposition of starting material or product and no corresponding chlorin was detected in UV-Vis spectrum. Selena/telluraporphyrins with phenyl group at meso position are less prone to reduction and require excess of the reducing agent and longer reaction time. The reaction was monitored by TLC and UV-Vis spectroscopy.

The fully meso-substituted core-modified porphyrinogens (5g–i) were synthesized by condensation of dipyrranes (4g–h) with corresponding diols (3g–h) in the presence of BF₃–etharate in dichloromethane (3.2 mM concentration) at room temperature for 30 min (method B). These porphyrinogens (5g–i) are highly stable and can be stored for over 1 year at room temperature. To obtain the higher analogue (Fig. 1), similar reactions were carried out in acetonitrile at 0 °C with the same concentration (∼3.2 mM) but not much difference in cyclic tetramer yields were observed and no higher ordered species were detected (Table 1). The increase in reactant concentration (∼16 mM) favored the formation of higher ordered species and gave 5g in 23% yield, N-confused cyclic tetramer (6g) in 5% yield, hexaphyrinogen (7g) in 13% yield and octaphyrinogen (8g) in 11% yield. Increasing the concentration upto 81 mM gave hexaphyrinogen (7g) and octaphyrinogen (8g) as the major product. The formation of hexaphyrinogen (7g) involves the condensation of diol (3g) with two molecules of 4g, in which one of the 4g undergo pyrrolic group cleavage under acidic condition and followed by subsequent cyclization gave hexamer.¹⁶ Further the nature of other acid catalysts were investigated in two different concentrations 3.2 mM and 16 mM. In 3.2 mM concentration, the above reaction in halogenated acid such as TFA and in nonhalogenated acid such as methane sulfonic acid, exclusively gave 5g. While the same reaction in 16 mM concentration, 5g, 6g, 7g and 8g were obtained in 19%, 3%, 16%, 10% yields respectively in TFA, whereas with methane sulfonic acid 5g and 8g were obtained as the major products (Table 1). These results imply that acid-catalysed cleavage of pyrryl group in the reaction increases on increasing the concentration of reactant and the template effect of trihalogenated acid (TFA and BF₃–etharate) assists the formation of hexaphyrinogen. The template effect of trihalogenated acid in the cyclization reactions, have been previously reported with dipyrromethane and acetone.¹⁷ Similar concentration studies were carried out with selenophene diol (3h) and dipyrrane (4h) to get 5h, 6h, 7h and 8h. The yield of these reactions did not vary much in comparison to tellurophene condensation reactions as reported above.

Fig. 1 Structure of N-confused and expanded porphyrinogens.

Table 1 Yield of the condensation reaction between 3g and 4g in dry acetonitrile in different reactant concentration and in the presence of different acid catalyst^a

Entry	Conc. of reactant (mM)	Catalyst	5g^b	6g^b	7g^b	8g^b
a Reaction condition: 3 g (0.5 mmol), 4 g (0.5 mmol) and acid catalyst (0.092 mmol) at 0 °C for 30 min.b Isolated yields in percentage (%).
1	3.2	BF3·OEt2	41	0	0	0
2	5.3	BF3·OEt2	42	0	0	3
3	16	BF3·OEt2	23	5	13	11
4	40	BF3·OEt2	15	7	19	17
5	81	BF3·OEt2	10	10	26	21
6	3.2	TFA	42	0	Trace	0
7	16	TFA	19	3	16	10
8	3.2	MeSO3H	43	6	0	0
9	16	MeSO3H	41	8	3	16

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The structures of all the products were characterized by IR, ¹H NMR, ¹³C NMR, CHN analysis and mass spectrometry (ESI†). In the ¹H NMR spectra of 5a, 5c and 5g in CDCl₃, tellurophene protons are more deshielded and appeared at δ 7.24–7.14 ppm compared to the selenophene protons (δ 6.84–6.82 ppm) of 5b, 5d and 5h due to better acceptor ability of σ*(Te–carbon) bond of tellurium than selenium.^11a In the ¹²⁵Te NMR (ESI†), chemical shift values of porphyrinogens 5a, 5c and 5g were appeared as singlet at δ 758.91, 755.08 and 754.88 ppm respectively. These values are more similar to the ¹²⁵Te chemical shift for tellurophene (δ 775.1 ppm).¹⁸ While in case of ⁷⁷Se NMR of porphyrinogen 5b, 5d and 5h, it showed upfield as compared to corresponding tellurophene porphyrinogens and exhibited ⁷⁷Se NMR signal at δ 601.02, 596.16 and 593.75 ppm respectively. Suitable crystal of 5g for single-crystal X-ray diffraction was obtained as shown in Fig. 2.¹⁹ The analysis of the above result revealed that 5g adopt an 1,3-alternate conformation in the solid state. Porphyrinogen 5g has a Te⋯Te distance of 4.61 °A and the two Te atoms occupy the same side of core and no flipped tellurophene was found in the crystal.^12c

Fig. 2 X-ray crystal structure of 5g (ORTEP plot).

The electronic absorption spectroscopic titration of 5,10,15,20-meso-octamethyl-21,23-ditelluraporphyrinogen (5g) with “soft” Lewis acid, mercuric perchlorate Hg(ClO₄)₂ were performed in dry acetonitrile at room temperature. Porphyrinogen 5g gives a characteristic absorption maxima at 284 nm, on the addition of Hg²⁺ in to acetonitrile solution of 5g (1 : 1 complex), the absorption peak was bathochromically shifted to 296 nm with the increase in relative intensity (Fig. 3). The binding constant K_a of 5g–Hg²⁺ complex was found to be 1.45 × 10⁵ M⁻¹. The method of continuous variation (Job's plot) was also used to prove the 1 : 1 stoichiometry (ESI†). A comparative study for the binding of Hg²⁺ cation with porphyrinogen 5g and 5,10,15,20-meso-octamethyl-21,23-dithiaporphyrinogen (N₂S₂) was performed and it was observed that porphyrinogen 5g showed more bathochromic shift (12 nm) than 5,10,15,20-meso-octamethyl-21,23-dithiaporphyrinogen (3 nm). The electronic absorption spectroscopic titration of Hg²⁺ with 7g and 8g were also performed and their respective preliminary peaks were shifted by 10–12 nm. UV-Vis spectra of 7g and 8g with Hg²⁺ are given in ESI.†

Fig. 3 The absorption spectra of 5g (4.2 × 10⁻⁶) in acetonitrile upon the addition of 20 μL stock solution (4.2 × 10⁻⁶) of mercuric perchlorate.

The ¹H NMR data of Hg²⁺ with tellurium and selenium porphyrinogen at 298 K are reported in Table 2. A significant downfield chemical shift change was observed for tellurophene and selenophene protons of 5g and 5h, which suggests that the interaction takes place through the tellurium and selenium donor atom of the tellurophene and selenophene rings. Furthermore, the involvement of tellurium of tellurophene and selenium of selenophene in binding with Hg²⁺ ion, was confirmed by ¹²⁵Te and ⁷⁷Se NMR. The 5g exhibited a signal at 754.88 ppm in ¹²⁵Te NMR which was significantly downfield shifted by 1.21 ppm and appeared at 756.09 ppm (ESI†). Similarly selenium of porphyrinogen 5h in the ⁷⁷Se NMR showed downfield shift by 0.71 ppm in the presence of Hg²⁺. The calix[4]pyrrole was also investigated for its interaction with the Hg²⁺ cation at 298 K and no significant chemical shift change was observed for the NH of calix[4]pyrrole, suggesting the absence of complexation between pyrrolic ligand and Hg²⁺ cation. It indicates the incapability of the NH groups of the pyrrolic units to bind with the Hg²⁺ cation.

Table 2 Changes in the ¹H, ¹²⁵Te and ⁷⁷Se NMR chemical shifts (δ in ppm) for 5g, 5h and calix[4]pyrrole complexes in CDCl₃ at 298 K

Porphyrinogen	^1H				^125Te	^77Se
	HMethyl	HPyrrole	HTelluro/Seleno	HNH
5g	1.63	5.81	7.24	7.26	754.88	—
5g + Hg^2+	−0.03	+0.11	+0.31	+0.11	+1.21	—
5h	1.61	5.85	6.83	7.19	—	593.75
5h + Hg^2+	−0.02	+0.09	+0.24	+0.09	—	+0.71
Calix[4]pyrrole	1.51	5.91	—	7.06	—	—
Calix[4]pyrrole + Hg^2+	−0.04	+0.02	—	+0.06	—	—

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Conclusion

In summary, the selenium and tellurium N₂Te₂, N₂Se₂, N₃Te and N₃Se meso-unsubstituted core-modified porphyrinogens and N₂Te₂, N₂Se₂, N₂TeSe meso-substituted core-modified porphyrinogens have been synthesized for the first time. The meso-unsubstituted porphyrinogens are oxidized by chloranil to corresponding porphyrins and porphines which are easily reduced to chlorin and bacteriochlorin. The meso-substituted porphyrinogens are used for the complexation of Hg²⁺ ion. The reaction approach using readily available conjugated diynes as starting materials make this strategy highly attractive in diversity oriented synthesis.

Experimental section

General

Melting points were determined on a capillary melting point apparatus and are uncorrected. The ¹H NMR and ¹³C NMR spectra 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 ⁷⁷Se NMR and ¹²⁵Te NMR spectra were recorded on a DPX-300 Bruker spectrometer at 57.24 and 94.69 MHz and the chemical shifts were reported relative to Me₂⁷⁷Se and Me₂¹²⁵Te. The mass spectra of selected compounds were recorded on Agilent tech. high resolution Q-TOF mass spectrometry. The column chromatography was carried out using silica gel (100–200 mesh). 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. 2,5-Bis(hydroxymethyl)pyrrole and 2,5-bis(2-pyrrolylmethyl)pyrrole were prepared according to the literature method.^2a

Synthesis of diynediols (2a–d). The diynediols (2a–d) were synthesized by slightly modifying the literature procedure.¹⁷ The solution of TMEDA (0.4 mmol), CuI (0.1 mmol), and NiCl₂·6H₂O (0.1 mmol) in THF (5 mL) was stirred at room temperature. Terminal alkynes (1a–d) (2 mmol) were dissolved in THF (5 mL) and added slowly with the bubbling of oxygen in the solution. The reaction was monitored by TLC. After completion of the reaction (1 h), the mixture was concentrated under reduced pressure and purified by column chromatography on silica gel. The product was eluted with 20% ethyl acetate in hexane to give diynediol as pale yellow solid (93–89%).

Hexa-2,4-diyne-1,6-diol (2a). White solid (92%), mp 112 °C; ¹H NMR (400 MHz, DMSO): δ = 4.31 (s, 4H, CH₂), 2.11 (s, 2H, OH); ¹³C NMR (75 MHz, DMSO): δ = 78.11, 70.21, 49.3; HR Q-TOF MS, m/z 111.09 (M + 1) (calcd for C₆H₆O₂ 110.03); analysis: calcd for C₆H₆O₂ C, 65.45; H, 5.49; found: C, 65.25; H, 5.16%.

Octa-3,5-diyne-2,7-diol (2b). White solid (97%), mp 94–96 °C; ¹H NMR (400 MHz, CDCl₃): δ = 4.57–4.54 (q, 2H, CH), 2.15 (s, 2H, OH), 1.45–1.44 (d, 6H, CH₃); ¹³C NMR (75 MHz, CDCl₃): δ = 79.4, 65.1, 59.4, 20.9; HR Q-TOF MS, m/z 139.09 (M + 1) (calcd for C₈H₁₀O₂ 138.06); analysis: calcd for C₈H₁₀O₂ C, 69.54; H, 7.30; found: C, 70.24; H, 7.21%.

1,6-Diphenyl-hexa-2,4-diyne-1,6-diol (2c). White solid (92%), mp 102 °C (lit. mp 102);^{12c 1}H NMR (CDCl₃, 400 MHz): δ = 7.50–7.30 (m, 10H, Ar-H), 5.50 (s, 2H, CH), 2.13 (br, 2H; OH); ¹³C NMR (CDCl₃, 75 MHz): δ = 139.75, 129.17, 127.50, 127.01, 80.35, 70.90, 65.35; HR Q-TOF MS, m/z 263.09 (M + 1) (calcd for C₁₈H₁₄O₂ 262.09); analysis: calcd for C₆H₆O₂ C, 82.42; H, 5.38; found C, 83.01; H, 5.01%.

2,7-Dimethylocta-3,5-diyne-2,7-diol (2d). White solid (93%), mp 134 °C; ¹H NMR (400 MHz, CDCl₃): δ = 1.98 (s, 2H, OH), 1.52 (s, 12H, CH₃); ¹³C NMR (75 MHz, CDCl₃): δ = 83.89, 66.33, 65.93, 30.96; IR (KBr) ν_max 3221, 2982, 2935, 1449, 1381, 1364, 1210, 1170, 954, 889, 731, 553; HR Q-TOF MS, m/z 167.089 (M + 1) (calcd for C₁₀H₁₄O₂ 166.099); analysis: calcd for C₁₀H₁₄O₂ C, 72.26; H, 8.49; found: C, 72.02; H, 8.19%.

Synthesis of tellurophene and selenophene diols (3a–h). A 250 mL three-necked flask equipped with a nitrogen inlet, reflux condenser and dropping funnel was loaded with tellurium/selenium (4 mmol), NaBH₄ (16 mmol), and distilled water (10 mL). The mixture was vigorously stirred under nitrogen for 30 min. A deoxygenated solution of diynediol (2a–d) (3.1 mmol) in MeOH (12 mL) with AgOAc (50 mg) was added drop wise and the mixture was stirred overnight. After completion of the reaction, water (100 mL) was added and the product was extracted with benzene–diethylether (1 : 1, 150 mL). The organic layer was washed with water (2 × 20 mL), dried over Na₂SO₄ and evaporated under reduced pressure to give (3a–h) as a off white solid (86–91%).

2,5-Bis(hydroxymethyl)tellurophene (3a). Off white solid (85%), mp 106 °C (lit. mp 106–107 °C);^{20a 1}H NMR (CDCl₃, 400 MHz): δ = 7.35 (s, 2H, tellurophene), 4.77 (s, 4H, CH₂), 2.02 (s, 2H, OH); ¹³C NMR (75 MHz, CDCl₃): δ = 158.6, 130.35, 64.97; HR Q-TOF MS, m/z 243.12 (M + 1) (calcd for C₆H₈O₂Te 241.95); analysis: calcd for C₆H₈O₂Te C, 30.06; H, 3.36; found C, 30.36; H, 3.61%.

2,5-Bis(hydroxymethyl)selenophene (3b). Light yellow solid (82%), mp 119 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 6.98 (s, 2H, selenophene), 4.79 (s, 4H, CH₂), 2.06 (brs, 2H, OH); ¹³C NMR (75 MHz, CDCl₃): δ = 151.6, 126.5, 62.5; HR Q-TOF MS, m/z 193.55 (M + 1) (calcd for C₆H₈O₂Se 191.96); analysis: calcd for C₆H₈O₂Se C, 37.71; H, 4.22; found C, 38.11; H, 4.47%.

2,5-Bis(hydroxy(methyl)methyl)tellurophene (3c). White solid (87%), mp 98 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.31 (s, 2H, tellurophene), 4.89–4.87 (m, 2H, CH), 2.24 (s, 2H, OH), 1.49 (s, 6H, CH₃); ¹³C NMR (75 MHz, CDCl₃): δ = 157.4, 131.2, 71.2, 27.2; HR Q-TOF MS, m/z 271.08 (M + 1) (calcd for C₈H₁₂O₂Te 269.99); analysis: calcd for C₈H₁₂O₂Te C, 35.88; H, 4.52; found C, 35.30; H, 4.33%.

2,5-Bis(hydroxy(methyl)methyl)selenophene (3d). White solid (83%), mp 109 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 6.86 (s, 2H, selenophene), 4.94–4.92 (m, 2H, CH), 3.48 (s, 2H, OH), 1.47–1.45 (s, 6H, CH₃); ¹³C NMR (75 MHz, CDCl₃): δ = 156.5, 124.2, 124.1, 67.9, 25.8, 25.7; HR Q-TOF MS, m/z 221.01 (M + 1) (calcd for C₈H₁₂O₂Se 220.00); analysis: calcd for C₈H₁₂O₂Se C, 43.85; H, 5.52; found C, 43.95; H, 5.10%.

2,5-Bis(hydroxy(phenyl)methyl)tellurophene (3e). White solid (86%), mp 146 °C (lit. mp 145–146 °C);^{12c 1}H NMR (CD₃OD, 400 MHz): δ = 7.36–7.23 (m, 10H, Ar-H), 7.23 (s, 2H, tellurophene), 5.68 (s, 2H, CH); ¹³C NMR (CD₃OD, 75 MHz): δ = 156.4, 141.7, 136.3, 131.3, 126.5, 128.3, 76.5; IR (KBr) ν_max 3403, 2925, 1626, 1111, 1005, 812, 775, 700; HR Q-TOF MS, m/z 395.02 (M + 1) (calcd for C₁₈H₁₆O₂Te 394.02); analysis: calcd for C₁₈H₁₆O₂Te C, 55.16; H, 4.11; found C, 55.46; H, 4.30%.

2,5-Bis(hydroxy(phenyl)methyl)selenophene (3f). White solid (83%), mp 136 °C (lit. mp 136–137 °C);^{20b 1}H NMR (CD₃OD, 400 MHz): δ = 7.35–7.21 (m, 10H, Ar-H), 6.81 (s, 2H, selenophene), 5.69 (s, 2H, CH); ¹³C NMR (CD₃OD, 75 MHz): δ = 157.4, 143.7, 139.3, 129.3, 128.5, 126.3, 73.5; IR (KBr) ν_max 3378, 1492, 1452, 1394, 1282, 1255, 1121, 998, 835, 815, 780, 725, 699, 610, 497; HR Q-TOF MS, m/z 345.12 (M + 1) (calcd for C₁₈H₁₆O₂Se 344.03); analysis: calcd for C₁₈H₁₆O₂Se C, 62.98; H, 4.70; found C, 62.36; H, 4.51%.

2,5-Bis(hydroxy(dimethyl)methyl)tellurophene (3g). White solid (88%), mp 95 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.24 (s, 2H, tellurophene), 2.17 (s, 2H, OH), 1.58 (s, 12H); ¹³C NMR (75 MHz, CDCl₃): δ = 161.74, 130.35, 74.97, 32.25; IR (KBr) ν_max 3326, 2970, 2924, 1497, 1458, 1373, 1360, 1244, 1207, 1147, 1094, 933, 811, 799, 663, 552; HR Q-TOF MS, m/z 299.01 (M + 1) (calcd for C₁₀H₁₆O₂Te 298.02); analysis: calcd for C₁₀H₁₆O₂Te C, 40.60; H, 5.45; found C, 40.20; H, 5.15%.

2,5-Bis(hydroxy(dimethyl)methyl)selenophene (3h). Off white solid (81%), mp 114 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 6.89 (s, 2H, selenophene), 2.15 (s, 2H, OH), 1.63 (s, 12H); ¹³C NMR (75 MHz, CDCl₃): δ = 160.55, 123.23, 72.78, 32.28; IR (KBr) ν_max 3307, 2979, 2930, 2867, 1451, 1408, 1357, 1358, 1245, 1167, 1153, 1047, 9470, 794, 603; HR Q-TOF MS, m/z 249.03 (M + 1) (calcd for C₁₀H₁₆O₂Se 248.03); analysis: calcd for C₁₀H₁₆O₂Se C, 48.59; H, 6.52; found C, 49.09; H, 6.25%.

Synthesis of dipyrranes (4a–h). Borontrifluoride-etherate (0.3 mmol) was added to solution of diols (3a–h) (0.6 mmol) in degassed pyrrole (6 mL), and the resulting mixture was stirred for 1 h under argon. The reaction was monitored by TLC. After completion of the reaction, it was diluted with CH₂Cl₂ (30 mL) and 40% NaOH (25 mL). The organic layer was separated and washed with water (3 × 100 mL) and brine (100 mL). It was dried over MgSO₄ and concentrated under reduced pressure. The product was purified over silica gel (70 : 30 hexanes–EtOAc) to give (89–94%) of (4a–h) as a light brown solid.

2,5-Bis(2-pyrrolylmethyl)tellurophene (4a). White solid (75%), mp 98 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.95 (br, 2H, NH), 7.20 (s, 2H, tellurophene), 6.66 (m, 2H, pyrrol), 6.11–6.03 (m, 4H, pyrrol), 4.11 (s, 4H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 163.1, 138.3, 129.7, 116.91, 108.7, 103.35, 31.9; IR (KBr) ν_max 3367, 2831, 1499, 1379, 1360, 1233, 1236, 991, 1031, 1010, 98,1 807, 723; HR Q-TOF MS, m/z 341.11 (M + 1) (calcd for C₁₄H₁₄N₂Te 340.02); analysis: calcd for C₁₄H₁₄N₂Te C, 49.77; H, 4.18; N, 8.29; found C, 49.81; H, 4.15; N, 8.69%.

2,5-Bis(2-pyrrolylmethyl)selenophene (4b). White solid (73%), mp 56 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.93 (br, 2H, NH), 6.81 (s, 2H, selenophene), 6.67–6.65 (m, 2H, pyrrol), 6.11–6.03 (m, 4H, pyrrol), 4.12 (s, 4H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 148.79, 130.1, 126.8, 117.1, 108.4, 106.2, 31.5; HR Q-TOF MS, m/z 291.03 (M + 1) (calcd for C₁₄H₁₄N₂Se 290.03); analysis: calcd for C₁₄H₁₄N₂Se C, 58.14; H, 4.88; N, 9.69; found C, 58.52; H, 4.51; N, 9.38%.

2,5-Bis(2-pyrrolyl(methyl)methyl)tellurophene (4c). Brown oil (91%); ¹H NMR (CDCl₃, 400 MHz): δ = 7.86 (br, 2H, NH), 7.15 (m, 2H, tellurophene), 6.58–6.57 (m, 2H, pyrrol), 6.06–6.03 (m, 2H, pyrrol), 5.95 (m, 2H, pyrrol), 4.13–4.08 (m, 2H, CH), 1.54–1.51 (m, 6H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 162.41, 140.73, 131.27, 116.91, 107.71, 103.35, 42.04, 31.92; HR Q-TOF MS, m/z 367.10 (M + 1) (calcd for C₁₆H₁₈N₂Te 368.05); analysis: calcd for C₁₆H₁₈N₂Te C, 52.52; H, 4.96; N, 7.66; N, 7.11; found C, 52.86; H, 4.91; N, 7.21%.

2,5-Bis(2-pyrrolyl(methyl)methyl)selenophene (4d). Light brown oil (88%); ¹H NMR (CDCl₃, 400 MHz): δ = 7.86 (br, 2H, NH), 7.15 (m, 2H, tellurophene), 6.58–6.57 (m, 2H, pyrrol), 6.06–6.03(m, 2H, pyrrol), 5.95 (m, 2H, pyrrol), 4.13–4.08 (m, 2H, CH), 1.54–1.51(m, 6H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 171.2, 155.6, 135.2, 125.0, 117.6, 116.8, 108.0, 104.3, 60.3, 36.4, 23.14, 21.0, 14.1; HR Q-TOF MS, m/z 319.32 (M + 1) (calcd for C₁₆H₁₈N₂Se 318.06); analysis: calcd for C₁₆H₁₈N₂Se C, 60.57; H, 5.72; N, 8.83; found C, 59.91; H, 5.12; N, 8.53%.

2,5-Bis(2-pyrrolyl(phenyl)methyl)tellurophene (4e). Light brown oil (85%); ¹H NMR (CDCl₃, 400 MHz): δ = 7.95 (br, 2H, NH), 7.30 (m, 10H, Ar-H), 7.24 (s, 2H, tellurophene), 6.65 (br, 2H, pyrrol), 6.10 (m, 2H, pyrrol), 5.96 (m, 2H, pyrrol), 5.46 (m, 2H, CH); ¹³C NMR (CDCl₃, 75 MHz): δ = 154.3, 144.5, 134.8, 134.4, 138.6, 128.2, 127.0, 117.0, 108.3, 107.1, 50.9; HR Q-TOF MS, m/z 493.26 (M + 1) (calcd for C₂₆H₂₂N₂Te 492.08); analysis: calcd for C₂₆H₂₂N₂Te C, 63.72; H, 4.52; N, 5.72; found C, 63.92; H, 4.65; N, 5.42%.

2,5-Bis(2-pyrrolyl(phenyl)methyl)selenophene (4f). Brown oil (83%); ¹H NMR (CDCl₃, 400 MHz): δ = 7.95 (br, 2H, NH), 7.30 (m, 10H, Ar-H), 7.24 (s, 2H, selenophene), 6.65 (br, 2H, pyrrol), 6.10 (m, 2H, pyrrol), 5.96 (m, 2H, pyrrol), 5.46 (m, 2H, CH); ¹³C NMR (CDCl₃, 75 MHz): δ = 156.2, 143.8, 133.9, 134.8, 139.1, 127.5, 126.9, 117.5, 107.8, 107.3, 50.5; HR Q-TOF MS, m/z 443.26 (M + 1) (calcd for C₂₆H₂₂N₂Se 442.09); analysis: calcd for C₂₆H₂₂N₂Se C, 70.74; H, 5.02; N, 6.35; found C, 70.36; H, 5.13; N, 6.65%.

2,5-Bis(2-pyrrolyl(dimethyl)methyl)tellurophene (4g). White solid (93%), mp 148 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.93 (br, 2H, NH), 7.10 (s, 2H, tellurophene), 6.65 (m, 2H, pyrrol), 6.10–6.03 (m, 4H, pyrrol), 1.66 (s, 12H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 162.41, 140.73, 131.27, 116.91, 107.71, 103.35, 42.04, 31.92; IR (KBr) ν_max 3367, 2964, 2861, 1559, 1381, 1361, 1254, 1236, 1112, 1037, 1010, 965, 807, 723, 581; HR Q-TOF MS, m/z 397.08 (M + 1) (calcd for C₁₈H₂₂N₂Te 396.08); analysis: calcd for C₁₈H₂₂N₂Te C, 54.87; H, 5.63; N, 7.11; found C, 54.30; H, 5.93; N, 7.41%.

2,5-Bis(2-pyrrolyl(dimethyl)methyl)selenophene (4h). White solid (91%), mp 97 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.91 (br, 2H, NH), 6.71 (s, 2H, selenophene), 6.65 (m, 2H, pyrrol), 6.12–6.05 (m, 4H, pyrrol), 1.69 (s, 12H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 160.94, 139.68, 124.10, 116.78, 107.72, 103.54, 39.68, 31.61; IR (KBr) ν_max 3372, 3099, 2967, 1559, 1439, 1383, 1312, 1296, 1231, 1112, 1036, 949, 803, 722, 583; HR Q-TOF MS, m/z 347.11 (M + 1) (calcd for C₁₈H₂₂N₂Se 346.09); analysis: calcd for C₁₈H₂₂N₂Se C, 62.60; H, 6.42; N, 8.11; found C, 62.56; H, 6.22; N, 8.51%.

Synthesis of porphyrinogens (5a–i)

Synthesis of porphyrinogens 5a–d (method A). To a degassed solution of dipyrranes (4a–d) (2 mmol) in CH₂Cl₂ (200 mL), the methanolic solution (5 mL) of corresponding diols (3a–d) (2 mmol) were added. The reaction was carried out at 0 °C in a dark room under argon. After 20 min, 20% BF₃·CH₃OH solution (0.4 mmol) was added with a micropipette and the reaction mixture was stirred for 1 h. Further stirring for 1 h was continued at room temperature. After completion of the reaction, the solvent was evaporated under reduced pressure to give viscous colorless oil. The viscous oil was purified by column chromatography on silica gel using CH₂Cl₂ as an eluent to afford a colorless solid 5a–d (15–27%).

Synthesis of porphyrinogens 5g–i (method B). To a degassed solution of 4g–h (0.5 mmol) and corresponding diol 3g–h (0.50 mmol) in CH₂Cl₂ (250 mL), BF₃·OEt₂ (0.091 mmol) was added and the reaction mixture was stirred for 1 h at room temperature. The reaction was monitored by TLC. After completion of the reaction, a saturated aqueous Na₂CO₃ solution (200 mL) was poured into the reaction mixture. The organic phase was separated and washed with aqueous Na₂CO₃ (3 × 100 mL) and brine (200 mL). It was dried over Na₂SO₄ and evaporated under reduced pressure. The residue was subjected to silica gel column chromatography (hexane–CH₂Cl₂ = 3/2). The (R_f 0.5) was collected and evaporated to afford 5g–i as a white solid (39–43%).

5,10,15,20-Meso-octahydro-21,23-ditelluraporphyrinogen (5a). White solid (15%); mp 171 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 7.34 (br, 2H, NH), 7.14 (s, 4H, tellurophene), 5.71–5.55 (d, J = 2.2 Hz, 4H, pyrrol), 3.92 (s, 8H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 150.6, 130.9, 130.0, 105.2, 31.5; ¹²⁵Te NMR (CDCl₃): δ = 758.91 (s); IR (KBr) ν_max 3412, 2991, 2923, 2841, 1598, 1481, 1451, 1416, 1267, 1222, 1101, 1045, 987, 784, 771, 720; HR Q-TOF MS, m/z 546.961 (M + 1) (calcd for C₂₀H₁₈N₂Te₂ 545.959); analysis: calcd for C₂₀H₁₈N₂Te₂ C, 44.36; H, 3.35; N, 5.17; found: C, 44.56; H, 3.41; N, 5.19%.

5,10,15,20-Meso-octahydro-21,23-diselenaporphyrinogen (5b). Off white solid (11%); mp 162 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 7.38 (br, 2H, NH), 6.82 (s, 4H, selenophene), 5.87–5.86 (d, J = 2.2 Hz, 4H, pyrrol), 4.04 (s, 8H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 150.4, 130.8, 126.5, 105.6, 31.6; ⁷⁷Se NMR (CDCl₃): δ = 601.02 (s); IR (KBr) ν_max 3444, 2915, 1642, 1420, 1381, 1230, 1049, 799, 741, 715, 581; HR Q-TOF MS, m/z 446.981 (M + 1) (calcd for C₂₀H₁₈N₂Se₂ 445.980); analysis: calcd for C₂₀H₁₈N₂Se₂ C, 54.07; H, 4.08; N, 6.31; found: 53.93; H, 4.09; N, 6.22%.

5,10,15,20-Meso-tetramethyl-21,23-ditelluraporphyrinogen (5c). Light brown solid (27%), mp 130 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 7.33 (br, 2H, NH, stereoisomer A), 7.35 (br, 2H, NH, stereoisomer B), 7.38 (br, 2H, NH, stereoisomer C), 7.24 (s, 4H, tellurophene, stereoisomer A), 7.25 (s, 4H, tellurophene, stereoisomer B), 7.29 (s, 4H, tellurophene, stereoisomer C), 5.83 (s, 4H, pyrrol, stereoisomer A), 5.86 (s, 4H, pyrrol, stereoisomer B), 5.87 (s, 4H, pyrrol, stereoisomer C), 4.02–3.97 (m, 4H, CH, stereoisomer A), 4.14–4.11 (m, 4H, CH, stereoisomer B), 4.14–4.11 (m, 4H, CH, stereoisomer C), 1.54 (s, 12H, CH₃, stereoisomer A), 1.56–1.58 (s, 12H, CH₃, stereoisomer B), 1.58 (s, 12H, CH₃, stereoisomer C); ¹³C NMR (CDCl₃, 75 MHz): δ = 158.7, 158.6, 157.1, 156.7, 154.7, 137.3, 137.2, 137.1, 131.8, 131.7, 131.6, 102.5, 102.4, 102.3, 40.5, 40.3, 39.9, 23.6, 23.5, 23.4; ¹²⁵Te NMR (CDCl₃): δ = 755.08 (s); HR Q-TOF MS, m/z 603.025 (M + 1) (calcd for C₂₄H₂₆N₂Te₂ 602.022); analysis: calcd for C₂₄H₂₆N₂Te₂ C, 48.23; H, 4.38; N, 4.69; found: C, 48.44; H, 4.59; N, 4.93%.

5,10,15,20-Meso-tetramethyl-21,23-diselenaporphyrinogen (5d). Off white solid (22%), mp 186 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 7.26 (br, 2H, NH, stereoisomer A), 7.29 (br, 2H, NH, stereoisomer B), 7.29 (br, 2H, NH, stereoisomer C), 6.82 (s, 4H, selenophene, stereoisomer A), 6.85–6.84 (m, 4H, selenophene, stereoisomer B), 6.86–6.87 (m, 4H, selenophene, stereoisomer C), 5.86 (m, 4H, pyrrol, stereoisomer A), 5.87 (m, 4H, pyrrol, stereoisomer B), 5.89 (m, 4H, pyrrol, stereoisomer C), 4.17 (m, 4H, CH, stereoisomer A), 4.19 (m, 4H, CH, stereoisomer B), 4.20 (m, 4H, CH, stereoisomer C), 1.54 (s, 12H, CH₃, stereoisomer A), 1.55 (s, 12H, CH₃, stereoisomer B), 1.57 (s, 12H, CH₃, stereoisomer C); ¹³C NMR (CDCl₃, 75 MHz): δ = 157.5, 157.3, 156.4, 155.3, 136.2, 136.1, 124.6, 124.4, 103.6, 103.2, 103.1, 37.0, 36.9, 36.5, 22.5, 22.3, 22.2; ⁷⁷Se NMR (CDCl₃): δ = 596.16 (s); HR Q-TOF MS, m/z 503.045 (M + 1) (calcd for C₂₄H₂₆N₂Se₂ 502.042); analysis: calcd for C₂₄H₂₆N₂Se₂ C, 57.61; H, 5.24; N, 5.60; found: C, 57.81; H, 5.32; N, 5.55%.

5,10,15,20-Meso-octamethyl-21,23-ditelluraporphyrinogen (5g). White solid (41%), mp 226 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.27 (br, 2H, NH), 7.24 (s, 4H, tellurophene), 5.81–5.80 (d, J = 2.4 Hz, 4H, pyrrol), 1.63 (s, 24H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 163.64, 141.98, 130.02, 101.00, 42.22, 30.79; ¹²⁵Te NMR (CDCl₃): δ = 754.88 (s); IR (KBr) ν_max 3439, 2960, 2923, 2860, 1640, 1495, 1458, 1413, 1247, 1229, 1104, 1039, 997, 794, 769, 722, 700, 587, 520; HR Q-TOF MS, m/z 659.0894 (M + 1) (calcd for C₂₈H₃₄N₂Te₂ 658.084); analysis: calcd for C₂₈H₃₄N₂Te₂ C, 51.44; H, 5.24; N, 4.28; found: C, 51.41; H, 5.19; N, 4.23%.

Synthesis of N-confused cyclic tetramer (6g), cyclic hexamer (7g) and cyclic octamer (8g). The solution of the 2,5-bis(hydroxymethylethyl)tellurophene (3g) (113 mg, 0.381 mmol) and 2,5-bis(pyrrolylmethylethyl)tellurophene (4g) (151 mg, 0.381 mmol) in acetonitrile (20 mL, 40 mM concentration) was stirred under N₂ at 0 °C for 15 min and then BF₃·OEt₂ (0.076 mmol) was added. The reaction was monitored by TLC. After 30 min, it was diluted with CH₂Cl₂ (100 mL) and washed with aqueous NaOH (0.1 N, 10 mL) and water (2 × 10 mL). The organic layer was dried over NaHCO₃ and concentrated under reduced pressure. The products were separated on column chromatography (hexane–CH₂Cl₂ 1 : 1) to afford two products, (5g) and (6g). N-confused cyclic tetramer (6g): white solid 7% yield, mp 229 °C (decompose); R_f: 0.81 (hexane–CH₂Cl₂ 1 : 1); ¹H NMR (CDCl₃, 400 MHz): δ = 7.69 (br, 1H, NH), 7.49 (br, 1H, NH), 7.20–7.16 (m, 4H, tellurophene), 6.37 (s, 1H, pyrrol), 5.78 (m, 2H, pyrrol), 5.61 (s, 1H, pyrrol), 1.63–1.61 (m, 24H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 163.6, 161.9, 141.9, 139.6, 131.8, 130.0, 103.0, 101.0, 42.2, 32.5, 30.8, 29.6; ¹²⁵Te NMR (CDCl₃): δ = 753.11 (s); IR (KBr) ν_max 3436, 3331 2959, 2928, 2854, 1638, 1490, 1449, 1420, 1254, 1221, 1121, 1038, 996, 780, 760, 714, 701, 583, 521; HR Q-TOF MS, m/z 659.114 (M + 1) (calcd for C₂₈H₃₄N₂Te₂ 658.0846); analysis: calcd for C₂₈H₃₄N₂Te₂ C, 51.44; H, 5.24; N, 4.28; found: C, 51.41; H, 5.19; N, 4.23. Hexamer (7g): white solid 15% yield, mp 231 °C (decompose); R_f: 0.65 (hexane–CH₂Cl₂ 1 : 1); ¹H NMR (CDCl₃, 400 MHz): δ = 7.69 (br, NH), 6.95 (s, 6H, tellurophene), 5.94–5.93 (d, J = 2.24 Hz, 6H, pyrrol), 1.59 (s, 36H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 162.8, 141.5, 129.4, 99.8, 42.8, 31.5; ¹²⁵Te NMR (CDCl₃): δ = 754.65 (s); IR (KBr) ν_max 3436, 2954, 2914, 2878, 1649, 1485, 1428, 1421, 1230, 1228, 1104, 1025; HR Q-TOF MS, m/z 987.11 (M⁺) (calcd for C₄₂H₅₁N₃Te₃ 987.12); analysis: calcd for C₄₂H₅₁N₃Te₃ C, 51.44; H, 5.24; N, 4.28; found: C, 51.33; H, 5.19; N, 4.52%. Octamer (8g): white solid 8% yield, mp 223 °C (decompose); R_f: 0.54 (hexane–CH₂Cl₂ 1 : 1); ¹H NMR (CDCl₃, 400 MHz): δ = 7.64 (br, NH), 6.98 (s, 8H, tellurophene), 5.88 (d, J = 2.9, 8H, pyrrol), 1.57 (s, 48H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 163.0, 142.4, 129.6, 101.0, 42.5, 30.1; ¹²⁵Te NMR (CDCl₃): δ = 754.11 (s); IR (KBr) ν_max 3435, 2944, 2985, 2756, 1652, 1478, 1436, 1391, 1241, 1254, 1110, 995, 780; HR Q-TOF MS, m/z 1316.32 (M⁺) (calcd for C₅₆H₆₈N₄Te₄ 1316.169); analysis: calcd for C₅₆H₆₈N₄Te₄ C, 51.44; H, 5.24; N, 4.28; found: C, 51.61; H, 5.29; N, 4.46%.

5,10,15,20-Meso-octamethyl-21,23-diselenaporphyrinogen (5h). White solid (39%); mp 198 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.19 (br, 2H, NH), 6.83 (s, 4H, selenophene), 5.85–5.84 (d, J = 2.4 Hz, 4H, pyrrol), 1.61 (s, 24H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 162.64, 143.89, 131.02, 102.00, 41.22, 32.79; IR (KBr) ν_max 3444, 2967, 2925, 1630, 1412, 1359, 1235, 1041, 798, 762, 725, 596; ⁷⁷Se NMR (CDCl₃): δ = 593.75 (s); HR Q-TOF MS, m/z 559.108 (M + 1) (calcd for C₂₈H₃₄N₂Se₂ 558.105); analysis: calcd for C₂₈H₃₄N₂Se₂ C, 60.43; H, 6.16; N, 5.03; found: C, 60.51; H, 6.29; N, 5.01%.

5,10,15,20-Meso-octamethyl-21-tellura-23-selenaporphyrinogen (5i). Off white solid (43%); mp 241 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.27 (br, 2H, NH), 7.24 (s, 2H, tellurophene), 6.91 (s, 2H, selenophene) 5.89–5.85 (dt, 4H, pyrrol), 1.687–1.671 (d, J = 6.4, 24H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 164.34, 161.73, 142.32, 140.82, 129.47, 122.80, 101.59, 101.10, 42.20, 39.66 30.73, 30.35; ¹²⁵Te NMR (CDCl₃): δ = 753.96 (s); ⁷⁷Se NMR (CDCl₃): δ = 594.02 (s); IR (KBr) ν_max 3447, 2927, 2865, 1607, 1431, 1359, 1233, 1040, 799, 763, 593, 512; HR Q-TOF MS, m/z 609.093 (M + 1) (calcd for C₂₈H₃₄N₂SeTe 608.09); analysis: calcd for C₂₈H₃₄N₂SeTe C, 55.57; H, 5.66; N, 4.63; found: C, 55.68; H, 5.71; N, 4.49%.

5,10,15,20-Meso-octahydro-21-telluraporphyrinogen (9a). Synthesized by method A; white solid (7%); mp 158 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 7.88 (br, 2H, NH), 7.58 (br, 1H, NH), 7.18 (s, 2H, tellurophene), 5.95 (s, 2H, pyrrol), 5.84–5.77 (m, 4H, pyrrol), 3.99 (s, 4H, CH₂), 3.86 (s, 4H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 150.5, 130.8, 130.1, 129.9, 105.6, 104.9, 103.9, 31.8, 30.6; ¹²⁵Te NMR (CDCl₃): δ = 761.33 (s); IR (KBr) ν_max 3351, 3329, 1589, 1188, 1129, 1078, 1026, 1007, 768, 765, 759, 748, 706, 569; HR Q-TOF MS, m/z 431.081 (M⁺) (calcd for C₂₀H₁₉N₃Te 431.064); analysis: calcd for C₂₀H₁₉N₃Te C, 56.00; H, 4.46; N, 9.80; found: C, 56.11; H, 4.35; N, 9.73%.

5,10,15,20-Meso-octahydro-21-telluraporphyrinogen (N-confused) (9b). Synthesized by method A; white solid (8%); mp 161 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 7.79 (br, 1H, NH), 7.60 (br, 1H, NH), 7.41 (br, 1H NH), 7.09–7.08 (m, 1H, tellurophene), 7.00–6.98 (m, 1H, tellurophene), 6.46 (m, 1H, pyrrol), 5.90–5.78 (m, 5H, pyrrol), 3.87 (s, 2H, CH₂), 3.86 (s, 2H, CH₂), 3.78 (s, 2H, CH₂), 3.67 (s, 2H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 150.7, 150.5, 131.9, 131.2, 130.9, 129.5, 112.0, 105.0, 104.2104.1, 103.5, 99.2, 31.6, 30.8, 30.0; ¹²⁵Te NMR (CDCl₃): δ = 759.21 (s); IR (KBr) ν_max 3347, 3336, 3128, 3108, 1573, 1181, 1125, 1089, 1018, 1008, 799, 764, 751, 742, 711, 558; HR Q-TOF MS, m/z 431.061 (M⁺) (calcd for C₂₀H₁₉N₃Te 431.064); analysis: calcd for C₂₀H₁₉N₃Te C, 56.00; H, 4.46; N, 9.80; found: C, 56.15; H, 4.43; N, 9.91%.

5,10,15,20-Meso-octahydro-21-selenaporphyrinogen (10a). Synthesized by method A; white solid (11%), mp 153 °C; ¹H NMR (CDCl₃, 400 MHz): δ = 7.88 (br, 2H, NH), 7.54 (br, 1H, NH), 6.74 (s, 2H, selenophene), 5.84–5.81 (m, 6H, pyrrol), 4.07 (s, 4H, CH₂), 4.92 (s, 4H, CH₂); ¹³C NMR (CDCl₃, 75 MHz): δ = 150.2, 131.6, 130.9, 130.4, 126.3, 105.5, 104.8, 103.1, 31.7, 30.0; ⁷⁷Se NMR (CDCl₃): δ = 603.14 (s); IR (KBr) ν_max 3341, 3333, 3118, 1581, 1192, 1131, 1092, 1020, 1003, 791, 763, 758, 748, 710, 561; HR Q-TOF MS, m/z 381.011 (M⁺) (calcd for C₂₀H₁₉N₃Se 381.072); analysis: calcd for C₂₀H₁₉N₃Se C, 63.16; H, 5.04; N, 11.05; found: C, 63.21; H, 4.96; N, 11.01%.

21,23-Diselenaporphine. To a degassed solution of 5,10,15,20-meso-octahydro-21,23-diselenaporphyrinogen (5b) (1 mmol) in CHCl₃ (10 mL), 0.1% aqueous FeCl₃ solution was added at 0 °C in dark under argon and stirred for 10 min. The organic solvent was separated and evaporated under reduced pressure. The crude was purified by column chromatography on silica gel using CH₂Cl₂ as an eluent to afford light brown solid of 21,23-diselenaporphine; mp 68 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 10.49 (s, 4H, meso-CH) 10.29 (s, 4H, selenophene), 9.36 (s, 4H, pyrrol); ¹³C NMR (CDCl₃, 75 MHz): δ = 182.7, 158.7, 152.6, 144.8, 108.6; HR Q-TOF MS, m/z 440.976 (M + 1) (calcd for C₂₀H₁₂N₂Se₂ 439.933).

5,10,15,20-Meso-tetramethyl-21,23-diselenaporphyrin. To a degassed solution of 5,10,15,20-meso-tetramethyl-21,23-diselenaporphyrinogen (5d) (1 mmol) in CHCl₃ (10 mL), the p-chloranil (2 mmol) was added at 0 °C in a dark room under argon. After 10 min, the solvent was evaporated under reduced pressure to give brown solid. The crude was purified by column chromatography on silica gel using CH₂Cl₂ as an eluent to afford a orange solid of 5,10,15,20-meso-tetramethyl-21,23-diselenaporphine; mp 173 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 10.15 (s, 4H, selenophene), 9.26 (s, 4H, pyrrol), 4.52 (s, 12H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 181.6, 155.7, 152.6, 146.4, 134.2, 31.5; ⁷⁷Se NMR (CDCl₃): δ = 628.81 (s); HR Q-TOF MS, m/z 497.040 (M + 1) (calcd for C₂₄H₂₀N₂Se₂ 495.996).

Meso-tetramethyldiselenachlorin and meso-tetramethyldiselenabacteriochlorin. The 5,10,15,20-meso-tetramethyl-21,23-diselenaporphyrin (1 mmol) and anhydrous potassium carbonate (10 mmol) were dissolved in dry pyridine and p-toluenesulfonhydrazide (2 mmol) in dry pyridine was added. The mixture was heated at 100 °C under argon for 24 hours. Further quantities of p-toluenesulfonhydrazide (2 mmol in dry pyridine) were added after 2, 4, 6 and 8 hours. After cooling, the mixture was treated with ethyl acetate–water (2 : 1) and heated at 100 °C for 5 min. After cooling, the organic phase was separated and washed twice with aqueous HCl (2 M), twice with water then saturated aqueous sodium hydrogen carbonate solution. Organic layer was concentrated under vacuum. The crude product was chromatograph on silica gel eluted with a mixture of ethylacetate–hexane (0.1 : 9). Meso-tetramethyldiselenachlorin: R_f: 0.25 (ETOAc–hexane 0.1 : 9); brown-orange solid (5%), mp 48 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 9.64–9.63 (d, J = 4.8, 2H, selenophene), 9.27–9.26 (d, J = 4.8, 2H, selenophene), 9.07 (s, 2H, pyrrol), 4.61–4.58 (t, 2H, reduced pyrrol), 4.40 (s, 6H, CH₃), 4.31–4.27 (t, 2H, reduced pyrrol), 4.01 (s, 6H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 181.8, 180.7, 161.2, 158.5, 152.4, 143.5, 135.9, 134.7, 127.6, 73.1, 31.0, 29.5; HR Q-TOF MS, m/z 499.015 (M + 1) (calcd for C₂₄H₂₂N₂Se₂ 498.011). Meso-tetramethyldiselenabacteriochlorin: R_f: 0.20 (ETOAc–hexane 0.1 : 9); brown solid (2%), mp 46 °C (decompose); ¹H NMR (CDCl₃, 400 MHz): δ = 8.92 (s, 4H, selenophene), 4.20 (s, 8H, reduced pyrrol), 3.90 (s, 12H, CH₃); ¹³C NMR (CDCl₃, 75 MHz): δ = 179.6, 155.7, 143.5128.7, 69.2, 28.1; HR Q-TOF MS, m/z 501.045 (M + 1) (calcd for C₂₄H₂₄N₂Se₂ 500.027).

X-ray crystal structure of 5,10,15,20-meso-octamethyl-21,23-ditelluraporphyrinogen (5g). Crystal was grown as colorless prism by slow evaporation of a benzene solution of 5g layered with hexane. C₂₈H₃₄N₂Te₂, M = 658.08 crystal system = triclinic; space group = P1̄; unit cell dimensions a = 10.361(2) Å, b = 11.5721(3) Å, c = 11.8258(15) Å, alpha = 101.396(15)°, beta = 90.972(13)°, gamma = 104.51(2)°; Z = 2; calculated density = 1.613 mg m⁻³; goodness-of-fit on F² = 0.656; total of 13 052 reflections were measured, 6096 unique (R_int = 0.0748); final R indices R = 0.0713, R_w = 0.1865; F(000) = 636; the structure was refined on full-matrix least-squares on F². Complete crystallographic data are given in ESI.†

Acknowledgements

We thank the Department of Science and Technology and University of Delhi, Delhi for financial assistance. S. Ahmad and K. K. Yadav are thankful to UGC, New Delhi for senior research fellowship and junior research fellowship. S. J. Singh is thankful to UGC, New Delhi for Dr D. S. Kothari Post-doctoral fellowship.

Notes and references

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19. ESI.†.
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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra of all new compounds. ¹²⁵Te and ⁷⁷Se NMR spectra of all porphyrinogens. UV-Vis spectra of porphyrinogens in the presence of Hg²⁺. ¹²⁵Te, ⁷⁷Se NMR of 5g and 5h in the presence of Hg²⁺. CCDC 932049. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra46491a

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