Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Tellurabenziporphyrin and Its Pd(II) Complex Sunit Kumar,† Way-Zen Lee,‡ and Mangalampalli Ravikanth*,† †
Indian Institute of Technology, Powai, Mumbai, 400076, India Instrumentation Center, Department of Chemistry, National Taiwan Normal University, 88 Section 4 Ting-Chow Road, Taipei 11677, Taiwan
‡
S Supporting Information *
ABSTRACT: An unprecedented tellurabenziporphyrin containing C, N, and Te donor atoms was synthesized by condensing benzitripyrrane and tellurophene diol under acid catalyzed conditions. The tellurabenziporphyrin readily forms a Pd(II) complex when treated with PdCl2 in CHCl3/CH3CN. The crystal structures of tellurabenziporphyrin and its Pd(II) complex revealed that the benzene ring hinders the π-electron delocalization. An unusual five-membered ring formed inside the macrocycle due to the strong interaction between “Te” and “N” in the Pd(II) complex.
B
which one of the pyrrole rings was replaced by either thiophene IIIa and furan IIIb and characterized these macrocycles by detailed NMR spectroscopy studies.9,10 Subsequently, the same group also reported the synthesis of dimethoxythiabenziporphyrins in which two methoxy groups were present on the benzene ring and the X-ray structure revealed that the macrocycle was highly nonplanar. The authors also reported that the dimethoxythiabenziporphyrin upon reaction with Pd(II) salts underwent demethylation and formed a Pd(II) complex of thiaoxybenziporphyrin IV.10 (Figure 1) Thus, the limited study suggests that the heterobenziporphyrins are indeed interesting macrocycles for coordination chemistry. Tellurium-containing porphyrinoids such as 5,10,15,20tetraphenyl-21-telluraporphyrin have unusual properties and reactivity when compared to their lighter chalcogen analogues.11 For example, it was shown that the inner tellurium atom can be removed from the parent 21-telluraporphyrin without changing
enziporphyrins are benzene containing porphyrinoid systems where benzene and three pyrrole rings are connected via four meso carbons within a conjugated framework.1 The benziporphyrins are either m-benziporphyrins I, which possess a 1,3-phenylene unit,1,2 or p-benziporphyrins II, which possess a 1,4-phenylene unit.3 Benziporphyrinoids are, in general, nonaromatic in nature, but it is possible to change these macrocycles to aromatic by introducing suitable substituent(s) on the benzene moiety or by protonating benziporphyrins with mild acids.4,5 For example, the protonated benziporphyrins exhibit slightly more diatropic character than benziporphyrins, but the presence of methoxy groups on the benzene ring significantly enhances the diatropic character of protonated benziporphyrins.4 However, the introduction of a hydroxy substituent at the 2-position of benzene triggers the keto−enol tautomerization to form a fully aromatic system known as oxybenziporphyrins.5 In addition to the above-mentioned substituted benziporphyrinoids, further modified highly diatropic benziporphyrinoid systems have also been available in the literature.6 The variations in aromatic character are insightful and provide a deeper understanding of aromaticity and conjugation in benziporphyrins. Furthermore, the benziporphyrins are found to be very interesting ligands to form stable organometallic derivatives.7 Benziporphyrins have been shown to have applications in the development of chemical sensors and in molecular recognition studies.8 Heterobenziporphyrins are a subclass of benziporphyrins in which one of the pyrrole rings is replaced by other heterocycles, such as thiophene, furan, selenophene, tellurophene, etc.1,8,9 The introduction of other heterocycles in the place of pyrrole is expected to alter the electronic properties of benziporphyrins significantly. Modification of the inner core also introduces new donor coordinating atoms, and thus the metal coordinating chemistry of such modified benziporphyrins will be worth exploring. A perusal of the literature revealed that there are only a few recent reports on heterobenziporphyrins by Lash and coworkers who reported the synthesis of heterobenziporphyrins in © XXXX American Chemical Society
Figure 1. Structures of benziporphyrinoids. Received: December 5, 2017
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DOI: 10.1021/acs.orglett.7b03715 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters the integrity of the porphyrin skeleton. The extrusion of the tellurium atom from 21-telluraporphyrin under oxidative conditions afforded 21-oxaporphyrin,12 whereas, in a strongly acidic solution, an unusual vacataporphyrin was formed which has a vacant space in place of a bridging tellurium atom and two hydrogen atoms substituting for one tellurium atom.13 Although the tellurium-containing porphyrinoids showed unique features, the reports on tellurium-containing porphyrinoids are limited because of inaccessibility of starting precursors, synthetic procedures, and their instability toward air and light. In this paper, we report the synthesis of first example of benziporphyrin containing tellurophene ring 1 (tellurabenziporphyrin) and its unusual Pd(II) complex 1-Pd(II) using readily available precursors. The tellurabenziporphyrin 1 was synthesized as shown in Scheme 1. The desired diol, 2,5-bis(hydroxymethyl)tellurophene Scheme 1. Synthesis of Tellurabenziporphyrin 1
Figure 2. 1H−1H COSY NMR spectrum of tellurabenziporphyrin 1 recorded in CDCl3.
The structure of tellurabenziporphyrin 1 was obtained by X-ray crystallography, and the crystal structure is presented in Figure 3.
214 and benzitripyrrane 310a were synthesized by following the reported procedures. The diol 2 and tripyrrane 3 were characterized by mass and NMR techniques, and the data were in agreement with the reported data. The tellurabenziporphyrin 1 was synthesized by condensing 1 equiv of tellurophene diol 2 with 1 equiv of benzitripyrrane 3 in CH2Cl2 by using a catalytic amount of boron trifluoride diethyl etherate (0.1 equiv) for 1 h under an inert atmosphere followed by oxidation with DDQ in open air for an additional 30 min. The TLC analysis showed one major spot corresponding to the desired benziporphyrin along with two unidentified minor spots. The crude compound was subjected to basic alumina column chromatographic purification and afforded the pure compound 1 as a green solid in 13% yield (Scheme 1). Tellurabenziporphyrin 1 was freely soluble in a wide range of organic solvents, and its identity was confirmed by a molecular ion peak at 741.1538 [M + H]+ in HR-MS. Compound 1 was thoroughly characterized by 1D, 2D NMR, and X-ray crystallography. The 1H−1H COSY NMR spectrum of compound 1 recorded in CDCl3 is presented in Figure 2. The four pyrrole protons appeared as two doublets at 6.76 and 7.49 ppm, and these resonances showed cross-peak correlation in the COSY spectrum. The inner aryl −CH proton (type c) and the two tellurophenyl protons along with meta and para meso-phenyl protons appeared as a multiplet in the region 7.30−7.45 ppm. The outer aryl ring type a and type b protons showed cross peak correlation in the COSY spectrum and appeared at 7.25 and 7.54 ppm, respectively. The ortho protons of the meso-phenyl groups appeared as a multiplet in the 7.50−7.65 ppm region. Thus, the NMR spectrum of compound 1 is simple and all resonances were easily identified. The NMR spectrum also indicated that compound 1 showed the absence of a global diatropic ring current confirming that the macrocycle is not aromatic.
Figure 3. Single crystal X-ray structure of tellurabenziporphyrin 1 with top (a) and side view (b). For clarity, all of the H-atoms, solvent molecule, and phenyl ring have been omitted.
The important crystallographic data are enlisted in Table S1. Compound 1 crystallizes in the P1̅ space group of the triclinic system, containing two independent molecules in a unit cell. The structural analysis of compound 1 indicates that it exhibits similar structural conformation as that of IIIa9b with a relatively flat macrocycle containing a moderately tilted m-phenylene ring making room for the meso-phenyl groups, which are almost coplanar with the macrocycle. In the crystal structure, the mphenylene group is deviated by an angle of 79.77° from the common plane defined by N1, N2, and Te1 atoms. This dihedral angle was identical to the corresponding angle in thiabenziporphyrin (IIIa).9b The C−C bond lengths within the m-phenylene moiety exhibits characteristic benzene-like behavior (C−C bond length varies from 1.371(9)Å to 1.384(7) Å with a slight deviation at the C28−C29 bond (1.416(7)Å) which specifically suggests more single-bond-like character and thus confirms the lack of conjugation. The rest of the molecule displays typically conjugated bond features within the macrocycle as confirmed from C−C and C−N bond lengths while the average C−Te bond length of 2.11 Å fairly matches with the C−Te single bond length in reported compounds.12,13 The meso-phenyl rings that were adjacent to the porphyrinato phenylene group have torsion angles of (C24C16C17C18) −142° and (C27C29C30C35) 134°, which were much higher than those of the phenyl rings on the tripyrrene unit (C1C40C41C42) −118° and (C4C5C6C11) B
DOI: 10.1021/acs.orglett.7b03715 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
an irregular fashion which induces strong distortion in the tellurabenziporphyrin macrocycle. One of the interesting features that was noticed is the close interaction between pyrrole N1 and tellurophene Te1 atoms with the bond distance of N1−Te1 being 2.510(4) Å. Thus, the very long N to Te distance (2.510(4) Å) found for 1-Pd(II) resembles the analogous distance of 2.661 Å observed for the reported Pd(II) complex of vacataporphyrin.15 Furthermore, such interactions were also noted earlier in other tellurium containing porphyrinoids.16 The strong interaction between inner core pyrrole N1 and tellurophene Te results in the formation of one five-membered ring inside the tellurabenziporphyrin macrocycle. The Pd(II) ion lies above the main plane defined by C5, C16, C29, and C40 meso-carbons. The binding of the Pd(II) atom affected the structure of the parent tellurabenziporphyrin. In the free base macrocycle 1, the benzene was tilted by an angle of 79.7° above the plane of the core which was decreased to 59.4° in 1-Pd(II). Furthermore, the pyrrole ring which was bonded to the Pd(II) ion was more deviated from the main plane compared to the other pyrrole which was in close interaction with tellurophene tellurium. The bond distances of Pd1−Cl1, Pd1−Cl2, Pd1− Te1, and Pd1−N2 were 2.2798(11), 2.3501(12), 2.051(4), and 2.4961(4) Å, respectively. Overall, the crystal structure shows that the coordination of Pd(II) with the macrocycle 1 altered the structural features of the tellurabenziporphyrin macrocycle. The 1-Pd(II) complex was further characterized by HR-MS, 1D and 2D NMR techniques. The molecular ion peak at 881.0179 [M − Cl]+ in HR-MS was in agreement with the structure of 1Pd(II). The 1H NMR spectrum of 1-Pd(II) recorded in CDCl3 at room temperature was broad. Hence, we recorded the 1H NMR spectra of 1-Pd(II) at different low temperatures, and a comparison of 1-Pd(II) recorded at 233 K with free base macrocycle 1 at room temperature is presented in Figure 5a. The unsymmetric nature of 1-Pd(II) was clearly evident in its 1H NMR spectrum which showed more resonances compared to macrocycle 1. In 1H NMR of 1-Pd(II), the four pyrrole protons are nonequivalent and appeared as four sets of resonances (type d, e, h, i); the two tellurophene protons are also nonequivalent and appeared as two doublets at 7.76 ppm (type g) and 7.90 ppm (type f), and the three aryl protons (type a, b and c) appeared as three sets of resonances. It is worth mentioning that the outer type b protons remain equivalent in both compounds 1 and 1-Pd(II). Furthermore, the pyrrolyl, tellurophenyl and outer aryl protons of 1-Pd(II) experienced downfield shifts whereas the aryl inner core CH proton experienced an upfield shift compared to tellurabenziporphyrin 1 which supports the presence of a recognizable diatropic ring current in 1-Pd(II). The comparison of absorption spectra of 1 and 1-Pd(II) recorded in chloroform is presented in Figure 5b. The tellurabenziporphyrin 1 showed one broad Soret type band centered at 372 nm and one broad Q-type like band at 668 nm, and these absorption features were in line with the reported thiabenziporphyrin. The 1-Pd(II) complex showed ill-defined broad absorption bands at 367, 418, and 629 nm. The electrochemical properties of 1 and 1-Pd(II) were investigated by cyclic voltammetry in CH2Cl2 containing TBAP (0.1 M) as the supporting electrolyte (see Supporting Information). Macrocycle 1 showed three quasi-reversible reductions at −0.80, −1.15, and −1.38 V but did not show any oxidations supporting the electrondeficient nature of macrocycle 1. In 1-Pd(II), the reductions were ill-defined but shifted to less negative reduction potentials indicating that 1-Pd(II) is more electron-deficient than tellurabenziporphyrin 1.
123°. These structural features show that the benzene ring incorporated in the framework of 1 completely blocks the macrocyclic delocalization while retaining its unperturbed aromaticity. Thus, the X-ray structure clearly reveals that the macrocycle 1 is nonaromatic in nature. Since tellurabenziporphyrin 1 contains three types of coordinating donor core atoms, N, C, and Te, we explored the coordination ability of the neutral macrocycle 1 with different types of metal salts under varied reaction conditions. Our attempts were successful in the synthesis of a novel Pd(II) complex of macrocycle 1. Macrocycle 1 was treated with PdCl2 in CHCl3/CH3CN (1:1) at room temperature for 2 h followed by simple silica gel column chromatographic purification which afforded 1-Pd(II) in 72% yield (Scheme 2). Fortunately, suitable Scheme 2. Synthesis of Pd(II)-Tellurabenziporphyrin 1Pd(II)
crystals of 1-Pd(II) were obtained for X-ray diffraction studies. The suitable crystals of 1-Pd(II) were obtained via slow evaporation of a hexane diffused chloroform solution of the compound at room temperature for over a period of 3 days. The molecular structure of 1-Pd(II) is shown in Figure 4, and the
Figure 4. Single crystal X-ray structure of macrocycles 1-Pd(II) with top (a) and side view (b). For clarity, all of the hydrogen atoms, solvent molecule, and phenyl ring have been omitted. Bond lengths (Å): Te1− C1 2.096(4), Te1−C4 2.137(5), Te1−N1 2.510(4), Te1−Pd1 2.4961(4), Pd1−N2 2.051(4), Pd1−Cl1 2.2798(11), Pd1−N2 2.351(12).
relevant crystallographic parameters are listed in Table S1. The compound crystallizes in a P1̅ space group with a triclinic crystal system. The unit cell contains two independent molecules along with three solvent molecules. The X-ray structure presented in Figure 4 shows that the palladium(II) was in a distorted square planar geometry. The distorted square planar geometry around the Pd(II) was due to the short distance between Te1 and N2 (2.834 Å), which was also evident from the small Te1−Pd1−N2 angle (76.43(10)°). In Pd(II) complex 1-Pd(II), the Pd(II) ion was bonded to one of the pyrrole nitrogens (N2) and tellurophene tellurium (Te1), and the other two coordinating sites were occupied by two chlorides (Cl1, Cl2) in a cis arrangement. Thus, the solid state structure shows that the metal was bonded with the two donor atoms of the macrocycle in C
DOI: 10.1021/acs.orglett.7b03715 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mangalampalli Ravikanth: 0000-0003-0193-6081 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.R. thanks the Department of Science and Technology, Govt. of India (File No. EMR/2015/002196 to M.R.), and S.K. thanks the UGC for a fellowship.
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Figure 5. (a) Comparison of 1H NMR spectra of macrocycle 1 at 300 K and 1-Pd(II) at 233 K recorded in CDCl3. (b) Comparison of normalized absorption spectra of compound 1 (black line) and compound 1-Pd(II) (red line) recorded in chloroform.
In summary, the first example of a tellurabenziporphyrin was obtained by condensing tellurophene diol with benzitripyrrane under mild acid catalyzed conditions. The X-ray structure revealed that the macrocycle is nonaromatic, and the benzene ring blocks the macrocyclic delocalization. The tellurabenziporphyrin is a neutral ligand and readily forms a Pd(II) complex in which Pd(II) is coordinated to one pyrrole nitrogen, and the tellurium of the tellurophene and two chloride ligands. Also, in the Pd(II) complex, the strong interaction between the tellurium and the adjacent pyrrole nitrogen led to formation of a five-membered ring inside the macrocycle. Both the free base tellurabenziporphyrin and its Pd(II) complex show ill-defined broad absorption bands in the region 300−800 nm, and these macrocycles were found to be electron-deficient in nature.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03715. Characterization data (including HRMS, 1H, and 13NMR spectra, crystal data) for all the reported compounds; absorption and crystallographic data (PDF) Accession Codes
CCDC 1588213−1588214 contain the supplementary crystallographic data for this paper. These data can be obtained free of D
DOI: 10.1021/acs.orglett.7b03715 Org. Lett. XXXX, XXX, XXX−XXX
