β-Functionalized Dibenzoporphyrins with Mixed Substituents Pattern

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

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β‑Functionalized Dibenzoporphyrins with Mixed Substituents Pattern: Facile Synthesis, Structural, Spectral, and Electrochemical Redox Properties Nitika Grover, Nivedita Chaudhri, and Muniappan Sankar* Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

Inorg. Chem. Downloaded from pubs.acs.org by LMU MUENCHEN on 02/05/19. For personal use only.

S Supporting Information *

ABSTRACT: A new series of mixed β-substituted dibenzoporphyrins were synthesized, and the effect of β-substitution on the spectral and electrochemical redox properties of the macrocycle was elucidated. The synthetic route to β-tetrasubstituted dibenzoporphyrins begins with the regioselective bromination of NiTPP(Benzo)2 to afford NiTPP(Benzo)2Br4, followed by Pd catalyzed coupling reaction to access NiTPP(Benzo)2(R)4 (R = phenyl (Ph) and phenylethynyl (PE)). Synthesized benzoporphyrins exhibited redshifted absorption spectral features with tunable redox properties. These benzoporphyrins displayed pronounced electronic effects of βsubstituents on the macrocyclic skeleton. NiTPP(Benzo)2(PE)4 exhibited the lowest HOMO−LUMO gap among the series due to extended π-conjugation. Intrestingly, metal-centered oxidation of Ni(II)/Ni(III) was observed for NiOPP(Benzo)2 and NiOPP(Benzo)Br2 after an initial conversion of the neutral porphyrin to its dicationic form under electrochemical conditions.

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INTRODUCTION Synthetic modification of porphyrin macrocycles through their peripheral functionalization has been an active area of research over the past few years.1−3 The functionalization at the βposition allows the manipulation of macrocycle conformation which results into the fine-tuning of their electronic and photophysical properties.1d In recent years, a number of research articles have been published which illustrate the advances in porphyrin functionalization.1e,f The development of synthetic routes has given access to the asymmetric substitution of porphyrinoids and made these molecules available for various practical applications.1g,h β-Functionalization at the porphyrin skeleton via existing substituents provides an easy access to a wide range of functionalized tetrapyrrolic systems as compared to the synthesis using functionalized pyrroles. Porphyrin derivatives having extended π-conjugation at β-pyrrolic positions attracted attention because of their promising applications in medicine and material science.2−4 The transformation of β-bromo porphyrins into benzoporphyrin is an apparent way to extend the π-conjugation of the porphyrin macrocycle.4 Benzoporphyrins exhibit sharper and red-shifted absorption spectral features due to their high molecular symmetry and extended π-conjugation.5a Structurally, tetrabenzoporphyrins are an intermediate between regular porphyrins and phthalocyanines.5b,c Benzoporphyrins have unique photophysical6 and electrochemical7 redox properties; therefore, they were utilized for various applications such as PDT agents,8 nonlinear optical materials,9 luminescent markers for oxygen,10 and pH sensors in biomedical imaging.11 © XXXX American Chemical Society

Further, benzoporphyrins are having good thermal and photochemical stability; therefore, they find potential application in DSSCs. In recent years, benzoporphyrins have been used as a sensitizer in DSSC and shown promising results.12 Although several methods have been reported for the total synthesis of benzoporphyrins, whereas functionalization and modification of benzoporphyrins (BPs) are less explored due to their multistep synthesis, and tedious purification procedures.13 Brominated porphyrins are utilized as a useful synthon for the synthesis of mixed substituted porphyrins. Furthermore, βbromo porphyrins exhibit varying degrees of nonplanar conformation which results into the alteration of photophysical properties.14 In 2006, Smith and co-workers reported the synthesis of mono/di/tribenzoporphyrins via Suzuki coupling, followed by olefin ring-closure metathesis.2c Jux and coworkers have synthesized π-extended porphyrins for efficient energy transfer and DSSC applications.4g,h Recently, Wang and co-workers reported the synthesis of dibenzoporphyrins via Heck coupling, followed by electrocyclization.3b,4a The majority of benzoporphyrins known in the literature are mono, di, or tetra benzoporphyrins. β-Functionalized dodecasubstituted dibenzoporphyrins with mixed substituents pattern are largely unexplored due to their synthetic difficulties. β-Substitution of dibenzoporphyrins is expected to have a pronounced impact on the electronic spectral and electroReceived: November 4, 2018

A

DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

preparation of novel β-substituted benzoporphyrins (Schemes 1 and 2) and provides an easy access to mixed substituted benzoporphyrins (Chart 1) and having scope in NLO and DSSC applications.

chemical redox properties. Being aware of the challenge of βfunctionalization and significance of β-substituted dibenzoporphyrins, we developed a new synthetic route to dodecasubstituted dibenzoporphyrins (Chart 1) for the first time in porphyrin chemistry.

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RESULTS AND DISCUSSION Synthesis and Characterization. Benzoporphyrins in which aromatic rings are fused at the β,β′-positions of porphyrins demonstrate distinctive photophysical and electrochemical redox properties. However, the synthesis and functionalization of these porphyrins are very demanding. In this work, β-functionalized benzoporphyrins were carried out by a facile synthetic route in good yields (62−87%) as shown in Schemes 1 and 2. These porphyrins were synthesized with the help of palladium catalyzed coupling reaction using βbromoporphyrins. Ni(II) dibenzoporphyrin was synthesized from Ni(II) tetrabromoporphyrins using a literature method.4a Ni(II) dibenzoporphyrin was further brominated using liquid bromine in dry CHCl 3 in order to obtain Ni(II) tetrabromodibenzoporphyrin (NiTPP(Benzo) 2 Br 4 ). The fused benzene ring of dibenzoporphyrins localized the aromatic pathway; therefore, it was possible to brominate regioselectively at the antipodal β,β′-positions of the porphyrin macrocycle. The success of bromination reaction allowed us to further introduce a wide variety of functional groups at the β,β′positions of benzoporphyrin. NiTPP(Benzo)2Br4 was utilized for Suzuki coupling reaction in order to access Ni(II) octaphenyldibenzoporphyrin [NiOPP(Benzo)2]. NiOPP(Benzo)2 was demetalated using conc. H2SO4 to obtain H2OPP(Benzo)2. NiOPP(Benzo)2 was also prepared by an alternative route using Heck coupling which involved the reaction of methylacrylate and NiOPPBr4. In this route, a small amount of monobenzo-dibromo-octaphenylporphyrin (NiOPP(Benzo)Br2) was also obtained (Scheme 2). Notably,

Chart 1. Molecular Structures of Synthesized β-Substituted Benzoporphyrins

We developed a facile synthetic route for the regioselective tetrabromination of dibenzoporphyrins using liquid bromine. The tetrabromodibenzoporphyrin was further utilized for the synthesis of mixed β-substituted “push−pull” dibenzoporphyrins via Suzuki cross-coupling reaction. In addition, the πconjugation at the β-position was further extended by Stille coupling reaction. Notably, this approach offers the selective Scheme 1. Synthesis of β-Substituted Ni(II) Dibenzoporphyrins

B

DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2. Alternative Synthetic Routes to MOPP(Benzo)2

nonplanar conformation of the macrocyclic core. The upfield shift of fused-H followed the order:

Heck coupling on NiOPPBr4 proceeded rapidly as compared to NiTPPBr4 possibly due to the more electron rich nature of NiOPPBr4.15 The introduction of phenyl groups at β-positions of benzoporphyrin led to an easy access to “push−pull” dibenzoporphyrins which are not reported in the literature. The introduction of phenylethynyl (PE) groups at the βpyrrolic position of porphyrin induces a remarkable red shift in optical absorption and emission spectra. In general, βphenylethynyl substituted porphyrins exhibit planar conformation of the macrocyclic core due to minimal steric interaction between the meso-phenyls and β-phenyls.3a The distinctive structural and electrochemical redox properties16 of β-phenylethynyl substituted porphyrins encouraged us to synthesize and investigate the unique properties of β-tetraphenylethynyldibenzoporphyrins. Reaction of NiTPP(Benzo)2Br4 with tributyl(phenylethynyl)tin using Pd(0) catalyst in distilled 1,4-dioxane led to tetraphenylethynyldibenzoporphyrin (NiTPP(Benzo)2(PE)4) in 71% yield. The demetalation of NiTPP(Benzo)2(PE)4 was not successful. The synthesized dibenzoporphyrins were characterized by UV−vis, fluorescence, and 1H NMR spectroscopy, MALDI-TOF-MS spectrometry, elemental analysis, and single crystal XRD and electrochemical studies. 1 H NMR spectra of synthesized porphyrins were recorded in CDCl3 at 298 K. All synthesized porphyrins have shown protons signals arising from β-phenyl, meso-phenyl, fusedbenzene, and ester groups. The integrated intensities of proton signals were in close agreement with the proposed structures. Figures S1−S6 in the Supporting Information (SI) represent the 1H NMR spectra of synthesized benzoporphyrins. Fused-H of β-substituted Ni(II) dibenzoporphyrins were upfield shifted as compared to unsubstituted dibenzoporphyrins due to

NiTPP(Benzo)2 < NiTPP(Benzo)2 (PE)4 < NiTPP(Benzo)2 Br4 < NiOPP(Benzo)Br2 < NiOPP(Benzo)2

MALDI-TOF mass spectra of synthesized porphyrins are shown in Figures S7−S14 in the SI. The observed molecular ion peak of synthesized porphyrins are in good agreement with the proposed structures. Crystal Structure Discussion. The selective formation of tetrabromodibenzoporphyrinatonickel(II) (NiTPP(Benzo)2Br4) was confirmed by the single crystal X-ray analysis (Figure 1). The porphyrin core adopted a nonplanar conformation due to the presence of four bromo substituents at the β-positions as shown in Figure 1. The crystallographic details are listed in Table S1 in the SI. The selected average bond lengths and bond angles of NiTPP(Benzo)2Br4 are listed in Table S2 in the SI. Figure S15 in the SI shows the deviation of 24 core atoms from the mean plane. The nonplanarity of the macrocyclic core

Figure 1. ORTEP diagrams showing top and side views of NiTPP(Benzo)2Br4. C

DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry was induced due to the steric repulsion among the β-bromo and meso-phenyl substituents. The C−C bond distances of βpyrrolic carbons bearing bromo substituents were shorter as compared to the Cβ′−Cβ′ distances where antipodal pyrroles bear fused benzene rings. NiTPP(Benzo)2Br4 exhibited larger Cβ−Cα−Cm angles (∼127°), lower N−Cα−Cm (∼123°) and Ni−N−Cα (∼125°) angles which clearly reflected the saddle shaped conformation of the macrocyclic core. The displacement of 24 core atoms from the mean plane (Δ24 = 0.548 Å) as well as the mean plane displacement of eight β-pyrrole carbons (ΔCβ = 0.987 Å) was higher as compared to the NiTPP(Benzo)2.4a DFT Studies. DFT calculations were performed for H2OPP(Benzo)2, NiTPP(Benzo)2Br4, and NiTPP(Benzo)2(PE)4 to gain insights into the conformational features of the synthesized macrocycles. The ground state geometry of H2OPP(Benzo)2 was optimized in the gas phase using the B3LYP functional and 6-31G basis set, whereas ground state geometries of Ni(II) complexes were optimized in the gas phase using the B3LYP functional and LANL2DZ basis set.17 The frontier molecular orbitals (FMOs) of H2OPP(Benzo)2 are shown in Figure 2. The electron density on the HOMO

Table 1. Electronic Spectral Data of Dibenzoporphyrins in CH2Cl2 at 298 K λabsa

porphyrin H2OPP(Benzo)2 CoOPP(Benzo)2 CuOPP(Benzo)2 ZnOPP(Benzo)2 NiOPP(Benzo)2 NiOPP(Benzo)Br2 NiTPP(Benzo)2Br4 NiTPP(Benzo)2(PE)4 a

468(5.56), 563(4.29), 638(4.01), 698(3.06)sh 455(4.96), 586(4.01), 628(4.11) 456(5.12), 584(4.15), 635(4.06) 469(5.48), 601(4.31), 644(4.20) 459(5.27), 582(4.13)sh, 632(4.38) 453(5.32), 570(4.25), 611(4.29) 460(5.26), 554(4.03), 591(4.17), 640(4.37) 477(5.31), 564(4.20), 687(4.73)

λem 729, 797

661, 719

Values in parentheses refer to log ε.

absorption spectra of ZnOPP(Benzo)2 and H2OPP(Benzo)2 in CH2Cl2, whereas Figure S17 in the SI shows the comparative UV−vis spectra of Ni(II) benzoporphyrins. The absorption spectra of benzoporphyrins depend upon the electronic nature of β-substituents. The synthesized benzoporphyrins were significantly red-shifted as compared to their parent porphyrin due to the combined effect of extended π-conjugation and electron-withdrawing or electron-donating substituents at βpositions. The Soret band of NiTPP(Benzo)2Br4 was ∼10 nm redshifted as compared to NiTPP(Benzo)2, whereas the Qx(0,0) band was 33 nm red-shifted. A remarkable red shift in the Qx(0,0) band was observed due to the nonplanar conformation of NiTPP(Benzo)2Br4 which resulted into the significantly reduced HOMO−LUMO gap. The Soret band of NiOPP(Benzo)2 was ∼9 nm red-shifted as compared to NiTPP(Benzo)2. The last Q-band of NiOPP(Benzo)2 was 25 nm redshifted as compared to NiTPP(Benzo)2 due to the presence of electron-donating phenyl groups which resulted in the destabilization of HOMOs. A remarkable red shift was observed in the Soret and Q-bands of MOPP(Benzo)2 as compared to MOPP19e due to extended π-conjugation in MOPP(Benzo)2. The Soret band of H2OPP(Benzo)2 was 34 nm red-shifted as compared to H2OPP, whereas the Qx(0,0) band was 21 nm red-shifted in contrast to H2OPP. NiTPP(Benzo)2(PE)4 exhibited 17 nm bathochromic shift in the Soret band and 47 nm red shift in the Qx(0,0) band as compared to NiTPP(Benzo)2Br4. Moreover, the intensity of the Qx(0,0) band was higher as compared to other synthesized benzoporphyrins. The pronounced red shift and high intensity of the Qx(0,0) band was observed due to extended πconjugation raised by β-phenylethynyl substituents. Emission spectra of H2OPP(Benzo)2 and ZnOPP(Benzo)2 were recorded in CH2Cl2 at 298 K (Figure 3b). H2OPP(Benzo)2 and ZnOPP(Benzo)2 exhibited feeble emission intensity and red-shifted spectral bands in their emission spectra due to the combined effect of conformation of porphyrin skeleton and electronic nature of β-substituents. We were unable to record quantum yield and singlet state lifetime of H2OPP(Benzo)2 and ZnOPP(Benzo)2 due to their weak emission. Electrochemical Redox Properties. To determine the combined influence of macrocyclic conformation, extended πconjugation, and the effects of electron-donating and/or electron-withdrawing substituents at the β-pyrrolic positions, the electrochemical studies of synthesized β-substituted

Figure 2. Frontier molecular orbitals (FMOs) of H2OPP(Benzo)2.

and LUMO in optimized structures was distributed over the porphyrin core and the two fused benzene rings. Notably, LUMO+1 does not exhibit electron density on unfused pyrrole which shows the localization of the aromatic pathway. In the case of H2OPP(Benzo)2, electron density on the electrondonating phenyl groups and the electron-withdrawing ester groups did not contribute significantly to FMOs possibly due to the perpendicular orientation with respect to the porphyrin mean plane. The electron density at electron-withdrawing ester groups did not appear in the LUMO; however, LUMO+1 and LUMO+2 contributed to ester groups. Similar characteristics were observed for Ni(II) complex as shown in Figure S16 in the SI. Electronic Spectral Studies. The absorption spectra of synthesized benzoporphyrins were recorded in CH2Cl2 at 298 K. All synthesized porphyrins exhibited their characteristic spectral features. The electronic absorption spectra of porphyrinoids are influenced by the presence of electrondonating or electron-withdrawing β-substituents.18 Table 1 lists the electronic spectral data of synthesized dibenzoporphyrins in CH2Cl2 at 298 K. Figure 3a shows the comparative D

DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Comparative absorption spectra of H2OPP(Benzo)2 and ZnOPP(Benzo)2. (b) Comparative emission spectra of H2OPP(Benzo)2 and ZnOPP(Benzo)2.

donating phenyl groups at the β-pyrrolic positions resulted in the cathodic shift in the first ring oxidation potentials (i.e., pushing effect) as compared to MTPP(Benzo)2, whereas the introduction of electron-withdrawing substituents such as Br and PE to the MTPP(Benzo)2 backbone led to a dramatic anodic shift in oxidation potentials, i.e., pulling effect. The first ring oxidation potential of NiOPP(Benzo)2 was found to be the least among all Ni(II) benzoporphyrins which was observed due to the +I effect of four β-phenyl groups. On other hand, the first ring reduction potential of H2OPP(Benzo)2 was 110 mV anodically shifted as compared to H2OPP, whereas a minimal cathodic shift (40 mV) in the first oxidation potential was observed. These shifts in the redox potentials are anticipated for MOPP(Benzo)2’s which were observed due to extended π-conjugation upon fusion of two benzene rings onto the porphyrin skeleton. The introduction of four electron-withdrawing bromo groups significantly influenced the reduction potentials, whereas the oxidation potentials were altered due to destabilized HOMOs. The first ring oxidation and reduction potentials of the NiTPP(Benzo)2Br4 were 260 and 310 mV anodically shifted as compared to NiOPP(Benzo)2, respectively. The first ring oxidation potential of NiTPP(Benzo)2(PE)4 was 150 mV anodically shifted as compared to NiOPP(Benzo)2, whereas the first ring reduction potential was 340 mV anodically shifted as compared to NiOPP(Benzo)2. Notably, NiTPP(Benzo)2(PE)4 exhibited the lowest HOMO−LUMO gap among all synthesized Ni(II) porphyrins which was probably due to the electron-withdrawing nature of four phenylethynyl substituents and extended π-conjugation.

benzoporphyrins were examined using cyclic voltammetry in CH2Cl2 containing 0.1 M TBAPF6. The comparative cyclic voltammograms of Ni(II) benzoporphyrins are shown in Figure 4, whereas Figure S18 in the SI depicts the comparative

Figure 4. Comparative cyclic voltammograms of Ni(II) benzoporphyrins in CH2Cl2. The Ni(II)/Ni(III) redox processes are shown in the dotted box.

cyclic voltammograms of MOPP(Benzo)2. The oxidation and reduction potentials for each redox reaction are summarized in Table 2. The redox potentials of benzoporphyrins are known to be influenced by the nature of the substituents, the solvent polarity, and the core metal ion. The introduction of electron-

Table 2. Electrochemical Redox Potentialsa of Synthesized Porphyrins oxidation (V) porphyrins

I

H2OPP(Benzo)2 CoOPP(Benzo)2 NiOPP(Benzo)2 CuOPP(Benzo)2 ZnOPP(Benzo)2 NiOPP(Benzo)Br2 NiTPP(Benzo)2Br4 NiTPP(Benzo)2(PE)4

0.87 0.79 0.92 0.78 0.76 0.97 1.18 1.07

II

reduction (V) III

0.95 1.19 1.08

1.10 1.84

1.17

1.83

1.24

IV 1.38

I

II

ΔE

−1.08 −0.56 −1.13 −1.11 −1.19i −1.10 −0.82 −0.79

−1.30 −1.10

1.95 2.05 2.05 1.90 1.96 2.07 2.01 1.86

−1.48 −1.43 −1.16 −1.06

a

vs Ag/AgCl, scan rate = 0.1 V/s, i = irreversible. E

DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

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in comparison to other synthesized nickel(II) benzoporphyrins possibly due to extended π-conjugation of the porphyrin skeleton. Figure S19 shows the variation in HOMO−LUMO levels of different metal complexes of H2OPP(Benzo)2. CuOPP(Benzo)2 has shown the least HOMO−LUMO gap among all other metal derivatives, i.e., MOPP(Benzo)2 (M = 2H, Co(II), Ni(II), Zn(II)).

Interestingly, the synthesized Ni(II) porphyrins have shown the following trend in the anodic shift of their first ringcentered oxidation potentials: NiOPP(Benzo)2 < NiOPP(Benzo)Br2 < NiTPP(Benzo)2 (PE)4 < NiTPP(Benzo)2 Br4

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The observed trend clearly suggest that, as the number of electron-withdrawing substituents increased, the ring oxidation was found to be difficult. By appending the phenyl groups at the β-position of dibenzoporphyrins, we were able to modulate the frontier molecular orbitals (FMOs) which resulted in multiple oxidations in the case of NiOPP(Benzo)2, NiOPP(Benzo)Br2, and CoOPP(Benzo)2. In 1993, Kadish and co-workers reported the Ni(II)/Ni(III) reaction after electrogeneration of a Ni(II) porphyrin dication.19a Although few reports are available in the literature for Ni(II)/Ni(III) conversion in porphyrins,19b−e however, multiple oxidations for benzoporphyrins are so far not reported in the literature. The observed oxidation potential for the elcetrogeneration of Ni(II)/Ni(III) was anodically shifted (0.11−0.22 V) as compared to MOPP(R) (R = H, CHO, and CH2OH)19e which clearly revealed the effect of electronwithdrawing ester groups appended at fused rings. Overall, a variation in the electronic nature of β-substituents makes a large impact on the redox potentials for these βfunctionalized dibenzoporphyrins. Intrestingly, the HOMO− LUMO gap of synthesized β-functionalized dibenzoporphyrins were lower as compared to previously reported mono- and dibenzoporphyrins. The decrement in the HOMO−LUMO gap indictated the pronounced steric/electronic effect of four β-substituents. Interestingly, substantial shifts were observed in the first ring redox potentials of synthesized nickel(II) complexes of dibenzoporphyrins as referenced from NiTPP. The first ring oxidation and reduction potentials of β-synthesized dibenzoporphyrins varied significantly with respect to electronic effect, extended π-conjugation, and enhanced nonplanarity produced by β-substituents. This β-substitution resulted in variation in the HOMO−LUMO gap (Figure 5) of highly substituted dibenzoporphyrins as compared to NiTPP and NiTPP(Benzo)2. By means of unsymmetrical substitution, we were able to achieve the redox tunability with decrement in HOMO−LUMO gap as compared to NiTPP. NiTPP(Benzo)2PE4 exhibited highly reduced HOMO−LUMO gap

CONCLUSIONS In summary, the synthesis and characterization of novel βsubstituted dibenzoporphyrins with mixed substituents pattern have been described. NiTPP(Benzo)2Br4 exhibited a saddle shaped conformation of the macrocyclic core. The spectral and electrochemical redox properties of the synthesized dibenzoporphyrins were tunable according to the electronic nature and size of the β-substituents. The observed weak fluorescence in free base porphyrin (H2OPP(Benzo)2) and its Zn(II) complex suggests that these porphyrins may have a higher degree of nonplanar conformation of the macrocyclic core as compared to NiTPP(Benzo)2. Further, the redox tunability was achieved by introducing β-substituents onto the MTPP or MOPP skeleton. Notably, a tunable metal-centered, Ni(II) to Ni(III), oxidation was observed after electrogeneration of a porphyrin dication for NiOPP(Benzo)2 and NiOPP(Benzo)Br2 which was possibly due to modulated HOMOs. The utilization of these porphyrins in nonlinear optics (NLO) is in progress.

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EXPERIMENTAL SECTION

Reagents. Pyrrole and methyl acrylate were purchased from Alfa Aesar and used as received. Benzaldehyde, N-bromosuccinimde (NBS), M(OAc)2·nH2O, where M = CoII, CuII, and ZnII, TBAPF6, CaH2, P2O5, and Br2 were purchased from HiMedia. Ni(OAc)2· 4H 2O, Pd(PPh3)4, Pd(OAc)2, and phenylboronic acid were purchased from Sigma-Aldrich and used as received. NBS was recrystallized from hot water and dried for 8 h at 70 °C under vacuum. TBAPF6 was recrystallized twice from hot ethanol and dried under vacuum. All solvents (like hexane, ethanol, CHCl3, etc.) were distilled and dried prior to use. Precoated thin layer silica gel chromatographic plates were purchased from Merck and used as received. H2TPP, H2TPPBr4, H2OPP, NiOPPBr4, and NiTPP(Benzo)2 were synthesized using literature methods.4a,14,15 Instrumentation and Methods. The electronic absorption spectra were recorded on an Agilent Cary 100 spectrophotometer using a pair of quartz cells of 10 mm path length and 3.5 mL volume. The emission spectra were recorded on a Hitachi F-4600 spectrofluorometer using a quartz cell of 10 mm path length. All the NMR studies have been carried out on a JEOL ECX 400 MHz using CDCl3 as a solvent at 298 K. MALDI-TOF-MS spectra were recorded on a Bruker UltrafleXtreme-TN MALDI-TOF/TOF spectrometer using 2-(4-hydroxyphenylazo)benzoic acid (HABA) as a matrix in positive ion mode. The cyclic voltammetric studies were carried out using CH instruments (CHI 620E). A three electrode assembly was used that consisted of a platinum working electrode, Ag/AgCl as a reference electrode, and Pt wire as a counter electrode. The porphyrin concentration was maintained ∼ 1 mM during electrochemical measurements. The whole experiment was performed under an inert atmosphere. Synthesis of 7,8,17,18-Tetrabromo-2 2,2 3 ,12 2 ,12 3 -tetra(carbomethoxy)-benzo[b]-5,10,15,20-tetraphenylporphyrinatonickel(II) [NiTPP(Benzo)2Br4]. 100 mg (0.099 mmol) of NiTPP(Benzo)2 was dissolved in dry CHCl3. 80 μL (1.58 mmol) of liq. bromine in 20 mL of CH2Cl2 was added dropwise over the period of 10 min, and the resultant reaction mixture was stirred for 4 h at room temperature. Reaction mixture was immediately washed with a saturated solution of Na2S2O5. The organic layer was dried over anhydrous sodium sulfate, and solvent was removed under reduced pressure. The residue was chromato-

Figure 5. Variation in HOMO−LUMO energy gap of synthesized porphyrins. F

DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

7.27(m, 4H, meso-m,p-Ph), 7.18(t, 3JH,H = 8 Hz, 8H, meso-m,p-Ph), 6.88−6.80 (m, 20H, β-Ph), 6.73(s, 4H, fused-H), 3.71(s, 12H, −CH3). MALDI-TOF-MS (m/z): found 1308.80 [M]+ 1308.05; found 1397.41 [M + THF + H2O]+ calcd 1397.17. Anal. Calcd for C84H56N4NiO8: C, 77.13; H, 4.32; N, 4.28. Found: C, 77.86; H, 4.43; N, 4.64. Synthesis of 5,7,8,10,15,17,18,20-Octaphenyl-22,23,122,123tetra(carbomethoxy)-benzo[b]porphyrin [H2OPP(Benzo)2 ] and Its Metal Complexes [MOPP(Benzo)2]. 80 mg (0.064 mmol) of NiOPP(Benzo)2 was dissolved in a minimum amount of CHCl3. To this, a few drops of conc. H2SO4 were added slowly, and the reaction mixture was stirred at 0 °C for 20 min. The reaction mixture was allowed to warm to room temperature, and then 20 mL of distilled water was added. The reaction mixture was washed with NH4OH solution, followed by distilled water (100 mL) in order to remove excess ammonia. The organic layer was separated and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure. The crude solid was purified by column chromatography using CHCl3. The title compound was obtained as a brown solid. Yield: 86% (66 mg, 0.053 mmol) UV/vis (CH2Cl2): λmax(nm) (log ε): 468(5.56), 563(4.29), 638(4.01), 698(3.06). 1H NMR (400 MHz, CDCl3) δ: 7.87(d, 3 JH,H = 8 Hz, 8H, meso-o-Ph), 7.41(t, 3JH,H = 8 Hz, 4H, meso-m,p-Ph), 7.31(t, 3JH,H = 8 Hz, 8H, meso-m,p-Ph), 6.95(d, 3JH,H = 8 Hz, 8H, βPh), 6.89−6.75(m, 20H, β-Ph), 6.75(s, 4H fused-H), 3.73(s, 12H, −CH3), −1.51(s, 2H, −NH). MALDI-TOF-MS (m/z): found 1252.77 [M + H]+ calcd 1252.38. Anal. Calcd for C84H58N4O8: C, 80.62; H, 4.67; N, 4.48. Found: C, 80.78; H, 4.75; N, 4.62. 20 mg of H2OPP(Benzo)2 was dissolved in 15 mL of CHCl3. To this, 10 equiv of M(OAc)2·nH2O (M = Co(II), Cu(II), and Zn(II)) in 5 mL of MeOH solution was added, and the resulting mixture was refluxed for 30 min. The organic layer was washed with water to remove excess of metal salt. The crude porphyrins were purified by column chromatography. CoOPP(Benzo)2. Yield: 87% (18 mg, 0.014 mmol). UV/vis (CH2Cl2): λmax(nm) (log ε): 455(4.96), 586(4.01), 628(4.11). MALDI-TOF-MS (m/z): found 1308.56 [M]+ calcd 1308.29. Anal. Calcd for C84H56N4CoO8: C, 77.12; H, 4.31; N, 4.28. Found: C, 77.52; H, 4.45; N, 4.32. CuOPP(Benzo)2. Yield: 84% (18 mg, 0.013 mmol). UV/vis (CH2Cl2): λmax(nm) (log ε): 456(5.12), 584(4.15), 635(4.06). MALDI-TOF-MS (m/z): found 1312.47 [M]+ calcd 1312.19. Anal. Calcd for C84H56N4CuO8: C, 76.84; H, 4.30; N, 4.27. Found: C, 76.52; H, 4.55; N, 4.85. ZnOPP(Benzo)2. Yield: 76% (16 mg, 0.012 mmol). UV/vis (CH2Cl2): λmax(nm) (log ε): 469(5.48), 601(4.31), 644(4.20). 1H NMR (400 MHz, CDCl3) δ: 7.76(d, 3JH,H = 8 Hz, 8H, meso-o-Ph) 7.39(t,3JH,H = 8 Hz, 4H, meso-m,p-Ph), 7.28(d, 3JH,H = 8 Hz, 8H, mesom,p-Ph), 6.84−6.74(m, 24H, β-H and Fused-H), 3.67(s, 12H, −CH3). MALDI-TOF-MS (m/z): found 1315.75 [M + H]+ calcd 1315.75. Anal. Calcd for C84H56N4ZnO8: C, 76.74; H, 4.29; N, 4.26. Found C, 76.92; H, 4.54; N, 4.06.

graphed on silica gel column and eluted with CHCl3 as eluent. The titled compound was recrystallized from CHCl3/CH3OH. Yield: 83% (109 mg, 0.083 mmol) UV/vis (CH2Cl2): λmax(nm) (log ε): 460(5.26), 557(4.02), 593(4.17), 640(4.36). 1H NMR (400 MHz, CDCl3) δ: 7.95(d, 3 JH,H = 8 Hz, 8H, meso-o-Ph), 7.82(t, 3JH,H = 8 Hz, 4H, meso-m,p-Ph), 7.75(t, 3JH,H = 8 Hz, 8H, meso-m,p-Ph), 7.09(s, 4H, fused-H), 3.81(s, 12H, −CH3). MALDI-TOF-MS (m/z): found 1342.87 [M + Na]+, calcd 1342.24. Anal. Calcd for C60H36Br4N4NiO8: C, 54.62; H, 2.75; N, 4.25. Found: C, 54.89; H, 2.85; N, 4.56. Synthesis of 7,8,17,18-Tetraphenylethynyl-22,23,122,123tetra(carbomethoxy)-benzo[b]-5,10,15,20-tetraphenylporphyrinatonickel(II) [NiTPP(Benzo)2(PE)4]. 50 mg (0.038 mmol) of NiTPP(Benzo)2Br4 and 20 mol % Pd(PPh3)4 were dissolved in 20 mL of 1,4-dioxane, and the solution was purged with Ar for 20 min. 133 μL (0.38 mmol) of tributyl(phenylethynyl)tin was dissolved in 10 mL of distilled dioxane and added to the above solution under an inert atmosphere. Reaction mixture was stirred at 80 °C for 3 h under an inert atmosphere. Then the solvent was removed under reduced pressure and redissolved in a minimum amount of CHCl3. The crude reaction mixture was subjected on a silica gel column, and the desired product was eluted using CHCl3/ C6H14 (1:1, v/v). NiTPP(Benzo)2(PE)4 was recrystallized from CHCl3/CH3OH. Yield: 71% (38 mg, 0.027 mmol) UV/vis (CH2Cl2): λmax(nm) (log ε): 477(5.31), 564(4.20), 687(4.73). 1H NMR (400 MHz, CDCl3) δ: 8.10−8.08(m, 8H, meso-o-Ph), 7.76−7.74(m, 12H, meso-m,p-Ph), 7.29−7.27(m, 6H, βPh), 7.25−7.23(m, 14H, β-Ph), 7.12(s, 4H, fused-H), 3.83(s, 12H, −CH3). MALDI-TOF-MS (m/z): found 1404.66 [M]+, calcd 1404.14. Anal. Calcd for C92H56N4NiO8: C, 78.69; H, 4.02; N, 3.99. Found: C, 78.85; H, 4.22; N, 4.02. Synthesis of 5,7,8,10,15,17,18,20-Octaphenyl-22,23,122,123tetra(carbomethoxy)-benzo[b]porphyrinatonickel(II) [NiOPP(Benzo)2]. Procedure A. 50 mg (0.038 mmol) of NiTPP(Benzo)2Br4 was dissolved in distilled toluene. 125 mg (0.94 mmol) of K2CO3 and 55 mg (0.46 mmol) of phenyl boronic acid were added to the above solution and purged with Ar for 25 min. 20 mol % Pd(PPh3)4 was further added, and the reaction mixture was stirred at 100 °C for 24 h under an argon atmosphere. After 24 h, the solvent was evaporated to drynees and the residue was redissolved in CHCl3. The chloroform solution was washed with NaHCO3 solution, followed by brine solution. The organic layer was separated and dried over anhydrous sodium sulfate. Solvent was reduced under pressure and subjected to silica gel column. The target porphyrin, NiOPP(Benzo)2, was eluted with CHCl3. Yield: 65% (32 mg, 0.025 mmol) Procedure B. 150 mg (0.12 mmol) of NiOPPBr4 was dissolved in a DMF/toluene (1:1, v/v) mixture. 64 mg of K2CO3, 40 mg (0.153 mmol) of PPh3 and 523 μL (50 fold excess) of methyl acrylate was added to the above solution, and Ar was purged for 30 min. To this, 14 mg (0.058 mmol) of Pd(OAc)2 was added, and the reaction mixture was stirred at 120 °C for 20 h under an inert atmosphere. After completion of the reaction, the reaction mixture was cooled to room temperature and diluted with CHCl3. Solvent was evaporated to dryness, and the crude porphyrin was subjected to a silica gel column to obtain the desired porphyrin. The first minor fraction was identified as NiOPP(Benzo)Br2, and the second major fraction was found to be NiOPP(Benzo)2. Both the porphyrins were recrystallized with CHCl3/CH3OH. NiOPP(Benzo)Br2. Yield: 8% (12 mg, 0.009 mmol). UV/vis (CH2Cl2): λmax(nm) (log ε): 453(5.32), 570(4.25), 611(4.29). 1H NMR (400 MHz, CDCl3) δ: 7.53−7.47(m, 8H, meso-o-Ph), 7.32(t, 3 JH,H = 8 Hz, 2H, meso-m,p-Ph), 7.19(t, 3JH,H = 8 Hz, 6H, meso-m,pPh), 7.09(t, 3JH,H = 8 Hz, 4H, meso-m,p-Ph), 6.80−6.76(m, 20H, βPh), 6.71(s, 2H, Fused-H), 3.71(s, 6H, −CH3). MALDI-TOF-MS (m/z): found 1298.79 [M]+ calcd 1298.13; found 1141.51 [M]+ − 2Br calcd 1141.92. Anal. Calcd for C76H48Br2N4NiO4: C, 70.23; H, 3.72; N, 4.31. Found: C, 70.64; H, 3.84; N, 4.42. NiOPP(Benzo)2. Yield: 60% (91 mg, 0.070 mmol). UV/vis (CH2Cl2): λmax(nm) (log ε): 459(5.27), 582(4.13)Sh, 632(4.38). 1 H NMR (400 MHz, CDCl3) δ: 7.51−7.47(m, 8H, meso-o-Ph), 7.32−

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03106. 1

H NMR spectra, MALDI-TOF mass spectra, deviation of the core atoms from the mean plane, crystal structure data, selected average bond lengths and bond angles, frontier molecular orbitals, comparative absorption spectra, comparative cyclic voltammogram, variation of the HOMO−LUMO gap (PDF) Accession Codes

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DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2011, 13, 1597−1605. (e) Lin, V. S. Y.; DiMagno, S. G.; Therien, M. J. Highly Conjugated, Acetylenyl Briged Porphyrins: New Models for Light-Harvesting Antenna Systems. Science 1994, 264, 1105−1111. (f) Nath, M.; Huffman, J. C.; Zaleski, J. M. Ambient Temperature Activation of Haloporphyrinic-Enediynes: Electronic Contributions to Bergman Cycloaromatization. J. Am. Chem. Soc. 2003, 125, 11484− 11485. (g) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. BisAnthracene Fused Porphyrins: Synthesis, Crystal Structure, and NearIR Absorption. Org. Lett. 2010, 12, 2124−2127. (h) Ishizuka, T.; Saegusa, Y.; Shiota, Y.; Ohtake, K.; Yoshizawa, K.; Kojima, T. Multiply-Fused Porphyrins-Effects of Extended π-Conjugation on The Optical and Electrochemical Properties. Chem. Commun. 2013, 49, 5939−5941. (4) (a) Deshpande, R.; Jiang, L.; Schmidt, G.; Rakovan, J.; Wang, X.; Wheeler, K.; Wang, H. A Concise Approach to the Synthesis of oppDibenzoporphyrins through the Heck Reaction. Org. Lett. 2009, 11, 4251−4253. (b) Jinadasa, R. G. W.; Fang, Y.; Kumar, S.; Osinski, A. J.; Jiang, X.; Ziegler, C. J.; Kadish, K. M.; Wang, H. β-Functionalized Push−Pull opp-Dibenzoporphyrins. J. Org. Chem. 2015, 80, 12076− 12087. (c) Jinadasa, R. G. W.; Fang, Y.; Deng, Y.; Deshpande, R.; Jiang, X.; Kadish, K. M.; Wang, H. Unsymmetrically Functionalized Benzoporphyrins. RSC Adv. 2015, 5, 51489−51492. (d) Waruna Jinadasa, R. G.; Thomas, M.; Hu, Y.; D’Souza, F.; Wang, H. Investigation of the Push-Pull Effects on β-Functionalized Benzoporphyrins bearing Ethynylphenyl Bridge. Phys. Chem. Chem. Phys. 2017, 19, 13182−13188. (e) Jiang, L.; Engle, J. T.; Zaenglein, R. A.; Matus, A.; Ziegler, C. J.; Wang, H.; Stillman, M. J. Pentacene-Fused Diporphyrins. Chem.Eur. J. 2014, 20, 13865−13870. (f) Jiang, L.; Engle, J. T.; Sirk, L.; Hartley, C. S.; Ziegler, C. J.; Wang, H. Triphenylene-Fused Porphyrins. Org. Lett. 2011, 13, 3020−3023. (g) Lungerich, D.; Hitzenberger, J. F.; Marcia, M.; Hampel, F.; Drewello, T.; Jux, N. Superbenzene-Porphyrin Conjugates. Angew. Chem., Int. Ed. 2014, 53, 12231−12235. (h) Lodermeyer, F.; Costa, R. D.; Malig, J.; Jux, N.; Guldi, D. M. Benzoporphyrins: Selective Co sensitization in Dye Sensitized Solar Cells. Chem.Eur. J. 2016, 22, 7851−7855. (5) (a) Lash, T. D. Modification of The Porphyrin Chromophore by Ring Fusion: Identifying Trends due to Annelation of the Porphyrin Nucleus. J. Porphyrins Phthalocyanines 2001, 05, 267−288. (b) Andrianov, D. S.; Rybakov, V. B.; Cheprakov, A. V. Between Porphyrins and Phthalocyanines: 10,20-Diaryl-5,15-tetrabenzodiazaporphyrins. Chem. Commun. 2014, 50, 7953−7955. (c) Carvalho, C. M. B.; Brocksom, T. J.; de Oliveira, K. T. Tetrabenzoporphyrins: Synthetic Developments and Applications. Chem. Soc. Rev. 2013, 42, 3302− 3317. (6) (a) Aartsma, T. J.; Gouterman, M.; Jochum, C.; Kwiram, A. L.; Pepich, B. V.; Williams, L. D. Porphyrins. 43. Triplet Sublevel Emission of Platinum Tetrabenzoporphyrin by Spectrothermal Principal Component Decomposition. J. Am. Chem. Soc. 1982, 104, 6278−6283. (b) Nguyen, K. A.; Pachter, R. Ground State Electronic Structures and Spectra of Zinc Complexes of Porphyrin, Tetraazaporphyrin, Tetrabenzoporphyrin, and Phthalocyanine: A Density Functional Theory Study. J. Chem. Phys. 2001, 114, 10757−10767. (c) Sommer, J. R.; Shelton, A. H.; Parthasarathy, A.; Ghiviriga, I.; Reynolds, J. R.; Schanze, K. S. Photophysical Properties of NearInfrared Phosphorescent π-Extended Platinum Porphyrins. Chem. Mater. 2011, 23, 5296−5304. (7) (a) Liou, K. Y.; Newcomb, T. P.; Heagy, M. D.; Thompson, J. A.; Heuer, W. B.; Musselman, R. L.; Jacobsen, C. S.; Hoffman, B. M.; Ibers, J. A. Preparation and Characterization of (Tetrabenzoporphyrinato)cobalt(II) Iodide, a Ring-Oxidized Molecular Conductor. Inorg. Chem. 1992, 31, 4517−4523. (b) Cheng, R.-J.; Lin, S.-H.; Mo, H.-M. Spectroscopic and Oxidation Studies of meso-Tetraphenyltetrabenzoporphyrin Carbonyl Complexes of Ruthenium(II): CO as the Probe to Elucidate the Bonding Characteristics of Porphyrins. Organometallics 1997, 16, 2121−2126. (c) Ye, L.; Fang, Y.; Ou, Z.; Xue, S.; Kadish, K. M. Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions.

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Corresponding Author

*E-mail: [email protected]. Tel: +91-1332-284753. Fax: +911332-273560. ORCID

Muniappan Sankar: 0000-0001-6667-3759 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support provided by Science and Engineering Research Board (EMR/2016/4016). N.G. and N.C. thank MHRD and CSIR India, respectively, for their senior research fellowship.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03106 Inorg. Chem. XXXX, XXX, XXX−XXX