Effects of Core Modification on Electronic Properties of para

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

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Effects of Core Modification on Electronic Properties of paraBenziporphyrins Rima Sengupta, Kishor G. Thorat, and Mangalampalli Ravikanth* Indian Institute of Technology, Powai, Mumbai 400076, India

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S Supporting Information *

ABSTRACT: In contrast to the nonaromatic meta-benziporphyrins, the para-benziporphyrins possess aromatic character depending on the type of five-membered ring present in the macrocyclic core. The effects of changing the para-benziporphyrinic core from C2N3 to C2NSN, C2NSeN, and C2NTeN by replacing the pyrrole with other five-membered heterocycles such as thiophene, selenophene, and tellurophene on aromatic properties of p-benziporphyrins are described here using spectral, electrochemical, X-ray, and density functional theory (DFT) studies. The missing core-modified p-benziporphyrins with C2NSeN and C2NTeN cores were synthesized by condensing 1 equiv of benzitripyrrane and 1,3-benzene-bis((4-phenyl)methanol with an appropriate diol such as 2,5-bis[(p-tolyl)hyroxymethyl]selenophene and 2,5-bis(hydroxymethyl)tellurophene under mild acid-catalyzed conditions at room temperature and characterized in detail by high-resolution mass spectrometry (HR-MS), one- & two-dimensional NMR, and X-ray crystallography of the one of the macrocycles, Selena p-benziporphyrin. The X-ray structure of Selena pbenziporphyrin revealed that the macrocycle was almost planar apart from the p-phenylene ring, which was deviated by 49.71° from the mean plane of the macrocycle defined by four meso carbons, unlike Selena m-benziporphyrin, which is relatively more distorted. NMR studies revealed that, as the core changes from C2N3 to C2NSN, C2NSeN, and C2NTeN, the diatropic ring current decreases, indicating that the aromatic character also decreases in the same order. X-ray structure and DFT studies also revealed that the distortion in the macrocycle increases as the pyrrole ring of p-benziporphyrin was replaced with other heterocycles such as furan, thiophene, selenophene, and tellurophene and that the tellura p-benziporphyrin was the most distorted macrocycle among core-modified p-benziporphyrins. Absorption and electrochemical properties were in agreement with these observations. Our repeated attempts on metalation of these p-benziporphyrins resulted in the successful synthesis of a Pd(II) complex of tellura p-benziporphyrin. The Pd(II) complex was characterized by HR-MS and NMR techniques, and the structure was optimized by DFT. The studies indicated that the Pd(II) ion was bonded to one of the pyrrolic nitrogens, tellurophene, tellurium, and two chloride ions in distorted square-planar geometry.

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INTRODUCTION Benziporphyrins1−3 result from replacement of one of the pyrrole rings of tetrapyrrole porphyrins by a benzene ring, and the resulting porphyrinoids with N3C core have unique ability to form organometallic complexes unlike regular porphyrins.4,5,14,6−13 Furthermore, the presence of a CH in the inner coordination sphere of benziporphyrins may stabilize metals in unusual oxidation states.10−12,15 These special features of benziporphyrinoids have attracted a lot of attention from researchers to explore the chemistry of these systems. The research groups of Lash,16−18 Latos-Grazynski,9,14,19−21 and others22 have extensively investigated synthetic methodologies for benziporphyrins and explored their spectral and electrochemical properties. The studies also showed that benziporphyrins have applications in the development of chemical sensors and in molecular recognition studies.23 The other interesting feature of these porphyrinoid macrocycles is their variation in aromaticity, which varies from nonaromatic to highly aromatic systems, and in a few cases antiaromatic systems are also formed.9,19,21,23,24 The benziporphyrins are broadly classified into meta-benziporphyrins 1, where benzene © XXXX American Chemical Society

subunit (m-phenylene) is connected to three pyrroles in a 1,3fashion1−3 and para-benziporphyrins 2, where the benzene subunit (p-phenylene) is connected to three pyrroles in a 1,4fashion.3,9 The studies indicated that simple m-benziporphyrins are nonaromatic but can be converted into aromatic systems by suitable modifications/substitutions on the mphenylene subunit, whereas p-benziporphyrins exhibit diatropic ring current and show more aromatic features.3,9,22,25−28 The electronic properties of m- and p-benziporphyrins can be altered further by replacing one of the pyrrole rings that is across the phenylene subunit of benziporphyrins by other fivemembered heterocyclic such as thiophene, furan, selenophene, and tellurophene, and the resulting core-modified benziporphyrins or hetero benziporphyrins with N2C2X (X = S, O, Se, and Te) cores are expected to exhibit different physicochemical and coordination properties compared to benziporphyrins with a N3C2 core.3,16,22,29 A perusal of literature revealed that the all core-modified derivatives of aromatic p-benziporphyrins Received: May 10, 2019

A

DOI: 10.1021/acs.inorgchem.9b01374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Chart 1. Structures of m-Benziporphyrins (1a−1c), p-Benziporphyrins (2−6), and Their Metal Complexes 1c-Pd(II) and 6Pd(II)

Scheme 1. Synthesis of p-Benziporphyrins 4−6

have not been synthesized, and no systematic investigation of the effect of core modification on electronic properties of pbenziporphyrins has been investigated. Herein, we report the synthesis of the missing hetero p-benziporphyrins such as Selena p-benziporphyrin 5 and tellura p-benziporphyrin 6 by

adopting a 3 + 1 synthetic strategy (Chart 1). However, we did not succeed in the synthesis and isolation of stable oxa pbenziporphyrin 3 due to its inherent unstable nature. We studied the effect of variation of the core on structural, spectral, and electrochemical properties of p-benziporphyrins using B


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Inorganic Chemistry

Figure 1. (A) Partial 1H NMR spectra of compound 6. Resonances were assigined on the basis of correlations observed in 1H−1H COSY (B) and 1 H−1H NOESY (C) spectra.

2,5-bis(hydroxymethyl)phenyl furan (9) did not yield the expected oxa p-benziporphyrin 3 but resulted in formation of calix oxa p-benziporphyrin as judged by mass spectral analysis. However, this macrocycle was not characterized further, because it belongs to a different class of macrocycle, and the properties are expected to be different from benziporphyrins. The aza analogue, meso-tetraaryl p-benziporphyrin 2, was synthesized by condensing benzitripyrrane 8 with pyrrole and aldehyde under mild acid-catalyzed conditions as reported earlier by Latos-Grazynski and co-workers.9 Compounds 2 and 4−6 are freely soluble in common organic solvents such as nhexane, CHCl3, CH2Cl2, toluene, ethyl acetate, methanol, etc. The identities of the compounds were confirmed by highresolution mass spectrometry (HR-MS) and one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy. The partial 1H, 1H−1H COSY, and NOESY NMR spectra of the tellura p-benziporphyrin 6 are presented in Figure 1. All the protons in compounds 4−6 were identified based on their peak positions, integrations, coupling constants, and cross-peak correlations observed in COSY and NOESY spectra. Because of C2 symmetry, a smaller number of resonances was observed in the 1H NMR spectra as shown for compound 6 in Figure 1A. In 1H NMR of macrocycle 6, we noted a singlet resonance for four protons at 6.51 ppm, which we assigned as type a protons of the p-phenylene ring. The type a protons showed nuclear Overhauser effect (NOE) correlation with doublet resonance at 7.88 ppm, which we assigned as type e protons of meso phenyl groups. The type e protons further showed NOE correlation with β-pyrrole protons appeared at 7.83 ppm, which we assigned as type b protons. The type b protons showed COSY correlation with the doublet resonance due to

various spectral and electrochemical techniques, X-ray crystallography, and DFT studies. In addition, we also succeeded in the synthesis of a Pd(II) complex of tellura pbenziporphyrin under standard metalation conditions.

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RESULTS AND DISCUSSION Synthesis and Characterization of Hetero p-Benziporphyrins 2 and 4−6. The known thia p-benziporphyrin22 4 and unknown Selena p-benziporphyrin 5 and tellura pbenziporphyrin 6 were synthesized over a sequence of steps as depicted in Scheme 1. The readily available precursors pbenzitripyrrane19 8 and the diols29−33 10−12 were prepared by adopting the reported methods, and the characterization data were in agreement with the reported ones. The hetero pbenziporphyrins 4, 5, and 6 were prepared by treating equimolar mixture of benzitripyrrane 8 and appropriate diol 10/11/12 in CH2Cl2 in the presence of 1 equiv of trifluoroacetic acid (TFA) as catalyst under nitrogen atmosphere for 30 min followed by oxidation using 2,3dichloro-5,6-dicyanobenzoquinone (DDQ) in open air for an additional 30 min. The reaction progress was followed by thinlayer chromatography (TLC) analysis, which indicated the disappearance of the spots corresponding to both the precursors and appearance of one new green colored spot corresponding to the desired macrocycles 4/5/6. The crude compounds were subjected to basic alumina column chromatographic purification using petroleum ether−dichloromethane (80:20) and afforded the pure hetero p-benziporphyrins 4, 5, and 6 as green solids in 4−6% yield. Interestingly, under similar experimental conditions as well as under modified reaction conditions, the condensation of benzitripyrrane with C


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Inorganic Chemistry

Figure 2. Comparison of 1H NMR spectra of compounds 2 and 4−6 in CDCl3.

of p-phenylene ring in NMR time scale in compound 29 was downfield-shifted in tellura p-benziporphyrin 6 and appeared as a singlet at 6.51 ppm. For example, the type a protons of pphenylene ring, which appeared as singlet at 5.20 ppm due to exchange between two equivalent conformations that differ by a flip of the p-phenylene ring in NMR time scale in compound 2,9 were downfield-shifted in tellura p-benziporphyrin 6 and appeared as a singlet at 6.51 ppm. Similarly, the type b and type c protons of pyrrole rings of compound 2 appeared at 8.24 and 7.62 ppm, respectively. It experienced upfield shifts in compound 6 and appeared at 7.83 and 7.24 ppm. These observations indicate that the electronic properties were significantly altered as the core is modified by replacing the pyrrole ring with other heterocyclic rings such as thiophene, selenophene, and telluorophene in compounds 4−6. Furthermore, the shifts in the NMR resonances of various protons of macrocycles indicate that the diatropic ring current is decreasing as pyrrole ring is replaced with other heterocycles, and the maximum effect was observed for tellura p-benziporphyrin 6. Stepien and LatosGrazynski also recorded the NMR spectrum of compound 2 at supercooled temperature of 168 K and observed two resonances for four protons of p-phenylene ring at 2.32 and 7.68 ppm, which were attributed to two inner and two outer pphenylene ring protons, respectively. The highly differentiated chemical shifts observed for p-phenylene ring at 168 K were due to the restriction of the rotation of p-phenylene ring. However, we recorded the 1H NMR spectra of reported compound 2 and compounds 4−6 at 233 K, which was our instrument limit, and provided the spectra in the Supporting Information (Figures S7−S10). In compounds 2 and 4−6 at 233 K, the type a protons of p-phenylene ring appeared as slightly broadened singlet with negligible shift in their chemical shifts compared to room-temperature spectra, indicating that the rotation of p-phenylene ring was not restricted in reported

two protons at 7.24 ppm, which was identified as type c protons of the pyrroles. The type c protons in turn showed NOE correlation with a triplet resonance due to four protons at 7.52 ppm, which were identified as type h protons of meso phenyl groups. The type h protons in turn showed NOE correlation with a singlet resonance at 8.05 ppm due to two protons, which we identified as type d protons of the tellurophene moiety. The multiplet resonance appeared in region of 7.46 to 7.47 ppm, which we assigned for type j protons, and the multiplet resonance in the region of 7.56− 7.62 ppm was assigned to the type g, f, and type i protons. Thus, all resonances in NMR spectrum of compound 6 were identified and assigned using 1D and 2D NMR spectroscopy. We adopted a similar approach to assign and identify all resonances observed in NMR spectra of other core-modified pbenziporphyrins. The comparison of 1H NMR spectra in the selection region of compounds 2 and 4−6 is presented in Figure 2. The selected peak resonances for the compounds 2 and 4−6 for comparison of their 1H NMR were included in Table 1. It is clear from Figure 2 and Table 1 that, as the heterocycle of the p-benziporphyrins varies from pyrrole to thiophene, selenophene, and tellurophene, the porphyrin core protons such as p-phenylene and pyrrole protons experienced either downfield or upfield shifts depending on the diatropic ring current. For example, the type a protons of p-phenylene ring, which appeared as singlet at 5.20 ppm due to fast rotation Table 1. Comparison of the Selected Chemical Shifts (1H) of the Compounds 2 and 4−6 (in ppm) type of proton

compound 2

compound 4

compound 5

compound 6

A B C D

5.20 8.24 7.62 7.82

5.56 8.10 7.59 8.21

5.85 8.03 7.52 8.23

6.51 7.83 7.24 8.05 D


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Inorganic Chemistry

Figure 3. Single-crystal X-ray structures of macrocycle 5 (A) top view and (B) side view and 1b (C) top view and (D) side view. Note: Hydrogen atoms in side views were omitted for clarity.

Figure 4. Conjugated pathways of compounds 1, 2, and 4−6 determined from the crystal structure.15

planar apart from the p-phenylene unit (ring a), which makes an angle of 49.7° with respect to the molecular plane defined by four meso carbons (C5, C10, C15, and C20). The atoms of the p-phenylene moiety showed deviations in the range of 0.797 and 1.03 Å from the mean plane, whereas the β carbon atoms of the pyrroles and the selenophene moieties are located within 0.008−0.028 Å from the mean plane. The reported selena m-benziporphyrin 1b was a bowl-shaped structure with m-phenylene unit tilted by an angle of 60.47°. The two pyrrole rings (ring b) point downward with respect to the mean plane in both selena m-benziporphyrin 1b and Selena p-benziporphyrin 5. The angle of deviation of the phenylene unit (ring a, 60.47°) and pyrrole ring (ring b, 29.01°) in selena mbenziporphyrin 1b was more pronounced compared to the selena p-benziporphyrin 5, where the ring a was deviated by 49.71°, and rings b were deviated by 6.90°. Following the same trend, the β carbon atoms of the pyrroles and the selenophene moieties (0.142−0.908 Å) and the phenylene moiety (0.052− 2.465 Å) are more deviated from the mean plane in the case of

compound 2 and compounds 4−6 under our experimental conditions. At this stage, we assume that compounds 4−6 also may show similar NMR features for p-phenylene ring as observed for the reported compound 2 if NMR of compounds 4−6 were recorded at supercooled temperature. X-ray Crystallography. We attempted to grow crystals of compounds 5 and 6, since the crystal structures of compounds 29 and 422 are already available in the literature. We successfully obtained good-quality crystals for the compound 5 by slow evaporation of chloroform solution of 5 into nhexane over a period of 3−4 d at room temperature. The selena p-benziporphyrin 5 crystallized in a triclinic system with a P̅ 1 space group. The X-ray structure of compound 5 is presented in Figure 3, and relevant data are summarized in Tables S1−S3. The X-ray structure confirmed the expected chemical compostition and connectivities. The molecular structure of the compound 5 contains one p-phenylene unit, two pyrrole rings, and one selenophene ring, connected via four methine bridges. The Selena p-benziporphyrin 5 is almost E


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Inorganic Chemistry selena m-benziporphyrin 1b compared to selena p-benziporphyrin 5. This supports the fact that the selena pbenziporphyrin 5 is aromatic, whereas the selena mbenziporphyrin 1b is nonaromatic. On the one hand, the C− Se single bond length of 1.91(6) Å in compound 5 fairly matches with the C−Se single bond length of reported selenophene-containing porphyrinoids,29,34,35 which in turn is slightly longer than the free selenophene (1.85 Å). On the other hand, the C12−C13 bond length (1.355 (9) Å) in compound 5 is shorter compared to typical selenophene molecule (1.433 Å),36 whereas the C11−C12 (1.415(9) Å) and C13−C14 (1.439(9) Å) bond lengths are found to be longer than that of selenophene (1.369 Å). TheC11−Se24− C14 bond angle of the macrocycle 5 remained almost the same as that of the free selenophene, whereas the other bond angles of the selenophene moiety were changed by an angle of 2°−3°. The close analysis of the bond lengths of the crystal structures of the selena m- and p-benziporphyrins 1b and 5, respectively, revealed that the C7−C8 and C17−C18 bond lengths showed more double bond character, whereas the C6− C7, C8−C9, C16−C17, and C18−C19 bonds showed more single bond character, indicating that these bonds are not involved in conjugation. The bond lengths of the rest of the macrocycle displayed conjugated bond behavior within the macrocycle. Thus, the probable conjugated pathways such as benzene-like structure (I) and the quinoid structure (II) of the macrocycles, determined from the bond length analysis of the crystal structure as well as NMR studies, are shown in Figure 4. Furthermore, the C4−C5 and C1−C20 bond lengths of selena m-benziporphyrin 1b are longer (1.483 and 1.504 Å, respectively) than that of the selena p-benziporphyrin 5 (1.478(9) and 1.468(8) Å, respectively), which indicates that p-phenylene moiety in 5 involves in π-conjugation more effectively with the remaining part of the macrocycle compared to m-phenylene moiety in selena m-benziporphyrin 1b. Overall, the macrocyclic core in selena para-benziporphyrin 5 is more planar compared to the selena meta-benziporphyrin 1b. Thus, the more planar nature of selena p-benziporphyrin 5 supports its more aromatic nature compared to nonaromatic nature of selena meta-benziporphyrin 1b. We further compared the crystal structures of compounds 2, 4, and 5, and the relevant parameters are presented in Table 2.

be attributed to the increase in the size of the heteroatom, which induces distortion in the macrocycle and distortion increases with the increase of the size of the heteroatom. Thus, aza-p-benziporphyrin is more planar, and selena pbenziporphyrin is more distorted. This can be further supported by the C4−C5 and C1−C20 bond lengths, which increase with the increase of the size of the heteroatom. Thus, the crystal structure analysis clearly indicated the decrease in the diatropic ring current as pyrrole ring (ring c) in 2 was replaced with thiophene in 4 and selenophene in 5. These observations were in line with NMR data. Density functional theory (DFT) calculations were performed to understand the structural aspects of compounds 2−6, and the B3LYP/6-31G (d,p) optimized structures in ground state (S0) are presented in Figure 5. The studies indicated a close match between optimized structures and Xray crystal structures for the compounds 2, 4, and 5. As compound 3 was not isolated, the proposed structure had to be optimized to predict and compare its electronic properties with other carbaporphyrins, namely, 2 and 4−6. The predicted structure of macrocycle 3 displayed similar planar conformation with tilted p-phenylene unit (ring a) by an angle of 44.5°, and the two pyrrole rings (ring b) point downward by an angle of 2.6° from the plane of macrocycle as observed for the other three macrocycles 2, 4, and 5. The optimized structure of tellura p-benziporphyrin 6 also indicated that the macrocycle exhibits planar structure apart from the more deviated pphenylene ring from the plane of the porphyrin compared to other p-benziporphyrins 2−5. Thus, DFT studies clearly indicated that the increased size of the heteroatom of ring c increases the deviation of the p-phenylene ring (ring a), and the highest deviation of the p-phenylene unit was observed for the tellurium-containing p-benziporphyrin 6 (54.8°). These observations were in line with the crystal structures obtained for 2, 4, and 5. Furthermore, the bond length analysis of the crystal structures of 2, 4, and 5 showed that C10−C11 and C16−C17 bond lengths increase with the increase in size of heteroatom of ring c. DFT also displayed the same trend from macrocycle 2 to macrocycle 6, and the longest bond length was found for the tellura p-benziporphyrin 6 (1.474 Å). Overall, DFT and crystal structure analysis indicated increased deviation of ring a with respect to the mean plane, with increase in size of the heteroatom leading to reduced diatropic ring current from compound 2 to compound 6 as reflected in the NMR data. Further, the nucleus independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA)37 values of the optimized structures were calculated to estimate the trend of aromaticity among the benziporphyrins 2 and 4−6. The NICS(0) values for the compounds 2 and 4−6 were predicted to be −17.29, −14.64, −14.37, and −10.10 ppm, and the HOMA values 0.867, 0.866, 0.865, and 0.834, respectively (Table S4), also suggest decrease in magnitude of aromaticity with increase in size of the heteroatom. Absorption and Electrochemical Properties of Compounds 2 and 4−6. The absorption and electrochemical properties of p-benziporphyrins 2 and 4−6 were investigated, and these data are represented in Table 3. The comparison of absorption spectra of p-benziporphyrins 2 and 4−6 recorded in toluene are presented in Figure 6A. All p-benziporphyrins showed one typical sharp Soret-like band in the region of 420− 450 nm and one relatively broad Q-type band in the region of 590−660 nm. Furthermore, as the heterocycle in p-

Table 2. Comparison of Bond Angle and Bond Lengths of para-Benziporphyrins (2, 4, and 5) deviation from the mean plane of (in deg) ring a ring b ring c bond lengths (in Å) C10−C11 C16−C17

compound 2 compound 4 compound 5 45.84 1.55 2.41 1.462 1.458

46.50 6.89 4.93 1.466 1.463

49.71 6.90 4.79 1.478 1.467

It is clear from the data presented in the Table 2 that, as the heterocycle (ring c) of p-benziporphyrins (2, 4, 5) was changed from pyrrrole to thiophene and to selenophene, the deviation of the para-phenylene moiety and the two pyrrole rings (ring b) increases. The deviation of the outer carbon atoms of the p-phenylene rings from the mean plane was found to be lowest for the aza p-benziporphyrin 2 (0.594 Å) and highest for the selena p-benziporphyrin 5 (1.094 Å). This can F


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Inorganic Chemistry

Figure 5. Ground-state optimized structures of the compounds 2−6.

Table 3. Absorption and Electrochemical Data of the Compounds 2, 4−6, and 6-Pd(II) absorption data compound 2 2.2H2+ 4 4.2H2+ 5 5.2H2+ 6 6.2H2+ 6-Pd(II)

λ1 (nm) (log ε) 431 453 439 460 442 466 421 452 437

(5.10) (4.64) (4.92) (4.33) (4.19) (4.32) (4.23) (4.33) (3.17)

redox data

λ2 (nm) (log ε) 600 (4.69) 715 (3.98) 593 (4.59) 735 (4.15) 600(4.01) 757 (4.19) 658 (4.10) 814 (4.22) 605 (2.89)

oxidations (V)

reductions (V) −1.05

−1.72

1.13

1.48

1.01

1.40

1.65

−0.95

−1.08

−1.40

−1.65

1.04

1.37

1.60

−0.90

−1.15

−1.32

−1.59

0.97

1.44

1.63

−0.82

−1.13

−1.41

−1.67

1.35

1.52

−0.31

−0.73

for all other protonated species of p-benziporphyrins (Table 3). The redox properties of p-benziporphyrins 2 and 4−6 were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using 0.1 M tetra-n-butyl ammonium perchlorate (TBAP) as supporting electrolyte in dichloromethane, and data are included in Table 3. A representative reduction wave of compound 6 is shown in Figure 6C. In general, compounds 2 and 4−6 showed two to three irreversible oxidations and two to four quasi-reversible/ reversible reductions. For example, tellura p-benziporphyrin 6 showed two reversible reductions at −0.82 and −1.13 V and two quasi-reversible/irreversible reductions at −1.41 and −1.67 V. A close inspection of the data presented in Table 3 showed that, as the pyrrole ring was replaced with other heterocycle ring, the porphyrins became easier to oxidize and easier to reduce. Thus, among p-benziporphyrinoids, the tellura p-benziporphyrin 6 was the easiest to oxidize and reduce. The time-dependent density functional theory (TD-DFT) showed a very close match between predicted and experimental absorption spectra (Figures S13A). The analysis

benziporphyrin was changed from pyrrole to thiophene, selenophene, and tellurophene, the absorption bands experienced bathochromic shifts as shown in Figure 6A. Thus, the Qtype band at 600 nm observed for aza p-benziporphyrin 2 was shifted to 658 nm in tellura p-benziporphyrin 6. Furthermore, the absorption bands of tellura p-benziporphyrin 6 were broader compared to those of the other three macrocycles 2, 4, and 5, which were tentatively attributed to a decrease of πconjugation and aromaticity because of the increased distortion. Protonation studies on compounds 2 and 4−6 were performed by adding 100 equiv of trifluoroacetic acid to the toluene solutions of macrocycles 2 and 4−6. Upon protonation, the green colored solutions of neutral macrocycles were turned to yellow due to formation of their corresponding diprotonated species. The absorption bands experienced bathochromic shifts upon protonation (Figure 6B). For example, the absorption bands at 439 and 593 nm in compound 4 were bathochromically shifted and appeared at 460 and 735 nm, respectively, in its deprotonated species 4.2H2+. A similar trend was observed G


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Inorganic Chemistry

Figure 6. (A) Comparison of absorption spectra of the compounds 4−6 (10 μM) recorded in toluene. (B) Comparison of absorption spectra of the compounds 4 and 4.2H2+ (10 μM) recorded in toluene. (C) Cyclic voltammogram of compound 6 recorded in CH2Cl2 containing 0.1 M TBAP as the supporting electrolyte and a saturated calomel electrode as the reference electrode at scan rates of 50 mV s−1.

of selected frontier molecular orbitals (FMOs) showed that the highest occupied molecular orbitals (HOMOs) in all the compounds 2−6 (Figures 7) are located mainly over the predicted conjugation pathway (Figure 4), leaving little orbital density on meso substituents. Also, the lowest unoccupied molecular orbitals (LUMO) located on predicted conjugation pathway with more uniform distribution of orbital densities supports the aromatic nature of the macrocycles 2−6. The LUMOs were found to get stabilized as the thiophene ring was replaced by selenophene and tellurophene, suggesting that compound 6 is easy to reduce compared to compound 5, which is again easy to reduce compared to compound 4. Though the LUMOs were stabilized in compounds 5 and 6 compared to compound 2 the energy levels of HOMOs in compounds 2−6 were consistent, resulting in the decrease in the band gaps for the compounds 5 and 6. This is in line with the observed red shift in the absorption bands of compounds 5 and 6 compared to those of compound 2. In the case of compound 6, the extent of stabilization of LUMO is more (band gap; ΔE = 2.11 eV), resulting in the absorption at longer wavelength compared to that of compound 5 (band gap; ΔE = 2.19 eV). Pd(II) Complex of Compound 6 (6-Pd(II)). We explored the coordination behavior of compounds 4−6 by reacting them with different metal salts under various reaction conditions. Compounds 4 and 5 did not form any metal complex after our repeated attempts, but fortunately, tellura p-

benziporphyrin 6 formed stable Pd(II) complex 6-Pd(II). The Pd(II) insertion was performed by treating the compound 6 with PdCl2 in CH3CN/CHCl3 (1:1 v/v) at room temperature for 2 h under inert atmosphere (Scheme 2). TLC and UV−vis analysis indicated the possible insertion of Pd(II) into the core of the macrocycle 6. The crude compound was passed through a bed of diatomaceous earth, and the compound was recrystallized from petroleum ether/CH3OH to afford pure 6-Pd(II) in 70% yield. The molecular ion peak at m/z = 881.0186 with expected isotopic pattern corresponding to [MCl]+ in HR-MS spectrum confirmed the formation of the desired compound 6-Pd(II). Our recent report on Pd(II) complex of tellura m-benziporphyrin (1c-Pd(II)) revealed that the palladium(II) is tetracoordinated and that the metal was bound to one of the pyrrole nitrogens, telluorophene tellurium, and two axial chlorides in a distorted square-planar fashion.3 Unfortunately, we did not get suitable crystals of 6-Pd(II) for X-ray diffraction. However, the HR-MS analysis of 6-Pd(II) showed a molecular ion peak at 881.0188 corresponding to [M-Cl]+, which is in agreement with the HR-MS observed for the reported compound 1c-Pd(II). On the basis of this observation, we tentatively assume that, in 6-Pd(II), Pd(II) ion was also bound to macrocycle 6 in the similar tetracoordinated distorted square-planar geometry observed for the reported 1c-Pd(II). Thus, the Pd(II) ion in 6-Pd(II) was bonded to one of the pyrrole nitrogens, one tellurophene tellurium, and two axial chlorides in distorted square-planar H


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Inorganic Chemistry

Figure 7. Energy-level diagram (selected FMOs) of compounds 2 and 6 calculated by the B3LYP/6-31g(d,p) method.

Scheme 2. Synthesis of Pd(II) Complex of Tellura p-benziporphyrin 6-Pd(II)

due to two inner p-phenylene protons, whereas the resonances at 7.47 and 8.03−8.08 ppm were due to outer p-phenylene protons. Thus, the p-phenylene moiety, which undergoes rapid exchange in a seesaw fashion in compound 6, was restricted in 6-Pd(II) due to binding of Pd(II) ion to macrocycle. The two tellurophene protons that appeared as singlet at 8.05 ppm in compound 6 became nonequivalent in 6-Pd(II) ion and appeared as two resonances in the region of 8.00−8.20 ppm. Furthermore, in 6-Pd(II), the inner p-phenylene protons experienced upfield shifts, whereas the outer p-phenylene protons, two pyrrole protons, and the tellurophene protons experienced downfield shift compared to tellura p-benziporphyrin 6, indicating an increase in the diatropic ring current in 6-Pd(II). DFT (B3LYP/6-31g (d,p)) studies were performed to further understand the structural aspects of palladium complexes of the tellura meta-benziporphyrin 1c-Pd(II) and tellura para-benziporphyrin 6-Pd(II), and the predicted

geometry, which was thoroughly investigated by NMR, absorption, electrochemical techniques, and DFT studies. The partial 1H, 1H−1H COSY NMR spectra of 6-Pd(II) recorded in CDCl3 at room temperature are presented in Figure 8. The fact that an increased number of resonances were observed in NMR spectrum of 6-Pd(II) compared to 6 indicates that 6-Pd(II) is unsymmetrical in nature. In the 1H NMR of compound 6-Pd(II), the four pyrrole protons were nonequivalent and appeared as three sets of resonances at 7.09 (doublet), 7.20 (doublet), and 7.79−7.81 ppm (multiplet), unlike in compound 6, where the four pyrrole protons appeared as two sets of doublet resonances at 7.24 and 7.83 ppm. The four protons of the p-phenylene moiety, which appeared as a singlet at 6.51 ppm for tellura p-benziporphyrin 6, appeared as three doublets at 5.51, 6.44, and 7.47 ppm, and the fourth proton appeared in multiplet along with tellurophene protons in the region of 8.03−8.08 ppm in 6Pd(II). The resonances observed at 5.51 and 6.44 ppm were I


Article

Inorganic Chemistry

Figure 8. 1H NMR and 1H−1H COSY spectra of Pd(II) complex 6-Pd(II).

Figure 9. DFT (B3LYP/6-31g (d,p)) optimized structures of compound 6-Pd(II) ((A) side view and (B) top view) and compound 1c-Pd(II) ((C) side view and (D) top view).

of 72.6° in 1c-Pd(II) and 74.9° in 6-Pd(II). The Pd(II) ion lies above the mean plane by a distance of 1.91 Å in 1c-Pd(II) and 1.42 Å in 6-Pd(II). The para and meta phenylene rings in compounds 6 and 1c were deviated from mean plane making angles of 54.8° and 65.6°, whereas these angles are 54.8° and

structures are represented in Figure 9. DFT-optimized structures of compounds 6-Pd(II) and 1c-Pd(II) show that, in both compounds, the palladium(II) lies above the mean plane (defined by four meso carbons C5, C10, C17, C22) with distorted square-planar geometry with the Te1−Pd1−N2 angle J


Article

Inorganic Chemistry

shifted to 605 nm along with a shoulderlike band at 712 nm in 6-Pd(II). Furthermore, compound 6-Pd(II) exhibited irreversible oxidations and reductions compared to free base macrocycle 6. Macrocycle 6 showed two reversible reductions at −0.82 and −1.13 V and two quasi-reversible/irreversible reductions at −1.41 and −1.67 V. In 6-Pd(II), the reductions were shifted to less negative, whereas the oxidations were shifted to more positive compared to 6, indicating that 6Pd(II) is more electron-deficient than tellura p-benziporphyrin 6. The TD-DFT predicted absorption spectra for 6-Pd(II) show close match with the measured one (Figure S13B). Experimentally, the Pd(II) ion insertion resulted in blue-shifts of absorption bands, which is evident from the increased band gap for compound 6-Pd(II) (ΔE = 2.15 eV) compared to the band gap of the macrocycle 6 (ΔE = 2.12 eV). Figure 11 represents the HOMO and LUMO for the compounds 6, 1c, 6-Pd(II), and 1c-Pd(II). The HOMO of compound 1c is localized mainly on the tellurophene ring with considerable orbital density on the meso carbons (C10 and C17) and the pyrrole nitrogen atoms with no orbital density on metaphenylene ring. In compound 6-Pd(II), HOMO is localized on the Pd(II) atom, pyrrole N atoms (N1 and N2), meso carbons, as well as on the tellurophene ring, whereas in compound 1cPd(II), HOMO is present mainly on the Te atom and Pd(II)Cl2 moiety. LUMOs of all compounds are distributed all over the macrocyclic core. Also, the observed redox properties of 1c, 1c-Pd(II), and 6-Pd(II) were in agreement with energy levels of their HOMO and LUMO orbitals. Both the HOMO and LUMO of 6-Pd(II) were energetically stabilized compared to that of tellura p-benziporphyrin 6, which supports the difficult oxidation and easier reduction of compound 6-Pd(II) compared to compound 6.

54.1° in 6-Pd(II) and 1c-Pd(II), respectively (Figures 5 and 9). This in turn suggests that there is no change in the angle of deviation of p-phenylene moiety upon binding to Pd(II) ion in 6-Pd(II), whereas the angle of deviation of meta phenylene ring in 1c-Pd(II) decreased by 11°. DFT studies indicated that the pyrrole ring that bound to Pd(II) was more deviated from the mean plane in both 6-Pd(II) and 1c-Pd(II). Furthermore, the C10−C11 and C16−C17 bond lengths were slightly decreased in the Pd(II) complexes 1c-Pd(II) (1.49 and 1.49 Å) and 6-Pd(II) (1.47 Å) compared to their free bases 1c and 6 supporting the increase in diatropic ring current upon Pd(II) complexation. Absorption and Electrochemical Properties of Compound 6-Pd(II). The absorption and electrochemical properties of compound 6-Pd(II) were investigated, and a comparison of absorption spectra of 6 and 6-Pd(II) is represented in Figure 10. The tellura p-benziporphyrin 6

Figure 10. Comparison of absorption spectra of the compounds 6 and 6-Pd(II) recorded in toluene.

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showed a Soret-like band at 421 nm along with a Q-band at 658 nm. However, 6-Pd(II) showed ill-defined broad absorption bands. The Soret-like band at 421 nm of compound 6 was slightly red-shifted to 437 nm, and the Q-band was blue-

CONCLUSIONS In benziporphyrins, the phenylene ring either participates or hinders the π-delocalization of the porphyrin macrocycle,

Figure 11. Selected FMOs of the compounds (A) 6, (B) 1c, (C) 6-Pd(II), (D) 1c-Pd(II) (EH = energy of HOMO and EL = energy of LUMO). K


Article

Inorganic Chemistry

non-hydrogen atoms were refined anisotropically. The X-ray data for compound 5 was collected on a Bruker Kappa CCD diffractometer equipped with a graphite-monochromated Mo Kα radiation source at 200 K using the θ−2θ scan mode. An empirical absorption correction by multiscans was applied, and all of the non-hydrogen atoms were refined with anisotropic displacement factors. The hydrogen atoms were placed in ideal positions and fixed with relative isotropic displacement parameters. The solvent molecules that could not be identified or modeled were found and eventually squeezed using PLATON. The corresponding loop of the residual electron-voids (from PLATON) was appended in the corresponding cif file. CCDC No. 1914824 (for compound 5) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. All the calculations were done using the Gaussian 09 program package. The DFT38 method, with hybrid functional B3LYP in conjunction with basis set 6-31G(d,p),39 was used to optimize the structures of the compounds 6 and 8 in ground (S0) states. With the same hybrid functional and basis set the vertical excitation energies and oscillator strengths were obtained for the 50 lowest S0 → Sn transitions at the optimized S0-state equilibrium geometries using TD-DFT method.40−45 All the computations in the toluene media were performed using the self-consistent reaction field (SCRF) under the polarizable continuum model (PCM).46,47 The electronic absorption spectra, including oscillator strengths, were systematically investigated using TD-DFT with PCM model on the basis of the S0-optimized structures. General Procedure for Synthesis of p-Benziporphyrins (4−6). The benzitripyrrane48 8 and the thiophene, selenophene, and tellurophene diols29−33 10/11/12 were synthesized by the reported methods. Samples of 1,3-benzene-bis((4-phenyl)methanol 8 (0.50 mmol) and appropriate diol 10/11/12 were dissolved in 200 mL of dichloromethane, and nitrogen was bubbled through the solution for 15 min. After 15 min, TFA was added to the same solution and allowed to stir for 1 h under inert atmosphere. DDQ (2.5 mmol) was added to the solution and stirred in open air for 30 min. The solvent was removed on a rotary evaporator under vacuum. The crude compound was purified by basic alumina column chromatography using petroleum ether/dichloromethane (80/20) as eluent affording the dark green solid in 4−6% yield. Synthesis of Compound 5. Compound 5 was synthesized from 8 (194 mg, 0.50 mmol) and 11 (185 mg, 0.50 mmol) by following the general procedure reported above for compounds 4−6. Yield 6% (20 mg); 1H NMR (400 MHz, CDCl3) δ 8.23 (s, 2H), 8.08−7.97 (m, 6H), 7.72−7.57 (m, 10H), 7.52 (d, J = 4.6 Hz, 2H), 7.39 (d, J = 7.8 Hz, 4H), 5.85 (s, 4H), 2.54 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 167.0, 156.2, 152.1, 149.1, 146.9, 140.0, 137.9, 137.5, 137.3, 135.5, 135.5, 133.0, 132.2, 131.2, 130.7, 129.3, 128.7, 128.4, 21.6. HRMS (electrospray ionization (ESI)): calcd for C48H35N2Se[M + H]+ m/z 719.1965; found m/z 719.1992. Synthesis of Compound 6. Compound 6 was synthesized from 8 (194 mg, 0.50 mmol) and 12 (196 mg, 0.50 mmol) by following the general procedure reported above for compounds 4−6. Yield 4% (15 mg); 1H NMR (400 MHz, CDCl3) δ 8.05 (s, 2H), 7.88 (dd, J = 7.8, 1.5 Hz, 4H), 7.83 (d, J = 4.7 Hz, 2H), 7.62−7.55 (m, 10H), 7.52 (t, J = 7.4 Hz, 4H), 7.46 (t, J = 7.3 Hz, 2H), 7.24 (d, J = 4.7 Hz, 2H), 6.51 (s, 4H). 13C NMR (101 MHz, CDCl3) δ 169.6, 167.9, 154.8, s149.1, 146.0, 142.5, 139.8, 135.5, 134.6, 132.6, 132.1, 131.4, 131.0, 130.4, 129.5, 128.9, 128.7, 128.4, 128.1, 127.3, 117.4, 114.2. HRMS (ESI): calcd for C46H31N2Te[M + H]+ m/z 741.1549; found m/z 741.1541. Synthesis of Compound 6-Pd(II). Tellura p-benziporphyrin 6 (10 mg, 0.013 mmol) and palladium(II) chloride (9.8 mg, 0.055 mmol) in acetonitrile/chloroform (10:10 mL) were stirred at room temperature for 2 h. The solution was washed with water and back extracted with dichloromethane; the organic layers were combined, and the solvent was evaporated under reduced pressure. The compound was recrystallized with CH2Cl2−pet ether to give the 6-Pd(II) (9 mg, 72%) as a dark green solid. 1H NMR (400 MHz, CDCl3) δ 8.19 (d, J = 7.1 Hz, 1H), 8.10−8.02 (m, 2H), 7.79−7.81 (m, 4H), 7.74−7.51

depending on the way it is connected with the rest of the macrocycle, which affects macrocyclic aromatic character. In meta-benziporphyrins, the meta-phenylene ring hinders the πdelocalization of the macrocycle; hence, it is nonaromatic, whereas in para-benziporphyrins, the p-phenylene ring is involved in π-conjugation of the macrocycle to some extent, and the extent of π-conjugation depends on the heteroatom present in the macrocycle, making the p-benziporphyrins aromatic in nature. We prepared the two missing coremodified p-benziporphyrins, namely, selena p-benziporphyrin and tellura p-benziporphyrin, by condensing benzitripyrrane with appropriate diols, 2,5-bis(hydroxymethylphenyl) selenophene/2,5-bis(hydroxymethylphenyl) tellorophene in a 1:1 ratio under mild acid-catalyzed conditions followed by purification to afford pure selena p-benziporphyrin/tellura pbenziporphyrin in decent yields. To find the effects of change of modifying macrocyclic core by replacing the pyrrole ring that is across the p-phenylene ring moiety with other heterocycles such as thiophene, selenophene, and tellurophene on structure, spectral and electrochemical properties were investigated using various spectral and electrochemical techniques and DFT studies. The crystallographic and DFT studies clearly indicated that the distortion in the macrocycle increases as we change the five-membered pyrrole ring with other five-membered heterocycles, and maximum distortion in the macrocycle was observed for tellura p-benziporphyrins. NMR studies revealed that the diatropic ring current decreases as the distortion in the macrocycle increases. Absorption, electrochemical, and DFT studies support these observations. Thus, while p-benziporphyrins exhibit diatropic ring currents, the diatropic ring current decreases as the size of the core atoms increases. Furthermore, the tellura p-benziporphyrin readily forms a stable Pd(II) complex with distorted squareplanar geometry. Further studies on the coordination behavior of core-modified p-benziporphyrins with various metal salts are currently under investigation in our laboratory.

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EXPERIMENTAL SECTION

General Experimental. The chemicals such as BF3·OEt2 and DDQ were used as obtained from Aldrich. All other chemicals used for the syntheses were reagent grade unless otherwise specified. Column chromatography was performed on silica gel and basic alumina. Compounds 2,9 4,22 and 7−12 were synthesized by the reported methods. The 1H and 13C NMR spectra were recorded in CDCl3 on Bruker 400 and 500 MHz instruments. The frequencies for 13 C nucleus are 100.06 and 125.77 MHz for 400 and 500 MHz instruments, respectively. Tetramethylsilane [Si(CH3)4] was used as an internal standard for 1H and 13C NMR. Absorption spectra were obtained with Shimadzu UV−vis−NIR (NIR = near-infrared) spectrophotometer. CV studies were performed with BAS electrochemical system utilizing the three-electrode configuration consisting of a glassy carbon (working electrode), platinum wire (auxiliary electrode), and saturated calomel (reference electrode) electrodes. The experiments were done in dry dichloromethane using 0.1 M tetrabutylammonium perchlorate as supporting electrolyte. The HR mass spectra were recorded with a quadrupole time-of-flight (QTOF) micro mass spectrometer. X-ray Crystal Structure Analysis. Single-crystal X-ray structure analysis was performed on a Rigaku Saturn 724 diffractometer that was equipped with a low-temperature attachment. Data were collected at 293 K using graphite-monochromated Mo Kα radiation (λα = 0.710 73 Å) by ω-scan technique. The data were reduced by using Crystal Clear-SM Ex-pert 2.1 b24 software. The structures were solved by direct methods and refined by least-squares against F utilizing the software packages SHELXL-97, SIR-92, and WINGX. All L


Article

Inorganic Chemistry (m, 18H), 7.47 (d, J = 8.0 Hz, 1H), 7.20 (d, J = 4.8 Hz, 1H), 7.09 (d, J = 4.7 Hz, 1H), 6.44 (d, J = 8.0 Hz, 1H), 5.51 (d, J = 8.0 Hz, 1H). 13 C NMR (101 MHz, CDCl3) δ 153.34, 140.79, 137.16, 135.45, 134.86, 129.84, 129.13, 128.86, 128.51. HRMS (ESI): calcd for C46H30ClN2PdTe[M + H]+ m/z 881.0188; found m/z 881.0186.

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(9) Stȩpień, M.; Latos-Grazyński, L. With Two Linked Carbon Atoms in the Coordination CoreTetraphenyl-p-Benziporphyrin: A Carbaporphyrinoid. J. Am. Chem. Soc. 2002, 124, 3838−3839. (10) Stȩpień, M.; Latos-Grazyński, L.; Szterenberg, L.; Panek, J.; Latajka, Z. Cadmium(II) and Nickel(II) Complexes of Benziporphyrins. A Study of Weak Intramolecular Metal-Arene Interactions. J. Am. Chem. Soc. 2004, 126, 4566−4580. (11) Stȩpień, M.; Latos-Grazyński, L.; Szterenberg, L. Conformational Flexibility of Nickel(II) Benziporphyrins. Inorg. Chem. 2004, 43, 6654−6662. (12) Idec, A.; Pawlicki, M.; Latos-Grazyński, L. Ruthenium(II) and Ruthenium(III) Complexes of p-Benziporphyrin: Merging Equatorial and Axial Organometallic Coordination. Inorg. Chem. 2017, 56, 10337−10352. (13) Hung, C.-H.; Chang, F.-C.; Lin, C.-Y.; Rachlewicz, K.; Stȩpień, M.; Latos-Grażyński, L.; Lee, G.-H.; Peng, S.-M. Iron and Copper Complexes of Tetraphenyl- m -Benziporphyrin: Reactivity of the Internal C-H Bond. Inorg. Chem. 2004, 43, 4118−4120. (14) Stȩpień, M.; Latos-Grazyński, L. Tetraphenylbenziporphyrin A Ligand for Organometallic Chemistry. Chem. - Eur. J. 2001, 7, 5113−5117. (15) Stȩpień, M.; Latos-Grazyński, L. Tetraphenyl-p-Benziporphyrin: A Carbaporphyrinoid with Two Linked Carbon Atoms in the Coordination Core. J. Am. Chem. Soc. 2002, 124, 3838−3839. (16) Lash, T. D.; Toney, A. M.; Castans, K. M.; Ferrence, G. M. Synthesis of Benziporphyrins and Heterobenziporphyrins and an Assessment of the Diatropic Characteristics of the Protonated Species. J. Org. Chem. 2013, 78, 9143−9152. (17) Lash, T. D. Out of the Blue! Azuliporphyrins and Related Carbaporphyrinoid Systems. Acc. Chem. Res. 2016, 49, 471−482. (18) Lash, T. D.; Young, A. M.; Rasmussen, J. M.; Ferrence, G. M. Naphthiporphyrins. J. Org. Chem. 2011, 76, 5636−5651. (19) Stȩpień, M.; Szyszko, B.; Latos-Grazyński, L. Steric Control in the Synthesis of P-Benziporphyrins. Formation of a Doubly nConfused Benzihexaphyrin Macrocycle. Org. Lett. 2009, 11, 3930− 3933. (20) Szyszko, B.; Latos-Grazyński, L.; Szterenberg, L. A Facile Palladium-Mediated Contraction of Benzene to Cyclopentadiene: Transformations of Palladium(II) p-Benziporphyrin. Angew. Chem., Int. Ed. 2011, 50, 6587−6591. (21) Szyszko, B.; Sprutta, N.; Chwalisz, P.; Stȩpień, M.; LatosGrazyński, L. Hückel and Möbius Expanded Para-Benziporphyrins: Synthesis and Aromaticity Switching. Chem. - Eur. J. 2014, 20, 1985− 1997. (22) Hung, C. H.; Lin, C. Y.; Lin, P. Y.; Chen, Y. J. Synthesis and Crystal Structure of Core-Modified Benziporphyrin: Thia-p-Benziporphyrin. Tetrahedron Lett. 2004, 45, 129−132. (23) Lash, T. D. Benziporphyrins, a Unique Platform for Exploring the Aromatic Characteristics of Porphyrinoid Systems. Org. Biomol. Chem. 2015, 13, 7846−7878. (24) Stȩpień, M.; Latos-Grazyński, L.; Sprutta, N.; Chwalisz, P.; Szterenberg, L. Expanded Porphyrin with a Split Personality: A Hückel-Möbius Aromaticity Switch. Angew. Chem., Int. Ed. 2007, 46, 7869−7873. (25) Szymanski, J. T.; Lash, T. D. Dimethoxytetraphenylbenziporphyrins. Tetrahedron Lett. 2003, 44, 8613−8616. (26) Lash, T. D.; Szymanski, J. T.; Ferrence, G. M. Tetraaryldimethoxybenziporphyrins. At the Edge of Carbaporphyrinoid Aromaticity. J. Org. Chem. 2007, 72, 6481−6492. (27) Lash, T. D. Oxybenziporphyrin, a Fully Aromatic Semiquinone Porphyrin Analog with Pathways for 18π-Electron Delocalization. Angew. Chem., Int. Ed. Engl. 1995, 34, 2533−2535. (28) Lash, T. D.; Rasmussen, J. M.; Bergman, K. M.; Colby, D. A. Carbaporphyrinoid Chemistry Has a Silver Lining! Silver(III) Oxybenzi-, Oxynaphthi-, Tropi-, and Benzocarbaporphyrins. Org. Lett. 2004, 6, 549−552. (29) Kumar, S.; Thorat, K. G.; Lee, W. Z.; Ravikanth, M. Synthesis, Structural, Spectral, and Electrochemical Studies of Selenabenziporphyrin and Its Pd(II) Complex. Inorg. Chem. 2018, 57, 8956−8963.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01374. HRMS spectra, NMR spectra, absorption, electrochemical, and computational data (PDF) Accession Codes

CCDC 1914824 contains the supplementary crystallographic data for this paper. These data can be obtained free of 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

*Fax: 91-22-5723480. Phone: 91-22-5767176. 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, Government of India (File No. EMR/2015/002196 to M.R.). R.S. thanks the CSIR India for research fellowship, and K.G.T. thanks IIT Bombay for a postdoctoral fellowship.

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REFERENCES

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Effects of Core Modification on Electronic Properties of para

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