Synthesis, Structural, Spectral, and Electrochemical Studies of

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

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Synthesis, Structural, Spectral, and Electrochemical Studies of Selenabenziporphyrin and Its Pd(II) Complex Sunit Kumar,† Kishor G. Thorat,† 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

‡

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S Supporting Information *

ABSTRACT: A new nonaromatic selenabenziporphyrin was synthesized by (3 + 1) condensation of m-benzitripyrrane and 2,5-bis[(p-tolyl)hydroxymethyl] selenophene under mild trifluoroacetic acid-catalyzed reaction conditions. The selenabenziporphyrin was characterized by high-resolution mass spectrometry, one- and two-dimensional NMR spectroscopy, and X-ray crystallography. The crystal structure revealed that the macrocycle was planar with moderately tilted m-phenylene ring and that the phenylene ring completely blocks the macrocyclic π-delocalization. The selenabenziporphyrin exhibits one broad absorption band at 645 nm along with one sharp band at 415 nm, and electrochemical studies revealed that the macrocycle was electron-deficient. The selenabenziporphyrin readily forms organometallic Pd(II) complex when treated with PdCl2 in CH3CN/CHCl3 at reflux followed by recrystallization. The X-ray structure revealed that the Pd(II) ion was coordinated with two pyrrole “N”s, selenophene “Se”, and m-phenylene ring “C” in square-planar fashion, and the complex retained its nonaromatic nature. The Pd(II) complex exhibits ill-defined absorption bands, and it was more electron-deficient than free-base selenabenziporphyrin macrocycle. Time-dependent density functional theory studies supported the experimental observations.

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arrays were synthesized, and their properties were explored.12 New heterocorroles, heterocalix[4]phyrins, and metal derivatives of heteroporphyrins were synthesized and characterized.11,13 During our course of investigations, we realized that the reports on the heterobenziporphyrins, which are a subclass of benziporphyrins in which one of the pyrrole rings is replaced with other heterocycles such as thiophene, furan, etc., are very few.1c,4 Lash and co-workers reported the first synthesis of nonaromatic heterobenziporphyrins in which one of the pyrrole rings was replaced either by thiophene Ia or furan Ib (Chart 1).14 Furthermore, they subsequently synthesized dimethoxythiabenziporphyrin, in which two methoxy groups were present on benzene ring and the crystal structure that obtained revealed that the macrocycle was nonaromatic and highly nonplanar.15 We recently reported the synthesis of tellurabenziporphyrin16 Ic by condensing 2,5-bis[(phenyl)hydroxymethyl]tellurophene and m-benzitripyrrin under trifluoroacetic acid (TFA)-catalyzed conditions, and X-ray structure revealed that the tellurabenziporphyrin retained same structural features as observed for thiabenziporphyrin Ia. Lash and co-workers did not report the metal complexes of nonaromatic thiabenzi- and oxabenziporphyrins (Ia & Ib), but

INTRODUCTION Carbaporphyrinoid macrocycles, which are resulted by replacing one or two nitrogen atoms in the porphyrin core with carbons, exhibit significant differences in their physicochemical and coordination properties compared to porphyrins.1 A large number of carbaporphyrinoid macrocycles have been synthesized that includes N-confused porphyrins,2 azuliporphyrins,3 benziporphyrins,4 N-confused pyriporphyrins,5 O- and S-confused heteroporphyrins,6 pyrazoloporphyrins,7 naphthiporphyrins,8 and neo-confused porphyrins,9 and their applications have been explored in a wide range of fields that includes catalysis, material science, and medicine.1c Among carbaporphyrinoids, benziporphyrinoids containing one or two benzene rings along with other five-membered heterocycles are the most widely studied carbaporphyrinoids and known to exhibit intriguing spectroscopic, structural, and coordination properties.1c,4 Our research group has been involved in the synthesis and studies of new core-modified porphyrins or heteroporphyrins that resulted from replacing one or two pyrroles with other five-membered heterocycles such as thiophene, furan, selenophene, tellurophene, etc.10 The core-modified porphyrins or heteroporphyrins possess very interesting physicochemical properties that are quite different from regular tetrapyrrolic porphyrins.11 Several covalently and noncovalently linked heteroporphyrin-based multiporphyrin © XXXX American Chemical Society

Received: April 6, 2018

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DOI: 10.1021/acs.inorgchem.8b00914 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

analysis and absorption spectroscopy indicated the formation of the desired selenabenziporphyrin 1. The crude compound was subjected to basic alumina column chromatographic purification and afforded the pure compound 1 as green solid in 17% yield. The selenabenziporphyrin 1 was freely soluble in wide range of organic solvents, and its identity was confirmed by high-resolution mass spectrometry (HR-MS), one-dimensional (1D) and 2D NMR, and X-ray crystallography. The 1D and 2D NMR spectroscopic techniques were used to characterize the selenabenziporphyrin 1, and Figure 1 displays the 1H NMR and partial 1H−1H COSY and 1H−1H NOESY spectra of macrocycle 1. The inspection of 1H NMR spectrum of macrocycle 1 indicates that the number of resonances were consistent with the presence of twofold symmetry along the axis passing through the centers of heterocycle and benzene moieties. To assign all of these resonances, we picked the tolyl-CH3 resonance of meso-tolyl groups of selenabenziporphyrin 1, which appeared as singlet at 2.45 ppm corresponding to six protons. The tolyl-CH3 protons showed NOE correlation with ortho protons (type a) of the meso-tolyl group, which appeared as multiplet in the region of 7.20−7.27 ppm. The type a protons resonance showed crosspeak correlation with a multiplet, which appeared in the region of 7.31−7.35, which we identified as type b protons of mesotolyl group of the selenabenziporphyrin 1. The type b protons resonance showed NOE correlation with a singlet and doublet at 7.27 and 6.68 ppm, which we identified as type c and type d pyrrole protons, respectively. The type d pyrrole protons at 6.68 ppm showed cross-peak correlation with the multiplet that appeared in the region of 7.31−7.35 ppm, which we assigned to type e pyrrole protons. The multiplet signal that appeared in the region of 7.44−7.46 ppm corresponding to 10 protons was assigned to meso-phenyl group (f, g, and h types), which in turn showed NOE correlation with a resonance at 7.11 ppm corresponding to phenylene ring type i protons. The type i protons showed cross-peak correlation with a multiplet appearing in the region of 7.31−7.35 ppm, which we assigned as type j phenylene ring proton. The type k proton of phenylene ring of macrocycle was expected to be singlet, and it appeared at 7.20 ppm. Thus, 1D and 2D NMR spectroscopy was very useful in deducing the molecular structure of selenabenziporphyrin 1. The molecular structure of selenobenziporphyrin 1 was unambiguously determined by X-ray crystallography, and the crystal structure is shown in Figure 2. The important crystallographic data and parameters are given in Tables S1 & S2. The chloroform/petroleum ether solution of compound 1, on slow evaporation at room temperature, afforded suitable single crystals for X-ray diffraction studies. The selenabenziporphyrin 1 crystallizes in a triclinic system with a P1̅ space group, and the unit cell contains two independent molecules. The structural analysis indicates that the macrocycle adopts a slightly bowl-shaped structure with a moderately tilted mphenylene ring, which is in agreement with the recently reported structures of thia-Ia and tellurabenziporphyrin Ic.16,19 However, the m-phenylene group in selenabenziporphyrin 1 was more deviated compared to thiabenziporphyrin Ia and less deviated as compared to tellurabenziporphyrin Ic. In thiabenziporphyrin Ia, the m-phenylene group shows deviation by an angle of 39.15°, and in tellurabenziporphyrin, the angle was 65.60° from the mean plane defined by meso-carbon atoms, whereas in case of compound 1, it was deviated by an angle of 60.70° from the mean plane. The macrocycle 1 adopts

Chart 1. Structures of Heterobenziporphyrins and Pd(II) Complexes

they succeeded in the synthesis of aromatic, neutral Pd(II) complex of dioxythiabenziporphyrin II, which was formed by demethylation of dimethoxythiabenziporphyrin upon insertion of Pd(II) ion (Chart 1).15 Recently, we showed that the nonaromatic tellurabenziporphyrin Ic forms neutral nonaromatic Pd(II) complex III readily, and the X-ray structure revealed the formation of an unusual five-membered ring inside the macrocycle due to strong interaction between tellurophene tellurium and pyrrolic nitrogen.16 Except one report17 on selenacarbaporphyrin and its Pd(II) complex, there is no report on selenabenziporphyrins to explore its potential to form metal complexes. In this paper, we report the synthesis of new selenabenziporphyrin 1 and its ability to form nonaromatic organometallic Pd(II) cation complex, Pd(II)-1. The absorption and electrochemical properties of 1 and Pd(II)-1 were studied and compared with the other reported heterobenziporphyrins.

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RESULTS AND DISCUSSION The selenium-containing m-benziporphyrin, selenabenziporphyrin 1 was synthesized by using the readily available precursors 2,5-bis[(p-tolyl)hyroxymethyl]selenophene 217 and m-benzitripyrrane 3.14 The precursors 2 and 3 were prepared by following the reported procedures,14,18 and characterization data were in agreement with the reported data. The selenabenziporphyrin 1 was prepared by condensing mbenzitripyrrane 3 with selenophene diol 2 in dichloromethane in the presence of catalytic amount of BF3·OEt2 for 1 h under inert atmosphere followed by oxidation with 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) in open air for an additional 30 min (Scheme 1). Thin-layer chromatography (TLC) Scheme 1. Synthesis of Selenabenziporphyrin 1

B


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Inorganic Chemistry

Figure 1. (a) Comparison of partial 1H NMR spectra of compounds 1 and Pd(II)-1. (b) 1H−1H COSY and (c) 1H−1H NOESY for 1 recorded in CDCl3 at room temperature.

Figure 2. Single-crystal X-ray structure of macrocycle 1. (a) Top and (b) side views (for clarity, all of the hydrogen atoms and meso aryl rings were omitted).

a saddle conformation with a maximum displacement of βpyrrole carbons by a dihedral angle of 28.47−28.52° from the mean plane (N1,N2,Se1,C29), whereas the selenophene ring lies almost in the mean plane. The molecular structure of 1 shows that the two pyrrole groups, one selenophene ring, and one benzene ring were linked by phenyl and tolyl groups at

C17/C30 and C5/C41, respectively, maintaining a coplanarity with the macrocycle. The m-phenylene moiety in macrocycle 1 displays characteristic benzene-like features that were evident from the C−C bond lengths within the ring, which varies from 1.371(9) to 1.384(7) Å. The slight deviation in bond length was observed for C28−C29 bond (1.416 (7)Å), which was C


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Inorganic Chemistry

appeared at 7.20 ppm in macrocycle 1, experienced ∼0.6 ppm downfield shift in Pd(II)-1 and appeared at 7.80 ppm. Similarly, the type j proton of outer phenylene ring that appeared at 7.32 ppm in macrocycle 1 experienced upfield shift and appeared at 7.00 ppm. Thus, upon complexation with Pd(II) ion, the electronic properties of the selenabenziporphyrin macrocycle were significantly altered. The coordination mode of the Pd(II) complex Pd(II)-1 was also elucidated by X-ray analysis. The front and side views of molecular structure of Pd(II)-1 were shown in Figure 3, and the relevant crystallographic parameters are included in Tables S1 & S3. The suitable crystals of Pd(II)-1 for X-ray diffraction were obtained via slow evaporation of hexane diffused into the chloroform solution of the compound at room temperature for over a period of 4 d. The compound Pd(II)-1 crystallizes in P1̅ space group with triclinic crystal system. The solid-state structure analysis indicates that the complex adopts a distorted square-planar geometry in Pd(II)-1. The Pd metal was coordinated to two pyrrole nitrogens, one inner phenylene ring carbon, and selenium of selenophene ring in slightly distorted square-planar geometry. The distortion minimizes the steric hindrance between the selenophene ring, the two pyrroles, and the phenylene ring. The Pd(II)-1 has a saddle conformation, in which the two pyrrole rings, phenylene ring, and selenophene ring were deviated alternately upward and downward, with dihedral angles of 1.13°, 1.53°, 19.29°, and 32.93° with respect to the mean plane (C5−C17−C30-C41). The saddling dihedral angles C1−Se1−C29−C28, C4− Se1−C29−C24, C16−N1−N2−C37, and C13−N1−N2-C40 were 47.87°, −51.11°, 2.99°, and 1.55°, respectively. However, the two Pd−N (Pd1−N1 2.080(8), Pd1−N2 2.076(7)) bonds were equivalent, while Pd−C and Pd−Se bond lengths were 2.077(10) and 2.381(13) Å, which matches fairly well with the reported Pd-selenacarbaporphyrin [Pd−Ns (2.063(4) and 2.072(4)), Pd−C (1.967(2)) and Pd−Se (2.3487(3)) Å].17 The meso-phenyl substituents were almost in perpendicular orientation with the macrocycle. Absorption and Electrochemical Properties of 1 and Pd(II)-1. The absorption and electrochemical properties of compounds 1 and Pd(II)-1 were investigated, and the comparison of absorption spectra of 1, its protonated derivative 1H22+, and Pd(II)-1 recorded in CHCl3 is shown in Figure 4a. The free-base macrocycle 1 showed one broad absorption band at 645 nm along with a sharp absorption band at 415 nm, which is in agreement with the absorption features of our recently reported tellurabenziporphyrin (Figure S12). The diprotonated species 1H22+ generated by addition of small amount of TFA to the CHCl3 solution of 1 showed more absorption bands at 413, 476, 820, and 960 nm. The Pd(II)-1 showed very ill-defined absorption bands in the region of 350− 900 nm unlike the Pd(II) complex of tellurabenziporphyrin III, which showed very similar absorption features to the free-base tellurabenziporphyrin. The electrochemical properties of 1 and Pd(II)-1 were investigated by cyclic voltammetry in CH2Cl2 containing tetrabutylammonium perchlorate (TBAP; 0.1M) as the supporting electrolyte, and the comparison of reduction waves of 1 and Pd(II)-1 was presented in Figure 4b. The selenabenziporphyrin showed only two reversible reductions at −0.73 and −1.04 V and one irreversible reduction at −1.58 V, but no oxidation was noted indicating the electron-deficient nature of macrocycle 1. Furthermore, the comparison of reduction potentials of 1 and its tellurium analogue Ic indicated that the selenabenziporphyrin 1 was somewhat easier

more like single-bond character confirming the absence of conjugation. The respective C−C and C−N bond lengths in the rest of the macrocycle other than m-phenylene ring displays typical conjugated bond behavior within the macrocycle. The average C−Se bond length of 1.91 Å matches fairly well with the C−Se single-bond length reported for other selenacarbaporphyrin in the literature.17 The macrocyclic phenylene group exhibited the torsion angles of −141° (C24C17C18C19) and 139° (C28C30C31C36) with respect to the meso-phenyl rings, which are much higher than the mesotolyl rings (C1C41C42C43−128° and C4C5C6C12 128°) of the tripyrrine unit. Overall, it is evident from the X-ray structure that the macrocyclic delocalization was completely blocked by the m-phenylene ring incorporated in the framework of 1, but the aromaticity of m-phenylene ring was retained in the macrocycle 1. Palladium(II) Complex of 1 (Pd(II)-1). The nonaromatic dimethoxythiabenziporphyrin formed neutral aromatic Pd(II) complex II because of ring aromatization by demethylation.15 In this complex, the Pd(II) ion was bound to two pyrrole “N”s, one thiophene “S”, and one phenylene “C” in square-planar fashion. The tellurabenziporphyrin formed neutral Pd(II) complex III by undergoing ring fusion of tellurophene tellurium with pyrrole nitrogen. In complex III, the Pd(II) ion was coordinated with pyrrole “N”, tellurophene “Te”, and two chloride ions in square-planar geometry. We thought that the selenabenziporphyrin 1 with one ionizable inner metaphenylene ring −CH proton may not prefer to form Pd(II) complex like thiabenziporphyrin Ia and tellurabenziporphyrin Ic. However, we performed the reaction of selenabenziporphyrin 1 with PdCl2 in CH3CN/CHCl3 at reflux for 2 h (Scheme 2). As the reaction progressed, we observed distinct Scheme 2. Synthesis of Pd(II)-Selenabenziporphyrin Pd(II)-1

color change from dark green to yellow, and absorption spectroscopy showed ill-defined absorption features unlike free-base macrocycle 1. We added NH4PF6 to the reaction mixture, and after standard workup and recrystallization it afforded Pd(II)-1 complex as brown solid in 58% yield. The formation Pd(II)-1 was confirmed by HR-MS, which showed a peak at 823.0870 [M-PF6]+. The compound Pd(II)-1 was characterized by 1D and 2D NMR spectroscopy and X-ray crystallography. The 1H NMR spectrum of Pd(II)-1 was included in Figure 1. As clearly observed from Figure 1, the inner phenylene −CH proton, which was observed at 7.20 ppm as broad singlet, was completely absent in Pd(II)-1 due to its involvement in coordination with Pd(II) ion. The β-pyrrole, β-selenophene, and outer phenylene protons experienced either upfield or downfield shifts in Pd(II)-1 compared to selenabenziporphyrin 1. For example, the type c β-proton of selenophene ring, which D


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Inorganic Chemistry

Figure 3. Single-crystal X-ray structure of macrocycle Pd(II)-1. (a) Top and (b) side views (for clarity, all of the hydrogen atoms, selenium atom, PF6− ion, and meso aryl rings were omitted). Selected bond lengths: Se1−C1 1.894(8), Se1−C4 1.905(9), Pd−C29 2.077(10), Pd1−N1 2.080(8), Pd1−N2 2.076(7), Pd1−Se1 2.381(13).

Figure 4. (a) Comparison of the absorption spectra of compound 1 (black line), 1-Pd(II) (red line), and 1-TFA (blue line) recorded in chloroform and (b) comparison of cyclic voltammograms of compound 1 (black line) and compound Pd(II)-1 (red line) and the differential pulse voltammogram (dotted blue line) recorded in CH2Cl2 with 0.1 M TBAP as the supporting electrolyte at a scan rate of 50 mV s−1.

Figure 5. Calculated excitations (black vertical lines) and experimental UV/vis absorption spectra (colored lines) for the compounds 1 and Pd(II)1 (A and B, respectively) along with optimized geometries (hydrogen atoms are omitted for clarity) and pictures of the calculated HOMO, LUMO, and LUMO+1 orbitals (isovalue = 0.02) for each compound (note: H = HOMO; L = LUMO; EH= energy of HOMO and EL= energy of LUMO).

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Article

Inorganic Chemistry to reduce than tellurabenziporphyrin Ic (Figure S14). The Pd(II)-1 also showed two reversible reductions at −0.10 and −0.57 V and one irreversible reduction at −1.03 V. The redox waves were shifted toward less negative in Pd(II)-1 compared to 1 indicating that Pd(II)-1 was much more electron-deficient than macrocycle 1. Thus, the macrocycle 1 and its Pd(II) complex exhibited novel absorption and redox features. Computational Studies. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to further probe the structural and spectroscopic features of the compounds 1 and Pd(II)-1. The optimized geometries (B3LYP/6-31G(d,p)), comparison of calculated excitation energies (black lines) and experimental UV−vis spectra (colored lines), along with pictures of the frontier molecular orbitals (highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and LUMO+1) for the compounds 1 and Pd(II)-1 were presented in Figure 5. The DFT-optimized structures of the compounds 1 and Pd(II)-1 suggest similar structural parameters as those obtained by X-ray single-crystal analysis. The calculated oscillator strengths (right axes) for the compounds represent the probability of the occurrence of corresponding transitions and are proportional to the observed molar extinction coefficients (left axes). The calculated absorption spectra of the compounds 1 and Pd(II)-1 match closely with those of measured UV−vis spectra. The HOMO of the compound 1 was mainly localized on selenophene ring, whereas LUMO and LUMO+1 were distributed uniformly all over the core of macrocycle. This suggests that the transitions from the HOMO → LUMO and HOMO → LUMO+1 were responsible for the low-energy prominent broad band in the UV−vis spectrum of the compound 1. The HOMO of the compound Pd(II)-1 was located mainly on phenylene side including meso substituents with little charge density on palladium atom, whereas LUMO was mainly localized on palladium atom. Furthermore, LUMO +1 was located mainly on meso substituents present at phenylene side of the macrocycle. Thus, the transitions HOMO → LUMO and HOMO → LUMO+1 were responsible for low-energy ill-defined band in the UV−vis spectrum of the compound Pd(II)-1.

synthesis of wide variety of metal complexes is under investigation in our laboratory.

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MATERIALS AND METHODS

General Experimental. The chemicals such as BF3·OEt2 and DDQ were used as obtained from Aldrich. All other chemicals used for the synthesis were reagent grade unless otherwise specified. Column chromatography was performed on silica gel. The 1H, 13C, 19 F, and 31P NMR spectra were recorded in CDCl3 on Bruker 400 and 500 MHz instruments. The frequencies for 13C 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 spectrophotometer. The HR mass spectra were recorded with a Q-TOF micro mass spectrometer. For UV−vis, the solution for all compounds (1 × 10−5 M) were prepared by using spectroscopic grade CHCl3 solvent. X-ray Crystallographic Method. Single crystals of suitable size for X-ray diffraction were selected under a microscope and mounted on the tip of a glass fiber, which was positioned on a copper pin. The X-ray data for compounds 1 and Pd(II)-1 (CCDC 1834547, 1834546) were collected on a Bruker Kappa CCD diffractometer, employing graphite-monochromated Mo Kα radiation at 200 K and the θ−2θ scan mode. The space group for compounds 1 and Pd(II)-1 was determined on the basis of systematic absences and intensity statistics, and the structure of compound 1 and Pd(II)-1 was solved by direct methods using SIR92 or SIR97 and refined with SHELXL97.20 An empirical absorption correction by multiscans was applied. All non-hydrogen atoms were refined with anisotropic displacement factors. Hydrogen atoms were placed in ideal positions and fixed with relative isotropic displacement parameters. Detailed crystallographic information for compound 1 and Pd(II)-1 are provided in the Supporting Information. Computational Details. DFT and TD-DFT calculations were performed using Gaussian 09 program package.21 For compound 1, the hybrid functional B3LYP with 6-31G(d,p)22 basis set was used for all atoms, whereas for compound Pd(II)-1, hybrid functional B3LYP coupled with LANL2DZ23 basis set for Pd and 6-31G(d,p) for all other atoms were used for optimization in ground (S0) state. The stability of the optimized geometries of the compounds 1 and Pd(II)1 was established with vibrational analyses, and no imaginary frequency was found. With the same hybrid functional and basis sets, 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. All the computations in the chloroform media were performed using the self-consistent reaction field (SCRF) under the polarizable continuum model (PCM).24 6,21-Diphenyl-11,16-ditolyl-24-selenobenziporphyrin 1. In a 250 mL one-necked round-bottom flask fitted with nitrogen bubbler, 2,5bis[(p-tolyl)hydroxymethyl] selenophene17 2 (191 mg, 0.514 mmol) and benzitripyrrane15 3 (200 mg, 0.514 mmol) were dissolved in 150 mL of CH2Cl2. After the flask was purged with nitrogen for 10 min, the reaction was initiated by adding BF3·OEt2 (0.013 mL, 0.103 mmol), and the reaction mixture was stirred at room temperature for 1 h. The progress of the reaction was checked by taking aliquots of the reaction mixture at regular intervals and oxidized with DDQ, and the absorption spectra was recorded, which clearly confirmed the formation of macrocycle 1. After 1 h, DDQ (235 mg, 1.03 mmol) was added, and the reaction mixture was stirred in air for an additional 30 min. The solvent was removed under reduced pressure, and purification of the crude compound was done by alumina column chromatography using petroleum ether/dichloromethane (80:20) as the eluent, and the product was collected as a dark green band. The compound was collected (65 mg, 17%) as a dark green solid. 1 H NMR (500 MHz, CDCl3, δ in ppm): 2.43 (s, 6H, tolyl −CH3), 6.68 (d, J = 5.0 Hz, 2H, pyrrolyl d), 7.11 (dd, J = 8.0 Hz, 2H, aryl (i), 7.20 (bs, 1H, aryl k), 7.20−7.27 (m, 6H, tolyl a, selenophenyl c), 7.31−7.35 (m, 7H, tolyl b, aryl j, and pyrrolyl e), 7.44−7.46 (m, 10H,

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CONCLUSIONS We successfully synthesized the missing selenabenziporphyrin by condensing benzitripyrrane with 2,5-bis[(p-tolyl)hydroxymethyl] selenophene under mild acid-catalyzed reaction conditions. The crystal structure revealed that the macrocycle adopts a slight bowl-shaped structure with slight deviation of m-phenylene ring, and the π-delocalization in macrocycle was completely hindered by the m-phenylene ring. The selenobenziporphyrin readily forms an organometallic palladium(II) complex when treated with PdCl2 in CH3CN/ CHCl3 at reflux, followed by recrystallization. The X-ray structure revealed that the Pd(II) ion was coordinated with two pyrrole “N”s, selenophene “Se”, and m-phenylene ring “C” in square-planar fashion. The selenabenziporphyrin showed one broad absorption band at 645 nm and one sharp absorption band at 415 nm, whereas the Pd(II) complex exhibited featureless absorption bands in the region of 300− 900 nm. Both selenabenziporphyrin 1 and its Pd(II) complex Pd(II)-1 are electron-deficient in nature. The computational studies were in agreement with the experimental observations. Currently, the potential of selenabenziporphyrin for the F


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Inorganic Chemistry phenyl f, g, and h); 13C NMR (100 MHz, CDCl3, δ in ppm): 21.5, 29.9, 113.5, 128.0, 129.0, 129.9, 130.9, 131.6, 132.5, 133.2, 135.6, 136.6, 137.4, 137.5, 140.6, 142.4, 147.4, 154.3, 158.0, 172.8; UV−vis (in CHCl3, λmax/nm, log ε) = 341 (4.6), 415 (4.8) and 645 (4.3); HRMS mass calcd. for C48H35N2Se[M + H]+ 719.1965, Found 719.1976. [6,21-Diphenyl-11,16-ditolyl-24-selenobenziporphyrin] palladium(II) (Pd(II)-1). Selenobenziporphyrin 1 (10 mg, 0.013 mmol) and palladium(II) chloride (9.8 mg, 0.055 mmol) in acetonitrile/chloroform (10:10 mL) were refluxed for 2 h. The solution was allowed to cool to room temperature followed by the addition of ammonium hexafluorophosphate to the reaction mixture. The reaction mixture was extracted with dichloromethane and water. The organic layers were combined, and the solvent was evaporated under reduced pressure. The residue was recrystallized with dichloromethane−petroleum ether to give the Pd(II)-1 (6.4 mg, 58%) as brown solid. 1H NMR (500 MHz, CDCl3, δ in ppm): 2.45 (s, 6H, tolyl −CH3), 6.82 (d, J = 5.0 Hz, 2H, pyrrolyl d), 7.00 (t, J = 8.0 Hz, 1H, aryl j), 7.10 (d, J = 5.0 Hz, 2H, pyrrolyl e), 7.27 (d, J = 8.0 Hz, 2H, aryl (i), 7.33−7.58 (m, 18H, tolyl a and b; phenyl f, g, and h), 7.80 (s, 2H, selenophenyl c); 19F NMR (376.4 MHz, CDCl3, δ in ppm): −72.19 (d, J (F−P) = 715.0 Hz, 6F), 31P NMR (161.9 MHz, CDCl3, δ in ppm): −144.43 (septet, J (P−F) = 715.0 Hz, P); 13C NMR (100 MHz, CDCl3, δ in ppm): 21.6, 29.9, 127.5, 127.6, 129.5, 130.2, 131.2, 131.5, 131.9, 134.3, 135.5, 137.4, 139.8, 140.3, 141.0, 142.9, 143.1, 145.0, 147.0, 159.8, 170.3; UV−vis (in CHCl3, λmax/nm, log ε) = 401 (4.7), 448 (4.4), 615 (3.9) and 796 (3.7); HRMS mass calcd. for C48H33N2PdSe [M-PF6]+ 823.0860, found 823.0870.

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fellowship, and K.G.T. thanks IIT Bombay for Institute PostDoctoral Fellowship.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00914. Characterization data (HRMS, 1H, 13C, 19F, 31P, and 2D NMR data) for all the reported compounds, absorption, electrochemical data, and DFT-optimized coordinates. Crystallographic data for compounds 1 and Pd(II)-1. (Both of the crystals contain alert B. This arises due to the presence of some disordered solvent molecules that could not be identified; therefore, Platon squeeze program was used to remove solvent molecules present at the void) (PDF) Accession Codes

CCDC 1834546−1834547 contain 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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REFERENCES

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Corresponding Author

*E-mail: [email protected]. Fax: 91-22-5723480. Phone: 91-22-5767176. ORCID

Mangalampalli Ravikanth: 0000-0003-0193-6081 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.R. thanks Science & Engineering Research Board, Government of India (EMR/2015/002196). S.K. thanks UGC for G


Article

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Synthesis, Structural, Spectral, and Electrochemical Studies of

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