meso-Tetrakis[4-(methoxycarbonyl)phenyl]porphyrinatopalladium(II) supported on graphene oxide nanosheets (Pd(II)-TMCPP-GO): synthesis and catalytic activity

Abstract

In this study, meso-tetrakis[4-(methoxycarbonyl)phenyl]porphynatopalladium(II) as a macrocyclic palladium complex was covalently grafted to the surface of graphene oxide (Pd-TMCPP-GO). The Pd-TMCPP-GO was characterized using microscopic and spectroscopic techniques for confirmation of functionalization. The prepared catalyst was checked in a Suzuki reaction. The catalytic system showed high catalytic activity in this reaction and the yields of the products were good to excellent. Also, the proposed catalyst was reusable for seven runs with no significant decrease in catalytic activity.

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1. Introduction

Nowadays, one of the greatest challenges in organic synthesis is the development of environmentally friendly, low-costing and efficient processes.^1–4 As a matter of fact, the discovery of efficient protocols has become even more important than high yielding synthetic methodologies.⁵ There are many advanced materials including natural products, drugs, and pharmaceuticals with biaryl structural units which possess interesting biological and pharmaceutical properties.^6–9 Some of the mentioned materials are shown in Scheme 1. The cross-coupling reactions such as Suzuki–Miyaura,¹⁰ Heck,¹¹ Stille,^12–14 Kumada,¹⁵ Negishi,¹⁶ Hiyama,¹⁷ and Sonogashira^18,19 catalyzed by palladium have revolutionized the chemical industry related to the synthesis of different natural products, agrochemicals, medicines, and supramolecular compounds. Recently, the Suzuki–Miyaura reaction has become one of the most important reactions to access the synthesis of organic compounds including the biaryl structural unit.

Scheme 1 The chemical structure of biological active materials based on biaryl unit.

The most important palladium catalysts for Suzuki reaction that have been applied in 2016 are Pd-NPs–PAN composite,²⁰ γ-Fe₂O₃–Pd,²¹ Pd(OAc)₂-SPhos,^22,23 Pd@C-dots@Fe₃O₄ NPs,²⁴ Pd–Fe₃O₄@SiO₂,²⁵ Pd(PPh₃)₄,²⁶ nano-tetramine-Pd(0),²⁷ Fe₃O₄/Py/Pd,²⁸ PFG-Pd,²⁹ and Pd/C (ligand-free).³⁰ Also, in recent years some other catalytic systems have been developed for the synthesis of biaryl compounds using Suzuki reaction.^31–40 In addition, El-Shall and co-workers reported the application of Pd nanoparticles and Pd/Fe₃O₄ supported on reduced graphene oxide in carbon–carbon cross coupling reactions.^41–43 In these studies the utility of prepared catalysts were checked in Heck, Suzuki and Sonogashira cross coupling reaction. Although these catalysts are effective catalytic systems, but these methodologies have limitation such as reaction medium, recovery of the catalyst and long reaction times.^44–46 Recently, the palladium porphyrin complexes were applied in different field of science.^47–52 In the last decade some palladium porphyrins as homogeneous catalyst have been used for the C–C coupling reactions. In 2007, Kostas and his co-workers have reported the successful synthesis of a water soluble palladium complex with a porphyrin ligand for the Suzuki–Miyaura reaction.⁵³ Subsequently in 2008, Wan and his co-worker have reported the ionic palladium porphyrin, which showed high activity in the Heck coupling reaction.⁵⁴ Although these homogeneous palladium porphyrins catalysts have contributed to improve catalytic activity significantly as a result of electron-donating functionality, they often suffered from the separation of these expensive catalysts after the reactions have completed. Furthermore, these homogeneous catalysts often result in heavy metal contamination of the desired isolated products.

In order to overcome these problems mentioned above, immobilization of homogeneous palladium porphyrins onto solid materials has been extensively employed in the field of the coupling reactions, since immobilized palladium porphyrins catalysts have the fewer of the drawbacks of homogeneous catalysts, such as the difficulties in recovery and regeneration. In 2010, Liu and co-workers have reported Pd-porphyrin supported on SBA-15 as an efficient catalyst for solvent-free Heck reaction.⁵⁵ The prepared Pd-porphyrin@SBA-15 was efficient and recyclable catalyst for Heck reaction without activity loss and Pd leaching even after nine runs. Moghadam and co-workers have studied Pd-porphyrin supported on Dowex 50WX8 and Amberlite IR-120 as efficient and reusable catalysts in C–C coupling reactions.^56,57 After reviewing the mentioned literature, the protocols were accomplished in high temperature, very long reaction times, and low active sites on catalyst support.

Recently, graphene oxide is the applicable and interesting support materials for synthesis of heterogeneous catalysts.^58–66 Therefore, the development of functionalization of graphene oxide and graphene derivatives is a hot topic to many researcher of heterogeneous catalysis. Due to the chemical, thermal, and physical properties of graphene derivatives^67,68 especially graphene oxide,⁶⁶ it is possible to immobilize metal nanoparticles,⁶⁹ metal complexes,^70,71 metal oxides,^72,73 ionic liquid,⁷⁴ and acidic sites^75,76 on the surface for application as catalyst in organic transformations.

To the best of our knowledge there is no original paper on the Pd-porphyrin supported on graphene oxide nanosheets that was applied as heterogeneous catalyst in the Suzuki reaction. Herein, in continuation of our previous studies on heterogeneous catalysis,^29,77,78 we would like to introduce a novel and versatile catalyst for the application in cross-coupling reaction. In this paper, the Pd(II)-TMCPP complex was immobilized on the surface of graphene oxide nanosheets through covalent bonding.

2. Experimental

2.1. General remarks

The chemicals used in this study, were purchased from Merck and Aldrich companies and used without further purification. NMR spectra were recorded on a Bruker DPX-400 MHz NMR spectrometer with CDCl₃ and DMSO-d₆ as solvents. FT-IR spectra were recorded using the KBr pellets on a Magna 550 Nicolet spectrometer. The field emission scanning electron microscopy (FESEM) was performed on Hitachi S4160. Transmission electron microscopy (TEM) was recorded with a Zeiss-EM10C with an acceleration voltage of 80 kV. X-ray photoelectron spectroscopy spectra (XPS) were measured on an ESCA-3000 electron spectrometer with non-monochromatized Mg Kα X-rays as the excitation source. Thermogravimetric analysis (TGA) was carried out on a Mettler TG50 instrument under air flow at a uniform heating rate of 5 °C min⁻¹ in the range 30–600 °C. Diffuse reflectance Ultraviolet-visible (DRS UV-Vis) spectra were recorded on a JASCO, V-670 spectrophotometer. The amount of Pd in the prepared catalyst was determined by a Perkin-Elmer ICP analysis. The crystallographic structure of Pd-TMCPP-GO was investigated on a Philips instrument with 1.54 Å wavelengths of X-ray beam and Cu anode material, at a scanning speed of 2° min⁻¹ from 10° to 80° (2θ). The elemental analyses (C, H, N) were obtained from a Carlo ERBA Model EA 1108 analyzer. The UV-Vis spectra of the solutions were measured on a GBC Cintra-6 UV-visible spectrophotometer. Melting points were determined with a Stuart Scientific SMP2 apparatus.

2.2. Catalyst preparation

2.2.1. Synthesis of porphyrin ligand. First of all, meso-tetrakis[4-(methoxycarbonyl)phenyl]porphyrin (H₂TMCPP) was provided according to literature.⁷⁹ In detail, freshly distilled pyrrole (1.4 mL, 20 mmol) was added to a mixture of 4-formylmethylbenzoate (3.42 g, 20 mmol) and nitrobenzene (15 mL). The mixture was refluxed for 4 h in the presence of propionic acid (70 mL) and then cooled to room temperature. The purple crystals of the ligand were filtrated and washed with distilled water and dried in oven at 80 °C (20.4%, 0.9 g). ¹H NMR (400 MHz, DMSO-d₆) for [H₂TMCPP]: δ (ppm) = 8.85 (s, 8H, β-hydrogen pyrrolic), 8.47 (d, 8H, aromatic), 8.32 (d, 8H, aromatic), 4.14 (s, 12H, OCH₃), −2.77 (s, 2H, pyrrolic hydrogens) (see ESI, Fig. 1S and 2S†), FT-IR spectra: 3310 (N–H), 2944 (C–H, sp³), 1722 (C=O), 1604 (C=N), 1275 (C–N) cm⁻¹; UV-Vis (solvent: DMF) (see ESI, Fig. 3S†): 419 (Soret band), 519, 554, 598 and 653 nm (Q bands).

2.2.2. Synthesis of Pd-porphyrin complex. The Pd-TMCPP was prepared by refluxing of H₂TMCPP (0.30 g, 0.354 mmol) and PdCl₂ (0.135 g, 0.763 mmol) in DMF (100 mL) for 12 h.⁸⁰ The purification of Pd-TMCPP was performed by repeated recrystallization and precipitation from DMF–H₂O solutions (86.1%, 0.29 g), ¹H NMR (400 MHz, DMSO-d₆) for Pd-TMCPP δ (ppm): 8.86 (s, 8H, β hydrogens pyrrolic), 8.47 (d, 8H, aromatic), 8.32 (d, 8H, aromatic), 4.11 (s, 12H, OCH₃) (see ESI, Fig. 4S†), FT-IR spectra: 2944 (C–H, sp³), 1724 (C=O), 1607 (C=N), 1275 (C–N) cm⁻¹; UV-Vis (solvent: DMF) (see ESI, Fig. 5S†): 416 (Soret band), 527 and 562 nm (Q bands).

2.2.3. Immobilization of Pd-porphyrin complex on graphene oxide. The Pd(II)-TMCPP-GO was prepared according to the following method. First, the graphene oxide was synthesized according to the previous work.⁸¹ Then, the Pd-porphyrin complex (500 mg) was dissolved in 20 mL DMF and 5.0 g of graphene oxide was added to this solution. The mixture was heated at 120 °C for 36 h. Then, the reaction mixture was slowly cooled to room temperature and the catalyst was filtered over sinter-glass (G-4) and washed exhaustively with DMF and deionized water. The prepared catalyst was dried and stored under vacuum.

2.3. Spectroscopic and physical data

2.3.1. 4-Methoxy-1,1′-biphenyl (Scheme 3, compound 3a). Mp: 87–89 °C; IR (KBr), ν (cm⁻¹): 3060, 3055, 2959, 1605, 1485, 1249, 1036. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 3.85 (3H, s), 6.98 (2H, d, J = 8.4 Hz), 7.30 (1H, t, J = 7.2 Hz), 7.42 (2H, t, J = 7.6 Hz), 7.55 (4H, t, J = 7.6 Hz). Anal. calcd for C₁₃H₁₂O: C, 84.75; H, 6.57. Found: C, 83.56; H, 5.86.

2.3.2. 2-Methoxy-1,1′-biphenyl (Scheme 3, compound 3b). Mp: oil; IR (KBr), ν (cm⁻¹): 3059, 2932, 1597, 1481, 1259, 1028. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 3.78 (3H, s), 6.98 (1H, d, J = 8.8 Hz), 7.02 (1H, t, J = 8.8 Hz), 7.28–7.33 (3H, m), 7.40 (2H, t, J = 7.2 Hz), 7.53 (2H, d, J = 5.2 Hz). Anal. calcd for C₁₃H₁₂O: C, 84.75; H, 6.57. Found: C, 82.95; H, 5.66.

2.3.3. 4-Phenylphenol (Scheme 3, compound 3c). Mp: 156–159 °C; IR (KBr), ν (cm⁻¹): 3406, 3036, 1603, 1486. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 6.93 (2H, d, J = 8.4 Hz), 7.33 (1H, t, J = 7.2 Hz), 7.44 (2H, t, J = 7.6 Hz), 7.51 (2H, d, J = 8.4 Hz), 7.56 (2H, t, J = 7.2 Hz), 8.28 (1H, d, J = 6.8 Hz). Anal. calcd for C₁₂H₁₀O: C, 84.68; H, 5.92. Found: C, 83.95; H, 5.66.

2.3.4. 1,1′-Biphenyl (Scheme 3, compound 3d). Mp: 68–69 °C; IR (KBr), ν (cm⁻¹): 3033, 1567, 1477. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.33–7.38 (2H, m), 7.45 (4H, t, J = 7.2 Hz), 7.61 (4H, d, J = 8 Hz). Anal. calcd for C₁₂H₁₀: C, 93.46; H, 6.54. Found: C, 92.75; H, 5.86.

2.3.5. 4-Nitro-1,1′-biphenyl (Scheme 3, compound 3e). Mp: 107–109 °C; IR (KBr), ν (cm⁻¹): 3098, 1597, 1513, 1476, 1344. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.46–7.53 (3H, m), 7.64 (2H, d, J = 6.8 Hz), 7.75 (2H, d, J = 8.8 Hz), 8.31 (2H, d, J = 6.8 Hz). Anal. calcd for C₁₂H₉NO₂: C, 72.35; H, 4.55; N, 7.03. Found: C, 71.18; H, 4.66; N, 6.44.

2.3.6. 2-Hydroxy-5-phenylbenzaldehyde (Scheme 3, compound 3f). Mp: 96–99 °C; IR (KBr), ν (cm⁻¹): 3419, 3043, 2856, 1673, 1603, 1465. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 6.94 (1H, d, J = 8.8 Hz), 7.54 (2H, t, J = 7.6 Hz), 7.63 (2H, m), 7.70 (1H, d, J = 2.8 Hz), 8.26 (1H, d, J = 1.2 Hz), 8.29 (1H, d, J = 1.2 Hz), 10.96 (1H, s). Anal. calcd for C₁₃H₁₀O₂: C, 78.77; H, 5.09. Found: C, 77.97; H, 4.76.

2.3.7. 1,4-Diphenylbenzene (Scheme 3, compound 3g). Mp: 189–192 °C; IR (KBr), ν (cm⁻¹): 3030, 1621, 1461. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.27–7.31 (2H, m), 7.39 (4H, t, J = 7.6 Hz), 7.57 (4H, d, J = 7.2 Hz), 7.61 (4H, s). Anal. calcd for C₁₈H₁₄: C, 93.87; H, 6.13. Found: C, 92.97; H, 5.36.

2.3.8. 5-Phenylnicotinic acid (Scheme 3, compound 3h). Mp: 252–254 °C; IR (KBr), ν (cm⁻¹): 3431, 3076, 1676, 1603, 1442. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.51 (3H, t, J = 7.6 Hz), 7.59 (2H, t, J = 7.6 Hz), 7.69 (1H, s), 8.23 (1H, s), 8.25 (1H, s). Anal. calcd for C₁₂H₉NO₂: C, 72.35; H, 4.55; N, 7.03. Found: C, 71.97; H, 4.36; N, 6.98.

2.3.9. 4-Phenylbenzene carboxylic acid (Scheme 3, compound 3i). Mp: 217–219 °C; IR (KBr), ν (cm⁻¹): 3431, 1679, 1603, 1442, 1306. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.32 (1H, t, J = 7.4 Hz), 7.40 (2H, dd, J = 7.3 Hz), 7.55 (2H, d, J = 6.9 Hz), 7.71 (2H, d, J = 8.2 Hz), 8.15 (2H, d, J = 7.5 Hz). Anal. calcd for C₁₃H₁₀O₂: C, 78.77; H, 5.09; found: C, 77.67; H, 4.36.

2.3.10. 3-Phenylbenzaldehyde (Scheme 3, compound 3j). Mp: oil; IR (KBr), ν (cm⁻¹): 3060, 2826, 2728, 1697, 1594, 1476. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.41–7.45 (1H, m), 7.48–7.52 (2H, m), 7.63 (2H, d, J = 7.2 Hz), 7.66 (1H, t, J = 1.6 Hz), 7.87–7.89 (2H, dd, J = 6.4 Hz), 8.13 (1H, s), 10.11 (1H, s). Anal. calcd for C₁₃H₁₀O: C, 85.69; H, 5.53; found: C, 85.17; H, 4.96.

2.3.11. 4-Phenylbenzaldehyde (Scheme 3, compound 3k). Mp: 56–58 °C; IR (KBr), ν (cm⁻¹): 3066, 1710, 1588, 1476, 765, 691. ¹H NMR (400 MHz, CDCl₃) δ (ppm): 7.45 (1H, t, J = 6.8 Hz), 7.53 (2H, t, J = 7.1 Hz), 7.77 (2H, d, J = 7.8 Hz), 7.91 (2H, d, J = 7.8 Hz), 7.99 (2H, d, J = 8.2 Hz), 10.06 (1H, s, CHO). Anal. calcd for C₁₃H₁₀O: C, 85.69; H, 5.53; found: C, 84.67; H, 4.76.

2.3.12. Quaterphenyl (Scheme 3, compound 3l). Mp: 298–300 °C; IR (KBr), ν (cm⁻¹): 3031, 1620, 1479. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.36–7.40 (2H, m), 7.48 (4H, t, J = 7.2 Hz), 7.66 (4H, d, J = 9.2 Hz), 7.70 (8H, s). Anal. calcd for C₂₄H₁₈: C, 94.08; H, 5.92; found: C, 93.17; H, 4.76.

2.3.13. 2-Hydroxy-3,5-diphenylbenzaldehyde (Scheme 3, compound 3m). Mp: 114–116 °C; IR (KBr), ν (cm⁻¹): 3426, 3047, 2848, 1651, 1607, 1459. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 7.38–7.44 (2H, m), 7.46–7.50 (4H, m), 7.59–7.67 (4H, m), 7.78 (1H, d, J = 2.4 Hz), 7.88 (1H, d, J = 2.4 Hz), 10.05 (1H, s), 11.52 (1H, s). Anal. calcd for C₁₉H₁₄O₂: C, 83.19; H, 5.14; found: C, 83.01; H, 4.96.

2.3.14. (4-Hydroxy-[1,1′-biphenyl]-3,5-diyl)dimethanol (Scheme 3, compound 3n). Mp: 90–93 °C; IR (KBr), ν (cm⁻¹): 3412, 3028, 1601, 1480. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 4.90 (2H, s), 5.23 (2H, s), 7.13 (1H, s), 7.44 (2H, t, J = 7.2 Hz), 7.49 (1H, s), 7.54 (2H, t, J = 7.6 Hz), 7.93 (1H, s), 7.95 (1H, s). Anal. calcd for C₁₄H₁₄O₃: C, 73.03; H, 6.13; found: C, 72.17; H, 5.36.

3. Results and discussion

3.1. Preparation of Pd(II)-TMCPP-GO

At first, we prepared Pd(II)-TMCPP-GO as a novel catalyst for application in Suzuki reaction (Scheme 2). The porphyrin ligand was synthesized according to the literature⁷⁹ (see ESI, Scheme 1S†). Then, the porphyrin ligand was metalated using PdCl₂.⁸⁰ The synthesized porphyrin and Pd-porphyrin complex were successfully achieved and confirmed using ¹H NMR, FT-IR and UV-Vis spectroscopy.

Scheme 2 The synthetic strategy for the preparation of Pd(II)-TMCPP-GO.

In the following, the Pd-porphyrin complex was reacted with hydroxyl groups on the surface of graphene oxide. According to the literature,⁸² the covalent bonds is a useful linkage for stabilization of organic or ligand on solid support. After the preparation of Pd(II)-TMCPP-GO catalyst, it was characterized using FT-IR, FESEM, TEM, EDX, XPS DRS UV-Vis, XRD, TGA and ICP.

3.2. Characterization of Pd(II)-TMCPP-GO

FT-IR spectroscopy was applied for characterization of porphyrin, Pd-porphyrin, graphene oxide and Pd(II)-TMCPP-GO (Fig. 1). As shown in Fig. 1, the IR absorption frequencies were different for porphyrin and Pd-porphyrin complex (Fig. 1a and b). It was found that the N–H bond stretching and bending frequencies of porphyrin ligand located at ∼3310 cm⁻¹ and 960 cm⁻¹. While, the palladium ion was inserted into the porphyrin ring, the N–H bond vibration frequency of porphyrin ligand was disappeared and the characteristic functional groups of Pd–N bond formed at ∼1000 cm⁻¹ which indicated that the formation of Pd-porphyrin complex.⁸³ The bands as 1722 and 1604 cm⁻¹ were assigned to carbonyl of ester and C=N, respectively. It also could be observed that the FT-IR spectrum of graphene oxide was confirmed using previous work.⁸¹ Moreover, it could be well observed that the Pd-complex was successively immobilized on the surface of graphene oxide via covalent linkages (Fig. 1d).

Fig. 1 FT-IR spectra of (a) synthesized porphyrin (b) Pd-porphyrin complex (c) graphene oxide (d) Pd(II)-TMCPP-GO.

Fig. 2 shows SEM images and EDX of graphene oxide and Pd(II)-TMCPP-GO catalyst. The FESEM image of graphene oxide was confirmed the exfoliated graphene sheet with lateral dimensions of several micrometer plates. The morphology of the prepared catalyst was changed relative to pristine graphene oxide. Also, the EDX of graphene oxide and Pd(II)-TMCPP-GO prove the elemental analysis of desired materials. Fig. 2e represents TEM image of the Pd(II)-TMCPP-GO catalyst. It can be seen the complex Pd(II)-TMCPP was supported on the surface of graphene oxide that assigned by red circle and this is good reason for the presence of Pd in this catalyst.

Fig. 2 SEM images of (a) graphene oxide (b) Pd(II)-TMCPP-GO catalyst and EDX of (c) graphene oxide (d) Pd(II)-TMCPP-GO catalyst (e) TEM image of Pd(II)-TMCPP-GO catalyst.

Fig. 3 shows the XRD patterns of graphene oxide and Pd(II)-TMCPP-GO. The peaks appeared at 40.11°, 43.61° and 68.22° could be assigned to the diffraction of (111), (200) and (220) plane of the face centered cubic palladium (card no. 87-0637).^84,85 The peak at ∼25° attributed to the (002) plane of hexagonal graphite structure, indicating that the Pd-porphyrin complex were introduced to the structural of graphene sheets.

Fig. 3 XRD pattern of (a) graphene oxide (b) Pd(II)-TMCPP-GO catalyst.

Additional confirmation of the successful chemical attachment of Pd-TMCPP on graphene oxide was provided using XPS spectrum. Fig. 4a depicts the XPS spectrum of the C 1s region for graphene oxide. The peaks appeared at 289, 287.5, 285.7 and 283.8 eV can be assigned to –COOH, O–C–O, C–OH, and C(sp²)–C(sp²) bonded.^86–88 Comparison between the XPS spectrum of C 1s region for graphene oxide and Pd(II)-TMCPP-GO (Fig. 4b) showed that all of binding energy of catalyst slightly shift to higher energy. These observation shows that the chemical attachment of Pd-TMCPP on the surface of graphene oxide was occurred via esteric bonds and they should be caused higher binding energy of C 1s. Fig. 4c shows the XPS spectrum of Pd 3d region for the Pd(II)-TMCPP-GO. The binding energies of this region show that all of the Pd are as Pd(II) consistent with the observed binding energies of 338 eV (Pd(II) 3d_5/2) and 344.3 eV (Pd(II) 3d_3/2).⁸⁹ The Pd 3d_5/2 binding energy of 338 eV is in agreement with Pd bonded to nitrogen found for similar compounds.^90,91

Fig. 4 XPS spectra of (a) C 1s region for graphene oxide (b) C 1s region for Pd-TMCPP-GO catalyst (c) Pd 3d region for Pd-TMCPP-GO catalyst.

The DRS UV-Vis spectra of graphene oxide and Pd(II)-TMCPP-GO were shown in Fig. 5. The spectrum of graphene oxide shows a strong UV absorption at 239 nm and a higher wavelength and lower energy at 277 nm.⁹² The spectrum of Pd(II)-TMCPP-GO was shown in Fig. 5b. The strong absorption of graphene oxide at 239 nm is blue-shift to 221 nm. The other peak due to porphyrin ligand is very weak. Also, all of the peaks related to the Pd-complex are visible at 416, 527 and 562 nm. These data reveal that the Pd-porphyrin complex was immobilized on graphene sheets.^93,94

Fig. 5 DRS UV-Vis of (a) graphene oxide (b) Pd(II)-TMCPP-GO catalyst.

In the UV-Vis spectrum, during the synthesis of Pd-TMCPP the initial absorption bands at 419, 519, 554, 598 and 653 nm were shifted and disappeared and new bands appeared at 416 (Soret band), 527 and 562 nm (Q band) in DMF solution, indicating that palladium ion was inserted into the porphyrin ring (see ESI, Fig. 3S and 5S†). In the ¹H NMR spectrum of Pd-TMCPP in DMSO-d₆, the signal at −2.77 ppm corresponding to pyrrolic hydrogens in H₂TMCPP was disappeared in Pd-TMCPP which confirmed the synthesis and metalation of porphyrin (see ESI, Fig. 1S, 2S, and 4S†).

Fig. 6 shows the thermogram of Pd(II)-TMCPP-GO. The TGA analysis of Pd(II)-TMCPP-GO shows four main weight losses. The first one is attributed to adsorbed water in the structural of catalyst (4.95%). The second and third one are occurred at 212–313 °C and they related to the decomposition of Pd-porphyrin (5.03% & 4.04%). Thus, TGA analysis indicates the high thermal stability of Pd(II)-TMCPP-GO. The later loss weight is attributed to decomposition of graphene sheets at ∼450 °C (8.59%).

Fig. 6 TGA of Pd-TMCPP-GO catalyst.

3.3. Catalytic evaluation of Pd(II)-TMCPP-GO

After preparation and characterization of Pd(II)-TMCPP-GO, the Suzuki reaction was checked for catalytic evaluation of the catalytic system. The Pd(II)-TMCPP-GO-catalyzed reaction between phenylboronic acid and 4-iodoanisole as a model reaction was employed to obtain excellent optimum condition. As shown in Table 1, different solvents were selected for evaluation of medium reaction (Table 1, entries 1–7). The H₂O : DMF (1 : 2) was selected as the best media for the reaction of solvent. It seems that the water attack to phenylboronic acid to form Ph-B(OH)₃⁻ and facilitates the elimination of B(OH)₃.⁵⁷ As reported in the literature, a suitable amount of water is very important for improving the reactivity of Suzuki reactions.^95,96 For this purpose, we studied the impact of the water and volume ratio of water–DMF on the yield of the Suzuki reaction. 75% and 65% of coupling product were formed when just water and/or DMF were used as the solvent. Upon changing the volume ratio of water to DMF from 2 : 1 to 1 : 1, the yield of the product increased from 79 to 88%. The best yield (96%) was obtained when the volume ratio of water–DMF reached 1 : 2.

Table 1 Optimization of Pd-TMCPP-GO-catalyzed Suzuki reaction between 4-iodoanisole and phenylboronic acid^a

Entry	Base	Solvent	Catalyst (mol%)	T (°C)	Yield^b (%)
a Reaction conditions: 1a (1 mmol), 2 (1.1 mmol), base (1.5 mmol), solvent (3 mL).b Isolated yield.c 2 mmol of base was used.d 1 mmol of base was used.e 1.3 mmol of phenylboronic acid was used.f 1.0 mmol of phenylboronic acid was used.
1	K2CO3	H2O	0.7	80	75
2	K2CO3	DMF	0.7	100	65
3	K2CO3	EtOH : H2O	0.7	80	70
4	K2CO3	NMP : H2O	0.7	80	85
5	K2CO3	H2O : DMF (2 : 1)	0.7	80	79
6	K2CO3	H2O : DMF (1 : 1)	0.7	80	88
7	K2CO3	H2O : DMF (1 : 2)	0.7	70	96
8	Na2CO3	H2O : DMF (1 : 2)	0.7	70	63
9	NaOH	H2O : DMF (1 : 2)	0.7	70	58
10	K3PO4	H2O : DMF (1 : 2)	0.7	70	65
11	K2CO3	H2O : DMF (1 : 2)	0.7	60	90
12	K2CO3	H2O : DMF (1 : 2)	0.9	70	96
13	K2CO3	H2O : DMF (1 : 2)	0.5	70	80
14	K2CO3	H2O : DMF (1 : 2)	0.7	70	96^c
15	K2CO3	H2O : DMF (1 : 2)	0.7	70	70^d
16	K2CO3	H2O : DMF (1 : 2)	0.7	70	89^e
17	K2CO3	H2O : DMF (1 : 2)	0.7	70	73^f

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Also, some bases were investigated and K₂CO₃ was chosen as the excellent base for this reaction (Table 1, entries 8–10). By change of reaction temperature the reaction yield was changed and the temperature 70 °C was selected as the best temperature (Table 1, entries 10, 11). The catalytic amount of catalyst was also optimized (Table 1, entries 11–13). The amount of catalyst loading was selected 30 mg which is equal to 0.7 mol% of Pd. In these experiments, the entry 7 was the best condition for Suzuki reaction. The byproduct (biphenyl) of Suzuki reaction is about 1% (GC analysis), demonstrating that this catalytic system is highly efficient for Suzuki reaction under heterogeneous conditions. In entries 14–17, the different mmol of base and phenylboronic acid were investigated and 1.1 mmol and 1.5 mmol of phenylboronic acid and base were the best amount, respectively.

After optimization of Suzuki reaction in the presence of Pd(II)-TMCPP-GO several compounds including biaryl units using phenylboronic acid and aryl halides were synthesized (Scheme 3). As can be seen, different aryl halides with electron-donating and electron-withdrawing groups were resulted to products in good to excellent yields and normally time of the reaction.

Scheme 3 Products of Suzuki reaction in the presence of Pd(II)-TMCPP-GO; all yields are isolated; general reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.1 mmol), base (1.5 mmol), solvent (3 mL).

One of the ways to stabilization of Pd could be achieved by a constant but reluctant release of Pd from the catalyst. If the release of Pd from catalyst controllable and reversible, only low amounts of “free” Pd will be present in solution. Since the probability of agglomeration and deactivation of Pd is then significantly lower, overall catalytic activity should be increased.⁹⁷ This principle is commonly not considered. Because releasing and destruction of the initial Pd compound is something intrinsic to many homogeneous and heterogeneous Pd catalysts.^98,99 To overcome this problem, it is necessary to find a catalyst that remains stable even under the harsh conditions that needed for the activation and conversion of even aryl chlorides, but generates sufficient amounts of coordinatively unsaturated Pd. The Pd porphyrins are thermal and chemical stable complexes, complete coordinative saturation of the central atom, and a preference for the Pd(II) oxidation state. Such compounds possess an extraordinarily high complex stability, so the release of Pd under coupling reaction conditions would be controllable. In Scheme 4, the mechanism of production of free Pd(0) and continued it application in catalytic cycle was represented. Depending on the reaction and catalytic systems such as complex stability and reduced reaction conditions, route A or route B could be occurred. After releasing of free Pd(0), the first step is oxidative addition of Pd(0) to the Ar–X to form the organopalladium specie (I). The reaction with KOH that it generated from aqueous K₂CO₃, gives intermediate (II) which via transmetalation with the boronate complex (III) forms the organopalladium specie (IV). The reductive elimination of the biaryl product (final product) restores original free Pd(0) which completed catalytic cycle and to start next catalytic cycle or go back to the hole of porphyrin.

Scheme 4 The proposed pathway for Suzuki reaction in the presence of Pd-TMCPP-GO.

Also, the reusability and recoverability of the Pd(II)-TMCPP-GO catalyst was also evaluated in the preparation of compound 3a using 4-bromoanisole as a starting aryl halide for 60 min. At the end of the each run, the reused catalyst was isolated using simple filtration and washed completely using DMF, water and acetone. The results show that the Pd(II)-TMCPP-GO catalyst could be recovered seven runs without any loss of catalytic activity. The analytical amount of Pd leached was determined using ICP analysis (Table 2). The porphyrin ligand prevents the realizing Pd ion in any obvious amount.¹⁰⁰ The structural and nature of the reused catalyst was investigated by DRS UV-Vis spectroscopy, FT-IR and FESEM analysis (see ESI, Fig. 6S–8S†).

Table 2 Reusability of the Pd(II)-TMCPP-GO catalyst in Suzuki reaction^a

Entry	Yield^b (%)	Pd-leached^c (%)
a Reaction conditions: 4-bromoanisole (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), DMF : H2O (2 : 1) (3 mL), T: 70 °C, time: 60 min.b Isolated yield.c Determined by ICP.
1	92%	0.65
2	92%	0.60
3	91%	—
4	90%	—
5	87%	—
6	86%	—
7	85%	1.12%

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The XPS of the reused catalyst after 7^th run was provided (Fig. 7a). The binding energies of the Pd(II) 3d_5/2 and 3d_3/2 electrons at 337.2 and 343.8 eV, corresponding to the Pd(II) ion. Based on proposed mechanism, during the cross-coupling reaction, partial in situ reduction of Pd(II) to Pd(0) occurs most likely by the solvent under basic conditions. This low amount of in situ reduction is confirmed by the measured binding energies of the Pd(0) 3d_5/2 and 3d_3/2 electrons after the reaction at 334.6 and 341.1 eV, respectively. In contrast to the initial catalyst, both oxidation states of Pd(0) and Pd(II) was observed in the reused catalyst. The remarkable activity and recyclability of this catalyst appears to be clearly related to the high concentration of Pd(II) that control releasing and reduction of Pd(II) in the supported catalyst. If these reductive Pd cannot go back the porphyrin ring and metalated again, they will be aggregate and produce Pd black and occupy surface of GO that caused deactivation of catalyst. However, the low release of Pd by porphyrin ligand retards the agglomeration and deactivation of this catalyst.

Fig. 7 (a) XPS spectra of Pd 3d region for Pd-TMCPP-GO (7^th run) (b) TEM image of reused Pd-TMCPP-GO (7^th run).

To confirm the XPS results, the TEM image of the reused catalyst after 7^th runs was provided to investigate the possible deactivation mechanism of the Pd-TMCPP-GO catalysts following the recycling experiments. The comparison between TEM images of initial catalyst and reused Pd-TMCPP-GO (Fig. 7b) showed slight degree of agglomeration of in reused catalyst. This result indicates that the mechanism of deactivation is likely to involve the formation of agglomerated Pd black.

The efficient utility of this protocol in Suzuki reaction was compared in Table 3. The suggested catalyst is robust and superior to some of the previously reported heterogeneous catalyst in reaction time, yield of the reaction, Pd leaching, and recoverable catalytic systems. These results are eventuated (i) Pd insertion between four nitrogen atoms (no/less Pd leached) (ii) high specific surface area of graphene oxide (iii) strong covalently linkage of Pd-complex on the support (iv) high thermal stability of the prepared catalyst.

Table 3 Comparison of the results of synthesis of compound 3a, using Pd(II)-TMCPP-GO with those obtained by the reported catalyst

Entry	Catalyst	Reaction conditions	Yield^a (%)	Halide	Reference
a Isolated yield.
1	GO-NHC-Pd	1 mol% Pd, Cs2CO3, DMF : H2O, 50 °C, 1 h	81	Br	101
2	GO-NHC-Pd(II)	0.25 mol% Pd, K2CO3, EtOH : H2O, reflux, 18 h	89	Br	102
3	Pd-Schiff base@MWCNT	0.1 mol% Pd, K2CO3, DMF : H2O, 60 °C, 3 h	99	Br	103
4	G-NH2-Pd(0)	1 mol% Pd, K2CO3, EtOH : H2O, 60 °C, 0.5 h	90	I	104
5	Pd(II)-TMCPP-GO	0.7 mol% Pd, K2CO3, DMF : H2O, 70 °C, 15 min	96	I	This work
6	Pd(II)-TMCPP-GO	0.7 mol% Pd, K2CO3, DMF : H2O, 70 °C, 40 min	92	Br	This work

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4. Conclusion

In conclusion, we have introduced a novel and efficient Pd-macrocyclic complex supported on the surface of graphene oxide. The provided catalyst was characterized using FESEM, TEM, EDX, XPS, XRD, FT-IR, TGA and ICP analysis. The Pd(II)-TMCPP-GO catalyst was used in the carbon–carbon coupling reaction using aryl halide and phenylboronic acid (Suzuki reaction). The suggested heterogeneous catalytic system was efficient catalyst for Suzuki reaction because less leaching of Pd, high specific surface area of graphene oxide, thermal and chemical stability is worthwhile advantages the suggested protocols. Also, the Pd(II)-TMCPP-GO was reusable for seven times without significant in decreasing in its catalytic activity and leaching of Pd. We believed that the Pd(II)-TMCPP-GO can be used in other Pd-catalyzed organic reactions.

Acknowledgements

The financial support from the research council of Malek-Ashtar University of Technology is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21223a

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