N-Heterocyclic carbene palladium complex supported on ionic liquid-modified graphene oxide as an efficient and recyclable catalyst for Suzuki reaction

Abstract

An N-Heterocyclic carbene palladium complex immobilized on graphene oxide (GO) with an ionic liquid framework (NHC-Pd/GO-IL) was synthesized by the modification of GO through a silylation reaction. The obtained catalyst displayed a high activity toward the Suzuki reaction in EtOH–H₂O (1 : 1).

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The interest in ionic liquids (ILs) has increased dramatically in the past decade due to their unique properties such as low vapour pressure, low toxicity, high chemical and thermal stabilities, and wide liquid range and favourable solvating properties.¹ Mehnert et al. described the concept of supported ionic liquid catalysis, which involves the treatment of a monolayer of a covalently attached ionic liquid on the surface of silica gel with an additional ionic liquid.² These layers serve as the reaction phase in which the homogeneous catalyst is dissolved. When ionic-liquid-supported (ILS) catalysts are used for organic reactions that are carried out in a homogeneous system, they may have a similar or even higher catalytic activity and selectivity, compared to the homogeneous parent catalyst system. The active species – although the resulting material is a solid – is dissolved in the ionic liquid and acts like a homogeneous catalyst.³ Several studies have successfully confined various ionic liquid phases to the surface of different supports and applied them in some catalytic processes.⁴

Recently, graphene which is a single layer of sp² carbon atoms bonded in a hexagonal lattice has attracted great attention from scientists all over the world. The extraordinary properties of these exfoliated graphene sheets, such as their extremely large surface area,⁵ fast charge mobility,⁶ remarkably high mechanical strength and Young's modulus,⁷ excellent chemical stability⁸ and low manufacturing cost make graphene a promising candidate to disperse or immobilize catalytically active species for heterogeneous catalysis.⁹ Recently, the silylation modification technique on graphene oxide was reported and can provide graphene nanocomposites with catalytic activities.¹⁰

Aryl–aryl bond formation via the palladium-catalyzed Suzuki cross coupling reaction is one of the most important tools of modern organic synthesis. The Suzuki cross coupling reaction is one of the most general and powerful tools for the synthesis of pharmaceuticals, herbicides, polymers, liquid crystals, natural products, ligands for catalysis and advanced materials.¹¹

Herein, we report a novel protocol for the Suzuki cross coupling catalyzed by an N-heterocyclic carbene palladium complex immobilized on GO with an ionic liquid framework (NHC-Pd/GO-IL). Additionally, the effects of solvent polarity, base, and temperature on the yield and recycling potential of the catalyst have all been assessed.

The process for the preparation of the NHC-Pd/GO-IL nanocomposite is schematically described in Scheme 1. First, the reaction of a sub-stoichiometric amount of Pd(OAc)₂ with an excess of imidazolium ionic liquid (IL) afforded the N-heterocyclic carbene palladium complex (NHC-Pd)-imidazolium ionic liquid (IL). In order to graft the NHC-Pd-IL onto the surface of the graphene oxide, a reaction of the NHC-Pd complex in the IL and graphene oxide was carried out in EtOH under reflux conditions for 12 h.

Scheme 1 Schematic description for the preparation of NHC-Pd/GO-IL.

The NHC-Pd/GO-IL nanocomposite was characterized by FT-IR, SEM, TEM, Raman, XPS, TGA and EDS mapping measurements. The FT-IR spectra of GO and the NHC-Pd/GO-IL nanocomposite are shown in (Fig. 1). In the spectrum of GO, strong absorption bands at 1732, 1068 and 3428 cm⁻¹ correspond to the stretching vibrations of C=O, C–O (epoxy) and O–H, respectively. The peaks at 1623 and 1286 cm⁻¹ correspond to the vibration of the carboxyl groups.¹² The FT-IR spectrum of NHC-Pd/GO-IL exhibited three new peaks around 1567, 1036 and 751 cm⁻¹, compared with the FT-IR spectroscopy of GO. These peaks corresponded to the ring vibration of the imidazole,¹³ and stretching vibrations of Si–O and Si–O–C, respectively, and provide direct evidence for the successful silylanization of GO.¹⁴

Fig. 1 FTIR spectra of GO and the NHC-Pd/GO-IL nanocomposite.

A scanning electron microscopy (SEM) image of the GO displayed two-dimensional structures with a crumpling feature (Fig. 2a). The density and distribution of the NHC-Pd and IL groups on the NHC-Pd/GO-IL nanocomposite were evaluated by quantitative energy dispersive X-ray spectroscopy (EDS) mapping. As can be seen in Fig. 2b–d, rather than only being located at the edges of the graphene sheets, the elements N, Pd and Si were found to be uniformly dispersed on the whole surface of the NHC-Pd/GO-IL nanocomposite indicating the homogeneous distribution of the NHC-Pd and IL groups.

Fig. 2 SEM image of (a) NHC-Pd/GO-IL, and corresponding quantitative EDS elemental mapping of (b) N, (c) Pd and (d) Si.

The Raman spectra of GO and NHC-Pd/GO-IL (Fig. 3) show an obvious blue shift of the G band from 1588.52 to 1594.83 cm⁻¹, probably due to the gradually increased compressive local stress induced by the attached NHC-Pd and IL on the surface of graphene oxide.¹⁵

Fig. 3 Raman spectra of GO and NHC-Pd/GO-IL nanocomposite.

The electronic properties of the nanocomposite were probed by X-ray photoelectron spectroscopy (XPS) analysis. As shown in Fig. 4, the peaks corresponding to Cl 2p & 2s, N 1s, Pd 3s & 3p & 3d, and Si 2s & 2p can be clearly observed in the XPS full spectrum. A pronounced peak at 101.9 eV (in the Si 2p XPS spectrum) which corresponds to the bonding energy of Si–O–C can be easily observed. The N 1s spectra could be divided into two peaks, one peak at 399.2 eV ascribed to sp² nitrogen in the aromatic ring of the NHC-Pd(II) complex and another peak at 401.2 eV attributed to the quaternary nitrogen of the IL.¹⁶ By integrating the area in the XPS the ratio of sp² nitrogen : quaternary nitrogen was found to be 1 : 9 which supported the hypothetical ratio used for the preparation of the catalyst NHC-Pd/GO-IL nanocomposite. The XPS spectrum of the Pd 3d core level region for the NHC-Pd/GO-IL nanocomposite displays main peaks at 337.7 and 342.9 eV which can be attributed to the binding energy of Pd 3d_5/2 and Pd 3d_3/2, respectively. These values correspond to the Pd(II) binding energies of the NHC-Pd(II) complex. Additionally, the formation of the NHC-Pd complex was confirmed by the Pd 3d_5/2 binding energy values changing from 338.8 eV to 337.7 eV after the ionic liquid was reacted with Pd(OAc)₂.¹³

Fig. 4 Full range XPS spectrum (a), and the Si 2p (b), N 1s (c), Pd 3d (d) core level region XPS spectra of the NHC-Pd/GO-IL nanocomposite.

Thermogravimetric analysis (TGA) was further used to study the composition of the NHC-Pd/GO-IL nanocomposite. TGA plots of GO and NHC-Pd/GO-IL nanocomposite are shown in Fig. 5. For GO, the weight lost was about 57.7% at temperatures below 154 °C, due to pyrolysis of the labile oxygen-containing functional groups. In contrast, the NHC-Pd/GO-IL nanocomposite showed good thermal stability. The total weight lost was only 6.8% at temperatures below 169 °C.

Fig. 5 TGA plots of GO and the NHC-Pd/GO-IL nanocomposite.

The catalytic activity of the NHC-Pd/GO-IL nanocomposite as a precatalyst was then tested in the Suzuki reaction. To optimize the reaction conditions, phenyl boronic acid and phenyl iodide were selected as model substrates in the presence of various bases and solvents (Table 1). By using H₂O as the solvent, the reaction was tested employing various bases such as K₂CO₃, Et₃N, K₃PO₄, and KOH at 60 °C (Table 1, entries 1–4). A superior yield was obtained when K₂CO₃ was used as the base (Table 1, entry 1). Then, different solvents such as EtOH, MeOH, CH₂Cl₂, CH₃CN and EtOH–H₂O (1 : 1) were screened in the model reaction using K₂CO₃ as base (Table 1, entries 5–9). It was found that the reaction using EtOH–H₂O (1 : 1) after 2.5 h resulted in a higher yield. The effect of NHC-Pd/GO-IL loading was also investigated under the optimum reaction conditions (Table 1, entries 5, 10 and 11). It was found that when the amount of NHC-Pd/GO-IL increased from 0.02 to 0.05 and 0.1 mol%, the yields increased from 42 to 58 and 98%, respectively. It was found that 0.1 mol% NHC-Pd/GO-IL is sufficient to push this reaction forward (Table 1, entry 5). To optimize the reaction temperature, we also performed several experiments at 50 °C, 60 °C and 70 °C in the presence of K₂CO₃ in EtOH–H₂O (1 : 1) using 0.1 mol% NHC-Pd/GO-IL (Table 1, entries 5, 12 and 13). As can be seen from Table 1, the most suitable reaction temperature is 60 °C.

Table 1 Screening of the reaction conditions^a

Entry	Solvent	Base	Temp (°C)	Yield (%)
a Phenyl iodide (1.0 mmol), phenyl boronic acid (1.3 mmol), base (3.0 mmol), NHC-Pd/GO-IL (0.1 mol% Pd), 60 °C, 2.5 h and solvent (2 ml). GC yield, n-dodecane was used as an internal standard.b Catalyst (0.02 mol% Pd).c Catalyst (0.05 mol% Pd).
1	H2O	K2CO3	60	76%
2	H2O	Et3N	60	Trace
3	H2O	K3PO4	60	15%
4	H2O	KOH	60	36%
5	H2O–EtOH (1 : 1)	K2CO3	60	98%
6	EtOH	K2CO3	60	64%
7	MeOH	K2CO3	60	22%
8	CH2Cl2	K2CO3	Reflux	Trace
9	CH3CN	K2CO3	60	Trace
10^b	H2O–EtOH (1 : 1)	K2CO3	60	42%
11^c	H2O–EtOH (1 : 1)	K2CO3	60	58%
12	H2O–EtOH (1 : 1)	K2CO3	50	81%
13	H2O–EtOH (1 : 1)	K2CO3	70	98%

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With the optimized reaction conditions in hand, we next examined the scope and limitations of the Suzuki reaction with various types of iodo-, bromo-, and chloroaryl derivatives and arylboronic acids (Table 2).

Table 2 NHC-Pd/GO-IL nanocomposite catalyzed Suzuki reaction^a

Entry	Aryl halide	Aryl boronic acid	Product	Yield^b
a Reaction conditions: aryl halide (1.0 mmol), aryl boronic acid (1.3 mmol), K2CO3 (3.0 mmol), NHC-Pd/GO-IL (0.1 mol% Pd), 60 °C, 2.5 h and EtOH–H2O (1 : 1) (2 ml).b Isolated yield by column chromatography.c TBAB (1 mmol), catalyst (0.2 mol% Pd) and 90 °C, 12 h.
1				98%
2	1a			98%
3		2a	3b	98%
4		2a		82%
5		2a		61%
6		2a		98%
7		2a	3a	92%
8		2a	3b	85%
9		2a	3e	94%
10		2a		98%
11		2b		93%
12^c		2a	3a	42%
13^c		2a	3b	57%
14^c		2a	3e	69%
15^c		2a	3f	74%

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It can be seen from Table 2 that the Suzuki reaction proceeded in good yields with most substrates. Aryl bromides and iodides bearing electron-donating and electron-withdrawing groups reacted well and gave good yields. The hindered substrates such as 2-iodo-1,3,5-trimethylbenzene converted to the corresponding product with a moderate yield (Table 2, entry 5). Unfortunately, the catalytic system was less effective for the reaction of aryl chlorides and moderate yields were obtained in the presence of tetra butyl ammonium bromide (TBAB) (Table 2, entries 13–15).

The reusability of the NHC-Pd/GO-IL nanocomposite was examined in the Suzuki reaction of phenyl boronic acid with phenyl iodide. It was found that recovery can be successfully achieved in five successive reaction runs (Table 3).

Table 3 Reusability of the NHC-Pd/GO-IL nanocomposite in the Suzuki reaction of phenyl boronic acid with phenyl iodide^a

a Phenyl iodide (1.0 mmol), phenyl boronic acid (1.3 mmol), K2CO3 (3.0 mmol), NHC-Pd/GO-IL (0.1 mol% Pd), 60 °C, 2.5 h and EtOH–H2O (1 : 1) (2 ml).b GC yield, n-dodecane was used as an internal standard.
Reaction cycle	1st	2nd	3rd	4th	5th
Yield^b (%)	98	97	95	92	90

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The heterogeneous nature of the precatalysis was proved using a hot filtration test and AAS analysis. To determine whether the catalyst was actually functioning in a heterogeneous manner or whether it was merely a reservoir for more active soluble palladium species, we performed a hot filtration test in the Suzuki reaction of phenyl boronic acid with phenyl iodide after ∼50% of the coupling reaction had completed. The hot filtrates were then transformed to another flask containing K₂CO₃ (3 equiv.) in EtOH–H₂O (1 : 1) at 60 °C. Upon further heating of the catalyst-free solution for 3 h, no considerable progress (∼5% by GC analysis) was observed. Moreover, AAS of the same reaction solution at the midpoint of completion indicated that no significant quantities of palladium were lost to the reaction liquors during the process.

A TEM image of the fresh NHC-Pd/GO-IL nanocomposite displays a crumpled and layer like structure without any nanoparticles (Fig. 6a). As shown in Fig. 6b and c, well distributed fine Pd nanoparticles (Pd NPs) with sizes of less 5 nm dispersed on the GO-IL sheets can be observed after one and three runs of the recycled NHC-Pd/GO-IL nanocomposite. This is in agreement with the previously reported results that the formation of Pd nanoparticles as an active spices were evolved during the catalytic reaction. The formation of Pd nanoparticles during the Suzuki reaction is supposed to be due to the reduction of Pd(II) by phenyl boronic acid.²¹ To demonstrate this assumption, TEM images after experiments without boronic acid (PhI and K₂CO₃) and without all reagents (only the K₂CO₃) show that the Pd nanoparticles are not formed during these experiments (Fig. S1a and b, ESI†).

Fig. 6 TEM images of (a) the fresh NHC-Pd/GO-IL nanocomposite, (b) the NHC-Pd/GO-IL reused after one run and (c) the NHC-Pd/GO-IL reused after three runs.

The imidazolium IL plays an important role in improving the dispersibility of Pd NPs and preventing the aggregation or agglomeration of Pd NPs on the GO-IL sheets.¹⁷ Additionally, Pd NPs are not found outside of the GO-IL sheets.

Table 4, compares the efficiency of the NHC-Pd/GO-IL nanocomposite (time, yield, reaction conditions) with efficiencies of other reported heterogeneous palladium catalysts in the Suzuki reaction. It is clear from Table 4 that our method is simpler, more efficient, and less time consuming for the Suzuki reaction.

Table 4 Comparison of efficiency of various heterogeneous palladium catalysts in Suzuki reaction

Catalyst	Conditions	Yield	Time	Ref.
NHC-Pd/GO-IL (0.1–0.2 mol%)	K2CO3, EtOH–H2O (1 : 1), 60–90 °C	42–98 (%)	2.5–12 h	This work
NHC-Pd/SBA-16-IL (0.01 mol%)	K3PO4, EtOH–H2O (1 : 1), 50 °C	87–99 (%)	5–10 h	13
GO-NHC-Pd^2+ (0.25 mol%)	K2CO3 EtOH–H2O (1 : 1), 80 °C	23–96 (%)	1–24 h	17
MPS-NHC-Pd (1–2 mol%)	K2CO3, DMF–H2O (2 : 1), 50–100 °C	31–99 (%)	0.5–24 h	18
Pd@PMO-IL (0.1–1 mol%)	K2CO3, H2O, 60–90 °C	60–99 (%)	2–20 h	19
Pd NPs-IP-IL (1 mol%)	Na2CO3, H2O, 100 °C	6–99 (%)	2.5–10 h	20

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Conclusions

We have demonstrated for the first time that an ionic liquid supported on graphene by grafting an N-heterocyclic carbene palladium complex on the surface of graphene oxide modified with ionic liquid is an effective and powerful support for the immobilization and stabilization of Pd nanoparticles for the Suzuki reaction. The nanocomposite shows good catalytic activity for the Suzuki coupling of a variety of activated and deactivated aryl halides with aryl boronic acids. Leaching tests such as hot filtration and AAS analysis indicate that the catalytic reaction is mainly heterogeneous in nature. The reusability of this catalyst is high and the catalyst can be reused several times without a significant decrease in its catalytic activity. Furthermore, the TEM image of the recovered catalyst showed the presence of well-distributed Pd NPs on the GO-IL sheets without any aggregation. Further applications of the catalytic system in other palladium catalyzed reactions are under investigation in our laboratory.

Notes and references

1. N. Liu, F. Luo, H. Wu, Y. Liu, C. Zhang and J. Chen, Adv. Funct. Mater., 2008, 18, 1518.
2. C. P. Mehnert, R. A. Cook, N. C. Dispenziere and M. Afeworki, J. Am. Chem. Soc., 2002, 124, 12932; C. P. Mehnert, E. J. Mozeleski and R. A. Cook, Chem. Commun., 2002, 3010.
3. L.-L. Lou, K. Yu, F. Ding, W. Zhou, X. Peng and S. Liu, Tetrahedron Lett., 2006, 47, 6513; M. Gruttadauria, S. Riela, P. L. Meo, F. D'Anna and R. Noto, Tetrahedron Lett., 2004, 45, 6113; R. Ciriminna, P. Hesemann, J. J. E. Moreau, M. Carraro, S. Campestrini and M. Pagliaro, Chem. - Eur. J., 2006, 12, 5220; B. Ni and A. D. Headley, Chem. - Eur. J., 2010, 16, 4426; K. V. S. Ranganath, S. Onitsuka, A. K. Kumar and J. Inanaga, Catal. Sci. Technol., 2013, 3, 2161.
4. Y. Gu and G. Li, Adv. Synth. Catal., 2009, 351, 817; H. Li, P. S. Bhadury, B. Song and S. Yang, RSC Adv., 2012, 2, 12525.
5. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498.
6. K. I. Bolotin, K. J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P. Kima and H. L. Stormer, Solid State Commun., 2008, 146, 351.
7. C. Lee, X. D. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385.
8. D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101.
9. R. X. Huang, X. Qi, F. Boey and H. Zhang, Chem. Soc. Rev., 2012, 41, 666.
10. W. Zhang, S. Wang, J. Ji, Y. Li, G. Zhang, F. Zhang and X. Fan, Nanoscale, 2013, 5, 6030; Y. Li, Q. Zhao, J. Ji, G. Zhang, F. Zhang and X. Fan, RSC Adv., 2013, 3, 13655.
11. N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457; J. Hassan, M. SRvignon, C. Gozzi, E. Schulz and M. Lemaire, Chem. Rev., 2002, 102, 1359; V. Farina, Adv. Synth. Catal., 2004, 346, 1553; F. Bellina, A. Carpita and R. Rossi, Synthesis, 2004, 2419; F.-S. Han, Chem. Soc. Rev., 2013, 42, 5270; H. Li, C. C. C. Johansson Seechurn and T. J. Colacot, ACS Catal., 2012, 2, 1147; S. Kotha, K. Lahiri and D. Kashinath, Tetrahedron, 2002, 58, 9633.
12. Q. Chen, L. Zhang and G. Chen, Anal. Chem., 2012, 84, 171.
13. H. Yang, X. Han, G. Li and Y. Wang, Green Chem., 2009, 11, 1184.
14. S. Hou, S. Su, M. L. Kasner, P. Shah, K. Patel and C. J. Madarang, Chem. Phys. Lett., 2010, 501, 68.
15. Q. Zhao, D. Chen, Y. Li, G. Zhang, F. Zhang and X. Fan, Nanoscale, 2013, 5, 882.
16. Z. Li, J. Liu, Z. Huang, Y. Yang, C. Xia and F. Li, ACS Catal., 2013, 3, 839; G. Buscemi, M. Basato, A. Biffis, A. Gennaro, A. A. Isse, M. M. Natile and C. Tubaro, J. Organomet. Chem., 2010, 695, 2359; S. Souto, M. Pickholz, M. C. dos Santos and F. Alvarez, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 57, 2536.
17. N. Shang, S. Gao, C. Feng, H. Zhang, C. Wang and Z. Wang, RSC Adv., 2013, 3(44), 21863.
18. D.-H. Lee, J.-H. Kim, B.-H. Jun, H. Kang, J. Park and Y.-S. Lee, Org. Lett., 2008, 10, 1609.
19. B. Karimi, D. Elhamifar, J. H. Clark and A. J. Hunt, Chem. - Eur. J., 2010, 16, 8047.
20. X. Yang, Z. Fei, D. Zhao, W. H. Ang, Y. Li and P. J. Dyson, Inorg. Chem., 2008, 47, 3292.
21. M. T. Reetz and E. Westermann, Angew. Chem., Int. Ed., 2000, 39, 165; J. Nasielski, N. Hadei, G. Achonduh, E. A. B. Kantchev, C. J. O'Brien, A. Lough and M. G. Organ, Chem. - Eur. J., 2010, 16, 10844; N. T. S. Phan, M. V. D. Sluys and C. W. Jones, Adv. Synth. Catal., 2006, 348, 609; N. Marion and S. P. Nolan, Acc. Chem. Res., 2008, 41, 1440–1449.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46056h

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