Ce_0·98Pd_0·02O_2-δ: Recyclable, ligand free palladium(II) catalyst for Heck reaction

Abstract

Palladium substituted in cerium dioxide in the form of a solid solution, Ce_0·98 Pd_0·02 O_1·98 is a new heterogeneous catalyst which exhibits high activity and 100% trans-selectivity for the Heck reactions of aryl bromides including heteroaryls with olefins. The catalytic reactions work without any ligand. Nano-crystalline Ce_0·98 Pd_0·02 O_1·98 is prepared by solution combustion method and Pd is in +2 state. The catalyst can be separated, recovered and reused without significant loss in activity.

1 Introduction

Palladium catalysed Heck reaction is well-known in organic synthesis.1 Recent review article2 has summarized the developments in homogeneous catalysed Heck reaction. Heck reaction is known for its high tolerance of functional groups and general applicability.3 The palladium catalysed coupling reaction of organic halides with olefins allows a one-stepsynthesis of aromatic olefins4, which are used extensively as biologically active compounds, natural products, pharmaceuticals and precursors of conjugated polymers.5 The Heck reaction proceeds in the presence of homogeneous as well as heterogeneous palladium catalysts, generated from either palladium(0) compounds or palladium(II) acetate or chloride salts.6 Several ligands such as phosphines, phoshites, carbenes, thioethers have been successfully employed for this reaction.7 However, homogeneous catalysis results in problems of recovery and reuse of the catalyst and also might result in palladium contamination of the product. Use of phosphines which are generally used as electron donating ligands in the Heck reactions is undesirable because they are toxic as well as moisture sensitive. Therefore, there is a need to develop catalysts which do not need any ligand for Heck coupling.

In recent years, Heck reaction has been catalysed by palladium metal supported on charcoal8, mesoporous carbon9, magnesium oxide10, silica, alumina or titania11, palladium/Nb-MCM-4112, polymers13, zeolites14, polyionic resins.15 Basic supports such as layered double hydroxide16, basic zeolites17, alkaline exchanged sepiolites18, mixed oxide19, flourapatite20 have been used because Pd on these supports shows considerably higher activity towards Heck reaction compared to neutral supports. Noble metal ion substitution in reducible oxides such as CeO₂, TiO₂ showed extraordinarily high activity for a variety of catalytic reactions.21 We considered it worthwhile to see if Pd ion substituted CeO₂ can be used for carbon–carbon coupling reactions of aryl halides and substituted alkenes in N,N-Dimethylformamide(DMF) using K₂CO₃ as a base and without any ligands. We show here that Pd²⁺ ion in CeO₂ is an excellent Heck coupling heterogeneous catalyst without requiring any ligand. A wide range of substrates has been screened including the less active bromoaryl compounds. The product is 100% trans selective.

2 Experimental

Ce_0·98Pd_0·02O_1·98 is prepared by solution combustion method as described earlier.22 In a typical preparation 9·8mmol of (NH₄)₂Ce(NO₃)₆ (Loba Chemie Pvt. Ltd., Mumbai, India), 0·20mmol of PdCl₂ (Sigma Aldrich), 23·56mmol C₂H₆N₄O₂ (oxalyldihydrazide) were dissolved in 10ml of water in a borosilicate dish with 200cm³ capacity. The dish containing the redox mixture was introduced into a muffle furnace maintained at 350°C. The solution boiled with frothing and foaming and ignited to burn with a flame (~1000°C) yielding a voluminous solid product within 5 min. Catalysts before and after reactions were characterized by powder XRD (Phillips X’Pert diffractometer using Cu Kα radiation at a scan rate of 2θ = 2°/min) and XPS (Thermoscientific Fisher Multilab 2000 using Al Kα radiation (1486·6eV)).

All the aryl halides and olefins were obtained from S.D. Fine Chemicals Ltd. and Spectrochem Pvt. Ltd. Mumbai, India. ¹H NMR and ¹³C NMR spectra were recorded using Bruker AV-400MHz spectrometer. Chemical shifts were given in parts per million and coupling constants (J) in Hertz. Chemical shifts of ¹H NMR are given with respect to the TMS chemical shift and of ¹³C NMR are given with respect to CDCl₃ chemical shift. Flash column chromatography was performed on silica gel (100–200 mesh) using ethyl acetate and hexane as eluents. The products were analysed employing gas chromatograph (Varian CP 3800 instrument). The capillary column used was Varian CP-Sil 8CB Fused silica column 5% Phenyl, 95% dimethylpolysiloxane Size: 30m × 0·25mm (ID), 0·25 μm (Df).

2.1 Typical procedure for the Heck reaction

Under an atmosphere of nitrogen, an oven-dried two-necked 25ml round-bottom flask containing a stir bar was charged with an aryl halide (0·50mmol), olefin (2·25mmol), catalyst Ce_0·98Pd_0·02O_1·98 (0·38mmol, containing 7·6μmol of Pd⁺² ions) and oven dried K₂CO₃ (1·0mmol). The mixture was heated and stirred at 120–130°C. The progress of the reactions was monitored by TLC. After completion of the reaction, the reaction mixture was cooled to room temperature and filtered. The filtrate is extracted with hexane and ethyl acetate (3:1) and water. The organic layer was washed with water and brine, dried with sodium sulphate and evaporated under reduced pressure. The residue was finally purified by flash chromatography on silica gel. All the products have been fully characterized by NMR studies.

3 Results and discussions

Substitution of Pd⁺² ion in CeO₂ in the form of Ce_0·98Pd_0·02O_2-δ solid solution has been shown earlier by XRD, XPS, EXAPS studies.22,23 Rietveld refined XRD profile of freshly prepared compound is shown in figure 1(a). X-ray pattern corresponds to single phase Ce_0·98Pd_0·02O_1·98 as reported earlier.23 From the FWHM of diffraction lines, crystallite size is estimated employing Schirrer formula (d = 0·95λ/βcosθ). Average size is 10  ±  0·5nm and colour of the catalyst is pale yellow. XPS of the Pd(3d) is shown in figure 2(a). Binding energy of Pd(3d_5/2) at 337eV confirms Pd in +2 state.22,23 Cerium is in +4 oxidation state in this compound and is confirmed from the XPS of Ce(3d) (not shown). Thus the formula of the catalyst is Ce_0·98Pd_0·02O_1·98. Due to lower valent Pd ^+ 2 ion substitution for Ce ^+ 4 ion in CeO₂, oxide ion vacancies₂ are created. EXAFS study has confirmed that the oxide ion vacancy is found next to Pd ^+ 2 ion in the lattice.23

Figure 1.

The Rietveld refined XRD patterns of Ce_0·98Pd_0·02O_1·98; (a) before reaction, (b) after reaction.

Figure 2.

XPS of Pd(3d) core level region of Ce_0·98Pd_0·02O_1·98; (a) before reaction, (b) after reaction.

The activity of nano-crystallite Ce_0·98Pd_0·02O_1·98 was examined for the Heck coupling reaction. Effect of reaction parameters such as time and temperature on the yield was investigated to determine the optimal condition with N,N-dimethylformamide (DMF), dimethylsulphoxide (DMSO), N-methylpyrrolidine (NMP) as solvents. The highest yield was obtained in DMF and it was used as a solvent for all the reactions. All the reactions were carried out using potassium carbonate (K₂CO₃) as a base except the reactions with styrene, for which sodium acetate was used. Iodobenzene and methylacrylate were chosen as model compounds for the initial study. A number of reactions have been carried out with Ce_0·98Pd_0·02O_1·98 catalyst and the reactions were performed with 5% molar excess of methyl acrylate at different temperatures. The aryl halides, olefins, products, time, temperature and yields are summarized in table 1. Heck reaction of iodobenzene with methyl acrylate over pure CeO₂ as catalyst was carried out and there was no conversion. Thus, Pd ion in the catalyst is the active site for the Heck reaction in this catalyst. The C–X bond strength affects the activity towards the Heck reaction as the general cross coupling reactions follow C–I > C–Br > C–Cl. Yields of coupling reactions with alkyl acrylates were found to be more compared to styrene. Bromobenzenes with electron withdrawing groups have shown comparable activity with that of iodobenzene towards Heck reaction. Bromobenzenes substituted with electron donating group have shown lesser activity. Exclusively trans-coupled product is obtained in each of the reaction as confirmed from NMR studies (see Supporting Information). The reaction of para-substituted bromobenzenes was studied with acrylates (table 2). 4-Formylbromobenzene and 4-acetylbromobenzene underwent the reaction in 6 h with 95% and 93% yields respectively. But 4-bromoaniline and 4-bromophenols were relatively less reactive and the Heck product yields were 65% and 62%, respectively. However, unsubstituted bromobenzene gave more than 72% yield. The effect of temperature on conversion was studied with the iodobenzene and methyl acrylate as shown in figure 3. While ~100% conversion was obtained in 30 min at 150°C, it required nearly one hour when temperature was 125°C. However, at 80°C, the reaction was very slow that the conversion was ~ 60% even after 7h.

Table 1. Reactions of different aryl halides with different vinyl compounds.

Table 2. Reaction of acrylates with substituted bromobenzenes.

Figure 3.

Conversion efficiency of reaction of iodobenzene and methyl acrylate at different temperatures. Yields are calculated based on GC results with respect to iodobenzene.

It is important to compare the activity of this catalyst with those Pd substituted ligandless heterogeneous catalysts for Heck coupling in the literature. Turnover frequency is one simple way to compare the activity of the catalysts. Turnover frequency (TOF) is defined here as the ratio of moles of conversion per unit time per mole of palladium ion in the catalyst. TOF values were obtained for the reports in the literature for the reaction of iodobenzene and methyl acrylate and the values are compared with our catalyst in table 3. Clearly TOF of the present catalyst is higher compared to other Pd substituted ligandless heterogeneous catalysts for carbon–carbon cross coupling reactions reported in the literature.24 TOF is comparable with recently reported immobilized Pd(OAc)₂ catalyst in ionic liquid on silica.25 TOF of our catalyst reported here is for the first cycle. The same catalyst is reused and accordingly TOF will be much higher if we take into account the number of times the same catalyst is reused for the reaction.

Table 3. Performance of the catalyst in the reaction of iodobenzene and methyl acrylate.

The catalyst can be recycled without significant loss of activity. After the completion of the reaction, the catalyst was recovered by filtration or by centrifugation. The powder catalyst was washed with hexane to remove organic impurities and then with water to remove DMF and K₂CO₃. The dried catalyst was reused in a fresh reaction. There was no significant loss in the catalytic activity for the model reaction of iodobenzene and methyl acrylate (see table 4). In the same way we could recover and reuse the catalyst in all the reactions which are listed in table 1. The catalyst was examined after several cycles of reaction. The Rietveld refined XRD profile of this reused catalyst is shown in figure 1(b). Clearly, fluorite structure of the catalyst remains intact. Thus the catalyst is stable in the reaction condition. Further, XPS of the spent catalyst was recorded and Pd(3d) spectrum given in figure 2(b) shows that Pd remains in +2 oxidation state. Pd ion is not reduced to Pd⁰ state and Ce also remains in +4 state. Further, surface concentration of Pd ion has not changed indicating that Pd is not leached out. Pd ion is not leached out from the catalyst even in acid (0·1N HCl) solution.

Table 4. Reuse of catalyst.

Catalyst surface contains two distinct sites: (i) electron deficient Pd²⁺ ion for adsorption of donor molecules and (ii) oxide ion vacant sites next to Pd²⁺ for the adsorption of acceptor molecules. There was no Heck reaction on pure CeO₂ clearly suggests that Ce⁴⁺ and O²⁻ sites are not active for this catalytic reaction. There are no other sites for the adsorption of donor and acceptor molecules in the catalyst. We have extensively shown that in Ce_1−xPd_xO_2−x the Pd ion and the oxide ion vacancies next to each other act as red-ox sites and dual site mechanism has been established.23 Further, redox catalytic reaction such as CO+O₂ over this catalyst is shown to follow modified Langmuir–Hinshelwood mechanism where, CO is exclusively adsorbed on Pd²⁺ site and oxygen on oxide ion vacancy site.21

A mechanism proposed for this ionic catalyst is given in figure 4. It is well known that olefins bind with Pd⁺² ion.26 Thus, normal site for adsorption of olefin is Pd⁺² ion in Ce_0·98Pd_0·02O_1·98. Oxide ion vacancy is a nucleophilic site ideal for accommodation of –Br, −I of the aryl halide. Elimination of HBr facilitated by the base leads to C–C coupling. Further studies are essential to see why only trans coupling is observed over this catalyst.

Figure 4.

Mechanism for carbon–carbon cross coupling reactions with Ce_0·98Pd_0·02O_1·98 catalyst.

4 Conclusion

A recyclable Pd²⁺ ion substituted CeO₂ (Ce_0·98Pd_0·02O_1·98) catalyses the carbon–carbon cross coupling of substituted iodobenzene, substituted bromobenzenes and bromoheteroaryls with alkylacrylates and styrenes with good yields.