Bio-supported palladium nanoparticles as a phosphine-free catalyst for the Suzuki reaction in water

Abstract

Bio-supported Pd nanoparticles were prepared as a robust catalyst for aqueous Suzuki coupling.

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The palladium catalyzed Suzuki reaction is one of the most powerful and versatile methods for the generation of unsymmetrical biaryls from arylboronic acids and aryl halidesvia a single C–C coupling.^1–3 In traditional protocols, however, the Pd catalyst chelates with organophosphine ligands or later developed nitrogen- or sulfur-containing ligands^4–6 and N-heterocyclic carbenes.^7–11 Such ligands and the corresponding catalysts are often difficult to prepare and incompatible with 100% water media, which significantly limits their industrial applications. Development of phosphine-free palladium catalysts has thus attracted much attention due to environmental and economical concerns.^12–16

Recent endeavours to immobilize palladium nanoparticles directly onto supports including carbon,¹⁷silica,¹⁸zeolites¹⁹ and polymers²⁰ are quite straightforward to solve the problems of not only heterogeneous catalysis in recovery and recyclability but homogeneous catalysis in catalyst loading and selectivity. The ever-intensifying appeal for cleaner, sustainable chemistry is driving the selection of supports from petrochemical-based feedstocks to environmentally benign materials. When bacterial cellulose (BC) is used as the support, this eco-friendly biomaterial can offer both a large surface area and stable heterogeneous interface to construct semi-heterogeneous catalysts.^16,21,22 Produced by fermentation of Acetobacter xylinum with D-glucose as carbon source, BC nanofibres boast a variety of notable properties such as an ultrafine fiber network structure, high water retention capability, good mechanical properties and high chemical stability due to their unique specific structure.^23,24 The ultrafine three-dimension networks endow BC with well-separated nanofibrils (width < 100 nm) to create extensive specific area, which is advantageous for BC to function as a matrix or support in creating hybrid nanomaterials.^25,26

Taking advantage of BC's characteristics, we employed BC nanofibers as the support for palladium nanoparticles. Our one-pot synthesis of the catalyst was straightforward, simply mixing palladium nitrate (or palladium chloride) and reducing reagent with BC nanofibres for a hydrothermal reaction.‡ Scanning and transmission electron microscopy (SEM and TEM) images showed that the deposited Pd nanoparticles formed polydisperse small islands of ∼20 nm dimension on BC nanofibres (Fig. 1b, c). The Pd/BC catalyst was further characterized with XRD, TGA and XPS experiments (see ESI†). The maximum palladium loading on dry BC we have been able to achieve is 0.31 mmol g⁻¹ with a BET surface area of 24.8 m² g⁻¹, which is a 7-fold improvement over pristine BC fibres.

Fig. 1 SEM image of BC nanofibers prior to palladium growth (a), TEM image of catalyst after 2 h hydrothermal reaction (b), after 3 h hydrothermal reaction for palladium growth (c), and catalyst after two catalytic cycles (d).

With the BC nanofiber-supported palladium nanoparticles in hand, we explored their catalytic performance in Suzuki coupling. We first studied the effect of the palladium source [Pd(NO₃)₂ or PdCl₂] on the catalytic activity of Pd/BC nanocomposites in aqueous Suzuki coupling (Scheme S2†). Starting with the easiest coupling reaction between iodobenzene and phenyl boronic acid,²⁷Pd nanoparticles prepared from different sources delivered similarly high yielding couplings (ca. 98%). The catalyst was highly recyclable, maintaining over 94% yields after two-cycle reuses (see ESI†). Even after heating at 85 °C for two cycles (3.5 h each), no large palladium agglomerates were observed (Fig. 1d). We did notice a decline in the quality of the nanofiber support along with a change in the distribution of nanoparticle sizes that might be responsible for the decrease in yield over extended recycling.

We also explored the above-mentioned coupling reaction in DMF and organic base for comparison. The reaction proceeded in less than 3.5 h at 85 °C. Surprisingly, the coupling with DMF and organic base achieved a lower yield of 78%, probably due to a decreased heterogeneous catalysis efficiency with the dispersion of reactants into the BC network. Lower yields were also found in the cycled reactions in DMF (see ESI†). This is significant since we eliminated the need to employ an organic solvent as the reaction medium and therefore could develop environmentally friendly processes. As we know, water is an attractive “green” solvent and very inexpensive.^31,32

The effect of temperature for Suzuki coupling was also considered for the Suzuki coupling in water. As expected,¹ the temperature-dependent coupling harvested increased conversion from 79% to 98% when the temperature increased from 65 °C to 85 °C (Table 1). The yield levelled off at reaction temperatures over 85 °C.

Table 1 Suzuki coupling at varying temperatures

Entry	T/°C	Yield/%
1	65	79
2	70	83
3	75	83
4	80	89
5	85	98
6	90	97
7	95	97

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With respect to the rate of Pd/BC catalysed Suzuki couplings in water, the model reaction in Scheme S2† was explored at increasing reaction times. The results (Fig. S5†) showed that the yield of the coupling reaction in water increased from 76% to 98% with reaction time before reaching its maximum at 3.5 h. Thus, we set our standard coupling conditions as: 0.05 mol% Pd/BC catalyst, K₂CO₃ (2 equiv.) base, water media and 85 °C for 3.5 h.

With the satisfactory protocol in hand, we obtained high yielding coupling with iodobenzene and arylboronic acids bearing a variety of substituents (Table 2). Excellent results were obtained for the coupling of electron-neutral 1, -deficient 3–4 and -rich 5–7 with electron-neutral iodobenzene. The recyclability of the Pd/BC catalyst was tested for all couplings in Table 2 for five-cycles with a low loading of 0.05 mol% equiv. We obtained yields of over 86% after five cycles (see ESI†), for all couplings except for the reaction with entry 4. Importantly, from a mechanistic point of view, it has been shown that in many couplings between aryl iodides and arylboronic acids, concentrations as low as parts per billion of palladium suffice for product formation.³³

Table 2 Suzuki coupling of iodobenzene and arylboronic acids

Entry	Reaction	Yield/%
1		98
2		92
3		95
4		88
5		99
6		99
7		98

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Pd leaching was studied by ICP-AES analysis of the catalyst before and after five reaction cycles.^28–30 The Pd concentration was found to be 5.29% before reaction and 5.26% after five reaction cycles, which confirmed negligible Pd leaching; this is probably due to the protection from the ordered water layers covering BC fibers via H-bonding (Scheme S1†).²⁶ Also, no Pd metal was detected in the final oxidation product.

Table 3 shows the results of extending this methodology to more challenging couplings with aryl chloride. The couplings with less reactive chlorobenzene exhibited slightly lower yields in comparison to iodobenzene. Nevertheless, good to excellent yields were achieved except for with electron-deficient 11. Especially, the electron-rich arylboronic acids 12–14 showed excellent yields. This phenomenon can be explained with the Suzuki reaction mechanism (see ESI†).

Table 3 Suzuki coupling of chlorobenzene and arylboronic acids

Entry	Reaction	Yield/%
8		89
9		90
10		92
11		75
12		94
13		91
14		90

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Inspired by the research of Clark et al.,³⁴ we also tested our catalyst in Heck couplings (Table 4). Outstanding yields were achieved for the coupling between iodobenzene and acrylates in organic base. The catalyst was also highly recyclable in the couplings, with yields over 84% guaranteed after 3-cycle reuse.

Table 4 Heck coupling of iodobenzene and acryl esters

Entry	Substrate^a	Yield/%
		Cycle 1	Cycle 2	Cycle 3
a 0.1 mol% Pd/BC, 2 equiv. triethylamine, DMF, 120 °C, 3.5 h.
15	COOCH3	92	90	86
16	COOC2H5	91	89	85
17	COOnC4H9	94	89	84
18	COOiC4H9	93	90	84

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In conclusion, BC nanofiber supported palladium nanoparticles have been prepared as a robust catalyst for 100% aqueous Suzuki coupling and for Heck coupling in DMF. The prepared Pd/BC catalyst presents good recyclability, with coupling yields maintained over five catalytic cycles. Extension of this catalyst to further couplings with aryl chlorides are in development and will be reported in due course.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Detailed experimental procedures and characterization for catalyst and products. See DOI: 10.1039/c2ra01015a

‡ Synthesis of Pd/BC nanocomposites: the two-step preparation protocol involves firstly the biosynthesis of BC nanofibers²⁷ followed by a hydrothermal reaction. With the purified BC nanofibers in hand, a typical hydrothermal reaction proceeded as follows: immersing PdCl₂ or Pd(NO₃)₂ (containing 0.1 g Pd) and freshly-prepared BC nanofibers (3 g) into 50 mL DI water; degassing for 0.5 h before heating to 140 °C with vigorous stirring (1200 rpm) under a N₂ atmosphere; adding potassium borohydride (2 g in 50 mL water) over 5 h; centrifuging and washing with water to obtain the titled composites. General protocol for Suzuki couplings: aryl halide (1.2 equiv.), aryl boronic acid (1 equiv.), and base (2 equiv.) were added to 15 mL water. The solution was stirred at 60–100 °C. Pd/BC (0.05 mol%) catalyst was added and the reaction mixture was stirred for 3.5 h or for the time indicated. Cooling down to room temperature prior to filtration, Pd/BC nanocomposites were washed with ethyl acetate (3 × 30 mL) and then deionized water for recycling. The collected filtrate was extracted with ethyl acetate and washed with water. After drying the extract with anhydrous MgSO₄, the solvent was removed by evaporation. The final product was obtained by recrystallization over petroleum ether.

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