Amphiphilic Dendronized Copolymer-Encapsulated Au, Ag and Pd Nanoparticles for Catalysis in the 4-Nitrophenol Reduction and Suzuki–Miyaura Reactions

Abstract

The branched structures of dendronized polymers can provide good steric stabilization for metal nanoparticle catalysts. In this work, an amphiphilic dendronized copolymer containing hydrophilic branched triethylene glycol moieties and hydrophobic branched ferrocenyl moieties is designed and prepared by one-pot ring-opening metathesis polymerization, and is used as the stabilizer for metal (Au, Ag and Pd) nanoparticles. These metal nanoparticles (Au nanoparticles: 3.5 ± 3.0 nm; Ag nanoparticles: 7.2 ± 4.0 nm; Pd nanoparticles: 2.5 ± 1.0 nm) are found to be highly active in both the 4-nitrophenol reduction and Suzuki–Miyaura reactions. In the 4-nitrophenol reduction, Pd nanoparticles have the highest catalytic ability (TOF: 2060 h⁻¹). In addition, Pd nanoparticles are also an efficient catalyst for Suzuki–Miyaura reactions (TOF: 1980 h⁻¹) and possess good applicability for diverse substrates. The amphiphilic dendronized copolymer will open a new door for the development of efficient metal nanoparticle catalysts.

1. Introduction

Metal nanoparticle catalysts have been widely investigated in homogeneous and heterogeneous catalysis over the past few decades because of their superior catalytic activity derived from their high specific surface area and quantum size effect [1,2]. Along with decreasing the particle size of metal nanoparticles, more active metal atoms are exposed on their surface and can serve as active sites, which will greatly enhance atomic utilization efficiency and improve catalytic performance [3,4]. Consequently, metal nanoparticles with relatively small sizes (<10 nm) often have better catalytic ability compared to large metal nanoparticle catalysts. Nevertheless, metal nanoparticles with small sizes might result in serious aggregation owing to the high surface energy, which would cause a loss or reduction in the catalytic performance [5,6,7,8,9,10]. To solve this problem, great efforts have been made in the past decade. At present, various supporting materials including zeolites, metal–organic frameworks, carbon-based materials, inorganic oxides, covalent organic frameworks and polymers have been reported to immobilize small-sized metal nanoparticles for catalysis [1,2,3,4]. Metal nanoparticles are able to be stabilized via electron interactions or spatial limitation with supporting materials; thus, the synergistic effects between metal nanoparticles and supporting materials can enhance the catalytic ability.

Polymers have been considered as a kind of ideal supporting materials for metal nanoparticle catalysts, because their structures and functionalities are easy to design and all kinds of functional atoms including oxygen, phosphorus, sulfur and nitrogen can be introduced into polymers to interact with metal nanoparticles to control their growth [5]. So far, a variety of polymers such as homopolymers, block copolymers, dendrimers, dendronized polymers, hyperbranched polymers, star polymers, and polymer networks have been developed as supporting materials for metal nanoparticle catalysts. For example, Gao et al. designed hyperbranched polytriazoles to support Pd nanoparticles (ca. 10 nm) for catalyzing the Heck reaction [11]. Xie’s group reported that an amphiphilic star-like β-cyclodextrin-g-poly(4-vinyl pyridine)-b-polystyrene diblock copolymer was used for controllable synthesis of stable Au nanoclusters (<2 nm) whose stability was proved in the 4-nitrophenol reduction via NaBH₄ [12]. Peinemann and colleagues found that the cross-linked β-cyclodextrin polymer network could be applied to prepare various metal (Pd, Ag, Au, Pt and Rh) nanoparticles with narrow size distribution, and the obtained Pd nanoparticles might serve as a heterogeneous catalyst for the low-temperature hydrogenation of various nitroarenes and the Suzuki–Miyaura reaction [13]. Indeed, polymers have shown a promising prospect for supporting metal nanoparticle catalysts based on steric stabilization.

Dendronized polymers are a type of typical branched polymer with well-defined dendritic wedges [14,15,16,17,18]. Compared to traditional dendrimers, a significant advantage is that dendronized polymers with a low generation level even possess more branched structures than higher-generation dendrimers [14,15,16,17,18]. The branched structures of dendronized polymers can provide good steric stabilization for metal nanoparticle catalysts [19]. Frechet and coworkers developed the first dendronized polymer with polyester dendrons to create a microenvironment for catalyzing a difficult esterification [17]. In addition, amphiphilic copolymers have the ability to self-assemble into micelles in solution, offering protection and solubilization for metal nanoparticle catalysts and creating a unique molecular microenvironment for various chemical reactions [20,21]. Gu et al. found that a second-generation highly branched amphiphilic dendronized copolymer (ADC) can be used as a stabilizer for Au and Ag nanoparticles, and the resulting metal nanoparticles were efficient catalysts for the 4-nitrophenol reduction [21].

With continuous efforts to develop functional polymers for supporting metal nanoparticle catalysts, an amphiphilic dendronized copolymer (ADC) containing hydrophilic branched triethylene glycol moieties and hydrophobic branched ferrocenyl moieties was readily prepared by one-pot ring-opening metathesis polymerization (ROMP). The ADC polymer was used to support metal (Au, Ag and Pd) nanoparticles (ADC-AuNPs, ADC-AgNPs and ADC-PdNPs) for catalysis (Figure 1). The catalytic properties of these metal nanoparticles were first investigated and compared in the 4-nitrophenol reduction. Furthermore, Suzuki–Miyaura cross-coupling reactions were adopted to further detect the catalytic ability and applicability of ADC-PdNPs.

2. Experimental Procedure

2.1. Materials

Silver nitrate (AgNO₃), chloroauric acid (HAuCl₄), potassium chloride (KCl), potassium carbonate (K₂CO₃), Grubbs’ second-generation metathesis catalyst, 4-nitrophenol, dichloromethane (CH₂Cl₂), ethyl vinyl ether, sodium borohydride (NaBH₄), palladium chloride (PdCl₂) and other chemicals were provided by Energy Chemical (The Woodlands, TX, USA). Grubbs’ 3rd generation catalyst, hydrophilic monomer and ferrocene-containing hydrophobic monomer were synthesized according to previous report [22,23]. All chemicals are of analytical grade and were used directly.

2.2. Instruments

¹H NMR (Bruker AV Ⅱ-400MHz, Munich, Germany) was adopted to characterize the chemical structures of compounds. UV2550 spectrophotometer was used for UV–vis spectra analysis. Transmission electron microscopy (TEM, JEOL JEM-2100 F, Tokyo, Japan) was applied to analyze the microstructures of products. Gel permeation chromatography (GPC, Agilent waters 1515, Santa Clara, CA, USA) analysis was carried out in N, N-dimethylformamide, and poly(methyl methacrylate) standards were used.

2.3. Synthesis of Amphiphilic Dendronized Copolymer

The ROMP synthesis for amphiphilic dendronized copolymer was from the previous literature [21]. Hydrophilic monomer (500 mg, 0.63 mmol, 60 equiv) and ferrocene-containing hydrophobic monomer (263 mg, 0.21 mmol, 20 equiv) were dissolved in CH₂Cl₂ (3 mL). Then, Grubbs’ 3rd generation catalyst (9.3 mg, 0.0105 mmol, 1 equiv) was added into the above solution and the obtained solution was vigorously stirred at room temperature under N₂ for 8 h. The reaction was quenched with ethyl vinyl ether (0.4 mL). The reaction mixture was concentrated and precipitated with methanol, followed by washing with petroleum ether. An amphiphilic dendronized copolymer with a yellow-brown color was obtained and was dried in vacuum overnight (yield: 95%). ¹H NMR (400 MHz, CDCl₃, 25 °C, TMS), δ ppm: 7.78 ppm (triazolyl), 7.07 ppm (phenyl), 5.60 ppm (olefinic), 5.49 ppm (olefinic), 4.65 ppm (CH₂-triazolyl), 4.50 ppm (CH₂-triazolyl), 4.16–4.04 ppm (ferrocenyl), 3.81–3.52 ppm (OCH₂ and NCH₂CH₂N), 3.36 ppm (CH₃), 3.34 ppm (CH₃), 2.98 ppm (=CH-CH and CH-CO), 2.62–2.52 ppm (OCH₂CH₂CH₂), 1.52 ppm (CH=CHCHCH₂). GPC data: Mn = 85,834 Da, Mw = 96,298 Da, PDI = 1.12.

2.4. Preparation of Amphiphilic Dendronized Copolymer-Encapsulated Au, Ag and Pd Nanoparticles

Metal salt precursors (AgNO₃, HAuCl₄ and PdCl₂) with a concentration of 1 × 10⁻³ mol/L were first prepared in deionized water. Then, the metal salt solution (2 mL) was added to a mixed solution containing deionized water (18 mL), amphiphilic dendronized copolymer (20 mg) and excess NaBH₄ (2 × 10⁻² mmol, 10 eq of metal salt precursors), and then the obtained solution was stirred at 30 °C for 2 h. In the experiments, metal salts were reduced by excess NaBH₄ (a strong reducing agent) to ensure the full reduction process, which was consistent with the previous literature [21]. Meanwhile, no precipitation was generated. A uniform homogeneous solution was obtained. So, the concentration of amphiphilic dendronized copolymer-encapsulated metal nanoparticles (ADC-AuNPs, ADC-AgNPs and ADC-PdNPs) was determined to be 1.0 × 10⁻⁴ mol/L. For the preparation of the amphiphilic dendronized copolymer-encapsulated Pd nanoparticles, equimolar KCl with respect to PdCl₂ was supplemented to improve the solubility in water.

2.5. General Process for the 4-Nitrophenol Reduction Using Amphiphilic Dendronized Copolymer-Encapsulated Au, Ag and Pd Nanoparticles as Catalysts

Amphiphilic dendronized copolymer-encapsulated Au, Ag and Pd nanoparticles were used to catalyze the 4-nitrophenol reduction with NaBH₄ as the reductive agent. Concretely, 2.5 mL of deionized water was first used to dissolve 0.06 mg of 4-nitrophenol in a standard quartz cell. Whereafter, 1.6 mg of NaBH₄ was added into the 4-nitrophenol solution. Then, 2.0 mol% or 0.5 mol% of amphiphilic dendronized copolymer-encapsulated Au, Ag and Pd nanoparticles were quickly placed into the standard quartz cell to catalyze the 4-nitrophenol reduction. The reduction reaction was detected through UV–vis spectra. All the catalytic experiments were performed three times.

2.6. General Process for Suzuki–Miyaura Reactions Using Amphiphilic Dendronized Copolymer-Encapsulated Pd Nanoparticles as Catalyst

Aryl halides (1.0 mmol), arylboronic acids (1.1 mmol), amphiphilic dendronized copolymer-encapsulated Pd nanoparticles (10 ppm, 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, respectively) and K₂CO₃ (2.0 mmol) were placed into a small tube. Then, EtOH (2.5 mL)/H₂O (2.5 mL) system was used as the solvent. The obtained mixture was stirred at 85 °C for 10 h. Then, CH₂Cl₂ (3 mL) was used to extract the reaction mixture three times. Crude product was obtained by concentrating the collected organic phase and was purified through column chromatography. ¹H NMR spectrum was adopted to characterize the desired products. All the catalytic experiments were performed three times.

3. Results and Discussion

ROMP has been considered as a kind of controlled polymerization technique for synthesizing a variety of functional polymers with well-defined structures [24,25,26,27]. Here, ROMP was applied to prepare the ADC polymer containing hydrophilic branched triethylene glycol moieties and hydrophobic branched ferrocenyl moieties. As shown in Figure 2a, ROMP of hydrophilic monomer and ferrocene-containing hydrophobic monomer with the help of Grubbs’ 3rd generation catalyst resulted in the amphiphilic ADC polymer. The chemical structure of the ADC polymer was analyzed via ¹H NMR (Figure 2b). The typical triazolyl proton peak was found at 7.78 ppm, and the ferrocenyl proton peaks were located at 4.16–4.04 ppm. The peak at 7.07 ppm corresponded to the phenyl protons. The representative double peaks of olefinic protons in poly(norbornene) were observed at 5.60 ppm and 5.49 ppm. The peaks at 3.36 ppm and 3.34 ppm were assigned to the methyl proton peaks in the hydrophilic moieties. These results confirmed that the ADC polymer was synthesized successfully. The molecular weight of the ADC polymer was determined via GPC (Figure 2c). The ADC polymer had an Mn value of 85,834 Da, an Mw value of 96,298 Da and a PDI value of 1.12.

The ADC polymer with branched structures and amphiphilic feature should be a suitable supporting material for metal nanoparticle catalysts. Therefore, the ADC polymer was used as a stabilizer for the preparation of Au, Ag and Pd nanoparticles. Using the metal salt (AgNO₃, HAuCl₄ and PdCl₂) reduction strategy, ADC-AuNPs, ADC-AgNPs and ADC-PdNPs could be synthesized, using the ADC polymer as the stabilizer and NaBH₄ as the reductive agent. ADC-AuNPs showed a wine-red color, ADC-AgNPs exhibited a yellowish-brown color, and ADC-PdNPs presented a light-brown color (Figure 3). UV–vis spectra analysis manifested that ADC-AuNPs and ADC-AgNPs had a typical surface plasmon band at 520 nm (corresponded to Au⁰, Figure 3a) and 414 nm (corresponded to Ag⁰, Figure 3b), respectively. It is worth noting that ADC-PdNPs had no obvious surface plasmon band (Figure 3c), which was also observed in the previous literature [28,29].

TEM and HRTEM analyses were adopted to characterize the morphologies and sizes of ADC-AuNPs, ADC-AgNPs and ADC-PdNPs. As can be seen from Figure 4, these metal nanoparticles were quasi-spherical. The average sizes of ADC-AuNPs, ADC-AgNPs and ADC-PdNPs were 3.5 ± 3.0 nm, 7.2 ± 4.0 nm and 2.5 ± 1.0 nm, respectively. In HRTEM images, ADC-AuNPs, ADC-AgNPs and ADC-PdNPs had the clear lattice fringe with interplanar spacing of 0.23 nm, 0.20 nm and 0.23 nm, respectively. As a result, ADC-AuNPs were consistent with the Au (111) plane [30,31,32], ADC-AgNPs were in accordance with the Ag (200) plane [33], and ADC-PdNPs were derived from the Pd (111) plane [34,35]. TEM and HRTEM analyses demonstrated that these metal nanoparticles possessed small sizes, which should be beneficial to catalysis.

The catalytic activities of ADC-AuNPs, ADC-AgNPs and ADC-PdNPs were studied in the reduction of 4-nitrophenol. As is well known, the 4-nitrophenol reduction is a representative model reaction used to estimate the catalytic performance of metal nanoparticles [36,37]. Although the 4-nitrophenol reduction is thermodynamically possible, it is kinetically limited without catalysts [38]. A high potential difference between electron donors and electron acceptors would cause a kinetic barrier. Nevertheless, catalysts may serve as the electronic relay to induce the reduction reaction by overcoming the kinetic barrier. In this way, electron transfer would occur from BH₄⁻ to 4-nitrophenol [39]. The catalytic ability of metal nanoparticles is highly relevant to its high specific surface area. Therefore, electron transfer could induce the reduction reaction. The reduction reaction was easily detected through UV–vis spectra, as shown in Figure 5a–f. 4-Nitrophenol had a characteristic surface plasmon band at 317 nm, which rapidly red-shifted to 400 nm when NaBH₄ was added to the solution because of the generation of a 4-nitrophenolate ion [40]. The previous literature has demonstrated that the typical surface plasmon band at 400 nm would weaken gradually after adding catalysts due to the consumption of 4-nitrophenol [41,42,43,44]. Meanwhile, a new surface plasmon band of 4-aminophenol would appear in the UV–vis spectra. ADC-AuNPs, ADC-AgNPs and ADC-PdNPs were found to be able to smoothly catalyze the 4-nitrophenol reduction (Figure 5a–f). In the 4-nitrophenol reduction, NaBH₄ (100 eq) was in high excess relative to 4-nitrophenol; thus, the reaction rate did not depend on NaBH₄. Therefore, the 4-nitrophenol reduction was regarded as the pseudo-first-order kinetics. The concentration of the 4-nitrophenol was proportional to its absorbance. The concentration at starting (C0) and at time t (C) was considered to be equivalent to the absorbance at starting (A0) and at time t (A). The rate constant (K) could be confirmed by plots of [−ln(C/C0)] vs. reduction time. So, the corresponding K values may be described as follows:

$$-\ln \left(\frac{\mathrm{A}}{\mathrm{A}0}\right)=-\ln \left(\frac{\mathrm{C}}{\mathrm{C}0}\right)=\mathrm{K}\mathrm{t}$$

As shown in Figure 5g–i, the plots of [−ln(C/C0)] vs. reduction time exhibited a linear dependence, manifesting good pseudo-first-order kinetics. Turnover frequency (TOF) was used to compare the catalytic effects. K and TOF values of ADC-AuNPs, ADC-AgNPs and ADC-PdNPs are summarized and compared in

Table 1. 4-nitrophenol reduction with AuNPs, AgNPs or PdNPs.

Catalyst	Amount (mol%)	Diameter (nm)	K (s−1)	TOF (h−1)
AuNPs	2.0	3.5 ± 3.0	2.16 × 10−2	900
	0.5		6.70 × 10−3	1200
AgNPs	2.0	7.2 ± 4.0	1.13 × 10−2	750
	0.5		4.50 × 10−3	550
PdNPs	2.0	2.5 ± 1.0	2.60 × 10−2	1130
	0.5		1.11 × 10−2	2060

. When 2.0 mol% catalysts were used, the 4-nitrophenol reduction was completed rapidly. At this point, ADC-AuNPs, ADC-AgNPs and ADC-PdNPs had K values of 2.16 × 10⁻² s⁻¹ (TOF: 900 h⁻¹), 1.13 × 10⁻² s⁻¹ (TOF: 750 h⁻¹), 2.60 × 10⁻² s⁻¹ (TOF: 1130 h⁻¹), respectively. These results confirmed that ADC-PdNPs possessed the best catalytic activity. Furthermore, when the catalysts were reduced to 0.5 mol%, the same phenomenon was found. ADC-PdNPs (0.5 mol%) could accomplish the 4-nitrophenol reduction within 350 s with K and TOF values of 1.11 × 10⁻² s⁻¹ and 2060 h⁻¹, ADC-AuNPs could complete the 4-nitrophenol reduction within 600 s, with K and TOF values of 6.70 × 10⁻³ s⁻¹ and 1200 h⁻¹, while ADC-AgNPs finished the 4-nitrophenol reduction in a longer time (1300 s), with relatively low K and TOF values of 4.50 × 10⁻³ s⁻¹ and 550 h⁻¹. As a consequence, ADC-PdNPs showed relatively higher catalytic activity in the 4-nitrophenol reduction than ADC-AuNPs and ADC-AgNPs. The good catalytic activity of ADC-PdNPs could be attributed to its small sizes, approximately monodisperse morphologies (Figure 4g–i), and the intrinsic stability of the Pd species [45]. The catalytic ability of ADC-PdNPs was also compared with the published literature in the 4-nitrophenol reduction. As shown in

Table 2. Comparing the catalytic performance of ADC-PdNPs with published studies in the 4-nitrophenol reduction.

Catalyst	Amount	Diameter (nm)	K (s−1)	TOF (h−1)	Reference
Co3O4/HNTs	0.1 mg	9.4	4.42 × 10−3	262	[46]
AgPd-120	-	27.6 ± 3.1	4.07 × 10−3	765	[47]
Ag/ZnO	1 mg	-	4.92 × 10−3	-	[48]
Pd/Mo2N-TiO2	0.8 wt%	2.4 ± 0.33	1.96 × 10−2	698	[49]
Pd/3D-AC	0.1 g	2.97	1.50 × 10−2	101	[50]
AuAgPd PNFs	40 μg	-	1.04 × 10−3	-	[51]
Pd@CCTP	2.0 mg	20–25	2.08 × 10−2	883	[52]
Ag0.5/C15h	0.51 wt%	3.5 ± 1.1	1.22 × 10−3	-	[53]
Pd/TiO2	0.11 wt%	2.0–5.0	1.22 × 10−2	161	[54]
Pd/GO-P	0.00065 mg	3.9 ± 0.4	2.32 × 10−2	-	[55]
Cu/MXene/PAM	1.0 mg	-	1.26 × 10−2	-	[56]
UiO-66-NH2/Pd@HMSN	0.05 mg	5.07	9.24 × 10−3	214	[57]
ADC-PdNPs	2.0 mol%	2.5 ± 1.0	2.60 × 10−2	1130	This work
ADC-PdNPs	0.5 mol%	2.5 ± 1.0	1.11 × 10−2	2060	This work

, the k_a and TOF values of relevant studies are summarized. The comparison clearly indicates that the current ADC-PdNPs had comparable catalytic activity, with a TOF value of up to 2060 h⁻¹ in a relatively low dosage (0.5 mol%).

The possible reaction mechanism for the 4-nitrophenol reduction by ADC-encapsulated metal nanoparticles in the presence of NaBH₄ was also proposed. Many previous studies have demonstrated that the 4-nitrophenol reduction mechanism using metal nanoparticle catalysts with the help of NaBH₄ conforms to a Langmuir–Hinshelwood mechanism (Figure 6) [58,59,60,61,62]. The 4-nitrophenol reduction is completed on the metal nanoparticle surface via a hydrogen transfer process. Firstly, the hydrolysis of NaBH₄ can generate BO²⁻ and active H₂. Then, H₂ is adsorbed into the metal nanoparticle surface, resulting in the formation of metal hydride (metal−H) intermediates. At the same time, 4-nitrophenol is deprotonated to produce a 4-nitrophenolate ion that is also adsorbed into the metal nanoparticle surface. Metal nanoparticles have the ability to accelerate the hydrolysis of NaBH₄, thus promoting the metal−H generation. Subsequently, the metal−H intermediates are able to attack -NO₂ groups, which results in the fast reduction of -NO₂ to -NH₂. In the 4-nitrophenol reduction, three consecutive hydro-deoxygenation procedures occur. The first process is that -NO₂ groups are converted to -NO groups accompanied by dehydration, and the second process is that -NO groups are subjected to hydrogenation to produce -NHOH groups. The third process is that -NHOH groups suffer from further hydrogenation to generate -NH₂ groups, which is proved to become the rate-determination step. Eventually, 4-aminophenol is desorbed from the metal nanoparticle surface, leading to a free surface. As a result, the metal nanoparticle catalyst can be regenerated and cycled.

Pd-based catalysts are considered to be the most common species for accelerating Suzuki−Miyaura reactions because of their high activity. In recent years, Pd nanoparticles have developed into efficient catalysts for Suzuki−Miyaura reactions [63,64,65]. Therefore, the catalytic activity of ADC-PdNPs is further investigated in Suzuki−Miyaura reactions. In order to optimize the reaction conditions, a model reaction between phenyl boronic acid and bromobenzene was carried out in the EtOH/H₂O system. The parameters including the base and the catalyst loading were considered in the optimization process, as shown in

Table 3. Optimizing Suzuki–Miyaura reaction using ADC-PdNPs as the catalyst.

Entry	Catalyst (ppm)	Base	Time (h)	Isolated Yield (%)	TOF (h−1)
1	500	NaOH	10	66	132
2	500	K3PO4	10	48	96
3	500	KOH	10	69	138
4	500	K2CO3	10	99	325 a
5	400	K2CO3	10	99	394 a
6	300	K2CO3	10	99	483 a
7	200	K2CO3	10	99	700 a
8	100	K2CO3	10	99	1257 a
9	50	K2CO3	10	97	2250 a
10	10	K2CO3	20	77	3850

Reaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.2 mmol), ADC-PdNPs, base (2.0 mmol), EtOH (2.5 mL)/H₂O (2.5 mL), 85 °C. ^a TOF was calculated after 4 h.

. A diverse base (such as NaOH, K₃PO₄, KOH and K₂CO₃) was first used for the model reaction when 500 ppm ADC-PdNPs were employed as the catalyst (Table 3, entry 1–4). It was found that the isolated yields and TOF values were obtained at a relatively low level (yield: 48–69%; TOF: 95–138 h⁻¹) using NaOH, K₃PO₄ and KOH as the base. Fortunately, the Suzuki−Miyaura reaction gave a high isolated yield (99%) and TOF (198 h⁻¹) using K₂CO₃ as a base. As a consequence, K₂CO₃ was selected for further optimization reactions. The catalyst loading was gradually decreased from 500 ppm to 10 ppm. When 50–400 ppm catalysts were used for the Suzuki−Miyaura reaction, high isolated yields (>97%) could be achieved, and a high TOF value (1940 h⁻¹) was also obtained (Table 3, entry 5–9). When the catalyst loading was reduced to 10 ppm, the Suzuki−Miyaura reaction might still have reached a 77% isolated yield with a high TOF value of 3850 h⁻¹ but in a longer reaction time (20 h) (Table 3, entry 10). These results prove the high catalytic ability of ADC-PdNPs in the Suzuki−Miyaura reaction. Considering the isolated yields and reaction time, the optimum reaction conditions for the Suzuki−Miyaura reaction were labeled as a 50 pm catalyst and K₂CO₃ as the base.

The substrate scope of ADC-PdNPs was further evaluated in different substrates, as shown in

Table 4. Catalytic activities of ADC-PdNPs in Suzuki–Miyaura reactions based on all kinds of substrates.

Entry	Aryl Halide	Aryl Boronic Acid	Isolated Yield (%)	TOF (h−1)
1			99	1980
2			97	1940
3			95	1900
4			96	1920
5			93	1860
6			96	1920
7			98	1960
8			94	1880
9			95	1900
10			92	1840
11			91	1820
12			96	1920
13			92	1840
14			95	1900
15			93	1860

Reaction conditions: aryl halide (1.0 mmol), aryl boronic acid (1.2 mmol), ADC-PdNPs (50 ppm), K₂CO₃ (2.0 mmol), EtOH (2.0 mL)/H₂O (2.0 mL), 10 h, 85 °C.

. It was observed that all substrates containing electron-releasing or electron-withdrawing substituents could be translated into the desired biphenyl compounds in high isolated yields (91–99%) and TOF values (1820–1980 h⁻¹). The chemical structures of these desired biphenyl compounds were measured by ¹H NMR (Figures S1–S9). As a result, ADC-PdNPs were highly active for Suzuki–Miyaura reactions and had good applicability for different substrates.

The catalytic ability of ADC-PdNPs in the Suzuki–Miyaura reaction was also compared with published studies. As can be seen from

Table 5. Comparing the catalytic performance of ADC-PdNPs in Suzuki–Miyaura reactions with published studies.

Catalyst	Amount	Aryl Halide	Aryl Boronic Acid	Diameter (nm)	Yield (%)	TOF (h−1)	Reference
Fe3O4/SiO2-NH2@CS/Pd	0.1 mol%			5.0	98	980	[66]
Pd–Ag@PMFC	0.008 g			52.0	93	-	[67]
Fe3O4@H2L-Pd(0)	0.8 mol%			12.0–19.0	99	-	[68]
DMSTNs-SH-Pd	0.2 mol%			3.1	78	780	[69]
Pd@InOF-1	10.0 mg			3.14	94	-	[70]
Fe3O4@SiO2-DTPA-Pd	1.7 µmol			-	95	186	[71]
g-CN(G)-AgPd	5.0 mg			2.54 ± 0.76	94	-	[72]
PAF-SP@Pd	20 mg			1–10	98	12	[73]
Pd/COF-SMC2	0.5 mol %			5	96	192	[74]
Cu/Pd@Mod-PANI-3OH	0.094 mol%			<1	93	659	[75]
SBA-15/Et-CN/Pd	0.1 mol%			-	95	950	[76]
PdNPs	50 ppm			2.0 ± 1.0	98	2450	This work

, the catalyst loading, diameter, yield and TOF values of relevant studies were summarized based on different substrates. The comparison results distinctly demonstrated that ADC-PdNPs possessed superior qualities including small sizes (2.5 ± 1.0 nm), low catalyst loading (50 ppm), high yield (99%) and high TOF (1980 h⁻¹).

The potential reaction mechanism for the Suzuki–Miyaura reaction via ADC-PdNPs is proposed. Many previous studies have discussed the mechanism of Pd-catalyzed Suzuki–Miyaura reactions [77,78,79,80]. As shown in Figure 7, the reaction process mainly involves three steps: oxidative addition, transmetalation and reductive elimination. In the oxidative addition step, aryl halide would oxidize Pd(0) to form σ-arylpalladium intermediate, which is considered as the rate-determining step. The σ-arylpalladium compound is able to react with aryl boronic acid to generate a biaryl palladium complex in the presence of K₂CO₃ through a transmetalation step. Finally, the biaryl palladium complex undergoes a reductive elimination to release the desired biaryl product. Meanwhile, the active Pd(0) catalyst was rebuilt for the next catalysis.

4. Conclusions

In summary, the ADC polymer containing hydrophilic branched triethylene glycol moieties and hydrophobic branched ferrocenyl moieties was successfully synthesized by ROMP and was applied as the supporting material to prepare metal nanoparticle catalysts including ADC-AuNPs, ADC-AgNPs and ADC-PdNPs. ADC-AuNPs, ADC-AgNPs and ADC-PdNPs had average sizes of 3.5 ± 3.0 nm, 7.2 ± 4.0 nm and 2.5 ± 1.0 nm, respectively, and showed quasi-spherical morphologies and a clear lattice fringe. The catalytic activities of ADC-AuNPs, ADC-AgNPs and ADC-PdNPs were investigated in the reduction of 4-nitrophenol. It was found that ADC-PdNPs exhibited relatively higher catalytic activity (K: 1.11 × 10⁻² s⁻¹; TOF: 2060 h⁻¹) in the 4-nitrophenol reduction than ADC-AuNPs and ADC-AgNPs. In addition, ADC-PdNPs were also highly active for Suzuki–Miyaura reactions (low catalyst loading: 50 ppm; high yield: 99%; high TOF: 1980 h⁻¹) and had good applicability for different substrates. Compared to existing catalysts, ADC-encapsulated metal NPs have the advantages of a high reaction rate constant, low catalyst dosage, high yields and high TOF values. The ADC polymer reported in this article gives a new inspiration for the design of the efficient supporting materials for metal nanoparticle catalysts.