Phosphinite-functionalized silica and hexagonal mesoporous silica containing palladium nanoparticles in Heck coupling reaction: synthesis, characterization, and catalytic activity

Abstract

A series of mono, di and tri phosphinite ligands functionalized on modified silica and modified hexagonal mesoporous silica (HMS) were synthesized and characterized. The complexation of these ligands with PdCl₂ was carried out to obtain palladium supported on phosphinite functionalized silica. TEM images of the catalysts showed that Pd dispersed in nanoparticle size on these heterogeneous catalytic systems. Hexagonal and mesoporous structures with high surface area of HMS were examined by SEM, TEM and BET techniques. In a typical Heck coupling reaction (HCR), catalytic activity of the Pd catalysts was compared. It was shown that di-functionalized phosphinite ligands react better for both silica and HMS. Aryl halides (also known as haloarenes or halogenoarenes) of different varieties with olefinic substrates in the HCR exhibited high efficiency and stability for the selected catalyst. Repeating Heck reaction cycles illustrated that the catalyst could be recycled.

---

1. Introduction

Possessing a vast array of unique properties, specifically electronic and steric, a key type of ligands within organometallic chemistry are phosphorous-based ligands.¹ Recently, a sub category of these ligands, specifically phosphinites, have undergone further development and require greater attention from the research community. When compared to phosphines, phosphinites offer different desirable properties such as those which are chemical, electronic, and structural in nature, thus providing new opportunities within the field of ligand design.

Metal complexes of phosphorous-type ligands have been employed in various applications in transition metal catalyzed organic reactions.^2,3 Compared to relatable phosphines, the P-/OR group (i.e., electron-withdrawing) of phosphinites increases the strength of the metal phosphorous bond. Furthermore, the phosphinite's (specifically P (OR) R₂) empty σ*-orbital provides stabilization and in turn acts as a better acceptor. Phosphinite complexes of different metals such as ruthenium, rhodium and palladium in their homogenous or heterogeneous forms also show high catalytic activity in different reactions.⁴

Among various reported reactions catalyzed by the metal complex of phosphinite ligands, C–C coupling with the Pd complex is one of the most studied. For the first time, Bedford et al., proved to be successful in their application of phosphinite ligands in a HCR.^5,6 Some other catalytic systems reported in Pd catalyzed homogeneous cross coupling reactions (CCR) are as follows: nitrogen and sulfur containing phosphinite ligand,^7,8 phosphinite based ionic liquid,⁹ phosphinite based PCP and PCN pincer ligands,^10,11 2-diphenylphosphinoxynaphthyl,¹² amino acid-based phosphinite¹³ and phosphinite- and phosphite-based type 1 palladacycles.¹⁴

Loss of expensive metal complexes as well as difficulties associated with product separation from the reaction mixture, which results in inefficient and poor separation, are two common examples of limitations experienced in the application of homogeneous systems. The application of phosphinites as heterogeneous systems in coupling reactions is miniscule even though the homogeneous catalytic processes have been thoroughly explored. Examples of heterogeneous systems on organic or inorganic supports are as follows: a set of phosphorus ligands on a polystyrene support,¹⁵ polyacrylamide containing phosphinite ligand supporting palladium nanoparticles,¹⁶ aminophosphinites ligands supported on Wang resin,¹⁷ phosphinated poly(vinyl alcohol),¹⁸ modified Merrifield resin containing PCP-pincer ligand,¹⁹ diphenylphosphinite cellulose,²⁰ silicadiphenylphosphinite (SDPP) ligands^21,22 and phosphine functionalized magnetic nanoparticles.²³ The influence of these ligands over palladium, providing both stability and increased reactivity, is the key to their effective use in the formation of C–C bonds. Today, mesoporous silica, a highly dispersed material with adjustable and uniform pore sizes for surface functionalization, have found potential applications in sensors,²⁴ selective adsorption of metals²⁵ and as a drug adsorber.²⁶ Furthermore, it can be applied as a solid support in organic transformations²⁷ and CCR.^28,29 Different types of mesoporous silica like HMS, MCM-41 (mobil catalyst material) and SBA-15 (Santa Barbara amorphous) have been applied in catalytic reactions.³⁰

There are limited reports in the literature on phosphinite ligands immobilized on silica and no report was found for phosphinite ligands immobilized on mesoporous silica. Furthering past studies conducted by the author(s) on Pd catalyst based on phosphinite ligands supported by polymers,^15,16,18,19 the synthesis and characterization of phosphinite-functionalized silica and hexagonal mesoporous silica containing palladium nanoparticles is reported in this work. These catalysts were explored as heterogeneous catalytic systems in HCR with different substrates.

2. Materials, methods, and analytical techniques

2.1. Materials and general analytical techniques

Merck, Fluka and Aldrich companies supplied the commercial, reagent grade chemicals used in the study. Fluka supplied the aminopropyl silica gel, which consisted of particle sizes averaging from 0.015 to 0.035 mm (>400 mesh ASTA). FTIR spectroscopy, hydrogen, as well as carbon nuclear magnetic resonance (¹H and ¹³C-NMR) spectroscopy was used to characterize all of the products utilized. An FTIR spectrophotometer (Shimadzu FTIR-8300) was utilized to obtain FTIR spectrum. A Bruker Avance DPX (250 MHz) instrument was used to perform ¹H and ¹³C-NMR analyses. ³¹P-HPDEC was measured with a spectrometer “Avance 400” from Bruker-BioSpin (10 000 Hz). The yields in this work refer to those spectra of the isolated products. A GC instrument (Shimadzu model GC 10-A) or TLC on silica-gel plates (Polygram SIL/UV 254) was used to monitor the reaction's progress and completion. An inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyzer (Varian, Vista-Pro) was utilized for Pd analysis. An XRD (X-ray diffraction), (D8, Avance, Bruker, AXS) analysis was conducted to obtain the XRD data for sample IVB with X-ray diffractometer (Cu Kα radiation with λ = 1.541874 Å) and a 2θ scan range of 10–90° at room temperature. A more precise X-ray diffractometer, MPD 3000, with linear detector and X-ray diffractometer (Cu Kα radiation) at 40 kV and 30 mA and a 2θ scan range of 10–100° at room temperature were applied for monitoring the crystallinity of catalyst XB. A SEM (scanning electron microscopy) at 20 kV (XL-30 FEG SEM, Philips), was used to obtain SEM micrographs and images. Prior to the SEM observation, samples were sputter-coated with gold. Transmission electron microscopy (TEM) analysis was performed using a Philips model CM 10 instrument with an accelerating voltage of 100 kV. Each sample was prepared by dropping the nanoparticle solution on to TEM grid, followed by wicking the solution away and drying. A Chem BET-3000, Quantachrome instrument with N₂ as an adsorbent, was used to determine the Brunauer–Emmet–Teller (BET) surface area of the catalyst.

2.2. Methods: catalyst preparation

2.2.1. Mono, di and tri phosphinite ligand functionalized on modified silica. Preparation of acrylamidopropyl silica through the aminopropyl silica (AMPS) and acryloyl chloride reaction was performed according to a reported procedure.^31,32 Suspension of AMPS (10 g, 9.5 mmol amino groups) in 200 mL of dried tetrahydrofuran (THF) was completed and allowed to be cooled until it reached 0 °C. The initial addition of N (CH₂CH₃)₃ (i.e., trimethylamine: 15 mmol, 1.51 g) was followed by a 2-propenoyl chloride (i.e., acryloyl chloride: 12 mmol, 1.09 g) addition over a one hour period at 5 °C. The slurry was then mixed at 0 °C for another four hours, filtered off and washed with 50 mL of THF and 2 × 50 (mL) water, so as to isolate the modified silica. Oven drying of the solid took place at 110 °C and was carried out for 24 hours.

To prepare and synthesize hydroxyl functionalized silica, 10 (mmol) of alkanol amine (ethanol amine, diethanol amine, or TRIS (also known as 2-amino-2-hydroxymethyl-propane-1,3-diol)) was added to an acrylamidopropyl silica (2 g) suspension in ethanol (20 mL) and mixed for 20 hours at ambient temperature. The solid collected from the filtered slurry was ethanol washed and subsequently dried under reduced pressure.

To prepare and synthesize phosphinite functionalized silica, a solution of (C₆H₅)₃PCl (6 mmol, 1.320 g, in 15 mL THF) was added drop by drop to the stirred, ice cooled (bath at 0 °C) solution of hydroxyl functionalized silica (2 g in 30 mL of THF) while in the presence of triethylamine (12 mmol, 1.12 g) over 15 minutes while being stirred. Stirring of the reaction mixture was continued for 10 additional hours at ambient temperature and was followed by a filtration step. Prior to being dried under reduced pressure, both THF and water were used to wash the recovered white solid.

2.2.2. Mono, di, and tri phosphinite ligand functionalized on modified hexagonal mesoporous silica. Per the reported procedure,³³ aminopropyl hexagonal mesoporous silica (HMS) was prepared. Simultaneously and rapidly, TEOS (0.09 mol, 18.8 g) and γ-aminopropyl (trimethoxy) silane (0.01 mol, 1.79 g) were added separately to an n-dodecylamine (27.5 mmole, 5.08 g) solution at 20 °C in an aqueous ethanol solution (absolute ethanol (46 mL) and distilled water (53 mL)) while being stirred. This solution was cloudy and underwent mixing for 18 hours. At first the solution turned milky, and then became thick after 5 minutes. After 18 hours, the white solid that was recovered from the final filtered solution and was rinsed using 100 mL of ethanol. To remove the template from the damp solid product, it was Soxhlet extracted for 24 hours in methanol.

The procedure used to synthesize acrylamidopropyl hexagonal mesoporous silica was the same as outlined in Section 2.2.1 for the preparation of acrylamidopropyl silica. Here, aminopropyl hexagonal mesoporous silica was used as a precursor.

The procedure used to synthesize hydroxyl functionalized hexagonal mesoporous silica was the same as outlined in Section 2.2.1 for the preparation and synthesis of hydroxyl functionalized silica. Here, acrylamidopropyl hexagonal mesoporous silica was used as a precursor.

The procedure used to synthesize phosphinite functionalized hexagonal mesoporous silica was also the same as outlined in Section 2.2.1 for the preparation and synthesis of phosphinite functionalized silica. Here, hydroxyl functionalized hexagonal mesoporous silica used as a precursor.

To prepare palladium catalysts, mono, di and tri phosphinite ligands functionalized on modified silica or modified hexagonal mesoporous silica (1 g) were treated at 100 °C for 8 hours with a solution of PdCl₂ (1.5 mmol, 0.28 g) in DMF (30 mL). Then, under reduced pressure the mixture was dried after it had been rinsed with DMF, water and acetone.

2.2.3. Heck coupling reaction: general procedure. Styrene or n-butyl acrylate (1.2 mmol) was added to a mixture of haloarene (1.0 mmol), K₂CO₃ (2.0 mmol) and Pd catalyst (0.5 mol%) in NMP (5 mL). The reaction mixture was stirred at 120 °C and was monitored by TLC (or GC if necessary). Upon the reaction's completion, the filtered catalyst was captured after the mixture was cooled to ambient temperature. The filtrate mixture was decanted in water (50 mL) then extracted with CH₂Cl₂ (3 × 20 mL). After being dried over Na₂SO₄, the organic layer was filtered off and evaporated. To ensure purity of the product, chromatography over silica-gel was performed. By comparing the results from the FTIR, ¹³C-NMR and ¹H-NMR analyses along with the physical data and the available standards, the product was characterized.

3. Results and discussion

3.1. Palladium catalyst: preparation and characterization

3.1.1. Phosphinite functionalized silica palladium complex (Si-OPPh₂-Pd). Synthesis of phosphinite functionalized silica with a linker had been previously reported for the oxidation reaction in the presence of ruthenium.³⁴ Phosphinite ligands, in homogeneous catalytic transformations, have been thoroughly researched; however, their use as a heterogeneous type system in CR has not been fully explored. Here, the methodology for the synthetic design of a series of Pd catalysts based on phosphinite functionalized silica is shown in Scheme 1.

Scheme 1 Preparation of phosphinite functionalized silica palladium complex (Si-OPPh₂-Pd).

First, acrylamidopropylsilica was synthesized by reacting aminopropylsilica (AMPS) with acryloylchloride (I). The spectrum produced by FTIR analysis of (I) showed the absorption peaks at 1558 and 1662 (cm⁻¹) for the amide group and at 1627 (cm⁻¹) for the double bond. The resulting modified silica was then reacted with alkanol amine in a Michael type addition to form the corresponding hydroxyl modified silica substrates.³⁵ A series of mono, di and tri alkanol amines such as ethanol amine (A), diethanol amine (B) and TRIS (C) were used. The spectrum produced by FTIR analysis of (II) showed that the reaction had reached completion due to the disappearance of the double bond peak at 1627 (cm⁻¹). The polymeric phosphinite ligands (III) were prepared by reacting hydroxyl modified silica (II) with ClPPh₂ in THF. FTIR spectroscopy was utilized for product characterization. The appearance of bands at approximately 1440 (cm⁻¹) confirmed the presence of the phosphinite group (O-PPh). Using the iodometric titration method, the amount of phosphorous within the products was obtained and the results are presented in Table 1. The lowest amount of phosphorous content belonged to mono-phosphinated ligand (IIIA), but the highest amount belonged to the di-phosphinited ligand (IIIB), even though (IIIC) was expected to be the highest. This observation might be due to the bulky structure of (IIIC) with three hydroxyl groups and the bulky structure of chlorodiphenylphosphine, which inhibited the reaction with hydroxyl groups.

Table 1 Phosphorous content of (IIIA) to (IIIC) and palladium loading of (IVA) to (IVC)

Entry	Catalyst	Phosphorus loading (mmol g^−1)	Pd loading (mmol g^−1)
1		0.5	0.06
2		3.5	0.34
3		2.3	0.26

---

The resulting phosphinite ligands were then complexed with PdCl₂ in DMF to obtain heterogeneous catalysts (IVA–IVC). ICP-AES analysis was undertaken to determine the Pd content and the results are presented in Table 1. The highest Pd loading belonged to the structure (IVB). This was predictable according to the reported phosphorous content values. The purity and validity of phosphinite ligand was further characterized with solid ³¹P HPEDC NMR of the compound (IVB) and the characteristic band of P–Pd appeared at 31.81 ppm (Fig. 1).

Fig. 1 ³¹P HPEDC NMR of (IVB).

Visual images of the supported catalyst (IVB) were obtained via SEM and TEM. The SEM image of the catalyst (IVB) showed the surface of the silica support (Fig. 2) where the shiny metallic particles are gleaming on the surface with sizes in the range of 100–300 nm. In the TEM image and histogram based on this image, dispersion of Pd particles on the surface of the modified silica was observed (Fig. 3). The histogram showed that 50% of the particles have a 14 nm diameter and 35% are 7 nm in diameter (Fig. 4).

Fig. 2 SEM image of catalyst (IVB) displaying the surface of the silica support.

Fig. 3 TEM image of catalyst (IVB). Dispersion of Pd particles on the surface of the modified silica was observed.

Fig. 4 Histogram of Pd nanoparticle dispersion based on Fig. 3.

Further characterization of the resulting palladium complexes was carried out by the XRD technique. As expected, the amorphous silica and crystallinity of Pd nanoparticles were observed via the resulting XRD pattern for (IVB) (Fig. 5). The presence of the crystallographic planes for the Pd(0) nanoparticles in the indicated region is the assignable reason for the observed diffraction rings (111, 200, 220 and 311). On the other hand, the expected crystallinity of amorphous silica appeared at 22°.

Fig. 5 XRD of pattern of catalyst (IVB).

3.1.2. Phosphinite functionalized hexagonal mesoporous silica palladium complex (HMS-OPPh₂-Pd). The preparation of phosphinite-functionalized hexagonal mesoporous silica containing palladium is summarized in Scheme 2. Hexagonal modified aminopropylsilica (V) was synthesized by the reaction of tetraethoxysilane and γ-aminopropyl (trimethoxy) silane in the presence of dodecylamine as a templating agent according to the procedure given by Clark et al.³³

Scheme 2 Preparation of phosphinite functionalized hexagonal mesoporous silica palladium complex (HMS-OPPh₂-Pd).

All other steps were performed in the same manner as that for aminopropylsilica. First hexagonal mesoporous aminopropyl silica was reacted with acryloylchloride to yield HMS modified with a double bond (VI). The resulting modified silica was reacted with a series of alkanol amines in a Michael type addition reaction. The resulting silica with hydroxyl groups at it ends was phosphinated along with (C₆H₅)₃PCl. Phosphorous content was determined by iodometry and the results are presented in Table 2.

Table 2 Phosphorous content and surface area of (VIIIA) to (VIIIC), and palladium loading of (XA) to (XC)

Entry	Catalyst	Phosphorus loading (mmol g^−1)	Pd loading (mmol g^−1)	Surface area (m^2 g^−1)
1		5.6	0.15	612
2		6.7	0.3	550
3		7.2	0.22	514

---

From the mono-phosphinated to the tri-phosphinated ligand, the phosphorous content increased. FTIR characterization of all products was carried out and the results were the same as for modified silica. Fig. 6 shows the comparison between the FTIR for VI and VIIB. The vanishing peak of the double bond from 1628 (cm⁻¹) and the appearance of the O-PPh band at 1388 (cm⁻¹) proves a successful transformation.

Fig. 6 FTIR spectrum of VI and VIIB.

Then, the resulting phosphinite ligands were complexed with PdCl₂ to achieve heterogeneous catalysts. ICP-EAS analysis was used to ascertain the Pd content and the results are presented in Table 2. The surface area of these three catalysts was estimated using the so-called BET method and the results are presented in Table 2. From the mono-phosphinated Pd complex to the tri-phosphinated complex, the bulkiness increased and available surface area was reduced. A BET surface area of 612 (m² g⁻¹) was obtained for the di phosphinite ligand (VIIIB).

The resulting palladium complex (XB) with the highest metal loading was selected and further characterized by SEM, TEM and XRD techniques. SEM imaging of (XB) was carried out and the representative image is shown in Fig. 7, displaying a well-designed HMS particle formation in hexagonal format. The major percentages of the material consisted of aggregates of small particles that were roughly spherical and had dimensions of 200 to 500 nm. The presence of shiny metallic particles on this surface is not apparent and can be attributed to their location being hidden within the pores.

Fig. 7 SEM image of catalyst (XB) showing well-designed HMS particle formation in hexagonal format.

Pd nanoparticles were revealed in the TEM image of the catalyst (XB), 3–5 nm in size, through the porous surface of catalyst (Fig. 8). The TEM histogram of selected area showed that 80% of particles are 5 nm in diameter (Fig. 9).

Fig. 8 TEM image of catalyst (XB).

Fig. 9 Histogram of Pd nanoparticle dispersion based on selected area of Fig. 8.

Ordered mesoporous silica has a unique crystalline structure which forms distinguished peaks that are different from that of amorphous silica in X-ray diffraction. An instrument with low angle scanning capability is needed to determine the XRD pattern of HMS (i.e. a MPD 3000 diffractometer). As seen in Fig. 10, the catalyst (XB)'s XRD pattern is consistent with the required crystallographic planes of HMS which appeared as a sharp band at 1° 2-theta scale for the diffraction ring of 100. This is consistent with a mesoporous silica pattern³⁶ and broad peak below 10° 2-theta scale which is due to the presence of small nanoparticle formations³⁷ (Fig. 10a). Also, the broad peak that appeared around 20° (Fig. 10a) is responsible for SiO₂ phase and the crystallographic planes of the Pd(0) nanoparticles appeared according to index. These are all assignable reasons for the observed diffraction rings (Fig. 10b).

Fig. 10 XRD pattern of Pd complex (XB); indicating (a) mesoporous silica pattern, (b) Pd nanoparticles patterns.

In conclusion, the two types of heterogeneous systems based on silica and HMS functionalized with phosphinite ligands were characterized and showed the potent capability of Pd loading. Fundamental differences were expected and observed in the HMS-based structure such as high surface area (Table 2) and specific crystallinity (Fig. 7). The XRD pattern of both samples IVB and XB exhibited the diffraction pattern of Pd(0). The active Pd(0) catalyst required for these reactions, was typically created by the coordination of two P atoms to a Pd(II) center, which was subsequently reduced to Pd(0) via an additional phosphinite moiety. This particular methodology for reducing Pd(II) compounds has been previously reported in literature.^4,38 On the other hand, Pd complexation occurred in DMF media, which is a known reducing agent and supportive in Pd(0) formation.³⁹ However, Pd nanoparticle formation was more uniform and smaller in size when HMS was utilized.

3.2. Catalytic activity of the catalysts in HCR

To check the applicability of the synthesized catalysts, the HCR was performed. First, the reactivity of Pd catalysts for compounds (IVA), (IVB) and (IVC) were checked in the HCR for a model system of n-butyl acrylate and p-nitro bromobenzene, all while under the same conditions (Scheme 3). The findings are presented in Table 3.

Scheme 3 HCR reaction of p-nitro bromobenzene with n-butylacrylate in the presence of catalysts IVA, IVB or IVC.

Table 3 HCR of n-butyl acrylate with p-nitro bromobenzene with catalyst (IVA), (IVB) and (IVC)^a

Entry	Catalyst	Time (h)	Conversion^b
a Reactions were run using 1.0 equiv. p-nitro bromobenzene, 1.2 equiv. n-butyl acrylate, 2.0 equiv. K2CO3 and 0.005 equiv. Pd catalyst in 5 mL DMF as solvent at 120 °C.b Conversion based on p-nitro bromobenzene.
1	(IVA)	5	100
2	(IVB)	0.5	100
3	(IVC)	2	100

---

The results from Table 3 illustrate that catalyst (IVB) worked better than the other two catalysts; therefore catalyst (IVB) was selected for further reactions. Optimized reaction conditions with catalyst (IVB) were determined and the results are presented in Table 4. The ideal system was found to use 0.5 (mmol%) of catalyst (IVB) at 120 °C with NMP as a solvent and K₂CO₃ as a base.

Table 4 Salts' and solvents' optimization for HCR of p-nitro bromobenzene with n-butyl acrylate alongside of catalyst (IVB) (0.5 mol%)^a

Entry	Base	Solvent	Pd catalyst (mol%)	Time (h)	Conversion^b (%)
a Reactions were run using 1.0 equiv. iodobenzene, 1.2 equiv. n-butyl acrylate, 2.0 equiv. base and 0.005 equiv. Pd catalyst (IVB) in 5 mL solvent at 120 °C.b Conversion based on p-nitro bromobenzene.
1	K2CO3	NMP	0.5	0.5	100
2	K2CO3	EtOH	0.5	5	40
3	K2CO3	DMF	0.5	1	100
4	Et3N	DMF	0.5	5	60
5	LiF	DMF	0.5	5	50
6	NaOAC	DMF	0.5	1	80

---

Styrene or n-butyl acrylate, used as olefinic substrates with different haloarene, was utilized to complete other coupling reactions, thus showing the generality of the Pd catalyst (IVB) (Scheme 4 and Table 5).

Scheme 4 HCR reactions in the presence of IVB.

Table 5 Different haloarene reaction with n-butyl acrylate, or styrene alongside Pd catalyst (IVB)^a

Entry	Haloarene	R^2	Product^b	Time	Yield^c (%)
a Reactions were run using 1.0 equiv. haloarene, 1.2 equiv. olefinic substrates, 2.0 equiv. K2CO3 and 0.005 equiv. Pd catalyst (IVB) in 5 mL NMP as solvent at 120 °C.b Products characterization was performed by comparison of their FTIR, ^1H-NMR, ^13C-NMR, and physical data with those of the authentic samples.c Isolated yields.d With additional tetrabutylammonium bromide (TBAB) (0.01 mmol).
1		CO2Bu^n		1 h	88
2		CO2Bu^n		1.5 h	85
3		CO2Bu^n		0.5	87
4		CO2Bu^n		30 min	93
5		CO2Bu^n		30 min	94
6		CO2Bu^n		1.5 h	80
7		CO2Bu^n		2 h	90
8		CO2Bu^n		9 h	83
9		CO2Bu^n		24 h	55
10		CO2Bu^n		10 h	70^d
11		CO2Bu^n		8 h	85^d
12		CO2Bu^n		9 h	82^d
13		CO2Bu^n		12 h	88^d
14		Ph		12 h	75
15		Ph		5 h	70
16		Ph		15 h	82
17		Ph		4 h	75

---

Haloarenes containing electron withdrawing and electron donating substituents were reacted with n-butyl acrylate, thus generating the corresponding coupling products while maintaining high yields. Reactivity of aryl chlorides was also checked while in the presence of a small amount of tetrabutylammonium bromide (TBAB) (Table 5: entries 10–13). Furthermore, some haloarenes with styrene CRs were also carried out successfully (Table 5: entries 13–17). The recycling feasibility of the catalyst was also assessed by the CR of n-butyl acrylate with iodobenzene. The catalyst (IVB) showed good heterogeneous character in three repeated cycles, but afterwards it unexpectedly showed approximately 50% Pd leaching along with reduced catalytic activity.

In the same manner, the reactivity of Pd catalysts was also reviewed, where compounds (XA), (XB) and (XC) in a HCR were examined in a model system of n-butyl acrylate and p-nitro bromobenzene while under the same conditions. The results are given below in Table 6.

Table 6 HCR of n-butyl acrylate with p-nitro bromobenzene using catalyst (XA), (XB) or (XC)^a

Number	Catalyst	Time (h)	Conversion^b
a Reactions were run using 1.0 equiv. p-nitro bromobenzene, 1.2 equiv. n-butyl acrylate, 2.0 equiv. K2CO3 and 0.005 equiv. Pd catalyst.b Conversion based on p-nitro bromobenzene.
1	(XA)	12	90
2	(XB)	9	100
3	(XC)	10	100

---

The results presented in Table 6 show that (XB) worked better in Heck coupling when compared to the other two catalysts. The optimized reaction conditions for (IVB) (using 0.5 mmol% of catalyst at 120 °C, NMP as solvent and K₂CO₃ as a base) were used to verify the Pd catalyst (XB)'s generality in a HCR for a different haloarene and olefinic substrates; the results are given in Table 7. In order to produce high product yields of the coupling products, n-butyl acrylate was reacted with electron-neutral, rich and poor haloarenes. The reaction utilizing the mesoporous catalyst (XB) was sluggish when compared with the catalyst (IVB) since the penetration of substrate into pores takes time. The reactivity of aryl chlorides, even in the company of a miniscule quantity of tetrabutylammonium bromide (TBAB) did not give good results for the coupled product. Only 4-nitro chlorobenzene as an activated haloarene was promising (Table 7: entry 10).

Table 7 n-Butyl acrylate reactions with different haloarene in existence of Pd catalyst (XB)^a

Entry	Haloarene	Product^b	Time (h)	Yield^c (%)
a Reactions were run using 1.0 equiv. haloarene, 1.2 equiv. olefinic substrates, 2.0 equiv. K2CO3 and 0.005 equiv. Pd catalyst (XB) in 5 mL NMP as solvent at 120 °C.b The characterization of products was performed by comparison of their FTIR, ^1H-NMR, ^13C-NMR, and physical data with those of the authentic samples.c Isolated yields.d With additional tetrabutylammonium bromide (TBAB) (0.01 mmol).
1			3	80
2			5	80
3			3	87
4			4	75
5			3	85
6			4	85
7			18	75
8			24	40
9			36	40
10			18	60^d
11			24	75

---

In order to determine the catalyst's ability to be recycled, the catalyst underwent 5 cycles of testing, each with prolonged reaction time, but immediate leaching appeared in the third run.

4. Conclusion

A number of phosphinite ligands functionalized on modified silica and modified HMS were synthesized and characterized. The complexations of these ligands with PdCl₂ were performed to prepare heterogeneous catalytic systems. TEM images showed that Pd dispersed in nanoparticle size for both series of catalysts. Hexagonal structure and high surface area was verified for the catalysts based on HMS using SEM and BET techniques. High efficiency and stability of Pd catalysts were shown in the HCR. Generation of the resulting CR product in high yield confirmed the good reactivity of the electron-neutral, rich and poor haloarenes. Better reactivity was shown with diphosphinite ligands in both categories of Pd catalysts. Catalysts based on HMS showed lower efficiency, since the penetration of the substrate into the pores takes time. Simple filtration and reusability of these catalysts are a few of the advantages displayed by these catalytic systems.

Acknowledgements

The authors gratefully acknowledge the partial support of this study by Shiraz University Research Council. Authors would also thank Prof. Dr Mathias Ulbricht and Mr Manfred Zähres from Universität Duisburg-Essen and Dr Doroodmand from Shiraz University for their collaboration in analysis performing.

References

1. H. Hudson, Primary, secondary and tertiary phosphines, polyphosphines and heterocyclic organophosphorus(III) compounds, Wiley, New York, 1990, pp. 386–472.
2. J. A. Gillespie, E. Zuidema, P. W. van Leeuwen and P. C. Kamer, Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis, 2012, pp. 1–26.
3. T. M. Shaikh, C.-M. Weng and F.-E. Hong, Coord. Chem. Rev., 2012, 256, 771–803.
4. F. Ozawa, A. Kubo and T. Hayashi, Chem. Lett., 1992, 2177–2180.
5. D. A. Albisson, R. B. Bedford, P. N. Scully and S. E. Lawrence, Chem. Commun., 1998, 2095–2096.
6. R. B. Bedford and S. L. Welch, Chem. Commun., 2001, 129–130.
7. I. D. Kostas, B. R. Steele, A. Terzis and S. V. Amosova, Tetrahedron, 2003, 59, 3467–3473.
8. M. Diéguez, O. Pàmies and C. Claver, J. Organomet. Chem., 2006, 691, 2257–2262.
9. N. Iranpoor, H. Firouzabadi and R. Azadi, J. Organomet. Chem., 2008, 693, 2469–2472.
10. J. Aydin, J. M. Larsson, N. Selander and K. J. Szabo, Org. Lett., 2009, 11, 2852–2854.
11. Y. Motoyama, K. Shimozono and H. Nishiyama, Inorg. Chim. Acta, 2006, 359, 1725–1730.
12. B. Punji, C. Ganesamoorthy and M. S. Balakrishna, J. Mol. Catal. A: Chem., 2006, 259, 78–83.
13. P. W. Galka and H.-B. Kraatz, J. Organomet. Chem., 2003, 674, 24–31.
14. P. He, Y. Lu and Q.-S. Hu, Tetrahedron Lett., 2007, 48, 5283–5288.
15. B. Tamami and F. N. Dodeji, J. Iran. Chem. Soc., 2012, 9, 841–850.
16. B. Tamami and S. Ghasemi, J. Mol. Catal. A: Chem., 2010, 322, 98–105.
17. A. Mansour and M. Portnoy, Tetrahedron Lett., 2003, 44, 2195–2198.
18. F. Ebrahimzadeh and B. Tamami, Preparation and characterization of palladium nanoparticles supported on phosphinated poly(vinyl alcohol) as new recyclable catalyst and their application for Heck cross-coupling reactions, Phosphorus, Sulfur Silicon Relat. Elem., 2015, 190(2), 144–157.
19. B. Tamami, M. M. Nezhad, S. Ghasemi and F. Farjadian, J. Organomet. Chem., 2013, 743, 10–16.
20. Q. Du and Y. Li, Beilstein J. Org. Chem., 2011, 7, 378–385.
21. N. Iranpoor, H. Firouzabadi, S. Motevalli and M. Talebi, J. Organomet. Chem., 2012, 708, 118–124.
22. N. Iranpoor, H. Firouzabadi, S. Motevalli and K. Rajabi, A Green Approach for Copper-Free Sonogashira Reaction of Aryl Halides with Phenylacetylene in the Presence of Nano-Pd/Phosphorylated Silica (SDPP/Pd⁰), Aust. J. Chem., 2014, 68(6), 926–930,  DOI:10.1071/ch14332.
23. A. Khalafi-Nezhad and F. Panahi, J. Organomet. Chem., 2013, 741, 7–14.
24. K. Unger, D. Kumar, M. Grün, G. Büchel, S. Lüdtke, T. Adam, K. Schumacher and S. Renker, J. Chromatogr. A, 2000, 892, 47–55.
25. I. Sierra and D. Pérez-Quintanilla, Chem. Soc. Rev., 2013, 42, 3792–3807.
26. F. Farjadian, P. Ahmadpour, S. M. Samani and M. Hosseini, Microporous Mesoporous Mater., 2015, 213, 30–39.
27. A. Corma and H. Garcia, Adv. Synth. Catal., 2006, 348, 1391–1412.
28. M. Opanasenko, P. Štěpnička and J. Čejka, RSC Adv., 2014, 4, 65137–65162.
29. V. Polshettiwar, C. Len and A. Fihri, Coord. Chem. Rev., 2009, 253, 2599–2626.
30. R. Luque, A. M. Balu, J. M. Campelo, M. D. Gracia, E. Losada, A. Pineda, A. A. Romero and J. C. Serrano-Ruiz, Catalytic applications of mesoporous silica-based materials, Catalysis, 2012, vol. 24.
31. B. Tamami, F. Farjadian, S. Ghasemi and H. Allahyari, New J. Chem., 2013, 37, 2011–2018.
32. F. Farjadian and B. Tamami, ChemPlusChem, 2014, 79, 1767–1773.
33. D. Macquarrie, D. Jackson, J. G. Mdoe and J. Clark, New J. Chem., 1999, 23, 539–544.
34. E. Dulière, M. Devillers and J. Marchand-Brynaert, Organometallics, 2003, 22, 804–811.
35. S. Santra, C. Kaittanis and J. M. Perez, Mol. Pharm., 2010, 7, 1209–1222.
36. T. R. Pauly and T. J. Pinnavaia, Chem. Mater., 2001, 13, 987–993.
37. K. Möller, J. Kobler and T. Bein, Adv. Funct. Mater., 2007, 17, 605–612.
38. N. Iranpoor, H. Firouzabadi and R. Azadi, Eur. J. Org. Chem., 2007, 2197–2201.
39. I. Pastoriza-Santos and L. M. Liz-Marzán, Nano Lett., 2002, 2, 903–905.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16131b

---