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• The formation of carbon-carbon (C—C) bonds via coupling reactions has been applied to synthesize a wide range of organics, such as natural products, pharmaceuticals, perfumes and dyes [1-6]. Recently, photocatalytic C—C coupling reactions have attracted considerable attention due to its sustainability. Especially, Pd nanoparticles (NPs) displayed high conversion and selectivity in these photocatalytic C—C coupling reactions [7-9]. However, the relatively high cost of individual Pd NPs is disadvantageous to its widespread application [10, 11]. To improve the atomic utilization, Kim and coworkers studied the size effect of Pd NPs on visible-light-driven Suzuki coupling reaction and found that 5.5 nm Pd NPs supported on exfoliated 2H-WS₂ exhibited an extraordinary photocatalytic activity, where polyvinylpyrrolidone (PVP) was used to stabilize the highly active ultrafine Pd NPs [12]. Guo and coworkers reported that PdCu alloy with a Pd/Cu molar ratio of 3:1 could not only enhance the photocatalytic activity toward Sonogashira C—C coupling reaction but also reduce the use of metal Pd [13].

  Metal phosphides (MPs) are composed of zero-valent metal and phosphorus; metal-rich phosphides have similar characteristics to zero-valent metals [14]. We and others recently reported that Ni₂P, CoP, Cu₃P, etc. could act as excellent cocatalysts in a myriad of photocatalytic reactions for hydrogen evolution [15-17], reduction of nitroarenes [18] and CO₂ reduction [19]. In view of the fact that Pd₃P NPs have similar characteristics to Pd NPs, the following question naturally arises: Can Pd₃P NPs be used as cocatalysts for photo-driven C—C coupling reactions? Our present study shows that photocatalytic Suzuki coupling reactions are indeed highly efficient with Pd₃P as a cocatalyst, CdS as a photosensitizer, 4-iodotoluene and phenylboronic acid as the coupling partners and ethanol as an electron donor, in which a yield of up to 98% can be achieved under optimized conditions at room temperature. To the best of our knowledge, it is the first time that Pd₃P is used in catalyzing Suzuki coupling reactions. The high catalytic activity and excellent stability of this present system highlights its promising application in other cross-coupling reactions.

  Pd₃P was prepared by a one-step annealing reaction of Pd(Ac)₂ and NaH₂PO₂ with a mass ratio of 1:5 under Ar atmosphere. As revealed by X-ray diffraction (XRD) pattern in Fig. S2 (Supporting information), the sample synthesized at 400 ℃ has well-defined peaks of orthorhombic Pd₃P (JCPDS No. 089-3046) [20-22] without other impurities. Further, X-ray photoelectron spectroscopy (XPS) was performed to analyze the valence state and electronic properties of Pd₃P NPs. As shown in Fig. S3c (Supporting information), two discernible peaks of P 2p region located at 129.8 eV and 130.6 eV can be assigned to P 2p_3/2 and 2p_1/2 of Pd₃P, respectively. The peak at 134.6 eV can be attributed to P—O bond, which originates from surface oxidation of Pd₃P [23]. Compared with the P⁰ peaks at 130.1 and 131.4 eV [20-24], P element in Pd₃P owns a lower binding energy, indicating its slightly negative charge in Pd₃P NPs. In addition, the peaks of Pd 3d region located at 335.9 eV and 341.2 eV belong to Pd 3d_5/2 and Pd 3d_3/2, respectively. Compared with binding energies (335.2 eV and 340.5 eV) of metal Pd [12, 25], the higher binding energies of Pd species in Pd₃P NPs indicate that Pd element is in the form of slightly positive valence state. Finally, the effect of calcination temperature on the structure and composition of as-prepared samples was explored and corresponding XRD patterns of samples synthesized at 300 ℃, 400 ℃ and 550 ℃ are shown in Fig. S2. We failed to obtain pure Pd₃P above or below 400 ℃. For example, incomplete phosphidation exists in the sample synthesized at 300 ℃, in which two peaks at 39.4° and 45.8° ascribed to Pd NPs are observed [26]. However, PdO and other impurities such as PdP₂ and PdO are detected in the sample synthesized at 550 ℃.

  CdS was fabricated via a hydrothermal route according to literature methods. As shown in Fig. S3a (Supporting information), XRD pattern reveals that CdS is present in hexagonal phase (JCPDS No. 77-2306) [27]. Furthermore, hybrid Pd₃P/CdS was obtained by mechanical mixing of a certain amount of Pd₃P and CdS. In the XRD pattern of hybrid Pd₃P/CdS in Fig. S3a, the weak diffraction peaks of Pd₃P at 39.9°, 40.3° and 42.6° prove that Pd₃P NPs are successfully incorporated onto CdS. Finally, to clarify the optical property of CdS, Pd₃P and Pd₃P/CdS, ultraviolet-visible diffuse reflectance spectra were performed. As shown in Fig. S3b (Supporting information), Pd₃P/CdS shows an enhanced absorption than naked CdS in the visible region.

  The morphological information of CdS and hybrid Pd₃P/CdS was characterized by transmission electron microscopy (TEM). TEM image of Cds in Fig. 1a exhibits the flake-like shape. A high-resolution TEM (HRTEM) image (Fig. 1b) shows that CdS has the distinct lattice fringes, suggesting its excellent crystallinity. Obviously, as revealed by the TEM image of hybrid Pd₃P/CdS in Fig. 1c, ultrasmall Pd₃P NPs are uniformly dispersed on the surface of CdS nanosheets and the average size of Pd₃P NPs is about 5.2 ± 0.5 nm based on statistical analysis of sizes of 120 Pd₃P NPs. HRTEM image of hybrid Pd₃P/CdS is also shown in Fig. 1d. The high-resolved lattice fringes with distances of 0.316 nm and 0.356 nm can be well indexed to the (101) and (002) planes of CdS, respectively, and 0.226 nm corresponds to (112) plane of Pd₃P, which well correspond to XRD results. Meanwhile, the HRTEM result confirms the intimate contact of CdS nanosheets and Pd₃P NPs. To further prove the distribution of Pd₃P NPs on CdS nanosheet, the TEM-EDS elemental mapping images of Pd₃P/CdS are illustrated in Fig. 1f and indicate the existence and homogenous distribution of Cd, S, Pd and P elements in Pd₃P/CdS.

  Figure 1

  

  Figure 1.  (a) TEM image and (b) HRTEM image of CdS. (c) TEM image and (d) HRTEM image of Pd₃P/CdS (Inset (c): size distributions of Pd₃P NPs). (e, f) TEM image and corresponding TEM-EDS elemental mapping images of hybrid Pd₃P/CdS.

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  The performance of Pd₃P/CdS photocatalyst for photocatalytic Suzuki coupling reaction was assessed under the irradiation of white LED light (4 × 25 W) at room temperature. Typically, the system was constructed with 4-iodotoluene and phenylboronic acid as the coupling partners, K₂CO₃ as base, EtOH and H₂O with a volume ratio of 3:2 as solvent. As shown in Table 1, 4-iodotoluene can be converted into desirable 4-phenyltoluene with 98% yield (entry 1). Notably, the control experiments showed that the reaction could hardly occur when either components of Pd₃P, CdS, light or Pd₃P/CdS was absent (entries 2–5), which confirms that the Suzuki coupling reaction is photocatalyzed by hybrid Pd₃P/CdS under visible light. In the absence of K₂CO₃, no desirable product was detected (entry 6), because the Suzuki coupling reaction needs base to achieve deprotonation of aryl boric acid and by-product hydrogen halide [28]. Therefore, the addition of an appropriate amount of K₂CO₃ is necessary to drive this reaction. In addition, to eliminate the effect of photo-induced heat to Suzuki coupling reaction, the control experiment was carefully operated under heating condition without visible light irradiation (entry 7). Only trace desirable product was detected, which further suggests that the reaction is actually photo-driven. Moreover, the cycling experiment under the "standard condition" was performed carefully. Hybrid Pd₃P/CdS after 4 cycles can maintain the activity almost equivalent to the first cycle (Fig. S4 in Supporting information), indicating its excellent stability.

  Table 1

  

  Table 1.  Photocatalytic Suzuki coupling reaction over Pd₃P/CdS.^a

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  With the optimal conditions in hand, we sought to evaluate the generality of photocatalytic Suzuki coupling reaction by Pd₃P/CdS. Various aryl halides and boronic acids are cross-coupled under the standard condition (Tables 2 and 3). ¹H NMR spectroscopy was used for product detection. Overall, all aryl iodides and aryl bromides showed high yields (Table 2, entries 1–14). It can be found that the position of substituents on the benzene ring has a slight effect on the product (entries 1–4), in which the yield of para-substituted substrate is slightly higher than that of ortho- and meta-substrates (entries 2–4). Surprisingly, whether the substituent is electron-withdrawing or electron-donating, the reaction yields are all over 90% (entries 5–10). Aryl bromides are also evaluated under the same condition, but have moderate yield even with long reaction time (entries 11–14). Aryl chlorides result in very poor yields because the strong C-Cl bond is more difficult to break [12]. Additionally, three different aryl-boronic acid compounds and 4-iodotoluene can be successfully cross-coupled, and all of them have high yields (Table 3).

  Table 2

  

  Table 2.  Photocatalyzed Suzuki coupling reactions with various aryl halides by Pd₃P/CdS.^a

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  Table 3

  

  Table 3.  Photocatalyzed Suzuki coupling reactions with several aryl-boronicacids by Pd₃P/CdS.^a

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  To get an in-depth understanding of photocatalytic Suzuki coupling reaction mechanism over Pd₃P/CdS, reference experiments were performed with p-benzoquinone and diisopropylamine under standard condition, respectively [8, 12, 29]. The former is an electronic quenching agent, which competes with reactant for photoexcitation electrons from photosensitizers CdS, thus the photoexcitation electrons cannot successfully transfer to 4-iodotoluene to break C–I bonds. The latter is a good hole quenching agent, which can greatly quench the holes from photosensitizer CdS in the system and prevent the hole to transfer to phenylboronic acid. As shown in Table 4, the reaction with addition of p-benzoquinone can hardly proceed, whereas the yield of the corresponding product is still 71% upon addition of diisopropylamine. These results imply that photogenerated electrons play a crucial role in these photocatalyzed Suzuki coupling reactions. To further figure out the effect of solvents, the reactions in different solvents were carried out and the results were observed carefully. With ethanol and methanol as solvents, it has a high yield of more than 80%. However, there is almost no products when using DMSO and DMF, which are excellent solvents for the thermal-catalytic Suzuki coupling reaction (Table 4, entries 5 and 6) [30, 31]. The experimental results indicate that the alcohol solvents (methanol, ethanol, etc.) play a key role in photocatalytic Suzuki coupling reaction. We speculate that alcohols may act as sacrificial agents oxidized by photogenerated holes to promote photocatalytic reaction, but holes did not transfer to phenylboronic acid [32].

  Table 4

  

  Table 4.  Photocatalytic Suzuki coupling reaction over Pd₃P/CdS.^a

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  Based on the above experimental results, we propose a plausible mechanism of photocatalytic Suzuki cross-coupling reaction by Pd₃P/CdS (Fig. 2). Under visible light irradiation, photogenerated electron-hole pairs can be produced and separated within CdS. Then photogenerated electrons transfer to Pd₃P to break the C–X bonds, while photogenerated holes are consumed by alcohol solvent. At the same time, oxidation addition of aryhalide can occur on the surface of Pd₃P, and then arylboronic acid undergoes transmetalation. Finally, the corresponding products are obtained by reductive elimination of intermediate species [28, 33].

  Figure 2

  

  Figure 2.  Proposed mechanism responsible for photocatalytic Suzuki coupling reactions over Pd₃P/CdS.

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  In summary, a highly efficient catalytic system with hybrid Pd₃P/CdS as photocatalyst was established for Suzuki coupling reaction. The reaction product was obtained in a facile way. The introduction of Pd₃P cocatalyst displayed significantly enhanced activity with bare CdS. Moreover, compared with naked metal Pd, Pd₃P achieves a reduction in the usage of noble-metal Pd. Given the sustainability, high activity and economics, Pd₃P is expected to be extended to other similar C—C coupling reactions as a highly effective catalyst.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  We acknowledge the financial support from the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB17000000) and the National Natural Science Foundation of China (Nos. 21773275 and 21971250). Y. Chen acknowledges the financial support from K. C. Wong Education Foundation and the CAS-Croucher Funding Scheme for Joint Laboratories.

  Appendix A. Supplementary data

  Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.06.022.

  
  ^* Corresponding author at: Key Laboratory of Photochemical Conversion and Optoelecctronic Materials & CAS-HKU Joint Laboratory on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
  E-mail address: chenyong@mail.ipc.ac.cn (Y. Chen)
  
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• 

  Figure 1  (a) TEM image and (b) HRTEM image of CdS. (c) TEM image and (d) HRTEM image of Pd₃P/CdS (Inset (c): size distributions of Pd₃P NPs). (e, f) TEM image and corresponding TEM-EDS elemental mapping images of hybrid Pd₃P/CdS.

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  Figure 2  Proposed mechanism responsible for photocatalytic Suzuki coupling reactions over Pd₃P/CdS.

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  Table 1.  Photocatalytic Suzuki coupling reaction over Pd₃P/CdS.^a

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  Table 2.  Photocatalyzed Suzuki coupling reactions with various aryl halides by Pd₃P/CdS.^a

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  Table 3.  Photocatalyzed Suzuki coupling reactions with several aryl-boronicacids by Pd₃P/CdS.^a

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  Table 4.  Photocatalytic Suzuki coupling reaction over Pd₃P/CdS.^a

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