Minimalization of Metallic Pd Formation in Suzuki Reaction with a Solid-State Organometallic Catalyst

Abstract

Suzuki reaction usually uses palladium (Pd) complexes to accommodate a wide range of substrates. In pursuing greener synthesis, immobilization of Pd complexes with various support materials has shown promising potential. Although this approach can give stable conversion, initially immobilized Pd ions are largely reduced to Pd 0 aggregates and turned essentially into supported nanoparticles after use, which departs from its original intention of complex immobilization and thus hampers its activity. Herein, we immobilize noble metal ions into a spherical thiolated organosilica. This new type of catalysts can catalyze Suzuki reaction homogeneously via leaching out Pd ions and shuttling them back after the reaction. The excellent reusability attained can be attributed to minimalization of forming metallic palladium. Thus, the developed catalysts can be viewed as a two-way device to release and to restore metal ions for homogeneous catalysis.

An Imidazole‐Rich Pd(II)‐Polymer Pre‐catalyst for the Suzuki‐Miyaura Coupling: Stability Influenced by Dissolved Oxygen and Reactants Concentration

“…The pristine pre-catalyst Pd/PECIm displays two signals centered at 337.9 eV and 343.3 eV (Figure 3A), which agree with those observed for Pd(II). [27,50,55] This result confirms that, as expected, no reduction takes place during the binding of the Pd ions to the polymer support. After the SM reaction, new signals appear at 335.2 eV and 340.4 eV (Figure 3B) which can be attributed to formation of Pd(0).…”

“…Our results are in agreement with the current view that chemical reduction of Pd(II) into Pd(0) on the surface of the precatalyst diminishes its performance. [27] In fact, when fresh Pd/ PECIm was reduced with NaBH 4 it becomes inactive (results not shown), indicating that keeping a large Pd(II)/Pd(0) ratio is of paramount importance for catalyst performance. In line with our findings, Li and Zeng [27] reported that minimization of metallic palladium formation and aggregation is important to maintain catalyst activity in the Suzuky-Miyaura cross-coupling.…”

“…In addition, Pd nanoparticles are commonly formed in these reactions, [21][22][23] and the insoluble support can help hinder the formation of larger, less reactive, aggregates, [24][25][26] enhancing catalyst longevity. [25,27] Binding Pd species on polymers is an interesting strategy to generate recoverable catalysts. [27] Compared to traditional supports, such as metal oxides or carbon, the support can be more easily tuned by inserting coordinating groups using simple chemical steps.…”

“…[25,27] Binding Pd species on polymers is an interesting strategy to generate recoverable catalysts. [27] Compared to traditional supports, such as metal oxides or carbon, the support can be more easily tuned by inserting coordinating groups using simple chemical steps. [28][29][30][31][32][33][34] Among the myriad of heterogeneous supports for Pd immobilization, many contain the imidazole group as a key component.…”

Nanowire Networks of Metal–Organosilicates as Reversible Pd(II) Reservoirs for Suzuki Coupling Reactions

“…Along a similar line, in a very recent work by our group, spherical organosilica was prepared solely from 3-mercaptopropyl trimethoxysilane (MPTMS) and demonstrated to be an effective Pd(II) reservoir for Suzuki coupling reactions with decent recyclability and minimum metallic Pd formation. 18 However, presumably due to the strong binding between thiolate ligands and Pd(II) species, only a thickness of 40 nm from the surface could be effectively utilized for immobilization of Pd ions, while Au ions could be more evenly distributed across the entire sphere. 18 This largely limits the utilization of the support material, yet still inhibiting the Pd-ion shuttling and release, as Pd ions have to travel across a distance of 40 nm inside the organosilica matrix before reaching the reacting liquid phase.…”