Homogeneous and heterogeneous catalysts of organopalladium functionalized-polyhedral oligomeric silsesquioxanes for Suzuki–Miyaura reaction

• The Pd(II) complex of mono-pyridine–triazole functionalized octahedral T8 cage (T8-Pd) was synthesized and studied by NMR spectroscopy and X-ray diffraction analysis.
• The decahedral T10 cage with fully substituted pyridine–triazole groups accommodates 4.6 Pd ions (T10-Pd), as evidenced by ICP-OES.
• Both T8-Pd and T10-Pd complexes act as effective homogeneous and heterogeneous catalysts, respectively, for Suzuki–Miyaura coupling.
• The heterogeneous T10-Pd catalyst exhibits high activities (with TON up to 25,000) and is reusable up to at least five reaction cycles.

The mononuclear T8-Pd and multinuclear T10-Pd catalysts (3 and 6) were prepared from a reaction between Pd(COD)Cl2 (COD = 1,5-cyclooctadiene) and the corresponding octameric T8 and decameric T10 silsesquioxane cages, functionalized with pyridine–triazole ligands. The T8-Pd complex 3 featuring one Pd(II) center was employed as a homogeneous catalyst for Suzuki–Miyaura cross coupling in a 1:1 EtOH:H2O solvent. On the other hand, the multinuclear T10-Pd catalyst 6 (ca. 4.6 Pd for each T10 cage) was obtained as amorphous insoluble materials with exceptionally high molecular Pd loading (1.61 mmol Pd g−1). Under the same catalytic conditions, the homogeneous catalysts 3 exhibited slightly higher activity than the heterogeneous catalyst 6 (initial TOFs = 870 and 690 h−1, respectively). Furthermore, for 6, 4-(MeO)C6H4Br and PhB(OH)2 substrates were catalyzed in the presence of a low catalyst loading of 3.6 × 10−3 mol% Pd under aerobic conditions to afford the coupled product in 91% yield (i.e., turnover number (TON) = 2.5 × 104). The silsesquioxane-supported Pd catalyst 6 was recovered by simple centrifugation and reused for at least five catalytic cycles without a loss in activities.

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