Novel ordered β-ketoimine-palladium(ii) multilayers supported on a silicon wafer fabricated using layer-by-layer self-assembly for catalyzing Suzuki cross-coupling reactions

Abstract

Novel ordered β-ketoimine-palladium(II) multilayers supported on the surface of a silicon wafer (Si@[β-Ki-Pd][L₂-Pd]_n, n = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11) were fabricated using layer-by-layer (LbL) self-assembly and characterized using Raman spectroscopy (RS), ultraviolet-visible spectroscopy (UV-vis), X-ray diffraction (XRD), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Their catalytic properties were systematically investigated using the Suzuki–Miyaura cross-coupling reaction as a template. Among these catalytic multilayers, Si@[β-Ki-Pd][L₂-Pd]₁₁ exhibited high activity (TOF = 12 903.2 h⁻¹), which was ten times and sixty times higher than that of Si@[β-Ki-Pd] (TOF = 1171.7 h⁻¹) and Li₂PdCl₄ (TOF = 215.8 h⁻¹), respectively. It also showed good substrate adaptability and could be reused 13 times. Si@[β-Ki-Pd][L₂-Pd]₁₁ was a heterogeneous catalyst and catalysis occurred on the surface. Active sites formed in situ on the surface, including Pd(0) and Pd(0)/PdO, which exhibited a synergistic effect, such as electronegative Pd(0), were enhanced via the synergistic action between PdO and Pd(0). This synergistic effect made the oxidative addition of Pd reacting with aryl halide easy and boosted catalytic activity. Oxygen also played a crucial role in the formation of PdO, which not only exhibited important electron transferring synergy with Pd, but acted as a stabilizer for Pd(0). This could maintain the balance ratio of Pd/PdO to prevent Pd from aggregating. The above investigation is essential for the optimal design of highly active catalysts.