Suzuki Coupling Reaction Catalyzed By Polyoxometalate/Organic Complex Hybrids

Recent years, more and more attentions have been paid on design, synthesis and applicationof organic/inorganic hybrids. This kinds of hybrids own the advantages of both organic andinorganic moieties and the consequently synergistic effects. Especially, hybrids ofpolyoxometalate/organic complex are an unique composite materials combining rigidness, sizestability of inorganic components and flexility, modifiability of organic ones, which can beendowed with higher added values than each of the components. The hybrids have been widelyused in various fields, such as optical, magnetic, electronic and catalytic areas. In respect ofcatalysis, the study on polyoxometalate/organic complex hybrids can obtain much moreinformation about interactions between organic molecules and the surface of oxides. This ishelpful for fabrication of more efficient and multifunctional catalysts with good reusability.Cross-coupling of aryl halides with arylboronic acids, known as Suzuki–Miyaura coupling,has contributed greatly to the straightforward and facile construction of carbon–carbon bonds.This reaction is generally considered to be a homogeneous reaction most commonly catalyzed bysoluble Pd(II) species with a variety of ligands. However, separation of the expensive catalystfrom the product for reuse is often problematic in these homogeneous systems. Moreover,aggregation and precipitation of palladium metal in the homogeneous systems always leads tolost activity of the catalysts. Thus heterogeneous catalysts with high efficiencies, activities andgood reusabilities are highly desirable. This thesis aims at developing more efficient andrecyclable polyoxometalate/organic complex hybrid catalysts for Suzuki–Miyaura couplingreaction and following studies have been carried out.1. A hybrid compound Pd(salen)–POM was prepared by covalently grafting palladium(salen) complex to a lacunary Keggin-type polyoxometalate, K8[SiW11O39](POM). The hybridwas characterized by FT-IR, elemental analysis, inductively coupled plasma and diffusereflectance UV–Vis spectroscopy methods, and its catalytic performance on Suzuki couplingreaction was investigated in EtOH/H2O under mild reaction conditions. The results have shownthat the hybrid has greatly improved catalytic activity compared to the corresponding organicand inorganic moieties, and much higher yields for coupling products were obtained for variousaryl halides including unreactive and sterically hindered ones. The catalyst also exhibitedprominent recyclable performance and no obvious loss of activity was observed after sixconsecutive runs. 2. A series of N-heterocyclic carbenes/polyoxometalates hybrid catalysts were synthesizedand characterized by UV-vis, FT-IR and ICP, and their catalytic performances on Suzukicoupling reaction were investigated. The results have shown that the hybrids have greatlyimproved catalytic activities compared to the corresponding organic and inorganic moieties,especially the hybrid of (iPr HCl)6SiW11PdO39formed by cation exchange of iPr HCl withK6[SiW11PdO39]. This hybrid catalyst also has good catalytic activity on homocoupling reactionof arylboronic acids. The results also shown that the catalyst has a good recyclable performanceand only slight loss of activity was observed after five consecutive runs.3. Palladium doped polyoxometalates were supported on ionic liquid-modified SBA-15.The formed hybrids were characterized by FT-IR, ICP, UV–Vis, BET and TEM, and theircatalytic performances on Suzuki coupling reaction were investigated. The results have shownthat the hybrids have greatly improved catalytic activities compared to the correspondingunsupported ones, especially SBA-15-Im-SiW11Pd. This catalyst also showed good recyclableapplicability during five consecutive runs.