Novel Pd-BTP/SiO₂-and Pd-Graphene As Effective Heterogeneous Catalysts For Suzuki&Heck Reactions

Suzuki reaction, the biaryl formation of aryl halides with arylboronic acids and Heck reaction, the cross coupling of aryl halides and olefins are two of the most valuable synthetic protocols in organic chemistry where those are extensively used for synthesis of polymers, agrochemicals, and pharmaceutical intermediates. These reactions are being dominantly catalyzed by homogeneous palladium catalysts due to its excellent reactivity and selectivity over its substitutes for instance Ni, Cu and Fe. Although homogeneous palladium is superior, it consists of significant drawbacks in purification of the final product, recycle of the catalyst and rapid deactivation through aggregation into palladium (0) nano particles. Even though, palladium is not properly proven for its biological effects, strict government protocols on medicinal products especially parenteral Pharmaceuticals and the cost factor request researchers to gravely focus on these drawbacks of the catalyst. Therefore as one of the promising alternatives, heterogeneous palladium catalysis are being investigated in many approaches. The immobilization of palladium (Ⅱ) on supporting surfaces is such a promising strategy to avoid these problems; many researchers are focusing on wider range of prospects of this phenomenon; and therefore deserve much attention. In order to contribute this research movement with a novel approach, we synthesized two catalytic compounds by immobilizing palladium (Ⅱ) on solid supports, modified silica gel with a ligand and also on reduced graphene oxide (Graphene).The first one was to develop an efficient and reusable immobilized palladium catalyst with a ligand immobilized on silica gel that has the ability to combine tightly with palladium (Ⅱ). We used a molecule,2,6-bis(5,6-dimethyl-1,2,4-triazine-3-yl) pyridine (BTP) which is belongs to bis-triazinyl-pyridine group; a bidentate ligand which is widely used in chromatographic extraction of metal ions from high level liquid waste (HLW) in nuclear spent fuel reprocessing. It was unexpectedly found to have the highest decomposition constant with palladium (Ⅱ) than with any other metal ions. It is also demonstrated to have ability to tightly absorb into silica-based supporting compounds by simple physical adsorption interactions. To the best of our knowledge no efforts have been taken to evaluate the possibility to synthesis palladium catalysts. Therefore, we for the first time prepared a novel catalyst as palladium (Ⅱ)-BTP complex on silica gel.The second one was to develop an efficient and reusable immobilized palladium (Ⅱ) on graphene oxide which was intended to utilize the large surface area per unit mass of it. Graphene is considered a miracle material by the most of the researchers in the world. It is receiving huge attention and being evaluating in every dimensions. Our perspective was to utilize the unique properties of graphene to prepare heterogeneous catalyst by immobilizing and trapping palladium (Ⅱ) among the reactive groups on the surface of graphene oxide.Preparation and characterization of both catalysts were conducted in the facilities of our institute. The BTP molecule was synthesized according to the published procedures with few modifications. The Pd-BTP/SiO2catalyst was then prepared by the conventional incipient wetness impregnation method in which a proper amount of PdCl2and BTP were dissolved in dichloride methane and impregnated with silica gel. Graphene oxide was synthesized via modified Hummer’s method starting from fine graphite oxide powered and palladium (Ⅱ) was impregnated onto the surface area of graphene oxide via tedious preparation process. Both catalysts, Pd-BTP/SiO2and Pd-Graphene were subjected to characterization by XPS, FT-IR, XRD, TEM, TGA-DG/DTG and BET methods.The catalytic activity of the newly synthesized Pd-Graphene catalyst was evaluated by catalyzing four Suzuki reactions between several aryl halides and phenylboronic acid while conducting parallel reactions with equivalent amount of homogenous PdCl2as the control for each reaction. The Pd-Graphene showed good performance in Suzuki reaction and was successfully reused by simple filtration for5consecutive times without losing any reactivity. And meantime, the catalytic activity of the newly synthesized Pd-BTP/SiO2catalyst was evaluated by catalyzing four Heck reactions with various aryl halides with acrylic acid and styrene as substrates and four Suzuki reactions with various aryl halides and phenylboronic acid. Homogenous PdCl2with the same mole amount of Pd (Ⅱ) was used as control for each reaction. The conversion and yield of each reaction was quantified by HPLC and results showed the novel Pd-BTP/SiO2has slightly higher reactivity than the control homogenous PdCl2:for each examined Heck reaction, similar conversions were achieved which the selectivity catalyzed by Pd-BTP/SiO2was higher than those catalyzed by the control by2-10percent. More excitedly, the Pd-BTP-SiO2catalyst was successfully reused for5consecutive times without losing any reactivity by simple filtration.