Regionally Selective Hydrosilylation Of Alkynes Catalyzed By Single-atom Site Gold Supported On Carbon Nitride Nanosheets

Organosilicon compounds are a kind of important compounds,which play an irreplaceable role in the fields of rubber,resin and materials,etc.Silicohydride reaction is one of the important ways to produce organosilicon compounds in industry.At present,Speier catalyst or Karstedt catalyst is widely used in the industrial synthesis of organosilicon compounds.However,the loss of precious metals,residues in products and the high price of precious metals restrict the development of organosilicon industry.Therefore,whether in basic scientific research or application research,it is a key research direction to reduce the amount of precious metal and recycle performance of catalyst while maintaining excellent performance.In this paper,graphitic-phase carbon nitride(g-C3N4)nanosheets as the support,chlorauric acid as the active metal precursor,through impregnation method to prepare a single atom site Au catalyst(Au1/g-C3N4),where the amount of Au is 0.3 wt%.During the preparation of Au1/g-C3N4,a series of g-C3N4-supported Au nanoparticles were prepared as contrast catalysts.These catalysts were characterized by spherical aberration correction electron microscopy.X-ray photoelectron spectroscopy(XPS)and X-ray absorption fine structure spectroscopy(XAFS),and their morphology,metal species distribution and charge characteristics were studied.Au1/g-C3N4 showed a single atom distribution under HAADF-STEM,and Au nanoparticles were visible under high resolution electron microscopy.XPS characterization shows that the oxidation state of Au1/g-C3N4 is+1.Through XAFS,we further verified the oxidation state of Au,and clarified the structural characteristics of Au coordination with three nitrogen atoms in Au1/g-C3N4.In this paper,the catalytic performance of Au1/g-C3N4 and contrast catalysts in hydrosilylation of alkynes was further studied.Taking phenylacetylene as the reaction substrate,the optimum reaction conditions were determined by temperature and solvent screening with dioxane as the solvent,70℃ reaction temperature.In the study of the performance of single atom site Au catalyst and Au nanocatalysts,it was found that the single atom site Au catalyst has more stable activity and better selectivity for β-(E)alkenyl silicon compound.The cyclic reactivity of the catalyst was studied and it was found that the catalyst could be recycled at least five times,and no obvious degradation of the catalytic activity was found.Au1/g-C3N4 was used to catalyze internal alkynes,aromatic alkynes,aliphatic alkynes and heteratomic cycloalkynes to explore the adaptability of substrates.Alkynes with different substituents were able to transform corresponding β-(E)alkenyl silicon compounds with high activity and selectivity when Au1/g-C3N4 was used as catalysts.In terms of mechanism,through a series of control experiments,we speculated that Si-H bond was oxidized and added on AuⅠ to form silicon-based AuⅢ-H intermediate,and then coordinated with phenylacetylene.Silicon-based fund reaction was carried out through the transition state to obtain intermediate products,and β-(E)configuration products and singleatomic site Au were formed after reductive elimination.If hydrogen metallization occurs,the intermediate is reduced to β-isomer products.The unique coordination environment creates stable+1 valence gold,which makes Au catalyst at single atom site show advantages in activity,selectivity and stability in hydrosilylation reaction.