Controlled Encapsulation And Catalytic Performance Of Gold/Palladium Nanoparticles Based On Polycarbene Cage

In the past decade,N-Heterocyclic Carbenes?NHCs?,a new type of ligands,have shown great potential in the field of modification of metal nanoparticles due to their unique structures and electronic properties.According to previous reports,using N-heterocyclic carbene ligands to protect metal nanwoparticles can greatly improve the stability of nanoparticles through M-C_NHC bond,but there are some problems with uncontrollable size and poor solubility.At present,three-dimensional discrete organic cages have demonstrated particular advantages in the controllable preparation of metal nanoparticles and the improvement of the solubility of metal nanoparticles,but there are some defects in the stability of metal nanoparticles.Based on the above background,three flexible kinds of three-dimensional polyimidazolium cages?PICs:PIC 1-3?with N-heterocyclic carbene structure units and different cavity sizes are designed in this paper,which combine the advantages of N-heterocyclic carbene ligands and three-dimensional discrete organic cages in the preparation of metal nanoparticles.For the first time,the polyimidazolium cages were used as templates to realize the controllable encapsulation of metal nanoparticles?Au,Pd?,the mechanism of PICs stabilized metal nanoparticles was studied.In addition,the stability and catalytic performance of metal nanoparticles protected by polycarbene cages?PCCs?were investigated.This paper mainly includes the following aspects:1.Three kinds of N-heterocyclic carbene ligands containing cinnamate group were synthesized,and three different trinuclear silver carbene framework compounds were obtained.The intramolecular[2+2]cycloaddition reaction took place under the UV irradiation of?=365 nm,thereby forming a closed cyclobutane links nitrogen heterocyclic carbene framework,and removing the metal ions to obtain three-dimensional hexakisimidazolium cages with different cavity sizes.2.PIC-1 was first used as a template for the controllable encapsulation of gold nanoparticles.It is confirmed that there are multiple Au-C_NHC bonds between the surface of gold nanoparticles and PIC-1 by various methods.The formation of a core-shell Au@PCC-1,in which gold nanoparticles was wrapped by polycarbene cage?PCC-1?.The stability of Au@PCC-1 was monitored by UV-visible absorption and X-ray photoelectron spectroscopy,and the results show that Au@PCC-1 has ultra-high thermal and chemical stability.In the same time,it is found that Au@PCC-1 has excellent catalytic activity in the reduction of aromatic nitro compounds,especially in the reduction of 4-nitroaniline,and it only takes 1 min to achieve 100%conversion of 4-nitroaniline,and the catalytic rate constant is as high as 3.12 min^-1.Besides,it can also play a catalytic role in the degradation of organic dyes.3.PIC-1 continued to be applied as a template for the controllable encapsulation of palladium nanoparticles.It is proved that palladium nanoparticles are stabilized in the cavity of PCC-1 by Pd-C_NHC bonds to obtain a core-shell Pd@PCC-1 compound.Testing results illustrate that Pd@PCC-1 has superior chemical stability.Moreover,it is concluded that Pd@PCC-1 not only has excellent catalytic effect in the reduction of aromatic nitro compounds,but also shows unique advantages in Suzuki coupling reaction,which can achieve the target product with high yield under mild reaction conditions,and also has special catalytic activity in the reaction of 2-bromo-1,3,5-triisopropylbenzene and phenylboronic acid.4.PIC-2/PIC-3 was used as the template to encapsulate gold nanoparticles for the first time,the purpose of this part is to discuss the role of the cavities of PICs on the size-controlled of gold nanoparticles.The comparative experiments demonstrate that specific cavity of PICs is a necessary condition for the size regulation of gold nanoparticles.The testing results explain that the Au-C_NHC bonds are formed between the surface of gold nanoparticles and PIC-2/PIC-3,and then a core-shell shaped Au@PCC-2/Au@PCC-3compound is obtained.Through a series of tests on the stability of Au@PCC-2/Au@PCC-3,and it is concluded that both of them have extremely high thermal and chemical stability.In addition,Au@PCC-2/Au@PCC-3 has good catalytic activity for the reduction of aromatic nitro compounds.