Synthesis,Structures And Properties Of Calixarene/Alkynyl-Protected Coinage Metal Nanoclusters

Ligand-stabilized coinage metal nanoclusters have attracted more and more attention due to their applications in many fields such as optics,catalysis,biology,and sensing.Nanoclusters possess unique geometric and electronic structures.They are expected to show novel catalytic properties essentially different from those of the corresponding nanoparticles.Compared to large-sized nanoparticles,absolutely monodisperse and atomically precise nanoclusters are ideal models for studying structure-properties correlations.However,obstacles such as low yield,poor stability,and low catalytic activity restrict the development of nanoclusters.Aiming at solving above problems,in this thesis,the synthesis,separation,structure,characterization,stability and catalytic properties of ligand-protected coinage metal nanoclusters were studied.The research contents are as follows:(1)The calixarene ligand as the protective group has been used in the synthesis of a silver nanocluster[Ag35(C?CBut)16(H2BTCA)2(BTCA)](SbF6)3,which is the first calixarene-protected silver nanoclusters.The transformation process between Ag4@H2BTCA motif and Ags@BTCA motif in the cluster was observed.Upon addition or removal of one silver atom,the Ag35 cluster can be transformed to Ag36 or Ag34 species.Subsequently,the structure of Ag34 was experimentally determined by modifying the alkynyl ligand and the calixarene ligand,and the Ag37 cluster was also obtained.Theoretical studies have found that the low-energy absorption bands around 500 nm in Ag35,Ag36 and Ag37 originate from primarily ligand-to-metal electronic transitions.For Ag34,the absorption originates from primarily metal-to-metal transitions.(2)In order to improve the stability of the clusters,a rigid calixarene ligand(4-tert-Butylthiacalix[4]arene)was used to syntliesize nanoclusters.However,the stability of Ag35 was not good as expected.Therefore,two novel clusters were obtained through synthetic design,l.e.,Ag34-tfa and AuAg33-tfa.It was found that the destruction of the coordination open sites may significantly enhance the stability of metal nanoclusters.In addition,nanoclusters such as Ag37-OOCPh,Ag37-NO3,Ag35-Diclofenac,Ag21-OOPPh2,AuAg20-OOPPh2 were obtained by introducing oxyacid ligands.Silver nanocluster dimers Ag68-1,4-BDC and Ag68-1,3,5-BTC were obtained for the first time by the introduction of polycarboxylic acids.(3)Column chromatography was applied to improve the yields of alkynyl-protected coinage metal nanoclusters.Three species were isolated by this method,and two species were determined by single crystal X-ray diffraction,that is,Au36Ag36(C=CPh)32Cl6 and[Au24Ag20(C=CPh)24].The low yield of single species is mainly caused by three factors,1)competition of multiple dominant species,2)difficulty in crystallization,and 3)production of insoluble species.(4)A number of all-alkynyl-protected gold nanoclusters were obtained by three methods(direct reduction of alkyne gold precursor,one-pot method,solid-state route).We not only obtained Aux(C?CR)y as the counterparts of Aun(SR)m,also received Aux(C?CR)y nanoclusters whose metal kernel and metal-to-ligand ratio have not been observed in Aun(SR)m series.It was found that the alkynyl and thiolate have commonalities and differences in terms of serving as the surface ligand.Alkynyl ligands can adopt either ?-or ?-coordination modes,while the thiolate ligands only adopt ?-coordination.It is the first time to discover isomerism in alkynyl-protected gold nanoclusters,and the difference in the coordination of the surface ligands is responsible for the isomerism of two Au23 clusters.They exhibit different optical properties from each other,and the less stable Au23-2 cluster can be spontaneously transformed to the more stable Au23-1 under ambient conditions.(5)The effects of different gold/silver ratio in clusters on the catalytic reaction were studied.It was found that the silver-rich cluster catalyst was more effective for the addition of terminal alkynes and aldehydes,and gold-rich cluster catalyst showed higher activity in benzyl alcohol oxidation reaction.