Syntheses,Structures And Properties Of Ligand-Stabilized Coin Metal Clusters

As a kind of sub-nanometer material,metal nanoclusters are the bridge between organic molecule and conventional nanoparticle.Therefore,controllable syntheses,structures and properties of metal clusters can provide guidance for development of traditional metal nanomaterial at molecular level.Nanoclusters are monodisperse and thus can crystallize long-range ordered single crystals.In recent years,with the rapid development of structural characterization techniques,such as single crystal diffraction and mass spectrometry,more and more crystal structures of metal nanoclusters have been successfully characterized and reported.In the thesis,we demonstrated the syntheses of different metal nanoclusters by using different organic ligands(eg.thiols and phosphines et al)and crystallographically characterized by single-crystal diffractometer.Then,the chiral origins,asymmetric synthesis as well as enantioseparation of chiral nanoclusters were systematically studied.Finally,we focused on the hierarchical assembly between different size clusters and nanocluster-based chemistry,including exploring the structural transformation between different clusters and catalytic behavior of clusters based on their surface reactivity,and the synopsis and main content of this thesis were as follows:Firstly,chiral origins of clusters and asymmetric synthesis and enantioseparation of chiral clusters.(1)Pure-thiolate-stabilized silver-based[MAg24(SR)18]2-(M = Pd,Pt)nanoclusters have a metal framework structure similar to that of wellknown Au25(SR)18 nanocluster.In both clusters,a M@Ag12 icosahedral core is stabilized by six distorted dimeric[Ag2(SR)3]staple motifs which generate a geometric chirality distinguished from Au25(SR)18.Due to the?…? stacking effect of the benzene ring between surface thiolated ligands,these staple units exhibit an asymmetric arrangement,making the whole structure chiral.For the first time,these two cluster structures demonstrated that thiol-stabilized Ag clusters can have staple coordination patterns similar to those of Au clusters.And the asymmetric arrangement of surface[Ag2(SR)3]staple patterns resulted in chirality of nanoclusters.(2)Thiolate-stabilized chiral silver-copper alloy cluster,(AgCu)40.This work presented a facile ion-pairing strategy for asymmetric synthesis and enantioseparation of optically active anionic(AgCu)40 nanoparticles using chiral ammonium cations.Mass spectrometric measurements suggested relatively strong ion-pairing interactions between the negatively charged nanocluster and ammonium cation.Encouraged by this observation,the as-prepared racemic mixture were partially separated into enantiomers by employing chiral ammonium salts as chiral selectors.Subsequently,direct enantiosynthesis of optically active enantiomers of(AgCu)40 by employing appropriate chiral ammonium cations(N-benzylcinchoninium or N-benzylcinchonidinium).This work proposed the chiral separation and asymmetric synthesis method based on chiral counterion and a general idea of enantioseparation mechanism.At the same time,the theoretical calculations show that each of the absorption peaks in the circular dichroism spectrum of the(AgCu)40 cluster solution is mainly derived from electronic structure of Ag4@Ag24 core,as well as contributions of surface metal-ligand charge transfer and other components.(3)Chiral silver clusters co-stabilized by multiphosphines and thioaltes.We try to employ other organic ligands into the thiol-stabilized noble metal nanocluster,and further promote the formation of chiral nanocluster through various coordination modes and anisotropic distribution of different ligands.In this part,we firstly demonstrated a general synthetic strategy,using mixed ligands to synthesize atomically precise,to prepare intrinsically chiral nanoparticles.Tridentated phosphine and thiolate are readily as stabilized ligands to give rise to chiral Ag23 and Ag21 nanoclusters.At the same time,a series of optically pure nanoclusters co-stabilized chiral diphosphine and thiolate were obtained by tinily altering the thiolated species.Based on fine ligand and crystallographic engineering,ae well as by means of single crystal X-ray diffraction,mass spectrometry,absorption spectroscopy and other characterizations,we can study the evolution of geometry and electronic structure of chiral nanoclusters as well as their chiral origins at the atomic scale.Secondly,structural transformation and surface reactivities of metal clusters.(1)We studied the thermal stability of chiral silver-copper alloy clusters,(AgCu)40.Interestingly,upon heating at 50?,this cluster undergoes a structural transformation from the fcc/hcp(AgCu)40(a 20-electron cluster)to an Ag-rich(AgCu)44(an 18-electron cluster)with a hollow icosahedral core which is similar to the known[Ag44(SR)30]4-cluster and presumably,more stable.This work provides direct evidence of structural transformation between metal nanoparticles of different structures under relatively mild conditions.(2)Thiol-halide-stabilized Ag206 catalyzed reaction of cycloisomerization of alkynyl amines.The Ag206 nanoparticles with atomic precision were successfully prepared by employing cyclohexylmercaptan.With a four-shell Ag7@Ag32@Ag77@Ag90 Ino-decahedral structure having a nearly perfect D5h symmetry,the metal core of the nanoparticle is co-stabilized by 68 thiolate and 4 halide ligands.Both electrochemistry and plasmonic absorption reveal the metallic nature of the nanoparticles,which is explained by DFT calculations.Electronically,the nanoparticle can be considered as a superatom,just short of a major electron shell closing of 138 electrons(q=-4).More importantly,many of ligands capping on the nanoparticle are labile due to their low-coordination modes,leading to high surface reactivity for catalyzing the synthesis of indoles from 2-ethynylaniline derivatives.The results exemplify the power of the atomic-precision nanocluster approach to catalysis in probing reaction mechanisms and in revealing the interplay of heterogeneous reactivities,electronic and surface structural dynamics,thereby providing ways for optimization.Thirdly,self-assembly behavior of coin metal clusters.By using monophosphine and thiol ligands,we obtained two gold-silver alloy particles with well-defined structure in one pot.An unusual cocrystalization of two well-defined ligand-stabilized nanoclusters giving rise to a periodic superstructure and expounding a novel bimodal particle nucleation and growth process in solution,which is an ensemble of a conventionally multilayer icosahedral structure with fcc closest packing in spatial and an electronic structure satisfied an 18-electron superatom shell closure coexisting in synthesis of nanoparticles.This hierarchical-assembly superstructure consists of spherical(AuAg)267 nanoparticles and a trigonal-prismatic(AuAg)45 nanoclusters in 1:1 ratio.The(AuAg)267 nanoparticle exhibits aesthetic arrangement of a four concentric core-shell structure of Ag@M12@M42@M92@Agi20(SR)80(M=Au or Ag).The 147-metal-atom icosahedral Mackay core,packed in face-centered cubic(fcc)pattern,is encaged in an anti-Mackay-like 120-metal-atom surface shell which is further capped by 80 fullerene-like thiolate ligands,while the M92-Ag120 interface is in hexagonal close-packing(hcp).The(AuAg)267 nanoparticle thus represents a perfect growth model for icosahedral twinning particle.The partially exposed M atoms of the vertex of the 147-atom icosahedron raise the prospects of the utility of the(AuAg)267 nanoparticle in catalysis.The relative Au:Ag occupancies in the metal frameworks of(AuAg)267 revealed some regularites via crystallographic AuxAg1-x occupancies,that is,exhibit alternating Ag-and Au-rich shells(average occupancies in parentheses):?0(100%Ag)@t?1(ca.60%Au)@?2(ca.60%Ag)@?3(ca.80%Au;ca.90%Ag on C3 axes)@?4(100%Ag).45 metal atoms of(AuAg)45 cluster are arranged into a trigonal prism,whose faces and edges are capped by thiolates and the vertices terminated by phosphines.The packing constraints of(AuAg)267·(AuAg)45 co-crystal induced by the different ligand distributions and their symmetry-matching dispositions during nanoparticle self-organizaton and cocrystaliztion resulting in the intriguing unconventional anti-NiAs-type structure.The facile synthesis and unique optical and electrochemical properties make the(AuAg)267-(AuAg)45 co-crystal another promising candidate for future applications.