Synthesis And Luminescent Properties Of Gold Nanoclusters/Complexes Constructed By N-Heterocyclic Carbene Ligands

N-Heterocyclic Carbenes(NHCs)are a group of electrically neutral heterocyclic compounds with at least one carbene carbon and at least one nitrogen atom in the ring structure.They have excellent properties such as strong electron donor ability and easy structure modification,which are of great significance in many chemical processes.Free NHCs can be coordinated with most of the metals in the periodic table to form NHCs metal complexes,which have been widely used in the fields of catalysis,materials science and biomedicine,etc.Among them,the most convincing research achievement is the successful development of the second generation of Grubbs catalyst for catalyzing olefin double decomposition reaction.The main researcher Robert H.Grubbs won the Nobel Prize in Chemistry in 2005 for this reason.Monetary metal nanoclusters,usually composed of the metal core and protective ligands,are a new type of atomic-accurate nanomaterials,providing an ideal model for studying atomic quantum effects and structure-activity relationships.At present,mercaptan,organic phosphine,alkynyl derivatives,carboxylic acids and pyridine containing molecules are commonly used to prepare monetary metal nanoclusters,which are used in catalysis,sensing,medicine,data storage and optoelectronics.However,due to the poor stability of clusters,unclear structure-activity relationship and uncontrollable preparation process,there is still a long way to go for the rational design and functionalization of atomic-accurate monetary metal nanoclusters constructed with traditional ligands,such as mercaptan,organic phosphine,alkynyl derivatives,carboxylic acids and pyridine containing molecules.Compared with the large number of monetary metal clusters constructed by traditional ligands with rich properties,the number and properties of N-Heterocyclic Carbene-protected monetary metal clusters are rare and have not received valuable and meaningful achievements for a long time.This is mainly due to the strong activity of free carbenes,most of them tend to form simple stable linear or quadrilateral metal carbene complexes before forming clusters with multiple metal atoms/complexes,thus losing the opportunity window for further reaction to form clusters.Until 2019,Cathleen M.Crudden’s group adopted a ligand-exchange strategy,replacing triphenylphosphine ligands on the surface of gold cluster precursor with NHCs,and thus reported the first case of a superatomic monetary metal nanocluster[Au11(PPh3)7(NHCi-Pr)Cl2]+constructed by NHCs with larger than three nuclei.Subsequently,they prepared[Au13(NHCBn)9Cl3]2+by direct reduction method.Therefore,the superatomic monetary metal nanoclusters constructed by NHCs ligands are attracting more and more attention from cluster scientists.In this paper,we prepared a series of monodentate and bidentate metal carbene precursors and a series of N-Heterocyclic Carbene ligands constructed metal clusters by reduction method.The effects of different N-substituents on cluster structure and luminescence properties were systematically studied.The main contents are as follows:1.Firstly,we synthesized a series of bidentate gold carbene precursors with different substituents to explore their influence on the preparation of gold clusters.The conclusions are as follows:(1)When the flexible alkyl chain(propyl or butyl)is the bridging group,and the substituted group on nitrogen(benzyl)provides the appropriate steric hindrance,the formation of the associated gold clusters can be facilitated.Density Functional theory(DFT)analysis shows that the closer the Au-Au bond length of[BisNHC-Au2]2+intermediate is to 2.6-3.0 ?,the more favorable it is to the formation of gold clusters.If the substituent on nitrogen is methyl group,no cluster is formed.When the substituents on nitrogen are menaphthyl group,related gold clusters can be formed,but the crystallinity is poor.(2)When the rigid benzene ring is the bridging group,Au11 and Au13 clusters can be adjusted by modifying the length of the alkyl chain on the nitrogen atom.N-substituents with iso-pentyl,propyl and butyl groups gave clusters of Au11,while N-substituents with methyl and ethyl groups gave clusters of Au13.Among them,Au11 cluster with formula[Au11(NHCi-Pent)5]3+is the first reported atomically accurate homoleptic carbene-stabilized gold cluster.Rare Au…H-C interactions exist in[Au13(NHCMe)5Br2]3+ clusters.Experimental and theoretical studies show that the driving force of cluster core transformation can be attributed to the steric hindrance effect of ligand.Au11 nanoclusters are non-luminescent,while Au13 nanoclusters are luminescent,and larger substituents are more conducive to higher quantum yields due to structural rigidity and interactions between ligands.Overall,our study provides a new strategy to adjust the cores of bidentate carbene protected clusters according to substituent effects,thus providing a useful exploration for the development of novel cluster functional materials.2.We have successfully synthesized a pair of atomically accurate N-Heterocyclic Carbenes-protected chiral gold clusters(R/S)-[Au13(NHCPi)8Br4]+with eight electrons configuration(1S21P6)by screening the N-substituents on the chiral gold carbene precursors.The clusters showed good chirality in solution and has remarkable circular dichroic signal.The crystal exhibited circularly polarized luminescence properties.This is the first reported gold cluster with circularly polarized luminescent properties constructed by NHCs ligands,which provides useful guidelines for the development of novel chiral cluster materials.3.By preparing a series of carbene-gold-carbazole complexes,we systematically studied the effect of substituents on the luminescence properties of the complexes,and pointed out that the change of molecular configuration and stacking mode caused by substituents is the main reason affecting the luminescence behavior,thus providing a beneficial supplement for the research in the field of carbin-metal-carbazole.