Ligand engineering of coinage metal nanoclusters: The synthesis and properties of calixarene stabilized nanoclusters

Recent years have seen the increasing interest in the studies of metal nanoclusters especially on their structure-property relationships. It has been accepted that the ligands are able to exert impacts on the stability, catalysis as well as the optical properties of nanoclusters. Ever since the rational synthesis of colloidal gold nanoparticles, phosphines and thiols have been the most widely studied ligands in coinage metal nanoclusters and nanoparticles. Only within 10 years, carbon-bonded ligands including alkynes and N-heterocyclic carbenes (NHCs) are emerging prevalent and being systematically investigated. Afterwards, the ligating atoms of the same group in the periodic table with carbon, sulfur and phosphine are explored to passivate the metal clusters. Meanwhile, incorporation of ligating units onto various skeletons has also become a practical approach to broaden the ligand library of metal clusters. The enriched ligands provide unprecedented opportunities in studying such materials from another angle that was previously neglected. Hence, studies specifically on the ligands are increasing and indeed are providing important insight into this area. On the other hand, the investigation of ligand effects and the expansion of ligand library are also broadening the properties of the ligated clusters, thus expanding their applications. This review will first summarize the recent advances in the ligand engineering of metal nanoclusters, mainly focusing on the ligands for manipulating the stability, catalysis and optical properties of corresponding nanoclusters. The first section describes the ligand effects and lays the basis for understanding the second half of this review which presents the detailed description of calixarene stabilized coinage metal nanoclusters. Incorporation of functional ligands onto metal nanoclusters represents an important approach to broaden the applications of such materials and is gaining increasing attention. Owing to the versatility of skeletons, easy chemical modification combined with many other unique properties including robust host-guest interactions, fast stimuli-induced responses, intrinsic chirality, optical properties among others, calixarenes has long been investigated as functional molecules. On the other hand, these molecules are often decorated to synthesize polynuclear metal complexes as multidentate ligands. These features enable calixarenes attractive in producing multifunctional metal clusters. The second section summarizes the synthesis, stability studies and the function of calixarene-metal clusters, providing perspectives on the future of multi-functional materials based on functional ligand stabilized metal clusters. From the pioneer work of calixarene stabilized metal clusters, it shows that metal clusters stabilized by these ligands are unique in terms of their synthesis and stability. However, their capability in functioning metal clusters have not been widely proved yet.