Carborane Meets Metal Nanocluster: New Opportunities in Nanomaterials

ConspectusMetal nanoclusters, distinguished by their atom-precise structures and quantum size effect, are regarded as a crucial bridge between organometallic complexes and plasmonic metal nanoparticles. These nanoclusters are primarily composed of a metallic core enveloped by protective ligands, wherein the ligands play a vital role in determining the nanoclusters' synthesis, structural integrity, and physicochemical properties. Considerable efforts in ligand engineering have concentrated on exploring novel coordinating functional groups to advance nanocluster research, particularly in the precise and controlled synthesis of superatomic nanoclusters, fine-tuning their intrinsic properties, and subsequent assembly and application. However, the backbone of these ligands seems equally important but attracts less attention. It is reasonable that if the utility of the two moieties (coordinating functional group and backbone) provokes a profound synergistic effect, their contributions to the structures and properties of the resultant metal nanoclusters are extremely inestimable. In this context, carborane, with its spherical shape and three-dimensional aromaticity (electronic effect), has emerged as a promising candidate for ligand backbone design. Over the past two decades, the incorporation of carborane moieties into ligands has enabled the construction of various metal nanoclusters exhibiting distinct architectures, enhanced stability, and unique reactivity. Therefore, it is important to present the current status and challenges associated with carboranyl ligand-protected metal nanoclusters to guide their future development. This Account provides a comprehensive summary of the recent advances in carboranyl ligand-stabilized metal nanoclusters, with a primary focus on the contributions from our laboratory. We begin by discussing the unique advantages of introducing carborane-based ligands in metal nanocluster preparation, with particular emphasis on their virtues for the synthesis of superatomic nanoclusters, heterometal-doped nanoclusters, and isostructural nanoclusters. Subsequently, we summarize the carborane-based ligand engineering strategies for precise modification and hierarchical assembly of metal nanoclusters, elucidating how the incorporation of carborane facilitates the modulation of specific properties and promotes supramolecular and covalent assembly. Furthermore, we discuss the cooperativity achieved by carboranyl ligands and the metal nanocluster framework to broaden the scope of applications for these nanoclusters in versatile fields, including hypergolic fuels, a previously unexplored area. Finally, we discuss the challenges facing future research on carboranyl ligand-protected metal nanoclusters, including the incorporation of nido-carborane or metallocarborane, a fundamental understanding of structure-property relationships, and potential applications such as boron neutron capture therapy and radionuclide extraction. This Account aims to stimulate interest in the unique attributes of carborane-based ligands and their corresponding metal nanoclusters among students and researchers across diverse disciplines, including chemistry, crystal engineering, and materials science.