Synthesis and characterization of atomically precise nanoclusters with large footprint area and multipodal surface ligands

Abstract

Constructing rigid covalent linkages between atomically precise nanoclusters would enable the unique catalytic and optical properties of individual nanoclusters—as well as emergent behavior from well-ordered nanocluster ensembles—to be realized in porous frameworks. However, an extended framework of atomically precise noble metal clusters has not been demonstrated because nanoclusters are commonly protected by many equivalent, disordered ligands, which do not provide well-defined points of extension. To address these issues, we sought linking chemistries which were orthogonal to that of the cluster-protecting functional groups such that framework assembly could occur in a separate step from cluster synthesis. To reduce the number of ligands required to protect a single cluster, we targeted multidentate ligands that occupied more of the cluster surface area than monodentate ligands, ultimately focusing on di- and triphosphine ligands. The ligands feature an orthogonal tail site that could be modified on the clusters pre- or post-synthesis to link clusters together and form extended frameworks. In this dissertation, I discuss four approaches toward the synthesis and characterization of atomically precise nanoclusters with multipodal surface ligands. Chapter one provides an introduction to the synthesis of crystalline frameworks from atomically precise nanoclusters, including a brief review of porous materials, nanocrystals, and examples of assembled nanocrystals. In chapter two, I discuss our initial attempts to synthesize a large atomically precise Au92 nanocluster to perform an indirect ligand exchange on the nanocluster’s surface using large footprint area metallophthalocyanine ligands. This is followed by a discussion of attempts to synthesize nanocrystals with unfunctionalized and functionalized metallophthalocyanines to perform a direct synthesis of nanocrystals protected by the metallophthalocyanine ligand. In chapter three, I discuss syntheses of a thiolate-protected Au25 nanocluster and the phosphine-protected Au11 nanocluster as precursors for ligand exchange experiments. The descriptions of the syntheses and their characterization are followed up with a description of our rational behind and experimental attempts in replacing the starting monodentate thiolate and phosphine ligands with tridentate trithiolate and tris-N-heterocyclic carbene tripods. In chapter four, I discuss the direct synthesis of a new atomically precise gold nanocluster that was synthesized using a tripodal triphos ligand, demonstrating that ligand sterics can lead to unorthodox cluster geometry and surface ligand motifs. Experiments to determine if diphosphine ligands could bind to the nanocluster’s pendant surface motifs are described. Modifications of the ligand made it possible to sterically encumber the orthogonal head group site while retaining the tripodal phosphine moiety and attempts to synthesize an atomically precise nanocluster are discussed. Further attempted transformations, including the synthesis of a triphos ligand with different transition metals and anions are also discussed. An attempt to form a mixed ligand nanocluster through the reaction of the triphos-protected cluster with a different atomically precise nanocluster is described. In chapter five, the synthesis of gold sulfide nanocluster complexes protected by PNP-pincer type ligands with various linkable functional groups are detailed. Terpyridine- functionalized nanocluster complexes are joined using first-row transition metals to form extended solid gels. Preliminary solid-state spectroscopic studies on the terpyridine-transition metal environment and the phosphine-gold cluster environment in the gel are discussed, along with future steps towards the construction of nanocluster-based extended solids.