Crystal Structure of Faradaurate-279: Au₂₇₉(SPh-tBu)₈₄ Plasmonic Nanocrystal Molecules

We report the discovery of an unprecedentedly large, 2.2 nm diameter, thiolate protected gold nanocrystal characterized by single crystal X-ray crystallography (sc-XRD), Au₂₇₉(SPh-tBu)₈₄ named Faradaurate-279 (F-279) in honor of Michael Faraday's (1857) pioneering work on nanoparticles. F-279 nanocrystal has a core-shell structure containing a truncated octahedral core with bulk face-centered cubic-like arrangement, yet a nanomolecule with a precise number of metal atoms and thiolate ligands. The Au₂₇₉S₈₄ geometry was established from a low-temperature 120 K sc-XRD study at 0.90 Å resolution. The atom counts in core-shell structure of Au₂₇₉ follows the mathematical formula for magic number shells: Au@Au₁₂@Au₄₂@Au₉₂@Au₅₄, which is further protected by a final shell of Au₄₈. Au₂₄₉ core is protected by three types of staple motifs, namely: 30 bridging, 18 monomeric, and 6 dimeric staple motifs. Despite the presence of such diverse staple motifs, Au₂₇₉S₈₄ structure has a chiral pseudo-D₃ symmetry. The core-shell structure can be viewed as nested, concentric polyhedra, containing a total of five forms of Archimedean solids. A comparison between the Au₂₇₉ and Au₃₀₉ cuboctahedral superatom model in shell-wise growth is illustrated. F-279 can be synthesized and isolated in high purity in milligram quantities using size exclusion chromatography, as evidenced by mass spectrometry. Electrospray ionization-mass spectrometry independently verifies the X-ray diffraction study based heavy atoms formula, Au₂₇₉S₈₄, and establishes the molecular formula with the complete ligands, namely, Au₂₇₉(SPh-tBu)₈₄. It is also the smallest gold nanocrystal to exhibit metallic behavior, with a surface plasmon resonance band around 510 nm.