Greengold, a giant cluster compound of unusual electronic structure

We report the isolation of a previously elusive giant gold-cluster compound with unusual electronic structure and excellent solid-state ordering properties. Greengold is a water-soluble Au:PR₃ compound (R = p-C₆H₄ - CONHCH₃) formed in high yield in the solution-phase reduction of R₃PAuCl. Following chromatographic isolation, it has been investigated by electron microscopy (STEM), optical spectroscopy, mass spectrometry (MALDI), and X-ray diffraction, from which emerges a consistent picture of Greengold's intrinsic characteristics as (i) a single compourd of ∼22000 amu molecular weight, containing a strongly bound globular metal core of mass 14800 (Au₇₅, 1.3-nm equivalent diameter); (ii) a highly ordered solid, exhibiting diffraction through 21 orders of a 2.47-nm spacing; and (iii) a carrier of a highly structured optical absorption across the entire visible and near-infrared spectrum (to ∼1.0 eV), including relative transparency in the blue and yellow regions, giving rise to its eponymous green tint.