Naked Au-55 clusters: Dramatic effect of a thiol-terminated dendrimer

Reaction of the thiol-terminated fourth-generation dendrimer 2-G(4) (96 SH groups) with the gold cluster compound Au-55(PPh3)(12)Cl-6 in a 3:1 molar r atio in dichloromethane results in the formation of bare Au-55 clusters. Th e cuboctahedrally shaped Au-55 particles coalesce to well-formed microcryst als (Au-55)(infinity). The role of the dendrimer is not only to remove the phosphine and chlorine ligands but also to act as an ideal matrix for perfe ct crystal growth. Transmission electron microscopy (TEM), small- and wide- angle X-ray diffraction (SAXRD and WAXRD) measurements indicate a structure where rows of edge-linked Au-55 building blocks form a distorted cubic lat tice. The X-ray data fit best if a 5% reduction of the Au-Au bond length in the Au-55 clusters is assumed, in agreement with previous extended X-ray a bsorption fine structure (EXAFS) measurements. Energy-dispersive X-ray spec troscopy (EDX) analyses and IR investigations show the absence of PPh3 and Cl in the microcrystals.