Mass Spectrometry of Ligand Exchange Chelation of the Nanoparticle [Au25(SCH2CH2C6H5)18]1- by CH3C6H3(SH)2

Mass spectrally detected products of ligand exchange reactions of the nanoparticle [Au-25(SC2H4C6F15)(18)](1-), (abbrev. Au-25(SC2Ph)(18)), where the dithiol is toluene-3,4-dithiol, CH3C6F13(SH)(2), include nanoparticles containing both doubly (bidentate, or chelating) and singly bonded dithiol. The bidentate binding displaces two of the original -SC2Ph ligands, and singly bonded dithiol displaces one -SC2Ph ligand, while maintaining, for mass spectrally detected species, occupancy of 18 ligation sites. Extended exchange reaction times result in an apparent maximum of six chelated dithiolates. In the Au-25(SC2Ph)(18) naroparticle, six semi-rings of -S(R)-Au-S(R)-Au-S(R)- act as the protecting ligand shell surrounding a Au-13 core; the chelation is suggested to involve binding of dithiolates to adjacent semi-rings, rather than to a single semi-ring. Both high resolution ESI and lower resolution MALDI spectra support the product assignments. A minor extent of bidentate ligand incorporation is sufficient to severely compromise the well-known Au-25(SC2Ph)(18) UV-vis fine structure and to alter its voltammetric pattern, reflecting either associated semi-ring distortion and/or decay of the exchange product.