Core Size Interconversions of Au-30(S-tBu)(18) and Au-36(SPhX)(24)

We report for the first time the interconversion between two nanomolecules: Au-36(SPhX)(24), (where X = -H or -tBu) and Au-30(S-tBu)(18). This is accomplished through thermochemical etching, with HSPhX on Au-30(S-tBu)(18) which converts the nano molecule to Au-36(SPhX)(24). We further show that the conversion is reversible from Au-36(SPhX)(24) to Au-30(S-tBu)(18) with tert-butyl thiol (HS-tBu) under the same thermochemical conditions. Not only is this the first reported interconversion between two nanomolecules, but this report further demonstrates the conversion from a smaller to a larger sized nanomolecule, which is a rare occurrence. Experimental evidence for the reversible conversion is provided using matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), nanoelectrospray ionization mass spectrometry (nESI-MS), and UV-vis-NIR spectroscopy. This discovery leads to valuable insight into the inherent nature of ligand dependency on the composition and atomic structure of gold thiolate nanomolecules. The nanomolecules, Au-36(SPhX)(24) and Au-30(S-tBu)(18), have cuboctahedral atomic structures with fcc arrangement but have entirely different staple arrangements, with Au-28 and Au-20 cores, respectively. Theoretical studies show that the difference between Au-30(S-tBu)(18) and Au-36(SPhH)(24) resides in the different fragmentation energy of the two species: conjugation in Au-36(SPhH)(24) weakens the strength of ligand bonding to the Au cluster and thus makes it somewhat less stable than Au-30(S-tBu)(18). Furthermore, Au-30(S-tBu)(18) appears to be slightly more stable than Au-36(SPhH)(24) also in terms of atomization energy of the Au cluster and ligand/ligand interactions.