Water-Soluble, Isolable Gold Clusters Protected by Tiopronin and Coenzyme A Monolayers

Isolable, water-soluble gold clusters protected by monolayers of tiopronin (tiopronin−MPCs) or coenzyme A (CoA−MPCs) were synthesized by a procedure of comparable simplicity to the Brust synthesis for alkanethiolate monolayer-protected gold clusters. High-resolution transmission electron microscopy shows that, like their alkanethiolate−MPC counterparts, the average core diameters of tiopronin−MPCs can be systematically controlled by varying the tiopronin:Au mole ratio employed in the reaction. The UV−vis spectra of tiopronin−MPCs exhibit pH and core size dependency of the surface plasmon band position and intensity, respectively. Thermogravimetric analysis of the tiopronin−MPCs gave average numbers of tiopronin ligands per cluster; for example, tiopronin−MPCs with an average core size of 1.8 nm (∼Au 201 ) are protected with an average of 85 tiopronin ligands. 1 H NMR reveals a size-dependent evolution of spectral features interpreted as reflecting differences in attachment sites (terrace, defects) and/or restriction in ligand mobility. Infrared spectroscopy reveals strong hydrogen bonding in the monolayer and provides evidence for intercluster association, and acid/base titrations produce p K A values similar to the free ligand in the presence of a charge-screening supporting electrolyte, but higher in its absence. The same analytical methods were also applied to CoA-MPCs.