Basic [Au25(SCH2CH2Py)18]-.Na+ Clusters: Synthesis, Layered Crystallographic Arrangement, and Unique Surface Protonation

The synthesis of high-purity and high-yield Au-25 clusters protected by the basic pyridyl ethanethiol (HSCH2CH2Py, 4-PyET and 2-PyET) is presented. Single-crystal X-ray diffraction of the [Au-25(4-PyET)(18)](-).Na+ clusters has revealed a structure similar to that known for the phenyl ethanethiolate analogue, but with pyridyl-N coordination to Na+, a more relaxed ligand shell, and a profoundly layered arrangement in the solid state. Because of the pendant Py moiety, the [Au-25(PyET)(18)](-) clusters are endowed with unique (de)protonation equilibria, which has been characterized in detail by UV/Vis absorption and H-1 NMR spectroscopy. [Au-25(PyET)(18)](-) clusters showed an unexpectedly H+-dependent solubility that is tunable in aqueous and organic solvents. The successful synthesis of the basic Py-terminated thiolate-protected Au-25 clusters paves the way to realize a new family of metalloid clusters possessing basic properties.