Same ligand--different binding: a way to control the binding of N-acetyl-cysteine (NAC) to Pt clusters

The functionalization of "unprotected" Pt clusters with N-acetyl-cysteine (NAC) at different pH-values is presented that allows for binding NAC either via the thiol or the amide group to the particle. NMR-spectroscopy was used to study the chemical nature of NAC at weakly acidic and alkaline conditions. The formation of a cyclic isomer of NAC was found at high pH-values which occurs through an intramolecular reaction between the thiol and the amide group delivering a cyclic thioether. The absence of the bare thiol groups in aqueous alkaline solutions leads to binding of the cyclic isomer of NAC to the Pt clusters via its nitrogen atom. IR spectroscopy was applied, which confirmed that the cyclic isomer is, however, not stable upon drying, but undergoes ring-opening yielding the "normal" non-cyclic form. This distinctive property of NAC in combination with the use of "unprotected" clusters allows for binding the same ligand to clusters from the same batch, but with different binding modes, while the particle size is preserved. As a consequence, differences in the cluster properties can be related exclusively to the influence of the binding properties of the ligand. Finally, the catalytic hydrogenation of 2-butanone was used as a probe reaction and the resulting differences in the enantioselectivity can thus be related to this particular change in the binding mode.