Formation of bioinorganic complexes by the corrosive adsorption of (S)-proline on Ni/Au(111)

Nickel nanoparticles modified by the adsorption of chiral amino acids are known to be effective enantioselective heterogeneous catalysts. The leaching of nickel by amino acids has a number of potential effects including the induction of chirality in the nickel atoms left behind in the nanoparticle and the creation of catalytically active nickel complexes. The adsorption of (S)-proline onto Au(111) precovered by two dimensional nickel nanoclusters was investigated by scanning tunnelling microscopy, X-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy. Adsorption of (S)-proline at 300 K resulted in the corrosion of the nickel clusters, the oxidation of the leached nickel and the on-surface formation of bioinorganic complexes, which are concluded to contain three prolinate species in an octahedral arrangement around the central Ni ion. Two distinguishable forms of nickel prolinate complexes were identified. One form self-assembles into 1-D chains and the other form gives rise to porous 2-D islands. Octahedral complexes of the type M(AB)3 are intrinsically chiral resulting in two pairs of enantiomers. The mirror symmetry of each pair of enantiomers is broken when, as in this study, the bidentate ligand itself possesses a chiral center. DFT calculations are used to examine the relative energies of each Ni(prolinate)₃ complex as isolated gas phase species and isolated adsorbed species.