Oligomerization of Proline Catalyzed by Nickel on a Au(111) Surface

The reaction of (S)-proline with two-dimensional Ni nanoclusters on Au(111) results in the formation of ordered structures constructed from two distinct types of nickel prolinate complex. Scanning tunnelling microscopy and density functional theory are combined to analyze the supramolecular arrangements produced as a function of annealing temperature. Octahedral Ni(Pro)(3) complexes are the fundamental building blocks of multiple phases identified by the formation of hexagonal pores of 1 nm diameter. Coexisting with the hexagonal porous phases is a honeycomb phase constructed from Ni(Pro)(2) complexes. In contrast to the thermal stability of the hexagonal porous phases, the honeycomb phase transforms on heating to a mesh phase which is concluded to be constructed from a Pro-Pro dipeptide species coordinated to Ni ions. At even higher temperature, more extensive oligomerization occurs to produce an ordered rose phase whose structure is consistent with the formation of oligoproline species.