Supramolecular assemblies of bioinorganic complexes formed via reaction of (S)-proline with nickel clusters on Au(111)

Abstract

Considerable research over several decades has been directed at the development of heterogeneous enantioselective hydrogenation catalysts via the chiral modification of Ni catalysts by chiral molecules such as α-hydroxy acids and α-amino acids. Some of these systems have proved to be very successful yielding catalysts achieving very high enantioselectivity. However, the modified surfaces display relatively poor stability under catalytic conditions, so there is a demand to produce more robust chiral architectures for application in this area of catalysis. The formation of metal organic coordination networks on surfaces has been the subject of intensive study in the last decade. This thesis reports the formation of metal organic structures via the reaction of (S)-proline with nanoscale Ni islands grown on Au(111). Initially, (S)-proline was studied Au(111) using low temperature scanning tunnelling microscopy (LT-STM), XPS and high resolution electron energy loss spectroscopy (HREELS). Adsorpton of (S)-proline at 300 K resulted in the formation of a mixture of the zwitterionic and anionic forms of proline. The former species condensed into ordered arrangements at 77 K giving a structure very similar to one crystal plane of solid proline. Proline species were relatively weakly bound and the vast majority of adsorbed proline desorbed by 370 K. (S)-proline adsorption was examined on Ni/Au(111) over a range of Ni coverages from 0.05 ML up to >1.0 ML STM, XPS, HREELS and DFT calculations. (S)-proline was found to oxidise Ni from the edges of the Ni islands producing nickel prolinate species. Five distinct classes of supramolecular assemblies were produced depending on Ni coverage and annealing temperature. The nature of each of these structures is described. It is concluded that one assembly is built up from Ni(prolinate)₂ (square planar) complexes, three of the assemblies have a fundamental building block consisting of a Ni(prolinate)₃ octahedral complex and the final structure, observed at high annealing temperatures, is constructed from oligomerized proline species. The interaction of the prochiral reagent, methyl acetoacetate, with some of these supramolecular assemblies is reported.