Gold(I) complexes and co oxidation catalysts

Abstract

Isonitrile gold(I) nitrates, (RNC)AuNO₃ (R = Buᵗ, Et or Xy) have been prepared and fully characterised. These join the rare set of gold(I) species exhibiting ligation to oxygen and are the first examples of gold(I) bonded to both carbon and oxygen. Molecular aggregations of the monomeric units were revealed by X-ray crystal structure determinations. The aggregations are the result of secondary Au--Au interactions. These interactions are also apparent in the structures of the analogous complexes (RNC)AuCl (R = Et or Xy), which were characterised for comparative purposes.

(EtNC)AuCl and (BuᵗNC)AuNO₃ formed zig-zag chain structures from which the other unprecedented structures can be derived. (EtNC)AuNO₃ formed a concertinaed chain, (XyNC)AuNO₃ (1) a compressed chain and (XyNC)AuCl a tetrameric chain. The formations of these supramolecular motifs are attributed to various steric and electronic ligand influences on the inherent Au--Au attraction. In addition, intermolecular π-π contacts were observed for the (XyNC)AuX (X = Cl or NO₃) structures.

In each instance, the Au--Au interactions for (RNC)AuNO₃ were significantly shorter than those of the analogous (RNC)AuCl structure. The results demonstrated that the non-polarisable nitrate anion enhances the Au--Au attraction. This is contrary to previous theoretical calculations, which predict that soft anions enhance the Au--Au attraction. The structure of the ionic [(XyNC)₂Au]⁺NO₃- complex was also characterised. This did not exhibit Au--Au contacts; the packing structure was instead based on infinite π-π stacking.

The potential of gold(I) complexes as precursors to supported gold catalysts was assessed. The complexes were adsorbed (from a solution phase) onto transition metal oxide supports which were subsequently vacuum dried and calcined. Scanning Electron Microscopy (SEM) revealed a range of gold particle sizes on the calcined materials. The extent of gold dispersion was dependent on the chemical properties of the complex employed.

The use of the stable (Ph₃P)AuCl, (Ph₃P)AuMe and (BuᵗNC)AuCl complexes resulted in relatively large gold particles (20-200 nm). In the case of (Ph₃P)AuMe, metallic gold crystals were observed. Under ambient conditions these materials were inactive for CO oxidation. Under the same testing conditions, efficient catalytic CO oxidation was observed over materials prepared by using the unstable (BuᵗNC)AuNO₃ complex as a precursor. The gold particles on the active catalyst were too small to be resolved by SEM.

The observed CO oxidation activity was proposed to be a two-component catalytic reaction involving gold nano-particles in intimate contact with a reducible oxide support. Reactant inhibition, which is commonly observed for catalysts with inert supports, is avoided in this system. This form of catalysis has received considerable recent attention for the CO oxidation reaction and provides guidance for attempts to enhance other heterogeneous catalytic processes.