Exciting developments in the chemistry of gold

Abstract

R—O—H ••• O—R H R—O •. H ••• O—R for example, is sufficient to dictate the properties and structures of many R-OH compounds significantly. Thus in the liquid state or in solution, it may give rise to intermolecular interactions with the formation, especially in the case of compounds containing more than one hydroxyl group, of effectively polymeric structures. In the solid state, similar intermolecular interactions have been found to dictate the crystal structures of many hydroxyl compounds. Perhaps the simplest and best known examples of this are the crystal structures of the straight chain primary fatty alcohols and fatty acids. Intramolecular hydrogen bonding is also well known. From the Inorganic Chemistry Institute of the Technical University of Munich there has now appeared a publication (H. SCHMIDBAUR, W. GRAF and G. MULLER, Angew. Chem., Int. Ed. Engl., 1988, 27(3), 417-419) which establishes an analogy between the role played by hydrogen bonding in determining the properties and structures of hydroxyl compounds, and that played by weak Au...Au bonding in determining the properties and structures of gold(I) compounds. The paper begins by drawing attention to the wealth of indirect evidence which has emerged from structural and spectroscopic studies for the existence of an attractive interaction between gold(l) atoms of d 10 configuration in both monoand poly-nuclear gold(1) compounds. This interaction is seen as stemming from a mixing of the 6s2 states, whose energy gap is reduced by relativistic effects. The phenomenon is a rare one, possibly exhibited most strongly by gold. In the case of the neighbouring element mercury such effects are already much less pronounced. The interaction L—Au—L L—Au—L + L —Au —L L —Au —L occurs perpendicular to the principal axis of the linearly two-coordinated gold(I) atoms. It can give rise either to weak intermolecular bonding between Au atoms in gold(I) compounds, with the formation of catenated or layered aggregations (effectively polymers) of molecules linked via Au' Au bonds, or to weak intramolecular bonding between Au(I) atoms in cases where close approach between such atoms within one and the same molecule is feasible. Structurally, such bonding manifests itself in the solid state in Au' «Au interatomic distances which are sometimes significantly less than 3.0 A. Apart from numerous such manifestations, however, the authors point out that they have found no recorded proof of the existence of such interactions between gold(I) atoms, and no successful attempts to measure their strengths. They then describe studies which provide such proof, and demonstrate that the energy of the Au'«Au interaction lies in the order of 7-8 kcal mol -1 , which is comparable with that involved in hydrogen bonding. They also show that the energy involved in such bonding is sufficient, as it is in the case of hydrogen bonding, to determine the conformation of some structures within which it occurs. This established analogy between the effects of hydrogen bonds in hydroxylic compounds and Au«••Au bonds in Au(1) compounds should do much to facilitate an understanding of the properties and behaviour of the latter. More importantly, however, greater appreciation of the implications of Au' «Au bonding has already provided a basis for further significant developments by Prof. Schmidbaur's research school on gold, where there is an active interest in the thermal, chemical and photochemical deposition of gold, for application in electronics and electron microscopy. From this point of view, not only gold clusters, in which gold atoms are directly bonded to one another, but also organogold compounds in which several gold atoms are bonded via a common carbon atom are of special importance. The close proximity of the gold atoms in such compounds should not only lead to clearly recognizable Au.. .Au bonding relationships but should also facilitate metal nucleation which is important for the controlled deposition of gold. Three further papers have been published in this connection, and a fourth is in press. The first of these (H. SCHMIDBAUR, F. SCHERBAUM, B. HUBER and G. MULLER,Angew. Chem., Int. Ed. Engl., 1988, 27(3), 419-421) describes the synthesis of the first triauriomethane compound. In this the three gold atoms were found to be about equidistant from the carbon atom. Pairwise, however, they form angles with the carbon atom which are significantly less than the tetrahedral angle of 109° and the Au-Au interatomic distances average 3.18 A, despite the repulsion to be expected from the large PPh 3 ligands to which they are