Synthetic and high field NMR studies of chiral clusters

Over the last few years, the development of logical routes to small organometallic clusters has opened the way towards the directed syntheses of chiral clusters. Tetrahedral systems comprised of four chemically different vertices are chiral. However, under the conditions required for catalytic action, clusters of this type generally racemize. The introduction of a chelating unsymmetrical ligand, such as Ph2PCH2CH2AsPh2, renders chiral a Co3(CO)9CR cluster but again such systems racemize on the NMR time scale via migration of the arsenic terminus from one cobalt vertex to another. It has been long suspected that cluster vertices such as CpMo(CO)2 could undergo localized rotation and this has now been unequivocally verified using high field NMR spectroscopy at low temperature. Furthermore, it is shown that in solution the predominant rotameric forms of the mixed metal clusters CpMoCo2(CO)8CR are not necessarily those found in the solid state. Finally, the syntheses of a series of mixed metal clusters of general formula CpMo(CO)2(CO)2(CO)6CR in which the apical substituents are derived from natural products, e.g. steroids or terpenes, are described. Treatment of these chiral clusters shows evidence of chiral discrimination in that preferential attack at one of the diastereotopic cobalt atoms is observed.