Ligands constitute useful and convenient tools for modifying the structures and properties of cluster molecules, with ligands capable of bridging between 2 or more metal atoms being particularly important. In this work, 2 types of phosphorus containing bridging ligands have been studied, the phosphido ligand and diphosphine ligands.
Interest in metal clusters with bridging phosphido ligands arises from their electronic properties as 3-electron donors and from their unique geometrical flexibility. Disphosphine ligands have a strong propensity for bridging between 2 metal centres to give a stabilising effect which has opened the way to the preparation of new heterogeneous, bimetallic iron palladium catalysts.
The nature and number of ligands and their electronic and steric properties effect the reactivity of the metal centres, as does the nature of the metal itself. Reactions with electrophiles and nucleophiles and cluster core isomerisation reactions have been studied. New bridging systems have been developed and bimetallic clusters have been investigated as precursors of catalytic materials.
Studies involving the synthesis of new homometallic and heterometallic substituted carbonyl clusters have included:
correlations between multisite coordination of hydrocarbyls and their reactivity patterns;
synthesis of new chiral clusters to be used in homogeneous catalysis;
alkyne substituted clusters as models of intermediates in catalytic reactions;
phosphido and phosphinidene substituted clusters and their role in homogeneous catalysis.
Polymetallic aggregates have also been studied as possible homogeneous catalysts and as precursors of heterogeneous catalysts or of new materials.
Work in this area has included:
homogeneous hydrogenation of dienes and alkynes;
characterisation of metal particles derived from clusters and showing catalytic activity;
Investigations have been carried out of the reactivity of functionalised alkynes and dialkynes towards Group VIII metal clusters in order to gain evidence of cyclisation, cocyclisation and cyclooligomerisation reactions of these molecules or to model catalytic intermediates of these reactions.
With the goal of improving productivity in ammonia synthesis, the activity of new supported catalysts, obtained from ruthenium clusters, was studied. The higher activity of the cluster derived catalyst is probably attributable to the influence of residual chloride ions on the conventional catalyst, or to the better dispersion of the metal particles on the support surface. The catalyst has been examined in order to determine the nature of the active metal species.
New methods have been developed for preparing bimetallic (nickel ruthenium) ultrafine metal and alloy powders by decomposing bimetallic carbonyl clusters. These techniques have applications in the fields of supported catalysis and industrial powder technology.
The main goal of this work was to gain a better understanding of stoichiometric and catalytic reactions in the ligand sphere of organometallic clusters. This work can be divided into 4 areas: attachment of substrates to clusters; simple substrate reactions; aspects of selectivity and cluster assisted substrate assembly.
A large number of molecules have been synthesised and studied. The number of compounds available and their reactivities can be varied further by metal exchange reactions. Investigations have included preferred substitution sites on heterometallic compounds and cluster chirality, as well as substrate connection on the cluster followed by product release.
Funding SchemeCSC - Cost-sharing contracts
79085 Freiburg (In Breisgau)