The synthesis, structure and chemistry of some novel organometallic dendrimers
The synthesis and chemistry of several series of novel organometallic dendritic wedges and small dendrimers containing iron, ruthenium, cobalt and tungsten have been investigated. The compounds have been synthesised using the convergent methodology of Hawker and Fréchet. Haloalkyl complexes such as Cp*Fe(CO)₂(CH₂)₃Br and CpRu(CO)₂(CH₂)₃Br were used for the synthesis of the dendrimers. In this approach, the metal and ligand system remain on the periphery of the dendrimer. The synthesis proceeds via successive coupling and activation steps. Many of the dendritic wedges decomposed during the activation procedure. All the dendritic wedges and first generation dendrimers were soluble in common organic solvents and were characterised by IR, ¹H and ¹³C NMR spectroscopy, mass spectrometry and microanalysis where possible. The organoruthenium dendrimers are significantly more stable at room temperature and in solution than all the iron analogues. The steric hindrance imparted by the cobaloxime ligand allowed only the zero-generation dendrimer to be synthesised. Several heterobimetallic dendritic wedges were synthesised as well as a hybrid block copolymer. The surface functionalisation of bromobenzyl-terminated dendrimers was attempted. The data are discussed and some properties and reactions of the compounds are described. The x-ray crystal structures of a dendritic precursor, [CpFe(CO)₂(CH₂)₃Br] (space group P2₁/c) and first generation ruthenium dendritic wedge (space group Cc) have been determined at low temperature. Calculated molecular structures were generated with the HYPERCHEM(TM) program; the dendritic wedges are fairly open structures while the first generation dendrimer adopts a more spherical shape. Cyclic voltammetry studies were carried out on several of the iron and ruthenium dendrimers. The cyclic voltammograms of all the complexes showed a single irreversible oxidation peak; this suggests that there is no interaction or communication between the metals. The thermal behaviour of some of the organoiron dendrimers was also investigated by thermogravimetric analysis and differential scanning calorimetry. The latter traces generally showed one endothermic peak which could be assigned to melting. The chemical reactivity of a first generation ruthenium dendrimer was investigated; this dendrimer was found to react more slowly than the mononuclear complexes. This first generation dendrimer, supported on silica, was tested as a catalyst in the Fischer-Tropsch synthesis. This mono-atomic ruthenium dendrimer was found not to have sufficient catalytic activity for this purpose. The chemical and physical properties of these dendrimers are discussed.
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