Design driven transition metal catalysts from atom-efficient catalysed C-X bond forming reactions

This thesis describes the synthesis and characterisation of a range of mono- and dirhodium(I) complexes, as well as their application as catalysts to promote C-X bond forming reactions (where X = Si or N). An emphasis is placed on the difference in reactivity between mono- and dirhodium(I) complexes in catalysis, as well as understanding reaction mechanisms using stoichiometric studies of catalysts with catalysis substrates.

Catalysis protocols for successful rhodium catalysed N-formylation and N-methylation of amines using CO₂ as the carbon source were established using a Rh(I) complex [RhCl{C(NCH₂PCy₂)₂C₁₀H₆] as catalyst. The catalysed N-formylation and N-methylation reactions each were successful in converting the amine substrates and CO₂ to formamide and methylamine products, forming new C-N bonds in the process. The reaction conditions—including the choice of reducing agent, reaction solvents and temperatures—were optimised, and the substrate scope showed that these reaction protocols were applicable to a range of substituted aromatic and aliphatic substrates. Reaction pathways for both the N-formylation and N-methylation reactions were proposed on the basis of stoichiometric studies and time course studies.

A range of new phosphine-imidazolium pro-ligands, as well as their mono- and dirhodium(I) complexes are presented. The rhodium(I) complexes are supported by both monotopic and homobitopic bidentate phosphine-NHC ligands, and contain 1,5-cyclooctadiene (COD) or carbonyl co-ligands. These new pro-ligands and metal complexes were structurally characterised using NMR spectroscopy, elemental analysis, high resolution mass spectrometry and IR spectroscopy. The mono- and dirhodium(I) complexes were found to have similar electronic environments. The dirhodium(I) complexes also demonstrated the potential to attain short Rh-Rh distances.

The use of the new mono- and dirhodium(I) complexes containing phosphine-NHC ligands prepared here as catalysts for both the N-formylation and N-methylation of aniline using CO₂ as the carbon source, as well as the hydrosilylation and hydroamination of alkynes was tested. The different catalytic efficiency arising from the varying nuclearity of the metal complexes and the identity of the co-ligand has been highlighted. The mono- and dirhodium(I) complexes lacked reactivity in the N-formylation and N-methylation reactions, but efficiently promoted the hydrosilylation and hydroamination of alkynes. Bimetallic cooperativity was manifested in the form of enhanced catalytic efficiency on using a dirhodium(I) complex as the catalyst to promote the hydrosilylation of diphenylacetylene. Stoichiometric studies of reactions of the rhodium complexes and catalysis substrates have been undertaken to investigate the lack of reactivity in the N-formylation and N-methylation reactions, the reaction pathways of the catalysed hydrosilylation and hydroamination reactions, and the origin of the observed bimetallic cooperativity.