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Zero valent n-heterocyclic carbene complexes of nickel, palladium and platinum : synthesis, structure, reaction chemistry and catalysis

Clement, Nicolas David Stephane 2005. Zero valent n-heterocyclic carbene complexes of nickel, palladium and platinum : synthesis, structure, reaction chemistry and catalysis. PhD Thesis, Cardiff University.

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This thesis describes the synthesis of coordinatively unsaturated bis- and mono-N-heterocyclic carbenes of Group 10 metal(0) complexes (Ni, Pd, Pt) and the study of their reaction chemistry and catalytic behaviour. Novel coordinatively unsaturated mono-NHC-Pd0 (NHC = N-heterocyclic carbene) complexes stabilised with activated olefin ligands (dimethylfumarate, DMFU and p-benzoquinone, BQ) have been synthesised and characterised, including by X-ray crystallography. The Pd (NHC)(DMFU)2 complexes were found to be the thermodynamic products of an equilibrium between bis- and mono-NHC-Pd (DMFU) species, the equilibrium reaction driven by the formation of an NHC-DMFU coupling product. The Pt (NHC)(DMFU)2 complex was also synthesised. Novel coordinatively unsaturated bis- and mono-NHC-Ni complexes stabilised with DMFU ligands were synthesised and crystallographically characterised. These Ni complexes have a dynamic behaviour in solution, and the same equilibrium observed for palladium between bis- and mono-NHC-M (DMFU) species with formation of an NHC-DMFU coupling product was also demonstrated to occur with nickel. Possible mechanisms for this unusual NHC-DMFU coupling process at these nickel and palladium(O) centres are discussed. The coordinatively unsaturated mono-NHC-Pd (DMFU/BQ) complexes, as well a bis-NHC-Pd complex, were tested as precatalysts in the telomerization of butadiene with methanol. All of these well-defined complexes were found to be extremely active and selective catalysts in the telomerization reaction. The C-H oxidative addition of imidazolium salts/ionic liquids to a coordinatively unsaturated mono-NHC-Pt0 complex and bis-NHC-Ni /Pd complexes was studied. These oxidative addition processes occur quickly under mild conditions and produce the corresponding bis- or tris-NHC-Mn-H complexes as surprisingly stable compounds. X-Ray crystallographic studies of the two tris-NHC-Mn-hydrido complexes (M= Ni, Pd) with the hydride ligands located are reported The combination of the imidazolium C2-H oxidative addition and the NHC-hydrocarbyl reductive elimination processes into a catalytic cycle to form a unique Ni-catalysed imidazo lium-alkene coupling reaction is reported. The scope of this new catalytic transformation that produces 2-alkylimidazolium salts under mild conditions is extended to other types of azolium salts. The implications of these results for NHC chemistry and for the use of imidazo Hum-based ionic liquids as solvents in transition-metal catalysed reactions is discussed.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
ISBN: 9781303201417
Funders: EPRSC
Date of First Compliant Deposit: 30 March 2016
Last Modified: 10 Oct 2017 15:29

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