Abnormally bound n-heterocyclic carbene ligands at group 10 metal centres : Synthesis, structure, reaction chemistry and catalysis

This thesis describes the synthesis of NHC ligands bound to the C4 or C5 atom of imidazolium ring at Pt and Ni metal centres and the study of their reaction chemistry and catalytic behaviour. In order to prevent deprotonation of the C2-position novel C2-blocked imidazolium salts have been synthesised and their behaviour towards C4.5-H oxidative addition to Pt complexes was investigated. New Pt and Ni hydride complexes bearing NHC ligands coordinated at the C4- or Cs-position were isolated and characterised. This work demonstrates that blocking the C2-position of imidazolium salts with alkyl groups does not necessarily prevent such salts interacting with low valent metals, which has implications for catalysis in imidazolium-based ionic liquid solvents. It was also shown that in the presence of alkene, oxidative addition products such as PtHBr(IMes)(1,2-dimethyl-3-propyl-imidazolin-5-ylidene) are susceptible to reductive elimination reactions to give back Pt(IMes)(alkene)2 complexes and C2- blocked imidazolium salts. Novel C2-blocked picolyl-functionalised imidazolium salts 1,2-dimethyl-3-(2-picolyl)imidazolium tetrafluoroborate and 1,3-di(2-picolyl)-2-methylimidazolium iodide were synthesised. Oxidative addition reaction of these salts to Pt afforded Pt(H)(IMes)(1,2-dimethyl-3-(2-picolyl)-2-methylimidazolin-5-ylidene) tetrafluoroborate and Pt(H)(IMes)(1,3-di(2-picolyl)-2-methylimidazolin-5-ylidene) iodide, respectively, with C4-bound abnormal carbene with chelated pyridyl groups isolated exclusively. The picolyl functionalised abnormal NHC Pt-H complexes are not prone to reductive elimination reactions of either the normal or abnormal carbene in presence of alkenes. Picolyl and aminoethyl functionalised blocked and unblocked imidazolium salts were employed in the hydrosilylation of styrene with triethylsilane by in situ generated Pt -catalyst, giving different product selectivity. When using the C2-blocked imidazolium salt the major product was found to be the dehydrogenative product triethyl-styryl-silane, while using the C2-H imidazolium salt the major product was the tftt-Markovnikov triethyl-(l-phenyl-ethyl)-silane. Several ultimately unsuccessful strategies to isolate a C4 free carbene are described. C2-Blocked 1,3-dimesityl-2-methylimidazolium chloride was synthesised but deprotonation of the C4-H position failed, with the C2-Me group deprotonated instead. It seemed necessary designing an imidazolium salt immune to deprotonation at the C2-position and it is also possible that blocking the C5 position with a o-donating alkyl bulky group will protect or stabilise the desired carbenic center.