Iridium and rhodium pyrazolyl and carbene based catalysts for X-C bond formation

Abstract

This thesis describes the investigation of Ir(I) and Rh(I) complexes as catalysts to facilitate the addition of X-H (X= Si, O and N) bonds across an alkyne C≡C triple bond. Towards this goal two new pincer ligands were prepared that contained a central NHC donor group linked to two pendent pyrazolyl donor groups by either a methyl (NCNMe) or ethyl (NCNEt) group. The longer ethyl linker of NCNEt led to a higher lability of one pyrazole donor. The coordination properties of the pincer ligands also varied depending on the nature of the metal centre and the complex with tridentate (κ3), bidentate (κ2) and monodentate (κ1) coordination modes all being observed. A bimetallic complex [Rh(NCNMe)CO]2(BPh4)2 was also prepared where the NCNMe ligand coordinated in an unusual bridging mode. The new pincer complexes were investigated as catalysts for the hydroamination, hydroalkoxylation and hydrosilylation of alkynes, and their reactivity was compared with two analogous complexes containing an NHC-pyrazolyl chelate. For the intra-molecular hydroamination of 5-phenyl-4-pentyn-1-amine the pincer ligand geometry was responsible for a decrease in catalyst activity, however, during the cyclisation of 4-pentynoic acid the pincer ligands appeared to stabilise the catalyst against deactivation resulting in higher reaction conversions. A marked difference was also observed between catalysts containing the NCNMe or NCNEt pincer ligands. A series of previously reported Ir(I) and Rh(I) complexes were also investigated as catalysts for the inter-molecular hydroamination of terminal alkynes with primary amines. Catalysts containing the weakly coordinating BArF anion were found to be much more effective than complexes containing BPh4-. In general, catalysts containing CO co-ligands were more beneficial than those containing COD, with iridium catalysts significantly more active than rhodium. The nature of the chelating ligand group, either bis(pyrazolyl)methane (bpm) or 1-[2-(diphenylphosphino)ethyl]pyrazole (PyP), had little impact on catalyst activity. The bimetallic system (μ-L)[Ir(CO)2]2(BArF)2 was found to be the most active catalyst of the series, particularly for the hydroamination of phenylacetylene and aniline, for which it is one of the most efficient catalysts known to date.