Synthesis and characterisation of chelating, hemilabile N-Heterocyclic carbene ligands : applications of their cobalt (II) and nickel (II) complexes as catalysts for aryl amination.
The chemistry and applications of N-heterocyclic carbenes (NHCs) as ligands for the preparation of transition metal complexes has remained at the forefront of research in organometallic chemistry and homogeneous catalysis worldwide. This is evidenced by the increasing number of research articles on the subject area. Hence, this thesis report on three different ligand frameworks consisting of a symmetric CNC pincer architecture and two non-symmetric N-substituted imidazoles bearing a picolyl and a ferrocenylphenyl imidazole functionalities. These were then used to synthesise new ligand precursors (imidazolium salts) which were later on complexed to Ni and Co via transmetalation of corresponding NHC-Ag complexes. The synthesis of the pincer imidazolium salts via a solvent free method led to the development of a new technique based on the use of a short plug of silica to purify such salts and isolate them in excellent yields. This method has been published and is now adopted in our laboratory and extended to the purification of a wider variety of salts including those based on the related triazolium moiety. All the compounds (salts and metal complexes) have been fully characterised by NMR (1H, 13C, 31P), IR, HRMS or MS-ES+, EA and in some cases single crystal X-ray diffraction analyses. Sharp melting points were also recorded for all air stable, none hygroscopic compounds. The characterisation data have established the chemical composition and purity of all the compounds reported in this thesis. All the complexes are active as catalysts for the activation of C-N coupling of aryl amines and aryl halides. The catalyst systems showed excellent selectivity to N-mono arylation as no trace of N,N-diarylated products were observed. The high selectivity and excellent yields to desired products achieved under mild reaction conditions and low catalyst loading of 1 mol% of Ni or Co has made the current catalyst systems competitive in comparison to classical Cu mediated or CuI catalysed Ullman coupling and Pd catalysed systems. A highly active in situ generated NiCl2/imidazolium salt catalyst was also developed, hence opening up a simple, robust and functional group tolerant system for C-N coupling reactions that may easily be generated from commercially available imidazolium salts in combination with over the counter metal salts such as NiCl2.