N-heterocyclic carbenes have been a huge talking point for the best part of six decades, with the past thirty years expediting this trend through the isolation of these species from their coexisting dimers. Throughout this period, it has been comprehensively understood that the electronic and steric properties composed within these compounds are quintessential for their intrinsic stability and characteristic reactivity. Furthermore, this recognition has foreseen an expansive library of NHCs to become unveiled.
The organic synthesises of NHCs has enabled their participation within coordination chemistry, purposing them to act as ancillary ligands towards less reactive metals. Moreover, it is unambiguous to state that a multitude of monodentate as well as multidentate NHC ligands have been discovered within many metal complexes, however sterically bulky polydentate NHC ligands (species that have more than one phenyl groups tethered to each N-site) have yet seen to be established. Herein, this research aims to investigate the formation of a new, hampered tridentate NHC/pyridine hybrid ligand via two similar synthetic workups. Ultimately, the formation of the anticipated ligand was unsuccessful in both attempts due to the extensive chemo-selective issues surrounding the synthesis of the imidazolium salt precursors. Nevertheless, in the second procedure, an unpredicted, never seen before derivative salt VI was produced via rearrangement of imidazolium salt V.
1H NMR spectroscopy was a vital analytical tool for this breakthrough as the integration values and the number of total proton environments present within the compound concluded that only one equivalent of the [Ph3C]BF4 salt reacted with 2,6-Bis[imidazol-1-yl]pyridine rather than two, which eventually bonded to the localised nitrogen within the pyridinyl moiety as opposed to the imidazolyl bounded nitrogen due being more thermodynamically stable. The bonding of the second trityl cation is suspected to have failed due to coulombic repulsion between itself and the positively charged nitrogen heteroatom derived within V.
Due to the limited timebound within the laboratory sessions, the aimed hybrid NHC/pyridine ligand observed no more progression within its synthesis and therefore complexation was never reached. Hypothetically, the multi-donor ligand would have made an appearance in a silver(I) complex in the attempt of functionalising within a variety of transmetaltion processes, to then support in subdivisions within catalysis. However, due to the countless chemo-selective issues surrounding the imidazolium salt’s synthesis in solution-state reactions, overall, the anticipated ligand is reported as a poor synthetic target.
PLEASE NOTE: You must be a member of the University of Lincoln to be able to view this dissertation. Please log in here.