UBC Theses and Dissertations
Synthesis and coordination chemistry of an N-heterocyclic carbene-containing tridentate ligand Shaw, Bryan Kenneth
One of the most exciting advances in transition-metal-catalyzed processes has been the move toward use of N-heterocyclic carbenes (NHCs) as ancillary ligands. The variation possible in NHC design is immense with one of the most important design aspects being the incorporation of NHC donors into polydentate arrays to generate tridendate pincer ligands. However, research into this class of compounds was limited until the isolation of the first stable crystalline NHC two decades ago. Therefore, understanding of the organometallic chemistry of this class of compounds remains limited in comparison to the closely related phosphine ligands. Thus, this thesis focuses on the preparation of a di-o-phenylene-bridged tridentate PCP donor set and the reactivity of coordination complexes of late transition metals containing this new ligand system. Incorporation of this tridentate ligand onto the group 10 triad of elements via oxidative addition proceeds smoothly to generate square-planar metal hydride complexes of the form [(PCP)MH][PF6] (where M = Ni(II), Pd(II) and Pt(II). Initiating reactivity studies of this series of complexes exposed a new type of non-innocent ligand ehaviour involving formation of a C-C bond between the carbene atom and an ethyl moiety. Isotopic labeling experiments and DFT calculations are used to provide mechanistic insight into this process that is viewed as a possible source of catalyst deactivation in similar systems. Group 9 metal complexes of rhodium and iridium incorporating tridentate ligand arrays have achieved considerable success in the activation and functionalization of C-H bonds. To examine the reactivity of pincer complexes utilizing a central NHC donor a variety of new rhodium and iridium complexes were synthesized. A rhodium hydride species (PCP)RhH showed catalytic activity in the dehydrogenation of ammonia borane as well as in the ydrosilylation of terminal alkynes. Complexes consisting of both metal centres also exhibited ligand rearrangement processes that challenge the presumed stability of rigid tridentate ligand frameworks.
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