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Page : 127 pages
File Size : 17,86 MB
Release : 2013
Category : Electronic books
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The objective of this research is to create unique and highly functional N-heterocyclic carbene complexes that provide maximum catalytic features in close proximity to the metal centers. The N-heterocyclic carbenes created over the past 20 years have generally consisted of imidazole moieties substituted at both nitrogen atoms with alkyl and aryl substitutents. These typical N-heterocylic carbenes have presented many highly stable and tunable complexes used in catalysis, however, some beneficial features would be imparted on these N-heterocyclic carbenes if one of the alkyl and aryl substituted sites were allowed to remain un-substituted. These unique atypical protic N-heterocyclic carbenes allow for acid base chemistry to occur in close proximity to the metal center while simultaneously providing the tunable, highly sigma-donating features of a typical N-heterocyclic carbene. The research presented in this thesis explores the synthetic approach to atypical protic N-heterocyclic carbene ligand precursors as well as their complexation and subsequent catalytic capabilities. Two ligand precursor families were created: one consisting of an iodobenzene moiety attached to a tert-butyl imidazole group, and one in which a pyridine ring moiety is attached to a tert-butyl imidazole group. The iodobenzene group yielded two ligand precursors, 2a and 2b, both containing methylene spacers between the auxiliary moiety and tert-butyl groups. The pyridine-containing family consisted of 3a, where the tert-butyl imidazole was directly attached to the pyridine ring, and 3c, where the tert-butyl imidazole was attached to the ring by a methylene spacer. Attempts to complex the first ligand precursor family 2a and 2b were unsuccessful and lead to the synthesis of the pyridine analogues 3a and 3c which were successfully complexed to both iridium and rhodium metal centers yielding: Ir-Phos- 3a, Ir-Phos-3c, Ir-S0-3a, Ir-S1-3c, Rh-S0-3a, and Rh-S1-3c. Catalytic studies carried out with the six complexes above consisted of the oxidation of alcohols to their corresponding ketones. These catalytic tests revealed a higher degree of successful turnover with the 3c containing complexes. Crystallographic data suggests that bond angles and bond distances found in 3c containing complexes give inherent advantages to the 3c containing complexes due to the flex allowed by the methylene spacer. Additionally, catalytic comparisons of methylated analogues of 3c with the atypical protic 3c complexes showed a decrease in catalytic activity suggesting that the presences of the un-substituted nitrogen in close proximity to the metal center yields the predicted benefit of using atypical protic N-heterocyclic carbenes.