Synthesis and Reactivity of Actinide Bis(N-Heterocyclic Carbene)Borate Complexes

Chapter 1: A new, chelating, monoanionic bis(N-heterocyclic carbene)borate mesityl ligand was synthesized and used to prepare a thorium bis(iodo) complex as well as a direduced-bpy derivative (Th-NHC-bpy). Complete Active Space Self-Consistent Field (CASSCF) calculations suggested the involvement of a multi-configurational open-shell singlet in the behavior of Th-NHC-bpy, with the main configuration corresponding to a Th(III)-bpy(-1) (f1π*1) electronic structure. The reactivity of this unique system was explored in transformations with carbonyls and organic azides, and the latter gave rise to the first example of a terminal Th-NHC-imido bpy complex./nChapter 2: The unusual reactivity of the direduced Th-NHC-bpy complex was investigated with organic isocyanides (R—NC). Th-NHC-bpy cleaved the C—N single bond within a series of alkyl isocyanides leading to the formation of thorium complexes containing terminal cyano groups and alkyl-substituted bpy ligands. The dealkylation event performed by Th-NHC-bpy is remarkably general and yielded very well-defined products that assisted in product identification and mechanistic elucidation. Single crystal X-ray diffraction analyses of the isolated thorium-cyano complexes revealed an unusual N-binding mode of the terminal cyanide ligand. The isolation of a rearranged but-3-enyl product from the reaction of Th-NHC-bpy and cyclopropylmethyl isocyanide supported the notion that a radical-based mechanism was operative./nChapter 3: The reactivity of the direduced Th-NHC-bpy complex with either propylene oxide or cyclohexene oxide to form ring-opened addition products is described. These C−O bond cleavage events proceeded cleanly and afforded well-defined products characterized by NMR, IR, and UV-Vis spectroscopy. The synthesis and characterization of a new reduced 6,6’-dimethyl-2,2’-dipyridyl thorium-bis(NHC)borate mesityl complex (Th-NHC-Me2bpy) is also described. The steric congestion present within Th-NHC-Me2bpy led to divergent reactivity compared to that of Th-NHC-bpy. Instead of clean bond making and breaking chemistry with various organic substrates such as tert-butyl isocyanide, propylene oxide, benzophenone, and p-tolylazide, Th-NHC-Me2bpy either did not react, or succumbed to decomposition./nChapter 4: The synthesis, characterization, and reductive elimination reactivity of a bis(NHC)borate-supported thorium-bis(mesitylphosphido) complex are described. Treating the complex with 2,2’-bipyridine induces the ligand-based reductive elimination of dimesityldiphosphine concomitant with the formation of the previously studied Th-NHC-bpy species. The kinetics of the reductive elimination were studied by 31P-NMR and suggest the presence of an intermediate. Treatment with alternative oxidants also led to diphosphine elimination, but the corresponding thorium species could not be isolated. Additional primary and secondary Th-bis(phosphido) complexes were synthesized but do not demonstrate the same facile reductive elimination./nChapter 5: The ability of the bis(NHC)borate-supported thorium-bis(mesitylphosphido) complex to undergo reversible intramolecular C−H bond activation and phosphine elimination was thoroughly investigated. Isotopic labelling combined with 2D NMR experiments and X-ray diffraction studies confirmed the complex’s proclivity for bond activation and cyclometalation. The dynamic process was leveraged towards the catalytic hydrophosphination of unactivated internal alkynes. Preliminary substrate screening indicated the catalysis was exceptionally clean, produced exclusively secondary vinyl phosphine products, did not require light, radical initiators or elevated temperature, and was relatively selective for the E isomer. Catalytic kinetics and stoichiometric experiments supported an unusual mechanism involving reactive Th-NHC metallacycle intermediates.

Advisor: Arnold, John