THE DADI LIGAND AND BEYOND! BENZIMIDAZOLE-DIAMIDE (BIDA) COMPLEXES OF CHROMIUM AND TITANIUM AND HOMOLEPTIC COMPLEXES FEATURING NITROXYL LIGANDS
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Transition metal catalysis has proven to be vital in organic synthesis and in the production of commodity chemicals. 2nd and 3rd-row metal species are typical catalysts due to their stability, low-spin electron configurations, and ready engagement in 2-electron chemistry. Unfortunately, they are less abundant, more expensive, and more toxic than their 1st-row analogues, which have been underutilized due to weak ligand fields and high-spin electron configurations. This makes characterization difficult and typically leads to unproductive 1-electron chemistry, thereby spoiling catalytic applications. One approach to circumvent unproductive 1-electron chemistry in 3d metals involves the use of redox non-innocent ligands. Redox non-innocence (RNI) mediates reactivity of metal complexes through a synergistic relationship between the metal center and ligand framework, typically acting as electron reservoirs or participating in bond functionalization events. Consequently, RNI ligands are excellent candidates for expanding chemistry in 1st-row metals.The RNI of a four-coordinate diamide-diimine (dadi) ligand was responsible for undesired reactivity at its C-C backbone, and a methylated version of the ligand was synthesized to attempt to block reactivity at the backbone. (medadi)M complexes were prepared and compared to the previously reported system. A benzimidazole-diamide pincer ligand (bida) that features potential RNI behavior, was coordinated to Cr and Ti to yield (bida)CrCl(THF) and (bida)TiCl2. For Cr, (bida)CrCl(THF) was successfully oxidized with organic azides, generating a collection of Cr(V) imido compounds, and resulting in a nitrene insertion. Alkylation of (bida)CrCl(THF) with alkyllithiums resulted in the clean formation of (bida)CrR(THF) compounds, and the discovery of a single component system for the polymerization of ethylene. For Ti, attempts were made to generate metal-ligand multiple bonds. Simple salt metathesis successfully yielded (bida)TiR2 compounds which thermally rearrange to produce a tuck-in species. Mechanistic investigations were initiated to explore the formation of the cyclometallated compound. TEMPO may also be considered as a redox active ligand with its ability to access three redox states. As such, examination of the coordination of TEMPO to first row metals was also investigated, leading to the isolation of chromium-TEMPO complexes featuring a rare-coordination geometry and unique iron-oxide clusters.
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Lancaster, Kyle