REDOX-ACTIVE LIGAND-INDUCED RADICAL REACTIVITY AT HIGH-VALENT OXORHENIUM AND OXOVANADIUM COMPLEXES
Hill-Lumm, Jennifer A.
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Metal-oxyl radicals might be the active species in many important oxygen-atom transfer and H-atom abstraction processes, ranging from industrial petroleum processing to biological detoxification, to energy conversion and storage in natural and synthetic water oxidation catalysis. However, their highly reactive nature makes them extremely challenging to both prepare and isolate. My PhD thesis research presents an entirely new strategy for preparing stabilized metal-oxyl radicals, which relies on ancillary redox-active ligands to impart radical character to terminal oxo groups. This approach was applied to two new classes of complexes. First, the synthesis and reactivity of a new d0 oxorhenium complex, one oxidation level above Re(VII), [ReVII(O)2(apPh)(isqPh)] ([apPh]2- = 2,4-di-tert-butyl-6-(phenylamido) phenolate, [isqPh]•1- = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate), are described. This S=1/2 rhenium complex shows both closed- and open-shell O-atom transfer reactivity with small molecule substrates, including stable carbon radicals. Radical C–O coupling is indicative of the amidophenolate ligand imparting radical-like character to the oxo ligand. Mechanistic experiments were performed, and electronic structure-property relationships were developed to rationalize the observed reactivity. Extensions to new d0 oxovanadium complexes VVO(OCO)X (OCO=di-tert-butylphenolate N,N′-disubstituted imidazoline) complexes (X- = Cl-, OMe-, OBn-) led to the discovery of catalytic, aerobic alcohol oxidations that apparently result from intramolecular H-atom transfer to coordinated O2- derived ligands, as well as other oxidation reactions relevant to lignin degradation. Divergent reactivity based on ligand oxidation state is shown to impart selectivity in oxidation of C–O, C–C, or C–H bonds in lignin model compounds.