Thermodynamic Studies of Aerobic O-Atom Transfer at a Redox-Active Ligand Oxovanadium Complex
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Castillo, Mariah C.
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Selective oxidations of small molecules are a cornerstone of synthesis and energy conversion in natural and synthetic systems. The use of air as a source of an O-atom in oxygenase-type redox catalysis is attractive economically and environmentally. However, metals that can cleanly homolyze O2 to generate oxidizing oxo complexes are rare and limited to later, more electronegative metals. The tendency of early, electropositive transition metals to resist reduction introduces a barrier to O-atom transfer or oxygenase-type redox catalysts that rely on reduced metals to transfer the cleaved O2. We proposed that redox-active ligands might facilitate multielectron O-atom transfer in Group 3-5 metals. A five-coordinate chlorovanadium complex supported by redox-active N-phenyl amidophenolate ligands was prepared. Structural and computational data suggest that the formally vanadium(V) species is better formulated as [(Phap)(Phisq)VIVCl] with an iminoseminquinonate radical. Exposure of [(Phap)(Phisq)VIVCl] to O2 readily cleaves the O=O bond, affording [(Phisq)(Phibq)VIV(O)Cl], with a doubly oxidized iminobenzoquinone ligand. Accordingly, the two-electron O-atom addition occurs without a change in experimental oxidation state at the vanadium center. The thermodynamics of these O-atom transfers were studied through reactions with different substrates with known X–O bond dissociation enthalpies (BDEs). The V=O bond strength was estimated to be 73 ± 14 kcal/mol, which is significantly weakened relative to similar V(IV) and V(V) oxo complexes with redox-innocent ligands. This thesis presents details of the O-atom transfer scope and thermochemistry, as well as preliminary extensions to selective aerobic C-H oxidations.
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Undergraduate Research Option Thesis