Probing Proton Coupled Electron Transfer and Fluorine Atom Transfer in Co^II-Catalyzed Oxidative Hydrocarbon...

In exploring Co^II-catalyzed oxidative fluorination, we have synthesized a family of Co^II complexes supported by linear tetradentate ligands. Nuclear magnetic resonance and infrared spectroscopies, mass spectrometry, and X-ray crystallography provided insights into the structural and electronic properties of the catalysts. All complexes displayed a cis-α geometry at Co, while displaying variable Co^II/III E_1/2 values. Certain catalysts delivered near quantitative yields of fluorinated product in the selectfluor-mediated fluorination of 1-adamantanol. The rate of fluorination was impacted by the nature of the supporting ligands, with more electron-rich Co sites displaying faster rates of fluorination, but lower yields of fluorinated product. Kinetic analysis suggested that proton coupled electron transfer (PCET) C-H activation was rate-limiting (substrate kinetic isotope effects = 2). We concluded that a Co^IV(F)₂ entity was the likely PCET oxidant producing an alkyl radical, HF, and Co^III-F. In order to explore how the alkyl radical and Co^III-F reacted through fluorine atom transfer (FAT), we probed a series of radical clock bicyclic substrates in the Co^II-catalyzed reaction. Product analysis supported the occurrence of a fast FAT rebound, consistent with the alkyl radical being in close proximity to the fluorine donor.