Fluorocarbene, Fluoroalkyl, and Fluoride Complexes of First-Row Transition Metals

Abstract

Fluorinated organic compounds play important roles in our society, as these products range from life-saving pharmaceuticals and agrochemicals, to fluoropolymers with extremely high thermal and chemical stability. Although elemental fluorine (F2) is the most reactive element, some fluoro-organic compounds are chemically inert. As such, controlled reactivity of fluorine or highly-fluorinated organic fragments is a considerable, yet important challenge for synthetic chemists.

Fluoro-organometallic chemistry has been studied for decades, as researchers attempt to maximize the potential of metal mediated/catalyzed processes for the synthesis of fluorinated organic molecules. Within this framework, metal fluorocarbene complexes are particularly interesting because of their highly tunable reactivity, and are proposed for use in important metathesis/polymerization reactions of perfluorinated alkenes. While considerable work is still needed to make these proposed reactions a reality, this thesis outlines contributions from our research group. We showed that cobalt fluorocarbene complexes CpCo(=CFRF)(PPh2Me) (RF = F, CF3) undergo [2+2] cycloaddition reactions with tetrafluoroethylene (TFE) and phenylacetylene to form perfluorometallacyclobutane and partially fluorinated metallacyclobutene products, respectively. For both reactions, computational studies reveal a stepwise ring-closing mechanism, which proceeds through a singlet 1,4-diradical intermediate.

Next, the formation of CpCo(=CF2)(L) complexes is achieved via the direct addition of difluorocarbene, generated in situ, to a cobalt(I) precursor. Subsequent addition of CF2 to cobalt fluorocarbene complexes results in [2+1] cycloaddition and formation of perfluorinated alkene complexes. The [2+1] addition is highly favored as the cobalt fluorocarbenes readily react with electrophilic CF2. A series of experiments provide evidence for the stepwise nature of fluoroalkene complex formation.

From Co(I) fluorocarbene complexes, the focus shifts to preparing metal fluorocarbenes with electrophilic-type reactivity. The synthesis of bis(perfluoroalkyl) complexes serve as precursors for preparation of perfluoroalkyl cobalt(III) fluorocarbenes, which undergo migratory insertion reactions of the fluorocarbene into the perfluoroalkyl ligand. Using a similar synthetic approach, nickel(II) and palladium(II) difluorocarbene complexes are prepared from their corresponding trifluoromethyl precursors.

The synthesis, characterization and reactivity of cobalt(III) fluoride complexes is also described, including the catalytic fluorination of acyl chlorides, demonstrating the first example of a cobalt(III) catalyzed fluorination reaction. The effects of the various ancillary ligands on these cobalt catalysts are investigated using high-throughput experimentation technology, and the scope of the reaction is expanded to include the synthesis of a variety of acyl fluoride compounds.

Finally, the results and learnings from this work will be summarized and highlighted. The future directions and novel research which could result from the continuation of these projects is discussed, with an emphasis placed on the areas believed to have the highest potential impact.