Green Routes to Fluorocarbons

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Fluorocarbons are important materials for a variety of applications. Small molecule hydrofluorocarbons (HFCs), for example, have a low ozone depletion potential and have replaced banned chlorofluorocarbons (CFCs) as refrigerants, blowing agents, solvents, etc. More recently, unsaturated HFCs are being marketed that have greatly diminished global warming potential relative to their saturated analogs. Industrial production of these molecules, however, includes use of environmentally problematic chlorocarbons and toxic metals such as antimony. The unique properties of fluoropolymers such as low surface energy, high thermal and chemical stability make them especially useful as thermoplastics, coatings, elastomer membranes and functional materials. Unfortunately, these materials are prepared by energy-intensive radical processes or emulsion polymerization that employs environmentally persistent fluorosurfactants. As a result, for important monomers such as vinylidene fluoride (CH2=CF2) and hexafluoropropene [CF2=CF(CF3)] there is no way to control the insertion regio- and stereoselectivity and thus the polymer microstructure. Moreover, important materials derived from copolymerization of these monomers with tetrafluoroethylene (CF2=CF2) also suffer from a lack of consistent properties resulting from the random nature of the currently employed polymerization methods. In our research program we aim to: 1) Prepare hydrofluoroalkenes (HFAs) from co-pyrolysis of inexpensive bulk polytetrafluoroethylene (PTFE) and difluoromethane, acetylene or syngas (CO/H2) at low pressures (< 20 torr) using a commercial catalytic pyrolysis unit. The carbon-containing gases are activated by the heterogeneous catalyst and then combine with gas phase CF2 generated by the PTFE pyrolysis. Solid bases such as MgO or hydrotalcite will be used to remove HF from CH2F2 to generate CHF carbene, metal catalysts will activate acetylene for CF2 addition or insertion into a C-H bond, and Fisher-Tropsch catalysts will be used to generate CHn units on the catalyst surface for reaction with the gas phase CF2 carbenes. Optimized reaction conditions will be used for scaled-up production of selected HFAs for lab use and subsequent technology transfer to industry partners. 2) Develop base metal complex-catalyzed reactions of HFAs, proceeding through metallacyclopentane intermediates, that will offer greener routes to technologically valuable HFCs. Our promising results with low-coordinate organofluoronickelacycles will be extended to include hemilabile N-heterocyclic carbene-thioether ligands for stabilization of the low-valent catalyst under H2 and bifunctional P-N and P-B ligands for heterolytic cleavage of dihydrogen and reversible C-F bond activation, respectively. Low-coordinate Fe complexes will also be evaluated to compare the d5,6 organofluorometallacycle reactivity with our d7,8 nickel examples, particularly with regard to formation and functionalization of mixed HC/FC metallacycles and development of tandem processes that make productive use of the HF product. 3) Investigate nucleophilic metal fluorocarbenes as initiators for HFA metathesis and polymerization via metallacyclobutane intermediates. Building on newly developed CF2 transfer to low-valent metals using Me3SiCF3 and catalytic NaI, we will synthesize a number of new nucleophilic base metal fluorocarbenes and investigate their cycloaddition reactivity with HFAs and subsequent fluoride abstraction. Target systems include XL2 pincer ligands and metal hydrofluorocarbenes. These fundamental studies will be accompanied by rapid throughput catalytic testing of tandem processes using the silyl reagent to deliver CFRF and CHRF carbenes to low-valent metal precursors and subsequent catalyzed insertion into C-H bonds.