Studies of Iron Polyfluorometallacycle Complexes Enroute to the "Green" Catalytic Synthesis of Hydrofluorocarbons

Abstract

Chlorofluorocarbons were once commonly used as propellants and refrigerants due to their stability, low toxicity and excellent physical properties. They were phased out, however, after being identified as ozone-depleters, and have been replaced by hydrofluorocarbons (HFCs) and other fluorocarbon derivatives (FCDs). The high temperature-resistant characteristic of these materials is beneficial for many applications; however, the robust nature of the C-F bonds leads to their persistence in the environment. Frequently, the synthesis of HFCs and FCDs involves energy-intensive processes and toxic precursors, such as chlorocarbons and heavy metals. In accordance with the shift to greener, more sustainable chemistry, more energy efficient methods employing less hazardous and non-toxic materials for the generation of HFCs and FCDs need to be developed. Organometallic catalysis offers potential new routes to the synthesis of fluorinated compounds. We sought to study the reactivity of tetrafluoroethylene and 1,1-difluoroethylene with a variety of iron complexes to increase our understanding of the bonding and chemistry of coordinated fluoro-olefins. Crystal structures were obtained for the parent and dppe substituted metallaycles: Fe(CF2) 4(CO)4 and Fe(CF2)4(K2-dppe)(CO) 2. Sterics were found to be responsible for the preferential formation of three- versus five-membered metallacycles through investigating the reactivity of TFE with iron phosphine- and phosphite-carbonyl complexes, as well as with the homoleptic complex pentakis(2,6-dimethylphenylisocyanide)iron and the anionic complex K[FeCp(CO)2]. The five-membered metallacycle was selectively formed with the mono-substituted phosphine and phosphite complexes, whereas, the di-substituted complexes yield the olefin (three-membered metallacycle) complexes. With the goal of developing methods for iron-mediated fluoride abstraction, the reactivity of these iron fluoro-metallacycles with a variety of Lewis acids was probed. Extremely electrophillic fluoro-carbenes were generated. A more electron rich metal centre, as in the thermally synthesized K[FeCp(CF 2)4(CO)2], is required to facilitate fluoride abstraction and stabilize the fluor-ocarbene. The results that were obtained in this investigation set the stage well for further development of iron-based organofluorometallic chemistry and our desire to functionalize the alpha-carbon of the metallacycle enroute to the generation of novel FCs.