C–F⋯M+ interaction in anionic α-fluorovinyl rhenium oxycarbene complexes and their β-fluoroenolate analogs

The C–F⋯M+ interaction in anionic σ-(α-fluorovinyl)rhenium oxycarbene complexes, [RCFCFReC(O)R′(CO)4]M (1–6), M = Na, Li, K is studied by 19F NMR in THF and Et2O. The coordination of α-F to M+ results in an upfield shift of the corresponding 19F NMR signal and a decrease of 1JCF. The maximum shift is found for the Li salt of complex 4 in Et2O (ΔδFα = 36.4 ppm), in which case a 7Li–19F spin–spin coupling is also observed (JLiF = 40 Hz). The ΔE of C–F⋯M+ interaction and its effect on 19F shielding was further studied by DFT calculations using β-fluoroenolates as models, which confirmed a strong impact of CF-bond environment on the coordination ability of fluorine in these F,O-chelates. A compound with a β-fluoroenolate backbone but without rhenium, o-(α-fluorovinyl)phenolate 12, was prepared and studied by 19F NMR, and similarly showed indications of C–F⋯M+ interaction in THF solution. It is concluded that the donor ability of fluorine in the studied system is enhanced because of the conjugation of α-fluorovinyl group with the enolate π-system and back donation from the transition metal.