A (pentafluoroethyl)(trifluoromethyl)carbene complex of iridium and reductive activation of its sp3 α, β, and γ carbon–fluorine bonds to give perfluoro-2-butyne, perfluoro-1,2,3-butatriene and perfluoro-1-irida-2-methyl-2-cyclobutene) complexes

Abstract

The (pentafluoroethyl)(trifluoromethyl)carbene complex Cp*Ir(CO)[C(CF₃)(C₂F₅)] was synthesized by the reductive activation of the α-C–F bond in the perfluoro-sec-butyl-iridium complex Cp*Ir(CO)[CF(CF₃)(C₂F₅)](I) with Na/Pb alloy. This compound exists as two geometric isomers in solution; the structure of one isomer has been determined by a single crystal X-ray diffraction study and contains two independent molecules in the asymmetric unit. Further reduction of this carbene complex with Na/Pb alloy afforded the perfluoro-2-butyne iridium complex Cp*Ir(CO)(η²-CF₃CCCF₃) by an overall 2-electron reduction and elimination of two β-fluorides. When magnesium graphite was utilized as the reducing agent for the further reduction, Cp*Ir(CO)(η²-CF₃CCCF₃) was produced as a minor product and the major product was the perfluoroiridacyclobutene complex Cp*Ir(CO)(η^2,4-CF₃CCFCF₂) resulting from a γ-C–F bond activation. Direct reduction of the precursor Cp*Ir(CO)[CF(CF₃)(C₂F₅)](I) with magnesium graphite generated the tetrafluorobutatriene iridium complex Cp*Ir(CO)(η^2,3-CF₂CCCF₂) along with the perfluoro-2-butyne complex and perfluoroiridacyclobutene complexes in a ratio of 1 : 2 : 6. These reductive inner-sphere pathways to unsaturated fluorocarbon ligands illustrate that carbon–fluorine bond activation can take place at α-, β- and γ-carbons but that selectivity in these heterogeneous reductions is difficult to control, with a variety of fluoride eliminations possible for complex perfluoroalkyl ligands. Density Functional Theory (DFT/B3LYP-D3/LACV3P**++) is used to explore the relative energetics of products and intermediates in these reactions.