Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Abstract

Thermal decarbonylation of the acyl compounds [Mn(CO)₅ (COR_F )] (R_F =CF₃ , CHF₂ , CH₂ CF₃ , CF₂ CH₃ ) yielded the corresponding alkyl derivatives [Mn(CO)₅ (R_F )], some of which have not been previously reported. The compounds were fully characterized by analytical and spectroscopic methods and by several single-crystal X-ray diffraction studies. The solution-phase IR characterization in the CO stretching region, with the assistance of DFT calculations, has allowed the assignment of several weak bands to vibrations of the [Mn(¹² CO)₄ (eq-¹³ CO)(R_F )] and [Mn(¹² CO)₄ (ax-¹³ CO)(R_F )] isotopomers and a ranking of the R_F donor power in the order CF₃ <CHF₂ <CH₂ CF₃ ≈CF₂ CH₃ . The homolytic Mn-R_F bond cleavage in [Mn(CO)₅ (R_F )] at various temperatures under saturation conditions with trapping of the generated R_F radicals by excess tris(trimethylsilyl)silane yielded activation parameters ΔH^≠ and ΔS^≠ that are believed to represent close estimates of the homolytic bond dissociation thermodynamic parameters. These values are in close agreement with those calculated in a recent DFT study (J. Organomet. Chem. 2018, 864, 12-18). The ability of these complexes to undergo homolytic Mn-R_F bond cleavage was further demonstrated by the observation that [Mn(CO)₅ (CF₃ )] (the compound with the strongest Mn-R_F bond) initiated the radical polymerization of vinylidene fluoride (CH₂ =CF₂ ) to produce poly(vinylidene fluoride) in good yields by either thermal (100 °C) or photochemical (UV or visible light) activation.