Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives.

Abstract

Thermal decarbonylation of the acyl compounds [Mn(CO)5 (CORF )] (RF =CF3 , CHF2 , CH2 CF3 , CF2 CH3 ) yielded the corresponding alkyl derivatives [Mn(CO)5 (RF )], 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)(RF )] and [Mn(12 CO)4 (ax-13 CO)(RF )] isotopomers and a ranking of the RF donor power in the order CF3 <CHF2 <CH2 CF3 ≈CF2 CH3 . The homolytic Mn-RF bond cleavage in [Mn(CO)5 (RF )] at various temperatures under saturation conditions with trapping of the generated RF 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-RF bond cleavage was further demonstrated by the observation that [Mn(CO)5 (CF3 )] (the compound with the strongest Mn-RF 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.