Rates of halogen atom transfer to manganese carbonyl radicals

Article abstract of DOI:10.1021/om00120a008

Flash photogeneration of Mn(CO)₄L· radicals (L = CO, L) from 1,2-diax-Mn₂(CO)₈L₂ in the presence of organic halides, or physical mixing of Mn(CO)₃L₂· (L = PR₃) and organic halide solutions, produces Mn(CO)_5-yL_yX as the only product. Bimolecular atom transfer rate constants, k_T, were measured for several radical/halide pairs by monitoring the decay of the visible or infrared absorbance of the radical. The rate constants vary over a range of 10¹⁰. Both the phosphine and organic halide affect the magnitude of k_T. The rate of halogen atom transfer is accelerated by increased electron density at the metal center and is impeded by phosphine ligand bulk, as measured by the cone angle. The rate constants for reaction of CCl₄ with Mn(CO)₃[P(i-Bu)₃]₂ or Mn(CO)₃[P(i-Pr)₃]₂ are smaller than for reaction with Mn(CO)₄P(Z-Bu)₃ or Mn(CO)₄P(i-Pr)₃, respectively. For a given halogen X, the rate constant is faster for C-X bonds of lower bond dissociation energy.