On the nature of halogen atom transfer reactions of Re(CO)4L radicals

Abstract

Rhenium carbonyl radicals Re(CO)/sub 4/L (L = P(CH/sub 3/)/sub 3/, P(O-i-C/sub 3/H/sub 7/)/sub 3/), generated by N/sub 2/ laser flash photolysis, react with organic halides in a bimolecular atom transfer reaction to form XRe(CO)/sub 4/L (X = Cl, Br, I). Rates constants for reactions with 18 organic halogen atom donors were measured in toluene at 22 /sup 0/C. The rate constants do not correlate well with available thermochemical C-X bond energies. Rate constants for bromine atom transfer for a series of benzyl bromides with Re(CO)/sub 4/P(O-i-C/sub 3/H/sub 7/)/sub 3/ obey a Hammett equation correlation, with rho = 0.75. The data are well-fitted by the Marcus/Agmon-Levine equation for atom or electron transfer, when the half-wave reduction potentials for the organic halides are employed as a measure of relative overall free energy change in the rate-determining process. The data are similarly well accommodated by the Rehm-Weller equation. The stimulated intrinsic barriers, ..delta..G/sup double dagger/ (0), are 3.5 kcal mol/sup -1/ for L = P(CH/sub 3/)/sub 3/ and 5.0 for L = P(O-i-C/sub 3/H/sub 7/)/sub 3/. The experimental results provide the first extensive test of the Marcus/Agmon-Levine or Rehm-Weller equation for halogen atom transfer. They suggest that atom transfer processes could be accommodatedmore » within such a theoretical framework when variations in electronic or steric properties are not so large as to markedly alter the equilibrium constant for formation of the encounter complex or the magnitude of the intrinsic barrier. They also provide an indication that electron transfer is a significant component in the reaction process and that configurational changes (C-X bond stretch and X-Re bond formation) in attaining the transition state are not large, or largely compensating.« less