The direct, Ag+- and Co2+-catalyzed oxidations by Ce(IV) of pentaammine(4-pyridinemethanol)cobalt(III) (A) or pentaammine(3-pyridinemethanol)coball(III) (B) yield both the aldehyde complexes of Co(III) and [Co2+ + free aldehyde], the ratio of Co(III)-CHO/Co(II) in the products increasing with the concentration of the oxidizing agent. These observations, together with the fact that the rate of consumption of A or B is strictly first order in the concentration of the external oxidizing agent, show that an oxidized intermediate is produced in each system by the external oxidant. The kinetic isotope effects obtained for the CD2OH derivative of A (kh/kd = 2.1 ± 0.2 and 2.2 ± 0.2 for the Ce(IV) and Co3+ oxidations in 4 M HCIO4, respectively) indicate that the rate-determining step involves attack on the alcohol function. A detailed examination of the product ratio as a function of the concentration of external oxidant shows that, in fact, two intermediates are formed which differ in the rate of internal reduction yielding Co(II), relative to reaction with external oxidant yielding Co(III)-CHO (rate ratio KRKT). The ratio Kr/Kt is observed to be significantly greater for the radicals derived from A than for the radicals derived from B, showing that internal electron transfer is more facile from the 4 than from the 3 position. For A, the ratio in which the intermediates are formed varies as the oxidizing agent is changed; this observation suggests that the intermediates are generated by parallel processes. The rates of oxidation by Ce(IV) of A and B and the free but protonated ligands are similar, as are the kinetic isotope effects observed for the -CD2OH derivative of A and the analogous protonated free ligand, supporting the view that the Co(III) center of the complex does not assist the external oxidant, and the conclusion that radical intermediates of finite lifetime are generated in the rate-determining step of the reaction. © 1969, American Chemical Society. All rights reserved.
