ELECTRON-TRANSFER FROM ALIPHATIC RADICALS TO RING-SUBSTITUTED PYRIDINE N-OXIDES AND THEIR COBALT-AMMINE COMPLEXES IN AQUEOUS-SOLUTION

The reduction of pyridine N-oxides by 1-hydroxy-1-methylethyl radicals ,(.)CMe(2)OH, is acid-dependent, in accord with the greater reducibili ty of protonated pyridine N-oxides. The respective rate constants for the unprotonated (k(RO)) and protonated (k(R1)) pyridine N-oxides have the following values (L mol(-1) s(-1)): 8.1 x 10(7) and 2.8 x 10(9) ( X=4-CN), 7.5 x 10(6) and 3.7 x 10(8) (4-Cl), 7.8 x 10(6) and 8.4 x 10( 7) (H), and 1.7 x 10(6) and 4.1 x 10(7) (4-Me). The reduction of the C o(III) complexes (NH3)(5)Co(X-pyO)(3+) by (.)CMe(2)OH is rapid (k=10(7 )-10(8) L mol(-1) s(-1)) and acid-independent. On the basis of linear free energy relationships it is concluded that the reduction of the Co (III) complexes takes place by a chemical mechanism, whereby (.)CMe(2) OH reduces the coordinated X-pyO followed by an intramolecular electro n transfer to the cobalt center.