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, •CMe2OH, is acid-dependent, in accord with the greater reducibility of protonated pyridine N-oxides. The respective rate constants for the unprotonated (k R0) and protonated (k R1) pyridine N-oxides have the following values (L mol-1 s-1): 8.1 × 107 and 2.8 × 109 (X = 4-CN), 7.5 × 106 and 3.7 × 108 (4-Cl), 7.8 × 106 and 8.4 × 107 (H), and 1.7 × 106 and 4.1 × 107 (4-Me). The reduction of the Co(III) complexes (NH3)5Co(X-pyO)3+ by •CMe2OH is rapid (k = 107−108 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 •CMe2OH reduces the coordinated X-pyO followed by an intramolecular electron transfer to the cobalt center.