Reversible dimerization of anion radicals: Studies of two cyanopyridines

The electrochemical reduction of 3-cyanopyridine, 1, and 2,6-dicyanopyridine, 2, has been studied in N,N-dimethylformamide. Each compound undergoes a one-electron reduction to form the corresponding anion radical and the anion radicals dimerize to form σ-dimer dianions that are oxidized at potentials considerably less negative than the original reduction process. At 298 K the dimerization rate constants are 4.1 × 104 and 6.45 × 104 M−1 s−1 (3.7 × 104 M−1 s−1 in drier solvent) for 1 and 2 respectively. For both 1 and 2, the dimerization is completely irreversible at 298 K but for 2 reversibility begins to appear at slow scan rates at higher temperatures as evidenced by the growth of an anodic peak due to oxidation of the anion radicals to the original neutral 2.