Reversible dimerization of the anion radicals of some dicyanonaphthalenes

The electrochemical reduction of 7 of the 10 isomeric dicyanonaphthalenes has been studied in N,N-dimethylformamide. The studied compounds were 1,2-, 1,3-, 1,4-, 2,3-, 1,8-, 2,6- and 2,7-dicyanonaphthalene. For most of the isomers, the one-electron reduction to the anion radical occurred as a simple, reversible reduction without complications from coupled chemical reactions. However, for 1,3-, 2,3- and 2,7-dicyanonaphthalene, the reduction was affected by a reversible dimerization of the anion radicals. Dimerization equilibrium constants were determined at various temperatures. DFT calculations of the anion radicals of all 10 isomers provided spin densities at ring carbons 1–8. The highest spin density for carbons 1–8 not bearing a cyano group was largest for the three isomers with detectable dimerization. Not only do these calculations suggest the site of dimerization, it was found that the free energy change for dimerization varies in an approximately linear fashion with spin density.