The Divergent Dimerization Behavior of N-Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals.

Herein, we demonstrate that the dimerization behavior of amine-substituted dicyanomethyl radicals can be switched from σ- to π-dimerization simply by varying the electron-donating substituents. For dicyanomethyl radicals with a 4,4'-ditolylamine (DT. ) or a phenothiazine (PT. ) substituent, the monomeric radical form and the corresponding dimer connected by a reversible C-C bond (σ-dimer) are in equilibrium in solution. On the other hand, the radical with the julolidine skeleton (JD. ) does not undergo σ-dimerization and was isolated as a stable radical in spite of the absence of bulky protecting groups. X-ray single-crystal analysis revealed that JD. forms the π-dimer in the crystalline state, and variable-temperature spectroscopy showed that JD. is in equilibrium with the π-dimer in toluene solution. DFT calculations point to the importance of electrostatic interactions as a driving force for the π-dimerization of JD. because of its polarized structure.