l-Alkyl-2-(carbomethoxy)pyridinyl Radicals: Monomers and Dimers Defined through Chemical and Photochemical Properties and Electron Paramagnetic Resonance

l-Methyl-2-(carbomethoxy)pyridinium ion (2⁺) is reduced to the radical dimer (2-2) which is in equilibrium with a small amount of l-methyl-2-(carbomethoxy)pyridinyl radical (2). Distillation of the dimer leads to thin films (at 77 K) of the radical 2., which dimerizes in solution. Chemical properties and EPR spectra of dimer solutions reflect the presence of the monomeric radical in equilibrium with the dimer; 2⁺ polarography also indicates a monomer-dimer equilibrium. Laser pulse dissociation of the dimer in solution gives the monomer reversibly, with k_assoc of ca. 10⁶ M^-1 s^-1. The dimerization constant, K_a(titration method), is 4 X 10⁵ M^-1; electrochemical techniques lead to a K_a value of at least 7 X 10⁴ M^-1, a k_assoc value of ca. 1-4 X 10⁴ M^-1 s^-1, and a k_disooc value of 1 s^-1. The activation energy for dimer dissociation (from EPR spectral broadening) is 7.7 kcal/mol (CH₃CN) or 9.4 kcal/mol (2-MTHF). The 1-methyl-2-acetylpyridinyl radical [(CH₃CO)₂.] is easily prepared and distilled and has a stability similar to that of the 1 -methyl-4-(carbomethoxy)pyridinyl radical (4.). EPR spectra of the 2-acetyl and 2-(carbomethoxy)pyridinyls are presented and analyzed. A convenient electrochemical cell for the generation and analysis of air-sensitive one-electron reduced species (such as the pyridinyl radicals or unstable pyridinyl radical dimers) is described.