Stable Free Radicals. 13. A Reinvestigation of the Pyridinyl Diradical, 1,1'-T rimethy lenebis(4-(carbomethoxy )pyridinyl)

The absorption spectrum attributed to l,l-trimethylenebis(4-(carbomethoxy)pyridinyl) (3) includes major contributions from intramolecularly cyclized forms, “cyclomers”: cis-A-3 and trans-3. The broad, temperature-sensitive, and medium-sensitive absorption bands of the cis A and trans cyclomers arise from π-σ→π-σ* transitions. The 3 cyclomers are transformed into diradical metal ion complexes, 3Mⁿ⁺, by MC10₄, and the cyclomers are reformed by removal of the metal ion with donors. Diradical oxidation level species belong to the (3) system. The components are in equilibrium with one another and can be converted into the (3)Mⁿ⁺ complexes. Reduction of the bispyridinium ion 3²⁺ with Na(Hg) leads mainly to the trans cyclomer; electrochemical reduction in the presence of LiC10₄ or Mg(C10₄)₂ yields the diradical complexes, 3Mⁿ⁺, but with n-Bu₄NC10₄ it yields the trans cyclomer. An initial preponderance of another cyclomer, cis-B-3, is replaced by trans-3 (t_1/2, 1 s). Isopentane extraction of trans-3 yields a variable (usually 1:2) mixture of cis-A-3 and trans-3. The spectrum of a thin film of distilled (3) resembles that of the isopentane extract. Partial reduction of 3²⁺, either with l-methyl-4-(carbomethoxy)pyridinyl (4) or electrochemically, gives the radical cation 3⁺, which exhibits an intervalence transition at 1360 nm. The disproportionation constant, K = [3⁺]2/[3²⁺][3] = ~0-18, depends on medium, with low values found when 3-Mg²⁺ is present. The previously reported spectrum of 3⁺. must include a contribution from trans-3. A complete scheme is given which includes 3²⁺, 3⁺., and various (3) species: cyclomers, open diradicals, and diradical metal ion complexes.