Generation and reactivity of 2,4,6-triphenylpyridinyl radicals linked by alkyl chains

A series of bis(2,4,6-triphenylpyridinyl) diradicals 1-6 linked by alkanediyl [1 for n (number of methylene) = 2, 2 for n = 3, 3 for n = 4, 4 for n = 5, S for n = 8, 6 for trans-1,4-cyclohexanediyl] were prepared by reduction of the corresponding bispyridinium salts with 3%-sodium amalgam. The diradicals 1–5 were moderately stable at room temperature under inert atmosphere. Although the diradical 6 was unstable at room temperature and underwent a facile C-N bond cleavage, the diradical persisted when the reduction of the bispyridinium salt was carried out at −30 °C under inert atmosphere. The triplet states for all these diradicals were observed and the exchange interaction was studied by temperature dependence of the ESR signal intensity for 1–3 and 6. The results of the exchange interaction are summarized as follows: 1 in singlet ground state with ΔES-T > 1.7 kJ/mol, 2, 3 in the singlet ground state with ΔES-T < 230 J/mol, and 6 in the triplet ground state or in a degenerate singlet state.