An ESR study of dynamic biradicals of two TEMPOs bridged with –(SiMe2)n– (n=1–4) in liquid solution

Biradicals composed of two TEMPOs (2,2,6,6-tetramethylpiperidinol-oxyl) bridged with –(SiMe2)n– (n=1–4) to each other were synthesized and subjected to an ESR study. A quintet spectrum of an isotropic 14N-hyperfine splitting of 21.6 MHz (g=2.0070) was observed for biradical 1 (n=1) and a triplet of 43.3 MHz (g=2.0072) for biradical 4 (n=4) in 3-methylpentane at 190 K. The Si-linkage was found to effectively enhance the spin–spin exchange interaction compared with the corresponding carbon fragments. The reversible temperature dependence of the spectra was analyzed based on a model of a modulated spin exchange interaction due to a tumbling motion. The apparent activation energies for the intramolecular dynamics were found to be 5.1–10.6 kJ mol−1 in 3-methylpentane and 2-methyltetrahydrofuran solutions.