Preparation and Spectroscopic, Magnetic, and Electrochemical Studies of Mono-/Biradical TEMPO Derivatives

A comparison set of mono-/biradical
TEMPO derivatives was prepared,
novel compounds were fully characterized, and their physicochemical
properties were determined. Cyclic voltammetry revealed reversible
redox behavior for all studied nitroxides. Moreover, the electron-withdrawing
substituents increased the oxidation potential of the respective nitroxides
in comparison to electron-donating groups. While EPR spectra of monoradicals
in dichloromethane at 295 K reveal the expected three-line signal,
spectra of biradicals show more complex features. DFT and MP2 calculations
indicate that the EPR splitting pattern of dinitroxide 7 could be explained by its interactions with solvent molecules. In
the solid state, mononitroxides 4 and 5 behave
as a Heisenberg antiferromagnetic chain, whereas dinitroxides 6–8 are almost isolated paramagnetic diradicals
coupled in an antiferromagnetic manner.