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.

本研究制备了单/双自由基TEMPO衍生物的对照数据集，对新型化合物进行了全面表征，并测定了其理化性质。循环伏安法测试显示，所有受试氮氧自由基均表现出可逆的氧化还原行为。相较于给电子基团，吸电子取代基可提升对应氮氧自由基的氧化电位。在295K的二氯甲烷溶剂中，单自由基的电子顺磁共振（EPR）谱呈现预期的三线信号，而双自由基的谱图则展现出更为复杂的特征。密度泛函理论（DFT）与MP2计算表明，二氮氧自由基7的电子顺磁共振分裂模式可通过其与溶剂分子的相互作用得到合理解释。在固态状态下，单氮氧自由基4与5表现为海森堡反铁磁链，而双氮氧自由基6–8则为近乎孤立的、以反铁磁方式耦合的顺磁性双自由基。