Chameleonic Behavior of the α-Methylcyclopropyl Group and Its Through-Space Interactions: A Route to Stabilized Three Redox States in Diarylnitroxides.

ABSTRACT

The α-methylcyclopropyl (MCP) group with conformationally dependent electronic properties is suggested as an additional structural "instrument" for stabilization of both open-shell and ionic states of diarylnitroxides, to be used as "ambipolar" redox active materials. New MCP-substituted diphenylnitroxides (fully characterized by electrochemical, spectral, and X-ray data) are the most stable nitroxides of the diaryl type known to date [τ1/2 in benzene exceeds three months (2310 h)]. The radicals are capable to reversible oxidation and reduction, yielding stable oxoammonium cations and aminoxyl anions. DFT investigation of the electronic structure and geometry of the compounds confirmed the conformational switching of the cyclopropyl orientation relative to the adjacent aromatic π system is dependent on the nitroxide's redox state. Additional through-space stabilizing interaction between the π-acceptor orbital of the NO+ moiety and the cyclopropyl "banana" bond orbital was also detected, highlighting its good hyperconjugative ability. The estimated σ(para)MCP value (-0.32) confirms its strong electron-donating properties.