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.

Pyridine‐Containing Donor‐Acceptor Diarylnitroxides: Noncovalent Stabilization of the Redox States

“…This creates a precondition for a molecule to change the CCNO dihedral angle, i. e., to vary the impact of the electronic effects of the ring in stabilization of the redox state: to increase it if it is favorable or to minimize if it destabilizes the redox state (see Scheme 1). It really works, as it was shown in our recent publications [20–23] . The other possibility is the insertion of the “chameleonic” substituent [23] (which electronic properties can be changed via self‐rotation over the C−C σ bond) in the aromatic ring.…”

“…Comparison of the reduction behavior of 1 and 4 illustrates “chameleonic” nature of the para‐MCP group: the anionic and cationic species exist in different conformations [23] . Donating π‐interaction from the HOMO of the MCP (having a significant π‐character) to arene is switched off/on in the reduced/oxidized form, respectively, due to rotation over the C Ph ‐C MCP σ‐bond.…”

Diarylamines with the Neighboring Pyridyl Group: Synthesis and Modulation of the Amine Functionality via Intramolecular H-Bonding

“…Synthesis and characterization of the reference compounds can be found in our previous publication. 30,40 Pairwise comparison reveals dramatic difference in the (NH) as well as in the (NH) values within the corresponding couples of the amines providing a direct evidence for the strong intramolecular H-bonding between the amine's proton and the nitrogen lone pair of the pyridine fragment. Notably, the observed downfield shift is maximal in case of the 2-CF3-4't-Bu-substituted compounds among the couples compared.…”

Regio‐ and Chemoselectivity of Oxidative Conversion of Diarylamines to N,N’‐Diaryldihydrophenazines and N,N’‐Diarylbenzidines: DFT and Experimental Study

“…This result provides an additional support to the conformational behavior of the MCP group attached to the aromatic fragments, providing the optimal balance between π-stabilizing interactions (which are dependent on the charge state of the molecule) and sterical penalty. [30] Notably, no noticeable contacts between atoms of the neighboring molecules can be detected in the crystal unit.…”