Radical Relatives: Facile Oxidation of Hetero-Diarylmethene Anions to Neutral Radicals

Furan and thiophene diarylmethenes are potential redox-active ligands for metal centers that could be exploited in the development of non-traditional, stoichiometric, and catalytic redox reactions. As such, we describe here the selective meso-deprotonations of the dithiophene, difuran, and diimine-difuran diarylmethanes to form the -conjugated anions, for which only the diimino-difuryl anion is truly isolable and studied by X-ray crystallography. In all cases, facile one-electron oxidation of these anions occurs which allows the isolation of the neutral dithienyl and diimino-difuryl radicals. UV-visible and TD-DFT studies reveal that the oxidation of the dithienyl anion to its radical is associated with an increase in the H(S)OMO-LUMO gap, evident through a hypsochromic shift of the main absorption band in the electronic spectrum, whereas oxidation of the diimino-difuryl anion causes only minor spectroscopic changes. Electrochemical studies support the stability of the radicals with respect to the anion, showing strongly negative oxidation potentials. The control of the redox activity of these diarylmethene carbanions through variation of the nature of the substituents, donor-atom, and the conjugated pi-system and their potential as ligands for redox-inert metal centers makes them intriguing candidates as non-innocent partners for redox reactions.