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

Abstract

Furan and thiophene diarylmethenes are potential redox-active ligands for metal centers that could be exploited in the development of nontraditional, stoichiometric, and catalytic redox reactions. As such, we describe here the selective meso-deprotonations of dithiophene, difuran, and diimine-difuran diarylmethanes to form the π-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 time-dependent density functional theory studies reveal that the oxidation of the dithienyl anion to its radical is associated with an increase in the highest (singly) occupied molecular orbital-lowest unoccupied molecular orbital 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 π-system and their potential as ligands for redox-inert metal centers makes them intriguing candidates as noninnocent partners for redox reactions.

Synthesis and complexes of a constrained-cavity Schiff-base dipyrrin macrocycle

“…The energies of the transition states are over estimated, as seen with previously reported hetero-diarylmethene ligands and their complexes. 43 The electronic density of both the HOMO and the LUMO of HL and ZnBr(L) comprise π orbitals that are centred on the dipyrrin unit and the imines, highlighting the extended conjugation in these molecules. The dominant absorption band in HL at 277 nm is assigned as a mixture of HOMO to LUMO+3 (41%) and HOMO to LUMO+1 (13%) transitions, with the HOMO-LUMO transition appearing at 512 nm.…”