Isolation of Elusive Fluoflavine Radicals in Two Differing Oxidation States

Facile access and switchability between multiple oxidation states are key properties of many catalytic applications and spintronic devices yet poorly understood due to inherent complications arising from isolating a redox system in multiple oxidation states without drastic structural changes. Here, we present the first isolable, free fluoflavine (flv) radical flv(^1-•) as a bottleable potassium compound, [K(crypt-222)](flv^•), 1, and a new series of organometallic rare earth complexes [(Cp*₂Y)₂(μ-flv^z)]X, (where Cp* = pentamethylcyclopentadienyl, X = [Al(OC{CF₃}₃)₄]^- (z = -1), 2; X = 0 (z = -2), 3; [K(crypt-222)]⁺ (z = -3), 4) comprising the flv ligand in three different oxidation states, two of which are paramagnetic flv^1-• and flv^3-•. Excitingly, 1, 2, and 4 constitute the first isolable flv^1-• and flv^3-• radical complexes and, to date, the only isolated flv radicals of any oxidation state. All compounds are accessible in good crystalline yields and were unambiguously characterized via single-crystal X-ray diffraction analysis, cyclic voltammetry, IR-, UV-vis, and variable-temperature EPR spectroscopy. Remarkably, the EPR spectra for 1, 2, and 4 are distinct and a testament to stronger spin delocalization onto the metal centers as a function of higher charge on the flv radical. In-depth analysis of the electron- and spin density via density functional theory (DFT) calculations utilizing NLMO, QTAIM, and spin density topology analysis confirmed the fundamental interplay of metal coordination, ligand oxidation state, aromaticity, covalency, and spin density transfer, which may serve as blueprints for the development of future spintronic devices, single-molecule magnets, and quantum information science at large.