Isolation\nof Elusive Fluoflavine Radicals in Two Differing\nOxidation States

Facile access and\nswitchability between multiple oxidation states\nare key properties of many catalytic applications and spintronic devices\nyet poorly understood due to inherent complications arising from isolating\na redox system in multiple oxidation states without drastic structural\nchanges. Here, we present the first isolable, free fluoflavine (flv)\nradical flv(<sup>1–•</sup>) as a bottleable potassium\ncompound, [K(crypt-222)](flv<sup>•</sup>), <b>1</b>,\nand a new series of organometallic rare earth complexes [(Cp*<sub>2</sub>Y)<sub>2</sub>(μ-flv<sup>z</sup>)]X, (where Cp* = pentamethylcyclopentadienyl,\nX = [Al(OC{CF<sub>3</sub>}<sub>3</sub>)<sub>4</sub>]<sup>−</sup> (z = −1), <b>2</b>; X = 0 (z = −2), <b>3</b>; [K(crypt-222)]<sup>+</sup> (z = −3), <b>4</b>) comprising\nthe flv ligand in three different oxidation states, two of which are\nparamagnetic flv<sup>1–•</sup> and flv<sup>3–•</sup>. Excitingly, <b>1</b>, <b>2</b>, and <b>4</b> constitute\nthe first isolable flv<sup>1–•</sup> and flv<sup>3–•</sup> radical complexes and, to date, the only isolated flv radicals of\nany oxidation state. All compounds are accessible in good crystalline\nyields and were unambiguously characterized via single-crystal X-ray\ndiffraction analysis, cyclic voltammetry, IR-, UV–vis, and\nvariable-temperature EPR spectroscopy. Remarkably, the EPR spectra\nfor <b>1</b>, <b>2</b>, and <b>4</b> are distinct\nand a testament to stronger spin delocalization onto the metal centers\nas a function of higher charge on the flv radical. In-depth analysis\nof the electron- and spin density via density functional theory (DFT)\ncalculations utilizing NLMO, QTAIM, and spin density topology analysis\nconfirmed the fundamental interplay of metal coordination, ligand\noxidation state, aromaticity, covalency, and spin density transfer,\nwhich may serve as blueprints for the development of future spintronic\ndevices, single-molecule magnets, and quantum information science\nat large.