Stable Diradical to Pentaradical Cobalt-Dithiolene Complexes: Toward Cobalt-Based Multinuclear-Multiradical Complexes

The stabilization and isolation of multiple radical centers within inorganic molecular complexes remain one of the most demanding challenges in modern chemistry, particularly for first-row transition metals, where low-lying metal d orbitals readily engage in electron exchange with ligand-centered radicals. This interaction often quenches ligand radical character, perturbs metal oxidation states, and exacerbates structural instability. Here, we address these limitations by employing redox-active N-heterocyclic carbene (NHC)-based anionic dithiolene radical ligands, LE^•- (E = S, Se), enabling the isolation of four unprecedented cobalt-based multiradical complexes spanning mono- to penta-nuclear complexes. Controlled variation of LE^•-: CoCl₂ stoichiometry proves crucial for regulating nuclearity and incorporation of multiple radicals in the same complex. [Co(II)₄Cl₄(LS^•-)₄] (1) features an exceptionally rare μ₃-Cl-bridged Co₄Cl₄ tetraradical cubane, in which weakly coordinating, redox-innocent chloride bridges, unlike the strongly covalent O^2- or S^2- linkers in Co₄O₄/Co₄S₄ cubanes, impose minimal electronic stabilization, rendering the Co₄Cl₄ core intrinsically labile yet isolable. A mononuclear square-pyramidal low-spin Co(III) complex, [Co(III)(LSe^•-)₂(SSS-NHC═Se)] (2), features a rare trisulfide-NHC coordination motif. In contrast to the octahedral Co(III)-triradical complexes [Co(III)(LE^•-)₃] (3, 3a, E = S; 5, E = Se), complex 4, [Co(III)₃Co(II)₂(S₂)₄(LS^•-)₅] constitutes a mixed-valence Co(II)/Co(III)-pentaradical captivating system with a pentanuclear metal-disulfide core architecture. Single-crystal X-ray diffraction reveals near-ideal radical C-C distances (≈1.37-1.40 Å), confirming robust ligand-centered radical character. Comprehensive spectroscopic (IR, UV-vis, Raman, XPS, EPR) and DC magnetic studies establish the spin ground states and elucidate metal-radical and radical-radical interactions.