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$^{•–}$: CoCl$_2$ stoichiometry proves crucial for regulating nuclearity and incorporation of multiple radicals in the same complex. [Co(II)$_4$Cl$_4$(LS$^{•–}$)$_4$] (1) features an exceptionally rare μ$_3$-Cl-bridged Co$_4$Cl$_4$ tetraradical cubane, in which weakly coordinating, redox-innocent chloride bridges, unlike the strongly covalent O$^{2–}$ or S$^{2–}$ linkers in Co$_4$O$_4$/Co$_4$S$_4$ cubanes, impose minimal electronic stabilization, rendering the Co$_4$Cl$_4$ core intrinsically labile yet isolable. ... mehrA 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)$_3$Co(II)$_2$(S$_2$)$_4$(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.