Square-Planar Cobalt(III) Pincer Complex

A series of square-planar cobalt­(II) complexes with pincer ligands {N­(CH₂CH₂P<i>t</i>Bu₂)₂}⁻ ({L₁^tBu}⁻), {N­(CH₂CH₂P<i>t</i>Bu₂)­(CHCHP<i>t</i>Bu₂)}⁻ ({L₂^tBu}⁻), and {N­(CHCHP<i>t</i>Bu₂)₂}⁻ ({L₃^tBu}⁻) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri<i>-tert</i>-butylphenoxy radical as hydrogen acceptor. [CoCl­{L_<i>n</i>^tBu}] (<i>n</i> = 1–3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl­(L₁^tBu)] and [CoCl­(L₂^tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt­(II) amine and imine complexes. In contrast, oxidation of [CoCl­{L₃^tBu}] with Ag⁺ enabled the isolation of thermally stable, square-planar cobalt­(III) complex [CoCl­{L₃^tBu}]⁺, which adopts an intermediate-spin (<i>S</i> = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry