Square-Planar Cobalt(III) Pincer Complex

A series of square-planar cobalt­(II) complexes with pincer ligands {N­(CH2CH2PtBu2)2}− ({L1tBu}−), {N­(CH2CH2PtBu2)­(CHCHPtBu2)}− ({L2tBu}−), and {N­(CHCHPtBu2)2}− ({L3tBu}−) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri-tert-butylphenoxy radical as hydrogen acceptor. [CoCl­{LntBu}] (n = 1–3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl­(L1tBu)] and [CoCl­(L2tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt­(II) amine and imine complexes. In contrast, oxidation of [CoCl­{L3tBu}] with Ag+ enabled the isolation of thermally stable, square-planar cobalt­(III) complex [CoCl­{L3tBu}]+, which adopts an intermediate-spin (S = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry.