Square-Planar Cobalt(III) Pincer Complex.

A series of square-planar cobalt(II) complexes with pincer ligands {N(CH2CH2PtBu2)2}(-) ({L1(tBu)}(-)), {N(CH2CH2PtBu2)(CHCHPtBu2)}(-) ({L2(tBu)}(-)), and {N(CHCHPtBu2)2}(-) ({L3(tBu)}(-)) 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{Ln(tBu)}] (n = 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 (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.