Square-Planar Cobalt(III) Pincer Complex

Abstract

A series of square-planar cobalt(II) complexes with pincer ligands {N(CH2CH2PtBu2)(2)}(-) ({L-1(tBu)}(-)), {N(CH2CH2PtBu2)(CHCHPtBu2)}- ({L-2(tBu)}(-)), and {N(CHCHPtBu2)(2)}(-) ({L-3(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{L-n(tBu)}] (n = 1-3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl(L-1(tBu))] and [CoCl(L-2(tBu))] 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{L-3(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.