Oxygen-cobalt chemistry using a porphyrinogen platform

Abstract

The tetraethylammonium salt of the Co(II)porphyrinogen complex, [Et₄N_]2[LCo(II)], 1, (L = tetrakis(cyclohexyl)porphyrinogen tetraanion) is oxidized by atmospheric oxygen to form [Et₄N][LCo(III)], 2, in contrast to the ligand oxidation of Co(II)porphyrinogen, [Li(THF)₂]₂[L'Co(II)] (L' = octaethylporphyrinogen tetraanion) with pure oxygen (Angelis, S. D.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1994, 116, 5702). Substitution of the η 4-bound lithium cation ([Li(THF)₂]⁺) on the periphery of the pyrollyl ring of [Li(THF)₂]₂[L'Co(II)] with a tetraethylammonium cation ([Et₄N]⁺) resulted in a metal-centered redox reaction analogous to that of 1 with oxygen, and thus demonstrated that the difference in reactivity is due to the change in the countercation rather than the substitution of the peripheral ligand. Reaction of [Et4N][LCo(III)], 2, with elemental iodine produced [LΔ Δ Co(II)-I](I₃)(I₂), 3. This reaction is viewed as an iodine-induced electron transfer reaction across Co(III)-porphyrinogen, in which the four electrons that originated from the oxidation of the internal reductant (porphyrinogen) to porphodimethene are shared by the internal oxidant (Co(III)), and the external oxidant (iodine), resulting in the reduction to Co(II) and iodide, respectively. Complexes 2 and 3 were characterized by spectroscopic studies, cyclic voltammetry (CV), magnetic moment measurements, and by single-crystal X-ray diffraction studies. The structural properties of 2 were analyzed based on the density functional theoretical (DFT) framework.