Oxygen-cobalt chemistry using a porphyrinogen platform

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Redox Chemistry

Background:

• Cobalt porphyrinogen complexes exhibit diverse reactivity with oxygen.
• The role of countercations in modulating redox behavior is not fully understood.
• Previous studies indicated ligand oxidation for lithium-substituted cobalt porphyrinogen.

Purpose of the Study:

• To investigate the influence of the tetraethylammonium countercation on the redox reaction of a Co(II)porphyrinogen complex with atmospheric oxygen.
• To compare the reactivity of tetraethylammonium and lithium countercations in cobalt porphyrinogen complexes.
• To elucidate the mechanism of iodine-induced electron transfer in a Co(III)-porphyrinogen complex.

Main Methods:

• Synthesis and characterization of tetraethylammonium cobalt porphyrinogen complexes.
• Oxidation reactions with atmospheric oxygen and elemental iodine.
• Spectroscopic studies (NMR, UV-Vis), cyclic voltammetry (CV), magnetic moment measurements.
• Single-crystal X-ray diffraction and Density Functional Theory (DFT) analysis.

Main Results:

• The tetraethylammonium salt ([Et(4)N](2)[LCo(II)]) undergoes metal-centered oxidation to [Et(4)N][LCo(III)] upon exposure to oxygen.
• This contrasts with ligand oxidation observed for the lithium salt ([Li(THF)(2)](2)[L'Co(II)]).
• Substitution of lithium with tetraethylammonium demonstrates the countercation's critical role in directing the reaction pathway.
• Reaction of [Et(4)N][LCo(III)] with iodine results in an iodine-induced electron transfer, forming [L(DeltaDelta)Co(II)-I](I(3))(I(2)).

Conclusions:

• The tetraethylammonium countercation promotes metal-centered redox reactions in cobalt porphyrinogen complexes.
• The observed reactivity difference is attributed to the countercation, not peripheral ligand substitution.
• The study provides insights into electron transfer mechanisms in cobalt porphyrinogen systems.