Electrocatalytic production of hydrogen peroxide enabled by post-synthetic modification of a self-assembled porphyrin cube

Abstract

Self-assembled metallacyles and cages formed via coordination chemistry have been used as catalysts to enforce 4H⁺/4e⁻ reduction of oxygen to water with an emphasis on attenuating the formation of hydrogen peroxide. That said, the kinetically favored 2H⁺/2e⁻ reduction to H₂O₂ is critically important to industry. In this work we report the synthesis, characterization, and electrochemical benchmarking of a hexa-porphyrin cube which catalyses the electrochemical reduction of molecular oxygen to hydrogen peroxide. An established sub-component self-assembly approach was used to synthesize the cubic free-base porphryin topologies from 2-pyridinecarboxaldehyde, tetra-4-aminophenylporphryin (TAPP), and Fe(OTf)₂ (OTf⁻ = trifluoromethansulfonate). Then, a tandem metalation/transmetallation was used to introduce Co(II) into the porphyrin faces of the cube, and exchange with the Fe(II) cations at the vertices, furnishing a tetrakaideca cobalt cage. Electron paramagnetic resonance studies on a Cu(II)/Fe(II) analogue probed radical interactions which inform on electronic structure. The efficacy and selectivity of the CoCo-cube as a catalyst for hydrogen peroxide generation was investigated using hydrodynamic voltammetry, revealing a higher selectivity than that of a mononuclear Co(II) porphyrin (83% versus ∼50%) with orders of magnitude enhancement in standard rate constant (k_s = 2.2 × 10² M⁻¹ s⁻¹versus k_s = 3 × 10⁰ M⁻¹ s⁻¹). This work expands the use of coordination-driven self-assembly beyond ORR to water by exploiting post-synthetic modification and structural control that is associated with this synthetic method.