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 (ks = 2.2 × 102 M−1 s−1versus ks = 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.