Lowering the Symmetry of Cofacial Porphyrin Prisms for Selective Oxygen Reduction Electrocatalysis

Unsymmetric Co-Facial "Salixpyrrole" Hydrogen Evolution Catalysts: Two Metals are Better than One

Designing ligand architectures that can mimic enzyme active sites is a promising approach for developing efficient small molecule activation catalysts for sustainable energy applications. Some key design features include chemically distinct binding pockets for multiple metal centers and a three-dimensional structure that controls the positioning of catalytic sites. With these principles in mind, mono- and bimetallic unsymmetric cofacial palladium complexes, 2 and 3, respectively, bearing ligands with calixpyrrole and salen coordination sites, or "salixpyrrole" ligands, are reported. These species were accessed in a straightforward Schiff-base reaction with appreciable yields. In addition, both 2 and 3 were found to be active hydrogen evolution electrocatalysts using para-toluenesulfonic acid monohydrate as the proton source. The two salixpyrrole species displayed different mechanisms of action, with 2 showing a second-order dependence on acid concentration, whereas 3 exhibited a first-order dependence. Moreover, the bimetallic catalyst was significantly more efficient, with higher turnover frequencies, 4640 s-1 vs 1680 s-1 for 2, and lower overpotentials, 0.39 V vs 0.69 V for 2. The results reported herein provide proof-of-concept that bimetallic catalysts with chemically distinct binding sites demonstrate enhanced catalytic properties in comparison to monometallic or symmetric analogues.

Electrocatalytic Production of Hydrogen Peroxide Enabled by Post-Synthetic Modification of a Self-Assembled Porphyrin Cube

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 oxgyen 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-1s-1 versus ks = 3 × 100 M-1s-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.

The rigidity of self-assembled cofacial porphyrins influences selectivity and kinetics of oxygen reduction electrocatalysis

We report the electrocatalytic Oxygen Reduction Reaction on a rigid Co(II) porphyrin prism scaffold bridged by Ag(I) ions. The reactivity of this scaffold differs significantly from previous prism catalysts in that its selectivity is similar to that of monomer (∼35% H₂O) yet it displays sluggish kinetics, with an order of magnitude lower k_s of ∼0.5 M^-1 s^-1. The deleterious cofacial effect is not simply due to metal-metal separation, which is similar to our most selective prism catalysts. Instead we conclude the structural rigidity is responsible for these differences.