Post-synthetic modification of bis-iron(iii)-μ-oxo-porphyrin prisms to enhance oxygen reduction electrocatalysis

Abstract

Bis-iron(III)-μ-oxo porphyrins are known electrocatalysts for Oxygen Reduction Reaction (ORR) that favor four-electron four-proton chemistry. The facile formation of the μ-oxo motif makes forming cofacial structures straightforward unlike for other metalloporphyrins where the face-to-face geometry must be enforced by tethering groups. We have pioneered the use of coordination chemistry to obtain cofacial porphyrin ORR catalysts containing Co(II) ions. Here, we adapt our use of molecular clips for the post-synthetic modification of μ-oxo porphyrin dimers rather than as self-assembly building blocks. Although the cofacial geometry is inherent to the μ-oxo core, under ORR catalysis conditions, the dimer is rapidly cleaved, resulting in reactivity differences between traditional unclipped bis-iron(III)-μ-oxo porphyrins and our post-synthetically tethered architectures. We demonstrate our approach using two molecular clips that differ in length. The bis-iron(III)-μ-oxo precatalysts were characterized by ¹H NMR, ESI-MS, and molecular modeling to support the formation of cofacial prisms. Catalytic activity was studied electrochemically using cyclic voltammetry and hydrodynamic voltammetry. Whereas untethered bis-iron(III)-μ-oxo porphyrins are limited to heterogenous catalysis so that the dimeric structure is not lost upon the removal of the oxo bridge, our clipped architectures show significant catalytic current response under homogeneous conditions. Furthermore, when a shorter molecular clip is used as a post-synthetic tether, the selectivity under heterogeneous conditions is significantly enhanced: monomeric Fe(III) tetraphenylporphyrin (TPhP) generates 64.3% H₂O₂. When this same porphyrin is templated into a cofacial environment (Fe₂O TPhP) the selectivity improves to 15.8% H₂O₂. When a tetrapyridyl porphyrin analogue is post-synthetically tethered through a oxalate-bridged Rh₂ molecular clip, the selectivity improves further to 7.2% H₂O₂. In contrast, when a longer bis-hydroxybenzoquinato bridge is used in the clip, the selectivity drops back to 14.5% H₂O₂. Our most selective system also shows the highest current response and therefore our post-synthetic modification provides a kinetic enhancement as well. These results establish that straightforward coordination chemistry can be used for post-synthetic tuning of molecular ORR catalysts and when proper molecular clips are selected, marked enhancements to both selectivity and activity may be realized.