Targeted proton delivery in the catalyzed reduction of oxygen to water by bimetallic pacman porphyrins

A combined experimental and theoretical investigation of the role of proton delivery in determining O₂ reduction pathways catalyzed by cofacial bisporphyrins is presented. A homologous family of dicobalt(II) Pacman porphyrins anchored by xanthene [CO₂(DPX) (1) and CO ₂(DPXM) (3)] and dibenzofuran [CO₂(DPD) (2) and CO ₂(DPDM) (4)] have been synthesized, characterized, and evaluated as catalysts for the direct four-proton, four-electron reduction of O₂ to H₂O. Structural analysis of the intramolecular diiron(III) μ-oxo complex Fe₂O(DPXM) (5) and electrochemical measurements of 1-4 establish that Pacman derivatives bearing an aryl group trans to the spacer possess structural flexibilities and redox properties similar to those of their parent counterparts; however, these trans-aryl catalysts exhibit markedly reduced selectivities for the direct reduction of O₂ to H ₂O over the two-proton, two-electron pathway to H₂O ₂. Density functional theory calculations reveal that trans-aryl substitution results in inefficient proton delivery to O₂-bound catalysts compared to unsubstituted congeners. In particular, the HOMO of [CO₂(DPXM)(O₂)]⁺ disfavors proton transfer to the bound oxygen species, funneling the O-O activation pathway to single-electron chemistry and the production of H₂O₂, whereas the HOMO of [CO₂(DPX)(O₂)]⁺ directs protonation to the [CO₂O₂] core to facilitate subsequent multielectron O-O bond activation to generate two molecules of H₂O. Our findings highlight the importance of controlling both proton and electron inventories for specific O-O bond activation and offer a unified model for O-O bond activation within the clefts of bimetallic porphyrins.