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 Co₂(DPXM) (3)] and dibenzofuran [Co₂(DPD) (2) and Co₂(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 H₂O over the two-proton, two-electron pathway to H₂O₂. 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.