Targeted Proton Delivery in the Catalyzed Reduction of Oxygen to Water by Bimetallic Pacman Porphyrins

Abstract

A combined experimental and theoretical investigation of the role of proton delivery in determining O_2 reduction pathways catalyzed by cofacial bisporphyrins is presented. A homologous family of dicobalt(II) Pacman porphyrins anchored by xanthene [Co_2(DPX) (1) and Co_2(DPXM) (3)] and dibenzofuran [Co_2(DPD) (2) and Co_2(DPDM) (4)] have been synthesized, characterized, and evaluated as catalysts for the direct four-proton, four-electron reduction of O_2 to H_2O. Structural analysis of the intramolecular diiron(III) μ-oxo complex Fe_2O(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 O_2 to H_2O over the two-proton, two-electron pathway to H_2O_2. Density functional theory calculations reveal that trans-aryl substitution results in inefficient proton delivery to O_2-bound catalysts compared to unsubstituted congeners. In particular, the HOMO of [Co_2(DPXM)(O_2)]+ disfavors proton transfer to the bound oxygen species, funneling the O−O activation pathway to single-electron chemistry and the production of H_2O_2, whereas the HOMO of [Co_2(DPX)(O_2)]+ directs protonation to the [Co_2O_2] core to facilitate subsequent multielectron O−O bond activation to generate two molecules of H_2O. 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.

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