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.

本文提出了一种结合实验与理论的研究，探讨了在决定由面对面双卟啉催化的氧还原途径中，质子传递所扮演的角色。通过合成、表征和评估一系列同系物二钴(II)Pacman卟啉，这些卟啉分别由香豆素[Co2(DPX) (1) 和 Co2(DPXM) (3)]以及二苯并呋喃[Co2(DPD) (2) 和 Co2(DPDM) (4)]锚定，并被用作直接四质子、四电子还原氧气至水的催化剂。对分子内二铁(III)μ-氧化物配合物Fe2O(DPXM) (5)的结构分析以及1-4的电化学测量结果表明，带有对位芳基基团的Pacman衍生物具有与其母体相似的构象灵活性和氧化还原性质；然而，这些对位芳基催化剂在直接将氧气还原为水的过程中，对于两个质子和两个电子途径至过氧化氢的选择性显著降低。密度泛函理论计算揭示，与未取代的同类物相比，对位芳基取代导致质子向氧气结合催化剂的传递效率降低。特别是，[Co2(DPXM)(O2)]+的HOMO不倾向于将质子传递给结合的氧物种，从而将O−O活化途径导向单电子化学，并产生过氧化氢，而[Co2(DPX)(O2)]+的HOMO则将质子化导向[Co2O2]核心，以促进后续的多电子O−O键活化，生成两个水分子。我们的研究强调了控制质子和电子库存以实现特定的O−O键活化的重要性，并为双金属卟啉裂缝中的O−O键活化提供了一个统一模型。