Targeted proton delivery in the catalyzed reduction of oxygen to water by bimetallic Pacman porphyrins
Targeted proton delivery in the catalyzed reduction of oxygen to water by bimetallic Pacman porphyrins
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DOI：
10.1021/ja049115j
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发表时间：
2004-08-18
影响因子：
15
通讯作者：
Nocera, DG
中科院分区：
文献类型：
--
作者：
Chang, CJ;Loh, ZH;Nocera, DG
结合实验和理论研究了质子传递在确定共面双卟啉催化的O-2还原途径中的作用。本文合成了以杂蒽[Co-2(DPX)(1)和Co-2(DPXM)(3)]和二苯并呋喃[Co-2(DPD)(2)和Co2(DPDM)(4)]为锚定的二钴（II） Pacman卟啉同源家族，并对其作为O-2直接四质子四电子还原为H2O的催化剂进行了表征和评价。分子内二铁（III） mu-o配合物Fe2O(DPXM)(5)的结构分析和1-4的电化学测量表明，带有芳基的Pacman衍生物具有与母体相似的结构柔韧性和氧化还原性能；然而，这些反式芳基催化剂通过双质子、双电子途径将O-2直接还原为H2O的选择性明显降低。密度泛函理论计算表明，与未取代的同系物相比，反式芳基取代导致质子传递到o -2结合催化剂的效率低下。特别是，[Co-2(DPXM)(O-2)]（+）的HOMO不利于质子向结合氧转移，将O-O激活途径引向单电子化学并生成H2O2，而[Co-2(DPX)(O-2)]（+）的HOMO将质子化作用引向[Co2O2]核心，从而促进后续多电子O-O键激活，生成两分子H2O。我们的研究结果强调了控制质子和电子库存对特定O-O键激活的重要性，并提供了双金属卟啉间隙内O-O键激活的统一模型。
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 Co2(DPDM) (4)] have been synthesized, characterized, and evaluated as catalysts for the direct four-proton, four-electron reduction of O-2 to H2O. Structural analysis of the intramolecular diiron(III) mu-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 O-2 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 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 H2O2, whereas the HOMO of [Co-2(DPX)(O-2)](+) 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.