Fukuzumi Shunichi, Okamoto Ken, Gros Claude P, Guilard Roger
Contribution from the Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
J Am Chem Soc. 2004 Aug 25;126(33):10441-9. doi: 10.1021/ja048403c.
The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co(2)(DPX)]. The other cofacial dicobalt porphyrins [Co(2)(DPA), Co(2)(DPB), and Co(2)(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co(2)(DPX). The micro-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting micro-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the micro-superoxo species. The superhyperfine coupling constant of the micro-superoxo species of Co(2)(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co(2)(DPX) for the four-electron reduction of dioxygen by Fe(C(5)H(4)Me)(2) results from the strong binding of the reduced oxygen with Co(2)(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP)(+), whereas the turnover-determining step in the Co(2)(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co(2)(DPX), depending on the electron donor ability of ferrocene derivatives.
对于单钴卟啉[Co(OEP)]，会发生选择性的双电子还原氧气反应；而对于共面双钴卟啉[Co₂(DPX)]，则会发生选择性的四电子还原氧气反应。其他共面双钴卟啉[Co₂(DPA)、Co₂(DPB)和Co₂(DPD)]也能催化双电子还原氧气反应，但四电子还原反应的效率不如Co₂(DPX)。共面双钴卟啉的微超氧物种是通过共面二价钴卟啉在大体积碱存在下与氧气反应，以及随后所得微过氧物种被碘进行单电子氧化而产生的。在微超氧物种的电子顺磁共振谱中，在室温下观察到了由两个等效钴核引起的超超精细结构。Co₂(DPX)的微超氧物种的超超精细耦合常数在共面双钴卟啉中是最大的。这表明Co₂(DPX)对Fe(C₅H₄Me)₂催化的氧气四电子还原反应的高效催化作用源于还原态氧与Co₂(DPX)的强结合，Co₂(DPX)的两个钴核之间存在适合氧结合的微妙距离。将基于对整个催化反应以及催化循环中每个氧化还原反应的详细动力学研究，讨论二茂铁衍生物催化氧气双电子和四电子还原反应的机理。Co(OEP)催化氧气双电子还原反应中的周转决定步骤是二茂铁衍生物向Co(OEP)⁺的电子转移，而Co₂(DPX)催化氧气四电子还原反应中的周转决定步骤根据二茂铁衍生物的电子供体能力，从电子转移变为Co₂(DPX)过氧物种的O - O键断裂。
