Kadish Karl M, E Wenbo, Zhan Riqiang, Khoury Tony, Govenlock Linda J, Prashar Jognandan K, Sintic Paul J, Ohkubo Kei, Fukuzumi Shunichi, Crossley Maxwell J
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA.
J Am Chem Soc. 2007 May 23;129(20):6576-88. doi: 10.1021/ja070759b. Epub 2007 May 1.
Porphyrin-2,3-diones and porphyrin-2,3,7,8- and porphyrin-2,3,12,13-tetraones were shown to have a redox-active unit that can function independently of the macrocycle at large. Electroreduction of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin-2,3-diones [(P-dione)M] and the corresponding -2,3,12,13-tetraones [L-(P-tetraone)M] and -2,3,7,8-tetraones [C-(P-tetraone)M], where M = 2H, CuII, ZnII, NiII, and PdII was investigated and the products were characterized by ESR and thin-layer UV-visible spectroelectrochemistry. Electrochemical and spectroelectrochemical data show that the first two reductions of the porphyrin-diones and the first three reductions of the porphyrin-tetraones occur at the dione units. This was confirmed by ESR spectra of first reduction products which show that the electron spin is totally localized on a semidione unit, independent of the central metal ion and of the number and location of dione units. ESR spectra of the radical anions derived from free-base porphyrin-2,3-dione [(P-dione)2H] and porphyrin-2,3,12,13-tetraone [L-(P-tetraone)2H] confirm the trans-arrangement of the two inner protons and their location on nonsubstituted pyrrolic rings, thereby maintaining an 18-atom 18-pi electron bacteriochlorin-like aromatic delocalization pathway. The redox unit is not similarly isolated in the corner free-base porphyrin-2,3,7,8-tetraone [C-(P-tetraone)2H]. A one-electron reduction of C-(P-tetraone)2H leads to the formation of a tautomer with trans inner hydrogens with one residing on the N of the ring with the reduced unit as the only detectable product. This process is favorable because it creates a more delocalized 18-atom 18-pi electron aromatic pathway. This result is consistent with the measured redox potentials which show the first reduction of C-(P-tetraone)2H to be substantially easier than (P-dione)2H or L-(P-tetraone)2H.
卟啉 - 2,3 - 二酮以及卟啉 - 2,3,7,8 - 四酮和卟啉 - 2,3,12,13 - 四酮被证明具有一个氧化还原活性单元，该单元在很大程度上可以独立于大环发挥作用。对5,10,15,20 - 四（3,5 - 二叔丁基苯基）卟啉 - 2,3 - 二酮[（P - 二酮）M]、相应的 - 2,3,12,13 - 四酮[L - （P - 四酮）M]和 - 2,3,7,8 - 四酮[C - （P - 四酮）M]进行了电还原研究，其中M = 2H、CuII、ZnII、NiII和PdII，并用电子顺磁共振（ESR）和薄层紫外 - 可见光谱电化学对产物进行了表征。电化学和光谱电化学数据表明，卟啉 - 二酮的前两次还原以及卟啉 - 四酮的前三次还原发生在二酮单元上。这一点通过首次还原产物的ESR光谱得到证实，该光谱表明电子自旋完全定域在半醌单元上，与中心金属离子以及二酮单元的数量和位置无关。由游离碱卟啉 - 2,3 - 二酮[（P - 二酮）2H]和卟啉 - 2,3,12,13 - 四酮[L - （P - 四酮）2H]衍生的自由基阴离子的ESR光谱证实了两个内环质子的反式排列及其在未取代吡咯环上的位置，从而维持了一个18原子18π电子类细菌叶绿素的芳香离域途径。在角位游离碱卟啉 - 2,3,7,8 - 四酮[C - （P - 四酮）2H]中，氧化还原单元没有类似地被隔离。C - （P - 四酮）2H的单电子还原导致形成一种互变异构体，其内环氢为反式，其中一个位于带有还原单元的环的氮原子上，这是唯一可检测到的产物。这个过程是有利的，因为它创造了一个更离域的18原子18π电子芳香途径。这一结果与所测量的氧化还原电位一致，该电位表明C - （P - 四酮）2H的首次还原比（P - 二酮）2H或L - （P - 四酮）2H要容易得多。
