Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile
Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile
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DOI：
10.1039/b403760j
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发表时间：
2004-01-01
影响因子：
4
通讯作者：
Takagi, HD
中科院分区：
文献类型：
--
作者：
Inamo, M;Kumagai, H;Takagi, HD
用分光光度法研究了5、10、15、20-四苯基卟啉（TPP）和2、3、7、8、12、13、17、18-辛基卟啉（OEP）的Cu（II）配合物在乙腈中与不同氧化剂的均相电子转移反应。根据电子能谱和Cu(II)-卟啉配合物的氧化还原电位，确定反应产物为相应Cu(II)-卟啉配合物的π -阳离子自由基。在Cu(II)-卟啉配合物中Cu2+大量过量的伪一级条件下，确定了Cu(II)-卟啉配合物与溶剂化Cu2+之间的电子转移反应速率是水浓度的函数。伪一阶速率常数随水浓度的增加而降低，这是由于乙腈在[Cu(AN)(6)](2+)（AN=乙腈）中被水逐步取代，得出只有被乙腈完全溶剂化的Cu2+组分[Cu(AN)(6)]（2+）才具有足够高的氧化还原电位来氧化Cu(II)-OEP和Cu(II)TPP。Cu(II)-卟啉配合物与其他氧化试剂如[Ni(tacn)(2)](3+) （tacn=1,4,7-三氮杂环酮）和[Ru(bpy)(3)](3+) （bpy=2,2'-联吡啶）的反应速度太快，无法用常规的停流技术跟踪。利用外球电子转移反应的Marcus交联关系估计了Cu(II)-卟啉与其阳离子自由基之间电子自交换反应的速率常数：在25.0℃下，TPP为log(k/M-1 s(-1))=9.5+/-0.5， OEP为log(k/M-1 s(-1))=11.1+/-0.5。如此大的电子自交换速率常数是典型的以卟啉为中心的氧化还原反应，这种反应需要非常小的内外球重组能。
Homogeneous electron transfer reactions of the Cu(II)complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the pi-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu2+ was determined as a function of the water concentration under the pseudo first-order conditions where Cu2+ is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)(6)](2+)(AN=acetonitrile) by water, and it was concluded that only the Cu2+ species fully solvated by acetonitrile, [Cu(AN)(6)](2+), possesses sufficiently high redox potential for the oxidation of Cu(II)-OEP and Cu(II)TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)(2)](3+) (tacn=1,4,7-triazacyclononane) and [Ru(bpy)(3)](3+) (bpy=2,2'-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its pi-cation radical: log(k/M-1 s(-1))=9.5+/-0.5 for TPP and log(k/M-1 s(-1))=11.1+/-0.5 for OEP at 25.0degreesC. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.