Bougher Curt J, Abu-Omar Mahdi M
Department of Chemistry Purdue University 560 Oval Drive West Lafayette IN 47906 USA.
Department of Chemistry Purdue University 560 Oval Drive West Lafayette IN 47906 USA; Current address: Department of Chemistry and Biochemistry Department of Chemical Engineering University of California Santa Barbara CA 93106 USA.
ChemistryOpen. 2016 Oct 28;5(6):522-524. doi: 10.1002/open.201600117. eCollection 2016 Dec.
The kinetics of formation of the valence tautomers (tpfc)Mn(O-LA)] [where LA=Zn, Ca, Sc, Yb, B(CF), and trifluoroacetic acid (TFA); tpfc=5,10,15-tris(pentafluorophenyl) corrole] from (tpfc)Mn(O) were followed by UV/Vis spectroscopy, giving second-order rate constants ranging over five orders of magnitude from 10 for Ca to 10 m s for Sc. Hydrogen atom transfer (HAT) rates from 2,4-di--butyl phenol (2,4-DTBP) to the various Lewis acid valence tautomers of manganese oxo corrole complexes were evaluated and compared. For LA=TFA, Sc, or Yb, the rate constants of HAT were comparable to unactivated (tpfc)Mn(O). However, with LA=B(CF), Zn, and Ca, 6-, 21-, and 31-fold rate enhancements were observed, respectively. Remarkably, [(tpfc)Mn(OCa)] gave the most enhancement despite its rate of formation being the slowest. Comparisons of HAT rate constants among the various Lewis acid tautomers revealed that both size and charge are important. This study underscores how valence may affect the reactivity of high-valent manganese-oxo compounds and sheds light on nature's choice of Ca in the activation of Mn-oxo in the oxygen-evolving complex.
通过紫外可见光谱跟踪了由(tpfc)Mn(O)形成价互变异构体[(tpfc)Mn(O-LA)]（其中LA = Zn、Ca、Sc、Yb、B(CF)以及三氟乙酸(TFA)；tpfc = 5,10,15-三(五氟苯基)卟啉）的动力学过程，得到的二级速率常数范围跨越五个数量级，从Ca的10到Sc的10 m s。评估并比较了氢原子从2,4-二叔丁基苯酚(2,4-DTBP)转移至锰氧卟啉配合物的各种路易斯酸价互变异构体的速率。对于LA = TFA、Sc或Yb，氢原子转移的速率常数与未活化的(tpfc)Mn(O)相当。然而，对于LA = B(CF)、Zn和Ca，分别观察到速率增强了6倍、21倍和31倍。值得注意的是，[(tpfc)Mn(OCa)]尽管其形成速率最慢，但增强效果最为显著。对各种路易斯酸互变异构体之间氢原子转移速率常数的比较表明，尺寸和电荷都很重要。这项研究强调了价态如何影响高价锰氧化合物的反应活性，并揭示了自然界在光解水复合物中选择Ca激活锰氧的原因。
