Lahaye Dorothée, Groves John T
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA.
J Inorg Biochem. 2007 Nov;101(11-12):1786-97. doi: 10.1016/j.jinorgbio.2007.07.017. Epub 2007 Jul 21.
The manganese meso-dimethylimidazolium porphyrin complex Mn(III)[TDMImP] reacted with HOBr/OBr(-) to generate the corresponding oxo-Mn(V)[TDMImP] species. The rate of this process accelerated with increasing pH. A forward rate constant, k(for), of 1.65x10(6)M(-1)s(-1) was determined at pH 8. Under these conditions, the oxo-Mn(V) species is short-lived and is transformed into the corresponding oxo-Mn(IV) complex. A first-order rate constant, k(obs), of 0.66 s(-1) was found for this reduction process at pH 8. The mechanism of this reduction process, which was dependent on bromide ion, appeared to proceed via an intermediate Mn(III)-O-Br complex. Thus, both a fast, reversible Mn(III)-O-Br bond heterolysis and a slower homolytic pathway occur in parallel in this system. The reverse oxidation reaction between oxo-Mn(V)[TDMImP] and bromide was investigated as a function of pH. The rate of this oxo-transfer reaction (k(rev)=1.4x10(3)M(-1)s(-1) at pH 8) markedly accelerated as the pH was lowered. The observed first-order dependence of the rate on [H(+)] indicates that the reactive species responsible for bromide oxidation is a protonated oxo-hydroxo complex and the stable species present in solution at high pH is dioxo-Mn(V)[TDMImP], O=Mn(V)=O. The oxo-Mn(V) species retains nearly all of the oxidative driving force of the hypohalite. The equilibrium constant K(equi)=k(for)/k(rev) for the reversible process was determined at three different pH values (K(equi)=1.15x10(3) at pH 8) allowing the measurement of the redox potentials E of oxo-Mn(V)/Mn(III) (E=1.01 V at pH 8). The redox potential for this couple was extrapolated over the entire pH scale using the Nernst relationship and compared to those of the manganese 2- and 4-meso-N-methylpyridinium porphyrin couples oxo-Mn(V)[2-TMPyP]/Mn(III)[2-TMPyP], oxo-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP], OBr(-)/Br(-) and H(2)O(2)/H(2)O. Notably, the redox potential of oxo-Mn(V)/Mn(III) for the imidazolium porphyrin approaches that of H(2)O(2)/H(2)O at low pH.
中-二甲基咪唑啉卟啉锰配合物Mn(III)[TDMImP]与HOBr/OBr(-)反应生成相应的氧代-Mn(V)[TDMImP]物种。该过程的速率随着pH值的升高而加快。在pH 8时测定的正向速率常数k(for)为1.65×10(6)M(-1)s(-1)。在这些条件下，氧代-Mn(V)物种寿命较短，并转化为相应的氧代-Mn(IV)配合物。在pH 8时，该还原过程的一级速率常数k(obs)为0.66 s(-1)。该还原过程的机制依赖于溴离子，似乎是通过中间体Mn(III)-O-Br配合物进行的。因此，在该体系中，快速、可逆的Mn(III)-O-Br键异裂和较慢的均裂途径同时发生。研究了氧代-Mn(V)[TDMImP]与溴离子之间的反向氧化反应与pH值的关系。该氧转移反应的速率（在pH 8时k(rev)=1.4×10(3)M(-1)s(-1)）随着pH值的降低而显著加快。观察到的速率对[H(+)]的一级依赖性表明，负责溴离子氧化的活性物种是质子化的氧代-羟基配合物，在高pH值下溶液中存在的稳定物种是二氧代-Mn(V)[TDMImP]，即O=Mn(V)=O。氧代-Mn(V)物种保留了次卤酸盐几乎所有的氧化驱动力。在三个不同的pH值下测定了可逆过程的平衡常数K(equi)=k(for)/k(rev)（在pH 8时K(equi)=1.15×10(3)），从而可以测量氧代-Mn(V)/Mn(III)的氧化还原电位E（在pH 8时E=1.01 V）。利用能斯特关系将该电对的氧化还原电位外推到整个pH范围，并与锰2-和4-中-N-甲基吡啶卟啉电对氧代-Mn(V)[2-TMPyP]/Mn(III)[2-TMPyP]、氧代-Mn(V)[4-TMPyP]/Mn(III)[4-TMPyP]、OBr(-)/Br(-)和H(2)O(2)/H(2)O的氧化还原电位进行比较。值得注意的是，咪唑啉卟啉的氧代-Mn(V)/Mn(III)氧化还原电位在低pH值时接近H(2)O(2)/H(2)O的氧化还原电位。
