A "Push-Pull" Mechanism for Heterolytic O-O Bond Cleavage in Hydroperoxo Manganese Porphyrins
A "Push-Pull" Mechanism for Heterolytic O-O Bond Cleavage in Hydroperoxo Manganese Porphyrins
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DOI：
10.1021/ic1015274
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发表时间：
2010-12-20
影响因子：
4.6
通讯作者：
Groves, John T.
中科院分区：
文献类型：
--
作者：
Jin, Ning;Lahaye, Dorothee E.;Groves, John T.
一种水溶性的锰卟啉5，10，15，20-tetrakis-(1，3-dimethylimidazolium-2-yl)porphyrinatomanganese(III)(Mn(III)TDMImP)与H_2O_2反应生成相对稳定的二氧锰(V)卟啉配合物(化合物I类似物)。停流动力学研究揭示了H_2O_2的Michaelis Menton饱和动力学。在饱和的H_2O_2条件下，可直接观察到化合物0型中间体的可见光谱(Soret带位于428 nm，Q带位于545和578 nm)，命名为Mn-III(OH)(OOH)TDMImP。速率控制的O-O异相分解步骤具有很小的活化热(H不等于=4.2+/-0.2千卡摩尔(-1))和大的负活化熵(S-不等于=-36+/-1卡摩尔(-1)K-1)。在8.8<；pH&10.4时，O-O键断裂反应与pH无关，一级反应速率常数为66+/-12 S(-1)。这些观察结果表明，Mn-III(OH)(OH)TDMIMP中的O-O键是通过协同的“推-拉”机制断裂的。在过渡态，随着水分子被释放到介质中，轴向(近端)-OH部分去质子化(“推”)，而末端氧In-OH部分质子化(“拉”)。这种机制类似于过氧化物酶和细胞色素P450等血红素酶的O-O键断裂，类似于类似的锰卟啉介导的快速、可逆的O-BR键断裂和形成反应。较小的活化热表明这种O-O键的断裂也是可逆的。
A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (Mn(III)TDMImP) is shown to react with H2O2 to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H2O2. The visible spectrum of a compound 0 type intermediate, assigned as Mn-III(OH)(OOH)TDMImP, can be directly observed under saturating H2O2 conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (Delta H-not equal=4.2 +/- 0.2 kcal mol(-1)) and a large, negative activation entropy (Delta S-not equal=-36 +/- 1 cal mol(-1) K-1). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 +/- 12 s(-1). These observations indicate that the O-O bond in Mn-III(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) -OH is partially deprotonated ("push"), while the terminal oxygen in -OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.