Unusual oxidation of phosphines employing water as the oxygen atom source and tris(benzene-1,2-dithiolate)molybdenum(VI) as the oxidant. A functional molybdenum hydroxylase analogue system.
Unusual oxidation of phosphines employing water as the oxygen atom source and tris(benzene-1,2-dithiolate)molybdenum(VI) as the oxidant. A functional molybdenum hydroxylase analogue system.
复制标题
使用水作为氧原子源和三(苯-1,2-二硫醇)钼(VI)作为氧化剂进行磷化氢的异常氧化。
作者：
A. Cervilla;F. Pérez;E. Llopis;M. Piles
研究了Mo(VI)(S2C6H4)3与有机膦反应生成阴离子Mo(V)络合物、Mo(V)(S2C6H4)3-和氧化膦的反应动力学。研究了在四氢呋喃/水介质中，磷化氢浓度、钼络合物浓度、pH值和水浓度对反应速率的影响。反应符合pH依赖的磷化氢饱和动力学，络合物浓度为一级反应。水的浓度显著提高了反应速率，这与作为关键中间体的Mo(VI)(S2C6H4)3(H2O)加合物的形成一致。观察到的反应速率的pH依赖关系来自于该加合物的酸和碱形式之间的分布。显然，膦对氧的亲电攻击要求配位水以未质子化的氢氧化物的形式存在，即Mo(VI)(S2C6H4)3(Ho)-。然后，Mo(VI)中心协同抽提2E-、H+，得到Mo(IV)(S2C6H4)3(2-)、H+和相应的氧膦。然而，这一Mo(IV)络合物产物通过与未反应的Mo(VI)(S2C6H4)3的同位异构化而被迅速氧化成Mo(V)(S2C6H4)3-。这样形成的Mo(V)络合物可以在通入O2时被氧化成起始的Mo(VI)络合物。因此，Mo(VI)(S2C6H4)3是磷化氢在水存在下自氧化的催化剂。此外，一系列叔膦的反应性也有明显的变化。(P-CH3C6H4)3P>；(C6H5)3P>；(p-ClC6H4)3P；(P-ClC6H4)3P随钼(VI)络合还原速率的增加而增大。氧同位素示踪证实，氧原子的来源是水而不是氧气，氧原子转移到磷化氢。这种反应活性与以膦为氧原子受体、Mo(VI)(S2C6H4)3为电子受体的黄嘌呤酶的酶活性相当。
The kinetics of the reaction of Mo(VI)(S2C6H4)3 with organic phosphines to produce the anionic Mo(V) complex, Mo(V)(S2C6H4)3-, and phosphine oxide have been investigated. Reaction rates, monitored by UV-vis stopped-flow spectrophotometry, were studied in THF/H2O media as a function of the concentration of phosphine, molybdenum complex, pH, and water concentration. The reaction exhibits pH-dependent phosphine saturation kinetics and is first-order in complex concentration. The water concentration strongly enhances the reaction rate, which is consistent with the formation of Mo(VI)(S2C6H4)3(H2O) adduct as a crucial intermediate. The observed pH dependence of the reaction rate would arise from the distribution between acid and basic forms of this adduct. Apparently, the electrophilic attack by the phosphine at the oxygen requires the coordinated water to be in the unprotonated hydroxide form, Mo(VI)(S2C6H4)3(HO)-. This is followed by the concerted abstraction of 2e-, H+ by the Mo(VI) center to give Mo(IV)(S2C6H4)3(2-), H+, and the corresponding phosphine oxide. However, this Mo(IV) complex product is oxidized rapidly to Mo(V)(S2C6H4)3- via comproportionation with unreacted Mo(VI)(S2C6H4)3. The Mo(V) complex thus formed can be oxidized to the starting Mo(VI) complex upon admission of O2. Consequently, Mo(VI)(S2C6H4)3 is a catalyst for the autoxidation of phosphines in the presence of water. Additionally, there was a detectable variation in the reactivity for a series of tertiary phosphines. The rate of Mo(VI) complex reduction increases as does the phosphine basicity: (p-CH3C6H4)3P > (C6H5)3P > (p-ClC6H4)3P. Oxygen isotope tracing confirms that water rather than dioxygen is the source of the oxygen atom which is transferred to the phosphine. Such reactivity parallels oxidase activity of xanthine enzyme with phosphine as oxygen atom acceptor and Mo(VI)(S2C6H4)3 as electron acceptor.