Khenkin Alex M, Kumar Devesh, Shaik Sason, Neumann Ronny
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100.
J Am Chem Soc. 2006 Dec 6;128(48):15451-60. doi: 10.1021/ja0638455.
A manganese(III)-substituted polyoxometalate, [alpha2-P2MnIII(L)W17O61]7- (P2W17MnIII), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F5PhI(TFAc)2) as a monooxygen donor. Pink P2W17MnIII turns green upon addition of F5PhI(TFAc)2. The 19F NMR spectrum of F5PhI(TFAc)2 with excess P2W17MnIII at -50 degrees C showed the formation of an intermediate attributed to P2W17MnIII-F5PhI(TFAc)2 that disappeared upon warming. The 31P NMR spectra of P2W17MnIII with excess F5PhI(TFAc)2 at -50 and -20 degrees C showed a pair of narrow peaks attributed to a diamagnetic, singlet manganese(V)-oxo species, P2W17MnV=O. An additional broad peak at -10.6 ppm was attributed to both the P2W17MnIII-F5PhI(TFAc)2 complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P2W17MnV=O were modeled by DFT calculations using the analogous Keggin compound, [PMnV=OW11O39]4-. Calculations with a pure functional, UBLYP, showed singlet and triplet ground states of similar energy. Further calculations using both the UBLYP and UB3LYP functionals for epoxidation and hydroxylation of propene showed lowest lying triplet transition states for both transformations, while singlet and quintet transition states were of higher energy. The calculations especially after corrections for the solvent effect indicate that [PMnV=OW11O39]4- should be highly reactive, even more reactive than analogous MnV=O porphyrin species. Kinetic measurements of the reaction of P2W17MnV=O with 1-octene indicated, however, that P2W17MnV=O was less reactive than a MnV=O porphyrin. The experimental enthalpy of activation confirmed that the energy barrier for epoxidation is low, but the highly negative entropy of activation leads to a high free energy of activation. This result originates in our view from the strong solvation of the highly charged polyoxometalate by the polar solvent used and adventitious water. The higher negative charge of the polyoxometalate in the transition versus ground state leads to electrostriction of the solvent molecules and to a loss of degrees of freedom, resulting in a highly negative entropy of activation and slower reactions.
一种锰（III）取代的多金属氧酸盐[α2-P2MnIII(L)W17O61]7-（P2W17MnIII）被用作氧化催化剂，使用碘五氟苯双（三氟乙酸酯）（F5PhI(TFAc)2）作为单氧供体。加入F5PhI(TFAc)2后，粉红色的P2W17MnIII变为绿色。在-50℃下，过量P2W17MnIII存在时F5PhI(TFAc)2的19F NMR谱显示形成了一种归因于P2W17MnIII-F5PhI(TFAc)2的中间体，该中间体在升温时消失。在-50℃和-20℃下，过量F5PhI(TFAc)2存在时P2W17MnIII的31P NMR谱显示一对窄峰，归因于抗磁性的单重态锰（V）-氧物种P2W17MnV=O。在-10.6 ppm处的另一个宽峰归因于P2W17MnIII-F5PhI(TFAc)2配合物和顺磁性的三重态锰（V）-氧物种。使用类似的Keggin化合物[PMnV=OW11O39]4-通过DFT计算对P2W17MnV=O的电子结构和反应性进行了建模。使用纯泛函UBLYP的计算显示了能量相似的单重态和三重态基态。使用UBLYP和UB3LYP泛函对丙烯环氧化和羟基化进行纯进行的进一步计算表明，两种转化的最低三重态过渡态能量最低，而单重态和五重态过渡态能量较高。特别是在对溶剂效应进行校正后的计算表明，[PMnV=OW11O39]4-应该具有高反应活性，甚至比类似的MnV=O卟啉物种更具反应活性。然而，P2W17MnV=O与1-辛烯反应的动力学测量表明，P2W17MnV=O的反应活性低于MnV=O卟啉。实验活化焓证实环氧化的能垒较低，但高度负的活化熵导致高的活化自由能。我们认为，这一结果源于所用极性溶剂和外来水对高电荷多金属氧酸盐的强烈溶剂化作用。与基态相比，过渡态中多金属氧酸盐的负电荷更高，导致溶剂分子的电致伸缩和自由度的丧失，从而导致高度负的活化熵和较慢的反应。
