Tiwari Chandan Kumar, Baranov Mark, Neyman Alevtina, Neumann Ronny, Weinstock Ira A
Department of Chemistry and the Ilse Katz Institute for Nanoscale Science & Technology, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100.
Inorg Chem. 2020 Sep 8;59(17):11945-11952. doi: 10.1021/acs.inorgchem.9b03747. Epub 2020 Mar 5.
Dissolution of the polyoxometalate (POM) cluster anion H[PVMoO] (; a mixture of positional isomers) in 50% aq HSO dramatically enhances its ability to oxidize methylarenes, while fully retaining the high selectivities typical of this versatile oxidant. To better understand this impressive reactivity, we now provide new information regarding the nature of (115 mM) in 50% (9.4 M) HSO. Data from V NMR spectroscopy and cyclic voltammetry reveal that as the volume of HSO in water is incrementally increased to 50%, V(V) ions are stoichiometrically released from , generating two reactive pervanadyl, VO, ions, each with a one-electron reduction potential of ca. 0.95 V (versus Ag/AgCl), compared to 0.46 V for in 1.0 M aq HSO. Phosphorus-31 NMR spectra obtained in parallel reveal the presence of PO, which at 50% HSO accounts for all the P(V) initially present in . Addition of (NH)SO leads to the formation of crystalline [NH][MoO(SO)] (34% yield based on Mo), whose structure (from single-crystal X-ray diffraction) features a corner-shared, permolybdenyl [MoO] core, conceptually derived by acid condensation of two MoO moieties. While in 50% aq HSO oxidizes -xylene to -methylbenzaldehyde with conversion and selectivity both greater than 90%, reaction with VO alone gives the same high conversion, but at a significantly lower selectivity. Importantly, selectivity is fully restored by adding [NH][MoO(SO)], suggesting a central role for Mo(VI) in attenuating the (generally) poor selectivity achievable using VO alone. Finally, P and V NMR spectra show that intact is fully restored upon dilution to 1 M HSO.
多金属氧酸盐（POM）簇阴离子H[PVMoO]（位置异构体的混合物）在50%的硫酸水溶液中的溶解显著增强了其氧化甲基芳烃的能力，同时完全保留了这种多功能氧化剂典型的高选择性。为了更好地理解这种令人印象深刻的反应性，我们现在提供了关于在50%（9.4 M）硫酸水溶液中（115 mM）的性质的新信息。钒核磁共振光谱和循环伏安法的数据表明，随着水中硫酸的体积逐渐增加到50%，V(V)离子按化学计量从 中释放出来，生成两个具有反应活性的过氧钒酰离子VO，每个离子的单电子还原电位约为0.95 V（相对于Ag/AgCl），而在1.0 M硫酸水溶液中的 为0.46 V。同时获得的磷-31核磁共振光谱表明存在PO，在50%硫酸中，PO占 中最初存在的所有P(V)。加入硫酸铵会导致形成结晶的[NH][MoO(SO)]（基于钼的产率为34%），其结构（来自单晶X射线衍射）具有一个角共享的、过钼酰基[MoO]核心，从概念上讲是由两个MoO部分的酸缩合得到的。虽然在50%硫酸水溶液中的 以大于90%的转化率和选择性将对二甲苯氧化为对甲基苯甲醛，但单独与VO反应时转化率相同，但选择性显著降低。重要的是，通过加入[NH][MoO(SO)]，选择性完全恢复，这表明Mo(VI)在减弱单独使用VO通常可实现的（一般）较差选择性方面起着核心作用。最后，磷和钒核磁共振光谱表明，稀释至1 M硫酸时，完整的 完全恢复。
