Kholdeeva Oxana A, Trubitsina Tatiana A, Maksimov Gennadii M, Golovin Anatolii V, Maksimovskaya Raisa I
Boreskov Institute of Catalysis, Lavrentieva 5, Novosibirsk 630090, Russia.
Inorg Chem. 2005 Mar 7;44(5):1635-42. doi: 10.1021/ic0490829.
Ti(IV)-monosubstituted Keggin-type polyoxometalates (Ti-POMs), mu-oxo dimer [Bu4N]8[(PTiW11O39)2O] (1), and three monomers [Bu4N]4[PTi(L)W11O39], where L = OH (2), OMe (3), and OAr (4, ArOH = 2,3,6-trimethylphenol (TMP)), have been prepared starting from mu-hydroxo dimer [Bu4N]7[(PTiW11O39)2OH] (5) or heteropolyacid H5PW11TiO40 or both. The compounds have been characterized by elemental analysis, IR, UV-vis, and multinuclear (31P, 1H, 183W) NMR. The interaction of 1 and 3-5 with H2O in MeCN produces 2. The hydrolysis constants, estimated from 31P and 1H NMR data, are 0.006 and 0.04 for 1 and 3, respectively. Studies by 31P NMR, IR, potentiometric titration, and cyclic voltammetry revealed that 1-3 and 5 afford the same protonated titanium peroxo complex [Bu4N]4[HPTi(O2)W11O39] (I) upon interaction with aqueous H2O2 in MeCN. The rates of formation of I correlate with the rates of hydrolysis of the Ti-POMs and follow the order of 5 > 1 > 3. A two-step mechanism of the reaction of Ti-POMs with H2O2, which involves hydrolysis of the Ti-L bonds to yield 2 followed by fast interaction of 2 with hydrogen peroxide producing I, is suggested. The equilibrium constant for the reaction of 2 with H2O2 to yield I and H2O, estimated using 31P NMR, is 10. The interaction of the Ti-POMs with TMP follows the trends similar to their interaction with H2O) and requires preliminary hydrolysis of the Ti-L bonds. All of the Ti-POMs catalyze the oxidation of TMP with H2O2 in MeCN to give 2,3,5-trimethyl-p-benzoquinone and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol. The product distribution is similar for all of the Ti-POMs. The catalytic activities of the Ti-POMs correlate with the rates of formation of I and follow the order of 2 > 5 > 1 > 3. The findings lay a basis for a better understanding of the nature of the reactivity of titanium in Ti-catalyzed oxidations.
以μ-羟基二聚体[Bu₄N]₇[(PTiW₁₁O₃₉)₂OH]（5）或杂多酸H₅PW₁₁TiO₄₀或两者为原料，制备了钛（IV）单取代的Keggin型多金属氧酸盐（Ti-POMs）、μ-氧二聚体[Bu₄N]₈[(PTiW₁₁O₃₉)₂O]（1）以及三种单体[Bu₄N]₄[PTi(L)W₁₁O₃₉]，其中L = OH（2）、OMe（3）和OAr（4，ArOH = 2,3,6-三甲基苯酚（TMP））。通过元素分析、红外光谱、紫外可见光谱和多核（³¹P、¹H、¹⁸³W）核磁共振对这些化合物进行了表征。1和3 - 5在乙腈中与水反应生成2。根据³¹P和¹H核磁共振数据估算，1和3的水解常数分别为0.006和0.04。通过³¹P核磁共振、红外光谱、电位滴定和循环伏安法研究表明，1 - 3和5在乙腈中与过氧化氢水溶液相互作用时会生成相同的质子化钛过氧配合物[Bu₄N]₄[HPTi(O₂)W₁₁O₃₉]（I）。I的形成速率与Ti-POMs的水解速率相关，且遵循5 > 1 > 3的顺序。提出了Ti-POMs与过氧化氢反应的两步机理，该机理包括Ti-L键水解生成2，随后2与过氧化氢快速相互作用生成I。使用³¹P核磁共振估算，2与过氧化氢反应生成I和水的平衡常数为10。Ti-POMs与TMP的相互作用遵循与其与水相互作用相似的趋势，并且需要Ti-L键的初步水解。所有的Ti-POMs都能催化TMP在乙腈中被过氧化氢氧化生成2,3,5-三甲基对苯醌和2,2',3,3',5,5'-六甲基-4,4'-联苯酚。所有Ti-POMs的产物分布相似。Ti-POMs的催化活性与I的形成速率相关，且遵循2 > 5 > 1 > 3的顺序。这些发现为更好地理解钛在Ti催化氧化反应中的反应性本质奠定了基础。
