Kholdeeva Oxana A, Trubitsina Tatiana A, Maksimovskaya Raisa I, Golovin Anatolii V, Neiwert Wade A, Kolesov Boris A, López Xavier, Poblet Josep M
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russia.
Inorg Chem. 2004 Apr 5;43(7):2284-92. doi: 10.1021/ic0354466.
The protonated titanium peroxo complex Bu(4)N[HPTi(O(2))W(11)O(39)] (1) has been first prepared via interaction of the micro-oxo dimeric heteropolytungstate Bu(4)N[(PTiW(11)O(39))(2)O] (3) with an excess of 30% aqueous H(2)O(2) in MeCN. Peroxo complex 1 has been characterized by using elemental analysis, UV-vis, IR, resonance Raman (RR), (31)P and (183)W NMR spectroscopy, cyclic voltammetry, and potentiometric titration. The electronic and vibrational spectra of 1 are very similar to those of the well-known unprotonated titanium peroxo complex Bu(4)N[PTi(O(2))W(11)O(39)] (2), while (31)P and (183)W NMR spectra differ significantly. A compilation of the physicochemical techniques supports a monomeric Keggin type structure of 1 bearing one peroxo ligand attached to Ti(IV) in a eta(2)-coordination mode. The protonation of the titanium peroxo complex results in an increase of the redox potential of the peroxo group, E(1/2) = 1.25 and 0.88 V relative to Ag/AgCl reference electrode for 1 and 2, respectively. In contrast to 2, 1 readily reacts with 2,3,6-trimethylphenol (TMP) at 40 degrees C in MeCN to give 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol (BP) and 2,3,5-trimethyl-p-benzoquinone (TMBQ). The proportion between BP and TMBQ in the reaction products depends on the TMP/1 ratio. When a 2-fold excess of TMP is used, the main reaction product is BP (90%), while using a 2-fold excess of 1 leads to TMBQ (95%). On the basis of the product study, a homolytic oxidation mechanism that implicates the formation of phenoxyl radicals is suggested. The RR deuterium labeling experiments show that the activating proton is most likely localized at a Ti-O-W bridging oxygen rather than at the peroxo group. Theoretical calculations carried out at the DFT level on the protonated and unprotonated titanium peroxo derivatives also propose that the most stable complex is formed preferentially after protonation of the Ti-O-W site; however, both Ti-OH-W and TiOO-H protonated anions could coexist in solution.
质子化钛过氧配合物Bu(4)N[HPTi(O(2))W(11)O(39)] (1)首次通过微氧二聚杂多钨酸盐Bu(4)N[(PTiW(11)O(39))(2)O] (3)与过量30%的H(2)O(2)水溶液在乙腈中的相互作用制备而成。过氧配合物1通过元素分析、紫外可见光谱、红外光谱、共振拉曼光谱(RR)、(31)P和(183)W核磁共振光谱、循环伏安法和电位滴定法进行了表征。1的电子光谱和振动光谱与著名的未质子化钛过氧配合物Bu(4)N[PTi(O(2))W(11)O(39)] (2)非常相似，而(31)P和(183)W核磁共振光谱有显著差异。一系列物理化学技术支持1具有单体Keggin型结构，其中一个过氧配体以η(2)-配位模式连接到Ti(IV)上。钛过氧配合物的质子化导致过氧基团的氧化还原电位升高，相对于Ag/AgCl参比电极，1和2的E(1/2)分别为1.25和0.88 V。与2不同，1在40℃下于乙腈中能与2,3,6-三甲基苯酚(TMP)迅速反应，生成2,2',3,3',5,5'-六甲基-4,4'-联苯酚(BP)和2,3,5-三甲基对苯醌(TMBQ)。反应产物中BP和TMBQ的比例取决于TMP/1的比例。当使用2倍过量的TMP时，主要反应产物是BP(90%)，而使用2倍过量的1则导致TMBQ(95%)。基于产物研究，提出了一种涉及苯氧基自由基形成的均裂氧化机制。RR氘标记实验表明，活化质子最可能位于Ti-O-W桥连氧上而非过氧基团上。在DFT水平上对质子化和未质子化钛过氧衍生物进行的理论计算也表明，Ti-O-W位点质子化后优先形成最稳定的配合物；然而，Ti-OH-W和TiOO-H质子化阴离子可能在溶液中共存。
