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首页> 外文期刊>Dalton transactions: An international journal of inorganic chemistry >Syntheses, molecular structures and pH-dependent monomer-dimer equilibria of Dawson alpha(2)-monotitanium(IV)-substituted polyoxometalates
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Syntheses, molecular structures and pH-dependent monomer-dimer equilibria of Dawson alpha(2)-monotitanium(IV)-substituted polyoxometalates

机译：道森α（2）-单钛（IV）-取代的多金属氧酸盐的合成，分子结构和pH依赖的单体-二聚体平衡

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摘要

The preparation and structural characterization of a novel, solid Bronsted acid based on Dawson alpha(2)-monotitanium(IV)-substituted polyoxometalates (POMs) are described. The free-acid form of the POM, i. e., 13H(+)-heteropolyacid with the formula H-13[1b] center dot 55H(2)O DH-1 (1b = [(alpha(2)-P2W17TiO61)(alpha(2)-P2W17TiO61H)(mu-O)](13-)), was prepared by passing an aqueous solution containing a potassium salt precursor, K-14[1a] center dot 17H(2)O DK-1 (1a = [(alpha(2)-P2W17TiO61)(2)(mu-O)](14-)), through a cation-exchange resin column. Compound DK-1 was obtained by a stoichiometric reaction of mono-lacunary Dawson POM [alpha(2)-P2W17O61](10-) with Ti(SO4)(2) in an aqueous solution. [Note: the abbreviations D, M, K and H stand for dimer, monomer, potassium salt and free-acid form, respectively.] Compounds DK-1 and DH-1 were characterized by using complete elemental analysis, thermogravimetric (TG) and differential thermal analysis (DTA), FTIR, solution (P-31 and W-183) NMR spectroscopy, pH-varied P-31 NMR spectroscopy, solid-state 31P CPMAS NMR, X-ray crystallography and acidity measurements in an organic solvent with a Hammett indicator for DH-1. The monomeric form, K-8[alpha(2)-P2W17TiO62] center dot 18H(2)O MK-1, was derived from DK-1. The molecular structure of 1b was composed of a dimer connected through one Ti-O-Ti bond between two alpha(2)-mono-Ti-IV-substituted Dawson POM subunits. BVS (bond valence sum) calculation showed that one oxygen atom (O(60A)) in one of the two Dawson subunits was protonated, therefore the two subunits were unequivalent. On the other hand, the molecular structure of 1a was a Ti-O-Ti bonding dimer of two equivalent Dawson subunits. The pH-varied 31P NMR spectra of DK-1 and DH-1 in aqueous solutions showed that the monomer at pH 7.0, the dimer at pH 1.0-3.0, and the mono-protonated species of the dimer at pH 0.5 were the predominant species in the solutions. The Hammett acidity constant (H-0) of DH-1 in CH3CN (-2.87) was estimated to be almost the same as that of homo-Dawson heteropolyacid H-6[P2W18O62] center dot 17H(2)O (-2.77).
机译：描述了基于道森α（2）-单钛（IV）取代的多金属氧酸盐（POMs）的新型固体布朗斯台德酸的制备和结构表征。 POM的游离酸形式，即例如，具有分子式H-13 [1b]中心点55H（2）O DH-1的13H（+）-杂多酸（1b = [（alpha（2）-P2W17TiO61）（alpha（2）-P2W17TiO61H）（mu -O）]（13-）），是通过使含有钾盐前体K-14 [1a]中心点17H（2）O DK-1（1a = [（alpha（2）-P2W17TiO61 ）（2）（mu-O）]（14-）），通过阳离子交换树脂柱。化合物DK-1是通过单腔道森POM [α（2）-P2W17O61]（10-）与Ti（SO4）（2）在水溶液中的化学计量反应获得的。 [注：缩写D，M，K和H分别代表二聚体，单体，钾盐和游离酸形式。]化合物DK-1和DH-1用完全元素分析，热重（TG）和差热分析（​​DTA），FTIR，溶液（P-31和W-183）NMR光谱，pH可变的P-31 NMR光谱，固态31P CPMAS NMR，X射线晶体学和在有机溶剂中的酸度测量DH-1的Hammett指示器。单体形式K-8 [α（2）-P2W17TiO62]中心点18H（2）O MK-1源自DK-1。 1b的分子结构由通过两个α（2）-单-Ti-IV-取代的Dawson POM亚基之间的一个Ti-O-Ti键连接的二聚体组成。 BVS（键合价和）计算表明，两个道森亚基之一中的一个氧原子（O（60A））质子化，因此两个亚基不等价。另一方面，1a的分子结构是两个等效道森亚基的Ti-O-Ti键合二聚体。水溶液中DK-1和DH-1的pH值变化的31P NMR谱图显示，主要组分是pH 7.0的单体，pH 1.0-3.0的二聚体和pH 0.5的二质子的单质子化物种在解决方案中。估计DH-1在CH3CN（-2.87）中的哈米特酸度常数（H-0）与均一道森杂多酸H-6 [P2W18O62]中心点17H（2）O（-2.77）几乎相同。

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