Borate complexes hydrolysis

The mode of dissolution of metal borates in aqueous solution is complex. Hydrolysis of the borate anion can result in completely different boron species that are stable only under particular conditions of pH, temperature, and concentration. 

Benzeneselenol, a representative selenol, is a colorless liquid of greater acidity than benzenefhiol [pK = 5.9 (PhSeH) 6.5 (PhSH)]. Benzeneselenol is usually synfhesized by the reaction of phenylmagnesium bromide with metallic selenium then quenching wifh aqueous hydrochloric acid [10]. Similar hydrolysis of the sodium phenylseleno(triethoxy)borate complex prepared by fhe reduction of diphenyl diselenide wifh sodium borohydride is a convenient alternative [11]. 

Mechanism of Sugar-Borate Complexation It has been suggested that borate leads to a shift in the equilibrium isomerization because of the binding of tetrahydroxyborate ions to aldose and ketose sugars. At near neutral pH, tetrahydroxyborate ions can be formed by hydrolysis of borax (Na2B4O5(OH)4-8H20) [44] ... 

Very important for the gel formation seems to be the molecular structure of the pol5rmer. Thus a reaction with borate is more likely with a linear molecule like that of salep mannan than with a molecule of branched structure like in the yeast mannan. With increasing molecular weight, the better is the gel formation. The reverse takes place when the pol5mier borate complexes are subjected to enzymatic hydrolysis (77). 

The relative stereochemistry of alditols can be deduced by formation of borate complexes, acetylation, partial hydrolysis to the polyol acetates, and then n.m.r. analysis. Borate complexes form preferentially in the sequence l,2-syn> 1,3-syn > 1,2-anti, 1,3-anti, and terminal diols (in staggered conformation). ... 

Cellodextrins (DP2-8), obtained by either acidic hydrolysis or acetolysis-deacetylatlon of cellulose and fractionated into components of defined molar mass by size-exclusion chromatography on Bio-Gel pi4, have been further fractionated by chromatography of their borate complexes on a borate-form anion-exchange resin. Components of as yet unknown structure were detected, in yields up to 30% for celloheptaose. Such heterogeneity was not revealed by chromatography on Ag - or Ca -form cation exchangers, or reversed... 

The whole series of pyrrol-l-ylborates, M[BH (NC4H4)4 ], has been synthesized (Tables 9 and 12). The mechanism of their formation and their hydrolysis kinetics have been studied in detail.100 The hydropyrazol-l-ylborates are very stable compounds and can be prepared in acid form.101 The di- and tri-pyrazol-l-ylborates are extensively used as complexing ligands (see Chapter 13.6). The photoelectron spectra of the sodium and thalliumfl) hydrotris(pyrazol-l-yl)borates and the electronic structure of the anion itself are reported.11 ... 

A vanadium borate of uncertain composition is obtained by fusing a mixture of vanadium pentoxidc and boric anhydride. It forms green or yellowish-green crystals which, however, undergo hydrolysis readily.2 A vanadyl borate is also obtained by the action of borax on vanadyl sulphate solution,3 and a complex vanadium borotungstate has been prepared. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

Unlike the harder congeners, the poly(mercaptoimidazolyl)borates react smoothly with the fac-[M(CO)3]+ moieties, yielding, in almost quantitative yield, complexes that are remarkably stable toward aerobic oxidation and hydrolysis. The 99mTc complexes are obtained with high radiochemical purity... 

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