Borate complexes in aqueous solution

Until recently, very little had been reported on the important area of metal borate complexation in aqueous solution. The effect of salts on the ionization of boric acid (358, 375) has been mentioned above, and subsequent research suggests that complexation of borate with, for example, calcium ions can account for the enhanced acidity of H O Literature on cationic complexes of boron was reviewed in 1970 (376). 

Ohman, L.-O. and Sjoberg, S. (1985) On the insignificance of aluminium-borate complexes in aqueous solution. Mar. Chem.,... 

In aqueous solution, riboflavin has absorption at ca 220—225, 226, 371, 444 and 475 nm. Neutral aqueous solutions of riboflavin have a greenish yellow color and an intense yellowish green fluorescence with a maximum at ca 530 nm and a quantum yield of = 0.25 at pH 2.6 (10). Fluorescence disappears upon the addition of acid or alkah. The fluorescence is used in quantitative deterrninations. The optical activity of riboflavin in neutral and acid solutions is [a]=+56.5-59.5° (0.5%, dil HCl). In an alkaline solution, it depends upon the concentration, eg, [a] J =—112-122° (50 mg in 2 mL 0.1 Ai alcohohc NaOH diluted to 10 mL with water). Borate-containing solutions are strongly dextrorotatory, because borate complexes with the ribityl side chain of riboflavin = +340° (pH 12). 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

The hydroxy a-amino acids l-serine and l-threonine, used as models for the 2-amino-2-deoxy glyconic acids, have been complexed with Ni(II) at 37 °C in aqueous solutions of 0.15M potassium nitrate. Values for the stability constants were obtained from iso-pH titration data which were collected by alternate, small, incremental additions of metal ion and potassium hydroxide being made such that the pH of the solution remained nearly constant. The data were consistent with the predominance of MLn species, along with additional protonated and hydrolyzed complexes. There was no evidence for the involvement of the hydroxyl group in chelation. By the same iterative computations the complexes formed between borate and mannitol have been analyzed, and the stability constants have been calculated. Complexes with mannitohborate stoichiometries of I.T, 1 2, 1 3, and 2 1 were proposed. 

Shiroishi and Hayakawa (18) have described the effect of sunlight irradiation on pyridoxine and related compounds in aqueous solution at various pH s. Pyridoxine and pyTidoxamine are relatively stable in acidic medium, while pyridoxal decomposes independently of pH. The authors suggested that the aldehydic 0 participates in the photolysis of pyridoxal. The presence of Cu++ had no effect on the stability of these substances. A borate complex of pyridoxine is stable to sunlight irradiation, heating, or autoclaving. Acetic acid and a reddish brown substance were separated as the decomposition products from an irradiated solution of pyridoxine (18). 

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. 

This chargeless molecule can be extracted by ether or amyl alcohol. In addition to this a set of complex ions, such as [Fe(SCN)]2+, [Fe(SCN)2]+, [Fe(SCN)4]-, [Fe(SCN)5]2-, and [Fe(SCN)6]3 are also formed. The composition of the product in aqueous solution depends mainly on the relative amounts of iron and thiocyanate present. Phosphates, arsenates, borates, iodates, sulphates, acetates, oxalates, tartrates, citrates, and the corresponding free acid interfere due to the formation of stable complexes with iron(III) ions. 

Similar behavior has been reported for electron transfer between photoexcited [(trimethylammonio)propyl]thiadicarbocyanine (TTDC +) and triphenyl benzyl borate (TPBB ) anions [94]. In aqueous solution, photoinduced charge separation was strongly attenuated by complex formation between the sensitizer and reducing... 

The reversible formation of borate esters by carbohydrates in aqueous solution has been known for over a century, and played a role in early conformational studies.Currently it is an ancillary in the separation of carbohydrates, and with its aid very fine separations can be achieved - the secondary tritum isotope effects at H2, H3, H5 and H6 (but not HI) of glucose on borate complexation permit complete isotopic separation to be achieved by ion-exchange chromatography. ... 

So, NMR spectra of 4,4 -bipyridyl, pyrazine, and ethylenediamine coordinated to undecatungstocobalto(III)silicate and -borate anions. Identification of 1 1 and dumbbell-shaped 12 complexes, Bull. Korean Chem. Soc, 14 759 (1993) (c) J.L. Samonte and M.T. Pope, Derivatization of polyoxotungstates in aqueous solution. Exploration of the kinetic stability ofcobalt(II)- and cobalt(III) derivatives of lacunary anions with pyridine and pyridine-type ligands. Can. J. Chem.. In press... 

In ocean water, boron occurs in two dominant species, boric acid B(OH)j and borate B(OH). The relative concentration of these two species is pH dependent such that the boric acid B(OH)3 is dominant at pH < 9.0, whereas the tetrahedral complex of B(OH) dominates at pH >9.0 (Fig. 10.12 Hemming and Hanson 1992). The principal process that causes fractionation of boron isotopes in aqueous solutions is the following exchange reaction ... 

Liquid crystalline mesophases can also be prepared in non-aqueous solution. Using ethanolic solutions of non-ionic block copolymers as a medium, Zhao et for example, have developed a route by which mesoporous metal phosphates and borates can readily be prepared. A mixture of the metal alkoxide and the acidic chloride is used to give a complex in ethanolic solution. This acid-base pair reacts in one component of the liquid crystalline assembly to give a mesophase solid of uniform composition. Evaporation of the ethanol leaves the mesostructure. This appears to be one of the most promising routes to the formation of non-silica mesoporous solids. Many reports of these solids have appeared, particularly of metal oxides such as titanium dioxide, but loss of ordering on the mesoscale frequently occurs upon template removal. The route of Sanchez, which involves the use of titanate precursor species in the sol-gel, is a promising approach to stable mesostructured titania that retains its porosity. 

The heat of formation of 2,8,9-trioxa-5-aza-l-boratricyclo[3,3,3,0 > ]undecane in the gas phase has been determined (191.5 0.4 kcalmol- ) as has the bond energy of the N B dative bond. A study describing the recrystallization of triarylboranes has appeared as has a report concerning the solvent isotope effect on the rate of methoxyl exchange between methyl borate and methanol. 2-[l-(o-dihydroxyborylphenyl)-2-phenylethyl]-2-imidazoline has been used for the selective determination of tartaric acid in aqueous solution and the absorption and emission properties of a series of phenyl-substituted dimesityl-boranes have been described. Synchronous spin- and stereo-isomerization of the complex cw-Fe(phen) a(NCBPh 3) 2 have been reported, The kinetics and mechanism of the protodeboronation of ferroceneboronic acid in moderately concentrated HaSO have been studied. ... 

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