Crystalline Borates

There are two broad categories of borates referred to as hydrated and anhydrous. Hydrated borates contain B-OH groups and may also contain interstitial H2O in their structures. Hence, the fully hydrated forms of the fundamental borate units above are B(OH)3 and [B(OH)4] , as found in aqueous solution. The majority of borate minerals and synthetic borates of commercial interest are hydrated. Compositions of borates can be resolved into the form a b B2O3 c H2O (x = 1 for dications x = 2 for 

Two triborate moieties may link together by sharing a common tetrahedral boron atom to form the pentaborate anions shown in Fig. 5. The pentaborate monoanion shown in Fig. 5a is observed in solution (see Fig. 2) and occurs in synthetic and mineral borates. Examples are the synthetic commercial ammonium pentaborate, NH4[Bs06(0H)4] 2 H2O [ = (NH4)20 5 B2O3  

4 H2O], and the mineral sborgite Na[B50g(0H)4] 3 H2O [25], which both contain isolated pentaborate monoanions. The pentaborate trianion (Fig. 5b) is found in the industrially important mineral ulexite [26], NaCa[B506(0H)g]  

Most of the isolated borate anions shown above may polymerize in a variety of ways to form extended chains, sheets, and networks. Polymerization occurs by either sharing an exocyclic oxygen atom or by sharing an intracyclic tetrahedral boron atom. These are exemplified by the structures of the important industrial minerals colemanite [29], Ca[6304(0H)3] H2O 

Borates can be viewed as complex salts in which the Lewis basicity of the structural unit must match the Lewis acidity of the interstitial complex to produce a stable structure. The relationship between basicity of the borate structural unit and its BO4/BO3 ratio was mentioned above, indicating that each specific structural unit has an associated basicity. It was recently shown that the Lewis basicity of borate structural units correlates with the average coordination number of the oxygen atoms they contain, counting H-bonds 

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The polyions postulated in solution all have known structural analogues in crystalline borate salts. Investigations of the Raman (66) and B nmr (67) spectra of borate solutions have confirmed the presence of three of these species the triborate (3), B202(0H) 4, tetraborate (4), [B40 (0H) 4], and pentaborate (5) B O (OH) 4, polyanions. Skeletal stmctures were assigned based on coincidences between the solution spectra and those soHd borates for which definitive stmctural data are available (52). These same ions have been postulated to be present in alkah metal borate glasses as well. 

Disodium Octaborate Tetrahydrate. The composition of a commercially available sodium borate hydrate, 66.3 wt % B203, POLYBOR (64), corresponds quite closely to that of a hypothetical compound, disodium octaborate tetrahydrate, Na2Bg013 TH O or Na O 4-B203 4H20. This product dissolves rapidly in water without the temperature decrease, which occurs when the crystalline borates dissolve, and easily forms viscous supersaturated solutions at elevated temperatures. The solution pH decreases as the concentration increases (Table 10). The solubility of the product is shown compared with that of borax (33) ... 

In the examination of crystalline borates, caution has been exercised over the structural interpretations of spectroscopic studies. Certain features pertaining to the boron coordination and water/hydroxyl entities can be deduced, but absolute configurations of molecules require more exacting techniques. 

Vibrational spectroscopy has proved to be the most useful technique yet for identifying which boron species are present in solution. Close agreement between solution spectra and those of crystalline borates of known structure have confirmed the presence of hitherto postulated polyborate ions. Details of the IR spectra of the NajO-BaOs-H O system at 26°C with absorptions assigned to polyborate species are shown in Table III (416, 417). In a more recent study (126), the major ions in 0.5 M boron solution were identified as BsOefOH), B405(0H)4 , and B(OH)4-. 

The structure determination of crystalline borates is well advanced, and it is hoped that the present enthusiasm will continue. There are many candidates for further investigation, particularly where spectroscopic or dehydration characteristics have suggested a particular boron anion. For example, the borate 2ZnO - 3B203 - 3.5H20 loses its water at temperatures in excess of 260°C, implying that its formula water is in the form of hydroxyl groups. 

Most nonmetal borates reported in the literature are ammonium and substituted ammonium salts, hi general, boric acid forms salts with alkyl- or aryl-amines that are roughly as or more basic than pyridine. Depending on the reaction conditions, a number of different crystalline borates might be isolated from a given amine. However, the chemistry of substituted amine borates is dominated by the occurrence of pentaborates, and relatively few compounds in this class have been reported containing more basic borates anions. 

Almost 40 years after the Lilly procedure was disclosed, a crystalline borate ester was prepared by an analogous method. Dissolution of 3-bromopyridine and triisopropyl borate in toluene-THF followed by the addition of n-BuLi in hexanes at 0°C delivered the borate ester in good yield (Scheme... 

The predominant polymeric species appears to be the ring polymer (8-1V). Rings of this sort have been characterized in crystalline borates such as Cs20-2B203. The equilibria in solution are rapidly established, as shown, for example, by rapid exchange between boric acid labeled with 180 and borates. 

Q Density Functional Theory Calculations of NMR Parameters in Crystalline Borates... 

P. J. Bray. NMR and NQR studies of boron in vitreous and crystalline borates. Inorg. Chim. Acta 289, 1999,158. 

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