Metaborates cyclic

The general formula for boric acid esters is B(OR)3. The lower molecular weight esters such as methyl, ethyl, and phenyl are most commonly referred to as methyl borate [121 -43-7], ethyl borate [150-46-9], and phenyl borate [1095-03-0], respectively. Some of the most common boric acid esters used in industrial applications are Us ted in Table 1. The nomenclature in the boric acid ester series can be confusing. The IUPAC committee on boron chemistry has suggested using thalkoxy- and triaryloxyboranes (5) for compounds usually referred to as boric acid esters, trialkyl (or aryl) borates, trialkyl (or aryl) orthoborates, alkyl (or aryl) borates, alkyl (or aryl) orthoborates, and in the older literature as boron alkoxides and aryloxides. Cyclic boric acid esters, which are trimeric derivatives of metaboric acid (HB02), are known as boroxines (1). 

Figure 8 Packing of the cyclic metaborate rings in the structure of /3-BaB204...

Ebelman and Bouquet prepared the first examples of boric acid esters in 1846 from boron trichloride and alcohols. Literature reviews of this subject are available. B The general class of boric acid esters includes the more common orthoboric acid based trialkoxy- and triaryloxyboranes, B(0R)3 (1), and also the cyclic boroxins, (ROBO)3, which are based on metaboric acid (2). The boranes can be simple trialkoxyboranes, cyclic diol derivatives, or more complex trigonal and tetrahedral derivatives of polyhydric alcohols. Nomenclature is confusing in boric acid ester chemistry. Many trialkoxy- and triaryloxyboranes such as methyl, ethyl, and phenyl are commonly referred to simply as methyl, ethyl, and phenyl borates. The lUPAC boron nomenclature committee has recommended the use of trialkoxy- and triaryloxyboranes for these compounds, but they are referred to in the literature as boric acid esters, trialkoxy and triaryloxy borates, trialkyl and triaryl borates or orthoborates, and boron alkoxides and aryloxides. The lUPAC nomenclature will be used in this review except for relatively common compounds such as methyl borate. Boroxins are also referred to as metaborates and more commonly as boroxines. Boroxin is preferred by the lUPAC nomenclature committee and will be used in this review. 

It is perhaps unnecessary to stress that a formula should correspond as closely as possible to the structure of the compound, that is, to the molecule or other grouping present, as, for example, NasBsOg for sodium metaborate, which contains cyclic B3O6 ions. Compounds containing metal atoms in two oxidation states are of interest in this connection. If the oxidation numbers differ by unity the formula does not reduce to a simpler form (for example, Fe304, Cr2ps), but if... 

As already noted, the structures of hydroxy-acids, which include B(0H)3 and Te(OH)g, are described in Chapter 14 with hydroxides, to which they are more closely related stmcturally, the molecules being linked by long 0-H—0 bonds (two from each OH) as in many organic hydroxy-compounds. One form of metaboric acid, consisting of cyclic 6303(011)3 molecules, is also of this type, for the three 0 atoms in the ring are not involved in hydrogen bonding. 

Compounds RB(0H)2 polymerize to cyclic trimers only, in contrast to R2Si(OH)2 which give polymeric linear siloxanes compare the formation of only one cyclic metaborate ion, 8306 . The formation of 6-membered rings is a prominent feature of boron chemistry, as in boroxine (p. 862) and the numerous substituted... 

I.r. spectra of the vapour over indium and thallium metaborates can be assigned to monomeric species MB02, which have a cyclic structure (76).556... 

Alkyl metaborate esters in combination with organic hydroperoxides are reported to be active for epoxidation [182]. The borate esters are cyclic and have general formula (ROBO)3, where R is an alkyl group like cyclohexyl (CeHn). They are six-membered ring compoimds formed from O-B units with a pendant RO, and are 1 1 dehydro adducts of alcohol and boric acid. 

The chemistry of cyclic oligomeric and high polymeric phosphazenes is dominated by substitution reactions, as is the chemistry of boron-nitrogen heterocycles. However, substitutive techniques play only a minor role in the chemistry of siloxane, silazane, phosphate or metaborate oligomers and polymers. It is not yet well developed in sulfur-nitrogen systems but may become more important in the future. 

The ease of condensation in this reaction decreases with increasing size of the side group, R. This type of condensation reaction is also used for the preparation of cyclic metaborates and cyclic metaphosphates ... 

Draw a well-labeled diagram showing the geometry and orbitals used in cyclic metaboric acid, H3B3O6. 

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