Zirconium borate

A variety of transition and other metal borates are reported in the literature. These include manganese borate, silver borate, iron borate, copper borate, nickel borate, strontium borate, lead borate, zirconium borate, double metal borates, and so on. Many of these borates have the potential to be flame retardants and/or smoke suppressants. 

Borides are inert toward nonoxidizing acids however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they ate also readily attacked by hot alkaline salt melts or fused alkaU peroxides, forming the mote stable borates. In dry air, where a protective oxide film can be preserved, borides ate relatively resistant to oxidation. For example, the borides of vanadium, niobium, tantalum, molybdenum, and tungsten do not oxidize appreciably in air up to temperatures of 1000—1200°C. Zirconium and titanium borides ate fairly resistant up to 1400°C. Engineering and other properties of refractory metal borides have been summarized (1). 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium, titanium, zirconium, and tin, ° silyl enol ethers, enol borinates,and enol borates, R CH=CR"—OB(OR)2. The nucleophilicity of silyl enol ethers has been examined. In general, metallic Z enolates give the syn (or erythro) pair, and this reaction is highly useful for the diastereoselective synthesis of these products. The ( ) isomers generally react nonstereoselectively. However, anti (or threo) stereoselectivity has been achieved in a number of cases, with titanium enolates, with magnesium enolates, with certain enol bor-inates, and with lithium enolates at — 78°C. ... 

A solid, encapsulated scale inhibitor (calcium-magnesium polyphosphate) has been developed and extensively tested for use in fracturing treatments [1451-1453]. The inhibitor is compatible with borate- and zirconium-crosslinked fracturing fluids and foamed fluids because of coating. The coating... 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

Among the preformed enol derivatives used in this way have been enolates of magnesium, lithium,526 titanium,527 rhodium,528 zirconium,522 and tin,529 silyl enol ethers,530 enol bori-nates,531 and enol borates R CH=CR"—OB(OR)2.532 In general, metallic Z enolates give... 

Zirconium Borohydride (Zirconium Tetrahydro-borate). Zr(BH4)4 mw 150.5 volatile pyrophoric cryst mp 28.7° bp 123° (extrapolated) d l.l3g/cc. Prepn is by reacting Zr tetrachloride with Li borohydride — the yield is 77%. Accdg to Gaylord (Ref 4) the compd is spontaneously flammable in air... 

Within this group, only lead and zirconium have borates of definite composition. The PbO 2B203 4H20 compound is formed by evaporation of acetic acid from lead acetate-boric acid mixtures (206). It dehydrates in three stages the first water molecule is lost at 50-95°C, two more between 130 and 170°C, and the remainder at over 300°C. The Pb0-B203-H20 system has been studied at 50, 75, and 100°C (385, 386), and the compounds detected were 1 2 3 at 50°C 4 5 2.5, 1 2 3, and 3 10 9 at 75°C and 4 5 2.5, 1 2.5 2, and 3 10 9 at 100°C. Postulated structures for some of these borates are outlined in Section III,D. 

This section does not review comprehensively all the tetrahydro-borate derivatives, since these have been adequately covered elsewhere (50). The aluminohydride compound Cp2Zr(H)AlH4 has been mentioned, but several similar hydrides have been identified. Such compounds are relevant in particular to known reactions where titanium(IV) or zirconium(IV) compounds catalyze the addition of Al-H bonds to alkenes and alkynes (114, 115). 

X-ray analysis of solid compounds such as ((CH3)2C5H3)2ZrCH3+ + H3CB(C6F5)3 (90) showed that a coordination bond still partly exists between the zirconocene and the borate. The olefin is tt-complex bonded into this compound and then inserted into the zirconium-methyl bond. 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

Zwitterionic structures with a borate anion covalently attached to a cyclopentadienyl donor can be obtained by a number of synthetic approaches. In the known compounds, the means by which the borate is tethered to the ring has a significant impact on the properties of the zwitterions. In particular, the length of the tether connecting the ring with the borate is crucial for the stability and intramolecular ion-ion contacts. For example tris cyclopentadienyl zirconium betaine complex in which boron atom is directly bonded to Cp can be generated from boron substituted cyclopentadienyl anion (equation 21). ... 

Hydroboration of coordinated alkenes has been achieved with the phosphine-stabilized olefin complexes, (775-C5H5)2Zr(772-CH2=CHR)(PPh2Me) (R = H, 78 Et, 79 Ph, 80), upon addition of the Lewis-acidic borane, HB(C6F5)2 (Equation (5)).37,65 Solid-state characterization indicates a weak interaction between the formally positively charged zirconium center and the carbon adjacent to the borate anion. This interaction is maintained in solution, as an upheld shifted 13C NMR resonance is observed for this carbon, which is in agreement with previous reports of metal-carbon interactions of this type. 

Treatment of a benzene solution of the sterically crowded bis(aryloxide)zirconium dibenzyl complex 5 with BIC F5)3 leads to the facile formation of the corresponding zwitterionic species 6 (Scheme 3).44 The molecular structure of 6 features a three-legged piano-stool geometry about Zr with the 7/ -bound benzyl group and the tightly associated benzyl borate anion through r/ -arcne coordination this complex is stable in solution in the presence of propylene and phenylpropyne. In contrast, treatment of the 2,6-di-Buc-substituted aryloxide dibenzyl derivative 7 with B(C6Fs)3 forms the cyclometallated compound 8 after elimination of toluene (Scheme 3) 45 compound 8 was structurally characterized. 

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