Containing Complex Boron Cations

Although anions containing boron have been known for many years, boron cations are much less common. The first such boron cation to be reported was bis(2,4-pentanedionato)-boron(III) found in 1906 by Dilthey1 in compounds of the types  

More recently Ryschkewitsch and associates6 prepared chelated boron cations with charges ranging from 1 to 3. Cations of 

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Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

The four-coordinate amine complexes are very stable to oxidation 34,143 they do not react with I2 and undergo only monosubstitution upon treatment with Br2 (equation 26). Monohalo-genation of bisamine complexes of the [H2BLL ]+ type affords cations containing chiral boron 34,148 the [XHB(NMe3)4-Mepy]+ (X = Cl, Br) cations were resolved149 into enantiomers. The crystal structure for X = Br has been reported.150... 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Cationic polymerization of coal-tar fractions has been commercially achieved through the use of strong protic acids, as well as various Lewis acids. Sulfuric acid was the first polymerization catalyst (11). More recent technology has focused on the Friedel-Crafts polymerization of coal fractions to yield resins with higher softening points and better color. Typical Lewis acid catalysts used in these processes are aluminum chloride, boron trifluoride, and various boron trifluoride complexes (12). Cmde feedstocks typically contain 25—75% reactive components and may be refined prior to polymerization (eg, acid or alkali treatment) to remove sulfur and other undesired components. Table 1 illustrates the typical components found in coal-tar fractions and their corresponding properties. 

Complex aluminum and boron hydrides can contain other cations. The following compounds are prepared by metathetical reactions of lithium aluminum hydride or sodium borohydride with the appropriate salts of other metals sodium aluminum hydride [55], magnesium aluminum hydride [59], lithium borohydride [90], potassium borohydride [9i], calcium borohydride [92] and tetrabutylammonium borohydride [95]. 

Simultaneous complexation of anions and cations, in a similar way to that reported for 15-crown-5 uranyl salene complexes, has been observed in the 21-membered ring boronate crown ether 40. Ligand 40 is capable of dissolving stoichiometric amounts of KF in dichloromethane at room temperature to give the KF adduct shown at Figure 13. The fluoride ion is bound to the boron atom out of the plane of the crown ether, which contains the ion. Ligand 40 fails to dissolve... 

Ogner [1] has described an automated analyser method for the determination of boron-containing anions in plants. This is based on the formation of a fluorescent complex between these anions and carminic acid at pH 7. The plant tissues are ashed at 550 °C and the residue dissolved in 0.5 N hydrochloric acid prior to adjustment to pH 6-7 with sodium carbonate solution. The solution is excited at 470 nm and fluorescence intensities measured at 585 nm. Interferences by the reaction of some cations with carminic acid are overcome by passing the solution through an ion exchange column to exchange the cations for sodium ions. Analytical recoveries of boron anions were in the range 98-104%. The detection limit of the method was 5 xg/l boron. 

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