Boron cations

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. 

Boron cations, Me2B+ and (MeO)2B+, are highly reactive towards carbonyl compounds, but are not easily studied in condensed phase, hi the gas phase, two pathways compete 183 (i) C=0 cleavage, with OH abstraction leaving behind a... 

An additional / -donor ligand results in the most stable boron cations because of the filled octet of boron and the complete coordination sphere.583 Boron in cation 240 has a distorted tetrahedral geometry and a N(l)—B—C angle of 99.5°.583 In cation 241 the boron atom is complexed to [Cp Fe(CO)2].595 The Cp group is bonded to boron in an r 5 fashion and the Cp-B-Fe vector is essentially linear (bond angle = 177.86°). Both spectroscopic data and DFT calculations (B3LYP with LANL2DZ and 6-31 +G ... 

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... 

The boron cation is stable in acidic aqueous solution, somewhat less stable in water, but decomposes in alkaline solution. The proton magnetic resonance spectrum in D20 relative to external TMS shows three bands —8.60 (broad), - —8.19 (doublet, J = 7 Hz.), and —2.85 (singlet), with the expected intensities corresponding to the a- and /6-ring hydrogen atoms and the methyl group, respectively. The bromide is very soluble in cold water and hot ethanol but insoluble in acetone. The hexafluorophosphate is slightly soluble in cold water but very insoluble in acetone. 

Whereas for the smallest member of Group 13, boron, cationic species of the type [B(H20)6]3+ are unknown, the remaining four elements have a rich cationic chemistry, in line with their metallic character. They also form many molecular compounds that are covalent. 

M = Nb, Ta) were obtained in an unusual process on interaction of K[HB(pz )3] with MClj. Their structure was again confirmed by X-ray diffraction and also NMR data Another series of interesting cyclic boron cations of tyj 15 has been reported... 

The boron cation, B, exists in aqueous solution as undissociated boric acid B(OH)3 and as the borate ion B(OH)4. Formation waters with pH values lower than 9 favor B(OH)3 as the predominant species, because the pK values for reaction (1)... 

Expected Optical Activity in Boron Cations. Through procedures outlined earlier, boron cations can be synthesized containing four different ligands. Miller and Muetterties reported HXB[N(CH3)3][N(C2H5)(CH3)2] +. 

The treatment of boron cations as typical coordination compounds... 

Optical isomerism for boron cations of the form [Babe d]" is predicted but, to the best of the author s knowledge, resolution of such complexes into optical isomers has not yet been reported. 

A significant contribution in the area of low-coordinate boron chemistry was realized by Corey with the synthesis of chiral tri-coordinate and cationic oxaborolidine complexes such as (4), for use in asymmetric catalysis. These compounds can be obtained by reaction of the desired chiral aminophenol with BBrs, and they are highly effective catalysts for enantioselective Diels-Alder reactions. Smith also reported the generation of the tri-coordinate boron cation HC(CMeNAr)2 BMe+, obtained by a Me abstraction reaction of HC(CMeNAr)2 BMe2 by B(C6F5)3. This boron cation appears to be much less Lewis acidic than its A1 counterpart. 

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