Tetraphenylborates

The larger cations of Group 1 (K, Rb, Cs) can be precipitated from aqueous solution as white solids by addition of the reagent sodium tetraphenylborate, NaB(C( H5)4. Sodium can be precipitated as the yellow sodium zinc uranium oxide ethanoate (sodium zinc uranyl acetate). NaZn(U02)3(CH3C00)y. 9H2O. by adding a clear solution of zinc uranyl acetate in dilute ethanoic acid to a solution of a sodium salt. 

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

One of the largest commercially used Grignard reagents is phenyhnagnesium chloride. Millions of kg per year of this Grignard react captively with inorganic haUdes. Some examples of these products are triphenylphosphine, triphenyl tin hydroxide, sodium tetraphenylborate, and triphenylantimony. 

In a similar manner tetraphenylbismuth chloride [42967-53-3], C24H2QBiCl, (162) and tetraphenylbismuthonium tetrafluoroborate [36682-02-7], C24H2qBB1F4, (163) are obtained from pentaphenjlbismuth and hydrogen chloride or hydrogen tetrafluoroborate, respectively. When triphenylboron is used, the tetraphenylborate is obtained (162) ... 

Tetraethylammonium tetraphenylborate [12099-10-4] M 449.4. Recrystd from aqueous acetone. Dried in a vacuum oven at 60° for several days. Similarly for the propyl and butyl homologues. 

Tetramethylammonium tetraphenylborate [15525-13-0] M 393.3. Recrystd from acetone, acetone/CCl4 and from acetone/1,2-dichloroethane. Dried over P2O5 in vacuum, or in a vacuum oven al 60° for several days. 

For purposes of characterization of enamines the perchlorate salts are preferred, as they crystallize well, and the perchlorate anion has no tendency to add to the iminium cation. Other salts, including hexachlorostannates (13), hexachloroantimonates (13), chlorides, bromides, tetraphenylborates, and nitrates, have also been used. Recently a method for the preparation of iminium salts directly from aldehydes or ketones and the amine perchlorate has been reported (16). 

It is noteworthy that carbene function may be manifested in the derivatives of tris(imidazol-l-yl)borate 44 (96AGE310). Its reaction first with n-butyllithium, then with iron(II) chloride, and finally with sodium tetraphenylborate gives the iron(III) carbene derivative 45. 

Bis(l-methylimidazol-2-yl)methane and -ketone with the dimer [Rh(CO)2Cl]2 in the presence of sodium tetraphenylborate give the dicarbonyl complexes 68 (X CHj, CO L = CO) where the carbonyl ligands may easily be substituted by the triphenyl phosphine ligands to yield 68 (X = CH, CO L = PPh ) (99JOM(588)69). The bis(l-methylbenzimidazol-2-yl)methane analogs of 68 (X=CH2 L=C0, PPhj) can be prepared similarly. 

Discussion. Potassium may be precipitated with excess of sodium tetraphenyl-borate solution as potassium tetraphenylborate. The excess of reagent is determined by titration with mercury(II) nitrate solution. The indicator consists of a mixture of iron(III) nitrate and dilute sodium thiocyanate solution. The end-point is revealed by the decolorisation of the iron(III)-thiocyanate complex due to the formation of the colourless mercury(II) thiocyanate. The reaction between mercury( II) nitrate and sodium tetraphenylborate under the experimental conditions used is not quite stoichiometric hence it is necessary to determine the volume in mL of Hg(N03)2 solution equivalent to 1 mL of a NaB(C6H5)4 solution. Halides must be absent. 

Procedure. Prepare the sodium tetraphenylborate solution by dissolving 6.0 g of the solid in about 200 mL of distilled water in a glass-stoppered bottle. Add about 1 g of moist aluminium hydroxide gel, and shake well at five-minute intervals for about 20 minutes. Filter through a Whatman No. 40 filter paper, pouring the first runnings back through the filter if necessary, to ensure a clear filtrate. Add 15 mL of 0.1M sodium hydroxide to the solution to give a pH of about 9, then make up to 1 L and store the solution in a polythene bottle. 

Standardisation. Pipette 10.0 mL of the sodium tetraphenylborate solution into a 250 mL beaker and add 90 mL water, 2.5 mL 0.1 M nitric acid, 1.0 mL iron(III) nitrate solution, and 10.0 mL sodium thiocyanate solution. Without delay stir the solution mechanically, then slowly add from a burette 10 drops of mercury(II) nitrate solution. Continue the titration by adding the mercury(II) nitrate solution at a rate of 1-2 drops per second until the colour of the indicator is temporarily discharged. Continue the titration more slowly, but maintain the rapid state of stirring. The end point is arbitrarily defined as the point when the indicator colour is discharged and fails to reappear for 1 minute. Perform at least three titrations, and calculate the mean volume of mercury(II) nitrate solution equivalent to 10.0 mL of the sodium tetraphenylborate solution. 

Pipette 25.0 mL of the potassium ion solution (about 10 mg K + ) into a 50 mL graduated flask, add 0.5 mL 1M nitric acid and mix. Introduce 20.0 mL of the sodium tetraphenylborate solution, dilute to the mark, mix, then pour the mixture into a 150mL flask provided with a ground stopper. Shake the stoppered flask for 5 minutes on a mechanical shaker to coagulate the precipitate, then filter most of the solution through a dry Whatman No. 40 filter paper into a dry beaker. Transfer 25.0 mL of the filtrate into a 250 mL conical flask and add 75 mL of water, 1.0 mL of iron(III) nitrate solution, and 1.0 mL of sodium thiocyanate solution. Titrate with the mercury(II) nitrate solution as described above. 

I. Sodium tetraphenylborate Na+ [B(C6H5)4] . This is a useful reagent for potassium the solubility product of the potassium salt is 2.25 x 10 8. Precipitation is usually effected at pH 2 or at pH 6.5 in the presence of EDTA. Rubidium and caesium interfere ammonium ion forms a slightly soluble salt and can be removed by ignition mercury(II) interferes in acid solution but does not do so at pH 6.5 in the presence of EDTA. 

Ammonium may be determined by predpitation with sodium tetraphenylborate as the sparingly soluble ammonium tetraphenylborate NH4[B(C6H5)4], using a similar procedure to that described for potassium it is dried at 100°C, For further details of the reagent, including interferences, notably potassium, rubidium, and caesium, see Section 11.38,... 

Precipitation reactions Dimethylglyoxime Lead nitrate Mercury(II) nitrate Silver nitrate Sodium tetraphenylborate Thorium(IV) nitrate Potassium dichromate DME DME DME Rotating Pt Graphite DME DME Ni2 + SO2", MoOj", F" r Cl", Br , I", CN", thiols K + F Pb2 +, Ba2 +... 

However, the energy difference between N- and S-bonded thiocyanate is very small and is influenced by an interplay of several factors steric effects, solvent and the counter-ion in ionic complexes. To illustrate the last point, in complexes [Pd[Et2N(CH2)2NH(CH2)2NH2]NCS]+, the PFg salt is N-bonded, as it is in the unsolvated BPhg salt. However, though the acetone solvate of the tetraphenylborate is N-bonded, the methanol solvate is S-bonded [126],... 

Fig. 2. ORTEP view of the cation in bis(trimethylphosphine)(diphenylsilanediyl)(pentame-thylcyclopentadienyl)ruthenium tetraphenylborate x acetonitrile 12 [37]...

Experience in PTC with cationic catalysts showed that, in general, the most suitable compounds have symmetrical structures, are lipophilic, and have the active cationic charge centrally located or sterically shielded by substituents. For anionic catalysis sodium tetraphenylborate fulfills these conditions, but it is not stable under acidic conditions. However, certain derivatives of this compound, namely sodium tetra-kis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB, 12.162) and sodium tetrakis[3,5-bis-(l,l,l,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]borate (HFPB) are sufficiently stable to be used as PTC catalysts for azo coupling reactions (Iwamoto et al., 1983b 1984 Nishida et al., 1984). These fluorinated tetraphenylborates were found to catalyze strongly azo coupling reactions, some of which were carried out with the corresponding diazotization in situ. 

Telomers, in Meerwein reactions 248 Tetrahydropyridazines 129 Tetraphenylborates, in phase transfer catalysis of azo coupling reactions 378 f. 

Nickel, methyltetrakis(trimethylphosphine)-tetraphenylborate stereochemistry, 1,44 Nickel, pentacyano-isomerism, 1, 206 structure, 1, 40 Nickel, tetracarbonyl-cxchangc reactions, 1,288 Nickel, tetracyano-, 5,67 Nickel, tetrahalo-, 5, 186 Nickel, tetrakis(dinitrogen)-syn thesis... 

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