Tetraphenylborate anions, lipophilicity

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. 

Lipophilic ion exchangers traditionally used for polymeric membrane preparation are the anionic tetraphenylborate derivatives and the cationic tetraalkylammonium salts. The charges on both lipophilic ions are localized on a single (boron or nitrogen) atom, but the steric inaccessibility of the charged center, due to bulky substituents, may inhibit ion-pair formation in the membrane and provide, when necessary, non-specific interactions between ionic sites and sample ions. 

E. Bakker and E. Pretsch, Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors. Anal. Chim. Acta 309, 7-17 (1995). 

Calcium phosphate sensors are commonly based on solvents with low permittivities, e.g. DOPP (s = 6.2), whereas neutral carrier types may require highly polar solvents, typically 2-nitrophenyl octylether (2-NPOE), e = 23.6 (28). Another characteristic feature of neutral carrier systems (Table 3.12) is the presence of lipophilic anions, e.g. tetraphenylborate (KTPB) which was first incorporated to reduce the interference by lipophilic sample anions (36) and so dislodge interferents from the membrane phase. Today tetrakis(4-chlorophenyl)borate, KTpClPB, is generally preferred because its water solubility is smaller than that of KTPB by a factor of about 1000, and its... 

Let us first discuss the interface between an oil phase contaiping a highly lipophilic electrolyte BiA, e.g., tetrabutylammonium tetraphenylborate (TBATPB) in nitro-. . benzene, and an aqueous phase containing a highly hydrophihc electrolyte B2A2, e.g., LiCl in water (hereafter Bj(j =1,2,...) represents a cation and Aj(j = 1,2,...) an anion and, unless otherwise stated, it is supposed that Zj = - -1 for M = Bj and —1 for M=Aj), as shown in Cell la ... 

The basis of most titration methods for nonionics is the ability of polyethers to form weak complexes with large cations such as Ba(II). The formation of the complex is governed by the presence of lipophilic anions, and the titrimetric finish usually involves the anion. Tetraphenylborate is the most commonly used anion, although substituted tetraphenylbo-rates, ferrocyanide, tetraiodobismuthate, and heteropoly acids are also used. Thorough discussions of these reactions have appeared. Generally, individual homologs containing more than four alkylene oxide units are titratible (60,61). 

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