Bifluoride anion

Other structural analyses of crystals in which the bifluoride is present are listed in Table 7. One compound, p-toluidinium fluoride [C7H,oN ][HF2 ], is worthy of further comment. The first X-ray diffraction study reported a symmetrical anion (Denne and MacKay, 1971), but a later analysis showed that the proton was not centred between the two fluorines and 7 f h values were 102.5 and 123.5 pm (Williams and Schneemeyer, 1973). This can be explained not by a double minimum potential energy well but by asymmetry due to other forces, such as secondary hydrogen bonding between one end of the bifluoride anion and the N—H group of the cation. An alternative explanation attributes the asymmetry of the bifluoride hydrogen bond to an unsymmetrical crystal field caused by the cation (Ostlund and Bellenger, 1975). 

Although anhydrous hydrogen fluoride is a very strong acid, its aqueous solution, hydrofluoric acid, is weakly acidic, particularly when dilute. The Ka value of aqueous acid at 25°C is 6.46x10- mol/L. It is an excellent solvent for many inorganic fluorides, forming bifluoride anion ... 

The best, and longest known, case is that of the bifluoride anion in NaHF2 and KHF2. In each the FHF ion lies across a centre of symmetry, and careful neutron-diffraction study finds the proton at the centre. However, neutron diffraction also shows, or appears to show, that the proton vibrates with a particularly large amplitude along the F H F bond. So there is a possibility that this may be partly due to disorder — qualitatively like that in ice (see also section 12), but now with the alternative proton sites not more than 0.1 A from the centre. The neutron-diffraction measurements can be equally well explained by two models the proton vibrates... 

Partially oxidized tetracyanoplatinates containing nonstoichiometric bifluoride anions are prepared electrochemically or via hydrogen peroxide oxidation, e.g. ... 

The Flood group further prepared macrocycle 86 for complexing bifluoride anion [158]. Anion binding was abetted by CH H-bonding provided by both the... 

A 10 g sample is roasted at 650°C and decomposed with hydrochloric acid/hydrogen peroxide. The Pt and Pd in the solution is pre-concentrated using adsorbent materials which are composed of active charcoal and anion resin. The adsorbent materials are washed sequentially with 2% ammonium bifluoride, 5% hydrochloric acid and distilled water, and subsequently ashed in a muffle furnace at 650°C. The total residue of ca. 0.25 mg is dissolved with 2 ml fresh aqua regia, then diluted to 5ml using 10% hydrochloric solution, and determined using ICP-MS, which has a detection limit of 0.2 ppb for Pt and Pd. The residue can also be mixed with a spectral buffer, and determined by DC-arc ES, which has detection limits of 0.3 ppb for Pt and 0.2 ppb for Pd. 

The interpretation is that in any of these bifluoride ions, the potential-energy well is of the flattened type shown in Fig 5. When the anion is at a symmetrical site, the minimum will be exactly at the centre as in curve (d). This occurs in NaHF2 and KHF2. When there is no symmetry, the curve will be tilted, as in curve (/), so that the minimum is considerably displaced to one side. (A close analogy would be a ball-bearing in an egg-cup or in a saucer, corresponding to curves (e) or (d). A 100 tilt of the egg-cup would cause only a small shift in the rest-position of the ball a 10° tilt of the saucer would cause a large shift. A big shift is therefore evidence of a saucer rather than an egg-cup.)... 

The preferred catalysts for GTP are nucleophilic anions. The most active catalysts are fluorides and bifluorides [1]. At above ambient temperatures, however, carboxylates and bicarboxlates are preferred [11]. A large counter ion is required for maximum efficiency. In the early work trisdimethylaminosul-fonium (TAS) was used, but later the more readily available tetrabutylam-monium (TBA) salts have gained favor. Since TBA slowly decomposes under the basic conditions used for GTP, other positive ions may work better. Quirk used cesium ion for his mechanistic studies and found it to be equivalent to TBA [6]. Bywater worked with the very stable Ph3PNPPh3+ bifluoride in his mechanistic probes [19] and Jenkins [21] showed that potassium com-plexed with 18-crown-6 was a possible alternative to TBA (Scheme 10). 

Early work on the GTP mechanism showed that the silyl groups on chain ends rapidly exchange in the presence of anionic catalysts [33, 34]. Without catalyst no exchange occurs [35]. No exchange occurred in the bifluoride catalyzed polymerization of MMA with dimethylphenylsilyl ketene acetal (Scheme 19a) in the presence of dimethyltolylsilyl fluoride [1]. However, in a similar experiment with trimethylsilyl acetate, TBA Ac, and dimethylphenylsilyl ketene acetal, complete exchange occurred within 5 min [36] (Scheme 19b). 

The fact that known anionic initiators for MMA can act as catalysts for GTP and the need for low amounts of catalysts in itself nearly puts to rest the associative mechanism. Seven of the other factors support the dissociative process. Except for the low temperature exchange studies, none supports the associative mechanism. Based on the lack of exchange of added silyl fluoride with silyl ketene acetal ends it looks like fluoride and bifluoride catalysts operate by irreversible generation of ester enolate chain ends [1] (Scheme 19b). On the other hand carboxylate catalysts appear to operate by reversible generation of ester enolate ends as evidenced by rapid exchange of silyl acetate with silyl ketene acetal ends [36] (Scheme 19c). 

The chain tacticity of PMMA synthesized by GTP catalyzed by nucleophiles at different temperatures was analyzed by Webster and coworkers The syndiotactic content increases from 50% at 60 °C up to 80% at —90°C in THF, using tris(dimethylamino)sulfonium bifluoride [(Me2N)3S+ HF2 ] as catalyst . In contrast to the anionic polymerization of MMA, the stereoselectivity of GTP is less sensitive to solvent. It must be noted that PMMA is less syndiotactic when the GTP is catalyzed by nucleophiles rather than by Lewis acids . GTP was extended to the living polymerization of many acrylates and methacrylates, such as nBuMA, glycidyl-MA, 2-ethylhexyl-MA, Me3SiOCH2CH2-MA, sorbyl-MA, allyl-MA, lauryl-MA), acrylates (EA, BuA), acrylonitrile, methacrylonitrile and Al,A-dimethylacrylamide . 

One main difference between anionic polymerization and GTP has to be found in the amount of enolates active in polymerization. In anionic polymerization, all the chains are end-capped by an enolate, which is the case for only a small part of the chains in GTP consistent with the very good control of GTP even at room temperature. In this respect, Brittain and Dicker showed that prop/ term is by far higher in GTP (250) than in classical anionic polymerization ( prop/ term = 8) . In line with slow termination compared to propagation in GTP, Bandermann and coworkers found that the amount of the nucleophilic catalyst is essential to the polymerization control. Indeed, as far as the tris(piperidino)sulfonium bifluoride-mediated GTP of MMA in THF is concerned, the polydispersity index increases with the amount of catalyst . 

By Bifluoride Ion. Trifluoromethyldifluorophosphine and bis-trifluoromethyl fiuorophosphine are oxidized to hexacoordinate phosphorus anions [(CF3) PF5 H] when heated with potassium bifluoride 235). The reactions occur even more readily in the presence of acetonitrile at room temperature. 

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