Complex ions counterions

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

A chelate compound may be either a neutral molecule or a complex ion associated with the appropriate counterions to produce electroneutraUty. 

The thermodynamic analysis of the selectivity of ion exchange with the participation of ions of quaternary ammonium bases [56--58] has shown that an increase in bonding selectivity, when metal ions are replaced by organic ions, which is usually accompanied by an increase in entropy of the system (Table 5). It follows from Table 5 that a drastic increase in bonding selectivity upon passing to a triethylbenzylammonium counterion (the most complex ion) is due to a considerable increase in the entropy of the system. 

The application of the HSAB principle is of considerable importance in preparative coordination chemistry in that some complexes are stable only when they are precipitated using a counterion conforming to the above rule. For example, CuCls3 is not stable in aqueous solution but can be isolated as [Cr(NH3)6][CuCl5]. Attempts to isolate solid compounds containing the complex ion Ni(CN)s3 as K3[Ni(CN)5] lead to KCN and K2[Ni(CN)4]. It was found, however, that when counterions such as Cr(NH3)63+ or Cr(en)33+ were used, solids containing the Ni(CN)53 anion were obtained. 

The results obtained with different counterions are shown In Table.II. Free alkoxlde Ions are about seventy times more reactive than cryptated Ion pairs. Cryptated Ion pairs are surprisingly slightly less or as reactive as the corresponding non complexed Ion pairs within the experimental errors though the... 

The solubility product is the equilibrium constant for the dissolution of a solid salt into its constituent ions in aqueous solution. The common ion effect is the observation that, if one of the ions of that salt is already present in the solution, the solubility of a salt is decreased. Sometimes, we can selectively precipitate one ion from a solution containing other ions by adding a suitable counterion. At high concentration of ligand, a precipitated metal ion may redissolve by forming soluble complex ions. In a metal-ion complex, the metal is a Lewis acid (electron pair acceptor) and the ligand is a Lewis base (electron pair donor). 

A key assumption implicit in the model is that isomerization and propagation take place only within the ion-counterion-monomer complex and that the ion-counterion pair is unreactive. Results of the (concurrent) composition and rate studies support this assumption. This is expected since the reactions of the ions are generally believed to proceed through charge-dispersed transition states. Formation of the ion-counterion-monomer complex provides intermediates which approach the energy level and charge dispersion of the transition states. In nonpolar solvent, stabilization of the ion-counterion pair provides the driving force for the formation of the complex. 

If these three reactions occur only within the ion-counterion-monomer complex then ... 

A hypothesis which may explain the experimental observations can be developed as follows Transfer has been assumed to occur by proton transfer to monomer. Previous studies (18,19) indicate that propagation and transfer have similar transition states in cationic polymerizations. For this reason it is possible that these two processes may both occur within the ion-counterion-monomer complex. Termination has been assumed to occur by ion-counterion collapse (20), for example, for EtAlCl2 ... 

These reactions have transition states different from those of propagation and transfer and can occur in the absence of monomer. Termination via ion collapse may be hindered in the ion-counterion-monomer complex by the presence of monomer, that is, the ion-counterion pair is in fact separated by complexed monomer. Reactions (17>—(19) may occur only in the uncomplexed ion-counterion pair. In that case ... 

The lines shown in Fig. 23 may be expressed by an equation of the form of (21). According to this concept termination occurs only in the ion-counterion pair, while transfer, propagation, and isomerization occur only within the ion-counterion-monomer complex. 

A model for the mechanism of the polymerization has been developed. In this model isomerization and propagation occur within an ion-counterion-monomer complex. The independence of J on monomer and coinitiator concentration and the first order dependence of the polymerization rate on monomer concentration have been accounted for by this model. 

The effect of monomer concentration on number average molecular weight indicates that while transfer to monomer occurs within an ion-counterion-monomer complex with an activation energy equal to that of propagation, termination occurs in an uncomplexed ion-counterion pair and decreases relative to propagation with decreasing temperature. 

The second approach involves the addition of a chelated form of the transition metal, for example, Ci2-dien plus zinc(II) (where Ci2-dien is 4-dodecyldiethylenediamine), into the mobile phase (Fig. 2.7). In this example, the triamine chelate binds strongly to the metal ion, forming a complex cationic counterion. The metal chelate represents a conformationally semirigid structure with a local polarized charge center. As such, not only is there the typical electrostatic attraction of anions for the positively charged... 

Azide ions are by far the most common nucleophilic species employed in substitution reactions for the preparation of amino sugars. An azido moiety is stable under many reaction conditions but can be reduced to an amino group by a variety of reagents. The nucleophilicity of azido ions can be increased by the addition of a suitable crown ether to complex the counterion.36c,63b In the past, ammonia and hydrazine were used as nucleophiles to overcome unfavourable dipolar interactions that arise when charged nucleophiles were used. However, a drawback of the use of these nucleophiles is that the product is still nucleophilic and can perform a second displacement. Phthalimide ions have successfully been applied in displacement reactions to yield a protected amino sugar derivative.58 63f... 

In addition to homogeneous complexation, ions may be extracted from various solvents or membranes by use of heteromacrocycles. Extraction may be quantified by monitoring a counterion. For example, a metal cation paired with picrate anion will be colored and can be assayed by visible spectrophotometry <1997MI931, 1998T13421,... 

Living anionic polymerization can also be used to produce well-controlled block copolymers. For PMMA, the best procedures need temperatures below O C and are therefore unlikely to be commercially attractive. Hiey are, furthermore, largely unsuccessful for the controlled polymerization of acrylates, which are far too reactive. The use of tetraalkyl ammonium ate complexes, in conjunction with an appropriate aluminum catalyst, solved fhis problem [225]. The function of the ammonium counterion is to promote dissociation of the complex ion to form the reactive ate complex of the aluminum enolate of the ester (Scheme 6.176). Thus, polymerization was initiated by the lithium enolate of isobutylate in the presence of the ate complex of Me3Al-R3NCl. A controlled block copolymer (PMMA-block-... 

The results on both the redox properties, IVCT bands, and EPR spectra of the trimers described earlier demonstrated a stronger electrostatic interaction of the complex anion with a smaller counterion that could locate more closely to the complex ion than the larger one, resulting in perturbation of the delocalization more significantly than larger anions. 

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