Counterions effect

Muto, N., Komatsu, T. Nakagawa, T. (1973). Counterion effect on the titration behaviour of poly(maleic acid). Bulletin of the Chemical Society of Japan, 46, 2711-15. 

In the radical anions of the norbornane-linked naphthalenes [37] mentioned earlier (Gerson et al, 1990) no counterion effects were detected for [37a], which has a small spatial separation, but the esr/ENDOR spectra of [37b]- and [37c]- indicate that the electron-spin transfer between the naphthalene moieties is determined by the rate of synchronous counterion migration (Gerson et al., 1990). For tight ion pairs the electron is localized, while for loose ion-pair conditions, e.g. by using solvents of high cation-solvating power, the transfer becomes fast on the hyperfine timescale (k > 107 Hz). 

It should be kept in mind that quantum chemical calculations of structures and magnetic properties generally are done for the isolated carbocation without taking into account its environment and media effects such as solvent, site-specific solvation or counterion effects. This is a critical question since NMR spectra of carbocations with a few exceptions are studied in superacid solutions and properties calculated for the gas-phase species are of little relevance if the electronic structure of carbocations is strongly perturbed by solvent effects. Provided that appropriate methods are used,... 

Before concluding this section, it has to be reminded that all the above calculations represent gas-phase processes without solvent and/or counterion effects, and without unfavorable entropic contributions. Recent studies have shown that they modify substantially the olefin-coordination energetics. 

Virtually every iridium catalyst of the formula [Ir(L )(COD)] [X] for asymmetric olefin hydrogenation that has appeared after the initial counterion effect studies was based on BArp as the preferred anion [14]. The anion effect is broadly applicable in iridium-catalyzed reductions as experiments with a direct analog of the Crabtree catalyst of the formula [Ir(pyridine)(Cy3P)(COD)]BArp indicates (Fig. 2). 

The authors conclude that the experimentally found energetic proximity of the two states is not an intrinsic property of the diaryldiazo cation-radicals, but must be due to some solvent or counterion effects acting to preferentially stabilize the a state by about 1 eV. These effects, however, failed to be identified in the quoted paper. Further development of the problem remains to be seen. 

The salt effects just considered are counterion effects. Sometimes, however, an added salt can induce electron transfer from a donor to an acceptor. Here are several examples. 

Ionic polymerizations are distinguished by their marked sensitivity to changes in the polarity and solvating ability of the reaction media and counterion effects. Radical polymerizations do not show such effects. 

Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbomene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbomene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 

Chen W-R, Porcar L, Liu Y, Butler PD, Magid LI. Small angle neutron scattering studies of the counterion effects on the molecular conformation and strucmre of charged G4 PAMAM dendrimers in aqueous solutions. Macromolecules 2007 40 5887-5898. 

Kanemasa, Curran, and co-workers did an extensive study on the metal and counterion effects in the Diels-Alder reaction using the DBF-box ligand Their results are summarized in Table 9.12 (Fig. 9.27). It has been shown that, for this ligand, the optimal conditions were use of nickel(II) perchlorate at 40 °C for 14 h leading to 96% yield of cycloadduct in a ratio of 97 3 (endo/exo) with >99% endo ee. 

The expansion of the film with Increasing acidity of the substrate may be due to the competition of counterions at the interface. The swamping amount of H ions in low pH subsolution competes with Na ions at the negatively charged interface. Such competition has been shown to exist between H ions and K" " ions at the negatively charged micelle-solution interface (11). Studies on the counterion effects in sodium docosyl sulfate monolayers (, 12) have shown that the film expansion follows the sequence Li > Na > K ". It follows that H" should give rise to the most expanded film. 

Three phenomena described above beg quantitative explanation. First, the buffer effects, which are readily explained qualitatively, require a quantitative theory. Second, the specific counterion effects need to be addressed. Third, there is the putative swelling phase transition that occurs at a critical pH, the latter depending on the composition of the geL As of this writing, only the buffer effects have been considered in detail, although work is in progress to understand the nature of the phase transition. 

Protein adsorption is dependent on the nature and concentration of the electrolyte. Norde and Lyklema have shown 61 > that adsorption generally increases with decreasing charge on the protein and on the surface, although counterion effects can often override this generalization. 

Finally, the effect of reaction conditions and solvents on chemoselectivity should also be considered.18 Empirically, polar, more basic solvents, e.g. HMPA, DMF, serve to minimize counterion effects by formation of solvent-separated ion pairs and promote electron-transfer processes which are conducive for... 

Evans, and later j0rgensen, studied the counterion effect of these Cj-symmetric bis(oxa-zoline)/Cu(II) complexes, and found that the counterion structure dramatically affected the catalytic efficiency, and SbFg- was the best among the anions examined (SbF6>PF6>OTf>BF4) [27,28] (Eq. 8A.15). This cationic bis(oxazoline)/Cu(II) catalyst has been successfully applied to asymmetric synthesis of enr-A1 -tetrahydrocannabinol [29] and enf-shikimic acid [30]. 

This electrophilic assistance could account for the observed solvent and counterion effects. The stereochemistry of substitution of alkoxy, mercapto and fluorosilanes with alkoxide ion depends dramatically on the composition of the solvent4. As the percentage... 

Photoinduced electron transfer rates can be considerably reduced when the counterion X- is changed from chloride to bromide. Charge transfer between the cationic part of a molecule and the bromide ion may be responsible to the reduction of photoinduced electron-transfer rates. Such a counterion effect on the photoinduced electron transfer and the reverse process has been demonstrated for examples of porphyrin-viologen-linked compounds (Mitsui et al. 1989). 

With these anthracene-linked dimeric cinchona-PTCs, the Najera group investigated the counterion effect in asymmetric alkylation of 1 by exchanging the classical chloride or bromide anion with tetrafluoroborate (BF4 ) or hexafluorophosphate (PF6-) anions (Scheme 4.10) [17]. They anticipated that both tetrafluoroborate and hexafluorophosphate could form less-tight ionic pairs than chloride or bromide, thus allowing a more easy and rapid complexation of the chiral ammonium cation with the enolate of 1, and therefore driving to a higher enantioselectivity. However, when... 

Boehme, C., Coupez, B., Wipff, G. 2002. Interaction of M3+lanthanide cations with malonamide ligands and their thia analogues A quantum mechanics study of monoden-tate vs bidentate binding, counterion effects and ligand protonation. J. Phys. Chem. A 106 6487-6498. 

Fig. 19 Counterion effects on the transfection by DOTAP lipoplexes as estimated from luciferase expression of NIH3T3 cells (a) and Balb-C murine lung (b) [60]...

---