Ion-pair complexes

The proton is not the only entity that can dissociate from a substrate or bond to it. We can enumerate other interactions, such as metal-ligand complexation, ion-pair formation, charge-transfer complex formation, etc. For the sake of brevity, we treat all of these as... 

The rate constants characterizing reactions of the uncomplexed ion pairs are given subscript 1, those of the complexed ion-pairs subscript 2. 

Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ("chemical bond"), an outer-sphere complex (ion pair) or be in the diffuse swarm of the electric double layer. (From Sposito, 1989)... 

The ability of quaternary ammonium halides to form weakly H-bonded complex ion-pairs with acids is well established, as illustrated by the stability of quaternary ammonium hydrogen difluoride and dihydrogen trifluorides [e.g. 60] and the extractability of halogen acids [61]. It has also been shown that weaker acids, such as hypochlorous acid, carboxylic acids, phenols, alcohols and hydrogen peroxide [61-64] also form complex ion-pairs. Such ion-pairs can often be beneficial in phase-transfer reactions, but the lipophilic nature of H-bonded complex ion-pairs with oxy acids, e.g. [Q+X HOAr] or [Q+X HO.CO.R], inhibits O-alkylation reactions necessitating the maintenance of the aqueous phase at pH > 7.0 with sodium or potassium carbonate to ensure effective formation of ethers or esterification [49,64]. 

It is noteworthy that benzyltriethylammonium chloride is a slightly better catalyst than the more lipophilic Aliquat or tetra-n-butylammonium salts (Table 5.2). These observations obviously point to a mechanism in which deprotonation of the amine is not a key catalysed step. As an extension of the known ability of quaternary ammonium halides to form complex ion-pairs with halogen acids in dichloromethane [8], it has been proposed that a hydrogen-bonded ion-pair is formed between the catalyst and the amine of the type [Q+X—H-NRAr] [5]. Subsequent alkylation of this ion-pair, followed by release of the cationic alkylated species, ArRR NH4, from the ion-pair and its deprotonation at the phase boundary is compatible with all of the observed facts. 

FIGURE 1.19 X-ray crystal structures of selector-selectand complexes (ion-pairs) (a) O-9-(P-chloro-fert-butylcarbamoyl)quinine with iV-(3,5-dinitrobenzoyl)-(5)-leucine, (b) tbe pseudoenantiomeric complex of 0-9-( 3-cbloro-tert-butylcarbamoyl)quinidine with N-(3,5-dinitrobenzoyl)-(i )-leucine, (c) 0-9-( 3-cbloro-terf-butylcarbamoyl)quinine with N-(3,5-dinitrobenzoyl)-(5)-alanyl-(5)-alanine, and (d) comparison of tbe complexes of (a) and (c). Most hydrogens have been omitted for the purpose of clarity. (Reprinted from C. Czerwenka et al., Anal. Chem., 74 5658 (2002). With permission.)... 

Type 111-E Extraction of ion pairs, and other unusual complexes Ion pair CIA2 (and counter species) in organic phase... 

Anion effects have been observed especially in relation to dissolution of the cation complexes in media of low polarity. Soft organic and inorganic anions (phenates, thiocyanate, permanganate etc) generally allow ready dissolution the pier ate anion has been much used (62, 66). The interaction between the anion and the complexed cation may affect the stability of the complex. Ion pairing may occur when the anion can contact the complexed cation, as in the case of macrocyclic complexes, where approach of the anion from top and bottom is possible. This is observed in the RbNCS complex of IS, but not in its NaNCS complex, as shown by the crystal structure data (100). With bromide as anion both a complexed ion pair and a complexed sodium cation are found in the solid state for (15, NaBr) (118). 

Fig. 19. The asymmetric unit in NaBr (XV) 2HaO showing A, a complex cation Na (XV) 2HaO and B, a complexed ion pair NaBr, (XV), HaO. The fourth water molecule forms a hydrogen bonded bridge between a coordinated water molecule and the coordinated bromide ion...

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

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... 

In the case of a polarizable monomer like propylene sulfide, cryptated ion pairs are not only much more reactive than the corresponding non-complexed ion pairs but they are even more reactive than free ions In THF. For ethylene oxide, the results are different since free alkoxide ions are significantly more reactive than cryptated Ion pairs which are themselves slightly less reactive than the corresponding non complexed ion pairs. 

The free energy change AG involved in the actual electron transfer process encounter complex - - ion-pair, can be calculated according to the expression 6.29. 

Increasing the amount of XIV Cl in a solution containing 2 + C60 complex also resulted in the rapid decrease of absorption at 452 nm (attributed to fullerene-fullerene interactions in a closed-packed one-dimensional array of C6o inside the nanotubular cavity), indicating that the encapsulation of ammonium ions led to partial disruption of the close-packed C60 array resulting in the formation of a mixed complex ion-pair/C6o host-guest complex, where the ion pair is intercalating between the fullerenes. 

In addition to the steric issues in chemoselectivity, which are intrinsic to the reaction of a given set of substrates, solvent and sensitizer effects are of importance for a direct control on selectivity because they can be controlled. The reaction of 8a and 13c catalyzed by sensitizer 6 shows an overall increase in cross-product formation with increasing solvent polarity. Entries 4 and 5 in Table 4.1 demonstrate that use of 3a reduces the formation of cross-products in polar solvents. This difference can be rationalized by the fact that after ET by 6, a strongly complexed ion pair is formed, whereas in the case of 3a, the sensitizer is neutral and can dissociate more easily from the radical cation formed. 

Completely unsolvated naked anions cannot be prepared in solution with coronands or even with cryptands as cation solvators. Even in this case ion-pairing still occurs leading to complexed ion pairs [646]. Totally unsolvated naked anions can exist only in the gas phase cf. Section 5.2. ). 

Figure 9.10. (a) Surface complex formation of an ion (e.g., cation) on the hydrous oxide surface. The ion may form an inner-sphere complex ( chemical bond ), an outer-sphere complex (ion pair), or be in the diffuse swarm of the electric double layer. (The inner-sphere complex may still retain some aquo groups toward the solution side.) (From Sposito, 1989.) (b) A schematic portrayal of the hydrous oxide surface, showing planes associated with surface hydroxyl groups ( s ), inner-sphere complexes ( a ), outer-sphere complexes ( /3 ), and the diffuse ion swarm ( d ). (Adapted from Sposito, 1984.)... 

Figure 6.5 Complex ion pair between the PTC 1 and the anion of 2 proposed by the Dolling group.

Contrast outer-sphere complexes (ion pairs) and inner-sphere complexes, using examples, in terms of the character and strength of the metal-ligand bond in solution (the value of for the complex) and its covalency and ionicity. 

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