Aggregated contact ion pairs

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

Ion pairs commonly exist as solvent separated ion pairs in polar media but may form contact ion pairs or aggregates in less polar solvents. 

Apart from free solvated radical ions (FRI), evidence was gathered for two kinds of ion pairs, which are referred to as tight ion pair (or contact ion pair, CIP) and loose ion pair (or solvent separated ion pair, SSIP) (Scheme 1, Eq. (1)) [3]. It is important to point out that CIP and SSIP are not the only species in solution. There are myriads of spatial cation-anion relationships that lie between them [4]. The SSIP is a pair of two ions of opposite sign with intact solvent shells. This is lacking in the CIP, anion and cation are in direct contact, the whole aggregate being surrounded by solvent molecules. 

The question concerning the nature of allyllithium in solution is of great interest (2). The interpretation of experimental studies is complicated because the bonding situation is considerably influenced by solvent and temperature effects (14,15). Possible structures for allyllithium species in solution are aggregates (A), contact ion pairs (B and C), solvent separated ion pairs (D) and free ions (E) (bases coordinated to the lithium atoms have been omitted for clarity). 

Winstein et al. [45] first presented evidence for the concept that different types of electrophilic species, each with distinct reactivities, may participate in reactions involving cationic intermediates. As shown in Eq. (36), Winstein et al. proposed that four species are in equilibrium, including covalent electrophiles, contact ion pairs, solvent-separated ion pairs, and free ions. In addition, ion pairs may aggregate in more concentrated solutions- According to this concept, electrophilic species do not react with a continuous spectrum of charge separation, but rather in well-quantified minima in the potential energy diagram. 

Whereas ionization and dissociation are clearly defined processes on the left side of Scheme 35, the situation is more complicated for carbanionic systems (Scheme 35, right). Organic alkali metal compounds, for example, which often exist as aggregates, are often described as covalent species with a certain percentage of ionic character [140-142]. If the formal carbanion is a resonance-stabilized species (e.g, diphenylmethyl lithium or sodium), the species with the closest interaction between the organic fragment and the metal is usually called a contact ion pair. In... 

NO3 in acetonitrile has been obtained by Janz and Muller. Associated structures of ions have been studied in nonaqueous solvents over a wide range of dielectric constants. LiCNS in solvents of low dielectric constant, such as ethers and thioethers, gives rise to several different types of ion aggregates. Many different types of contact ion pairs or agglomerates have been identified, and the role the solvent has in this association—whether the solvent separates the ions or not—has been determined. The Bjerrum critical distance, that is, the distance at which the ion is able to interact with other ions to form ion-pair stmctures (see Section 4.8.8), is of great use in these types of studies. Table 4.25 shows some values for 1 1, 2 2, and 3 3 electrolytes in different solvents. 

The propagating anion and its counterion exist in relatively nonpolar solvents mainly in the form of associated ion pairs. Different kinds of ion pairs can be envisaged, depending on the extent of solvation of the ions. As a minimum, an equilibrium can be conceived between intimate (contact) ion pairs, solvent-separated ion pairs, and solvated unassociated ions. The nature of the reaction medium and counterion strongly influences the intimacy of ion association and the course of the polymerization. In some cases the niicrostructure of the polymer that is produced from a given monomer is also influenced by these variables. In hydrocarbon solvents, ion pairs are not solvated but they may exist as aggregates. Such inter-molecular association is not important in more polar media where the ion pairs can be solvated and perhaps even dissociated to some extent. 

The addition of alkali and alkaline earth metal perchlorates to AN causes in the region of the C-N stretching band of AN the appearance of a band produced by acetonitrile molecules in the solvation shells of the cations (fig. 1 for NaClOt solutions) a band of the originally triply degenerate antisymmetric Cl-O stretching vibration splits at increased salt concentration due to ion-aggregate formation (fig. 2, for a solution of LiClO ). The bands (a) and (b) in fig. 2 tire assigned to the free anions, band (1) is due to the vibration of a contact ion pair and band (2) to that of a solvent separated ion pair. Alkaline earth metal perchlorate solutions reveal in addition to the free anion bands (a) and (b) three bands due to [M + C10 )2Y,[M - C10 ]+ and [M + AN)C10 ]+,M + = Mg +,Ca +,Ba +. 

The study of the effective volume of dipole rotation has proved to be a valuable tool for clearing up the structure of ionic aggregates in electrolyte solutions 9t 14,31,32 well as the investigation of the effective dipole moments of the aggregates revealing the transition from solvent separated to contact ion pairs, e. g. NaClOt solutions in AN at increasing electrolyte concentration, where LiBr reveals contact ion pairs down to the lowest measured concentrations. ... 

Metal enolate solutions consist of molecular aggregates (6) such as dimers, trimers and tetramers in equilibrium with monomeric covalently bonded species (7), contact ion pairs (8) and solvent-separated ion pairs (9), as shown in Scheme 1. The nature of the metal cation, the solvent and, to a degree, the structure of the enolate anion itself may significantly influence the extent of association between the anion and the metal cation. In general, the factors that favor loose association, e.g. solvent-separated ion pairs, lead to an increase in the nucleophilicity of the enolate toward alkylating agents and also its ability to function as a base, i.e. to participate in proton transfer reactions. 

Fig. 1 Formation of contact ion pair aggregates [6]. Copyright Wiley. Reproduced with permission...

The result with allyllithium 23 a differs only insignificantly from that of an earlier report (10.5 kcal/mol)25>. Complexation of 23 a with TMEDA does not influence the rate of exchange. Hexamethylphosphoric triamide (HMPT), 15-crown-5 ether and [2.1.1]cryptand in tetrahydrofuran (THF) led to rapid decomposition of 23a. Addition of n-butyllithium had essentially no effect on the barrier. Since the 13C NMR chemical shifts of 23 a are independent of the solvent, it is assumed that 23 a exists as a contact ion pair or higher aggregate in the NMR experiments. (The other alkali metals should also form contact ion pairs with the allyl anion because of their well-known tendency to form contact ion pairs even more readily than the lithium cation 26)). 

There is a considerable body of experimental and theoretical evidence for two types of ionic aggregates termed multiplets and clusters (95). The multiplets are considered to consist of small numbers of associated contact ion-pairs that are dispersed in the matrix of low dielectric constant, but do not themselves constitute a second phase. The number of ion-pairs in a multiplet is sterically limited by the fact that the salt groups are bound to the polymer chain. On the other hand, clusters are considered to be small microphase separated regions (<5 nm) of aggregated multiplets. Thus, the clusters are rich in ion-pairs, but they also contain an amount of the organic polymer. 

Under some circumstances ion-ion interactions can be more important than ion-dipole interactions. This is especially true when the valence of the ion is greater than one, and the electrolyte concentration is high. Then, the formation of ion pairs and higher aggregates is possible. Two types of ion pairs have been recognized, namely, contact ion pairs in which the cation and anion are in physical contact, and solvent-separated ion pairs in which one or two solvent molecules are situated between the cation and anion. Ion pairing must be considered in developing a complete picture of an electrolyte solution. 

The m-isomer must be obtained from contact ion pairs (or possibly aggregates) and loose ion pairs, whereas solvent-separated ion pairs and possibly even solvated ions yield the trans-isomer15. 

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