Alkali cations, solvation

Alkali aromatic pairs, single crystals, 39-51 Alkali cations, solvation, 23... 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

Note Added in Proof After we sent the manuscript to the publishers we became aware of CNDO studies on alkali ion solvation performed by Gupta and Rao 270> and Balasubramanian et al.271 >, which might be of some importance for readers interested in cation solvation by water and various amides. Another CNDO model investigation on the structure of hydrated ions was published very recently by Cremaschi and Simonetta 272> They studied CH5 and CH5 surrounded by a first shell of water molecules in order to discuss solvation effects on structure and stability of these organic intermediates or transition states respectively. 

The study of the interactions between organic compounds and aUtali-metal cations, in the gas phase, is related to many topics such as ion solvation, catalysis and molecular recognition. Furthermore, mass spectrometry has been used for the analyses of organolithium compounds and supramolecular assemblies that contain lithium cations. Alkali cationization is an important ionization technique, implemented for the analyses of a wide range of organic compounds. Finally, gas-phase studies are also useful for the quantitative determination of lithium cation affinity. The interaction between lithium cation and organic substances is thus related to different aspects of gas-phase chemistry and mass spectrometry. 

Similarly, a lipophilic crown ether is partitioned into the organic phase. Its complexing ability serves to transfer salts with alkali cations into the organic phase. The anions in the organic phase are poorly solvated and highly reactive. The overall reactivity in phase-transfer-catalyzed nucleophilic displacement reactions thus is a function of both the partition coefficient for extraction of the reactive anion into the organic... 

Furthermore, the equivalent conductivity is known to decrease with concentration as c1/2 for dilute solutions (Kohlrausch law). At higher concentrations the conductivity usually increases above the Kohlrausch law value [107]. Furthermore, in weakly polar solvents, there is extensive evidence that strong electrolytes do not dissociate completely, but neutral ion pairs remain in solution [107]. Indeed, solutions of alkali metals in ethers have received considerable attention and two forms of alkali-metal-cation—solvated electron ion pair have been characterised by Seddon et al. [108]. Reactions of an ion as an ion or when ion-paired should be considered as two totally different processes. 

Note Added in Proof Coordinated anionic polymerization of dienes by alkali metal alkyl catalysts and the effect of cation solvation on mechnism have been discussed in a similar manner by Medvedev and Gantmakher (231a). 

The ion-pair dissociation of ambident alkali enolates, which results in increasing 0/C alkylation ratios, can be promoted not only by dissociating solvents but also by specific cation solvation. In the latter case, EPD solvents cf. DMF and DMSO in Table 5-22b) or macro(poly)cyclic ligands such as coronands ( crown ethers ) or cryptands are used [376, 377, 660]. For example, the alkylation of sodium y9-naphtholate with (bromomethyl)benzene or iodomethane in the presence of benzo[18]crown-6 gives high O/C alkylation ratios when tetrahydrofuran or benzene are the solvents [660]. In dissociating solvents such as A,A-dimethylformamide or acetonitrile, however, so far no... 

Use of donicity values as a measure of cation solvation and cation stabilization has been demonstrated by polarographic measurements on alkali and alkaline earth metal ions, and various transition and rare earth metal ions (17,16,34). This is illustrated in Fig. 2 which shows the variation of half-wave potentials for the reduction of Tl -vTF, Zn " " -> Zn°, and Eu -> Eu " as a function of solvent donicities. 

In polar solvents such as DMF or acetonitrile, the interaction increases in the order tetrabutylammonium < K < Na < Li and, consequently, reduction potentials shift in a positive direction [6,45]. Obviously, ion pairing is greatest for the small lithium ions, in agreement with the prediction of Born s equation [46]. In the case of solvents with low dielectricity constants the pattern is different, and ion pairing becomes dominant as the radius of alkali cations increases [44,47,48]. The reasons for this behavior have not yet been studied in detail, but it has been proposed that in ethereal solvents the solvation of small cations remains stronger than that of larger ones, and therefore ion pairing of potassium should be more pronounced than that of lithium. 

The stability of halide complexes is known to increase in the sequence Cl-— Br — I- [81, 365-369]. This follows from the fact that the corresponding stability constants, estimated on the basis of molten alkali-metal nitrates, increase in the sequence given. However, alkali-metal nitrate melts are referred to media possessing the lowest ability of metal-cation solvation. Moreover, the molalities of the metal cations studied in the above-said works do not exceed 0.02 mol kg-1 and the corresponding molalities of halide ions are in the order of 0.3 mol kg-1. It is obvious enough that there is no appreciable association of halide ions in complexes with alkali-metal halides in such diluted solutions. 

Alkali metal chelates possess extraordinary anion reactivity which is attributed to deaggregated, cation-solvated ion pairs in hydrocarbon solvents where anion solvation is negligible. In contrast, solvated anions must undergo desolvation before reaction, making them less reactive than the naked anions obtained in the chelate systems. 

Powerful cation solvating agents were discovered in recent years the crown ethers and the kryptates. In their presence, the amount of dissolved alkali metals substantially increases65. Their association with cations,... 

The active species Nb=Nb, 1, can perform the six-electron reduction of N2 without the addition of any further reducing agent when the reaction is carried out in toluene. The reaction leads to the trinuclear 2-bisnitrido species, which is rather labile in the presence of solvents binding alkali cations, leading to the compounds 7 and 8 (Scheme 4). A detailed report has been published on why the solvent can drastically affect the reduction pathway of N2 mediated by metalla-calix[4]arenes. The bifunctionality of the complexes used is such that the solvation of the alkali cation can be an important driving force and at the same time the presence of tight-ion pair or separated-ion forms can affect the kinetic pathways (see Chart 3). 

An analysis of the transport properties of alkali metal halides in TFE led to association constants, which were interpreted in terms of greater anion solvation by TFE than by ethanol and also less cation solvation by TFE (19). Despite evidence that chloride ion is strongly solvated by TFE, HC1 is about 15 times more soluble in ethanol than in TFE this result emphasizes the weak solvation of protons by TFE (19). Continuing this trend, alkali metal salts are very sparingly soluble in HFIP (18). 

Product data cited previously (26, 35-37, 41) support independent evidence from kinetic studies (Table III) that HFIP is even less nucleophilic than TFE. Solubilities of alkali metal salts show the same trend in cation solvating power (18, 19). These diverse results warrant emphasis because Abraham et al. have implied (42) that the solvatochromic parameter P is a measure of solvent nucleophilicity. However, the P values for TFE and HFIP are both zero (43), and the relationship between p and solvent nucleophilicity is therefore questionable. 

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