Anion interaction acetonitrile

Overall, it is possible to divide the silyl Lewis acids into two groups, depending on how strong the counter anion interacts with the silicon atom as shown in Scheme 2. In the case where a very weakly coordinating anion is part of the compound, one could consider that a free silyl cation is present. However, the silyl cation is very strong and will be coordinated by solvent molecules like acetonitrile or toluene [25, 26]. This complex could activate, for example, a carbonyl group. Whether the carbonyl group replaces the solvent molecule is not known. In the case... 

Like calix[4]arenes, calix[4]pyrroles are versatile ligands to the extent that the composition of the anion receptor complex is solvent dependent. A representative example is that involving 8 and the fluoride anion. As shown in Fig. 4b, the well-defined change in curvature observed at 1 1 ligand fluoride mole ratio indicates that in acetonitrile one fluoride anion interacts per unit of receptor 8. However, in moving from acetonitrile to A/,A/-dimethylformamide, the noticeable changes in curvature observed at a ligand/anion mole ratio of 0.5 and 1, indicate respectively the formation of a 1 2 and 1 1 anion complexes, respectively, in this solvent. 

The anion-binding carcerand 11 was described by the Amouri group [31]. This complex contains a tetrafluoroborate anion coordinated to two cobalt(II) ions. Each cobalt ion adopts a square-pyramidal geometry. Four benzimidazole arms of the bridging ligands fill the equatorial positions, and solvent molecules (acetonitrile) coordinate to the outside axial positions. Inside the complex the included tetrafluoroborate anions interacts with the cobalt ions whose inside axial positions are otherwise coordinatively unsaturated. No exchange of the anion was observed even at 60 °C. A detailed study of the anion-binding properties in the crystal state of similar metalla-macrotricyclic cryptands has been performed by Adarsh et al. [32],... 

This equation accounts for experimental association constant values higher than theoretical ones because KA > Ki. Moreover, we may deduce that when cations are equal, KA is smaller for smaller anions, which means that they have a greater charge density and thus are more basic, since K2 is lower for them. On the contrary, KA values decreasing with an increase in the size of the anion have been observed in aprotic solvents like TMS (II), acetone (41), nitrobenzene (42), nitromethane (43), acetonitrile (44), and 1,1,3,3-tetramethylurea (45). This order can be understood if one considers that in this class of solvents the anions are scarcely solvated so that association is affected mainly by the strength of cation-anion interaction, which, with the cations being equal, increases with an increase in the anion charge density. 

The effect of the nature of the anion on y °° of the model solutes is displayed in Fig. 5. With regard to the molecular dipole interactions (acetonitrile), both acetate and chloride give higher y °° than in the polarisable methylsulfate, [BFJ- and [BTA]-, where the dipole is better accommodated. The relatively high y °° of acetonitrile in the acetate-based ionic liquid cannot be explained at present a lower value would have been expected as acetate should be more polarisable than chloride. 

The extent of radical anion-solvent interactions has been considered by Peover by comparing potentials in different organic solvents. Amongst aprotic solvents the variations can, with few exceptions, be explained purely in terms of solvation energy as predicted by the Born equation. However, when potentials of quinones in acetonitrile and in alkaline aqueous media are compared, the semiquinone radical anions are seen to be vastly more stable in water, indicating the specific nature of the solvent-radical anion interactions in that system. 

A comparison of the results obtained in acetonitrile and acetone solutions has shown [Po 64] that in acetonitrile the vibrations of the solvent are almost inde )endent of the anion, whereas in acetone they change considerably on the action of the anion. This can be attributed to the much weaker interactions of the anions with acetonitrile than with acetone. 

Cyclic voltammetric electrochemical investigations in acetonitrile revealed the respective ligand-centred reduction waves of (20), and (21) to undergo cathodic shifts of up to 130mV in the presence of H2PO4" and 65mV with Cl". No anion interactions with [Ru(bipy)3](PF6)2 were observed under analogous proton NMR and electrochemical experimental conditions. 

It may be assumed that with decreasing cation-anion interactions the free energy of activation of a displacement reaction will correspondingly decrease. This prediction is in agreement with a series of data For example. Table 4 reports the rate constants for nucleophilic displacement of halide in 1-bromobutane by and Bi N" phenoxides in dioxane and acetonitrile. The reactions of Bu4N" PhO always proceed more rapidly than the corresponding reactions involving K" PhO . 

Conductivity measurements demonstrated that both K" and BiVjN" phenoxides exist practically as an ion pair in dioxane while they are largely dissociated in acetonitrile Therefore, the fact that quaternary ammoniiun phenoxide in dioxane is more reactive than the corresponding potassium salt is mainly due to the greater interionic distance and, accordingly, to the lower cation-anion interaction energy. 

This kind of concept has been extended to imidazolium-based ligands, in which three 2,2 -bipyridyl substituents bind an Fe(II) center resulting in the formation of an anion-binding pocket capable of complexing Cl" and Br" in acetonitrile solution with log K >1. The X-ray crystal structure of the metaUocryptand shows that halides are bound solely by CH- anion interactions. Figure 17. ... 

The Outlook for Transition Metal Compounds. Relatively little is known about this problem among seven-coordinate transition metal complexes like ReP7, WF7 , NoF7 , and UF7-. We have crystallized the salts Cs WF7 and Cs MoF7 from acetonitrile and have found that both anions exhibit a monocapped octahedral structure (9). Whether this is the result of cation-anion interactions in the cubic lattice, or whether there is a different rule governing the structures of seven-coordinate transition metal compounds, remains to be seen. Further work is in progress. 

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