Solvation in complex solvents

The previous sections have focused on a generie model of a very simple solvent, in which solvation dynamics is determined by molecular translations and reorientations only. These in turn are controlled by the solvent molecular mass, moment of inertia, dipole moment and short-range repulsive interactions. When the solvent is more eomplex we may expect specific structures and interaetions to play signifieant roles. Still, numerical simulations of solvation dynamics in more eomplex systems lead to some general observations  

Numerical simulations of solvation dynamics in polar molecular solvents have been carried out on many models of molecular systems during the last decade. The study described in sections 4.3.4-4.3.5 focused on a generic model for a simple polar solvent, a structureless Stockmayer fluid. It is found that solvation dynamics in this model solvent is qualitatively similar to that observed in more realistic models of more structured simple solvents, including solvents like water whose energetics is strongly influenced by the H-bond network. In particular, the bimodal nature of the dynamics and the existing of a prominent fast Gaussian relaxation component are common to all models studied. 

Such numerical simulations have played an important role in the development of our understanding of solvation dynamics. For example, they have provided the first indication that simple dielectric continuum models based on Debye and Debey-like dielectric relaxation theories are inadequate on the fast timescales that are experimentally accessible today. It is important to keep in mind that this failure of simple theories is not a failure of linear response theory. Once revised to describe reliably response on short time and length scales, e.g. by using the full k and (O dependent dielectric response function e(k,o , and sufficiently taking into account the solvent structure about the solute, linear response theory accounts for most observations of solvation dynamics in simple polar solvents. 

Numerical simulations have also been instrumental in elucidating the differences between simple and complex solvents in the way they dynamically respond to a newly created charge distribution. The importance of translational motions that change the composition or structure near the solute, the consequent early failure of linear response theory in such systems, and the possible involvement of solvent intramolecular motions in the solvation process were discovered in this way. 

We conclude by pointing out that this report has focused on solvation in polar systems where the solvent molecule has a permanent dipole moment. Recently theoretical and experimental work has started on the dynamics of non-polar solvation. This constitutes another issue in our ongoing effort to understand the dynamics of solvation processes. REFERENCES 

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Solvent changes would be expected to result in changes in complex stability, especially where cations are strongly solvated in one solvent (e.g. Li+, Na+, Mg2+, Ca2+ in water) but not in another. The available data support this supposition, as seen in Table 14 where log K values for... 

The results in Table 11 also illustrate the dubious basis of the generalization (e.g. Pritt and Whiting, 1975) that Sn 1 reactions have AS 0, since t-butyl chloride in water has AS = +12-2, and 2-adamantyl tosylate in 97% hexafluoropropan-2-ol/water has AS = —17 cal deg 1 mol 1. Whilst it could be argued that solvation in mixed solvents is more complex than in pure solvents, it is nevertheless clear that there is a wide range of values of AS for these Sjjl solvolyses. 

A qualitative assessment of the relative ligand strengths of interaction between nitrate ion, water and four organic solvents and a number of 4-3 lanthanide ions was made from changes in f-f transitions. The affinity series of the lanthanides for the nitrate ion and the solvents was DMF > tributylphosphate > NO3 H2O > EtOH > dioxan. In DMF, hexadimethylformamide-lanthanides were the only complexes present, although conductivity measurements showed a major portion of the nitrate ion to be ion-paired, while in anhydrous dioxan the solvent-solute interaction is so weak that the rubidium-lanthanide nitrate double salts employed were not soluble. In water, in the absence of excess nitrate ion, hexaquo-lanthanide complexes predominate with little nitrate ion-association, in line with better nitrate solvation in protic solvents. 

Solvent exchange is merely a special case of substitution. However, this topic will be treated separately, as a more convenient arrangement of material can be thus achieved. Solvent exchange involving unmixed solvates in pure solvents will be discussed first, followed by mixed ligand complexes in pure solvents, then solvent exchange in mixed solvents. Proton exchange between bulk and co-ordinated solvents is covered in the final section. 

The reaction rates depend to a great extent on the nature of the organometallic compounds, such as polarity of the bonds and the degree of solvation. In polar solvents, where free solvated ion pairs predominate, the mechanism of initiation may simply consist of a direct addition of the anion to the monomer. If the solvents are nonpolar, on the other hand, the initiation is more complex. In these solvents the metal cation coordinates with the monomer first. This is followed by a rearrangement ... 

Solvation energy, a ligand that is mote strongly solvated in one solvent than another will be a poorer nucleophile when it is more strongly solvated because mote solvation energy will be lost during the formation of the metal complex. 

The water of hydration of these complexes can be replaced with other coordinating solvents. For example, the ethanol and methanol solvates were made by dissolving the hydrates in triethyl and trimethyl orthoformate, respectively (81,82). The acetic acid solvates are made by treating the hydrates with acetic anhydride (83). Conductivity and visible spectra, where appHcable, of the Co, Ni, Zn, and Cu fluoroborates in A/A/-dimethylacetamide (L) showed that all metal ions were present as the MLg cations (84). Solvated fluoroborate complexes of, Fe +, Co +, , Cu +, and in diethyl... 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

Phase-transfer catalysis succeeds for two reasons. First, it provides a mechanism for introducing an anion into the medium that contains the reactive substrate. More important, the anion is introduced in a weakly solvated, highly reactive state. You ve already seen phase-transfer catalysis in another fonn in Section 16.4, where the metal-complexing properties of crown ethers were described. Crown ethers pennit metal salts to dissolve in nonpolai solvents by sunounding the cation with a lipophilic cloak, leaving the anion free to react without the encumbrance of strong solvation forces. 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

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