Water interaction with Lewis bases

The divalent state of the metal in complexes is confirmed by the diamagnetism of ytterbium derivative and the EPR spectrum of Eu compound, which is characteristic of Eu(II) complexes. The presence or absence of coordinately linked NH3 does not essentially influence the spectrum. It testifies to the ionic character of complex. Both compounds are pyrophoric on air and quickly hydrolysed in water. They interact with Lewis bases to form the coordination complexes. It results in a change of the compound colour. Heating in vacuum (200°C/10 mm Hg) gives back unsolvated compound. Complexes of ytterbium with the substituted ligand t-BuCgH7 has been synthesized by the same method [21]. 

Unlike CNTs, BNNTs are less investigated in the biomedicinal applications. The major problan remains the insolubility of BNNTs in aqueous medium, as well as any other common organic solvents this can be overcome via surface functionalization of the BNNTs. There are a few published reports available on noncovalent, as well as covalent, functionalizations of BNNTs. Zhi et al. reported BNNTs, wrapped in a polymer, were soluble in organic solvents, for example, chloroform, tetrahydrofuran. Functionalization of BNNTs by amine-terminated oligomeric poly(ethylene glycol), stearoyl chloride, interaction with Lewis bases, ammonia plasma irradiation were also studied. Recently, Chen et al. cited the noncytotoxicity of BNNTs and emphasized the need for water-soluble BNNTs. ... 

Finally, the solvent also interacts with Lewis acid and Lewis base sites that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalization of charges onto the surrounding solvent molecules causes ions in solution to be softer than in gas phase241. Again, water is particularly effective in this respect because it can act as an efficient electron-pair acceptor and donor. 

Charged metals (cations) in water behave as Lewis acids (willing to accept electrons). Water on the other hand, because it is willing to share its two unshared oxygen-associated pair of electrons, behaves as a Lewis base. Strong H2Q-metal (Lewis base-Lewis acid) interactions allow H+ on the water molecule to dissociate, hence, low pH water is produced. The degree of dissociation of water interacting with a cation (Mn+) is described by the metal hydrolysis constant (Table 2A)... 

Because metal ions are positively charged, fliey attract the unshared electron pairs of water molecules. It is primarily this interaction, referred to as hydration, that causes salts to dissolve in water. (Section 13.1) The process of hydration can be thought of as a Lewis acid-base interaction in which the metal ion acts as a Lewis acid and the water molecules as Lewis bases. When a water molecule interacts with the positively charged metal ion, electron density is drawn from the oxygen, as illustrated in Figure 16.15 This flow of electron density causes the O—H bond to become more polarized as a result, water molecules bound to the metal ion are more acidic than those in the bulk solvent. 

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

The second important influence of the solvent on Lewis acid - Lewis base equilibria concerns the interactions with the Lewis base. Consequently the Lewis addity and, for hard Lewis bases, especially the hydrogen bond donor capacity of tire solvent are important parameters. The electron pair acceptor capacities, quantified by the acceptor number AN, together with the hydrogen bond donor addities. O, of some selected solvents are listed in Table 1.5. Water is among the solvents with the highest AN and, accordingly, interacts strongly witli Lewis bases. This seriously hampers die efficiency of Lewis-acid catalysis in water. 

Analogously, water is extremely efficient in weakening hard Lewis add - hard Lewis base interactions. Consequently, when aiming at catalysis by hard Lewis adds, the inefficiency of the interaction between the catalyst and the substrate is a serious problem. Strangely enough, this characteristic of water is not recognised by many researchers working with hard Lewis acids in... 

The parameters and Ca are associated with the Lewis acid, and Eg and Cb with the base. a and b are interpreted as measures of electrostatic interaction, and Ca and Cb as measures of covalent interaction. Drago has criticized the DN approach as being based upon a single model process, and this objection applies also to the — A/y fBFs) model. Drago s criticism is correct, yet we should be careful not to reject a simple concept provided its limits are appreciated. Indeed, many very useful chemical quantities are subject to this criticism for example, p o values are measures of acid strength with reference to the base water. 

In this section we report a detailed summary of the experimental studies on the interaction of TS-1 with H2O and NH3. The choice of these two molecules is far from random. Interaction with water is important since the catalyst works in aqueous solution (Sect. 2). The interest in the study of NH3 is twofold ammonia is a reactant in the ammoximation of cyclohexanone to give cyclohexanone oxime and it is a stronger base than water, thus allowing a direct comparison between the effects induced by Lewis bases of increasing strength. 

The second important solvent effect on Lewis acid-Lewis base equilibria concerns the interactions with the Lewis base. Since water is also a good electron-pair acceptor129, Lewis-type interactions are competitive. This often seriously hampers the efficiency of Lewis acid catalysis in water. Thirdly, the intermolecular association of a solvent affects the Lewis acid-base equilibrium242. Upon complexation, one or more solvent molecules that were initially coordinated to the Lewis acid or the Lewis base are liberated into the bulk liquid phase, which is an entropically favourable process. This effect is more pronounced in aprotic than in protic solvents which usually have higher cohesive energy densities. The unfavourable entropy changes in protic solvents are somewhat counterbalanced by the formation of new hydrogen bonds in the bulk liquid. 

In summary, water appears as an extremely unsuitable solvent for coordination of hard Lewis acids to hard Lewis bases, as it strongly solvates both species and hinders their interaction. Hence, catalysis of Diels-Alder reactions in water is expected to be difficult due to the relative inefficiency of the interactions between the Diels-Alder reactants and the Lewis acid catalyst. On the other hand, the high stereoselectivities and yields observed in biosyntheses, with water as the solvent, indicate that these rationalizations cannot entirely be true. As a matter of fact, we will demonstrate in the following that Lewis acid catalysis in water is not only possible, but also allows for effective as well as environmentally friendly reaction conditions. 

This definition is completely independent from water as the reaction medium and is more general than the previous ones. In terms of Lewis acidity, the Br0nsted-type acid HA is the resnlt of the interaction of the Lewis-type acid species H+ with the base A . According to the definitions given, Lewis-type acids (typically, but not only, coordinatively unsatnrated cations) do not correspond to Br0nsted-type acids (typically species with acidic hydroxyl groups). On the contrary, Lewis basic species are also Br0nsted bases. 

Figure 2.11, on the other hand, shows the acceptor numbers of mixtures of water and aprotic solvents. Because water is protic and selectively interacts with Et3P = 0 (strong Lewis base), many of the relations curve upward. However, with HMPA, the relation curves downward, because HMPA is a strong base and easily interacts with water to weaken the interaction between water and Et3P = O. The acidity and basicity of mixed solvents are influenced not only by the acidity and basicity of the constituent solvents but also by the mutual interactions between the molecules of constituent solvents. At present, however, this cannot be treated theoretically. 

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