Macrocyclic ligands solvation effects

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

The thermodynamic origins of the enhanced stabilities of macrocyclic ligands over their acyclic counterparts have been the subject of considerable debate since the term macrocyclic effect was first coined.83 Comparison of thermodynamic data for the several metal ion complexes of the [18]crown-6 and its acyclic counterpart are shown in Table 1. Enthalpy contributions to stabilization appear strongest for the K+ complex, while entropic contributions are stronger for the Na+ complex. Undoubtedly, the factors responsible for the thermodynamics will vary according to ion size, charge, solvation effects and structural preference. Hence, a single definable source of the macrocyclic effect is, in these systems at least, probably nonexistent. 

Since binding in solution results from a compromise between interaction with the ligand and solvation, new insights into the origin of the cation recognition process and of the macrocyclic and cryptate effects can be gained from experimental gas phase studies [2.34, 2.35] as well as from computer modelling calculations in vacuo or in a solvent [1.35b, 1.42, 1.43, 1.45, 2.36, 2.37, A.37]. In particular, molecular dynamics calculations indicate that complementarity is reflected in restricted motion of the ion in the cavity [1.45, 2.36]. 

Usually, there is a compensation effect, that is AHd —TAS11 so that AGd 0. Experimentally, AS1 is positive (especially for macrocyclic ligands), and so is very often A Hr (which means that usually, the Ln-L bonds are weaker than the Ln-OH2 ones) so that complexation reactions in water are entropy driven and, moreover, a linear relationship between AH and A. S 1 holds for the Ln(III) series of cations. One has, however, to be cautious when this approach is applied to polydentate ligands. The thermodynamic parameters may also reflect other factors such as the formation of stable 5-membered chelate rings. When another solvent is considered, the solvation enthalpy is much smaller than in water and the above considerations may no more hold. 

In solvent of low permittivity, where the cation will be very weakly solvated, there is an effective competition between a macrocyclic ligand, C, and the counter anion, A , for the metal, M. From ultrasonics and IR spectra on the Na /18-crown-6 system in propylene carbonate, which show a dependence on the nature as well as the concentration of the anion present, it has been proposed that such a competition process between the anion and the macrocycle is also relevant in solvents with a much higher permittivity it is suggested that this could account for the bimolecular mechanism proposed as a path for metal exchange and complex dissociation, given by Eq. (1). 

In some cases, the unidentate ligand is liberated at the end of the reaction. Usually, however, the ligand is found in both the reactant and the product. The effect has been most systematically examined for Ni(II). Coordinated NHj and polyamines have the largest accelerating influence. The rate acceleration induced by macrocycles resides primarily in reduced AFTI values (by 15-26 kJ mol ). The 6- and 5-coordination of solvated tetramethylcyclam complexes is controlled by the conformation at the 4 N-centers, 2 and 3. These complexes exchange by and 4 mechanisms, respectively, as indicated by positive and negative values (Table 4.9). Also Sec. 4.9. 

The introduction of -alkyl substituents to the secondary amine donors of the macrocycle results in anodic shifts in both oxidation and reduction potentials of the complexes relative to the parent ligand systems (Table II). The extent of anodic shifts depends on the number of alkyl groups introduced to the ligand (47,55a). That is, -alkylation makes the attainment of the Ni(I) state easier and the Ni(III) state more difficult. The stabilization of Ni(I) species by -alkylation is ascribed to solvation and stereochemical effects (55b, 60). -Ethyl groups have greater inductive effects than -methyl groups and yield less anodic shift in both oxidation and reduction potentials (47). This anodic shift of redox potentials may be attributed to weaker Ni-N interactions in the -alkylated complexes. The weaker Ni-N interaction for the tertiary amine results in the stabilization of antibonding o--orbitals of the Ni(II) complex, which makes it more favorable to add an electron, but less favorable to remove an electron. 

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