Complexation enthalpy driven

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

The literature on arene - arene interactions in ternary metal-ion complexes, as reviewed in Section 3.2.3, indicates that these interactions are generally enthalpy-driven and counteracted by a reduction... 

More subtle arguments have been invoked to rationalize the dichotomous behavior of so-called second-generation Mn-salen catalysts of type 7 toward unfunctionalized and nucleophilic olefins. For example, higher yields and ee s are obtained with the (i ,S)-complex for the epoxidation of indene (8). However, JV-toluenesulfonyl-l,2,3,4-tetrahydropyridine (10) gave better results using the (R,/ -configuration. An analysis of the transition-state enthalpy and entropy terms indicates that the selectivity in the former reaction is enthalpy driven, while the latter result reflects a combination of enthalpy and entropy factors <00TL7053>. 

In general, the (scarce) thermodynamic data for exchanges involving complexes leads us to conclude that the selectivity enhancement upon complexing is enthalpy driven and may be ascribed to enhanced charge dependent (primarily coulombic) interactions with the surface as compared with the aqueous ions. 

In comparing ArH°, we note that the values are more negative than are the TArS° values. The result is a negative ArG° for reaction (15.37), resulting in a stable complex for all four alkali metals with both macrocycles. We describe the formation of these complexes as energy (or enthalpy) driven. That is, the favorable ArH° more than compensates for the unfavorable TArS° to give a ArG° < 0, and hence, a favorable equilibrium constant K. 

A reaction is said to be enthalpy-driven if it involves a large, negative AH" with a smaller and usually unfavourable TAS at all accessible temperatures. In a thermochemical analysis of such a reaction, and in comparing several such related reactions, only the enthalpy terms need normally be considered. Most redox reactions and acid-base reactions come into this category. The latter term can be interpreted liberally to include many instances of complex formation, e.g. ... 

Metal complexes also can show contradictive behavior with benzo-15-crown-5 the complexation of lanthanide perchlorates is entropy-driven in acetonitrile, while the complexation of lanthanide nitrates with corresponding disubstituted derivatives is primarily enthalpy-driven, with small entropic differences.44... 

The exergonicity (AG°) is entirely enthalpy-driven, whilst a respectable negative entropy contribution testifies to the formation of a well ordered complex structure. The energetic signature is quite different from ordinary hydrophobic bonding in water which is commonly characterized by positive association entropies thereby reflecting the poor structural definition of the associate. 

A similar complexation study supported by thermodynamic parameters was performed by the same author for mono-(6-anilino-6-deoxy)- 3-cyclodextrin and mono-[ 6-( 1 -pyridinio)-6-deoxy]-P-cyclodextrin complexation with several amino acids in zwitterionic form [24]. The inclusion complexation was enthalpy driven for the former and entropy driven for the latter host. 

A large negative AH and a near zero AS were found for the complex. This result would suggest that inclusion of the adamantyl moiety in (3 - CD is an enthalpy-driven process and that Ka decreases when the temperature increases. 

Yui s group analyzed the thermodynamics on the inclusion complexation between the a-cyclodextrin-based nanotube and sodium alkyl sulfonate [148]. They prepared a supramolecular hydrogel utilizing enthalpy-driven complexation between the molecular tube and an amphiphilic molecule [147]. They carried out the thermodynamic analysis of inclusion complexation between a-cyclodextrin-based molecular tube and poly(ethylene oxide)-Wocfc-poly(tetrahydrofuran)-b/oc/c-poly(ethylene oxide) triblock copolymer in terms of isothermal titration calorimetry [157]. Furthermore, they incorporated the tube into gels that could recognize the length of alkyl chain [158]. 

In spite of very diverse successful practical applications, the mechanism of com-plexation and the relationship between structure and selectivity are still at best only partly solved and remain open for discussion. Thermodynamic studies could supply some valuable information facilitating an understanding of the physicochemical basis of the complexation processes. The GC modified with CyDs is one of a variety of experimental methods employed in the determination of thermodynamic quantities for the formation of CyD inclusion complexes (see Chapters 8-10). The thermodynamic parameters for separation of the enantiomers were determined for various derivatives of CyDs dissolved in various stationary phases [63-65] or as a Uquid derivatized form [66]. Interesting observations were made by Armstrong et al. [66]. The authors postulated two different retention mechanisms. The first involved classical formation of the inclusion complex with high thermodynamic values of AH, AAH, and AAS and a relatively low column capacity and the second loose, probably external, multiple association with the CyD characterized by lower AH, AAH, and AAS values. The thermodynamic parameters of complexation processes obtained from liquid and gas chromatography measurements are collected in Table 5.2. It is clear from those data that for all the compounds presented the complexation processes are enthalpy-driven since in all cases AH is more negative than TAS. 

The compiexation of A-7 is enthalpy driven. The compiexation entropy is negative, indicating a tight complex with a well defined geometry. According to the CIS values... 

We have previously found that solvophobic interactions between side chain moieties of two different ligands co-ordinated to die same metal ion favor mixed complex formation by means of a favorable enthalpy contribution [47-49]. In the CDhm systems the cavity seems to play the same role as a side chain moiety and stereoselectivity is seen to be enthalpy driven. The entropy change seems to be less favorable for the D-enantiomers, the side chains of which are most probably included in the cavity. This could also be interpreted as resulting from the loss of internal rotational freedom of the side chain which predominates over the effect of cavity desolvation due to inclusion. Thermodynamic stereoselectivity was also found in the copper(U) ternary complexes with 6-deoxy-6-[4-(2-aminoethyl)imidazolyl]-p-cyclodextrin 2 (CDmh) [29], an isomer of CDhm, where the histamine is linked by the imidazole nitrogen (Table IE). In this case the copper(II) mixed complex with the D-isomer of tryptophan is less stable than the complex with the L-isomer. 

Summarizing as follows, we could say that we learned the following from the thermodynamic studies (1) The different stability of the copper(H) ternary complexes with functionalized P-cyclodextrins is enthalpy driven, in accordance with what was previously found for the ternary system without the CD cavity (2) The cis-disposition of the amine groups of the two ligands seems to be the discriminating factor, in some cases allowing the interaction of the aromatic residues of one the two isomers with the cavity (3) The difference in AH° values between the two copper(II) diastereoisomeric complexes reflects the different interactions of the aromatic moieties of the two amino acid enantiomers. 

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