Supramolecular kinetic stability

For such supramolecular systems, diffusion NMR may be extremely important for the determination of the structure and dynamics of the system. Diffusion is also extremely important in determining the association constant in systems, which on their formation induce a small change in the NMR spectra, and in systems in which chemical shift changes occur for reasons other than complexation, for example when protonation takes place. Diffusion NMR can also easily be used to probe the kinetic stability of multicomponent systems just by monitoring the effect of a small excess of one of the components on the diffusion coefficient of the supramolecular system. 

It is important to have an understanding of both the thermodynamic and kinetic stability of axial coordination compoimds before attempting to construct a supramolecular assembly using these features. The simple reason is that if the mode of connectivity is badly chosen, the systems will not be stable in solution, and dissociation to yield the individual building blocks will be inevitable. The basic concepts are discussed in more detail in other chapters in this book therefore we will give only a brief overview. 

Owing to the thermodynamic and kinetic stability of the Ru-pyridyl bonds, solutions of 9Zn can be treated with excess of other N-ligands without the occurrence of decomposition or scrambling processes. Thus, 9Zn is a rigid 2D module with two central junctions that can be exploited for the construction of more elaborate supramolecular adducts upon treatment with appropriate polytopic ligands. 

When the coordination bonds that hold these assemblies together are both stable and inert, their formation from the components occurs under kinetic rather than thermodynamic control and it is thus disputable if they can be truly defined as supramolecular systems. On the other hand, owing to their kinetic stability, some of these species can be further exploited as building blocks in the construction of higher order architectures through a hierarchical self-assembly approach (see for example the molecular sandwiches 13-15). Through this modular approach, multichromophore systems become easily accessible on demand, with a limited synthetic effort. 

Terpin, A.J. Ziegler. M. Johnson. D.W. Raymond. K.N. Resolution and kinetic stability of a chiral supramolecular assembly made of labile components. Angew. Chem. Int. Ed. 2001, 40. 157-160. 

The cobalt(II) complex is labile, and self-assembles to triple-helical [Co2(5-bismbmp)3] [24] which is readily oxidized to the cobalt(III) complex [Co2(5-bismbmp)3] which is sufficiently inert for the two enantiomers to be separated by classical methods [25]. The transformation of labile species to kinetically stable ones offers the possibility of using these elements as building blocks for further supramolecular construction. On the basis of our results with sophisticated ligand systems such as L232, we suspect that complicated, multicomponent self-assembly reactions without some element of kinetic stability will be rather difficult to develop. 

Mechanically bonded complexes that undergo no dissociation have the same kinetic stability as covalent molecules, whereas their supramolecular structures generate a variety of dynamic and responsive functions. Use of such supramolecular motifs may prove a useful approach to construction of the cavity-shaped frameworks that are more relevant to the modeling of enzymes. 

Impressive, highly ordered centimetre-sized fibres are obtained whose synergistic growth mechanism based on the kinetic cross-coupling of a dynamical supramolecular self-assembly and a stabilizing silica mineralization may well be the basis of the synthetic paths used by Nature to obtain its materials with well-defined multiscale architectures in biological systems. 

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

The interplay of complex stability and cation exchange kinetics is very important in the uses of supramolecular cation hosts. On the basis of their behaviour, we may distinguish between cation receptors (slow kinetics, large stability constants) and cation carriers (fast kinetics, lower stability). In the next section we will see how fast exchange kinetics make cation carriers highly useful in applications such as phase transfer catalysis. 

Several thermodynamic and kinetic behaviors of enzyme-catalyzed reactions performed in ILs, with respect to enzymatic reactions carried out in conventional solvents, could lead to an improvement in the process performance [34—37]. ILs showed an over-stabilization effect on biocatalysts [38] on the basis of the double role played by these neoteric solvents ILs could provide an adequate microenvironment for the catalytic action of the enzyme (mass transfer phenomena and active catalytic conformation) and if they act as a solvent, ILs may be regarded as liquid immobilization supports, since multipoint enzyme-1L interactions (hydrogen. Van der Waals, ionic, etc.) may occur, resulting in a flexible supramolecular not able to maintain the active protein conformation [39]. Their polar and non-coordinating properties hold considerable potential for enantioselective reactions since profound effects on reactivities and selectivities are expected [40]. In recent years attention has been focused on the appUcation of ILs as reaction media for enantioselective processes [41—43]. 

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