Host-guest selectivity

It should be noted that not all host-guest phane complexes are of the type just described. Indeed, a significant number of lattice inclusion complexes also occur. In these, the host molecules stack such that a channel running between hosts is formed. Guest molecules occupy this channel. As expected, such an arrangement is usually reflected by relatively poor host-guest selectivity. 

The host-guest selectivity of macrocyclic ligands as measured in homogeneous solution can translate effectively into multiphase separations systems such as IC and liquid membranes, even when macrocyclic structures must be modified to accommodate system demands. Separations scientists have applied this selectivity in novel ways to these two methodologies to effect separations that have potential or realized practical uses, both in analytical chemistry and preparative separations. To date, only a fraction of the macrocyclic structures that exhibit such potential have been studied, and to the degree that this line of research is pursued vigorously, many further innovations can be expected. 

These effects appearance is explained by the nature of hydrophobic interactions and the stixrctural organization of micellar solutions. The host-guest phenomena determine the increase of the selectivity of analytical reactions in the surf actant-based organized systems. 

Attempts to realize enzymatic reactions have been reported over the past four decades in the context of host-guest chemistry, presently a well-established research field. In the field of CPOs, much attention has been paid to identical research objectives. The host-guest chemistry based on CPOs holds a special position, because specific selectivity and reactivity will be achieved using the coordination-bond-forming reactions between the substrate and the incorporated metals in the porphyrins, as well as the redox reaction associated with the porphyrin s rr-electron system. 

Figure 75 Selectivity in the formation of the two hosts. Flat guests select the asymmetric host, whereas...

The above selectivities are affected by changes in the molar ratio between the guests and between guest and host in the solution mixture. They show, however, consistent trends. Qualitatively similar results of guest selectivity were obtained with N,N -bis(tri-p-tolylmethyl)urea and a number of related hosts47. Additional experiments using ureas substituted with chiral spacers are readily envisioned. 

Combining features typical of both complexes and clathrates (coordinatoclathrate) should provide new possibilities of host-guest control2). They are indicated by the relations specified in Fig. 8, e.g. chemoselectivity or selectivity for functional groups on the one hand, caused by the complex part, and on the other hand constitutional selectivity or selectivity for molecular size and expanse due to the clathrate branche of the diagrammatic family tree of a coordinatoclathrate shown in Fig. 8. 

Enantioselective photocyclization of 74 occurred efficiently in the inclusion complex with 2a. In particular, the selectivity is very high in the case of 74a. However, control is inefficient in the 1 2 complex 85 composed of 2a and 74c. The host guest ratio probably depends on the packing of the components in the crystal. The packing is... 

Table 5.2. A selection of host-guest complexes between crown polyethers and thiourea (Pedersen, 1971).

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

---