Reversible Intermolecular Complexation

One of the most active areas in modem chemical research involves molecular recognition, the formation of so-called host-guest (or H-G) complexes (C) that arise when two molecules fit perfectly together, held in place predominantly by hydrogen bonding  

Such complexes are sometimes called supramolecules. The choice of which molecule is the host and which is the guest is somewhat arbitrary, but the larger molecule is usually deemed the host. Enzyme-substrate complexes are a prime biochemical example of such interactions. Cram, Pedersen, and Lehn shared the 1987 Nobel Prize in chemistry for their pioneering work in the area of molecular recognition and supramolecular chemistry. The recent chemical literature is replete with examples in which NMR has been used to study such complexa-tion.6 

Of particular interest is the strength of the complexation, as measured by equilibrium constant K  

With this information, we can determine the value of K under slow-exchange conditions by first rewriting Eq. (10.9) as 

Under fast-exchange conditions the observed chemical shift (8) of the nucleus of interest will be the population average of and 8(-  

From the signal integrations it is clear that the enol form constitutes 86% of the equilibrium mixture. Furthermore, at room temperature the rate of equilibration must be much slower than the NMR time scale to observe both sets of sharp signals.  

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In another study poly(acrylic acid-g-styrene) copolymers were also shown to have good emulsifying ability and high water absorbency [105]. Membranes produced from intermolecular complexes of the above materials with poly(ethylacry-late-g-ethylene oxide) copolymers behaved like chemical valves, whose perme-ativity could be controlled reversibly by changing the pH of the surrounding medium, since both graft copolymers behave as polyelectrolytes in aqueous solution. 

The formation of the dinuclear intermediate and its dissociation to two metal complexes in a concerted pathway suggest a reversible intermolecular exchange of the two ligands. Most of the transmetalation reactions, however, occur smoothly... 

At high concentrations of macroligand solutions there is an appreciable increase in the probability of the formation of the intermolecular (and not the intramolecular) MMC [48]. By varying the degree of conversion, it is possible to observe the reversible transition between intramolecular and intermolecular complexes. This is shown in Fig. 3-4 for the formation of complexes between... 

The need of the acylurea site participating in intermolecular hydrogen bonding (cf. Figs. 11 and 12) for the complex formation is exemplified by the fact that a 1 1 mixture of JV-(p-dimethylaminophenyl)phenylacetamide (21) and JV-isobutyl-p-nitro-benzamide (22) gives no crystalline complexes under the same conditions as with 19 and 20. The trend of the complex formation often changes, when the combinations of R7 and R8 are reversed 35). 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

Studies of the association of polyphenols with proteins have a long history (27). Loomis (28) has succinctly summarised the conclusions of this earlier work. The principal means whereby proteins and polyphenols are thought to reversibly complex with one another are (i) hydrogen bonding, (ii) ionic interactions and (iii) hydrophobic interactions. Whilst the major thrust in earlier work was to emphasize the part played by intermolecular hydrogen bonding in the complexation, Hoff (29) has drawn attention to the possibility that hydrophobic effects may dominate the association between the two species. 

Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO)4] with Ru(CO)4I2 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO)4]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO)4 fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO)4] is eventually converted to [HRu3(CO)),] [as in (64)] by reaction with further [HRu(CO)4], the second [HRu(CO)4]- ion may be involved in a kinetically significant trapping reaction. 

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